The Importance of the Motion of Water for Magnetic Resonance Imaging

Abstract

Since the water content of all soft tissues is about the same, contrast in magnetic resonance imaging depends principally on the parameters that govern nonequilibrium behavior of the nuclear spin system of the water protons of tissue, the longitudinal and transverse relaxation rates 1/T1 and 1/T2. A fundamental understanding of the determinants of both 1/T1 and 1/T2 at a cellular level, and ultimately at a molecluar level (so that contrast can be optimized and perhaps manipulated), will require a model of the behavior of water that describes the dynamics of the motion of water molecules throughout tissue. A particular model is presented here, one in which tissue water is relatively free to diffuse randomly throughout the intracellular and extracellular regions of tissue, colliding with cellular and subcellular constituents along the way; this motion dominates 1/T1 at higher fields. When not in actual contact with interfaces, i.e., within .apprx. 5 .ANG. of a macromolecular surface, the thermal motion of the water molecules is not influenced by the interfaces, but is altered slightly by the presence of solute macromolecules. However, this small difference is amplifield 106-fold, roughly the ratio of the macromolecular to solvent molecular weights, by a mechanism previously named the slosh effect; this effect dominates 1/T1 at low fields and 1/T2 at all fields. It is shown how the foregoing view of tissue water follows quite naturally from NMRD profiles (measurements of the magnetic field dependence of 1/T1 of water protons) of a wide variety of protein solutions and samples of tissue, both native and containing added paramagnetic (Mn2+)ions. Result from this model evidently lead to the inference that relaxation rates of protons in tissue depend mainly on the properties of individual cells and their constituents, rather than on the anatomic structure of the tissues formed by these cells.