Saturation pulse design for quantitative myocardial T1 mapping

Background: Quantitative saturation-recovery based T-1 mapping sequences are less sensitive to systematic errors than the Modified Look-Locker Inversion recovery (MOLLI) technique but require high performance saturation pulses. We propose to optimize adiabatic and pulse train saturation pulses for quantitative T1 mapping to have <1 % absolute residual longitudinal magnetization (vertical bar M-Z/M-0 vertical bar) over ranges of B-0 and <(B)over cap>(1) (B-1 scale factor) inhomogeneity found at 1.5 T and 3 T. Methods: Design parameters for an adiabatic BIR4-90 pulse were optimized for improved performance within 1.5 T B-0 (+/- 120 Hz) and (B) over cap (1) (0.7-1.0) ranges. Flip angles in hard pulse trains of 3-6 pulses were optimized for 1.5 T and 3 T, with consideration of T-1 values, field inhomogeneities (B-0=+/- 240 H-z and (B) over cap (1)=0.4-1.2 at 3 T), and maximum achievable B-1 field strength. Residual M-Z/M-0 was simulated and measured experimentally for current standard and optimized saturation pulses in phantoms and in-vivo human studies. T-1 maps were acquired at 3 T in human subjects and a swine using a SAturation recovery single-SHot Acquisition (SASHA) technique with a standard 90 degrees-90 degrees-90 degrees and an optimized 6-pulse train. Results: Measured residual M-Z/M-0 in phantoms had excellent agreement with simulations over a wide range of B-0 and (B) over cap (1). The optimized BIR4-90 reduced the maximum residual vertical bar M-Z/M-0 vertical bar to <1 %, a 5.8x reduction compared to a reference BIR4-90. An optimized 3-pulse train achieved a maximum residual vertical bar M-Z/M-0 vertical bar <1 % for the 1.5 T optimization range compared to 11.3 % for a standard 90 degrees-90 degrees-90 degrees pulse train, while a 6-pulse train met this target for the wider 3 T ranges of B0 and (B) over cap (1). The 6-pulse train demonstrated more uniform saturation across both the myocardium and entire field of view than other saturation pulses in human studies. T1 maps were more spatially homogeneous with 6-pulse train SASHA than the reference 90 degrees-90 degrees-90 degrees SASHA in both human and animal studies. Conclusions: Adiabatic and pulse train saturation pulses optimized for different constraints found at 1.5 T and 3 T achieved <1 % residual vertical bar M-Z/M-0 vertical bar in phantom experiments, enabling greater accuracy in quantitative saturation recovery T-1 imaging.