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

Purpose To characterize the effects of fat on commonly used T 1 mapping sequences and evaluate a new method of quantitative fat fraction (FF) imaging for low fractions based on the modulation of T 1 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) T 1 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 T 1 ‐based FF imaging method was evaluated using Monte Carlo simulations and phantom and in vivo experiments. Results SASHA and MOLLI had similar T 1 dependence on FF, with characteristic under‐ or overestimation of T 1 values as a function of off‐resonance frequency (30–70 ms variation in native T 1 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 T 1 measured with MOLLI and SASHA over a narrow range of off‐resonance frequencies; T 1 shifts from fat can be used to accurately quantify FF. Magn Reson Med 77:237–249, 2017.