Abstract
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.