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Optimized saturation pulse rrains for SASHA T1 mapping at 3T

Background

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 [3] 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 [4] will offer superior performance and reduce errors in SASHA T1 values due to incomplete saturation.

Methods

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.

Results

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.

Figure 1
figure1

Simulated and experimentally measured residual longitudinal magnetization for a commonly used 90°-90°-90° saturation pulse train and a proposed 6 pulse train. White boxes denote the 3T optimization range.

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).

Figure 2
figure2

A B1 map (bottom left) and T1 maps (top row) using SASHA with a reference 90°-90°-90° saturation pulse, SASHA with a proposed 6 pulse train, and MOLLI in a swine. A profile of T1 values along the left ventricular wall shows decreased T1 values in the lateral wall coinciding with reduced B1 values.

Conclusions

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.

Funding

Canadian Institutes of Health Research, Alberta Innovates - Health Solutions, NIH/NHLBI Intramural Research Program.

References

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Correspondence to Kelvin Chow.

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This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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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

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Keywords

  • Flip Angle
  • Intramural Research Program
  • Double Angle Method
  • Alberta Innovate
  • Bloch Equation Simulation