Background: T2 * anisotropy affects the clinical assessment of tendons (magic-angle artifact) and may be a source of T2 *-misinterpretation.
Purpose: To analyze T2 *-anisotropy and T2 *-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 (T2 *-mapping).
Assessment: All tendons were measured at 0° and 55° relative to B0 . Additional angles were measured for one Achilles and one patellar tendon for a total of 11 angles ranging from 0° to 90°. T2 *-decay was analyzed with mono- and bi-exponential signal fitting. Mono-exponential T2 *-values (T2 *m ), short and long T2 *-components (T2 *s , T2 *l ), and the fraction of the short component Fs of the bi-exponential T2 *-fit were calculated. T2 *-decay characteristics were compared with morphological MRI and histologic findings based on a region-of-interest analysis.
Statistical tests: Akaike information criterion (AICC ), F-test, and paired t-test. A P value smaller than the α-level of 0.05 was considered statistically significant.
Results: T2 *m -values between fiber-to-field angles of 0° and 55° were increased on average from T2 *m (0°) = 1.92 msec to T2 *m (55°) = 29.86 msec (15.5-fold) in the Achilles and T2 *m (0°) = 1.46 msec to T2 *m (55°) = 23.33 msec (16.0-fold) in the patellar tendons. The changes in T2 *m -values were statistically significant. For the whole tendon, according to F-test and AICC , 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 T2 *-decay (mono- vs. bi-exponential model) and tendon heterogeneity. Changes in fiber-to-field angle result in significant changes in T2 *-values. Thus, we conclude that awareness of T2 *-anisotropy should be noted in quantitative T2 *-mapping of tendons to avoid T2 *-misinterpretation such as a false positive detection of degeneration due to large fiber-to-field angles.
Evidence level: 2 TECHNICAL EFFICACY: Stage 2.
Keywords: MR microscopy; T2* anisotropy; T2* mapping; fiber-to-field angle dependence; magic angle artifact; magic angle effect; tendon.
© 2022 The Authors. Journal of Magnetic Resonance Imaging published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.