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 T ₁ 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.