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- Open Access
Optimized saturation pulse rrains for SASHA T1 mapping at 3T
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: W20 (2015)
SASHA and MOLLI T1 mapping sequences can have errors in calculated T1 values when their magnetization preparation pulses do not fully saturate/invert magnetization [1, 2]. The commonly used 90°-90°-90° saturation pulse train  has poor performance at 3T due to large B1 field inhomogeneities. We propose that a new hard RF pulse train with numerically optimized flip angles  will offer superior performance and reduce errors in SASHA T1 values due to incomplete saturation.
Flip angles for a 6-pulse train were optimized by minimizing the maximum residual longitudinal magnetization in Bloch equation simulations performed over ranges of values expected at 3T: 40-120% B1 scaling, -240-240 Hz off-resonance, 200-2000 ms T1, and 14 μT B1 strength. Complete spoiling of transverse magnetization was assumed during spoilers. Optimization code is available at https://bitbucket.org/kelvinc/pulsetrainopt.
Saturation performance for the 90°-90°-90° and the 6-pulse train was measured in a phantom with saturation recovery GRE. B0 and B1 maps were calculated using multi-TE and multiple flip angle GRE respectively. A magnetic field gradient was used to produce a range of off-resonance and experiments were repeated with the prescribed pulse train flip angles scaled by 40-120% to emulate B1 inhomogeneity.
SASHA and MOLLI T1 mapping were performed using investigational prototype sequences on a 66 kg swine in a 3T system (MAGNETOM Skyra, Siemens AG, Germany). SASHA was acquired using both the 90°-90°-90° and proposed pulse train with a 45° imaging flip angle. MOLLI used an optimized inversion pulse (2) with a 20° flip angle. A B1 map was acquired using a saturated double angle method with single-shot EPI readouts.
The optimized 6-pulse train flip angles were 115-90-125-85-176-223° with a 33 ms duration. The 6-pulse train had excellent performance (Fig. 1), with an average and maximum absolute residual longitudinal magnetization over the optimization range of 0.27% and 0.87% respectively. Experimental data had excellent agreement with simulations.
In the swine study, the B1 varied from 30-95% across the left ventricle (LV) profile (Fig. 2). MOLLI and 90°-90°-90° SASHA T1 maps show a >50% artifactual decrease in T1 values with reduced B1 values in the lateral wall. SASHA T1 values using the 6-pulse train are more spatially homogeneous (1386±70 ms across the entire LV profile).
A saturation pulse train optimized for B0, B1, and T1 ranges expected at 3T was shown to have residual longitudinal magnetization of <1%. In-vivo swine MOLLI and SASHA data with the commonly used 90°-90°-90° pulses had >50% T1 variation due to B1 inhomogeneity while 6-pulse train SASHA had a 5% coefficient of variation.
Canadian Institutes of Health Research, Alberta Innovates - Health Solutions, NIH/NHLBI Intramural Research Program.
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Chow, K., Kellman, P., Spottiswoode, B.S. et al. Optimized saturation pulse rrains for SASHA T1 mapping at 3T. J Cardiovasc Magn Reson 17, W20 (2015). https://doi.org/10.1186/1532-429X-17-S1-W20
- Flip Angle
- Intramural Research Program
- Double Angle Method
- Alberta Innovate
- Bloch Equation Simulation