Limitations of rapid myelin water quantification using 3D bSSFP

ObjectImaging of the myelin water fraction (MWF) is conventionally performed using a multi-echo spin-echo sequence. This technique requires long acquisition times and therefore often suffers from a lack of volume coverage. In this work, the application of 3D balanced steady-state free precession (bSSFP) sequences to extract high-resolution myelin water maps is discussed.Materials and MethodsBased on a two-pool water exchange model, an approximate bSSFP signal equation is derived and applied to fit the flip angle dependence of the in vivo bSSFP signal. Thereby, the MWF and signal amplitude are fitted, while a priori assumptions are made for the other parameters of the two-pool system.ResultsThe effects of magnetization transfer, finite RF pulses, B0 and B1 inhomogeneities, as well as variation of the constant fit parameters, are investigated. Acquisition and calculation of quantitative, high-resolution MWF maps from white matter of healthy volunteers based on bSSFP is feasible and averaged MWF fit results agree with literature. However, results from numerical simulations indicate a severe dependence of the derived MWF values on the constant two-pool parameters.ConclusionThe demonstrated dependence of the MWF on the two-pool parameters considerably impairs the applicability of the proposed method.