What is anisotropy, and how can this be used to differentiate among soft tissue structures?
Anisotropy refers to the variation of ultrasound interaction with fibrillar tissues and is primarily observed when evaluating tendons and ligaments. For example, when the ultrasound beam is directed perpendicular to a structure such as a tendon, the typical hyperechoic fibrillar appearance is generated. The tendon will appear speckled and hyperechoic in the short axis and striated and hyperechoic in the long axis.
Upon slight angulation of the transducer probe from this perpendicular plane, the hyperechoic appearance of the tendon will become hypoechoic. This artifact may be confused for pathology as tendon and ligament abnormalities are often hypoechoic. Therefore, when evaluating a tendon or ligament, it is important to focus on the segment that is perpendicular to the ultrasound beam to exclude anisotropy as the cause of a focal hypoechoic defect.
Anisotropy can be helpful in differentiating among normal soft tissue structures, especially in the ankle or wrist, where a hyperechoic tendon or ligament is often surrounded by hyperechoic soft tissues. Toggling or “heel-toeing” the transducer along the tendon or ligament will cause the structure to become hypoechoic, providing greater contrast from a background of hyperechoic fat which does not possess anisotropy