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

Purpose To investigate the effect of varying levels of T 1 ‐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‐ T 2 ∗ 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 T 1 ‐weighting on these signals, expecting that the previously reported, short T 1 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 T 1 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 T 1 ‐values. This suggests the presence of magnetization transfer and exchange effects which counteract the T 1 ‐weighting. Conclusion Variation of the B 1 + 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 T 1 ‐values.