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

Background: The reference standard for assessing water T (T ) at high fat fraction (FF) is H MRS. T (T ) dependence on FF (FF ) has recently been demonstrated in muscle at high FF (i.e. ≥60%).

Purpose: To investigate the relationship between T and FF in the thigh/leg muscles of patients with neuromuscular diseases and to compare with quantitative MRI.

Study Type: Retrospective case-control study.

Population: A 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/sequence: A 3-T; single-voxel stimulated echo acquisition mode (STEAM) MRS, multispin echo (MSE) imaging (for T mapping, T ), three-point Dixon imaging (for FF and mapping).

Assessment: Mono-exponential and bi-exponential models were fitted to water T decay curves to extract T and FF . Water resonance full-width-at-half-maximum (FWHM) and B spread (∆B ) values were calculated. T (mean), FF (mean, kurtosis, and skewness), and (mean) values were estimated in the MRS voxel.

Statistical Tests: Mann-Whitney U tests, Kruskal-Wallis tests. A P-value <0.05 was considered statistically significant.

Results: Normal T threshold was defined as the 90 percentile in healthy controls: 30.3 msec. T was significantly higher in all patients with FF  < 60% compared to healthy controls. We discovered two subgroups in patients with FF  ≥ 60%: one with T  ≥ 30.3 msec and one with T  < 30.3 msec including abnormally low T . The latter subgroup had significantly higher water resonance FWHM, ∆B , FF kurtosis, and skewness values but nonsignificantly different (P = 1.00) and long T component and its fraction (P > 0.11) based on the bi-exponential analysis.

Data Conclusion: The findings suggest that the cause for (abnormally) T at high FF is biophysical, due to differences in susceptibility between muscle and fat (increased FWHM and ∆B ), rather than pathophysiological such as compartmentation changes, which would be reflected by the bi-exponential analysis.

Evidence Level: 3 TECHNICAL EFFICACY: Stage 3.