Transverse Relaxation Anisotropy of the Achilles and Patellar Tendon Studied by MR Microscopy

BACKGROUND: T(2)* anisotropy affects the clinical assessment of tendons (magic‐angle artifact) and may be a source of T(2)*‐misinterpretation. PURPOSE: To analyze T(2)*‐anisotropy and T(2)*‐decay of Achilles and patellar tendons in vitro at microscopic resolution using a variable‐echo‐time (vTE) sequence. STUDY TYPE: Prospective. SPECIMEN: Four human Achilles and four patellar tendons. FIELD STRENGTH/SEQUENCE: A 7 T MR‐microscopy; 3D‐vTE spoiled‐gradient‐echo‐sequence (T(2)*‐mapping). ASSESSMENT: All tendons were measured at 0° and 55° relative to B(0). Additional angles were measured for one Achilles and one patellar tendon for a total of 11 angles ranging from 0° to 90°. T(2)*‐decay was analyzed with mono‐ and bi‐exponential signal fitting. Mono‐exponential T(2)*‐values (T(2)*(m)), short and long T(2)*‐components (T(2)*(s), T(2)*(l)), and the fraction of the short component F(s) of the bi‐exponential T(2)*‐fit were calculated. T(2)*‐decay characteristics were compared with morphological MRI and histologic findings based on a region‐of‐interest analysis. STATISTICAL TESTS: Akaike information criterion (AIC(C)), F‐test, and paired t‐test. A P value smaller than the α‐level of 0.05 was considered statistically significant. RESULTS: T(2)*(m)‐values between fiber‐to‐field angles of 0° and 55° were increased on average from T(2)*(m) (0°) = 1.92 msec to T(2)*(m) (55°) = 29.86 msec (15.5‐fold) in the Achilles and T(2)*(m) (0°) = 1.46 msec to T(2)*(m) (55°) = 23.33 msec (16.0‐fold) in the patellar tendons. The changes in T(2)*(m)‐values were statistically significant. For the whole tendon, according to F‐test and AIC(C), a bi‐exponential model was preferred for angles close to 0°, while the mono‐exponential model tended to be preferred at angles close to 55°. CONCLUSION: MR‐microscopy provides a deeper insight into the relationship between T(2)*‐decay (mono‐ vs. bi‐exponential model) and tendon heterogeneity. Changes in fiber‐to‐field angle result in significant changes in T(2)*‐values. Thus, we conclude that awareness of T(2)*‐anisotropy should be noted in quantitative T(2)*‐mapping of tendons to avoid T(2)*‐misinterpretation such as a false positive detection of degeneration due to large fiber‐to‐field angles. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 2