Chapter 5: Special Imaging Techniques

Plain radiographs are a sufficient adjunct to the history and physical examination for the evaluation of most acute extremity complaints. It must be stressed that this statement is true assuming that the quality of views is adequate. A minimum of two perpendicular views are required to adequately visualize and describe fractures. Oblique views are commonly included when imaging the wrist, hand, ankle, and foot. In addition, radiographs of the joints above and below a fracture should be considered to exclude the presence of a subluxation, dislocation, or a second fracture.

Several other imaging techniques exist that offer additional information not readily available when imaging with plain radiography. These techniques, which include ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and fluoroscopy, may be used in conjunction with plain radiographs and may be superior to plain radiographs for certain musculoskeletal disorders. These studies and the clinical situations in which they are useful are discussed in this chapter.

Ultrasonography is gaining an increasing role within the specialty of emergency medicine, and this role continues to grow for orthopedic conditions. Soft tissue and musculoskeletal ultrasound is now recognized as one of the eleven core emergency ultrasound applications.¹ This modality offers several advantages over traditional imaging methods including the ability to perform dynamic imaging of the affected body part, the ability to easily compare findings on the affected side with those on the unaffected side, and the lack of exposure to the harmful effects of radiation. The last advantage mentioned is particularly important when evaluating the pediatric population who are more susceptible to ionizing radiation, which is delivered in large doses with imaging techniques such as CT.^2,3

Common musculoskeletal ultrasound applications in the emergency department (ED) include tendon evaluation, muscle evaluation, joint evaluation for effusion, foreign body identification, and procedural guidance.^4–6 There are several studies documenting the usefulness of ultrasound in trauma, especially with regard to the evaluation of bony trauma. It may be used in conjunction with plain radiographs to evaluate for fractures and may even be superior to plain radiographs in certain types of fractures, including rib and scaphoid fractures (Fig. 5–1).^7–9 In addition, recent research has also suggested that this modality is useful in diagnosing extremity fractures in military or sideline settings where other imaging capabilities are not readily available.^10–11 This role of ultrasound in the acute setting is also expanding to include evaluation of musculoskeletal infections. The localization of soft-tissue collections by ultrasound helps narrow the differential diagnosis based on the finding of fluid in the dermis, joint, bursa, or muscle. For this reason, ultrasound can be used to detect simple abscesses, pyomyositis, septic bursitis and tenosynovitis, joint effusions, and subperiosteal fluid associated with osteomyelitis.¹²

Musculoskeletal Ultrasound Techniques

The following is an overview of the basic musculoskeletal imaging that may be useful and can be easily performed in the ED. There is a focus on how to obtain the images for evaluation, normal imaging findings, and how to identify deviations from the normal patterns. If your findings deviate from the normal appearance of the structure, pathology should be expected, and further investigation should be pursued with appropriate imaging techniques, lab studies, or expert consultation.

Most EDs utilize ultrasound imaging systems with a high-frequency 7 to 12 MHz linear transducer. This transducer is ideal for superficial (<3–4 cm deep) musculoskeletal imaging. It provides good resolution in the near field with less penetration into deeper structures (Fig. 5–2A,B). For imaging that requires deeper penetration (>3–4 cm deep), such as hip ultrasound, a lower-frequency 2 to 5 MHz curvilinear transducer should be used (Fig. 5–2C,D). The typical width of an ultrasound beam is 0.2 to 1 mm thick, therefore careful interrogation of musculoskeletal structures is imperative not to overlook a small abnormality. Ultrasound imaging, regardless of the structure being evaluated, should be performed in both the longitudinal and transverse axes of the structure. When evaluating small parts or parts with abnormal contours, a water bath or a stand-off pad will help increase through transmission of the ultrasound waves being transmitted and improve image quality (Fig. 5–3).¹³

Tendon Evaluation

Ultrasonographic evaluation of tendons is used to identify traumatic tendon rupture and infections of the tendon and tendon sheath. Tendons are best evaluated with the linear transducer in both the longitudinal and transverse axes. On ultrasound, tendons exhibit an echogenic fibrillar pattern that is linear in nature without disruption.¹⁴ This finding is more notable when imaging in the long axis of the tendon (Fig. 5–4A). Disruption of the normal linear pattern should prompt further evaluation for an acute tear (Fig. 5–4B). Dynamic evaluation is helpful when imaging tendons, as this may enlarge an area of hematoma or rupture not previously visualized on static imaging.

Of note, it is important to maintain a perpendicular angle between the ultrasound beam and the tendon being imaged, as tendons demonstrate anisotropy. Anisotropy is an artifact that occurs when the ultrasound beam and the tendon are not perpendicular to each other. It may create a dark area within the tendon that can be easily be mistaken for pathology (Fig. 5–4C).¹⁵

When there is concern for an infectious process involving the tendon, any amount of fluid within a tendon sheath (>2 mm) should be considered abnormal and may suggest a tenosynovitis (Fig. 5–5).

Muscle Evaluation

Ultrasound is used to detect muscle tears and infections such as myositis or abscess. Muscles may be evaluated with either the linear or the curvilinear transducer, depending on how deeply situated the muscle in question lies within the body. Again, the muscular tissue should be evaluated in the long and transverse axes. On ultrasound, muscle appearance ranges from hypoechoic to echoic in nature and is encased in a hyperechoic fascial sheath (Fig. 5–6).¹⁶ Dynamic imaging will provide important information regarding muscle structure and function. Disruption of the normal fiber organization or the inability to contract the muscle under ultrasound is suggestive of a tear (Fig. 5–7).⁵ Enlargement of the muscle belly as a whole, loss of the normal architecture, and diffuse hypoechogenicity when compared to the contralateral muscle is suggestive of myositis (Fig. 5–8A).¹² A well-circumscribed anechoic area or an area of hypoechogenicity within the muscle belly should be concerning for an abscess (Fig. 5–8B).¹⁷

Joint Evaluation

Ultrasonic evaluation of the joint is useful to identify joint effusions. For most joints, the linear transducer can be used, but evaluation of the hip joint will likely require the curvilinear transducer. Every joint contains a small amount of joint fluid for normal function, but any joint fluid in excess of normal should prompt the provider to pursue joint aspiration for fluid analysis if acute pathology is suspected (Fig. 5–9). The suggested norms for joint fluid, measured in millimeters, are provided in Table 5–1. Ultrasound is not only useful for identifying joint effusions but is also useful for distinguishing their presence from other soft-tissue abnormalities.¹⁸

Bone Evaluation

Ultrasound is used to detect fractures or the secondary signs of fractures. The high-frequency linear transducer should be used for bony evaluation and the bone should be evaluated in both the long and transverse axes. The cortex of the bone should appear as a hyperechoic linear structure without disruption (Fig. 5–10). When there is any disruption or buckling of the cortex a fracture should be suspected. Ultrasound is also useful for identifying secondary signs of fracture that are not readily visualized with plain radiography. These include soft-tissue edema overlying the bone and hematoma formation adjacent to the fracture site (Fig 5–11).

Foreign Body Identification

Ultrasound may be employed to identify foreign bodies within soft tissues. Using a high-frequency probe, ultrasound is better equipped to detect radiolucent foreign bodies (plastic and wood) than conventional radiography and fluoroscopy.¹⁹ In one experimental model, ultrasound identified wood and plastic foreign bodies with a sensitivity of 83% and a specificity of 59%.²⁰ Emergency physicians trained in this technique exhibit a similar rate of detection as ultrasound technologists and radiologists.²¹

Procedural Guidance

There is an increasingly large role for ultrasound when performing procedures of the musculoskeletal system. The procedures in which ultrasound may be useful include fracture reduction, joint aspiration, joint injection, hematoma blocks, and peripheral nerve blocks. A few recent studies have shown ultrasound to be useful in forearm fracture reduction in the pediatric population.^22–24 Ultrasound guidance of hematoma blocks has also been shown to be superior to a landmark-based approach, which should help facilitate adequate fracture reduction.²⁵ When ultrasound guidance has been studied for joint aspiration at the level of the knee, it was shown to cause less procedural pain, improve physician’s confidence with the procedure, and provide greater synovial fluid for analysis.^26,27 Peripheral nerve blocks under ultrasound guidance for acute bony trauma and joint dislocation are becoming more widely used as they have been shown to decrease ED length of stay when compared with procedural sedation for upper-extremity injuries.^28–32

Numerous advances in CT have expanded its uses for bone and soft-tissue injuries. With the advent of spiral CT scanning with multiple detectors, both speed and resolution have improved and three-dimensional computer reconstructions make diagnosis easier. The two major areas where CT is useful in emergency orthopedics are the evaluation of trauma and the evaluation of soft-tissue infections and tumors.

Trauma

Spiral CT has two major applications for the evaluation of a traumatized extremity¹: to detect a fracture that is suspected clinically but not visualized on plain radiographs and² to determine the extent of a previously identified fracture. Table 5–2 outlines specific areas where spiral CT is useful in the setting of trauma.^33–42 In addition, CT is useful for the detection of wood foreign bodies within the soft tissues of the extremities (Fig. 5–12).⁴³

CT has proved to be useful in the evaluation of pelvic fractures. The axial format allows better visualization of anterior and posterior displacement than do plain radiographs. The acetabulum is well visualized by this technique, and the data provided by the CT scan may influence the decision to proceed with open reduction and the type of procedure needed.⁴⁴ The cost and radiation exposure of this technique, however, should be borne in mind and it should not be used routinely on all pelvic fractures. Simple fractures not involving the acetabulum, which are stable on clinical examination, are usually adequately evaluated on plain films.

CT can evaluate non-displaced fractures of the femoral head and neck. The axial projection allows good visualization of the head of the femur and its relationship to the acetabulum. Bone fragments or distortions of the joint surface, which are not appreciated on plain films, are seen routinely on high-resolution CT.

The introduction of CT and MRI has led to improved assessment of the Salter injuries to the physis, epiphysis, and metaphysis. As well, the analysis of growth disturbances and injuries to these structures has clearly become much easier using these two imaging techniques.⁴⁵

Soft-Tissue Infections and Tumors

The advent of spiral CT has increased the sensitivity for detecting inflammatory and infectious processes in the soft tissues because the entire examination can be obtained at the peak of the intravenous contrast bolus.³⁶ CT will aid the clinician by demonstrating the extent of the process, including the compartments involved. This information will impact the need for surgical versus medical management. CT will assist in the diagnosis of necrotizing fasciitis, intramuscular abscess, myositis, pyomyositis, and osteomyelitis.³⁶

CT has been demonstrated to be an extremely valuable tool in the evaluation of bone and soft-tissue neoplasms in the extremities.^37,46,47 Ordinarily, the emergency physician will refer patients with suspected bone tumors, but the increasing availability of CT may make this a routine part of the initial evaluation. Although the CT scan may not be diagnostic, it often provides important information about the density of the mass, its relation to normal bone, nerves, and vessels, and the presence or absence of recurrence in patients who have been treated surgically.^48,49 The radionuclide scan and MRI are more sensitive tools for the detection of neoplasms of the extremities, whereas CT scan is superior in the detection of cortical destruction and lesion calcification.

In patients with extremity injuries, MRI remains a rarely ordered study from the ED. However, MRI in patients with acute musculoskeletal trauma has an increasing application that should be understood by the emergency physician.

MRI of bone identifies occult traumatic lesions, such as fractures of the scaphoid and femoral neck, not always seen with other imaging techniques.^50–52 The types of injury detected by MRI include bone bruises, stress or insufficiency fractures, and osteochondral fractures.⁵³ Increasing evidence suggests that occult fractures are detected by MRI sooner and with greater specificity than with bone scan.⁵¹

In addition, MRI is indispensable in the diagnosis of a number of soft-tissue problems. MRI is sensitive and therefore routinely used following knee trauma to detect ligamentous and meniscal injuries. At the shoulder, MRI is used to evaluate the integrity of the rotator cuff, glenoid labrum, and biceps tendon. And, at the ankle, it is used to detect ligamentous and syndesmotic injuries which are not as readily visualized with x-ray or ultrasound imaging.^52,54

Fluoroscopy uses x-ray beams that strike a fluorescent plate that is coupled to an image intensifier and monitor. For ED use, a fluoroscope, or “C-arm,” can be purchased for $30,000 to $60,000 (Fig. 5–13). The primary advantage of fluoroscopy is the ability to view anatomic structures in real time. Fluoroscopic films have been shown to be equal to plain radiographs in evaluating skeletal injuries. They may offer an advantage because they demonstrate motion of fracture fragments and the examiner can obtain multiple views.^55,56 In addition, fluoroscopy decreases the length of patient stay within the ED by eliminating the need to have a radiograph performed outside the department.

The advantage of real-time viewing using fluoroscopy is for foreign body removal, fracture reduction, and difficult arthrocentesis. In a similar manner to conventional radiography, fluoroscopy will reliably detect gravel, metal, and glass, but it cannot be used to identify plastic or wood.^57,58 The technique is easily learned by clinicians and retention rates are high.⁵⁹ For radiopaque foreign materials, removal is aided because the clinician can visualize both the instrument and the foreign body. Although the fluoroscope is turned on, the extremity can easily be manipulated to give a three-dimensional picture that allows the object to be located and removed.

The use of fluoroscopy is also advantageous for fracture reduction. Rather than sending the patient back to the radiology suite, confirmation of adequate reduction can be obtained immediately with a portable fluoroscope. Fracture reduction using a portable ED fluoroscope is more successful, thereby decreasing extra trips to radiology for postreduction films by 30%.⁵⁵ This technique will also reduce procedural sedation requirements by ensuring that the reduction is done correctly the first time.

The dose of radiation received from fluoroscopy is not negligible, but the patient receives a greater dose from a conventional radiograph than from a “spot film” taken by a fluoroscope by a ratio of 2:1. Estimates of whole-body radiation suggest that the clinician using a portable ED fluoroscope could work with the instrument at 1-m distance continuously for 2 hours every day and still be under the maximum permissible dose equivalent for radiation workers.⁵⁵ Although this statistic is encouraging, most work is performed at a distance closer than 1 m, and therefore lead aprons are recommended, as they reduce the radiation exposure by 85%.