Transverse Relaxation Anisotropy of the Achilles and Patellar Tendon Studied by <scp>MR</scp> Microscopy
Benedikt Hager, Markus Schreiner, Sonja M. Walzer, Lena Hirtler, Vladı́mir Mlynárik, Andreas Berg, Xeni Deligianni, Oliver Bieri, Reinhard Windhager, Siegfried Trattnig, Vladimír Juráš
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