Rapid myelin water content mapping on clinical MR systems.

We present an algorithm for the fast mapping of myelin water content using standard multiecho gradient echo acquisitions of the human brain. The method extents a previously published approach for the simultaneous measurement of brain T(1), T(2)(*) and total water content. Employing the multiexponential T(2)(*) decay signal of myelinated tissue, myelin water content was measured based on the quantification of two water pools ("myelin water" and "rest") with different relaxation times. As the existing protocol was focussed on the fast mapping of quantitative MR parameters with whole brain coverage in clinically relevant measurement times, the sampling density of the T(2)(*) curve was compromised to 10 echo times with a TE(max) of approx. 40ms. Therefore, pool amplitudes were determined using a quadratic optimisation approach. The optimisation was constrained by including à priori knowledge about brain water pools. All constraints were optimised in a simulation study to minimise systematic error sources given the incomplete knowledge about the real pool-specific relaxation properties. Based on the simulation results, whole brain in vivo myelin water content maps were acquired in 10 healthy controls and one subject with multiple sclerosis. The in vivo results obtained were consistent with previous reports which demonstrates that a simultaneous whole brain mapping of T(1), T(2)(*), total and myelin water content is feasible on almost any modern MR scanner in less than 10 minutes.