Probing the myelin water compartment with a saturation‐recovery, multi‐echo gradient‐recalled echo sequence

Purpose To investigate the effect of varying levels of T1‐weighting on the evolution of the complex signal from white matter in a multi‐echo gradient‐recalled echo (mGRE) saturation‐recovery sequence. Theory and Methods Analysis of the complex signal evolution in an mGRE sequence allows the contributions from short‐ and long‐T2∗ components to be separated, thus providing a measure of the relative strength of signals from the myelin water, and the external and intra‐axonal compartments. Here we evaluated the effect of different levels of T1‐weighting on these signals, expecting that the previously reported, short T1 of the myelin water would lead to a relative enhancement of the myelin water signal in the presence of signal saturation. Complex, saturation‐recovery mGRE data from the splenium of the corpus callosum from 5 healthy volunteers were preprocessed using a frequency difference mapping (FDM) approach and analyzed using the 3‐pool model of complex signal evolution in white matter. Results An increase in the apparent T1 as a function of echo time was demonstrated, but this increase was an order of magnitude smaller than that expected from previously reported myelin water T1‐values. This suggests the presence of magnetization transfer and exchange effects which counteract the T1‐weighting. Conclusion Variation of the B1+ amplitude in a saturation‐recovery mGRE sequence can be used to modulate the relative strength of signals from the different compartments in white matter, but the modulation is less than predicted from previously reported T1‐values.