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

Abstract

BackgroundT2* anisotropy affects the clinical assessment of tendons (magic‐angle artifact) and may be a source of T2*‐misinterpretation.PurposeTo analyze T2*‐anisotropy and T2*‐decay of Achilles and patellar tendons in vitro at microscopic resolution using a variable‐echo‐time (vTE) sequence.Study TypeProspective.SpecimenFour human Achilles and four patellar tendons.Field Strength/SequenceA 7 T MR‐microscopy; 3D‐vTE spoiled‐gradient‐echo‐sequence (T2*‐mapping).AssessmentAll 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 TestsAkaike information criterion (AICC), F‐test, and paired t‐test. A P value smaller than the α‐level of 0.05 was considered statistically significant.ResultsT2*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°.ConclusionMR‐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 Level2Technical EfficacyStage 2