Cross-relaxation imaging reveals detailed anatomy of white matter fiber tracts in the human brain

Cross-relaxation imaging is a new quantitative MRI modality, which allows mapping of fundamental parameters determining the magnetization transfer (NIT) effect in tissues, cross-relaxation rate constant (k) and bound pool fraction (f). This study introduces-a new time-efficient technique for cross-relaxation imaging, which obtains three-dimensional (3D) whole-brain k and f maps with scan time of <30 min and isotropic spatial resolution of 1.4 mm. The technical principle of the method is based on four-point fit of a matrix model of pulsed NIT to imaging data obtained with variable offset frequency saturation while using a complimentary R-1 (=1/T-1) map. Anatomical correlations of in vivo cross-relaxation parametric maps were evaluated in three healthy subjects. The f maps revealed correspondence of areas with highly elevated f=12-15% to major fiber tracts such as corpus callosum, anterior commissure, optic radiations, and major brain fasciculi. The rest of white matter (WM) demonstrated lower f=9-11%, resulting in clear visual contrast of fiber tracts. Even lower f=6.5-8.5% were found in gray matter (GM) with the highest f=8.5% in the anterior thalamus. Distribution of k was relatively uniform in WM and produced sharp contrast between GM and WM (k=1.6 and 3.3 s(-1), respectively). The most marked feature of k maps was their ability to visualize the corticospinal tract, which had elevated k=3.4-3.8 s(-1) but appeared invisible on f maps. The observed patterns on f maps can be explained by variations in the density of myelinated fibers, while the trends of k may reflect regional differences in axonal organization. Cross-relaxation imaging can be used in various clinical studies focused on brain development and white matter diseases. (C) 2004 Elsevier Inc. All rights reserved.