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The Speed of Sound

The Speed of Sound

Starting at the Very Beginning

Ultrasound for Soft Tissue

Butts, Christine MD

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doi: 10.1097/01.EEM.0000481771.71826.a5
    Image 1. Normal soft tissue. The thinnest and most superficial structure (arrow) is the skin, followed immediately by the subcutaneous tissue (star). The muscle is the next layer and appears more hypoechoic (darker). Bands of fascia can be seen as hyperechoic (white) separations between the layers. Note the discrete borders between the layers.
    Figure
    Figure

    PART 1 IN A FIVE-PART SERIES

    One of my favorite uses for ultrasound at the bedside is evaluating the soft tissue. EPs are not generally great at determining the presence of an abscess based on physical exam alone. Other times, patients may have limitations, such as habitus or severe pain, that may make examination of joints or bursa difficult. Or the diagnosis may be subtle and the standard signs may not be obvious, as in a case of possible infectious tenosynovitis.

    Bedside ultrasound can be extremely helpful in all of these situations by adding to the history and physical exam findings. We will explore over the next few months its role in evaluating the soft tissue, from the skin to the tendons, to the bursa and beyond. This month, let's start at the very beginning (a very good place to start), and look at normal soft tissue.

    With few exceptions, a linear transducer is best for evaluating the soft tissue. Most structures of interest will be located close to the surface, making a high-frequency transducer the best choice for maximum resolution.

    The skin will appear as the most superficial structure and should appear thin, with a clear demarcation between it and the underlying subcutaneous tissue. Clear demarcations between the layers are generally a key finding in normal tissue. (Image 1.)

    The next structure will vary depending on the location within the body. Muscle appears as a distinct layer, with the individual fascicles frequently visible. (Image 1.) Muscle is typically hypoechoic (dark gray) compared with the surrounding tissue because it has a higher water (blood) content. The fascial planes may also be visible as hyperechoic (white) bands within the muscle. Tendons appear most clearly in long axis as distinct hyperechoic, fibrillar structures. (Image 2.) They can frequently be followed to their point of origin.

    Image 3. The hand or foot is placed in a water bath, and the transducer is placed within the bath, hovering over (but not touching) the area of interest. We will touch on this technique in more detail later in this series.

    Knowing the underlying anatomy is helpful in identifying structures, particularly when they are abnormal. Clear demarcation should be seen between the layers, and a key sign of inflammation would be a loss of this demarcation. When in doubt, comparing the affected side to the unaffected side or to an area distal to the affected area can be helpful in identifying normal versus abnormal.

    Image 2. Normal tendon. The tendon is seen in its long axis (arrow) extending from the patella (star). Note the discrete borders and fibrillar texture. The individual fibers can be seen upon close inspection.

    Using a water bath is essential in evaluating the soft tissue of the extremities. It will overcome the near-field effect, in which structures within the first few millimeters of the transducer lack the resolution of deeper structures. Placing the hand or foot within a water bath (Image 3) gives a bit of step off from the most superficial structures, enhancing their resolution. In a pinch, a small bag of saline or a water-filled glove between the transducer and the extremity works as well because it enables the detection of small findings, such as felons within a fingertip.

    Next month: We'll examine that essential question with the base of a normal ultrasound: To cut or not to cut: Cellulitis or abscess?

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