Sodium-23 NMR relaxation times in body fluids

23Na longitudinal and transverse NMR relaxation times were measured in human serum, plasma, cerebrospinal fluid (CSF), and solutions of plasma proteins. The magnetization decay curves could not be resolved into two exponentials. A procedure to extract quantitative information from the measured relaxation rates in such a case was developed. The relaxation times of 23Na in serum and plasma were analyzed in terms of the different contributions from free Na+, Na+ bound to small molecules, and Na+ bound to various protein fractions in these body fluids. While T1 is essentially that of free Na+ in a solution which is slightly more viscous than salt solution, T2 is influenced by binding to proteins with the largest contribution from serum albumin. The effect of binding to small molecules on T1 and T2 is negligible. From measurements of the relaxation times at several magnetic field strengths a rotational correlation time of Na+ bound to serum albumin of 16 +/- 6 ns was obtained. The fraction of bound Na+ in serum and plasma was roughly estimated as 0.02% of the total sodium. The relaxation times in cerebrospinal fluid are very similar to those of NaCl solution.