In cartilage, sodium can exist in three different environments; 1) sodium ions that are in motional narrowing regime relax with single exponential and do not exhibit multiple quantum coherences, 2) in biological tissues although quadrupolar interaction of a sodium nucleus is averaged, it is in slow motion regime and its effect is manifested as biexponential relaxation and 3) in anisotropic environments in tissues where sodium ions exhibit local ordering the quadrupolar interaction is not completely averaged to zero but there exists a finite averaged quadrupolar interaction which we also refer to as residual quadrupolar interaction. In vivo studies of residual quadrupole interaction in human articular cartilage would provide information about the anisotropic environments in the tissue such as collagen ordering. We have obtained, for the first time, in vivo multiple quantum spectra from human articular cartilage. Flip angle independent triple quantum filtering sequence has been used to determine biexponential relaxation rates via triple quantum spectroscopy. We have studied sodium relaxation rates of articular cartilage in normal human volunteers. On-going studies are being conducted on osteoarthritic human knee in vivo to determine the status of bound sodium ions in cartilage.
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