Orientation Dependence and Decay Characteristics of T * Relaxation in the Human Meniscus Studied with 7 Tesla MR Microscopy and Compared to Histology

Purpose: To evaluate: (1) the feasibility of MR microscopy T * mapping by performing a zonal analysis of spatially matched T * maps and histological images using microscopic in-plane pixel resolution; (2) the orientational dependence of T * relaxation of the meniscus; and (3) the T * decay characteristics of the meniscus by statistically evaluating the quality of mono- and biexponential model.

Methods: Ultrahigh resolution T * mapping was performed with ultrashort echo time using a 7 Tesla MR microscopy system. Measurement of one meniscus was performed at three orientations to the main magnetic field (0, 55, and 90°). Histological assessment was performed with picrosirius red staining and polarized light microscopy. Quality of mono- and biexponential model fitting was tested using Akaike Information Criteria and F-test.

Results: (1) The outer laminar layer, connective tissue fibers from the joint capsule, and the highly organized tendon-like structures were identified using ultra-highly resolved MRI. (2) Highly organized structures of the meniscus showed considerable changes in T * values with orientation. (3) No significant biexponential decay was found on a voxel-by-voxel-based evaluation. On a region-of-interest-averaged basis, significant biexponential decay was found for the tendon-like region in a fiber-to-field angle of 0°.

Conclusion: The MR microscopy approach used in this study allows the identification of meniscus substructures and to quantify T * with a voxel resolution approximately 100 times higher than previously reported. T * decay showed a strong fiber-to-field angle dependence reflecting the anisotropic properties of the meniscal collagen fibers. No clear biexponential decay behavior was found for the meniscus substructures.