Chapter 19

### INTRODUCTION

Ultrasound is a wonderful imaging modality for the assessment of the soft tissues. It is noninvasive, cost-effective, and widely available. It allows real-time dynamic assessment of muscles, tendons, and nerves and can generate images with greater spatial resolution than that obtained with standard clinical MRI techniques. The aim of this chapter is to present the usefulness of ultrasound in the assessment of the main soft tissue disorders.

### ULTRASOUND TECHNIQUE

#### General Aspects

The development of compact real-time ultrasound systems in the 1980s and the subsequent availability of broadband high-frequency linear-array transducers in the 1990s have dramatically improved the accuracy of clinical assessment of musculoskeletal disorders.1 The choice of transducer frequency depends on the overall size of the structure, its position relative to the skin (deep or superficial), and the conspicuousness of the intervening soft tissue planes. Superficial structures require small-sized probes with high frequencies (7.5–20 MHz), while deep structures require low-frequency probes (3.5–5 MHz).1

The brightness of the images depends on the degree of reflection of the ultrasound waves and is defined as echogenicity. The echogenicity of a tissue depends on the characteristics of that tissue and on the transducer frequency. The terms used to describe such characteristics include hyperechoic (white signal), isoechoic, hypoechoic, and anechoic (black) echogenicity. In general, bones and gas-like structures are most echogenic and fluids, such as joint effusion, and are less echogenic.1

Assessing soft tissue disorders via ultrasound has many advantages as compared with MR imaging:15

• It is well tolerated and has no contraindication, unlike MR imaging.

• It can effectively image patients with orthopedic hardware that causes significant artifacts in MR imaging.

• It is noninvasive and does not require intravenous contrast administration in the clinical practice, as it can provide information about blood flow and the degree of tissue vascularity thanks to the use of color Doppler and power Doppler.13,6

• It is cost-effective and widely available. An ultrasound examination of an extremity costs about three times less than a magnetic resonance imaging (MRI) examination.7

• It allows real-time dynamic assessment of muscles, tendons, and nerves. Using passive and active movements, or isolated muscle contractions, is fundamental for the depiction of tendon and bursal impingements (such as subacromial impingement), nerve impingements, tendon and nerve instability and muscle herniation, as well as to improve the depiction of small lesion such as tears.8,9

• It facilitates bilateral comparison. A wide degree of anatomic variability exists in the musculoskeletal system. Consequently, the detection of bilateral asymmetry can be useful when deciding whether an abnormality is present.

• It has a more flexible field of view than MR imaging and is the preferred imaging technique for structures with a long course in the body, such as peripheral nerves.

• It can generate images with greater spatial resolution ...

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