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

Abstract

Purpose: 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. Methods: 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. Results: 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. Conclusion: 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.