Chapter 16. Soft Tissue and Extremity Ultrasound

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.

Bedside ultrasound evaluation of the musculoskeletal system should be performed in the following:

• 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

Linear Probe with a Frequency of 7.5 MHz or Higher

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.

Focal Zone

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.

Depth

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.

Gain or Time-Gain Compensation

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.

Color-Flow Doppler

Color-flow Doppler detects blood flow. This is useful in differentiating blood vessels from surrounding structures. It helps prevent inadvertent injury to blood vessels when retrieving a foreign body under ultrasound guidance.

Skin, Muscle, and Blood Vessels

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 ...