Orientation dependence and decay characteristics of T 2 * 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-2* mapping by performing a zonal analysis of spatially matched T-2* maps and histological images using microscopic in-plane pixel resolution; (2) the orientational dependence of T-2* relaxation of the meniscus; and (3) the T-2* decay characteristics of the meniscus by statistically evaluating the quality of mono-and biexponential model. Methods: Ultrahigh resolution T-2* 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 degrees). 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-2* 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 degrees. Conclusion: The MR microscopy approach used in this study allows the identification of meniscus substructures and to quantify T-2* with a voxel resolution approximately 100 times higher than previously reported. T-2* 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.