How does anisotropy help you tell tendon from muscle on ultrasound?

Tendons are made of tightly aligned collagen fibers that reflect sound in a very angle-dependent way. When the probe isn’t close to perpendicular, the tendon can look dark or even disappear—classic anisotropy.

Muscle, by contrast, has a more complex, less organized internal pattern, so it doesn’t fade in and out as dramatically with small angle changes.

This difference helps you distinguish the two. If a structure brightens sharply when you adjust the probe angle, you’re likely on tendon. If it stays relatively consistent in brightness as you tilt the probe, you’re more likely on muscle.