Water content changes in new multiple sclerosis lesions have a minimal effect on the determination of myelin water fraction values

ABSTRACT Background and Purpose Myelin water fraction (MWF) is a histopathologically validated in vivo myelin marker. As MWF is the proportion of water with a short T 2 relative to the total water, increases in water from edema and inflammation may confound MWF determination in multiple sclerosis (MS) lesions. Total water content (TWC) measurement enables calculation of absolute myelin water content (MWC) and can be used to distinguish edema/inflammation from demyelination. We assessed what influence changes in total water might have on MWF by calculating MWC values in new MS lesions. Methods 3T 32‐echo T 2 relaxation data were collected monthly for 6 months from six relapsing‐remitting MS participants. TWC was determined and multiplied with MWF images to calculate corrected MWC images. The effect of this water content correction was examined in 20 new lesions by comparing mean MWF and MWC over time. Results On average, at lesion first appearance, lesion TWC increased by 6.4% ( p = .003; range: –1% to +21%), MWF decreased by 24% ( p = .006; range: –70% to +12%), and MWC decreased by 20% ( p = .026; range: –68% to +21%), relative to prelesion values. Average TWC in lesions then gradually decreased, whereas MWF and MWC remained low. The shape of the MWF and MWC lesion evolution curves was nearly identical, differing only by an offset. Conclusion MWF mirrors MWC and is able to monitor myelin in new lesions. Even after taking into account water content increases, MWC still decreased at lesion first appearance attributed to demyelination.