Rapid Simultaneous Mapping of Total and Myelin Water Content , T 1 and T 2 * in Multiple Sclerosis

It is long discussed that quantitative magnetic resonance imaging might provide a more specific insight into disease process, progression and therapeutic response of multiple sclerosis. We present an extension of a previously published approach for the simultaneous mapping of brain , and total water content. In addition to those three parameters, the method presented in the current work allows for the measurement of myelin bound water content, a surrogate marker of tissue myelination. Myelin water was measured based on its distinct relaxation with reduced , resulting in a multiexponential decay signal. However, only 10 points could be acquired on the relaxation curve within a maximum echo time of <40ms as the quantitative protocol has been adapted previously for fast acquisitions with whole brain coverage. The sparse sampling required an adaption of the optimisation approach with additional constraints necessary in order to obtain reliable results. Therefore, the corresponding pool fractions were determined using linear optimisation instead of the standard nonnegative least squares (NNLS). The whole approach including the proper choice of constraints was optimised and validated in simulation studies. Furthermore, it was shown that total water content measurement based on a single exponential model is significantly biased in tissue which shows biexponential behaviour such as white matter. Therefore, the corresponding reconstruction algorithm was refined in the current work in order to reduce the systematic measurement error. Furthermore, the independent determination of total water content allows for an absolute quantification of myelin water content, resulting in a more reliable measurement in oedemateous tissue. Using the approach developed, whole brain maps of , , total and myelin water content were acquired in 12 patients suffering from