Frequency dependence of water proton longitudinal NMR relaxation times in mouse tissues around the freezing transition.

Water proton longitudinal NMR relaxation times were measured in various tissues of healthy and tumor-bearing mice. Measurements were performed as a function of the Larmor frequency nu in the range 6-90 MHz, and at two temperatures (theta + and theta -) bracketing the 'freezing transition', at which the major part of the water signal disappears. At both temperatures, 1/T1 behaves according to: 1/T1 = A/square root nu + B A and B are obtained at theta + and theta -, and yield the proportion of bound water, which is convincingly identified with non-freezable water. The proportions found lie around 6% for tumors and 12% for other tissues. Discrimination between tissues via T1 is demonstrated to be essentially due to the bound water proportion. Bound water on the one hand and free water on the other hand behave similarly in all tissues including tumors. The activation energy for free water is found to be identical to that of pure water, although relaxation times are markedly different. It is noticed that determining the bound water proportion by signal intensity measurements at theta + and theta - is less reliable than by the T1 method.