Characterization of T1 bias in skeletal muscle from fat in MOLLI and SASHA pulse sequences: Quantitative fat-fraction imaging with T1 mapping.

Abstract

To characterize the effects of fat on commonly used T1 mapping sequences and evaluate a new method of quantitative fat fraction (FF) imaging for low fractions based on the modulation of T1 values by the fat pool. Bloch equation simulations and phantom and in vivo (skeletal muscle) experiments were used to characterize the response of the modified Look-Locker inversion recovery (MOLLI) and saturation recovery single-shot acquisition (SASHA) T1 mapping sequences to fat-water systems with known FFs (0%-10%) at 1.5T. FFs were measured with single voxel spectroscopy and Dixon imaging methods. A new T1 -based FF imaging method was evaluated using Monte Carlo simulations and phantom and in vivo experiments. SASHA and MOLLI had similar T1 dependence on FF, with characteristic under- or overestimation of T1 values as a function of off-resonance frequency (30-70 ms variation in native T1 per 1% FF). FF maps generated from the SASHA method yielded a low variability of ±0.25% for a signal-to-noise ratio of 150:1 in the nonsaturation image, with good agreement with spectroscopy and a performance that is superior to that of Dixon methods at low FFs. Fat results in negative or positive shifts in native tissue T1 measured with MOLLI and SASHA over a narrow range of off-resonance frequencies; T1 shifts from fat can be used to accurately quantify FF. Magn Reson Med 77:237-249, 2017. © 2016 Wiley Periodicals, Inc.