Orientation dependence and decay characteristics of T₂* relaxation in the human meniscus studied with 7 Tesla MR microscopy and compared to histology

Abstract

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°). 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°. 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.

Magnetic Resonance Imaging of the Musculoskeletal System at 7T

“…Proper setup of sequence parameters, overcoming increased B 0 and B 1 inhomogeneities, and increased RF power deposition using dedicated coils at 7T was necessary to gain higher SNR at 7T versus 3T, as expected. Hager et al 27 used a micro-MRI insert at 7T to investigate the orientation dependence of T 2 ∗ in the knee meniscus. With a resolution approximately 100 times higher than that which is used clinically, they were able to describe the meniscal structures and the strong dependence of T 2 ∗ values on orientation toward B 0 .…”

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

“…In a previous study on human menisci, it was shown that the combination of ultra‐high field strength 7 T, an MR microscopy system and a 3D variable echo time sequence meets these requirements for ultra‐short echo times and microscopic resolution 24 and as such represents an ideal, unique setup for the angle‐dependent MR examination of tendons and their respective T 2 * characteristics, which we wish to demonstrate in this study.…”

Musculoskeletal MRI at 7 T: do we need more or is it more than enough?

“…UHF MRI also allows T2* quantification of the meniscal ultrastructure with a demonstrated relationship between T2* decay and collagen fibres orientation. Although further studies are needed to transfer this method into the clinical practice, these results might help in the interpretation of T2 and T2* quantitative images of early meniscal degeneration at clinical field strengths with lower resolution [ 65 ]. Important information on disc ultrastructure and degeneration was obtained ex vivo with a correlation between matrix degeneration and distribution of PG diffusivity and PG CEST.…”