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Anisotropy

Case contributed by Matt A. Morgan
Diagnosis certain

Presentation

Asymptomatic.

Patient Data

Age: 35 years
Gender: Male

Example #1

ultrasound

In this patient with a normal Achilles tendon, the transducer was rocked back and forth to demonstrate the appearance of anisotropy.

1. 15MHz transducer insonating the Achilles tendon (dark red arrow) at 90 degrees (perpendicular to the tendon fibers)

2. 15MHz transducer insonating the Achilles tendon (dark red arrow)  at approximately 60 degrees.

3. Cine clip with a 15MHz (hockey stick) transducer. The transducer is rocked between angle of 45 - 135 degrees with respect to the long axis of the tendon. The tendon is maximally brightest at 90 degrees.

Example #2

ultrasound

In this patient with a normal wrist, the transducer was rocked back and forth to demonstrate the appearance of anisotropy. (1: flexor digitorum tendons. 2: flexor carpi radialis. 3: palmaris longus. 4: median nerve). Note how anisotropy increases the contrast between the tendon and nerve against the surrounding fat.

1. 15MHz transducer insonating the wrist flexor tendons at 90 degrees (perpendicular to the tendon fibers).

2. 15MHz transducer insonating the wrist flexor tendons at 45 degrees.

3. Cine clip with a 15MHz (hockey stick) transducer. The transducer is rocked between angle of 45 - 135 degrees with respect to the long axis of the tendons and nerve. The tendon is maximally brightest at 90 degrees.

Case Discussion

When practicing musculoskeletal ultrasound, one should be aware of anisotropy.

Unlike many other structures in the body, the echo received from a compact fibrillar structure (such as a tendon or a nerve) is highly dependent on the angle of incidence. This has the potential to cause normal structures to appear abnormal.  For instance, a normal tendon may appear dark and tendinotic if it is insonated at less than 90 degrees (less than perpendicular to the long axis of the tendon).

Anisotropy is particularly important in evaluation of tendons and nerves that turn corners or dive beneath other structures; for instance, the peroneal tendons around the lateral malleolus, or the median nerve dipping below the flexor retinaculum.

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