Myelin water mapping by spatially regularized longitudinal relaxographic imaging at high magnetic fields

Purpose Magnetic resonance T1‐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 T1 contribution could be separated in geometrically sampled Look‐Locker trains of low flip angle gradient echoes. Methods T1 relaxograms from normal human brain were computed by a spatially regularized inverse Laplace transform after estimating the apparent inversion efficiency. Results With sufficiently long inversion‐time sampling (ca. 5 × T1 of cerebrospinal fluid), the T1 relaxogram revealed a short‐T1 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 T2* of the short‐T1 peak at 3 T was shorter, 27.9 ± 13.0 ms, than that of the long‐T1 peak, 51.3 ± 5.6 ms. Conclusion The short‐T1 fraction is interpreted as the water resident in myelin. Its detection is facilitated by longer T1 of axoplasmic water at higher magnetic field. Magn Reson Med 71:375–387, 2014. © 2013 Wiley Periodicals, Inc.