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Anisotropy is an artefact encountered in ultrasound, notably in muscles and tendons during a musculoskeletal ultrasound. In musculoskeletal applications, the artefact may prompt an incorrect diagnosis of tendinosis or tendon tear.
When the ultrasound beam is incident on a fibrillar structure as a tendon or a ligament, the organized fibrils may reflect a majority of the insonating sound beam in a direction away from the transducer. When this occurs, the transducer does not receive the returning echo and assumes that the insonated area should be hypoechoic.
This anisotropic effect is dependent on the angle of the insonating beam. The maximum return echo occurs when the ultrasound beam is perpendicular to the tendon. Decreasing the insonating angle on a normal tendon will cause it to change from brightly hyperechoic (the actual echo from tightly bound tendon fibers) to darkly hypoechoic. If the angle is then increased, the tendon will again appear hyperechoic.
If the artefact causes a normal tendon to appear hypoechoic, it may falsely lead to a diagnosis of tendinosis or tear.
In some situations, anisotropy may be useful in diagnosis. If a tendon is surrounded by other brightly hyperechoic structures (e.g. fat), then altering the angle of the transducer will cause the tendon to become hypoechoic, differentiating it from the other structures.