Myelin Water Mapping by Spatially Regularized Longitudinal Relaxographic Imaging at High Magnetic Fields

PurposeMagnetic resonance T-1-weighted images are routinely used for human brain segmentation, brain parcellation, and clinical diagnosis of demyelinating diseases. Myelin is thought to influence the longitudinal relaxation commonly described by a mono-exponential recovery, although reports of bi-exponential longitudinal relaxation have been published. The purpose of this work was to investigate if a myelin water T-1 contribution could be separated in geometrically sampled Look-Locker trains of low flip angle gradient echoes. MethodsT(1) relaxograms from normal human brain were computed by a spatially regularized inverse Laplace transform after estimating the apparent inversion efficiency. ResultsWith sufficiently long inversion-time sampling (ca. 5 x T-1 of cerebrospinal fluid), the T-1 relaxogram revealed a short-T-1 peak (106-225 ms). The apparent fraction of this water component increased in human brain white matter from 8.3% at 3 T, to 11.3% at 4 T and 15.0% at 7 T. The T-2* of the short-T-1 peak at 3 T was shorter, 27.913.0 ms, than that of the long-T-1 peak, 51.3 +/- 5.6 ms. ConclusionThe short-T-1 fraction is interpreted as the water resident in myelin. Its detection is facilitated by longer T-1 of axoplasmic water at higher magnetic field. Magn Reson Med 71:375-387, 2014. (c) 2013 Wiley Periodicals, Inc.