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 (MT) 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 MT to imaging data obtained with variable offset frequency saturation while using a complimentary R1 (=1 / T1) 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.
Department of Radiology, University of Washington, Seattle, WA 98195, USA. yarnykh@u.washington.edu
Vasily L
VL
Yarnykh
Chun
C
Yuan
D000704:Analysis of Variance; D000818:Animals; D001369:Axons / Q000473:pathology; D001921:Brain / Q000473:pathology*; D001931:Brain Mapping / Q000379:methods; D002417:Cattle; D004292:Dominance, Cerebral / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008279:Magnetic Resonance Imaging; D008432:Mathematical Computing; D009103:Multiple Sclerosis / Q000473:pathology; D009413:Nerve Fibers, Myelinated / Q000473:pathology*; D011712:Pyramidal Tracts / Q000473:pathology; D012016:Reference Values