  Since the inception of magnetization transfer (MT) imaging, it has been
widely assumed that Henkelman's two spin pools have similar longitudinal
relaxation times, which motivated many researchers to constrain them to each
other. However, several recent publications reported a $T_1^s$ of the
semi-solid spin pool that is much shorter than $T_1^f$ of the free pool. While
these studies tailored experiments for robust proofs-of-concept, we here aim to
quantify the disentangled relaxation processes on a voxel-by-voxel basis in a
clinical imaging setting, i.e., with an effective resolution of 1.24mm
isotropic and full brain coverage in 12min. To this end, we optimized a
hybrid-state pulse sequence for mapping the parameters of an unconstrained MT
model. We scanned four people with relapsing-remitting multiple sclerosis (MS)
and four healthy controls with this pulse sequence and estimated $T_1^f \approx
1.84$s and $T_1^s \approx 0.34$s in healthy white matter. Our results confirm
the reports that $T_1^s \ll T_1^f$ and we argue that this finding identifies MT
as an inherent driver of longitudinal relaxation in brain tissue. Moreover, we
estimated a fractional size of the semi-solid spin pool of $m_0^s \approx
0.212$, which is larger than previously assumed. An analysis of $T_1^f$ in
normal-appearing white matter revealed statistically significant differences
between individuals with MS and controls.
