Sodium magnetic resonance imaging and chemical shift imaging

Abstract

The study of sodium in biological systems is critical for an understanding of physiology and pathophysiology. NMR has proven to be an excellent tool for the study of sodium because of the relatively high concentration of sodium in the extracellular fluids of biological tissues, the inherent sensitivity of the 23Na quadrupolar nucleus to its environment, and the high contrast obtainable in 23Na MR images. The challenge has been to improve the instrumentation and to develop the data acquisition techniques to the point where high quality images and CSI data sets could be obtained in an acceptable time. As techniques have improved it has been demonstrated that 23Na MRI is indeed sensitive to tissue viability and pathologic state, yet clinical studies have not yet found unique information from 23Na MR images alone. (56, 75) The use of multiple echoes to provide relaxation information has greatly increased the utility of 23Na MRI in spite of the fact that intra- and extracellular sodium pools do not appear to be differentiable solely on the basis of the presence of a short component of T 2. The generation of images based on relaxation time thresholds (70) may prove valuable in providing more tissue-specific information. The use of paramagnetic shift reagents or contrast reagents has proved useful for the differentiation of intra- and extracellular sodium pools, but toxicity of the currently available shift reagents may be problematic. In instances where toxicity is not a severe problem, shift reagents can be used to monitor fluid dynamics as well as to distinguish different sodium pools.