What causes anisotropy?
In highly organized fibrillary structures (like tendons and larger nerves), when the sound beam strikes the surface at 90 degrees, it returns the maximum amount of sound directly to the transducer (i.e. the tendon is bright). When the sound beam strikes the surface at less than 90 degrees, some of the sound beam is reflected away and never returns to the transducer. The weakened returning echo makes the tendon look darker.
Why can anisotropy be a diagnostic problem?
Pathologic processes that disrupt the organized structure of a tendon, such as tendinosis, cause the tendon to be increasingly hypoechoic. Anisotropy can make a normal tendon look hypoechoic, leading to false diagnosis of tendinosis.
When can anisotropy be helpful?
Rocking the transducer slightly will cause fibrillar structures in the field of view to become hypoechoic. This can be useful for localization and confirmation that the structure one is insonating is, indeed, a fibrillar structure.
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.