What is anisotropy in ultrasound?

Anisotropy is a visual illusion that happens when the ultrasound beam doesn’t strike a structure at the right angle. Tendons, ligaments, and nerves can look dark or even seem to disappear, despite being completely normal.

The flashlight analogy helps explain it: when you shine a flashlight directly on something, it lights up clearly; when you sweep the beam off to the side, the object fades into shadow. In ultrasound terms, this beam alignment is called the insonation angle, and for many structures the best view comes when the beam hits them close to 90 degrees. If the angle is off, the structure looks falsely dark.

Adjusting the probe to restore that bright appearance confirms what’s real and prevents misinterpreting anisotropy as pathology.