Chapter 16. Soft Tissue

Beyond the well-known primary and secondary indications for emergency ultrasound lie a plethora of clinically useful soft tissue ultrasound applications. These offer the emergency care provider the ability to rapidly evaluate and better manage a wide array of common clinical problems. This chapter will focus on eight areas where the use of these soft tissue techniques can be of considerable value in the care of the emergency or ambulatory care patient. These include (1) evaluation of abdominal wall masses, (2) miscellaneous applications for the airway, (3) assessment of bony cortices for rapid fracture diagnosis, (4) foreign body localization, (5) imaging of tendons, joints, and muscles, (6) diagnosis of salivary gland disease, (7) bedside 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 pathological processes can occur in the abdominal wall and a patient's abdominal pain may, on occasion, be discovered to be 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 bedside 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, or is fluid collection present? Is a fascial defect present in the abdominal wall, and if so, is a loop of bowel seen passing through the defect? Armed with this additional anatomic knowledge, the site of the findings, and the history, the provider can then pursue a more targeted work-up of the abnormal process at hand.

Incisional hernias are said to occur as a delayed complication in up to 4% of abdominal surgeries1 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 always be found along the lateral border of the rectus muscle and a focal defect will be present in the aponeuroses of the transversus abdominus 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 said to occur at age 50, with men and women affected equally.2

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, lymphomas, soft tissue sarcomas, vascular bypass grafts, and pseudoaneurysms.3 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 has long been noted to be more effective at distinguishing adenopathy than clinical palpation alone.1

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. Patients on anticoagulant therapy are most prone to this malady, and bleeding may occur because of rupture of an epigastric artery or vein or because of a tear of the rectus muscle fibers.1 The resulting hematoma remains confined to the rectus sheath.

Abdominal wall endometriosis can occur at the site of a previous C-section or lapartomy and should be considered in the differential diagnosis of a women presenting with recurrent focal abdominal wall pain with menses. 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.4

Finally, 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 also been used to help localize the injection port of an intrathecal drug delivery pump whose location in the abdominal wall could not be found clinically.5

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 abdominus 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. The femoral artery, vein, nerve, and lymph nodes are found just anterior to the iliopubic eminence. 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.3 Since the anatomic structures of interest are all superficially located, a linear array transducer is most suited to this type of examination.

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. The rectus muscles appear hypoechoic and speckled in short axis 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 bowel 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 16-1 and 16-2).

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. This conjoined fascial layer is termed the Spigelian fascia. The hypoechoic lateral border of the rectus muscle serves as a localizing landmark for the region (Figures 16-3 and 16-4). The area of greatest sonographic interest will be in the region between the umbilicus and the pubic region where a Spigelian hernia is most likely to occur.

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 psoas muscle 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 beneath the anechoic femoral vessels (Figure 16-5).

Common and Emergent Abnormalities

Wound Abscess

As noted above, a wide range of pathology may occur in the abdominal wall. A postoperative wound abscess will appear as a hypoechoic fluid collection at the surgical site with clinical signs that suggest that a wound infection is present. A post-operative seroma, however, will manifest as an anechoic collection of easily compressible fluid with no associated clinical signs to suggest infection (Figure 16-6).

Lymph Node

A region of focal tenderness in the inguinal region may be discovered to be due to a reactive inguinal lymph node. A lymph node will appear as an elliptical structure in long axis, hypoechoic at the periphery with a variably hyperechoic fatty central hilum (Figure 16-7).

Rectus Sheath Hematoma

On occasion, a patient may present with a focal region of abdominal tenderness and swelling that is discovered to be due to a rectus sheath hematoma. Sonographically, the normally homogeneously hypoechoic rectus muscle will appear hyperechoic and a focal homogeneous collection consistent with a hematoma may be present (Figure 16-8).

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 16-9). 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 16-10A). In long axis, a linear region of “dirty shadowing” and reverberation artifacts will be seen (Figure 16-10B). When obstructed, small bowel loops will appear as hypoechoic tubular fluid-filled structures with prominent hyperechoic val-vuli conniventes.

Lipoma

These masses will appear as rounded or ovoid structure similar in echotexture to the surrounding subcutaneous tissue. Palpation of the lesion will guide the clinician to the region of sonographic interest. On the sonogram, 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 16-11.)

Common Variants and Selected Abnormalities

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 with sharply defined borders and scattered internal echoes similar to the endometriomas that occur in the abdominal cavity.4 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.

A pseudoaneurysm represents an area of fibrous encapsulation around a pulsatile and expansile 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 and Indications

The anatomic structures of the larynx and upper airway are superficially located and well suited for bedside sonographic assessment. The sonographer may thereby (1) obtain precise knowledge of the location of the thyroid, cricoid, and tracheal cartilages if landmarks are difficult to palpate and a surgical airway is planned, (2) visualize motion of the vocal cords and arytenoids when there is a question of a vocal cord palsy, or (3) visualize the real-time passage of an endotracheal tube to ascertain whether it is in the airway or esophagus, and confirm cuff location within the trachea. Ultrasound may also be used to assess for diaphragmatic movement or pleural gliding during the phases of respiration and can thereby provide indirect evidence of endotracheal, endobronchial, or esophageal intubation. A brief review of these airway-related ultrasound applications follows.

The distinctive ultrasound images that are obtained from endotracheal intubation compared were esophageal intubation were described in one report6; recognition of these characteristic sonographic patterns on a transverse view of the trachea can be of value for rapid identification of an inadvertent esophageal intubation. In a related report, 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. Of note, 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) instead of at the cricothyroid membrane (73% sensitivity and 56% specificity).7 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 usually ideally situated midway between the vocal cords and the carina. The authors concluded that this sonographic technique could be clinically useful for rapid assessment of endotracheal tube position in any situation where tube movement, near extubation, or endobronchial intubation might have occurred.8

In addition, ultrasound may be used for secondary confirmation of endotracheal tube position either by direct observation of diaphragm motion during ventilation or by evaluation for lung sliding. A 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 intubations, all correct tube placements, both esophageal placements, and all 8 right mainstem placements 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.”9 Using a cadaver model and a 4–2 MHz microconvex transducer, the performance of the lung sliding sign as a predictor of endotracheal tube placement was evaluated in 68 intubations in 9 cadavers.10 For differentiating esophageal vs. tracheal intubation sensitivity ranged from 95 to 100% and specificity was 100%.

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.11–13 Ultrasound has also been evaluated to assess the anteroposterior thickness of the epiglottis. One study examined 100 subjects, using a subhyoid window and a transverse scan plane, and the epiglottis was visualized in all cases. There is apparently 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.14 Its application for rapid bedside airway assessment of a patient with suspected epiglottitis is as of yet unclear.

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 neck. The thyroid cartilage is composed of two broad rectangular laminae that meet in the anterior midline at about a 90 degree 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 arytenoids 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 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.

Technique and Normal Ultrasound Findings

Transducers

The upper airway is best imaged with a 5–10 MHz linear array transducer. Long- and short-axis midline views may be used, depending upon 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 is long. A stand-off pad may be helpful if a lower frequency transducer is being used. Copious use of gel may help with obtaining images at the suprasternal notch.

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 range of echogenic appearances, from hyperechoic to nearly isoechoic with the adjacent strap muscles. When hyperechoic, the region beneath appears nearly anechoic (Figure 16-12); when isoechoic with the surrounding muscles, the laryngeal structures beneath are more readily identified (Figure 16-13). When scanning just above the superior thyroid notch, the anterior portion of the inverted V will always appear hypoechoic; a thin echogenic line that corresponds to the anterior portion of the thyrohyoid ligament may be noted (Figure 16-14). The hypoechoic strap muscles are seen overlying the laminae on each side of the thyroid cartilage.

Cricoid Cartilage and Cricothyroid Membrane

As the transversely oriented transducer 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 hyperechogenicity 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 simple (Figure 16-15). The cricothyroid muscles surround the cricoid cartilage at this level and appear as anechoic crescents on either side of the somewhat 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 cricoid ring 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 grey echotexture. When scanning the airway in long axis, the cricoid cartilage typically appears hypoechoic and ovoid in shape, and may contain some internal areas of calcification that 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 16-16). Either long- or short-axis views can be used for rapid localization of the cricothyroid membrane.

Vocal Cords

The patient can be seated or supine with the neck in a relaxed neutral position. The vocal cords are imaged with a linear array transducer in a transverse scan plane either through the thyroid cartilage itself if it is not echogenic or through the membranous portion of the thyrohyoid ligament immediately above the superior thyroid notch. The arytenoids appear as rounded echogenic structures that are easily identified by their posterior midline location within the larynx and their movements on abduction and adduction (Figure 16-17).

Trachea

In long-axis orientation the tracheal rings are identified as small hypoechoic rectangular structures that appear like a string of beads in the midline near field. They may be either completely hypoechoic (in which case no shadowing will be seen) or somewhat calcified and have an echogenic surface that is associated posterior acoustic shadowing. The tracheal lumen appears as a brightly echogenic line immediately beneath the cartilages (Figures 16-18 and 16-19). The cricoid cartilage can be identified by its larger size and more ovoid profile allowing for accurate identification and numbering of the tracheal rings if needed.

Confirmation of Endotracheal Intubation

For evaluation of endotracheal tube location and cuff position, both transverse and long-axis views may be utilized. A long-axis view is best for visualizing the passage of the endotracheal tube into the trachea at the time of intubation and slight to and fro motion of the endotracheal tube will further enhance its visualization (Figure 16-20). A foam filled cuff or 8–10 mL of saline within the cuff 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 tracheal wall will be seen as two parallel echogenic lines (curved in short axis and linear in long axis) that represent the inner and outer 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 16-21, 16-22, and 16-23). 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, after which the transducer is rotated 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 good for supervising resident intubations and can be used as a rapid confirmatory test in a postarrest situation in which capnography may not be useful.

Secondary Confirmation of Endotracheal Tube Placement

Diaphragmatic movement may be observed real-time from a subxiphoid window using a standard curved array abdominal transducer. A wide sector angle should be used so that both diaphragms may be easily visualized from a single scanning plane. When this is not feasible, a sagittal right or left chest view from the anterior to mid axillary line may be employed. A combined B- and M-mode image may be obtained and the direction and depth of diaphragm motion observed and in real-time and recorded. With normal respirations or ventilation, the echogenic line that represents the diaphragm on the M-mode tracing will be seen to move toward the transducer with inspiration and away with expiration (Figure 16-24). With correct endotracheal tube placement, symmetrical movement toward the transducer should be seen with a delivered breath at both diaphragms. With esophageal intubation, the air-filled stomach will push the diaphragm away from the transducer during inspiration. Asymmetry of movement of the two diaphragms is seen with inadvertent right mainstem intubation. In an analogous fashion, evaluation of pleural gliding may similarly be used to assess for correct tube placement. Absence of a gliding implies that the hemithorax being assessed is not being ventilated or that on pneumothorax is present.

Epiglottis

The epiglottis is reliably imaged from a subhyoid window in transverse orientation. The appearance of the sonogram is that of a face or mask; two ovoid hypoechoic “eyes” represent the sternohyoid muscle in cross section; the hyperechoic “nose” correlates with the fat pat beneath the thyrohyoid ligament, and the hypoechoic “mouth,” somewhat downturned in the center with a hyperechoic inferior border, represents the epiglottis in cross section (Figure 16-25). As noted above, the average AP diameter of the epiglottis in a series of 100 normal subjects was of 2.39 ± 0.15 mm.14

Common and Emergent Abnormalities

Esophageal Intubation

With an esophageal intubation, the otherwise flaccid and normally flattened esophagus will be stented open by the endotracheal tube. On a short-axis view, the endotracheal tube will appear as a series of parallel curved echogenic lines anteriorly with posterior acoustic shadowing. It may be seen either behind the posterolateral edge of the thyroid cartilage or lateral to the trachea at a level below the cricothyroid membrane. The lumen of the airway will not show evidence of endotracheal tube presence. Best visualization is reported to occur about 1 cm below the cricoid ring (Figure 16-26). A nasogastric tube will similarly stent open the normally flaccid and flattened esophagus and will be apparent on the sonogram as an additional hypoechoic circle with posterior acoustic shadowing adjacent to the airway (Figure 16-27).

Vocal Cord Palsy

Prolonged vocalization of a single vowel (such as “e”) will enhance visualization of both asymmetric vocal cord motion and abnormal arytenoid movement and should be 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.11

Pitfalls

Subcutaneous emphysema, significant neck edema or hemorrhage from trauma or recent surgery, or an open wound over the anterior neck can make sonographic imaging of the upper airway difficult to impossible. Significant neck flexion may also impair adequate imaging by making it difficult to place the transducer (especially in a long-axis orientation) on the anterior surface of the neck. A deep suprasternal notch may be difficult to image because of poor transducer skin contact; copious use of gel and an acoustic stand-off pad may help in this latter situation. An endotracheal tube cuff may be difficult to visualize if a large amount of bubbles have entered along with the saline. Real-time imaging during intubation can be difficult in a patient with a short neck and could interfere with unobstructed use of the laryngoscope handle.

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 or even for finding landmarks for ultrasound-guided lumbar puncture. With the bony cortex so readily visible, sometimes to less than a millimeter resolution, sonography also provides us with a rapid and portable means to assess for bony fractures. This section will focus on the identification of selected fractures in the ED, specifically, rib and sternal fractures, long bone fractures in the trauma patient, nasal bone fractures, and zygomatic arch fractures.

Ultrasound has long been known to be considerably more sensitive for diagnosing rib fractures than standard chest radiography.15–17 In one series of 50 patients with suspected rib fractures, sonography detected fractures in six times as many patients as radiography (using a standard PA chest radiograph and a single oblique view) and was found to detect 10 times as many fractures as radiography.15 In another review of 103 patients with suspected rib injury, rib fractures were diagnosed about twice as often with ultrasound when compared to standard chest radiography.16 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.

The time-consuming nature of the examination (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 reported for this particular ultrasound application, however.15 Nevertheless, the potential utility of this rapid, bedside application for the workup of the ambulatory patient with a suspected cough fracture or isolated rib injury is undeniable.

Although the diagnostic sensitivity of chest radiography and ultrasound for suspected sternal fracture is similar, the time required to make this diagnosis can be considerably shortened with the use of bedside 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 of the 16 patients within 1 minute.18

A number of authors have commented that ultrasound may be 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).19–21 In an effort to evaluate the test characteristics of ultrasound for fracture diagnosis, investigators trained cast technicians to assess ED patients for fractures after a 2-hour training program. One hundred fifty-eight ultrasound examinations were performed in 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 easy to diagnose with no missed injuries in patients with midshaft fractures of the radius, ulna, humerus, femur, tibia, or fibula. Of added 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 as well as Respiratory assessment for pneumothorax, coining an alternate acronym: the “FASTER” examination.19 Requiring little added time to perform, such an extension of the FAST examination might provide useful and timely diagnostic information in the ED trauma room setting.

The accuracy of physician-performed ultrasound in the detection of long bone fractures was investigated. With only 1 hour of training, physicians with minimal ultrasound experience evaluated 58 ED patients with bedside ultrasound and results were compared to plain films or CT as the gold standard. US provided improved sensitivity with less specificity compared with physical examination. For detecting humerus and midshaft femur fractures ultrasound was found to be 100% sensitive. Ultrasound was found to be limited in detection of fractures at or above the intertrochanteric line of the hip, however, with five false-positive readings reported in this subset of patients.20

The past decade has also seen ultrasound employed as a diagnostic tool in the assessment of patients with nasal trauma. In a report on 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. A 10 MHz linear array probe was used, and 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). Of the 63 patients evaluated, 42 (67%) were diagnosed with nasal fractures. Ultrasound was found to be statistically superior to radiography for assessment of the lateral nasal walls, and conventional radiography superior for evaluation of the nasal dorsum.22 Scanheads (20 MHz) normally used for evaluation of skin tumors and skin thickness assessment have also been utilized for evaluation of nasal bone fractures.23

Additional diagnostic applications reported in the ultrasound literature are many and include intraoperative postreduction confirmation of the position of zygomatic arch fracture fragments,24 diagnosis of infant hip dislocation, diagnosis of infant posterior shoulder dislocation,25 diagnosis of posterior sternoclavicular dislocation,26, 27 diagnosis of subtle fractures of the clavicle and femur in infants,28 and finally, use of ultrasound as a procedural aid for closed reduction of displaced extra-articular distal radius fractures29 or pediatric forearm fractures.29 For each of these applications, it is the identification of the bright bony cortical echo on the sonogram that allows the provider to assess whether the bone being imaged and its relationship to surrounding structures are 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 through the seventh ribs articulate with the manubrium and sternum laterally, and the manubrium articulates with the sternum 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 supports of the external nose are composed of two nasal bones along the dorsum of the nose and the frontal processes of the maxilla laterally. The nasal bones are contiguous with the frontal bone above via the nasofrontal suture and the maxillae laterally via the nasomaxillary sutures (Figures 16-28 and 16-29). 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

Evaluation of rib and costochondral cartilage fractures is usually undertaken with linear array transducers in the 7.5–12 MHz range. In short-axis orientation, a rib will be seen casting a dense posterior acoustic shadow beneath its echogenic superficial cortical surface (Figure 16-30). Slightly below the rib, the pleura will be seen as a brightly echogenic horizontal line. Pleural gliding and comet-tail artifacts will usually be seen at this interface on real-time scanning. At the site of maximal tenderness, the rib being evaluated should first be aligned in short-axis orientation in the center of the image. The transducer should then be turned 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 16-31). Care should be taken to 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 deeper and with pleural gliding and comet-tail artifacts usually apparent (Figure 16-32).

A 5.0–10 MHz linear array scanner is recommended for evaluation of a suspected sternal fracture. The sternum should be imaged 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. An acoustic stand-off pad may be useful to bring the sternal surface into an optimal focal zone.

Imaging of long bones in the trauma setting can be undertaken with the transducer 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 16-33). Once the lower end of the relevant bone has been located, the transducer is oriented sagittally and the transducer can be slid up the extremity to evaluate for any cortical irregularities along the shaft (Figure 16-34). Again, the cortical surface of the bone closest to the transducer will be seen as a brightly echogenic line on the sonogram.

For sonography of the nasal pyramid, a 10 MHz or higher frequency linear array transducer is typically utilized. It is placed along each side of the nose aiming superomedially 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 16-35 and 16-36). Imaging depth is typically set to 3 cm and, as with all bone imaging, the electronic focus should be adjusted to maximize resolution at the level of the cortex. Using transducers and settings similar to those used for nasal bone imaging, the zygomatic arch can be readily visualized with ultrasound by scanning the upper lateral cheek in a horizontal scan plane (Figure 16-37).

The same imaging techniques apply when ultrasound is used for reducing fractures (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 being scanned (Figure 16-38), 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 16-39) and a hypoechoic fracture hematoma may be noted 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 16-40).

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 being imaged. Examples of common long bone fractures are demonstrated in Figures 16-41, 16-42, and 16-43.

Nasal Bones

A nasal bone fracture is clearly demonstrated by the large hypoechoic gap in the normally echogenic cortical surface in Figure 16-44.

Zygomatic Arch

The normal contour of the zygomatic arch is clearly disrupted in Figure 16-45, and a prominent hypoechoic fracture hematoma is present.

Pitfalls

1. General pitfalls. An important pitfall of fracture sonography was illuminated 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.30 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.31

2. Rib imaging pitfalls. The pseudofracture. 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. The brightly echogenic pleural surface (seen when scanning along the long-axis of an intercostal space) should not be mistaken for the cortex of the rib. The rib in question should first be scanned in short-axis orientation and can be easily identified by its characteristic posterior acoustic shadowing. Note should be taken of the location of the superficial surface of the rib and its depth within the soft tissues of the chest wall. When the transducer is subsequently rotated into a long-axis orientation to assess the rib for the presence of a fracture, the sonographer will then be aware of the expected depth of the rib surface. 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 the to-and-fro gliding movements (the “gliding sign”) of the brightly echogenic pleural interface. Gliding comet-tail artifacts (arising from the visceral pleural surface and the adjacent air-filled alveoli) are also usually apparent at this location and can aid in its identification as pleura and not rib. Both these findings will be absent if a coexistent pneumothorax is present, however, and it may be the depth of this echogenic line alone that identifies the structure.

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.18

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.20 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 probe head may be difficult to place on a small nose. In such cases, copious use of gel or an acoustic stand-off 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 wide 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.32 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 medico-legal 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.33

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 also is frequently 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%.34, 35 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.34, 36, 37 In a case where a radiolucent foreign body is suspected, ultrasound equipment can be brought to the bedside at the time of the examination, functioning as an extension of the physical examination. With ultrasound equipment now readily available in many emergency departments, bedside sonographic assessment of wounds suspected of harboring a radiolucent foreign body should become increasingly common.

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 of a radiolucent foreign body,
• localization of a radiolucent or radio-opaque foreign body, and
• foreign body removal.

The literature on sonographic detection of soft tissue foreign bodies encompasses a wide range of specialties and methodologies.34, 36–49 The types of foreign bodies that have been studied include metal, wood, plastic, gravel, sand, and thorns or cactus spines. Sonographers with varying levels of skill perform the ultrasound examinations in these studies, ranging from emergency physicians with no prior formal training, credentialed sonographers, 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,39 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.36 In studies involving “freshly thawed” cadaver feet and hands, diagnostic sensitivities and specificities ranged from 90 to 94% and from 90 to 97%, respectively.42, 47 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 only 43% and 70%, respectively, with a sensitivity of only 50% for detecting a 1-cm-long piece of wood.41 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 difficult. Of note, vigorous wound irrigation itself can introduce subcutaneous gas bubbles that interfere with subsequent attempts to locate small glass fragments with ultrasound. However, in another 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.50

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.39 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.42 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 sonographer 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 sonographers to locate foreign bodies with ultrasound.36 It found no statistically significant difference in accuracy between a board-certified radiologist whose practice was limited to ultrasonography, two ultrasound technologists, 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 technologists.

In clinical case series, wood is the most common radiolucent material reported, hand and foot injuries predominate, and most foreign bodies are found to be superficial in location.34, 37, 51 One series of 50 patients evaluated for radiolucent foreign bodies noted that 45 of the 50 injuries involved the hand or foot.37 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.34

Anatomic 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

The highest frequency linear array transducer available should be used 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 7.5–10.0 MHz range is generally recommended. A 7.5 MHz curvilinear transducer, such as an endocavitary probe, may also function adequately for this application and has the added advantage of a smaller, rounded skin contact footprint for scanning in web spaces.38, 52 A 5.0 MHz transducer may be useful when searching for a deep foreign body. Higher frequency small parts transducers (typically in the 10–13.0 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. In recent years, small parts transducers have become a more common addition to ultrasound equipment purchased for use in the ED. The combination of high-image resolution and a small skin contact footprint make these transducers useful for imaging digits and Web spaces for foreign bodies, in addition to ocular imaging, and selected procedural and musculoskeletal applications.

Use of an acoustic stand-off pad may be necessary with some transducers to adequately image the superficial soft tissues. Stand-off 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 stand-off 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, 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 stand-off pad or the copious use of gel. Compared to direct contact with gel, the water-bath technique was easier to perform and provided superior images of tendons and foreign bodies in one report.53, 54 The water bath technique is also reported to cause less patient discomfort, since images may be obtained without direct contact between the patient and the transducer.55 The sonographer should ensure that only sealed portions of a transducer are immersed in the water bath.

Optimizing depth and focus settings is particularly important when using ultrasound to search for a small, subcutaneous foreign body. The transducer should be held perpendicular to the skin surface and area of interest systematically scanned 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 sonographer 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 stand-off is used, adjacent to the distant side of the anechoic stand-off 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 above the muscle layer immediately below. Muscle tissue appears relatively hypoechoic with regular internal striations (linear or pennate in long-axis, speckled in short-axis relative to the orientation of the muscle fibers). Tendons are moderately echogenic, appear ovoid and finely speckled in short-axis, and rectangular with a characteristic fibrillar sonographic pattern 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 musculotendon portion of this chapter. Tendon movement can be observed 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 16-46 and 16-47.

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 16-48, 16-49, 16-50, and 16-51). Linear metallic foreign bodies will typically display a reverberation artifact with bright, regularly spaced parallel lines seen distal to the actual object (Figures 16-52, 16-53, and 16-54). Metal objects that are small, or rounded, may display a comet-tail artifact (Figure 16-55). 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 16-56 and 16-57). 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 facilitates identification and localization of the foreign body (see Case Study). In a similar fashion, a local anesthetic injected adjacent to a foreign body may improve the ability to visualize it. Finally, 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 seen in Figures 16-58 and 16-59.

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 stand-off 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 transducer or an endocavitary probe, as well as stand-off 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. 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. In the case of a deep wound, a poorly accessible foreign body or closely adjacent neurovascular structures, consultation or referral to an appropriate surgical specialist is recommended.

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 area but no definite fluctuance (Figure 16-60a). 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 16-60b). An ultrasound was performed using a 7.5 MHz annular array transducer and an acoustic stand-off to obtain the images in both longitudinal (Figure 16-60c) and transverse (Figure 16-60d) planes. A hyperechoic foreign body with an associated hypoechoic surrounding inflammatory response was seen above the PIP joint. The wood fragment was easily removed in toto with a superficial skin incision along the PIP crease (Figure 16-60e).

Commentary

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

Musculotendinous Applications

Clinical Considerations

Technological advancements over the past decade have led to the increased use of diagnostic ultrasound for the 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, whether in the hands of radiologists, rheumatologists, or emergency physicians. Ultrasound has emerged as “a powerful extension of the physical examination”56 and nowhere more so than in the realm of musculotendinous assessment. Imaging of tendons, joints, and muscles can help the provider make a correct diagnosis for a host of painful musculoskeletal conditions; armed with this knowledge, optimal treatment and management can follow. 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.57

A myriad of musculoskeletal applications for diagnostic ultrasound have 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 muscle tears (specifically, the rectus femoris and gastrocnemius muscles), (3) diagnosis of occult ganglion cysts58 (wrist or fingers), (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 sonographers, or they may require specialized transducers that are not generally available in the emergency or urgent care settings. With the now common presence of linear array transducers available for compact ultrasound units, however, and the increasing appearance of small parts transducers available on newer equipment packages, a host of selected musculotendinous applications can easily be added to the diagnostic armamentarium of the emergency sonographer.

Clinical Indications

Clinical indications for performing a musculotendinous ultrasound examination may include

• assessment of suspected partial or complete tendon tears,
• evaluation of suspected tenosynovitis and selected tendinopathies,
• assessment of joint involvement in chronic arthritis, and
• precise guidance for aspiration and soft tissue injection procedures involving tendons and bursae.

A number of clinical studies have investigated the use of ultrasound to image rotator cuff tears with variable success.59–61 In an attempt to explain the wide range of reported accuracy for diagnosing rotator cuff tears with ultrasound (60–95%), a number of contributing factors were identified. The technical difficulty involved, the considerable experience required to perform the examination, 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.62 Because of these difficulties, only a few centers routinely use ultrasound to evaluate the shoulder for a suspected rotator cuff injury. MRI, with its excellent image quality and lack of operator dependence, 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; MRI has become the imaging technique of choice for these injuries as well.

Complete disruptions of the rotator cuff, the biceps, triceps, quadriceps, or Achilles tendons are usually reliably diagnosed clinically and imaging is usually not required. If desired, however, sonography can be used to rapidly demonstrate the site of the specific 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 ably assist the provider and help clarify the nature and extent of the suspected tendon injury. The Achilles tendon, quadriceps tendon, patellar ligament, and the triceps tendon all lend themselves to ready 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 or with plain radiography alone.

Tenosynovitis is also readily diagnosed with bedside ultrasound and its characteristic sonographic signature (hypoechoic fluid surrounding the tendon or tendons) makes arriving at this diagnosis simple. 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 appropriate therapy. Similarly, selected tendinopathies can be diagnosed with a sonographic evaluation that reveals a characteristic focal area of hypoechogenicity of the tendon being assessed. A suspected ganglion cyst can similarly be quickly diagnosed with the visualization of an anechoic sac-like 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 detail in the section on ultrasound guided arthrocentesis (Chapter 20).

Finally, ultrasound guidance may be used to advantage for precise percutaneous injections of painful conditions of the musculotendinous system, specifically, injections of tendon sheaths, joints, bursae, and peri-fascial injections for the treatment of common tendinopathies, bursitidies, arthritidies, and fasciitis.63 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 corticosteroids and local anesthesia used for treating these conditions. A specific example is the improved management of patients with subacromial bursitis. In a report investigating the treatment effectiveness of ultrasound-guided injections, a group of 40 patients with sonographically confirmed subacromial bursitis were divided into standard blind injection and ultrasound-guided injection groups.64 The outcome measure assessed 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 authors concluded that ultrasound could be used to advantage in guiding the injection needle accurately into the inflamed synovial bursa with significant therapeutic benefits as a result.

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 vinculae.65

Technique and Normal Ultrasound Findings

Musculotendinous ultrasound imaging should, in general, be performed with high-frequency linear array or small parts transducers in the 7.5–13.0 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.62 This drawback can be somewhat compensated for by narrowing the sector angle and using a stand-off pad. When imaging very superficial structures, such as the Achilles tendon, patellar ligament, or structures within the finger, the use of an acoustic stand-off pad may be helpful. Attention should be paid to proper frequency, focus, and depth settings to obtain optimal images. A split screen set up may be useful for comparing corresponding images from each side of the body.

Skin is typically seen as a thin, hyperechoic layer adjacent to the transducer or stand-off pad. Subcutaneous tissue appears hypoechoic, is of variable thickness depending on body location and habitus, and has a somewhat hyperechoic reticular pattern of 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 being imaged. 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 skin surface. 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 appear round, ovoid, or flat, and will usually appear as a cluster of multiple small echogenic dots. A subtle anechoic rim will be seen in tendons with a tendon sheath. Commonly imaged normal tendons are seen in Figures 16-61, 16-62, 16-63, 16-64, and 16-65.

Tendons exhibit an optical phenomenon known as anisotropy (i.e., the image obtained from the tendon is directionally 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.65 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 helps avoid this pitfall; the transducer is gently rocked in its long-axis plane from one end to the other while scanning long-axis over a tendon. 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 long-axis.

Specific Imaging Techniques

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.

The patellar ligament should be scanned in longitudinal and transverse scan planes with the knee held in 30 degrees 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.62

The Achilles tendon is best evaluated with the patient prone with the foot hanging over the edge of the examining table. Use of a stand-off 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 delivery of therapeutic injections of corticosteroids. A free-hand technique is most commonly used for injection of tendon sheaths, joints, and bursae. The guiding principle is that the medication delivery needle (typically a 1 1/2′′ 20–25 G spinal needle) should be oriented as perpendicular to the insonating beam as possible in order that the needle will appear maximally reflective. The needle's position will be apparent by its strong reverberation or ring-down artifact and its position can be further fine-tuned by injecting a small amount of anesthetic solution and observing the corresponding echoes. When a soft tissue injection procedure is undertaken, the tendons should be oriented 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.63

Common and Emergent Abnormalities

General Ultrasound Findings

Partial tendon tears are usually seen on ultrasound as hypoechoic to anechoic regions within the substance of the tendon. Complete tendon tears will demonstrate obvious tendon discontinuity during real-time scanning and the tendon sheath may be seen to be filled with a hypoechoic hematoma at the site of disruption. Tenosynovitis will sonographically manifest as tendon thickening, tendon sheath widening, and loss of the tendon's normal fibrillar echotexture. An associated anechoic collection of inflammatory synovial fluid will be seen surrounding the tendon. Synovial fluid is typically anechoic in acute inflammatory processes. Tendonitis or tendinosis usually manifests as areas of patchy hypoechogenicity and loss of fibrillar echotexture within the tendon. Tendons without a sheath will show thickening and altered echogenicity that varies according to the duration of the process.

Specific Clinical Scenarios

Tenosynovitis

DeQuervain's disease is an overuse injury involving the first dorsal compartment or the hand (specifically, the abductor pollicis longus and extensor pollicis brevis tendons). Although a positive Finkelstein's sign usually confirms the diagnosis, the tendon can be rapidly imaged in short-axis to confirm the diagnosis in ambiguous cases. Fluid within the tendon sheath, tendon thickening, and pain on “sonographic palpation” are typical (Figure 16-66). 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 16-67).

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”). 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 cheap, noninvasive, repeatable and accurate.”66 Sonographically, the lesion appears as a localized area of hypoechogenicity in the central portion of the patellar ligament near its insertion on the patella. The patient usually has exquisite tenderness with palpation at this site (Figure 16-68). Diagnosis and injection therapy of another common tendinopathy, lateral epicondylitis or “tennis elbow,” is discussed in detail below.

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 (“jumper's knee,” above) 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 to the patella. On sonography the discontinuity between the patellar ligament and the patella will be apparent. 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 16-69). 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 16-70). 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 16-71).

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 in normal subjects and a sign of bursitis is that the bursae are demonstrable by ultrasound at all (Figure 16-72).69 Injection of the retrocalcaneal bursa is sometimes performed to relieve the pain of a chronic Achilles tendon bursitis. The patient is scanned prone with the tendon and bursa in a transverse scan plane using a 7.5 or higher MHz transducer. Needle placement and steroid injection into the bursa are best accomplished with a lateral approach.

Common Variants and Selected Abnormalities

Ganglion Cysts

A patient with a palpable mass in the hand, wrist, or digit may be assessed with ultrasound for the presence of a ganglion cyst. 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 16-73).67

Ultrasound-Guided Steroid Injections

The techniques employed for ultrasound-guided soft tissue injection therapy have been reviewed above. Injection of a common tendinopathy, lateral epicondylitis, is demonstrated in Figure 16-74. 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.68, 70 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). Gouty tophi will demonstrate posterior acoustic shadowing, much like gallstones. Inflamed bursae (also discussed in the Arthrocentesis section) will appear as superficial hypoechoic fluid collections. Care should be taken to limit transducer pressure on smaller bursae to avoid causing them to collapse and thereby displacing the bursal fluid 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.62 Since areas of hypoechogenicity are 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, a heel–toe scanning technique should be employed to further clarify the true sonographic character of the area in question.

Salivary Glands

Clinical Considerations and Indications

When evaluating a patient with preauricular or submandibular swelling or tenderness, a bedside ultrasound examination of the area can rapidly clarify if the source of the problem lies within the parotid or submandibular gland. Although CT is the traditional imaging modality of choice for parotid and submandibular inflammatory conditions, and MR is considered the modality of choice for evaluation of parotid and submandibular tumors, ultrasound is considered by some to be the initial imaging modality of choice for assessment of any palpable abnormalities of the parotid gland.71 Ultrasound can clarify if an intra- or extraglandular lesion is present, provides information as to whether a focal process or diffuse glandular involvement is present, and can help guide aspiration if an abscess is found in a case of acute bacterial sialadenitis. Ultrasound is particularly useful in the evaluation of suspected sialolithiasis where it has a reported 96% diagnostic accuracy.72 Given the somewhat questionable reliability of the clinical examination (one report notes that 30% of a series of 38 preauricular masses were not intraparotid in location73) and the ready availability of portable ultrasound equipment in many emergency departments, ultrasound examination of the region of facial swelling or tenderness should be considered to be part of the bedside workup of any patient with suspected salivary gland disease in the ED.

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). The sonographic features of these diseases will be discussed in further detail below. The majority of parotid tumors (85–90%) are benign pleiomorphic adenomas. They are slow growing, painless masses seen most commonly in middle-aged patients and are usually found in the superficial lobe of the parotid gland. By contrast, 50% of tumors in the submandibular gland are found to be malignant. Salivary gland calculi may occur in the parotid gland, but they are most commonly found in the submandibular gland, presumably because of the more mucinous content of submandibular gland secretions. Most salivary gland calculi occur in the submandibular duct; the remainder are found within the gland or in the ductal hilum. Multiple stones are found in 25% of patients.72

Anatomical Considerations

The body of the parotid gland lies in a preauricular location, its uppermost portion roughly in line with the external acoustic meatus and from there the gland extends 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 2nd 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. Lymph nodes may be 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 lateral 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 lateral border of the mylohyoid muscle where it then courses medial to the sublingual gland to a papilla adjacent to the lingual frenulum in the anterior floor of the mouth. The duct is about 5 cm in length. Unlike the parotids, no intraglandular lymph nodes are found within the submandibular gland. The named 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.72

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

The parotid and submandibular glands are best imaged with a 7.5–12 MHz linear array transducer. The parotid gland is scanned in long-axis in a vertical scan plane in front of the lower ear. The superior portion of the transducer will need to be angulated somewhat anteriorly, however, 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, a transverse scan plane is employed 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 (Figures 16-75 and 16-76). Intraparotid ducts appear as echogenic linear structures within the substance of the gland. Intraparotid nodes are common, most commonly in a preauricular location. Lymph nodes are typically elliptical in shape with a hypoechoic periphery and a hyperechoic fatty central hilum. 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, the transducer should be placed in the submental region just medial to the mandible aiming toward the middle of the chin. From whatever side is being imaged, the orientation marker should always face left. By so doing, 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 appears posterolateral to the mylohyoid muscle and appears as a rounded and somewhat lobular structure with a finely granular echotexture identical to that of the parotid gland. The Küttner lymph node may be seen posterior and adjacent to the submandibular gland. The mylohyoid muscle appears as a horizontally oriented and somewhat striated appearing rectangular region of hypoechogenity in the near field of the image; the muscle tapers as it approaches its insertion on the symphysis menti. The inferior aspect of the symphysis menti appears as a slightly curved region of hyperechogenicity in the near field with a dense posterior acoustic shadowing beneath. 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, it is important to 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.

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, hypoechoic, and of heterogenous echotexture. 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. Chronic sialadenitis may occur as a result of infective or noninfective causes and has a sonographic appearance that varies with the type and stage of the disease.

Masses

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 encountered and comprises 85–90% of parotid tumors. Sonographically it appears rounded or lobular in shape, well circumscribed, and is homogeneously hypoechoic with posterior acoustic enhancement. Because the most common malignant solid tumor (mucoepidermoid carcinoma71) may initially appear sonographically similar to a pleiomorphic adenoma, all solid lesions should be referred for definitive assessment and diagnosis.

Sialolithiasis

A salivary duct stone or calculus will appear as a hyperechoic focus with prominent posterior acoustic shadowing. It will most typically be found within the salivary duct of the submandibular gland, and occasionally within the ductal hilum. Less commonly, a calculus may be encountered within the parotid duct or gland. The salivary duct in such cases will typically be quite dilated and will appear in long-axis as a prominent rectangular or tubular anechoic region leading up to the stone. The gland itself may appear enlarged with dilatation of the hilum of the gland giving the gland a somewhat “hydronephrotic” appearance (Figures 16-77 and 16-78).

Lymph Nodes

Finally, a preauricular mass or 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 16-79).

Pitfalls

The major pitfall with ultrasound imaging of the salivary glands is failure to consider a malignant etiology as a cause for a solid lesion. Since benign and malignant lesions may have similar sonographic appearances, all solid lesions should be referred for definitive workup. The role of the bedside ultrasound examination should be to identify if a lesion is present, to establish whether it is intraglandular or not, and to identify if calculus disease is the cause of the swelling or pain. Nearly all other pathologies should be considered for referral and further evaluation with the exception of diffuse polyglandular enlargement (as seen with mumps), diffuse uniglandular enlargement and tenderness (as seen in sialadenitis), sialolithiasis, an obvious abscess cavity, and a reactive lymph node.

Maxillary Sinusitis

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.74 Bedside diagnosis of bacterial sinusitis, for which antibiotics are recommended, is difficult by history and physical examination alone, however. At most, only 50% of patients presenting to emergency or general care settings with sinus complaints will actually have bacterial sinusitis.75 Signs and symptoms of bacterial sinusitis are nonspecific and may be 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 the additional time, cost, and radiation exposure produce little additional diagnostic accuracy.76 A fluid-filled maxillary sinus can readily be identified by ultrasound, however. Ultrasound may, therefore, be employed as a rapid and safe diagnostic aid at the bedside when evaluating a patient suspected of having maxillary sinusitis. Sinusitis is also an important occult cause of fever and nosocomial pneumonia in patients undergoing prolonged mechanical ventilation.77 In the ICU setting, bedside sinus ultrasound is a well-established screening test for detecting suspected maxillary sinusitis in intubated patients.

Clinical Indications

The primary indications for sinus ultrasound in emergency and ambulatory practice are as follows:

• 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 probably 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 methodologic 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.78 A systematic review of these early studies concluded that operator skill and experience had a large effect on how well ultrasound performed.79

Three contemporary studies have been conducted in the ambulatory and emergency care setting comparing bedside ultrasound to radiography or MRI for diagnosing acute maxillary sinusitis. A study found that, using MRI as the gold standard, ultrasound was 64% sensitive and 95% specific compared to 73% sensitive and 100% specific for plain film radiography.80 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-film radiographs be obtained. A study found that ultrasound was 92% sensitive and 95% specific compared to plain film radiography. In the primary care setting in which their study was conducted, the addition of bedside sinus ultrasound to the history and physical examination would have reduced antibiotic prescriptions for sinusitis by one half.81 Finally, Price and colleagues compared emergency physician–performed ultrasound to CT in 48 emergency department patients with suspected maxillary sinusitis. Sensitivity was 81% and specificity 89%.82

Maxillary Sinus Fluid in Intubated Patients

In the ICU setting, bedside ultrasound has emerged as a convenient screening test for nosocomial sinusitis, both because plain film radiographs are inaccurate assessing for sinus fluid in recumbent patients, and because obtaining CT imaging in intubated patients is particularly time and labor intensive. Ultrasound can be used at the bedside to assess for the presence of fluid in the maxillary sinus. If fluid is present, rhinoscopy or sinus puncture is subsequently performed to obtain fluid for culture. The reported prevalence of infected sinus fluid varies widely among different studies, ranging from 5% to 60% of cases.77, 83, 84 Among the numerous studies investigating the use of maxillary sinus ultrasound in the ICU setting, the three best compared ultrasound to radiography or sinus puncture (presence of fluid), and found a sensitivity of 67–100% and specificity of 86–97%.83, 85, 86 Specificity was 100% when all sinus walls were seen, which generally correlates with complete sinus opacification on radiography.

Anatomic Considerations

The maxillary sinuses are paired and somewhat pyramidal-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. The maxillary sinus is typically 2–4 cm in anteroposterior 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 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 3–10 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 will be apparent 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. The sinus should be scanned 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. Attention should be paid to proper depth settings (typically 5–7 cm) so that the image of the sinus fills about three fourths the screen. Comparison with the uninvolved sinus should be routine and can be useful 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. Of note, it is important not to confuse one of the deeper periodic resonance artifacts as representing the posterior wall of the sinus (Figure 16-80).

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.87 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 rhinosinusitis75 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, the curved posterior wall of the sinus will be clearly apparent in the far field of the image and no periodic resonance artifact will be noted (Figure 16-81). 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 lower fluid-filled portion of the sinus (right side of the image on a sagittal sonogram) and a periodic resonance artifact may be seen in the upper air-containing region of the sinus (left side of the image). An example of partial sinus opacification on CT and its corresponding sonographic representation are illustrated in Figure 16-82a,b. A continuous brightly echogenic line along the posterior wall of the sinus on sagittal imaging correlates well with complete sinus opacification on radiography. This has been referred to as a “sinusogram.”85 A discontinuous line, and some persistence of the periodic resonance artifact from the anterior wall, correlates with partial sinus opacification.83, 85

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 sensitivity of ultrasound for maxillary sinusitis is not more than 85% and involvement of other sinuses cannot be assessed. Thus, in the patient with severe symptoms, in the elderly or diabetic patient, or when the clinical suspicion for sinusitis is high, a more sensitive imaging study such as CT should be obtained for definitive diagnosis. On the other hand, evidence of maxillary sinus fluid in what otherwise appears to be an acute viral rhinosinusitis should be treated conservatively with decongestants. It should also be noted that a number of conditions other than sinusitis will allow transmission of the insonating beam through the sinus and will result in a positive scan of the posterior wall of the sinus. Significant mucosal thickening, polyps, fluid-filled cysts, occasional solid masses, as well as blood from facial trauma and sinus fractures can all give rise to a positive “sinusogram” (Figures 16-83 and 16-84). Technical pitfalls include scanning with the patient in a supine position, failure to accurately set the image depth to a level that would include the posterior sinus, and misinterpreting a periodic resonance artifact as an image of the posterior wall of the sinus.

Clinical Considerations

Emergency and ambulatory care physicians evaluate patients with skin and soft tissue infections on a daily basis. These infections include cellulitis, subcutaneous abscess, and, on rare occasion, 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 in that anatomic area. An occult abscess may coexist with what appears clinically to be a simple case of cellulitis. Ambiguous clinical findings may direct the clinician away from performing a drainage procedure when it is, in fact, indicated. Ultrasound has emerged as a valuable tool for assessing soft tissue infections at the bedside, and it is being used with increasing frequency in the emergency setting to correctly diagnose and localize occult abscesses, and to facilitate abscess drainage by providing the clinician with an anatomic road map for safely guiding the incision.

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 ambiguous clinical findings are present and
• localization of an optimal site for incision and drainage or aspiration of an abscess

Detection of Occult Abscess

The ability of ultrasound to detect soft tissue abscesses and guide subsequent incision and drainage has been appreciated since the 1980s.88–92 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.88, 91 Ultrasound has also been found to be of value in the diagnosis and treatment of odontogenic facial abscesses.93

Three studies conducted in the emergency department setting have examined the utility of bedside ultrasound for evaluation of skin and soft tissue infections.94–96 All three studies examined the impact ultrasound assessment has on the accuracy of abscess diagnosis, with cases stratified by pretest likelihood of an abscess being present. Overall, ultrasound assessment led to a change in management in 17–56% of cases. A consistent finding was that among cases categorized as unlikely to have an abscess present, ultrasound frequently revealed an unsuspected pus pocket that was confirmed by a drainage procedure (this scenario occurred in 14–58% of “low” likelihood cases). It has been demonstrated that ultrasound has utility when an abscess is clinically judged likely to be present. In such cases, a negative ultrasound evaluation often correctly excludes abscess, thus preventing an unnecessary drainage procedure. With the addition of ultrasound to physical examination alone, the positive predictive value increased from 81 to 93% and the negative predictive value increased from 77 to 97%. However, because of methodologic limitations, the actual false-negative rate for detecting abscess cannot be determined with confidence from these studies.

Anatomic Considerations

A thorough understanding of the regional anatomy of the area being scanned is essential for the clinician 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 the perianal region. They may be found in close proximity to veins, arteries, nerves, tendons, and within muscles. Awareness of the proximity of adjacent structures, familiarity with their normal sonographic appearance, and an understanding of preferred lines for elective incision are mandatory.

Technique and Normal Ultrasound Findings

Sonographic evaluation of the skin and subcutaneous tissue for a suspected subcutaneous abscess is optimally performed with a 5–10 MHz linear array transducer, although an annular array or sector transducer may also be utilized. An acoustic stand-off pad may improve image resolution if a lower frequency transducer is being used or if the abscess is very superficial and no linear array transducer is available. Depth and focus settings are adjusted to place the area of interest within the transducer's optimal focal zone. Transducer pressure should be kept to a minimum to avoid collapse and nonvisualization of superficial veins. The soft tissue area being evaluated should be systematically scanned in two perpendicular planes.

General soft tissue normal ultrasound findings are reviewed as follows. Normal skin 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 seen 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 rectangular structures, depending on the orientation of the transducer relative to the vessel; use of color flow or Doppler may aid in their identification. Veins typically compress easily with transducer pressure. 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 16-85.

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 sonographic findings in cellulitis likely arise from the accumulation of edema fluid. Findings include thickened and abnormally hyperechoic skin, swelling and diffusely increased echogenicity of the subcutaneous tissue, and areas of hypoechoic edema that traverse the subcutaneous fat in a reticular pattern (Figure 16-86).97–99 Comparison to the unaffected side may aid in the recognition of subtle findings. It should be emphasized that sonographic findings in cellulitis are simply indicative of edema and are therefore nonspecific; both necrotizing fasciitis and the inflamed tissue surrounding an abscess may take on the sonographic appearance of cellulitis. In a patient with chronic lymphedema, the subcutaneous tissue appears hyperechoic relative to normal tissue but usually not as echogenic as that seen in cellulitis. With lymphedema, prominent horizontally layered bands of hypoechoic edema fluid will be seen traversing the subcutaneous tissue (Figure 16-87).

Abscess

A subcutaneous abscess may have a variety of sonographic appearances91, 97, 99, 100 but will almost universally be surrounded by a rim of edematous soft tissue or cellulitis that appears hyperechoic relative to normal subcutaneous tissue (Figures 16-88a,b). Abscess cavities are most often spherical or elliptical-shaped and the liquefied contents of the abscess cavity will typically demonstrate posterior acoustic enhancement (Figures 16-89, 16-90, and 16-91). Not uncommonly, however, abscesses may also appear irregular or lobulated, and the abscess cavity may interdigitate between tissue planes or take an irregular path within the hyperechoic surrounding cellulitic tissues (Figures 16-92 and 16-93). Although the contents of an abscess cavity may exhibit a wide range of sonographic patterns, they will most commonly appear hypoechoic or anechoic relative to the surrounding tissues. In some cases, however, the purulent abscess cavity contents may be isoechoic or hyperechoic, making the collection more difficult to identify. Abscesses may contain hyperechoic debris, septae, or gas; the latter will appear as brightly hyperechoic regions within the abscess cavity with an associated ring-down or reverberation artifact (Figures 16-94 and 16-95). Gentle transducer pressure over the abscess site may induce motion of purulent material within the abscess cavity and helps confirm the liquid nature of the material being scanned, especially in cases where the abscess contents are relatively isoechoic with the surrounding tissue or muscle.97 Color flow Doppler may demonstrate hyperemia adjacent to the abscess cavity and can help confirm absence of flow within it.99 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, and tendons; recognition of the sonographic appearance of anatomically adjacent structures is essential prior to embarking on a drainage procedure (Figure 16-96).

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,101 making the diagnosis of necrotizing fasciitis on clinical grounds alone is often difficult. The role of ultrasound as a diagnostic test for necrotizing fasciitis has never been studied systematically, and it should not be used to exclude the diagnosis. However, sonographic findings thought to be 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.102, 103 Invariably, the subcutaneous fascia, normally seen as a thin, brightly hyperechoic line, was greatly thickened and edematous-appearing, as was the overlying subcutaneous tissue. An anechoic fluid layer measuring greater than 4 mm adjacent to the deep fascia is considered diagnostic of necrotizing fasciitis.102 In some cases, discreet masses were seen in and around the fascial plane from which pus was aspirated.16 The presence of subcutaneous gas (which typically appears as “dirty” acoustic shadowing with a reverberation artifact) may also be rapidly confirmed with bedside ultrasound in such cases. It should be borne in mind, however, that this is a nonspecific finding and that gas pockets may also be seen within an abscess cavity.

Pitfalls

1. Abscesses are not always hypoechoic. They can on occasion be sonographically subtle to detect. 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 any potential swirling or movement of the isoechoic region being evaluated may help avoid this pitfall. In the face of a strong clinical suspicion for an abscess and an apparently negative ultrasound result, an attempt at a needle aspiration for further assessment may be considered.

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. Studies examining the use of bedside ultrasound in the diagnosis of necrotizing fasciitis are limited. Ultrasound should not be used to exclude the presence of necrotizing fasciitis.

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 surrounded by these structures should be guided by the experience and comfort level of the sonographer.