Anisotropy in Musculoskeletal Ultrasound: How to Differentiate Artifact from Tendon Tear and Avoid Overdiagnosing Tendinosis
The Nature of Anisotropy
Anisotropy is a property dependent on direction (as opposed to isotropy). When positioned perpendicular to the beam, a tendon exhibits a characteristic hyperechoic fibrillar structure. If the beam deviates from perpendicular, the structure becomes hypoechoic. The artifact is particularly pronounced in tendons, ligaments, nerves, and to a lesser extent in muscles—due to their oblique course and parallel reflecting surfaces (Griffith, 2025).
Anisotropy can occur with a beam deviation of just 5° from perpendicular and can mimic tendinosis or tendon tear, as well as reduce the visibility of a biopsy needle (Griffith, 2025).
Where the Risk of Overdiagnosis is Greatest
The artifact is most pronounced where the structure bends: ankle tendons wrapping around the malleoli and supraspinatus fibers bending towards the attachment site (Griffith, 2025). When visualizing the distal supraspinatus tendon in the long axis, the articular fibers sharply change direction at the greater tuberosity, creating focal artifact hypoechogenicity that simulates a tear (Jacobson, 2026).
How to Differentiate Artifact from Pathology
The main technique is reorienting the beam perpendicular to the fibers:
| Situation | Maneuver |
|---|---|
| Tendon in long axis | Heel-toe (rocking motion) |
| Tendon in short axis | Toggling (transducer rocking) |
After angle correction, anisotropy disappears and the normal tendon becomes hyperechoic again (Jacobson, 2026). It is important to evaluate only the segment of the structure that is perpendicular to the beam and resist the temptation to diagnose pathology in adjacent non-perpendicular segments (Griffith, 2025). All pathology should be confirmed in two planes (Griffith, 2025).
Anisotropy as a Diagnostic Aid
The artifact can be used beneficially: when visualizing a tendon in the short axis, toggling the transducer makes it hypoechoic, distinguishing it from adjacent hyperechoic fat, which is not subject to anisotropy—relevant in the ankle and wrist areas. After identifying the structure, anisotropy must be corrected to exclude pathology (Jacobson, 2026).
In calcific tendinosis of the supraspinatus, directing the beam obliquely (heel-toe) makes the surrounding tendon hypoechoic due to anisotropy, enhancing the visibility of the calcification (Jacobson, 2026).
Other Pitfalls of the Rotator Cuff
Besides anisotropy, pathology can be mistakenly assumed for: tendon edges at the boundary with adjacent structures; the tendon interval between the anterior edge of the supraspinatus and the long head of the biceps (recognized by the ovoid shape of the biceps); the junction zone of the supraspinatus and infraspinatus (hypoechoic due to fiber interdigitation); fibrocartilaginous attachment (enthesis); muscle-tendon junction (Griffith, 2025).
Frequently asked questions
At what angle deviation does anisotropy appear?
Anisotropy can occur with a beam deviation of just 5° from perpendicular to the structure (Griffith, 2025).
What maneuver should be used to eliminate anisotropy in the long and short axis?
In the long axis—heel-toe (rocking motion), in the short axis—toggling (transducer rocking) to align the beam perpendicular to the fibers (Jacobson, 2026).
Which structures are most susceptible to anisotropy?
Tendons, ligaments, and nerves; to a lesser extent muscles. Especially where the structure bends—tendons at the ankles and supraspinatus fibers at the attachment (Griffith, 2025).
How to avoid overdiagnosing a supraspinatus tear?
The sharp bend of articular fibers at the greater tuberosity creates artifact hypoechogenicity. Reorienting the beam perpendicular to the fibers restores normal hyperechogenicity; confirm in two planes (Jacobson, 2026; Griffith, 2025).
Can anisotropy be used beneficially?
Yes: toggling the transducer makes the tendon hypoechoic, distinguishing it from adjacent hyperechoic fat, which is not subject to anisotropy (ankle, wrist). It also enhances the visibility of calcifications (Jacobson, 2026).