In vivo quantification of the bound pool T1 in human white matter using the binary spin–bath model of progressive magnetization transfer saturation

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The relative size and relaxation of the invisible pool of bound spins (T1b) underlying magnetization transfer (MT) was quantified in eight subjects in vivo at 1.5 T from progressive saturation experiments using repetitive MT pulses. The evolution of the binary spin–bath was sampled by increasing the repetition period from 8 to 200 ms. Single-shot echo-planar images at TE = 50 ms were evaluated in the central white matter. Three models were fitted: the general solution, and with constraints of equal relaxation and T1b = 1 s for the invisible pool. The general solution of unconstrained T1b provided a significantly better fit, indicating fast-to-intermediate exchange. The bound pool fraction was 17 ± 4%, the relaxation times T1f = 1.6 ± 0.2 s for free water and T1b = 171 ± 22 ms for the bound pool. The constrained models did not differ from each other, since here T1b was similar to the observed T1 of 1.1 ± 0.1 s. They underestimate the bound pool fraction and its relaxation. Thus, the standard assumption of continuous-wave MT models may underestimate the relaxation via the bound pool by more than a factor of five.