Normal-Appearing White Matter Changes in Multiple Sclerosis: Magnetization Transfer

Abstract

Magnetization exchange is a physicochemical mechanism which generally occurs between protons within a tissue. With appropriate techniques this mechanism can be used to augment tissue contrast and, even more importantly, to explore specific tissue characteristics [1]. One such approach is to saturate only a certain proton species and thereby to determine its contribution to a given tissue from the observed signal reduction, commonly known as “saturation transfer”. This is understood best when biological tissue is considered as consisting of at least two kinds of protons that differ in their molecular mobility, i.e. a pool of motionally restricted protons — those bound to macromolecules such as myelin proteins and lipids of brain tissue — and another pool of freely mobile protons in tissue water. The different mobility of these protons is reflected by different transverse relaxation times and hence by different spectral line widths. The free water protons with little spin-spin interaction have a very narrow spectral line width. In contrast, macromolecular protons, due to their diverse interactions, have a very broad line width which can be up to 100 kHz (Fig. 1). If a train of short radiofrequency (RF) saturation pulses is applied with a RF offset and band width that partly matches the shape of the absorption line of the macromolecular protons, these macromolecular protons will become saturated. By various mechanisms this saturation is partly transferred to the “MR-visible” water protons, thus “magnetization transfer” (MT).