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

The relative size and relaxation of the invisible pool of bound spins (T(1b)) 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 T(1b) = 1 s for the invisible pool. The general solution of unconstrained T(1b) provided a significantly better fit, indicating fast-to-intermediate exchange. The bound pool fraction was 17 +/- 4%, the relaxation times T(1f) = 1.6 +/- 0.2 s for free water and T(1b) = 171 +/- 22 ms for the bound pool. The constrained models did not differ from each other, since here T(1b) was similar to the observed T(1) 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.