Low-field NMR profiling and relaxometry of articular cartilage subject to loading and enzymatic degradation

The layered structure of mammalian articular cartilage is reflected by a pronounced depth dependence of T2, a consequence of varying order of the collagen fibers but also of a gradient of water and glycosaminoglycan (GAG) concentration. The orientational order results in an angular dependence of T2 that is less pronounced at greater distance from the joint surface. T1, however, at conventional laboratory field strengths shows little variation in comparison.
In this study, the dependence of magnetic resonance relaxation times in bovine and human articular cartilage is investigated by portable, single-sided scanners at magnetic field strengths of 0.27 T and 0.44 T, respectively. One-dimensional, depth-dependent scans were carried out with spatial resolutions between 20 and 50 µm. While a systematic variation of T2 is found that is in agreement to similar mammalian cartilage observed at high fields, T1 also shows a strong depth dependence that correlates with the separation of the tissue into three distinct zones. This pronounced effect is explained by the increased T1 contrast commonly found towards smaller magnetic field strengths, a consequence of slow and anisotropic molecular reorientations that dominate the relaxation dispersion at low magnetic resonance frequencies.
At even lower Larmor frequencies, the so-called quadrupolar dips are observed which indicate cross-relaxation of protons with the partially immobilized 14N nuclei in amino acids in collagen and GAGs. Varying the composition, water content or structural integrity of cartilage affects both the general frequency dependence of T1 and the shape of the quadrupolar dips, providing a possible diagnostic access to arthropathies such as osteoarthritis (OA).
The effect of enzymes onto cartilage and its constituents is investigated as a model for OA, and results are compared to measurements carried out at bovine and human healthy and osteoarthritic cartilage tissues, respectively. While trypsin is known to separate GAGs from the proteoglycan backbone, collagenase attacks the collagen structure exclusively. 14N nuclei in both substances are shown to contribute to the quadrupolar dips in a similar way. Experiments for both constituents as well as fresh and enzyme-treated bovine articular cartilage were carried out and the relaxivity in the region of the quadrupolar dips were quantified. The strong dependence on water concentration observed is in agreement with disease models of OA and is correlated with low-field imaging results. Loading of extracted cartilage tissue in a pressure cell up to the maximum physiological compression is reflected in a layer-dependent change of relaxation times, whereas the tissue's integrity is artificially affected by enzymatic treatment prior to the loading process.