Sodium NMR Relaxation Parameters in Cartilage: Implications for MR Imaging

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Abstract

Cartilage is a dense connective tissue which covers the opposing ends of bones in a joint and acts as lubricating and resilient surface. Cartilage consists of hydrated extracellular matrix and relatively few cells. The extracellular matrix is a network of collagen fibers and large proteoglycan aggregates. Proteoglycans have a net negative charge under physiological conditions known as fixed charge density (FCD). These negatively charged molecules preferentially attract positive sodium ions and thus are a source of osmotic pressure which gives cartilage its resilience to compression. These extracellular sodium cations can be measured by nuclear magnetic resonance (NMR) and used as a nondestructive technique to monitor the tissue glycosaminoglycan concentration. Arthritis is a degenerative disease of cartilage and is characterized by a decrease in proteoglycan concentration. Sodium MR imaging can be an early indicator of this degenerative process and may provide non-invasive means of measuring cartilage degradation in vivo. The essential parameters which effect the signal intensity in sodium MRI are sodium density, and sodium T₁ and T₂ relaxation times. Therefore the main goal of the present study was to determine these relaxation parameters. Calf epiphyseal (EP) cartilage was harvested from distal ulna joints. T₁ determined for calf EP cartilage consisted of a single exponential time constant with mean of 18.2 ms and increased to 33.0 ms after trypsin degradation to remove cartilage proteoglycans. Cartilage T₂ consisted of two well defined exponential components, T₂ fast = 0.96 ms and T₂ slow = 21.1 ms. After treating cartilage with trypsin the T₂ fast decreased considerably to 0.2 ms with slow component changing slightly to 24 ms. In summary, these data provide the fundamental tissue parameters necessary for designing magnetic resonance imaging protocols necessary for quantitative sodium density weighted images. These protocols will ultimately be useful in monitoring early degeneration of cartilage and in evaluating in vivo therapies.

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