Currently, our group is investigating the structure and physical and chemical properties of hyaline articular cartilage, mainly using low-field NMR techniques. Articular cartilage is a complicated, multicomponent, but well-ordered biological tissue. The structure is characterized by three layers of differently oriented collagen fibers, accompanied by a gradient of increasing proteoglycan as well as decreasing water concentration. As a consequence of these structural variations, there is a change in longitudinal and transverse relaxation times depending on the distance to the cartilage surface. This dependence, especially after treatment of the samples with e.g. enzymes, contrast agents, is investigated with the help of our Low Field NMR Mobile Universal Surface Explorer (NMR-MOUSE). A one-dimensional depth resolution of more than 50 µm is possible. With this method, an increased contrast of the longitudinal relaxation rate was found at lower magnetic field strengths, which could also be used to estimate the spatially dependent glycosaminoglycan concentration in the cartilage samples.
NMR-field-cycling relaxometry is another technique used in our group. It allows us to measure the longitudinal relaxation time as a function of magnetic field strength. Experiments on cartilage showed a longitudinal relaxation time dispersion superimposed by so-called quadrupolar dips. These dips are a consequence of hydrogen-nitrogen cross-relaxation in articular cartilage. The application of enzymes such as trypsin or collagenase changes the percentage composition of articular cartilage. These changes and in particular their influence on quadrupolar dips are now studied.
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