Chapter 18. Musculoskeletal, Soft Tissue, and Miscellaneous Applications

Beyond the well-known primary applications of emergency ultrasound lies a veritable smorgasbord of clinically useful ultrasound applications. Some of these applications allow emergency care providers to rapidly evaluate and better manage common clinical problems. This chapter will focus on eight applications of point-of-care ultrasound that are useful in the acute care setting. These include (1) evaluation of abdominal wall pain and masses; (2) airway assessment; (3) evaluation of bony cortices for rapid fracture diagnosis and postreduction alignment; (4) subcutaneous foreign body diagnosis and localization; (5) imaging of selected tendons, joints, and muscles for common musculoskeletal complaints; (6) diagnosis of salivary gland disease; (7) point-of-care detection of maxillary sinusitis; and (8) evaluation of soft tissue infections, particularly for detection and accurate localization of subcutaneous abscesses prior to drainage.

Clinical Considerations and Indications

A surprisingly wide range of pathologic processes can occur in the abdominal wall, and a patient's abdominal pain may, on occasion, be discovered due to a lesion or defect within this anatomic region. Since the area of anatomic interest is quite superficial and free of shadowing artifacts, it is well suited to sonographic evaluation with a linear array transducer. When a palpable or indistinct abdominal wall mass is found on physical examination, or when a focal area of abdominal wall tenderness is encountered, a point-of-care ultrasound examination of the affected area may help provide immediate answers to a number of clinical questions. Is the region of tenderness due to a lesion within the abdominal wall itself or does it appear that an underlying structure (e.g., a metastatic lesion in the liver) is causing the discomfort? If a lesion is present, where is it and what are its sonographic characteristics? Is it solid, cystic, hypo, or hyperechoic, and is it a vascular structure? Is a fluid collection present, and if so, is the fluid simple or complex? Is a fascial defect noted in the abdominal wall, and if present, is a loop of bowel seen passing through the defect? Armed with the additional anatomic knowledge of the site and character of the sonographic findings, as well as the clinical history, the provider can then pursue a more targeted workup.

Ultrasound examination of the abdominal wall can provide valuable information when the diagnosis of an abdominal wall hernia is unclear. In one clinical series, 39% of 144 patients with an abdominal wall mass of unclear etiology (with or without pain) were found to have a hernia.1 Incisional hernias occur as a delayed complication in up to 4% of abdominal surgeries2, and ultrasound can sometimes detect the fascial defect early in its development. While many abdominal wall hernias are apparent on clinical examination alone and do not require sonographic evaluation for diagnosis, others can be difficult to diagnose because the fascial defect is small and difficult to appreciate clinically. The fascial defect in a Spigelian hernia (also known as an interstitial hernia) will be found along the lateral border of the rectus muscle and a focal defect will be present in the aponeuroses of the transversus abdominis and internal oblique muscles, but not in the aponeurosis of the external oblique muscle. Since the fascial defect lies beneath the external oblique aponeurosis, the defect may not be apparent on clinical examination. A Spigelian hernia will typically be found where the lateral rectus sheath intersects with the inferior margin of the rectus sheath at a region termed the arcuate line, located about halfway between the umbilicus and the pubis. Signs and symptoms of a Spigelian hernia can be nonspecific and the pain may be poorly localized. Peak incidence is at age 50, with men and women affected equally.3 Additionally, ultrasound can play an important role in the evaluation of patients with inordinate pain or excessive swelling of the abdominal wall in the postoperative period after a herniorrhaphy.1

The femoral region may be host to a wide range of pathologic and postoperative entities ranging from inguinal and femoral hernias, reactive and metastatic lymph nodes, lipomas, abscesses, hematomas, seromas, lymphomas, soft tissue sarcomas, vascular bypass grafts, and pseudoaneurysms.4 Ultrasound examination of the groin can narrow the differential diagnosis and help differentiate among the many pathologic processes that occur in this anatomic region. Ultrasound is more accurate than the physical exam for distinguishing inguinal adenopathy from other inguinal pathology.2

A patient's abdominal pain is sometimes discovered to be due to a spontaneous or posttraumatic rectus sheath hematoma, most frequently caused by sudden vigorous abdominal contractions in the setting of a seizure, a coughing or sneezing paroxysm, direct trauma, or recent surgery. Older patients on anticoagulant therapy are most prone to this malady. In one series of 16 cases, 73% were on anticoagulant therapy and the mean age was 64.5 years old.5 Bleeding may occur because of rupture of an epigastric artery or vein or because of a tear of the rectus muscle fibers.2 The resulting hematoma remains confined to the rectus sheath.

Abdominal wall endometriosis can occur at the site of a previous C-section or laparotomy and should be considered in the differential diagnosis of women presenting with recurrent focal abdominal wall pain near a surgical scar during menses; the frequency of this disorder is estimated at 0.8% of all C-sections.6 In one series of 28 patients scar endometrioma sizes ranged from 0.7 to 6 cm and the average time since the last C-section ranged from 40 to 66 months. In the 12 patients with large scar endometriomas (3–6 cm), the hypoechoic lesions exhibited increased vascularity, solid and cystic portions, occasional fistulous tracts, and irregular shapes when compared with smaller scar endometriomas.7 Although the sonographic finding of a hypoechoic mass within the region of the operative scar is nonspecific, this finding coupled with a characteristic history can help make the diagnosis.8

Other abdominal wall masses such as lipomas, sebaceous cysts, subcutaneous abscesses, cutaneous metastases, a primary malignant melanoma, hemangiomas, and pseudoaneurysms of the epigastric artery may all occur in the abdominal wall and should also be included in the differential diagnosis of a palpable or tender abdominal wall mass. Ultrasound has been used to help localize the injection port of intrathecal drug delivery pumps whose location in the abdominal wall cannot be found by physical examination.9

Ultrasound also improves the performance characteristics of selected nerve blocks of the abdominal wall (the transversus abdominis plane or TAP block, as well as ilioinguinal nerve blocks), and has been used to guide injection therapy of entrapped abdominal wall cutaneous nerves at the lateral border of the rectus abdominis.10–12

Anatomical Considerations

The abdominal wall is composed of skin, subcutaneous tissue of varying thickness depending on patient habitus, muscular layers that also vary in thickness with patient habitus and conditioning, and finally a layer of extraperitoneal fat. The muscular layers are enclosed in fibrous fascial sheaths. The fascial sheaths or aponeuroses of the three lateral abdominal wall muscles (the external oblique, the internal oblique, and the transverses abdominis muscles) combine to form a thickened fascial layer known as the Spigelian fascia in the paramedian region just lateral to the paired midline rectus muscles. The region along the lateral border of the rectus muscles extending from the costal margin to the pubic bone is referred to as the linea semilunaris or Spigelius line. Moving medially, the Spigelian fascia divides into two layers to form the anterior and posterior rectus sheaths that surround the rectus muscles. In the midline, the anterior and posterior rectus sheaths from each rectus muscle combine and fuse into a single central fascial layer known as the linea alba. In long axis, the rectus muscles appear as paired bundles of muscle tissue with the muscle fibers aligned in a sagittal orientation, interrupted by three transversely oriented tendinous intersections. In cross section, the rectus muscles are ovoid in profile. Of note, the posterior layer of the each rectus sheath ends approximately midway between the umbilicus and the pubic symphysis. The thickened inferior edge of the rectus sheath at this level forms an anatomic region termed the arcuate line. Below the arcuate line, the posterior layer of the rectus sheath is composed only of a thin layer of tissue known at the transversalis fascia.

The anatomy of the inguinal region is more complex and the region immediately adjacent to the inguinal ligament is the area of anatomic interest. The inguinal ligament represents the thickened inferior border of the aponeurosis of the external oblique muscle and is located between the anterior superior iliac spine and the pubic tubercle of the pelvis. Beneath the inguinal ligament lie three important bony prominences. Moving medially from the laterally situated anterior superior iliac spine, the next bony ridge encountered will be the anterior inferior iliac spine. Continuing further medially, the large curved bony ridge of the iliopubic eminence will be noted. This ridge corresponds to the anterior rim of the acetabulum. Medial to the iliopubic eminence lies the bony prominence of the pubic crest, about 1 cm medial to the pubic tubercle. The iliopsoas muscle runs beneath the inguinal ligament in the space between the anterior superior and inferior iliac spines and the iliopubic eminence. Medial to lateral, the common femoral vein, the common femoral artery, and the femoral nerve are found just anterior to the iliopubic eminence. Lymph nodes can be found on either side of this neurovascular bundle. The deep inguinal ring lies superficial to the inguinal ligament in the region above the femoral vessels. From there, the inguinal canal courses medially and inferiorly toward the superficial inguinal ring that is found in close proximity to the pubic crest, still superficial to the inguinal ligament.

Technique and Normal Ultrasound Findings

The abdominal wall is divided into three sonographically distinct regions.4 Since the anatomic structures of interest are all superficially located, a high-frequency linear array transducer is best suited to this type of examination. If extended-field-of-view or panoramic imaging software is available, this feature can be used to demonstrate the lesion's anatomic relationship with adjacent structures in the abdominal wall (Figure 18-1).

The midline region is best scanned in short axis. The linea alba (representing the midline confluence of the anterior and posterior fascial sheaths from each rectus muscle) appears beneath the skin and subcutaneous tissue of the midline abdomen as a horizontally oriented and somewhat hyperechoic and thickened line. The linea alba is surrounded on either side by the hypoechoic triangular medial portions of each rectus muscle (Figure 18-2). The rectus muscles appear hypoechoic and speckled in short axis (Figure 18-3) and hypoechoic and striated in long axis. The anterior and posterior rectus sheaths appear as thin hyperechoic lines surrounding the muscle bundles. The underlying peritoneal interface will usually be apparent on real-time scanning. The adjacent anterior bowel wall surface appears hyperechoic, and gliding of the bowel is usually noted with respiration or with bowel peristalsis. Comet-tail artifacts, or dirty shadowing, arising from pockets of admixed air and fluid in the bowel loops may also be seen (Figures 18-1 and 18-3).

In the paramedian region, the sonographic area of interest is at the lateral border of the rectus muscle at the confluence of the aponeuroses of the lateral abdominal wall muscles. In the paramedian region, the fascial layers surrounding the three lateral wall muscles (external oblique, internal oblique, and transverse abdominal) are conjoined into one fascial layer called the Spigelian fascia. Therefore, the landmarks for ultrasound of the paramedian region are the lateral border of the rectus muscle, the Spigelian facia, and the medial border of the three lateral wall muscles (Figure 18-4A and B). Spigelian hernias occur in the Spigelian fascia between the umbilicus and the pubis.

In the inguinal region, the area of sonographic focus will be along an oblique plane between the palpable bony landmarks of the anterior superior iliac spine and the pubic crest. The region should be scanned in a series of successive parallel planes several centimeters above and below the inguinal ligament. In the normal patient, the hypoechoic iliopsoas muscles will be seen occupying the region bounded by the anterior superior iliac spine laterally, the anterior inferior iliac spine below, and the edge of the iliopubic eminence medially. The hyperechoic curve of the iliopubic eminence will be noted just beneath the anechoic femoral vessels (Figure 18-5).

Common and Emergent Abnormalities

Postoperative Abdominal Wound Evaluation

As noted earlier, a wide range of postoperative pathology may occur in the abdominal wall. A wound abscess will typically appear as a hypoechoic, sometimes isoechoic, fluid collection at or near the surgical site with clinical signs suggesting that a wound infection is present. Swirling of the abscess fluid may be noted with gentle transducer pressure on the site. Deeper abscesses may be connected by a thin column of fluid rising up to the skin surface. A postoperative seroma will manifest as an anechoic collection of easily compressible fluid with no associated clinical signs to suggest infection (Figure 18-6). Liquefying hematomas will often exhibit both simple and complex features with layering.

Lymph Node

Focal tenderness in the inguinal region may be due to a reactive inguinal lymph node. A lymph node will appear as a lobulated elliptical structure in long axis, hypoechoic at the periphery with a variably hyperechoic fatty central hilum (Figure 18-7).

Rectus Sheath Hematoma

On occasion, a patient may present with a focal region of abdominal tenderness and swelling due to a rectus sheath hematoma. Sonographically, the normally homogeneously hypoechoic rectus muscle will appear diffusely hyperechoic from hemorrhage into the muscle, and a focal homogeneous fluid collection consistent with a hematoma may also be present (Figure 18-8).

Hernia

A Spigelian hernia will appear as a hypoechoic fascial defect at or near the junction of the linea semilunaris and the arcuate line. A bowel loop may be seen extending laterally under the external oblique muscle. A small epigastric hernia will appear as a hypoechoic fascial defect in the linea alba; visualization of peristaltic movements in the herniated bowel loop during real-time scanning will help confirm that a hernia is indeed present (Figure 18-9). Seen in cross section, a herniated loop of small bowel will have a rounded target-like appearance with a hypoechoic outer muscular layer, followed by a hyperechoic mucosal layer and, on occasion, strongly reflective central echoes that arise from admixtures of air and fluid in the bowel lumen (Figure 18-10). In long axis, a linear region of “dirty shadowing” and reverberation artifacts may be seen (Figure 18-10B). When obstructed, small bowel loops will appear as dilated hypoechoic tubular fluid-filled structures with prominent hyperechoic valvulae conniventes.

Lipomas and Sebaceous Cysts

These masses typically appear as rounded or ovoid structures in the subcutaneous tissues. Palpation of the lesion will guide the clinician to the region of sonographic interest. Lipomas appear similar in echotexture to the surrounding subcutaneous tissue. A subtle curved region of echogenicity will outline the border of the lipoma in what otherwise appears to be a homogenous layer of subcutaneous tissue (Figure 18-11) Angiolipomas exhibit somewhat increased echogenicity. A sebaceous (or epidermoid) cyst appears as a heterogeneous hypoechoic cystic mass, filled with a fatty cheese-like material (Figure 18-12). If the contents extravasate into the adjacent soft tissues, an abscess will frequently form.

Common Variants and Selected Abnormalities

Endometrioma

An abdominal wall endometrioma will be found at the site of a prior caesarean section or laparotomy and will appear as a solid hypoechoic mass and scattered internal echoes similar to the endometriomas that occur in the abdominal cavity.8 An undescended testicle will appear as a homogenous mass smaller in size but similar in echotexture to a normal testicle, with its long axis parallel to the inguinal canal.

Pseudoaneurysm

A pseudoaneurysm represents an area of fibrous encapsulation around a pulsatile and expanding hematoma that occurs from arterial bleeding into adjacent soft tissue. Because there is a persistent communication between the vessel and the fluid space, to-and-fro flow will be noted between the mass and the adjacent artery and characteristic echogenic swirls will be seen on color Doppler examination. In contradistinction to a true aneurysm, the neck of a pseudoaneurysm is narrow.

Pitfalls

The major sonographic pitfalls of abdominal wall imaging are failure to consider a malignant etiology for any homogeneously hypoechoic solid lesion, especially in the groin, and failure to consider a vascular etiology for an anechoic lesion, particularly if aspiration is considered.

Clinical Considerations

The anatomic structures of the larynx and upper airway are superficially located and well suited for point-of-care sonographic assessment. Although airway ultrasound techniques are not widely used, there is growing evidence that ultrasound can provide valuable information about the anatomy of the upper airway and can be used to confirm proper placement of an endotracheal tube.

Clinical Indications

The indications for using point-of-care ultrasound for airway management are

1. Preintubation assessment of the upper airway

2. Assessment of anatomic structures for surgical airway management

3. Postintubation confirmation of endotracheal tube placement

4. Assessment of vocal cord function

5. Evaluation for epiglottitis

Preintubation Assessment of the Upper Airway

Airway ultrasound has been shown to be a useful tool for preintubation assessment in both adult and pediatric patients. Sonographic measurements of anterior soft tissue thickness at the level of the hyoid bone and thyrohyoid membrane can help predict difficult direct laryngoscopy better than clinical screening tests. Sonographic measurements of infrahyoid airway structures have been found to correlate well with CT or MRI.13 Ultrasound imaging of the width of the air column at the level of the cricoid cartilage was found to have a correlation coefficient of 0.99 with MRI measurements taken in the same group of 19 patients. This parameter could help clinicians estimate proper endotracheal tube size and avoid the complications that occur when an excessively large endotracheal tube is employed. In a study of 192 pediatric patients 1 month to 6 years of age, sonographically measured subglottic airway diameter was better predictor of proper endotracheal tube size than standard age and height based formulas.14

Assessment of Anatomic Structures for Surgical Airway Management

Ultrasound of the upper airway can also play an important role when placement of a percutaneous cricothyrotomy or tracheostomy is contemplated. Surface landmarks are notoriously unreliable, especially in obese patients. In one review, percutaneous identification of the cricothyroid membrane by 18 anesthesiologists was found to be poor. Only 30% of 108 landmark assessments were located over the cricothyroid membrane and only 10% were over the desired target site.15 In a study of percutaneous tracheotomies performed by 50 anesthesiologists on an inanimate model with unidentifiable anterior neck anatomy, ultrasound led to a significant increase in procedural success as well as a significant decrease in time to successful cannula placement (57 seconds vs. 110 seconds).16 Also, a large human study showed that the mean time for emergency physicians to accurately visualize the cricothyroid membrane with ultrasound was 24 +/− 20 seconds.17

Postintubation Confirmation of Endotracheal Tube Placement

Numerous studies discuss the role of ultrasound for endotracheal tube placement confirmation. Recognition of the characteristic sonographic patterns of endotracheal or esophageal intubation on a transverse or longitudinal view of the trachea can be of significant clinical value for rapid identification of tracheal versus esophageal intubation.18 Real-time imaging of the trachea on a transverse view at the level of the cricothyroid membrane was reported to be 99.7% sensitive and 97% specific for detection of endotracheal intubation. Real-time imaging during the intubation was found to be superior to a static imaging technique performed after the intubation. Evaluation of the static imaging technique alone revealed notably improved test characteristics when images were obtained at a suprasternal location (97% sensitivity and specificity) rather than at the cricothyroid membrane (73% sensitivity and 56% specificity).19 In another study, transverse imaging just superior to the suprasternal notch confirmed tracheal versus esophageal tube placement in 150 patients with 100% sensitivity and specificity within 3 seconds of tube insertion.20 In a cadaver model, a longitudinal scan plane was employed at the level of the cricothyroid membrane. Dynamic imaging was found to be 97% sensitive and 100% specific for endotracheal tube position confirmation. In contrast, static imaging at that site after the endotracheal tube had been placed was noted to be only 51% sensitive for confirming endotracheal tube location, which is consistent with other studies. In a randomized controlled trial, a transverse scan plane just above the suprasternal notch was employed dynamically during intubation, and correctly identified tracheal or esophageal intubations in all cases.21 In pediatric patients, visualization of widening of the glottis with endotracheal tube passage and identification of lung sliding with initial ventilation were found to be reliable indicators of tracheal tube placement.22 In one study, the combination of transverse dynamic ultrasound imaging at the cricothyroid membrane and lung sliding with initial ventilation was found to be 100% sensitive and 100% specific for endotracheal tube placement.23

Because of the large acoustic impedance mismatch between soft tissue and the air-filled trachea, visualization of an endotracheal tube within the airway may be difficult unless it is in direct contact with the tracheal wall. When a saline or foam-filled endotracheal tube cuff is utilized, the cuff will be in contact with the trachea and will exhibit a distinct sonographic pattern that assists in its identification. This technique was investigated in a series of 24 intubated patients and reached the following conclusions: (1) the saline or foam-filled cuff was best visualized in a long-axis view, (2) a slight longitudinal to-and-fro motion of the endotracheal tube further enhanced visualization of the cuff, and (3), when the cuff was visualized at the level of the suprasternal notch, the endotracheal tube was ideally situated midway between the vocal cords and the carina. The study concluded that this sonographic technique could be clinically useful for rapid assessment of endotracheal tube position in any situation where endotracheal tube movement, near extubation, or endobronchial intubation might have occurred.24

Ultrasound may also be used for secondary confirmation of endotracheal tube position either by direct observation of diaphragm motion or by identification of lung sliding during ventilation. One study of 59 emergently intubated patients ranging from newborn to 17 years of age utilized real-time B- and M-mode ultrasound and a subxiphoid window to evaluate diaphragm motion during ventilation. Of the 59 patients, 49 tracheal intubations, 2 esophageal intubations, and 8 right mainstem intubations were correctly identified with ultrasound. The authors concluded that ultrasound imaging of diaphragm motion was a “useful, quick, noninvasive, portable, and direct anatomic method for assessment of endotracheal tube position.”25 Diaphragmatic ultrasound has not been found to be a reliable indicator to distinguish mainstem endotracheal intubation, with only 50% specificity in one review.26

Using a cadaver model and a 4–2 MHz microconvex transducer, the identification of the lung sliding sign as a predictor of endotracheal tube placement was evaluated with 68 intubations in 9 cadavers.27 For differentiating esophageal versus tracheal intubation, the sensitivity was 95–100% and the specificity was 100%. Visualization of lung sliding to rule out or identify mainstem intubation has not been well studied, but is used extensively in clinical practice. However, the unilateral absence of lung sliding in patients with mainstem intubation should not be confused with a pneumothorax.28

Assessment of Vocal Cord Function

A real-time image of the vestibular folds (the false vocal cords), the vocal folds (the true vocal cords), and the arytenoids can be obtained by scanning transversely though the thyroid cartilage. Ultrasound has been found to be a useful tool for evaluation of vocal cord function by a number of investigators.29–31

Evaluation for Epiglottitis

Ultrasound has also been used to assess the anteroposterior (AP) thickness of the epiglottis. One study examined 100 normal subjects using a subhyoid window and a transverse scan plane, and the epiglottis was visualized in all cases. There was little variation in the AP diameter of the normal adult epiglottis, with an average AP dimension of 2.39 ±0.15 mm in this report.32 A long-axis view of the epiglottis may be obtained from either the midline or a paratracheal location at the level of the thyrohyoid membrane, but successful visualization of the epiglottis with this technique was noted to be only 71% compared with 100% when a transverse transducer orientation was employed. Another study touted the use of a long-axis sonographic technique as being a safe and practical way to noninvasively assess a patient for epiglottitis at the bedside. The study described a sonographic finding that they called the “alphabet P sign,” the ultrasound equivalent to the “thumb print sign” seen on the lateral neck radiograph.33 The use of ultrasound for rapid point-of-care assessment of a patient with suspected epiglottitis appears to be a promising technique.

Anatomical Considerations

The thyroid and cricoid cartilages, the cricothyroid membrane, and the upper trachea are located within the superficial subcutaneous tissues of the anterior midline of the neck. The thyroid cartilage is composed of two broad rectangular laminae that meet in the anterior midline at about a 90° angle. Superiorly, the thyroid cartilage attaches to the hyoid bone via the thyrohyoid membrane. Posteriorly, the superior and inferior horns of the thyroid cartilage connect the thyroid cartilage with the hyoid bone and cricoid cartilages, respectively. Inferiorly and anteriorly, the thyroid cartilage connects to the cricoid ring via the cricothyroid ligament or membrane; in an adult, it averages about 2 × 1 cm in size. A V-shaped gap separates the upper aspects of the thyroid laminae in the midline and the base of this gap forms the superior thyroid notch or the laryngeal prominence. The strap muscles (the sternohyoid, omohyoid, and thyrohyoid muscles) lie just anterior to the thyroid cartilage. The cricothyroid muscles extend from the lower border of the thyroid cartilage to the lower aspect of the cricoid ring and surround the anterolateral portions of the cricothyroid membrane and cricoid cartilage. The thyroid gland surrounds the lateral portions of the cricoid ring, and extends superiorly to the lower border of the thyroid cartilage and anteriorly over the upper tracheal cartilages. The narrow rectangular midline segment of the thyroid gland is known as the thyroid isthmus.

The vestibular folds (also known as the false vocal cords or ventricular folds) are composed of a thick fold of mucous membrane and connective tissue. They lie just above and protect the more delicate vocal cords below. The vocal folds (or true vocal cords) are composed of the vocal ligaments medially and the laterally adjacent vocalis and thyroarytenoid muscles. They are covered by a mucous membrane and extend from the level of the mid-thyroid cartilage anteriorly to the paired arytenoid cartilages posteriorly. The arytenoids rest on the broad posterior cricoid ring and are attached to the thyroarytenoid muscles that adduct the vocal folds. The midline gap between the vocal ligaments is referred to as the rima glottidis.

The base of the epiglottis attaches to the upper border of the thyroid cartilage via the thyroepiglottic ligament; more superiorly the hyoepiglottic ligament provides the anterior support for the epiglottis. A preepiglottic fat pad separates the epiglottis from the thyrohyoid membrane. The epiglottis approaches its widest dimension just below the level of the hyoid bone.

The cricoid cartilage is the only complete ring of cartilage around the trachea and attaches distally to the first tracheal ring via the cricotracheal ligament. The upper five or six tracheal rings of the trachea lie just beneath the skin in the region between the cricoid cartilage and the lower aspect of the suprasternal notch. The diameter of the airway at the level of the cricoid ring governs the choice of endotracheal tube size, as this is the narrowest point in the upper airway.

Technique and Normal Ultrasound Findings

Transducers

The superficial structures in the upper airway are best imaged with a high-frequency linear array, or hockey stick transducer. Use the short-axis midline, long-axis midline, or long-axis paratracheal views, depending on the airway application being performed. The length of the transducer face may limit its utilization in long axis if the neck is short or the transducer face is long. Generous application of ultrasound gel may help with obtaining adequate images at the suprasternal notch. For submandibular views, a curved array transducer may be more appropriate and will provide a wider field of view of the many structures being imaged at that level.

Thyroid Cartilage

The thyroid cartilage is best imaged in a transverse scan plane in the upper neck with the neck slightly extended. Beneath the skin and subcutaneous tissues the thyroid cartilage appears as an inverted V-shaped structure that exhibits a variety of echogenic appearances, ranging from nearly isoechoic in younger patients to hyperechoic if the cartilage has become calcified. The hypoechoic strap muscles are seen overlying the laminae on each side of the thyroid cartilage. When the thyroid cartilage is hyperechoic, the region beneath will appear nearly anechoic (Figure 18-13) and when the thyroid cartilage is isoechoic with the surrounding muscles, the laryngeal structures beneath (arytenoids, vocal folds, and vocal cords) can frequently be identified (Figure 18-14). When scanning in cross section just above the superior thyroid notch, the anterior portion of the inverted V appears hypoechoic; a thin echogenic line corresponding to the anterior portion of the thyrohyoid ligament may be noted (Figure 18-15). If the transducer is placed in long axis in the midline at the upper portion of the thyroid cartilage, the hyoid bone will be seen in cross section and its prominent posterior acoustic shadow will be noted (Figure 18-16)

Cricoid Cartilage and Cricothyroid Membrane

As the transversely oriented transducer face slides down the length of the thyroid cartilage, the inverted V-shape of the thyroid cartilage suddenly disappears, and the airway takes on a more rounded appearance. A prominent area of echogenicity will be noted in the anterior midline at the level of the cricothyroid membrane. When the insonating beam suddenly encounters the air-filled lumen of the airway, the large acoustic impedance mismatch gives rise to an echogenic periodic resonance artifact that makes identification of the cricothyroid membrane straightforward (Figure 18-17). The cricothyroid muscles surround the cricoid cartilage at this level and appear as anechoic crescents on either side of the slightly more echogenic outline of the cricoid ring. The cricoid ring is the only complete ring in the airway, is round in its transverse profile, wedge-shaped in a lateral profile, and becomes progressively taller as one moves laterally and posteriorly. The ring of the cricoid cartilage appears as a smaller circular structure in the center of the sonogram. The thyroid isthmus may be seen overlying the cricothyroid membrane and exhibits a homogenous mid-gray echotexture unless some thyroid gland pathology is present. The right and left lobes of the thyroid gland will be seen on either side of the trachea at this level and will typically exhibit a homogenous granular mid-gray echotexture (Figure 18-18). When scanning the airway in a longitudinal midline or paramedian axis, the cricoid cartilage typically appears hypoechoic and ovoid in shape. It may, on occasion, contain areas of calcification that will appear echogenic. The cricothyroid membrane appears as a hyperechoic horizontal line located between the downward slanting thyroid cartilage on the left of the image, and the ovoid hypoechoic cross section of the cricoid cartilage on the right (Figure 18-19). Typically, two hypoechoic ovals will be noted on the caudal side of the image; the one immediately below the brightly echogenic cricothyroid membrane represents a mirror image artifact. A combination of first short- then long-axis views can be used for rapid and precise localization of the cricothyroid membrane.

Vocal Cords

Place the patient in a seated or supine position with the neck in a relaxed neutral position. Assuming that the thyroid cartilage is hypoechoic, scan the arytenoids and vocal cords with a linear array transducer held in a transverse orientation over the lower portion of the thyroid cartilage. The arytenoids appear as rounded echogenic structures that are easily identified by their posterior paramedian location within the larynx and their characteristic movements on abduction and adduction (Figure 18-20).

The true vocal cords can be found by locating the hyperechoic anterior commissure; the inverted “V” of the vocal ligaments will appear as two centrally located narrow hyperechoic lines that abduct and adduct with respiration, and appear to flutter during phonation. The thicker hyperechoic false cords will be found somewhat more superiorly and will not move with phonation.

Trachea

In a midline or paratracheal long-axis orientation, identify the tracheal rings as small hypoechoic rectangular structures that appear like a string of beads in the near field. They may be either completely hypoechoic (in which case no shadowing will be seen) or somewhat calcified and have an echogenic surface with associated posterior acoustic shadowing. The tracheal lumen appears as a brightly echogenic line immediately beneath the cartilages (Figure 18-21). Identify the cricoid cartilage superiorly by its larger size and more ovoid profile allowing for accurate identification and numbering of the tracheal rings if needed.

Esophagus

Identify the esophagus just behind the left lobe of the thyroid gland. The normal gland has a uniformly fine-grained gray echotexture and the esophagus will be seen as a somewhat flaccid to flattened circular or ovoid structure with alternating rings of hypo- and hyperechogenicity. It will usually be seen just anterior to the echogenic surface of the adjacent vertebral body and just medial to the carotid artery (Figure 18-22).

Confirmation of Endotracheal Intubation

Use both transverse and long-axis views for evaluation of endotracheal tube location and cuff position. Some investigators believe the real-time long-axis view to be best for visualizing the passage of the endotracheal tube into the trachea at the time of intubation. Slight to-and-fro movement of the endotracheal tube will further enhance its visualization (Figure 18-23). Filling the endotracheal tube cuff with 8–10 mL of saline can also enhance cuff visualization and assist with its accurate placement in the suprasternal notch (typically located midway between the vocal cords and the carina). The portion of the endotracheal tube that is in contact with the anterior tracheal wall will appear as two closely spaced parallel echogenic lines (curved in short axis, linear in long axis) representing the outer and inner anterior surfaces of the endotracheal tube. The tube will typically demonstrate a distinct comet-tail or reverberation artifact in contradistinction to the periodic resonance artifact of the unintubated airway. Although less distinct than a fluid-filled cuff, an air-filled cuff may be apparent by its curved profile and associated comet-tail artifacts (Figures 18-24, 18-25, and 18-26). One recommended technique is to start in transverse orientation at the level of the cricothyroid membrane to quickly confirm that the endotracheal tube is not in the esophagus, then rotate to a long-axis orientation to confirm mid-tracheal cuff placement with a gentle to-and-fro movement of the tube. This technique has been touted as useful when supervising learner-performed intubations and might also be useful as a rapid confirmatory test in a postarrest situation when capnography is indeterminate.

Secondary Confirmation of Endotracheal Tube Placement

Diaphragmatic movement may be observed in real time from a subxiphoid window using a standard curved array abdominal transducer. Use a wide sector angle so that both diaphragms may be easily visualized from a single scanning plane. When this is not feasible, use an oblique sagittal right or left chest view from the anterior to midaxillary line. Obtain a combined B- and M-mode image and observe the direction and depth of diaphragm motion in real time. With normal respirations or mechanical ventilation, the echogenic line corresponding to the diaphragm on the M-mode tracing will be seen to move toward the transducer with inspiration and away with expiration (Figure 18-27). With correct endotracheal tube placement, observe symmetrical movement toward the transducer with a delivered breath at both diaphragms. With an esophageal intubation, the air-filled stomach will push the diaphragm away from the transducer during inspiration. Asymmetry of movement of the two diaphragms may be seen with inadvertent right mainstem intubation. In an analogous fashion, evaluation of lung sliding may similarly be used to assess for correct tube placement (See Video 7-1). Absence of lung sliding implies that the hemithorax being assessed is either not being ventilated or that a pneumothorax is present.

Epiglottis

Scan the epiglottis from a subhyoid window in transverse orientation. Its sonographic appearance is that of a bird-like face or mask; the two ovoid hypoechoic “eyes” represent three of the four strap muscles (sternohyoid, omohyoid, thyrohyoid) in cross section; the hyperechoic triangular “nose” correlates with the preepiglottic fat pad located just deep to the thyrohyoid ligament, and the downturned hypoechoic “beak” or “mouth” represents the lower epiglottis in cross section (Figure 18-28). The posterior surface of the epiglottis is seen as a hyperechoic line due to the impedence mismatch at the mucosa–air interface.

Common and Emergent Abnormalities

Esophageal Intubation

With an esophageal intubation, the otherwise flaccid esophagus will be stented open by the endotracheal tube. On a short-axis view, the anterior surface of the endotracheal tube will appear as two closely spaced parallel curved echogenic lines with posterior acoustic shadowing. It may be detected either behind the left posterolateral inferior edge of the thyroid cartilage or just lateral to the trachea at the level of the cricothyroid membrane, directly behind the left lobe of the thyroid gland. As additional confirmation of an esophageal intubation, the lumen of the airway will not show any evidence of endotracheal tube presence. Best visualization is reported to occur about 1 cm below the cricoid ring (Figure 18-29). Location of the esophagus can be somewhat variable, however. In two studies that commented on esophageal location at this level in the neck, the esophagus was noted to the left of the trachea in 85–88% of cases, and to the right or posterior to the trachea in 12–15% of cases.20,21 A nasogastric tube will similarly stent open the normally flaccid and flattened esophagus but will appear as a smaller diameter structure with similar parallel anterior echoes and posterior acoustic shadowing (Figure 18-30).

Epiglottitis

In a patient presenting with epiglottitis, the transverse sonogram through the thyrohyoid membrane may reveal a dramatically altered picture from the usual bird-like image obtained at this location. The cross-sectional view of the normally thin and hypoechoic lower epiglottis (the “beak”) will now be noted to be very thickened compared to normal, and the preepiglottic fat pad (representing the “nose”) may appear enlarged due to localized edema. Instead of appearing like a bird, the image will have the appearance of a dog's snout (Figure 18-31).

Vocal Cord Palsy

Prolonged vocalization of a single vowel (such as “e”) will enhance visualization of asymmetric vocal cord motion and abnormal arytenoid movement. Assuming the thyroid cartilage is not calcified, these findings should be readily apparent on a real-time examination. The affected vocal cord will appear shorter and in a lower position on the sonogram with anterior bowing of the flaccid vocal cord.29

Pitfalls

1. Subcutaneous emphysema, significant neck edema, hemorrhage, or an open wound over the anterior neck can make sonography of the upper airway difficult or impossible.

2. Significant neck flexion may impair adequate scanning by making it difficult to place the transducer (especially in a long-axis orientation) on the anterior surface of the neck.

3. It may be difficult to obtain good images in the suprasternal notch because of poor transducer skin contact. Using a copious amount of gel may significantly improve image quality. A saline-filled endotracheal tube cuff may be difficult to visualize if air bubbles have entered the balloon along with the saline.

4. Real-time scanning during intubation may be difficult in a patient with a short neck and may interfere with the laryngoscope handle or with external laryngeal manipulation.

Clinical Considerations and Indications

Ultrasound excels at identifying the interface between soft tissue and bone due to the large difference in acoustic impedance between the two tissues. When perpendicular to a given bony surface, most of the incident ultrasound beam will be reflected back to the transducer and the interface will be represented by a brightly echogenic line that follows the contour of bony cortex being imaged. These cortical outlines on the sonogram can be used to identify precise locations for arthrocentesis as well as for finding landmarks for ultrasound-guided lumbar puncture. With the bony cortex so readily visible, sometimes to less than a millimeter resolution, ultrasound can also provide a rapid and portable means to assess for bony fractures. This section will focus on the identification of selected fractures, specifically ribs, sternum, zygomatic arch, nasal bones, and long bones.

Ultrasound has long been known to be considerably more sensitive for diagnosing rib fractures than standard chest radiography.34–36 In one review of 103 patients with suspected rib injury, rib fractures were diagnosed about twice as often with ultrasound compared to standard chest radiography.35 Ultrasound was also found to be useful for detecting coexisting small pleural effusions that were not demonstrated on the chest radiograph. The authors of this latter report opined that the ability to provide a definitive diagnosis of rib fracture (and thus better estimate the duration of work disability) was an important advantage that supported the use of ultrasound in this clinical setting. Ultrasound has particular utility in the ED workup of a patient with an isolated area of chest wall tenderness after trauma. The time-consuming nature of the examination in patients with larger areas of chest wall tenderness (from 10 to 15 minutes/patient reported in one study) and the inability to visualize retroscapular and infraclavicular rib injuries are some of the disadvantages for this particular ultrasound application.34

Although the diagnostic sensitivity of chest radiography and ultrasound for suspected sternal fracture is reported to be similar, the time required to make this diagnosis may be considerably less with the use of point-of-care ultrasound. In one report on 16 patients with radiographically documented sternal fractures, an examiner unfamiliar with the chest radiograph results was able to locate and diagnose the sternal fracture with ultrasound in each patient within 1 minute.37 In a study of 31 patients with sternal fractures, ultrasound was found to be much more sensitive than radiography or bone scan. Twelve sternal fractures were noted on radiography, 18 with a bone scan, and 31 with ultrasound. Fracture distribution was noted as follows: 8 in the manubrium, 11 in the upper sternal body, 5 in the midsternal body, and 7 in the lower sternal body.38

Ultrasound may be most useful for diagnosis of bony fractures in “austere” environments where power, weight, and space requirements make conventional radiography impractical (e.g., battlefield or military settings, on a spacecraft or submarine, or in rural or wilderness medicine settings).39–41 In an effort to evaluate the test characteristics of ultrasound for fracture diagnosis, one group of investigators trained cast technicians to assess ED patients for fractures after a 2-hour training program. One hundred fifty-eight ultrasound examinations were performed on 95 patients; the diagnostic accuracy was found to be greater in midshaft locations and least in the metacarpals, metatarsals, proximal femur, and hip. Leg and forearm fractures were found to be straightforward to diagnose with no missed injuries in patients with midshaft fractures of the radius, ulna, humerus, femur, tibia, or fibula. Of note, no false positives were reported in any location. The authors suggested that the FAST examination could be expanded to include both extremity assessment for fractures and respiratory assessment for pneumothorax, coining an alternate acronym: the “FASTER” examination.39 Requiring little added time to perform, such an extension of the FAST examination may provide useful and timely diagnostic information in the initial management of trauma patients.

Several studies have investigated the performance characteristics of ultrasound for fracture diagnosis in a turkey bone model with ED nurses, EMTs, and medics performing the exam. Sensitivities in these reports ranged from 98% to 100% and specificities from 90% to 93%.42–44 Another group of investigators evaluated the accuracy of physician-performed ultrasound for the detection of long bone fractures. With only 1 hour of training, physicians with minimal prior ultrasound experience evaluated 58 ED patients using point-of-care ultrasound. Results were compared to plain films or CT as the gold standard. Ultrasound provided improved sensitivity with less specificity compared with physical examination. Ultrasound was found to be 100% sensitive for detecting humerus and midshaft femur fractures.40

In pediatric patients, overall sensitivity of ultrasound was only 73%, with the highest accuracy noted at the diaphysis of long bones. The majority of diagnostic errors in this study (>85%) occurred at the ends of bones or near joints.45 In a related review of physician-performed ultrasound for pediatric upper extremity fractures, there was 95% agreement between ultrasound and radiography for fracture identification.46 In a series of 31 patients with hand fractures, ultrasound was noted to be 90% sensitive and 98% specific when compared with radiography.47

Ultrasound has also been employed as a diagnostic tool in the assessment of patients with nasal trauma. In a study of 63 patients seen in an ENT clinic with clinical signs of a nasal bone fracture, standard radiography employing lateral and occipitomental views was compared with ultrasound. Using a 10 MHz linear array transducer, images were obtained in three locations: on the left and right lateral nasal walls (for evaluation of the frontal processes of the maxillary bone) and on the nasal dorsum (for evaluation of the nasal bones proper). Of the 63 patients evaluated, 42 (67%) were diagnosed with nasal fractures. Ultrasound was found to be superior to radiography for assessment of the lateral nasal walls and radiography was superior for evaluation of the nasal dorsum.48 Other authors have found ultrasound to be more sensitive than standard radiography for diagnosing fractures of the nasal dorsum (98% vs. 88%).49

High-resolution ultrasound is considered a reliable tool for the evaluation of nasal bone fractures, and several reports have advocated that this modality should be the first-line imaging modality in the setting of nasal trauma. Studies of high-resolution ultrasound have shown that it has better diagnostic accuracy than both CT50 and standard radiography.49 Higher frequency linear array transducers (20 MHz) normally used for the evaluation of skin tumors and skin thickness assessment have also been utilized for evaluation of nasal bone fractures.51 Also, in a series of 32 patients, ultrasound was found to be useful for evaluation of intraoperative repositioning of the nasal bones when a closed reduction was being performed.52

Several additional applications of bony ultrasound have been reported in the medical literature. These include intraoperative postreduction confirmation of the position of zygomatic arch fracture fragments,53 diagnosis of infant hip dislocation, diagnosis of pediatric skull fractures in children with scalp hematomas,54 diagnosis of infant posterior shoulder dislocation,55 diagnosis of posterior sternoclavicular dislocation,56,57 diagnosis of subtle fractures of the clavicle and femur in infants,58 and use of ultrasound as a procedural aid for closed reduction in displaced extra-articular distal radius fractures or pediatric forearm fractures.59 In a series of 62 adults with distal radius fractures, the rate of repeat manipulation and reduction was significantly reduced (1.6% vs. 8.8%) when ultrasound was used compared with a standard “blind” technique. The ultrasound group was also noted to have an improved volar tilt and a reduced operative rate.60 For each of these applications, identification of the bright bony cortical echo allows the provider to assess whether the bone and its relationship with surrounding structures is normal or abnormal.

Anatomical Considerations

The location for ultrasound of the ribs is usually guided by the patient's complaint of pain; the rib segment in question will be found beneath skin, subcutaneous tissue, and the relevant chest wall musculature at the site being investigated. It is important to remember the curved course of the ribs when scanning. The sternum is superficially located beneath skin, subcutaneous tissue, and the medial portions of the pectoralis major muscles, and is composed of two flat bones, the manubrium superiorly and the sternal body inferiorly. The first seven ribs articulate with the manubrium and sternal body laterally, and the manubrium articulates with the sternal body at the sternal angle. The shafts of the long bones (humerus, radius, femur, and tibia) are fairly rounded in cross-sectional profile and become wider and flatter on their distal aspects. The bony support of the external nose is provided by the two nasal bones along the upper dorsum of the nose and the frontal processes of the maxillae laterally. The nasal bones are contiguous with the frontal bone above via the nasofrontal suture and the maxillae laterally via the nasomaxillary sutures (Figures 18-32 and 18-33). The frontal processes of the maxillae are contiguous with the frontal bone via the frontomaxillary sutures. The bony zygomatic arch sits beneath skin and subcutaneous tissue and is formed anteriorly by the temporal process of the zygoma and posteriorly by the zygomatic process of the temporal bone. The masseter muscle originates from the edge of the zygomatic arch and inserts on the ramus of the mandible below.

Technique and Normal Ultrasound Findings

Use a linear array transducer (in the 7.5–12 MHz range) to assess for fractures of the ribs and costochondral cartilage. In short-axis orientation, a rib will be seen casting a dense posterior acoustic shadow beneath its echogenic superficial cortical surface (Figure 18-34). Slightly below the rib, the pleura will be seen as a brightly echogenic horizontal line. Pleural sliding and comet-tail artifacts will usually be noted at this interface on real-time scanning. At the site of maximal tenderness, align the transducer to evaluate the rib in the short-axis orientation in the center of the image. Then, turn the transducer parallel to the long axis of the rib. The superficial cortex of the normal rib and costal cartilage will appear as a thin echogenic line on the sonogram (Figure 18-35). Remain directly over the long axis of the rib since the pleural line will also appear as a horizontal echogenic line on the sonogram, albeit somewhat deeper and with pleural sliding and comet-tail artifacts usually apparent (Figure 18-36).

Use a 7.5–12 MHz linear array transducer for evaluation of a suspected sternal fracture. Scan the sternum in both long- and short-axis views, although the long-axis view is reported to be the most fruitful for fracture detection. The sternal surface will appear as a horizontal echogenic line with a slight elevation in the cortical surface noted at the level of the sternomanubrial junction. As with evaluation for suspected rib fractures, scanning at the area of maximal tenderness can help locate the fracture site quickly.

Ultrasound of long bones in the trauma setting can be undertaken with the same curved array transducer that is used for the FAST examination. A transverse orientation on the limb being scanned is best for quickly establishing the location and depth of the bone being examined (Figure 18-37). Once the lower end of the relevant bone has been located, rotate the transducer longitudinally and move the transducer up the extremity to evaluate for any cortical irregularities along the shaft (Figure 18-38). The cortical surface of the bone closest to the transducer will be seen as a smooth brightly echogenic line on the sonogram.

Use a 10 MHz or higher frequency linear array transducer for sonography of the nasal pyramid. Place the transducer along each side of the nose aiming medially along the lateral nasal pyramid to assess both the frontal process of the maxilla and ipsilateral proximal nasal bone, and along the left and right paramedian midline to assess the full length of the nasal bones proper (Figures 18-39 and 18-40). Set imaging depth to 3 cm and, as with all bone imaging, adjust the electronic focus to maximize resolution at the level of the cortex. Using transducers and settings similar to those used for nasal bone imaging, visualize the zygomatic arch by scanning the upper lateral cheek in a horizontal scan plane (Figure 18-41).

The same imaging techniques apply when ultrasound is used for reducing fractures (typically in the distal radius and forearm).

Common and Emergent Abnormalities

Rib Fractures

Fractures of the rib or costochondral junction will be recognized by a clear discontinuity of the anterior cortical echo of the rib, costochondral junction, or costal cartilage (Figure 18-42), or by real-time visualization of widening of the fracture line with local transducer pressure. Comet-tail artifacts may be noted to emanate posteriorly from the mobile fracture site (Figure 18-43), and a hypoechoic fracture hematoma will commonly be seen adjacent to the fracture.

Sternal Fractures

A sternal fracture will appear as a disruption in the cortical echo of the anterior sternum; movement of the sternum fracture fragments with respiration may be noted during real-time scanning. A hypoechoic fracture hematoma may be seen adjacent to the fracture site (Figure 18-44).

Long Bones

Fractures of the femur, tibia, and humerus are best appreciated with a long-axis scanning technique and will be apparent as an obvious disruption in the echogenic line that corresponds to the cortical surface of the bone. Examples of common long bone fractures are demonstrated in Figures 18-44, 18-45, 18-46, and 18-47.

Nasal Bones

A nasal bone fracture is clearly demonstrated by the large hypoechoic gap in the normally echogenic cortical surface in Figure 18-48.

Zygomatic Arch

The normal contour of the zygomatic arch is clearly disrupted in Figure 18-49, and a prominent hypoechoic fracture hematoma is present.

Pitfalls

1. General pitfalls. An important pitfall of fracture sonography was highlighted in an experimental study examining the sonographic profile of fractured cadaver bones. It was observed that fractures and bony defects were not well visualized when the transducer was oriented parallel to the fracture line or zone of bony impaction. Optimal imaging of a fracture and any associated bony displacement requires that the ultrasound transducer be oriented axially along the bone and, ideally, perpendicular to the fracture line.61 Characteristically, an interruption of the normal cortical echo reflection and its associated posterior acoustic shadow will be noted; additionally, a dorsal band of comet-tail echoes may be seen at the fracture site.62

2. Rib imaging pitfalls. If the transducer is located partly over the rib and partly over an intercostal space (or over a portion of the scapula), the image obtained on the sonogram may be interpreted as representing a fracture when, in fact, none is present. Costal cartilage calcifications may also give rise to this “pseudofracture” phenomenon.

   Misidentifying pleura for a rib. Do not mistake the brightly echogenic pleural surface (seen when scanning along the long axis of an intercostal space) for the cortex of the rib. Initially scan the rib in question in short-axis orientation; it will be easily identified by its characteristic posterior acoustic shadowing. Note the location of the superficial surface of the rib and its depth within the soft tissues of the chest wall. Next, rotate the transducer into a long-axis orientation directly over the rib surface to assess for the presence of a fracture. The brightly echogenic pleural line will usually be seen about a centimeter deep to the superficial surface of the rib. Careful observation during real-time scanning will typically reveal to-and-fro gliding movements (“lung sliding”) at the brightly echogenic pleural interface. The lung sliding sign will be absent if a coexistent pneumothorax is present, however, and it may be the depth of this echogenic line alone that clarifies its identity as rib or pleural surface.

3. Sternum fracture pitfalls. The hypoechoic sternomanubrial junction may be confused with a fracture on the long-axis view. In general, a fracture of the sternum will appear as a sharply defined area of cortical discontinuity, whereas the sternomanubrial junction will appear as a gentle and smoothly edged ridge with a small hypoechoic joint space in between. Another reported pitfall, when imaging the sternum with ultrasound, is mistaking the hypoechoic pectoralis muscles for a hematoma on a short-axis view.37

4. Long bone fracture pitfalls. While generally excellent for diagnosing midshaft fractures of the long bones, diagnostic accuracy for fracture detection with ultrasound is limited by a number of factors. Diagnostic accuracy is notably poorer in the metacarpals, metatarsals, with small avulsion injuries, and injuries involving the joint space. Notably, imaging “at or above the intertrochanteric line of the femur” is felt to be fraught with difficulty, with a propensity for false positive studies to occur in this area.40 These areas of poorer diagnostic accuracy are generally more challenging to image and interpret, likely due to the many irregular bony acoustic interfaces present. Subcutaneous air around an open fracture may also adversely affect image quality and therefore diagnostic accuracy.

5. Nasal bone fractures pitfalls. A large transducer head may be difficult to place on a small nose. In such cases, copious use of gel or an acoustic standoff such as a gel-filled portion of a rubber glove or a piece of a commercially available gel pad may prove helpful for obtaining an adequate image.

6. Zygomatic arch fracture pitfalls. The zygomatic arch is long and narrow and the transducer needs to be accurately aligned to image the arch fully. The length of the full contour of the arch may exceed the length of the transducer, and several images may be required to fully assess the arch for a fracture.

Clinical Considerations

Correctly diagnosing and managing a wound that harbors a soft tissue foreign body can be challenging, especially when the foreign body is radiolucent. To further complicate matters, wounds that harbor foreign bodies often occur in the hand or foot where the likelihood of iatrogenic injury from blind wound exploration and the potential for subsequent infectious complications is high. Although usually located in superficial soft tissues, foreign bodies may cause no symptoms initially and can easily be overlooked. In a retrospective review of 200 patients referred for retained foreign bodies, 38% were misdiagnosed on the index visit.63 Even with a high index of suspicion, liberal use of radiography, and exploration, a soft tissue foreign body can be missed. The possible infectious and medicolegal consequences of this missed diagnosis can be unfortunate for the patient and the provider alike. Missed foreign bodies have been reported to be one of the most common causes of malpractice claims against emergency physicians.64

While metal and glass are radio-opaque and usually apparent on standard two-view radiographs, other commonly encountered foreign bodies, particularly organic material such as wood or thorns, are nearly always radiolucent. Plastic is also typically radiolucent. CT or MRI may be useful in the assessment of suspected foreign bodies, but these modalities are expensive, time-consuming, and not always readily obtainable. Furthermore, the sensitivity of CT for the detection of wooden foreign bodies is low, reported to range from 0% to 60%.65,66 Ultrasound offers some decided advantages in this setting. For detecting wooden foreign bodies—nearly always missed with plain radiography—ultrasound is 79–95% sensitive and 86–97% specific.65,67,68 In a case where a radiolucent foreign body is suspected, an ultrasound evaluation of the wound should be considered in the ED workup. Whether radio-opaque or radiolucent, once a soft tissue foreign body has been identified, the next issue faced by the clinician is how best to remove it. As most experienced clinicians will confirm, removal of a subcutaneous foreign body can be enormously frustrating. Ultrasound can additionally be used to provide precise preoperative localization of the foreign body, or, if desired, the foreign body may be retrieved under direct sonographic guidance.

Clinical Indications

The clinical indications for the use of ultrasound in the management of a suspected soft tissue foreign body include:

• Detection and localization of a foreign body
• Foreign body removal

The literature on sonographic detection of soft tissue foreign bodies encompasses a wide range of specialties and methodologies.65,67–80 The types of foreign bodies that have been described include metal, wood, graphite, plastic, gravel, sand, thorns, cactus spines, and bamboo twigs. Clinicians with varying levels of skill perform the ultrasound examinations in these studies, ranging from emergency physicians with no prior formal training, credentialed clinicians, and radiologists specially trained in musculoskeletal ultrasound. The ultrasound machines and transducers are different in nearly every study. While this literature is therefore somewhat difficult to synthesize, a number of useful conclusions can be drawn.

Success in detecting foreign bodies varies widely in the experimental literature, depending in part on the tissue model employed and foreign body type. Using a homogenous beef cube as a tissue model, ultrasound was 98% sensitive and specific in identifying a variety of embedded foreign bodies in one report,70 whereas another study using a chicken thigh model (a model that more closely mimics the human hand) reported an overall sensitivity of only 79% for detecting a wooden foreign body.67 In studies involving freshly thawed cadaver feet and hands, diagnostic sensitivities and specificities ranged from 90% to 94% and from 90% to 97%, respectively.73,78 In contrast to such excellent results, another investigation that used ultrasound for detecting foreign bodies in chicken thighs reported an overall sensitivity and specificity of 43% and 70%, respectively, with a sensitivity of only 50% for detecting a 1-cm-long piece of wood.72 Review of the methods employed in this study revealed that the chicken thighs were incised and systematically opened with a hemostat prior to foreign body placement. Such tissue disruption with the likely introduction of subcutaneous air probably exceeds that which occurs in natural wounding and may have made subsequent sonography more difficult. In a cadaveric study where 6 emergency physicians performed a total of 900 assessments looking for a <5 mm foreign body, the diagnostic sensitivity and specificity were found to be 53% and 47% respectively.81 The model employed skin incision technique, depth of foreign body placement, and foreign body type and size as variables that can all dramatically affect outcome in such experimental studies. Of note, vigorous wound irrigation itself can introduce subcutaneous gas bubbles that can interfere with attempts to locate small glass fragments with ultrasound. However, in a study in which air was purposely injected into turkey breasts containing glass, metal, and bone, the soft tissue gas did not appear to diminish the ability to locate the foreign bodies.82

Success in soft tissue foreign body detection also depends on foreign body size. The test characteristics of ultrasound reported among various studies must therefore be interpreted with an awareness of the size of the experimental foreign body being imaged. Small glass fragments and cactus thorns were difficult to detect in one report, and may have exceeded the limits of the ultrasound transducer's resolution.70 Variations in detection rates with two differing lengths of wooden toothpicks inserted into freshly thawed cadaver feet were reported. Sensitivity decreased from 93% for detecting a 5.0-mm-long fragment to 87% for detecting one that was 2.5 mm long.73 Specificities were uniformly high across studies, indicating that it is uncommon to falsely identify a foreign body when none is present.

While it might appear intuitive that provider's experience and expertise would be a crucial determinant of success for foreign body localization, there is little experimental evidence to support this assumption. Only one study directly compared the ability of various types of clinicians to locate foreign bodies in a chicken thigh model with ultrasound.67 It found no statistically significant difference in accuracy between a board-certified radiologist whose practice was limited to ultrasonography, two sonographers, and three emergency medicine residents. Sensitivity was 74% in the hands of the emergency physicians compared to 83% and 85%, respectively, for the radiologist and sonographers.

In clinical case series, wood is the most common radiolucent material reported. Hand, foot, calf, and forearm injuries predominate, and most foreign bodies are found to be superficial in location.65,68,83 One series of 50 patients evaluated for radiolucent foreign bodies noted that 45 of the 50 injuries involved the hand or foot.68 All of the 21 foreign bodies retrieved at surgery were found less than 2 cm from the skin surface in this report. In another case series of patients evaluated for suspected wooden foreign bodies in the feet, all 10 of the wooden foreign bodies discovered with ultrasound were located between 0.4 and 1.4 cm from the skin surface.65 Ultrasound can also be utilized to detect and retrieve an errant piece of tongue or cheek piercing jewelry that has become accidently embedded beneath the skin surface or to detect a suspected radiolucent foreign body in a patient who has engaged in self-embedding behavior.

Anatomical Considerations

Since hand and foot wounds are the most common injuries that may harbor a subcutaneous foreign body, a thorough familiarity with the anatomy of the hands and feet is essential for the clinician scanning these regions. Given the relatively shallow depth of the soft tissues in these anatomically intricate regions and the multiple acoustic interfaces present, clinicians should practice scanning on normal hands and feet to gain familiarity with the normal sonographic appearance of these commonly injured areas. The utility of examining the contralateral uninjured extremity for comparison when a confusing sonographic finding is encountered cannot be overemphasized.

Technique and Normal Ultrasound Findings

Use the highest frequency linear array transducer available when searching for subcutaneous foreign bodies since most will be found located within 2 cm of the skin surface. A linear array transducer in the 7.5–12.0 MHz range is generally recommended. A 7.5 MHz curvilinear transducer, such as an endocavitary transducer, may also function adequately for this application and has the added advantage of having a smaller, rounded skin contact footprint for scanning in web spaces.69,84 A 5.0 MHz transducer may be useful when searching for a deep foreign body. Higher frequency small parts transducers (typically in the 10–15.0 MHz range) offer the ability to discern very small foreign bodies; a 12.0 MHz transducer can reportedly detect a 1–2 mm foreign body. Small parts —“hockey stick”—transducers have become a more common addition to ultrasound equipment. The combination of high-resolution imaging with a small skin contact footprint makes these transducers particularly useful for imaging digits and web spaces for foreign bodies.

Use of an acoustic standoff pad may be necessary with some transducers to adequately image the superficial soft tissues. Standoff pads provide a sonolucent acoustic window, raise the transducer 1–2 cm above the skin surface, and move the subcutaneous region of interest beyond the extreme near field (and beyond the transducer's “dead zone”) into a more suitable focal zone. Although incorporating the use of a standoff pad into the ultrasound examination requires additional technical agility and some practice, the effort can be amply rewarded with improved near-field image quality. Inexpensive and commercially available gel pads are available just for this purpose, and smaller chunks can be cut off for single patient use and then discarded. Other options include the use of a water- or gel-filled glove or glove finger. When using a water-filled glove, it is essential to exclude any air bubbles that may impede subsequent imaging. In the case of a large or gaping wound, copious sterile surgical gel can be applied onto the wound; after the ultrasound examination is completed, the wound should be thoroughly irrigated. A water bath technique, in which the affected extremity is submerged in a basin of water during scanning, represents an alternative to the use of a standoff pad or the copious use of sterile surgical gel. Compared to direct contact with gel, the water bath technique is easier to perform and provides superior images of tendons and foreign bodies.85,86 The water bath technique also causes less patient discomfort since images may be obtained without direct contact between the patient and the transducer.87 The provider should ensure that only the sealed portions of the transducer are immersed in the water bath or that a waterproof transducer cover is employed.

Optimizing depth and focus are particularly important when using ultrasound to search for a small, subcutaneous foreign body. Hold the transducer perpendicular to the skin surface and systematically scan the area of interest in two orthogonal imaging planes. Best visualization of a foreign body will occur when the transducer is aligned such that the long axis of the ultrasound beam is parallel to the long axis of the foreign body. Small objects can easily be missed when scanned in short-axis orientation alone. With a small wooden foreign body, however, it is sometimes the prominent posterior acoustic shadow on the short-axis view that alerts the provider to its presence. Because wounds containing foreign bodies can occur on any part of the body, a wide array of normal sonographic findings is therefore possible depending on the anatomic region being scanned. Most wounds suspected of harboring a foreign body occur in the hands and feet, however, where numerous anatomic structures and interfaces, each with a distinct sonographic appearance, will be encountered. The skin surface is the most superficial echogenic structure encountered, and is seen adjacent to the transducer surface (or, if an acoustic standoff is used, adjacent to the distant side of the anechoic standoff pad). In the hands and especially the feet, this layer is notably thicker than elsewhere on the body. Subcutaneous fat appears hypoechoic with a reticular pattern of echogenic connective tissue seen between the fat lobules. The thickness of this layer varies considerably with body location and habitus. Fascial planes appear as thin, echogenic, usually horizontal lines immediately adjacent to the muscle. Muscle tissue appears relatively hypoechoic with regular internal striations (linear or pennate in long axis and speckled in short axis due to the relative orientation of the muscle fibers). Tendons are moderately echogenic, appear ovoid and finely speckled in short axis, and rectangular with a characteristic fibrillar sonographic echotexture in long-axis orientation. Interestingly, tendons will appear considerably more hypoechoic when imaged obliquely; this characteristic of tendon imaging is known as anisotropy and is discussed in greater detail in the musculotendinous applications portion of this chapter. Tendon movement can be observed in real time when the corresponding joints are moved. Bone appears brightly echogenic on the cortical surface closest to the transducer, with prominent posterior acoustic shadowing. Joint spaces can be readily identified by a V-shaped discontinuity in the bright cortical echo of adjacent bones. Blood vessels are anechoic and have a circular or tubular profile when scanned in short or long axis, respectively. They can be further characterized with color flow Doppler, if necessary. In general, veins will compress easily with transducer pressure whereas arteries will remain pulsatile. Sonograms of the thenar eminence of a normal hand and a chicken thigh (commonly used as a tissue model for foreign body imaging) are shown in Figures 18-50 and 18-51.

Common and Emergent Abnormalities

Soft tissue foreign bodies exhibit a variety of sonographic patterns depending on the material involved, the size of the foreign body, and the length of time the foreign body has been present in the tissue. Common materials such as wood, glass, metal, plastic, and gravel will generally appear hyperechoic with variable amounts of posterior acoustic shadowing and associated artifacts that are material and shape dependent. A wooden foreign body typically casts a hypoechoic posterior acoustic shadow that often facilitates its discovery (Figures 18-52, 18-53, 18-54, and 18-55). Linear metallic foreign bodies will typically display a reverberation artifact with bright, regularly spaced parallel lines seen distal to the actual object (Figures 18-56, 18-57, and 18-58). Metal objects that are small, or rounded, may display a comet-tail artifact (Figure 18-59). The acoustic profile of glass is less consistent, however, and acoustic shadowing, reverberation artifact, or diffuse beam scattering may all be encountered during scanning (Figures 18-60 and 18-61). Foreign bodies retained for longer than 24 hours are frequently surrounded by a hypoechoic “halo,” resulting from edema, pus, or granulation tissue. This hypoechoic region around the foreign body often facilitates identification and localization of the foreign body. In a similar fashion, a local anesthetic injected adjacent to a foreign body may improve the ability to visualize it. Sonograms of less commonly encountered foreign bodies such as plastic (with a prominent reverberation artifact) and gravel (with a prominent posterior acoustic shadow similar to a gallstone) are demonstrated in Figures 18-62 and 18-63.

Common Variants and Selected Abnormalities

Various wound characteristics can complicate sonographic evaluation for a soft tissue foreign body. Air introduced into the wound by the injury itself, from the process of wound exploration or wound irrigation, or from bubbles inadvertently administered with the anesthetic agent can cause imaging difficulties. Air bubbles and associated artifacts may obscure the foreign body or may be mistaken for a foreign body when none is actually present. Air bubbles introduced during wound irrigation can complicate subsequent attempts to locate small glass fragments. Air pockets can sometimes be obliterated by compression with the transducer, thereby improving image quality. Ultrasound examination of large open wounds may be difficult because of bleeding, associated tissue distortion, or patient discomfort.

Pitfalls

1. Inadequate knowledge of the regional sonographic anatomy. Lack of familiarity with normal sonographic anatomy, particularly of the hand and foot, can make correct interpretation of the ultrasound image difficult. Normal acoustic shadows from bone, brightly echogenic tissue interfaces and fascia, and artifacts arising from vascular calcifications, can all lead to misinterpretation of the image. Sesamoid bones may be falsely interpreted as a foreign body. Bony shadowing of the multiple carpal and metacarpal bones (or tarsal and metatarsal bones) and phalanges should not be confused with a foreign body.

2. Failure to take necessary steps to optimize scanning of small superficial objects. Scanning for subcutaneous foreign bodies, particularly in hands and feet, requires a significant investment in time, patience, and attention to certain scanning principles. A standoff pad or water bath may be required. Attention to transducer frequency, depth, and focus adjustment is crucial. In addition, the location of the wound and the transducer's skin contact footprint may make adequate imaging technically difficult. Scanning small curvilinear regions, such as a web space, may require a small parts (hockey stick) or endocavitary transducer, as well as standoff pad and the help of an assistant.

3. Other pitfalls. Small foreign bodies may exceed the limits of the transducer's resolution. Tissue interfaces in close proximity to one another may distort the image. A small foreign body adjacent to a bone may be hidden by the bone's posterior acoustic shadow. Scar tissue, air, ossified cartilage, and keratin plugs may all appear as small hyperechoic structures and be mistaken for foreign bodies.

4. Foreign body removal. Once a soft tissue foreign body has been located, an important clinical decision must be made as to whether retrieval is appropriate or even technically feasible. It is wise to set a time limit for exploration and foreign body removal and to have a plan for further evaluation or referral.88 Various factors to consider include the skill of the operator, the body part injured, the time available, and the size and type of foreign body involved. With a deep wound, a poorly accessible foreign body, or closely adjacent neurovascular structures, consultation or referral to an appropriate surgical specialist is recommended. Advantages of real-time ultrasound use in this setting include knowledge of the exact depth and location of the foreign body as well as its proximity to other structures. Ultrasound-guided removal should be considered as “a first choice procedure for the extraction of foreign bodies” and can usually be successfully performed in 15–30 minutes.89

Case Study

Patient Presentation

A 42-year-old man presented with a 10-day-old puncture wound of the finger with associated soft tissue swelling. The patient reported getting a splinter in his hand while sitting on a park bench. He thought he had removed the entire splinter at the time of the injury.

On physical examination, there was swelling over the volar proximal third phalanx but no definite fluctuance (Figure 18-64A). Distal neurocirculatory examination was intact, and there were no signs of lymphangitis or tenosynovitis.

Management

A radiograph of the affected digit was obtained and notable only for some soft tissue swelling (Figure 18-64B). An ultrasound was performed using a 7.5 MHz annular array transducer and an acoustic standoff to obtain the images in both longitudinal (Figure 18-64C) and transverse (Figure 18-64D) planes. A hyperechoic foreign body with an associated hypoechoic surrounding inflammatory response was seen above the PIP joint. The wood fragment was easily removed with a superficial skin incision along the PIP crease (Figure 18-64E).

Commentary

This case emphasizes the advantage of sonography over plain radiography for identification of wooden (and other radiolucent) foreign bodies. Foreign bodies retained for longer than 24 hours will typically be surrounded by a hypoechoic “halo” resulting from edema, or early infection around the foreign body. This hypoechoic “halo” can facilitate identification and successful removal of a foreign body.

Clinical Considerations

Technological advancements over the past decade have led to the increased use of diagnostic ultrasound for evaluation of a wide range of musculotendinous and rheumatologic conditions. Smaller, more portable ultrasound units, high-frequency small parts transducers with resolutions to a fraction of a millimeter, tissue harmonics, compound imaging technology, and extended-field-of-view features have helped promote ultrasound as a diagnostic tool for evaluating musculotendinous complaints by radiologists, orthopedists, rheumatologists, and emergency physicians. Ultrasound has emerged as “a powerful extension of the physical examination,”90 especially in the setting of a musculotendinous complaint. Imaging of tendons, joints, and muscles can help the provider make a correct diagnosis for a host of painful musculoskeletal conditions and allow for optimal patient management. Self-teaching programs in musculoskeletal ultrasound have been promulgated for rheumatologists with excellent results reported after 24 hours of active scanning and 8–9 hours reviewing images with tutors.91 A similar self-teaching program has been incorporated into the educational curriculum at a residency program in Physical Medicine and Rehabilitation.92

A myriad of musculoskeletal applications for diagnostic ultrasound has been described in the ultrasound literature. These include (1) evaluation of suspected partial or complete tendon tears (rotator cuff, triceps tendon, distal biceps tendon, Achilles tendon, quadriceps tendon, patellar ligament, and flexor tendons of the hand), (2) evaluation for suspected muscle tears (typically in the rectus femoris and gastrocnemius muscles), (3) diagnosis of occult ganglion cysts in the wrists or fingers93 and diagnosis of soft tissue tumors of the hand,94 (4) dynamic evaluation of flexor tendons of the hand (for evaluation of annular pulley ligament disruption or assessment of tendon location after flexor tendon avulsion), and (5) assessment of selected nerves for suspected entrapment or compression (e.g., the ulnar and median nerves in cubital and carpal tunnel syndromes). Some of these applications require expertise in ultrasound beyond that of most clinicians or they may require specialized transducers that are not generally available in the acute care setting. Since linear array and specialized small parts (hockey stick) transducers are available on most point-of-care ultrasound units, a host of musculotendinous applications can be added to the diagnostic armamentarium of emergency care providers.

Clinical Indications

Clinical indications for performing a musculotendinous ultrasound examination may include

• Assessment of suspected partial/complete tendon or muscle tears
• Evaluation of suspected tenosynovitis and selected tendinopathies
• Precise needle guidance for aspiration and soft tissue injection procedures involving tendons and bursae

Assessment of Suspected Partial/Complete Tendon or Muscle Tears

The use of ultrasound to image rotator cuff tears has encountered variable success.95–97 A number of contributing factors have been identified to explain the wide range of reported accuracy for diagnosing rotator cuff tears with ultrasound (60–95%). The technical difficulty of the exam, the considerable experience required, the complex anatomy of the shoulder, and the occurrence of prominent beam propagation artifacts in the shoulder all combine to make sonographic evaluation of rotator cuff injuries a challenge.98 Because of these difficulties, routine use of ultrasound to evaluate the shoulder for a suspected rotator cuff injury is uncommon. MRI provides excellent image quality without operator dependence and has become the diagnostic imaging technique most commonly used for these injuries. In general, sonography of the rotator cuff has little utility for the emergency evaluation of a shoulder injury since the clinical examination and radiography will usually adequately guide the direction of clinical care. Similarly, ultrasound is generally considered unsuitable for evaluation of meniscal or other ligamentous injuries of the knee, and MRI has become the imaging technique of choice for these injuries as well.

Complete disruptions of the biceps, triceps, quadriceps, or Achilles tendons are usually reliably diagnosed clinically and imaging is usually not required. However, ultrasound can be used to rapidly demonstrate the specific site of a tendon disruption at the bedside. Partial tendon tears present more of a diagnostic challenge. These may be difficult to diagnose on clinical grounds alone, and may therefore be misdiagnosed altogether. It is in this clinical scenario that ultrasound can help clarify the nature and extent of the suspected tendon injury. The Achilles, quadriceps, patellar, and triceps tendons all lend themselves to a point-of-care sonographic evaluation when a partial tendon tear is being considered. Each of these tendons is located in the superficial soft tissues and can therefore be examined in detail with a high-frequency transducer. The ability to both visualize the substance of the tendon and perform dynamic assessment of its function and integrity in real time can offer important diagnostic information that may not be appreciated on physical examination.

Evaluation of Suspected Tenosynovitis and Selected Tendinopathies

Ultrasound is felt to be at least as good as MRI for imaging selected tendon abnormalities.99 In addition, ultrasound is very accurate for the diagnosis of tenosynovitis because its characteristic sonographic signature (hypoechoic fluid surrounding the tendon or tendons) is easy to recognize. In a patient with a swollen and painful hand or foot, the ability to rapidly and confidently make a diagnosis of tenosynovitis improves patient care and assists with the implementation of appropriate therapy. Similarly, selected tendinopathies can be diagnosed with a sonographic evaluation that reveals a characteristic focal area of hypoechogenicity of the tendon. Thickening of the plantar fascia insertion (more than 5 mm) on ultrasound is suggestive of plantar fasciopathy.100

Ultrasound is a useful diagnostic tool for screening soft tissue masses of the hand or forearm, allowing for characterization of a cystic versus solid mass, an accurate estimate of the lesion's volume, and precise 3D localization.94 A ganglion cyst can be quickly diagnosed with the visualization of an anechoic saclike structure with a thin pedicle connecting it to a joint space or a tendon sheath. Use of ultrasound for the assessment of a host of painful arthritic joint conditions is discussed in more detail in the section on ultrasound-guided arthrocentesis (Chapter 22, “Additional Ultrasound-Guided Procedures”).

Precise Needle Guidance for Aspiration and Soft Tissue Injection Procedures Involving Tendons and Bursae

Ultrasound guidance may be used for precise percutaneous injections of painful conditions of the musculotendinous system, specifically for injections of tendon sheaths, joints and bursae, and for fascial injections for the treatment of tendinitis, bursitis, arthritis, and fasciitis.101 Inadvertent administration of corticosteroids within the substance of a tendon has long been known to put the patient at risk of tendon rupture. Real-time ultrasound imaging allows for continuous observation of both needle placement and medication delivery and is therefore ideally suited for precise deposition of the corticosteroids and local anesthesia that are used for treating these conditions. A specific example is the improved management of patients with subacromial bursitis. In a study investigating the treatment effectiveness of ultrasound-guided injections, a group of 40 patients with sonographically confirmed subacromial bursitis were given either standard blind injection or an ultrasound-guided injection.102 The outcome measure was shoulder abduction range of motion preinjection compared with 1-week postinjection. No statistical differences were noted in shoulder range of motion at 1 week in the blind injection group, whereas a statistically significant difference was reported in the group that received the ultrasound-guided injections. The study concluded that ultrasound could be used to guide the injection needle accurately into the inflamed synovial bursa with significant therapeutic benefits. Ultrasound-guided injections may not be as useful for rotator cuff disease. In a randomized double blind study of 106 patients comparing ultrasound guided versus systemic steroid injection for rotator cuff disease, no significant difference in short-term outcome was detected.103

The sonoanatomy of trigger fingers has been described in detail by several investigators.104,105 Thickening and hypervascularization of the A1 pulley as well as distal flexor tendinosis (“dark tendon sign”)104 and tenosynovitis were found to be the sonographic hallmarks of this condition.105 An ultrasound-guided A1 pulley steroid injection technique has been reported in a series of 24 patients and 50 trigger fingers. The technique described was found to be highly effective with a 90% success rate at 1 year for complete resolution of symptoms compared to the 57% success rate at 1 year reported for a blind injection technique.106 In a cadaver model, the performance characteristics of peroneal tendon sheath injection were significantly improved when ultrasound was used for the procedure instead of a blind palpation technique (100% ultrasound vs. 60% with palpation).107 Ultrasound-guided steroid injection has also been effectively used for the treatment of tenosynovitis of the first dorsal compartment of the wrist (DeQuervain's disease). In this report on a series of 17 patients with DeQuervain's disease, ultrasound guidance facilitated correct needle placement, thereby avoiding intratendinous injection as well as local complications such as fat atrophy and skin depigmentation.108

Anatomical Considerations

Nearly all of the tendinous structures that are likely to be evaluated in an ED setting are superficial in location. Tendons consist of parallel fascicles of collagen fibers and may form a single homogenous bundle or they may be composed of multiple bundles or laminae in which case they are referred to as complex tendons. A thin layer of connective tissue, the peritenon, surrounds all tendons. A peritendinous synovial sheath usually surrounds tendons that take a more curvilinear course. This sheath contains a thin film of synovial fluid that helps reduce tendon friction and abrasions. Peritendinous bursae may additionally reduce friction between a tendon and adjacent bones during movements. Tendons are sparsely vascularized and receive their nourishment primarily from segmental vessels arising from the surrounding peritenon or through vincula.109

Technique and Normal Ultrasound Findings

In general, perform musculotendinous ultrasound with high-frequency linear array or small parts transducers in the 7.5–15.0 MHz range. Sector transducers are generally not recommended for this application because the diverging beam of sector transducers can create undesirable beam scattering artifacts that leave only a small central portion of the image unaffected.98 This drawback can be somewhat compensated for by narrowing the sector angle and using a standoff pad. When imaging very superficial structures, such as the Achilles tendon, patellar ligament, or structures within the finger, the use of an acoustic standoff pad may be helpful. Pay attention to proper frequency, focus, and depth settings to obtain optimal images. A split screen setup may be useful for comparing corresponding images from each side of the body.

The epidermis and dermis typically appear as a thin, slightly hyperechoic layer adjacent to the transducer or standoff pad. Subcutaneous tissue appears hypoechoic, is of variable thickness depending on body location and habitus, and has a fine reticular pattern of hyperechoic connective tissue between the fat lobules. Skeletal muscle is readily identified by its characteristic hypoechoic echotexture with echogenic internal striations that appear linear or pennate (feather-like) in long axis and speckled in short axis. Fascial planes are brightly echogenic and follow the surface contour of the muscle. Muscle fiber movement is often visible with muscle contraction in real time. In long axis, tendons appear as hyperechoic rectangular or linear structures that generally track on a parallel course with the long axis of the underlying bone. The tightly packed parallel echoes emanating from the collagen fibrils within the substance of the tendon give rise to a tendon's characteristic fibrillar echotexture. In short axis, tendons can appear round, ovoid, rectangular, or flat, depending on location, and will usually appear as a cluster of multiple small echogenic dots. A subtle anechoic rim will be seen in tendons that have an associated tendon sheath. Normal tendons are seen in Figures 18-65, 18-66, 18-67, 18-68, and 18-69. A normal A1 flexor tendon pulley is demonstrated in Figure 18-70.

Tendons exhibit an optical phenomenon known as anisotropy (i.e., the image obtained from the tendon is extremely dependent on the angle of the ultrasound beam) and a thorough understanding of this concept is essential for anyone engaged in musculotendinous imaging. A tendon will appear hyperechoic only when the insonating beam is precisely perpendicular to the tendon fibers. At all other angles, there is a significant reduction in the percentage of the ultrasound beam that is reflected back to the transducer and the tendon will therefore appear hypoechoic. Anisotropy is important because if a tendon is not properly imaged its internal structure cannot be adequately assessed.109 It should be noted that most significant tendon pathology manifests itself sonographically as a hypoechogenic defect. If inadequate attention is paid to the imaging technique, a focal area of false hypoechogenicity may be interpreted as representing evidence of tendinosis or tendon rupture when in fact the hypoechogenicity is simply due to the insonating beam not being perfectly parallel with the tendon fibers at that region. A heel–toe imaging technique in long axis helps avoid this pitfall; the transducer is gently rocked in its long-axis plane from one end to the other while scanning the long axis over a tendon. In short axis, a dynamic fanning technique will achieve the same result. A focal area of hypoechogenicity that disappears with this technique is due to anisotropy; a defect that persists throughout changes in the angle of insonation likely represents true tendon pathology. Anisotropy is present in both long- and short-axis planes, although it is more apparent when scanning the tendon in the long axis.

Specific Imaging Scenarios

The biceps tendon is most easily found by scanning the upper arm in short axis just above the level of the axillary crease with the arm slightly externally rotated. The normal biceps tendon will be seen as a hyperechoic circular structure resting within the echogenic bicipital groove of the proximal humerus. Once located in short axis, a long-axis view can easily be obtained by simply rotating the transducer.

The triceps tendon is best imaged from behind the patient with the transducer oriented in a midline long-axis orientation just above the olecranon process. This view is similar to that used when evaluating the elbow joint for an effusion. The triceps tendon will be seen in the near field as a moderately echogenic fibrillar structure that inserts on the echogenic olecranon process. The hypoechoic posterior fat pad and the echogenic outline of the posterior humerus, olecranon fossa, and olecranon will be seen in the far field of the image.

The lateral and medial epicondyles of the elbow are best imaged in long axis across the joint line with the arm and elbow in full extension, and the hand in either a “thumbs up” position for lateral epicondyle imaging or in a hypersupinated position for medial epicondyle imaging. The bony outlines of the respective epicondyles and either the radial head (lateral) or coronoid process (medial) will be noted on the sonogram. The radial and ulnar collateral ligaments are thin structures found adjacent to and crossing the joint line. The common extensor or flexor tendons will be seen lying superior to the collateral ligaments and their point of origin lies several centimeters above the joint line on their respective epicondyles.

Hand and foot tendons are quickly assessed for tenosynovitis by placing the transducer in the region of clinical interest in a short-axis orientation relative to the direction of the tendon. Individual tendons are best examined in long axis when evaluating for tendon disruption.

Scan the patellar ligament in longitudinal and transverse planes with the knee held in 30° of flexion to avoid the “false hypoechogenicity” artifact that can occur in this location. This phenomenon is due to tendon anisotropy in the region adjacent to the patella. The contralateral knee should always be scanned for comparison.98

The Achilles tendon is best evaluated with the patient prone with the foot hanging over the edge of the examining table. Use of a standoff pad can be helpful, and the foot should be plantarflexed and dorsiflexed to observe dynamic tendon movement.

Soft Tissue Injection Techniques

Ultrasound imaging can provide real-time guidance for therapeutic injection of corticosteroids. A freehand technique is most commonly used for injection of tendon sheaths, joints, and bursae. The guiding principle is that the medication delivery needle should be oriented as perpendicular to the insonating beam as possible so that the needle will appear maximally reflective. The needle's position will be apparent by its strong reverberation or ring-down artifact. Needle tip localization can be further enhanced by (1) placing the needle bevel face up or face down and (2) injecting a small amount of anesthetic solution (the “hydrolocation” technique). The echogenic needle tip will be more clearly visualized within the hypoechoic anesthetic fluid pocket where it can then be redirected as needed. When a soft tissue injection procedure is undertaken, orient the tendons so that they display maximal anisotropy for best visualization. A short-axis orientation over the tendon and a long-axis orientation over the injection needle are recommended for avoiding intratendinous injections.101

Common and Emergent Abnormalities

General Ultrasound Findings

Partial tendon tears will appear as hypoechoic to anechoic regions within the substance of the tendon. Complete tendon tears will demonstrate obvious tendon discontinuity during real-time scanning; the tendon sheath may additionally contain a hypoechoic hematoma at the site of disruption. Synovial fluid will typically appear anechoic in an acute inflammatory process. The hallmark of tenosynovitis is an anechoic collection of inflammatory fluid seen surrounding the affected tendon. Additional sonographic features of tenosynovitis include tendon thickening, tendon sheath widening, and loss of the tendon's normal fibrillar echotexture. Tendonitis or tendinosis usually manifests as areas of patchy hypoechogenicity with loss of fibrillar echotexture within the affected portion of the tendon. Tendons without a sheath will appear thickened with altered echogenicity that varies according to the duration of the process.

Tenosynovitis

DeQuervain's disease is an overuse injury involving the two tendons in the first dorsal compartment of the hand (specifically, the abductor pollicis longus and extensor pollicis brevis tendons). Although a positive Finkelstein's test usually confirms the diagnosis, the tendon can be rapidly imaged in short axis to confirm the diagnosis in ambiguous cases. Fluid around the tendons, tendon thickening, and pain on “sonographic palpation” over the first dorsal compartment are typical (Figure 18-71). Bicipital tendonitis typically occurs as the result of overuse and repetitive stress on the tendon of the long head of the biceps. Sonographically, a hypoechoic fluid collection will be seen surrounding the biceps tendon and tenderness will be noted along the course of the tendon (Figure 18-72). Septic or reactive tenosynovitis may be encountered in the hands or feet, most commonly in injection drug users. The affected tendons in both entities typically appear thickened and hyperechoic with variable amounts of surrounding inflammatory synovial fluid (Figure 18-73).

Tendinopathies

Ultrasound can play a useful role in the evaluation of the athlete with chronic localized knee pain suggestive of tendinopathy of the proximal patellar ligament (“jumper's knee” or patellar tendinopathy). In a review of 25 surgically proven cases of “jumper's knee,” ultrasound correctly identified the lesion in all patients. The authors advocated the use of ultrasound as “the method of choice for the evaluation of jumper's knee, as it is inexpensive, noninvasive, repeatable, and accurate.”110,111 Sonographically, the lesion appears as a localized area of hypoechogenicity in the central portion of the patellar ligament near its insertion on the patella. Patients with this condition will usually have exquisite tenderness to palpation at this site (Figure 18-74).

Partial and Complete Tendon Tears

The final common pathway for each of these lesions is excessive stress placed on the actively contracting muscle in question. Partial rupture of the proximal patellar ligament is apparent sonographically as a characteristic cone-shaped hypoechoic lesion, exceeding 0.5 cm in length, and found close to the origin of the patellar tendon at the inferior border of the patella. This hypoechoic lesion represents a focal discontinuity of the ligament in that anatomic area and an associated hematoma. An underlying tendonitis is often associated with this injury and is identified by thickening of the tendon and overall hypoechogenicity in the area of inflammation. A complete tear of the patellar ligament is usually clinically obvious. The disruption usually occurs at the point of attachment at the inferior edge of the patella. The discontinuity between the patellar ligament and the patella will be apparent on ultrasound. An associated avulsion fracture fragment originating from the inferior pole of the patella may be seen attached to the proximal end of the ligament (Figure 18-75).

A partial tear of the triceps tendon will appear as a focal area of hypoechogenicity seen along the anterior insertion of the triceps tendon on the olecranon process. The hypoechoic defect represents both torn tendon and an associated hematoma (Figure 18-76). Similarly, a partial tear of the Achilles tendon will appear as an obvious hypoechoic region of tendon discontinuity and hematoma. Dynamic imaging can help further clarify the extent and location of the tendon disruption (Figure 18-77).

Bursopathies

The Achilles tendon is surrounded anteriorly and posteriorly by two bursae near its point of insertion on the calcaneus. The superficial calcaneal bursa lies in the subcutaneous tissue posterior to the Achilles tendon, and the retrocalcaneal bursa is found between the distal Achilles tendon and the calcaneus. These bursae are not typically seen on ultrasound in normal subjects, but can be visualized sonographically when they are inflamed (Figure 18-78).112 Injection of the retrocalcaneal bursa is sometimes performed to relieve the pain of a chronic Achilles tendon bursitis. Scan the prone patient with the tendon and bursa in a transverse plane using a 7.5 or higher MHz transducer. Needle placement and steroid injection into the bursa are best accomplished with a lateral approach using an in-plane technique.

Common Variants and Selected Abnormalities

Ganglion Cysts

Assess a patient with a palpable mass in the hand, wrist, or digit with ultrasound to determine if a ganglion cyst is present. Ganglion cysts reportedly represent 50–75% of all soft tissue masses of the hand. They commonly appear as a sonolucent cystic or ovoid structure adjacent to a tendon sheath or the wrist joint. Typically, an anechoic linear duct will be seen extending from the ganglion cyst to the tendon sheath or joint giving the structure a “tadpole” appearance (Figure 18-79).113

Ultrasound-Guided Steroid Injections

The techniques employed for ultrasound-guided soft tissue injection therapy have been reviewed earlier. Injection of a common tendinopathy, lateral epicondylitis, is demonstrated in Figure 18-80. Medial or lateral epicondylitis is thought to occur as a result of chronic repetitive stress and microtrauma of the common flexor or common extensor tendons, respectively.114,115 Sonographically, there may be some swelling of the extensor or flexor tendon near its origin on the epicondyle. With lateral epicondylitis, there is a reported predilection for deep tendon fiber injury. Intratendinous hyperemia may additionally be noted with color flow assessment.

Ultrasound may also play a useful role in the management of patients with subacromial bursitis. On a subacromial scanning window (patient seated, arm behind the back with the elbow flexed), subacromial bursitis will appear as a hypoechoic fluid collection between the deltoid muscle and the supraspinatus tendon.

Other Musculotendinous Sonographic Findings

With biceps femoris, rectus femoris, or gastrocnemius muscle tears, a “clapper in the bell” sign may be seen on the sonogram where the retracted, ruptured upper portion of the muscle (the clapper) is surrounded by a hypoechoic hematoma (the bell). Obvious disruption of muscle fiber insertion on the adjacent aponeurosis may also be apparent in some of these injuries (Figure 18-81). A muscle tear due to penetrating trauma will appear as a focal region of discontinuity in the muscle. Dynamic imaging will widen the gap between the two ends of the now discontinuous muscle bundle, and a hypoechoic defect representing fresh blood or a hematoma will typically be noted at the site of the injury (Figure 18-82). Gouty tophi will demonstrate posterior acoustic shadowing, much like gallstones. Inflamed bursae will appear as superficial hypoechoic fluid collections. Limit transducer pressure on smaller bursae to avoid causing them to collapse. The diagnosis may be missed if the bursal fluid is displaced out of the field of view.

Pitfalls

The most important potentially misleading artifact in tendon sonography is the false hypoechogenicity that results from the slightest obliquity of the ultrasound beam in relation to the tendon fibers.98 Since areas of hypoechogenicity provide the sonographic clues to tendon pathology (specifically, tendon disruption and tendonitis), optimal tendon imaging is paramount. The angle of insonation should ideally be as parallel to the course of the tendon fibers as possible to avoid this significant pitfall. Where areas of tendon hypoechogenicity are encountered, use a heel–toe scanning technique (or fanning of the transducer if imaging in short axis) to further clarify the true sonographic character of the area in question.

Clinical Considerations and Indications

When evaluating a patient with preauricular or submandibular swelling or tenderness, a point-of-care ultrasound examination of the area can rapidly determine if the source of the problem lies within the parotid or submandibular glands or if the swelling is unrelated to these structures. Although CT is the traditional imaging modality of choice for parotid and submandibular inflammatory conditions and MRI is considered the modality of choice for evaluation of parotid and submandibular tumors, ultrasound is now considered by many to be the initial imaging modality of choice for assessment of any palpable abnormalities in the region of the salivary glands.116–118 Ultrasound can clarify if the lesion is intra- or extraglandular, provide information as to whether a focal process or diffuse glandular involvement is present, and help guide aspiration if an abscess is found in a case of acute bacterial sialadenitis. It can also demonstrate if there is involvement of the surrounding lymph nodes.

Salivary gland disease can be broadly classified into four categories: acute sialadenitis (with viral and bacterial etiologies), chronic sialadenitis (with infective and noninfective etiologies), sialolithiasis, and tumors (benign and malignant).

Sialolithiasis is the most common salivary gland pathology encountered in the ED. Ultrasound is particularly useful in the evaluation of suspected sialolithiasis, where it has 96% diagnostic accuracy.119 Salivary gland stones as small as 0.4 mm can be detected on ultrasound with 90% sensitivity.117 Salivary gland calculi may occur in the parotid gland, but they will most commonly be found (80% of the time) in the submandibular gland, presumably because of the more mucinous content of submandibular gland secretions. Most of these calculi will be found in the submandibular duct; the remainder will be located within the gland or in the ductal hilum. Multiple stones are found in 25% of patients.119 Most salivary gland tumors occur in the parotid gland. The vast majority (85–90%) of parotid tumors are benign pleiomorphic adenomas. They are slow growing, painless masses, most commonly seen in middle-aged patients, and they are usually found in the superficial lobe of the parotid gland. By contrast, 50% of tumors discovered in the submandibular gland are malignant. Of note, neither CT nor MRI has been found to be superior to ultrasound in distinguishing benign from malignant tumors of the salivary glands.117 In addition, ultrasound may be used to guide fine-needle aspiration of salivary gland tumors, and to guide intraglandular abscess drainage in advanced cases of bacterial sialadenitis. Finally, ultrasound-guided botulinum toxin injection into the salivary glands has recently been used as a therapy for patients with excessive salivation or drooling disorders.117 Given the somewhat questionable reliability of the clinical examination120 and the ready availability of point-of-care ultrasound equipment and high-frequency linear array transducers in most EDs, sonographic examination of the region of facial swelling or tenderness should be considered a routine part of the bedside workup of any patient with suspected salivary gland disease in the ED.

Anatomical Considerations

The body of the parotid gland lies in a preauricular location, with its upper portion roughly in line with the external acoustic meatus and extending inferiorly and posteriorly to the angle of the jaw. The parotid duct (Stenson's duct) arises from the anterior border of the gland, lies superficial to the masseter muscle and about 1–2 cm below the zygomatic arch, courses horizontally at earlobe level through the buccal fat pad, and pierces the buccinator muscle to enter the mouth at the parotid papilla opposite the upper second molar. The normal duct is approximately 2–3 mm in diameter and 4–6 cm in length. On occasion, an accessory parotid gland may be seen lying anterior to and following the course of the parotid duct. The gland is broad and flattened superficially and wedge-shaped on its posterior and deep aspects. The bulk of the gland overlies the masseter and mandible. The facial nerve, the retromandibular vein, and the external carotid artery lie in a vertical orientation immediately deep to the gland. Small lymph nodes are commonly found within the substance of the parotid gland.

The submandibular glands are found beneath the superficial subcutaneous tissues of the submandibular triangle, lateral to the anterior belly of the digastric muscle. The much larger superficial lobe and the much smaller and more posterior deep lobe are C-shaped in long-axis profile and connect where they wrap around the posterolateral border of the mylohyoid muscle. Intraglandular ducts drain into the submandibular (or Wharton's) duct that emerges from the hilum of the submandibular gland. The submandibular duct passes medially, then up and over the posterolateral border of the mylohyoid muscle where it then courses medial to the sublingual glands to a papilla adjacent to the lingual frenulum in the anterior floor of the mouth. The submandibular duct is about 5 cm in length. Unlike the parotids, no intraglandular lymph nodes are found within the submandibular gland. The “Küttner” lymph node is reliably found in the space between the posterior border of the submandibular gland and the anterior border of the inferior aspect of the parotid gland.119

The sublingual glands are located below the mucous membranes of the floor of the mouth adjacent to the mandible and the genioglossus muscle. They drain via numerous small caliber ducts either directly into the floor of the mouth or into the submandibular duct.

Technique and Normal Ultrasound Findings

Scan the parotid and submandibular glands with a 7.5–15 MHz linear array transducer. Scan the parotid gland in long axis in a vertical plane in front of the lower ear. Angle the superior portion of the transducer somewhat anteriorly to evaluate the portion of the gland that lies inferior to the ear at the angle of the jaw. The gland is predominantly superficial in location with some deeper portions of the gland hidden by the mandible. When evaluating the parotid duct in its long axis, use a transverse plane where transducer orientation is nearly horizontal from mid-earlobe level to the mid-cheek. The parotid duct appears as two closely spaced parallel echogenic lines with a thin region of lucency between them, and is found superficial to the masseter muscle. Because of the fatty glandular tissue composition of the gland, the normal parotid appears fairly homogeneous with a fine granular echotexture that appears similar to a fatty liver. Intraparotid ducts appear as echogenic linear structures within the substance of the gland. Intraparotid lymph nodes are common, especially in the preauricular region. Lymph nodes are typically elliptical in shape with a hypoechoic periphery and a hyperechoic fatty central hilum (Figures 18-83, 18-84, and 18-85). When scanning the parotid in a long-axis orientation, the retromandibular vein and the external carotid artery will appear as two parallel hypoechoic channels beneath the gland; they are best appreciated with the use of color Doppler imaging. The facial nerve may be seen as a thin fibrillar structure overlying the more superficially located vein.

When scanning the submandibular gland, place the transducer in the submental region just medial to the mandible aiming toward the middle of the chin. Aim the orientation marker toward the operator's left. By doing so, the chin will appear on the right side of the image when scanning the right submandibular gland and on the left side of the image when scanning the left submandibular gland. The bulk of the gland is located inferior to the mylohyoid muscle and appears as a rounded and somewhat lobular structure with a homogeneous finely granular echotexture similar to but more distinct than the appearance of the more fatty parotid gland. The Küttner lymph node may be seen posterior and immediately adjacent to the submandibular gland in the space between the submandibular and the parotid glands (Figure 18-86). The mylohyoid muscle appears as a horizontally oriented and somewhat striated appearing rectangular region of hypoechogenicity in the near field of the image, tapering as it approaches its insertion on the symphysis menti. The inferior aspect of the bony symphysis menti appears as a slightly curved region of hyperechogenicity in the near field with dense posterior acoustic shadowing below. The submandibular duct is somewhat narrower in caliber than the parotid duct, has a similar hyperechoic tubular appearance, and is best visualized when it is pathologically dilated. When scanning the submental region, remember that the patient's anatomy on the sonogram appears upside down; the image obtained represents a somewhat oblique sagittal section of a patient standing on his or her head.

The sublingual glands are best visualized by scanning the anterior underside of the chin in a short-axis orientation. They appear as paired homogeneous triangular structures, just deep to the hypoechoic mylohyoid muscle, similar in echotexture to the submandibular gland.

Common and Emergent Abnormalities

Sialadenitis

A variety of sonographic patterns may be encountered with parotid and submandibular gland disease. With acute viral sialadenitis (most commonly caused by mumps) and acute bacterial sialadenitis (most commonly caused by Staphylococcus aureus and Streptococcus viridans), the gland appears enlarged, hyperechoic compared to normal salivary glandular tissue, and has heterogeneous echotexture (Figures 18-87 and 18-88). With bacterial sialadenitis, air may be present within the intraglandular ducts and small hyperechoic foci with associated comet-tail artifacts may be seen. If an abscess has formed, it will usually appear hypoechoic or anechoic and demonstrate posterior acoustic enhancement similar to a subcutaneous abscess. Increased blood flow may be noted on color Doppler imaging with any acute inflammatory condition. Chronic sclerosing sialadenitis (also known as Küttner's tumor) occurs as a result of chronic infection and appears as a well-defined hypoechoic mass. The gland may appear normal to small in size and inhomogeneous in echotexture. In Sjögren's syndrome, a systemic autoimmune disorder that affects exocrine glands, the salivary glands will exhibit varying degrees of parenchymal inhomogeneity, there will be reduced salivary gland volume, and hypoechoic foci of sialectasia will be present. Sonographic textural inhomogeneity scores are used as a tool to establish the diagnosis of Sjögren's syndrome.117

Masses

Most salivary gland tumors will be found in the parotid gland and most of these will be benign. Solid lesions will typically appear hypoechoic and demonstrate posterior acoustic enhancement. Cystic lesions will typically appear anechoic. A pleiomorphic adenoma is the most common benign solid parotid tumor, comprising 85–90% of parotid tumors. Sonographically, it appears rounded or lobular in shape, well circumscribed, and homogeneously hypoechoic with posterior acoustic enhancement. Fewer tumors are found in the submandibular glands, but the rate of malignancy there is much higher. While the tumor shape, boundary, and the posterior echoes are different when comparing benign and malignant salivary gland tumors, their internal echogenicity is similar.121 Malignant neoplasms of the salivary gland are more likely to have irregular shapes, irregular borders, blurred margins, and a hypoechoic inhomogeneous structure, but they may be lacking these features and look similar to benign tumors.122 The most common malignant solid tumor (mucoepidermoid carcinoma)116 may initially appear sonographically similar to a pleiomorphic adenoma. Therefore, consider all solitary salivary gland masses to be neoplastic until proven otherwise and refer for definitive diagnosis. Prior trauma to the area and an occult salivary duct injury may give rise to a siaolocele, which will appear as a hypoechoic cystic mass.

Sialolithiasis

Salivary duct stones (or calculi) appear as hyperechoic foci with prominent posterior acoustic shadowing. Stones are usually found within the salivary duct of the submandibular gland and less frequently within the ductal hilum. Less commonly, calculi may be found within the parotid duct or gland. When complete obstruction occurs, the salivary duct becomes dilated and appears in long axis as a prominent anechoic tubular structure leading to the echogenic stone. The gland itself may appear enlarged with dilatation of the hilum giving the gland a somewhat “hydronephrotic” appearance (Figures 18-89 and 18-90).

Lymph Nodes

A preauricular mass or area of focal tenderness will sometimes be due to a reactive and enlarged lymph node. The lymph node has a characteristic elliptical shape with a hypoechoic periphery and an echogenic fatty central hilum (Figure 18-91).

Pitfalls

The major pitfall with ultrasound imaging of the salivary glands is failure to consider malignancy as the etiology for a solid lesion. The role of the point-of-care ultrasound for potential salivary gland pathology is to determine if a focal lesion is present, establish whether it is intraglandular or not, and to determine if a salivary duct stone is the cause of the swelling or pain. With the exception of diffuse polyglandular enlargement (as seen with mumps), diffuse uniglandular enlargement and tenderness (as seen in acute sialadenitis), and a locally reactive lymph node, nearly all other salivary pathologies should be referred for further evaluation.

Clinical Considerations

Rhinosinusitis is one of the 10 most common diagnoses in ambulatory practice and the fifth most common diagnosis for which antibiotics are prescribed.123 Accurately diagnosing bacterial sinusitis, for which antibiotics are recommended, is difficult with just the history and physical examination. At most, only 50% of patients presenting to acute care settings with sinus complaints have bacterial sinusitis.124 Signs and symptoms of bacterial sinusitis are nonspecific and indistinguishable from the clinically similar presentation that occurs with viral rhinosinusitis. Although radiography improves diagnostic accuracy somewhat, neither plain radiography nor CT is recommended for evaluating uncomplicated sinusitis in ambulatory patients because of the additional time, cost, and radiation exposure associated with these tests.125 Conversely, point-of-care ultrasound is a rapid and safe diagnostic tool that can be used by the provider to accurately identify a fluid-filled maxillary sinus.

Sinusitis is an important occult cause of fever and nosocomial pneumonia in patients undergoing prolonged mechanical ventilation.126 In this setting, point-of-care sinus ultrasound is a well-established screening test for detecting maxillary sinusitis.

Clinical Indications

The primary indications for sinus ultrasound are

• Confirm or exclude clinically suspected maxillary sinusitis in patients with upper respiratory symptoms
• Detect maxillary sinus fluid in intubated ICU patients to screen for suspected nosocomial sinusitis

Maxillary Sinusitis

Although CT and MRI are sensitive imaging modalities for diagnosing sinusitis, the presence of sinus fluid or mucosal thickening alone does not necessarily indicate bacterial infection (low specificity and low positive predictive value). The true “gold standard” for diagnosing bacterial sinusitis is a positive culture from fluid obtained on sinus puncture. Ultrasound, while less sensitive for mucosal changes and small amounts of fluid, is more specific for clinically important disease because a positive study requires the presence of a significant amount of fluid in the sinus. Studies evaluating the diagnostic characteristics of ultrasound for bacterial sinusitis vary substantially in terms of the population studied (general practice, subspecialty, or ICU), the criterion standard employed (sinus puncture, radiography, MRI), and methodological quality. A summary of studies conducted in the 1980s and 1990s in which sinus ultrasound was compared to sinus puncture found a weighted mean sensitivity and specificity for ultrasound of 85% and 82%, respectively. This compares favorably to the 87% sensitivity and 89% specificity found for plain radiography.127 A systematic review of these early studies concluded that operator skill and experience had a large effect on how well ultrasound performed.128

Four studies have been conducted in ambulatory and emergency care settings comparing point-of-care ultrasound to radiography or MRI for diagnosing acute maxillary sinusitis. In one study, which used MRI as the gold standard, ultrasound was found to be 64% sensitive and 95% specific, compared to 73% sensitivity and 100% specificity for plain radiography.129 The authors concluded that a positive ultrasound examination confirmed the diagnosis of maxillary sinusitis. If the ultrasound examination was negative and clinical suspicion was high, however, they recommended that plain radiographs be obtained. Another study found that ultrasound was 92% sensitive and 95% specific compared to plain radiography, and also noted that the addition of point-of-care sinus ultrasound to the history and physical examination would reduce antibiotic prescriptions for sinusitis by one half.130 One study compared emergency physician-performed ultrasound to CT in 48 ED patients with suspected maxillary sinusitis.131 It reported 86% agreement between the two techniques and noted that ultrasound was 81% sensitive and 89% specific compared to CT. Finally, in a prospective study of 67 pediatric patients (mean age 9 years 2 months +/– 3 years 9 months), 134 sinuses were imaged with both ultrasound and plain films.132 There was 84% agreement between the two techniques, and compared to the plain films, ultrasound was found to be 95% sensitive and 98% specific.

Maxillary Sinus Fluid in Intubated Patients

In the ICU setting, point-of-care ultrasound has emerged as a convenient screening test for nosocomial sinusitis because plain radiographs are not accurate for visualizing sinus fluid in recumbent patients and CT is particularly time and labor intensive in intubated patients. Ultrasound can be used at the bedside to quickly and accurately assess for fluid in the maxillary sinuses. If fluid is present, rhinoscopy or sinus puncture is subsequently performed to obtain fluid for culture. The reported prevalence of true bacterial sinusitis varies widely among different studies, ranging from 5% to 60% of cases.126,133,134 Among the three most robust studies of maxillary sinus ultrasound in the ICU setting, reported sensitivity ranged from 67–100% and specificity from 86–97%.133,135,136 Specificity was 100% when all sinus walls were seen, which generally correlates with complete sinus opacification on radiography.

It is considered normal when the maxillary sinus ultrasound shows only an echo emanating from the front wall of the maxillary sinus; in such cases, the diagnosis of sinusitis can usually be eliminated. A complete sinusogram is defined as the sonographic visualization of all maxillary sinus walls (posterior, medial, and lateral), and this finding has a high specificity for the presence of sinusitis.

A diagnostic dilemma occurs in the case of a partial sinusogram when only the anterior and a portion of the posterior walls of the sinus are visualized. In such cases, a postural change test can be performed to help determine if an air-fluid level is present or if mucosal thickening is the cause of the partial sinusogram. An ICU study evaluated the test characteristics of a postural change test in patients in whom a partial sinusogram was found.137 Maxillary sinus imaging was first performed with the patient in a “half-sitting” position. In patients with a partial sinusogram, a repeat sinusogram was subsequently performed with the patient supine. The test was considered positive for sinusitis if the partial sinusogram disappeared when the patient was supine (indicating the presence of fluid in the sinus). The test was considered negative if the partial sinusogram was visualized in both positions (indicating mucosal thickening). In the 300 sinuses scanned in this report, and using CT imaging as the criterion standard, results were reported as follows: (1) a complete sinusogram was found in 98 cases and all were radiologically confirmed to be sinusitis, (2) normal sinus ultrasound exams were found in 112 cases and CT confirmed the presence of a normal sinus in 95% of these patients, (3) a partial sinusogram was seen on ultrasound in 90 cases and CT confirmed the presence of sinusitis in 61%, and (4) the postural change test was performed in all patients who had a partial sinusogram, which increased the positive predictive value of sinus ultrasound from 61% to 91% in these patients. The same study also reported that proficiency in maxillary sinus imaging required performance of about 20 sinusograms.137

A-mode ultrasound has also been found to be useful for diagnosing maxillary sinusitis. In a series of 140 sinuses imaged in intubated ICU patients, A-mode sinus imaging was found to be 95% specific with a 92% negative predictive value.138 In addition, all empty sinuses were correctly identified as being empty using this technique.

Anatomical Considerations

The maxillary sinuses are paired and somewhat pyramid-shaped airspaces within the maxillary bone on either side of the nose. They are bordered by the orbital floor superiorly, the lateral nasal wall medially, the alveolar process and hard palate inferiorly, and the zygoma laterally. Mean maxillary sinus volume was noted to be 15.7 +/– 5.3 cm3 in an analysis of 120 maxillary sinuses measured on head CTs in one report.139 The maxillary sinus is typically 2–4 cm in AP depth, and fluid within the sinus renders the posterior wall of the sinus visible by ultrasound. The ethmoid sinuses lie superomedially, and the sphenoid sinus lies in the midline deeper within the skull. The frontal sinuses, which would seem to be sonographically accessible, are usually not well visualized by ultrasound, likely because the anterior surface of the frontal bone is quite thick.

Technique and Normal Ultrasound Findings

Maxillary sinus ultrasound may be performed with a wide array of transducers, ranging from a phased array or microconvex 3.5 MHz transducer (e.g., a typical cardiac transducer) to a 3–12 MHz linear array transducer. A small skin contact footprint is preferred, however, for scanning on the curved, relatively solid surface over the anterior maxilla. Although one might think that lower frequencies would be required to adequately penetrate the anterior bony wall of the sinus, this does not appear to be the case in practice, and a fluid-filled sinus can be visualized with either transducer type. The patient should sit upright or lean slightly forward to ensure that sinus fluid, if present, layers out against the anterior wall. Scan the sinus in both sagittal and transverse planes, midway between the nose and the zygoma, just below the orbital rim. When using a linear array transducer, the sagittal image is easier to obtain by angling the transducer slightly superomedially, parallel to the nasolabial fold. Pay attention to proper depth settings (typically 5–7 cm) so that the image of the sinus fills about 75% of the image. Adjust the transducer angle so that the anterior surface of the maxilla is represented as a horizontal line on the sonogram. Compare with the uninvolved sinus, especially in equivocal cases.

In the normal air-filled sinus, a prominent periodic resonance artifact will be apparent, consisting of an evenly spaced series of echogenic lines that parallel the shape of the anterior surface of the maxilla and diminish in intensity at increased depth. Beneath the distinct echo of the anterior wall of the sinus, an indistinct “snowstorm” appearance will be noted on the sonogram and the posterior wall of the sinus will not be apparent. It is essential not to confuse one of the deeper periodic resonance artifacts as representing the posterior wall of the sinus (Figure 18-92). These artifacts will always parallel the anterior surface of the maxilla, whereas the posterior wall of the sinus, when apparent, will always appear curved.

Common and Emergent Abnormalities

Acute viral rhinosinusitis produces abnormalities within the maxillary sinuses in up to 87% of cases, typically a thickening of the sinus mucosa along with some secretions. Occasionally, a significant amount of fluid will accumulate within the sinus and produce an air-fluid level on CT.140 An ultrasound examination of the sinus at this point would appear positive for fluid and would represent a false-positive result for the diagnosis of bacterial maxillary sinusitis. Bacterial sinusitis is said to supervene in 1–2% of cases of viral rhinosinusitis124 and typically occurs after 5–7 days of symptoms. It may be accompanied by mucopurulent nasal discharge and signs of maxillary inflammation (such as focal, often unilateral sinus pain, and tenderness). As the inflammatory process evolves, pus accumulates in the sinus, giving rise to air–fluid levels or complete opacification on radiography, decreased transillumination, and a positive sinus ultrasound examination.

In the patient with a completely fluid-filled sinus, a curved, continuous, and brightly echogenic posterior sinus wall will be apparent in the far field of the sagittal sonogram, and typically no periodic resonance artifacts will be noted (Figure 18-93). In the transverse orientation, the medial and lateral walls of the sinus will also be clearly visualized. This constellation of sonographic findings is referred to as a complete sinusogram.135 If the sinus is only partially filled with fluid, a mixed picture will be seen. The posterior wall of the sinus will be apparent only in the inferior fluid-filled portion of the sinus (right lower side of the image on the sagittal sonogram) and a periodic resonance artifact may be seen in the upper air-containing region of the sinus (left side of the image). A transverse view of the sinus will not show the presence of the medial or lateral walls of the sinus. This constellation of sonographic findings is referred to as a partial sinusogram.133,135 As discussed, dynamic testing in both sitting and supine positions would then be indicated to clarify if the partial sinusogram represented fluid or mucosal thickening. An example of partial sinus opacification on CT and its corresponding sonographic representation are illustrated in Figure 18-94.

Pitfalls

The most important potential pitfall when evaluating suspected maxillary sinusitis with ultrasound is failure to consider the results of imaging in the context of the pretest likelihood of disease and other clinical indicators of severity. The weighted mean sensitivity of ultrasound for maxillary sinusitis is not more than 85% and involvement of other sinuses cannot be assessed. Therefore, in cases where there is a high clinical suspicion for sinusitis and when there are severe symptoms, especially in elderly or diabetic patients, obtain a more sensitive imaging study (CT) to provide a definitive diagnosis.

Note that a number of conditions other than sinusitis will allow transmission of the insonating beam through the sinus and will result in visualization of the posterior wall of the sinus. Significant mucosal thickening, polyps, fluid filled cysts, solid masses, and blood from facial trauma and sinus fractures can all create a positive sinusogram (Figures 18-95 and 18-96).

Technical pitfalls include failure to accurately set the image depth to a level that includes the posterior wall of the sinus (usually 5–7 cm), misinterpreting a periodic resonance artifact for the posterior wall of the sinus (resonance artifacts are horizontal, the posterior sinus wall is curved), and only scanning the patient in a supine position (unless the sinusogram is clearly positive). A patient with a negative supine study may still have sinusitis, and further dynamic testing should be performed in such cases to see if a partial sinusogram is present in a “half-sitting” position as well.

Clinical Considerations

Emergency and ambulatory care physicians evaluate patients with skin and soft tissue infections on a daily basis. These infections include cellulitis, abscesses, and necrotizing fasciitis. The diagnosis of cellulitis by physical examination alone is usually straightforward when erythema, warmth, and tenderness are present. Similarly, determining that a subcutaneous abscess exists is simple when fluctuance or focal skin necrosis is present. Clinical findings can be misleading, however, when a cutaneous abscess is small, deep to the skin surface, early in its formation, or where there is preexisting scar tissue from a prior abscess drainage procedure. An occult abscess may occasionally be present when the clinical presentation is consistent with simple cellulitis. Ambiguous clinical findings may direct the clinician away from performing a much needed drainage procedure. Ultrasound has emerged as a valuable tool for assessing soft tissue infections at the bedside. It is increasingly used in emergency and ambulatory care settings to detect occult abscesses, facilitate abscess drainage, and help providers avoid an unnecessary procedure in patients without true abscesses.

Clinical Indications

The clinical indications for the use of ultrasound in the management of soft tissue infections include

• Detection of an occult subcutaneous abscess when clinical findings are ambiguous
• Localization of an optimal site for incision and drainage or aspiration of an abscess
• Assessment of soft tissue inflammatory processes to determine if they are the result of an adjacent arthritis or tendonitis rather than cellulitis

Detection of Occult Subcutaneous Abscess

The ability of ultrasound to detect soft tissue abscesses and guide subsequent incision and drainage has been appreciated since the 1980s.141–145 In reports focusing on injection drug users, particularly those with inflammatory lesions of the groin, ultrasound has been shown to successfully differentiate cellulitis from abscess, in addition to being useful for detecting adenitis, septic thrombophlebitis, and pseudoaneurysm.141,144 Ultrasound has also been found to be of value in the diagnosis and treatment of odontogenic facial abscesses,146 and in the assessment of patients with buccal space swelling.147

Four studies conducted in the ED setting have examined the utility of point-of-care ultrasound for evaluation of skin and soft tissue infections.148–151 All four studies examined the impact of ultrasound assessment on the accuracy of abscess diagnosis, with cases stratified by the pretest likelihood of an abscess. Overall, ultrasound assessment led to a change in management of 17–56% of cases. A consistent finding was that among patients clinically categorized as unlikely to have an abscess, ultrasound frequently revealed an unsuspected abscess that was confirmed by a drainage procedure. This occurred in 14–58% of “low” likelihood cases.

Ultrasound is also useful when an abscess is thought to be present based on clinical factors. In such cases, a negative ultrasound evaluation often correctly excludes abscess, thereby preventing an unnecessary drainage procedure.

With the addition of ultrasound to physical examination alone, the positive predictive value in one study increased from 81% to 93%, and the negative predictive value increased from 77% to 97%.149 Because of methodological limitations, the actual false-negative rate for detecting abscess cannot be determined with confidence from these studies. The conclusions that can be made from these four studies are as follows: (1) point-of-care ultrasound improves accuracy for detection of superficial abscesses in both adult and pediatric patients, (2) it helps prevent the performance of unnecessary invasive procedures, and (3) it provides guidance as to whether patients require further imaging or consultation.

Point-of-care ultrasound can also be used to guide other aspects of clinical care in patients with known or suspected cellulitis or soft tissue abscesses. In a study of 101 patients with superficial skin abscesses, incision and drainage were compared with ultrasound-guided needle aspiration. The success rate (sonographic and clinical resolution of the abscess by day 7) was reported to be 80% in the incision and drainage group and only 26% in the needle aspiration group leading to the clear conclusion that ultrasound-guided needle aspiration was insufficient therapy for skin abscesses.152 Another study analyzed the utility of venous ultrasound in 240 patients with lower extremity cellulitis and found a DVT prevalence of only 6.25%. The study concluded that concurrent DVT and cellulitis are rare and that investigation for DVT in such patients has a low yield unless other risk factors are present.153 In another report on patients with cellulitis, ultrasound examination of the affected area 4 days after initiation of antibiotic therapy revealed that patients with only subcutaneous thickening required a shorter duration of therapy and had a higher rate of early treatment response compared with patients who had subcutaneous cobblestoning.154

Assessment of Soft Tissue Inflammatory Processes

Localized pain and erythema over a joint may be due to an inflammatory condition in the joint rather than cellulitis. In a study of 54 patients with erythema, swelling, and joint pain, ultrasound was found to be a useful tool for differentiating soft tissue abnormalities from joint effusions and directing subsequent management. The study reported a statistically significant difference in treatment plans, with a 35% decrease in arthrocentesis procedures when ultrasound was used.155 Ultrasound is useful in the evaluation and diagnosis of other soft tissue infections, including 1) Ludwig's angina,156 (2) subcutaneous myiasis (maggot infiltration of human tissue),157 and (3) muscular or subcutaneous cysticercosis. In cysticercosis, the characteristic sonographic finding is a cystic lesion with a mean diameter of 6 mm with pericystic inflammatory changes and an echogenic, eccentrically located pedunculated nodule attached to the inner wall of the cyst.158

Anatomical Considerations

A thorough understanding of the regional anatomy of the area of interest is essential when evaluating for an abscess. Subcutaneous abscesses may be encountered nearly anywhere and are commonly seen on the hand, face, neck, forearm, groin, lower extremity, buttocks, and perianal region. They may be found in close proximity to veins, arteries, nerves, tendons, bones, and muscles. Awareness of the proximity of adjacent structures, familiarity with their normal sonographic appearance, and an understanding of the preferred lines for elective incision are essential.

Technique and Normal Ultrasound Findings

Perform sonographic evaluation of the skin and subcutaneous tissue for a suspected abscess with a 7–12 MHz linear array transducer, although an annular array or sector transducer may also be utilized. An acoustic standoff pad may improve image resolution if a lower frequency transducer is being used or if the abscess is very superficial and a linear array transducer is not available. Adjust depth and focus settings to place the area of interest within the transducer's optimal focal zone. Keep transducer pressure to a minimum to avoid collapse and subsequent nonvisualization of superficial veins. Systematically scan the area of interest in two perpendicular planes.

One study demonstrated that S. aureus (including MRSA) frequently contaminates uncovered ultrasound transducers used in the ED for skin and soft tissue infection scanning. MRSA was reliably removed when a three-step cleaning and disinfection protocol was employed (wiping first with a dry gauze towel, then wiping with a sterile saline moistened gauze towel, and finally wiping with a quaternary ammonia germicidal wipe.)159 If cleaning and disinfection is not consistent, it may be prudent to cover the ultrasound transducer with a sterile adhesive dressing when scanning patients with skin and soft tissue infections.

A study showed that pediatric emergency physicians without prior ultrasound training could learn to perform and interpret point-of-care ultrasound for assessment of skin and soft tissue infections with just a brief training program.160 After 6 hours of training, these physicians were able to perform technically adequate studies with excellent agreement with an ultrasound expert (the interrater reliability of the clinician with the expert resulted in a kappa statistic of 0.8). Another study found that a training simulator using a chicken breast injected with “mock purulent material” is valuable for teaching sonographic diagnosis and management of superficial abscesses.161

Normal soft tissue ultrasound findings are reviewed as follows. Normal skin (epidermis and dermis) typically appears as a thin, homogeneous, and somewhat hyperechoic layer immediately beneath the transducer. Subcutaneous tissue, composed primarily of subcutaneous fat, lies immediately below and appears hypoechoic with a reticular pattern of thin echogenic connective tissue between the fat lobules. Fascia and connective tissue planes are usually seen as echogenic horizontal or gently curved lines that follow the contour of the underlying muscle. Arteries and veins appear as anechoic circular or tubular structures, depending on the orientation of the transducer relative to the vessel. The use of color flow or Doppler allows for further vessel characterization. Veins typically compress with light or moderate transducer pressure. Arteries will compress with significant pressure and a rhythmic arterial wall pulsation will be noted. Muscle appears relatively hypoechoic with a regular pattern of internal striations in long axis and a speckled appearance in short axis. The anterior cortex of bone is brightly echogenic with far-field acoustic shadowing. The normal sonographic appearance of the various layers of soft tissue within the forearm is seen in Figure 18-97.

Common and Emergent Abnormalities

Cellulitis

Although nonspecific, familiarity with the sonographic appearance of cellulitis is important for two reasons. First, a rim of cellulitic tissue nearly always surrounds an abscess cavity, and second, cellulitis is the usual diagnosis of exclusion when performing ultrasound on an undifferentiated skin and soft tissue infection. Cellulitis is a diffuse infection of the skin and subcutaneous tissue, and the typical sonographic findings in cellulitis arise from the accumulation of edema fluid. Findings include thickened and abnormally hyperechoic skin and increased echogenicity of the subcutaneous tissue with poor detail resolution. “Cobblestoning” is also a common finding, characterized as areas of hypoechoic edema that traverse the subcutaneous fat in a reticular pattern (Figures 18-98 and 18-99).162–164 Comparison to the unaffected limb or adjacent normal soft tissue may aid in the recognition of subtle findings. Sonographic findings in cellulitis are simply indicative of edema and are therefore nonspecific, and both necrotizing fasciitis and the inflamed tissue surrounding an abscess may take on the sonographic appearance of cellulitis. In patients with chronic lymphedema, the subcutaneous tissue appears hyperechoic relative to normal tissue but the skin is not thickened or as indistinct as in cellulitis. Additionally, prominent horizontal bands of hypoechoic edema fluid will be seen traversing the subcutaneous tissue (Figure 18-100). Finally, regional lymph nodes in areas with soft tissue cellulitis will often become enlarged and very tender, and it is important to recognize their sonographic appearance and not confuse them with an abscess or other soft tissue mass (Figure 18-101).

Abscess

A subcutaneous abscess may have a variety of sonographic appearances,144,162,164,165 but will almost universally be surrounded by a rim of edematous soft tissue or cellulitis that appears hyperechoic relative to normal subcutaneous tissue. Abscess cavities are most frequently spherical or elliptical shaped and the liquefied contents of the abscess cavity will typically demonstrate posterior acoustic enhancement. An abscess cavity may exhibit a wide range of sonographic patterns from hypoechoic to isoechoic, but it will most commonly appear hypoechoic relative to surrounding soft tissues. On occasion the purulent abscess cavity may be isoechoic or even hyperechoic, making it more difficult to identify (Figures 18-102 to 18-105). Sometimes an abscess may appear irregular or lobulated, and the abscess cavity may interdigitate between tissue planes or take an irregular path within the surrounding hyperechoic cellulitic soft tissues (Figures 18-106 and 18-107). Occasionally, a subcutaneous abscess will be so large that the image will not fit within the field of view of the transducer. In such cases, an extended-field-of-view feature may be used to delineate the abscess's exact location and extent (Figure 18-108). Abscesses may contain hyperechoic debris, septae, or gas bubbles. Gas appears as brightly hyperechoic foci within the abscess cavity with an associated ring-down or reverberation artifact (Figure 18-109). Gentle transducer pressure over the abscess site may induce a swirling motion of the purulent material within the abscess cavity (referred to as “ultrasonic fluctuance”) and helps confirm the liquid nature of the material (Figure 18-110). An abnormal fluid collection can be difficult to detect in cases where the abscess contents are isoechoic, and ultrasonic fluctuance can be of particular diagnostic value in such cases.162 Color flow Doppler may demonstrate hyperemia adjacent to the abscess cavity and can help confirm absence of flow within it.164 If a hypoechoic fluid collection is seen adjacent to a long bone, the diagnosis of osteomyelitis should be entertained. Finally, abscesses may be found situated adjacent to anatomic structures such as blood vessels, muscles, tendons, and bones. Recognition of adjacent structures is essential prior to embarking on a drainage procedure (Figure 18-111).

Common Variants and Selected Abnormalities

Necrotizing Fasciitis

Necrotizing fasciitis is a rare soft tissue infection defined by tissue necrosis (particularly involving fascia) and a fulminant course. While rapid recognition and treatment can favorably affect outcome,166 making the diagnosis of necrotizing fasciitis on clinical grounds alone is often challenging. The role of ultrasound as a diagnostic test for necrotizing fasciitis has never been studied systematically, and it should never be used to exclude the diagnosis. However, sonographic findings characteristic of the disease have been described by two groups of investigators in Taiwan, reporting on a total of 21 cases of necrotizing fasciitis in adults and children.167,168 Invariably, the subcutaneous fascia, normally seen as a thin, brightly hyperechoic line, is greatly thickened and edematous appearing, as is the overlying subcutaneous tissue. An anechoic fluid layer measuring greater than 4 mm adjacent to the deep fascia is considered by some authors to be diagnostic of necrotizing fasciitis.167 In some cases, discreet masses are seen in and around the fascial plane, from which pus may be aspirated. Sonographic findings that may be present in cases of necrotizing fasciitis include (1) marked skin thickening, (2) swelling and increased echogenicity of the overlying fatty tissues with interlacing fluid collections, and (3) subcutaneous emphysema spreading along deep fascia.169,170 The presence of a subcutaneous gas collection may be rapidly confirmed with a point-of-care ultrasound examination and will typically appear as a focal region of high amplitude echoes with “dirty” posterior acoustic shadowing as well as a reverberation artifact (Figure 18-112). Although small gas bubbles may occasionally be noted within an abscess cavity, the presence of subcutaneous gas that is not within a discreet abscess or gas following fascial planes should raise the index of suspicion for necrotizing fasciitis.

Pitfalls

1. Abscesses are not always hypoechoic. They can on occasion be sonographically subtle. An isoechoic collection of pus, especially one that is in or around muscle tissue, or an abscess that is deep and obscured by overlying hyperechoic soft tissues, may give rise to a false-negative ultrasound examination. Operator experience may also play a role. Optimization of the image with appropriate frequency, focus, and depth settings, and gentle transducer pressure to assess for ultrasonic fluctuance may help avoid this pitfall. In the face of strong clinical suspicion for an abscess and an apparently negative ultrasound result, performing a needle aspiration may be reasonable.

2. Necrotizing soft tissue infections. An uncommon but potentially disastrous pitfall is failure to consider a necrotizing soft tissue infection once an abscess has been excluded by ultrasound. Although studies examining the use of point-of-care ultrasound in the diagnosis of necrotizing fasciitis are limited, and although ultrasound should never be used to exclude the presence of necrotizing fasciitis, subcutaneous gas spreading along a fascial plane is an ominous finding and indicates the need for definitive imaging with CT as well as emergent surgical consultation.

3. Failure to recognize adjacent structures. Abscesses may occur in close proximity to tendons, nerves, arteries, and veins. Although ultrasound is extremely helpful in diagnosing and mapping the location of an abscess, the decision to drain an abscess that is in close proximity to important structures should be guided by the experience and comfort level of the provider.

The author thanks Dr. Bradley W. Frazee for his contributions to this chapter in the previous edition.