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Clinical Summary

Soft-tissue ultrasound has been shown to increase diagnostic accuracy, speed disposition, and reduce complications of procedures.

Indications

  • Suspected fluid collection (abscess, hematoma, etc)

  • Localization of subcutaneous foreign body

  • Guidance for procedures

Equipment

  • High-frequency linear array

Patient Positioning

  • Place the patient in a position of comfort that allows maximum exposure of the area of interest.

Technique

  • Image the area of interest by gently sweeping the probe across the skin. This should be done in two perpendicular planes. If possible, orient the planes along conventional axes of the body (ie, transverse and longitudinal).

  • Identify the dermis, subcutaneous tissue, tendon, muscle, ligament, bone, and relevant neurovascular structures.

  • Waterbath evaluation technique (WET): Distal extremities can be placed in a tub of water covering the area of interest and the water can be used as a conductive media for the ultrasound (Fig. 24.131). Ultrasound waves travel well through water, permitting a standoff approach where the probe need not actually touch the skin. This is better tolerated by the patient as no contact with the painful area is necessary and this technique facilitates imaging of extremely superficial structures.

  • Apply superficial marking to overlying skin if necessary.

FIGURE 24.131

Waterbath Evaluation of the Extremity. Distal extremities can be placed in a tub of water covering the area of interest, and the water can be used as a conductive media for the ultrasound. The ultrasound image shows the volar aspect of the distal phalanx of the middle finger obtained using the waterbath technique. (Photo contributor: Lawrence B. Stack, MD; ultrasound contributor: Jordan Rupp, MD.)

Abnormal Findings

  • Edema/cellulitis: Increased interstitial fluid appears similar on ultrasound regardless of the cause (eg, cellulitis, lower extremity–dependent edema). Initially, increased echogenicity and loss of definition of the tissue are noted in the subcutaneous fat. This obscures deeper structures such as muscle and bone. As the condition progresses, fluid is noted to accumulate diffusely in a reticular pattern, creating a cobblestone appearance (Fig. 24.132).

  • Abscess: An abscess appears as a focal fluid accumulation, often with a significant amount of increased echogenicity in the surrounding tissues. The fluid may appear anechoic or have mixed complex echoes (Fig. 24.133). If drainage will be attempted, the depth and location of the abscess, any associated tracts, and other important and relevant adjacent structures should be noted and marked if appropriate (vessels, nerves, etc).

  • Joint effusion: Ultrasound can readily identify fluid accumulation within joints. A distended joint capsule will be visualized with anechoic or heterogeneous echogenic fluid (Fig. 24.134). Thickened synovial lining may be noted as well. Drainage can ...

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