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The purpose of this paper is to confirm previous reports that identifiedmagnetization transfer (MT) as an inherent driver of longitudinal relaxation inbrain tissue by asserting a substantial difference between the $T_1$ relaxationtimes of the free and the semi-solid spin pools. Further, we aim to identify anavenue towards the quantification of these relaxation processes on avoxel-by-voxel basis in a clinical imaging setting, i.e. with a nominalresolution of 1mm isotropic and full brain coverage in 12min. To this end, weoptimized a hybrid-state pulse sequence for mapping the parameters of anunconstrained MT model. We scanned 4 people with relapsing-remitting multiplesclerosis (MS) and 4 healthy controls with this pulse sequence and estimated$T_1^f \approx 1.90$s and $T_1^s \approx 0.327$s for the free and semi-solidspin pool of healthy WM, respectively, confirming previous reports andquestioning the commonly used assumptions $T_1^s = T_1^f$ or $T_1^s = 1$s.Further, we estimated a fractional size of the semi-solid spin pool of $m_0^s\approx 0.202$, which is larger than previously assumed. An analysis of $T_1^f$in normal appearing white matter revealed statistically significant differencesbetween individuals with MS and controls. In conclusion, we confirm thatlongitudinal spin relaxation in brain tissue is dominated by MT and that thehybrid state facilitates a voxel-wise fit of the unconstrained MT model, whichenables the analysis of subtle neurodegeneration.

On multi-path longitudinal spin relaxation in brain tissue