New Insights into the Spread of MRS‐Based Water T2 Values Observed in Highly Fatty Replaced Muscles

Abstract

BackgroundThe reference standard for assessing water T2 (T2,H2O) at high fat fraction (FF) is 1H MRS. T2,H2O (T2,H2O,MRS) dependence on FF (FFMRS) has recently been demonstrated in muscle at high FF (i.e. ≥60%).PurposeTo investigate the relationship between T2,H2O,MRS and FFMRS in the thigh/leg muscles of patients with neuromuscular diseases and to compare with quantitative MRI.Study TypeRetrospective case–control study.PopulationA total of 151 patients with neuromuscular disorders (mean age ± standard deviation = 52.5 ± 22.6 years, 54% male), 44 healthy volunteers (26.5 ± 13.0 years, 57% male).Field Strength/SequenceA 3‐T; single‐voxel stimulated echo acquisition mode (STEAM) MRS, multispin echo (MSE) imaging (for T2 mapping, T2,H2O,MRI), three‐point Dixon imaging (for FFMRI and mapping).AssessmentMono‐exponential and bi‐exponential models were fitted to water T2 decay curves to extract T2,H2O,MRS and FFMRS. Water resonance full‐width‐at‐half‐maximum (FWHM) and B0 spread (∆B0) values were calculated. T2,H2O,MRI (mean), FFMRI (mean, kurtosis, and skewness), and (mean) values were estimated in the MRS voxel.Statistical TestsMann–Whitney U tests, Kruskal–Wallis tests. A P‐value <0.05 was considered statistically significant.ResultsNormal T2,H2O,MRS threshold was defined as the 90th percentile in healthy controls: 30.3 msec. T2,H2O,MRS was significantly higher in all patients with FFMRS < 60% compared to healthy controls. We discovered two subgroups in patients with FFMRS ≥ 60%: one with T2,H2O,MRS ≥ 30.3 msec and one with T2,H2O,MRS < 30.3 msec including abnormally low T2,H2O,MRS. The latter subgroup had significantly higher water resonance FWHM, ∆B0, FFMRI kurtosis, and skewness values but nonsignificantly different (P = 1.00) and long T2,H2O,MRS component and its fraction (P > 0.11) based on the bi‐exponential analysis.Data ConclusionThe findings suggest that the cause for (abnormally) T2,H2O,MRS at high FFMRS is biophysical, due to differences in susceptibility between muscle and fat (increased FWHM and ∆B0), rather than pathophysiological such as compartmentation changes, which would be reflected by the bi‐exponential analysis.Evidence Level3Technical EfficacyStage 3