Background and Indications for Examination
Soft tissue and extremity conditions are common complaints encountered in the acute care setting. Traditionally, patient evaluation was performed using physical examination, radiographs, CT scans, MRI, and bone scans. More recently, these imaging modalities are now being augmented by the use of bedside ultrasound. Ultrasound has increasingly been shown to be useful in the evaluation of conditions such as cellulitis, abscesses, necrotizing fasciitis, foreign bodies, fractures, muscle and tendon inflammation, infection, or injury.
- Evaluation of presence, location, and extent of possible abscess or necrotizing fasciitis
- Identification of a suspected soft tissue foreign body not visualized on plain radiographs
- Dynamic assistance with removal of a foreign body visualized by ultrasound
- Assessment of muscles and tendons for inflammation or infection
- Evaluation of long bones for fracture and in guiding reduction
Probe Selection and Technical Considerations
Soft tissue and musculoskeletal structures are usually superficial, making a higher-frequency probe optimal for better resolution. In the case of deeper structures, a lower-frequency curvilinear probe may provide better penetration.
The focal zone should be adjusted and placed at the level of the structure being imaged. If the object of interest is too superficial, a standoff pad can be placed in between the probe and body part being imaged to increase the distance between the two and better the resolution. An IV fluid bag may be used as a standoff pad if a commercially produced one is not available. If possible, the body part being imaged can be submersed into a water bath, which also optimizes the focal zone and patient comfort.
Soft tissue and musculoskeletal injuries are usually superficial. The sonographer should decrease the depth in order to bring the area of interest into the center of the screen.
Total gain can be adjusted to optimize the strength of the signal returning from the object of interest. In deeper structures or obese patients, it may only be necessary to increase the far gain using time-gain compensation (TGC). Using too much gain can “wash out” the image and make it too bright; therefore, it should only be increased as necessary.
Normal Ultrasound Anatomy
The sonographer should be familiar with the appearance of normal soft tissue anatomy when evaluating for infections, injuries, inflammation, or foreign bodies (Fig. 16-1). The epidermis and dermis are seen most superficially with a more hypoechoic subcutaneous fatty tissue deep to the cutaneous layer. This adipose layer is highlighted with a reticular pattern of connective tissue between fat. The thickness of this layer varies with body location and habitus. Muscle is found deep to the subcutaneous fat layer and is also relatively hypoechoic with regular internal striations that will appear linear in long axis and punctate in short axis. Blood vessels are anechoic and have a circular appearance in short axis and a tubular appearance in long axis. They will display blue or red color patterns with Doppler, depending on the direction of blood flow relative to the probe. Veins are thin-walled and will collapse easily with pressure applied by the transducer, whereas larger arteries are thick-walled and remain patent and pulsatile with compression.
Normal longitudinal view of anterior thigh skin and muscle tissue. The top layer is the hyperechoic heterogeneous epidermis and dermis (D&E), which cannot be differentiated from each other at this frequency. The next layer is the hypoechoic heterogeneous subcutaneous layer (SC). Within the subcutaneous layer are fat globules (F) and the subcutaneous septa (S). The deepest layer is muscle (M), which has a hyper- and hypoechoic fibrinous pattern consistent with normal fascicles. The muscle fascia appears as a horizontal hyperechoic line (FS).
Normal tendons are echogenic and have a characteristic fibrillar pattern in both the longitudinal and transverse axis (Fig. 16-2a and b). In a transverse orientation, the fibrils have a punctuate echogenic pattern which is best achieved when the probe is directed perpendicular to the axis of the tendon. The slightest angle away from the perpendicular axis will result in an artifact referred to as “anisotropy” of tendons (Fig. 16-3a and b). This is a result of the scattering of the ultrasound beam producing an apparent hypoechoic tendon. This artifact is important to identify as most tendon pathology will also appear hypoechoic on ultrasound and can result in the misinterpretation of findings. Anisotropy and the dynamic property of tendons are two unique characteristics that allow the sonographer to differentiate them from other surrounding structures that may have similar appearances, such as nerve bundles (see Fig. 16-3a and b).
Transverse view of the mid-forearm illustrating the median nerve (N) and a tendon (T). The nerve has a more honeycomb appearance with distinct fascicles while the tendon has a more fibrillar pattern. In (a), the ultrasound beam is perpendicular to the nerve (N) and tendon (T) while in (b) the beam is at a 20° tilt decreasing the echogenicity of the tendon (T) but not the nerve (N). Also seen in (a) are muscle bundles (M) and the cortex of the radial bone (R).
Nerve bundles also demonstrate an echogenic fibrillar pattern on ultrasound; however, there are some differences that can help make the distinction between the two. Nerve axon bundles are also hypoechoic, but are much thicker than the fibrils that are visualized in tendons. On transverse scans, these bundles have a characteristic “honeycomb” appearance due to the presence of thick hypoechoic axons that are found within nerves. Furthermore, as described earlier, tendons display anisotropy and nerves do not (see Fig. 16-3a and b).
Bones are visualized on ultrasound as thin, brightly echogenic cortex with a prominent posterior acoustic shadow and oftentimes reverberation artifact (see Fig. 16-3a). The shape of the cortex will reflect the contour of the bone in the plane that is being scanned. A long bone will appear as a bright curved line in a cross-sectional profile and as a straight line in a longitudinal profile. Although ribs are smaller and follow a curved path around the thorax, they have a similar appearance on ultrasound as long bones do. Ribs are superficial to the parietal and visceral pleura, which can be observed sliding back and forth between two ribs’ shadows in cross-sectional profile (see Chap. 7).