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Fourier Transform based Velocity-Selective Labeling Pulses for Myocardial ASL

Vanessa Landes¹, Ahsan Javed², Terrence Jao³, Qin Qin^4,5, and Krishna Nayak²
¹Biomedical Engineering, University of Southern California, Los Angeles, CA, United States, ²Electrical Engineering, University of Southern California, Los Angeles, CA, United States, ³Keck School of Medicine, University of Southern California, Los Angeles, CA, United States, ⁴Radiology, Johns Hopkins University, Baltimore, MD, United States, ⁵F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States

Synopsis

We present an improved velocity-selective (VS) labeling pulse for myocardial arterial spin labeling (ASL) that addresses limitations of prior pulses. The proposed pulse is designed using a Fourier Transform based Velocity-Selective (FT-VS) pulse train and optimized to label coronary blood while not labeling myocardium. Myocardial VSASL experiments were performed in healthy adult volunteers. The proposed pulse provided comparable measurements to FAIR ASL and a 2.5-fold reduction in physiological noise compared to a prior VS-ASL pulse.

Introduction

Myocardial arterial spin labeling (ASL) is a promising non-contrast technique for myocardial perfusion (MP) imaging which can be used to detect coronary artery disease^1,2. Velocity-selective (VS) labeling was recently demonstrated for myocardial ASL³ to reduce sensitivity to transit delay, which can be a major source of error in perfusion quantification. The proposed VS labeling pulse had two major limitations which reduced the sensitivity of the myocardial VS-ASL; 1) A BIR8 saturation pulse was used which has half the labeling efficiency compared to inversion labeling used in FAIR and 2) VS-pulse design didn’t allow control over the velocity profile which increased the likelihood of spurious labeling of moving myocardium. Recently proposed Fourier Transform based velocity selective (FT-VS) pulses^4-9 can be used to overcome the limitations of the myocardial VS-ASL method proposed by Jao et al³. In this study, we present a framework for optimization of the velocity profile of FT-VS pulses and use a carefully designed FT-VS pulse to demonstrate improved sensitivity for myocardial VS-ASL at 3T.

Methods

Phantom and in-vivo experiments were performed on a 3T whole body scanner (Signa HDxT; General Electric Healthcare, Waukesha, WI) using an 8-channel cardiac coil receiver array. The human protocol was approved by our Internal Review Board and informed consent was obtained from all subjects.
VS Pulse Design
The proposed VS pulse (VS-prop) is based on designs by Shin et al and Qin et al^5,7. The initial guess for the sub-pulse envelope was a linear-phase inversion with time-bandwidth product=6. π phase was added to alternate sub-pulse amplitudes to shift FOVv such that coronary velocities were inverted, and static tissues was unaffected. Sub-pulse amplitudes were optimized using Bloch simulations with minimum norm solver in MATLAB. Optimization was performed over b1 scale of 0.5-1¹⁰ and off-resonance between ±125 Hz^10,11, to maximize coronary ASL signal and minimize labeling of moving myocardium. Velocity ranges of 10-70 cm/s at rest, and 10-130 cm/s during stress were considered for coronary blood flow^12-14,whereas a range of <±3 cm/s was used for myocardium^15,16. These are the expected velocities during mid-diastole, when VS-labeling is performed. Silicon oil phantom (T1/T2 = 1111/227 ms)^17,18 experiments were used to determine the gradient delays required to mitigate artifacts from gradient imperfections.
ASL experiments
The performance of myocardial VS-ASL, with original and proposed labeling pulses, was compared to the reference FAIR-ASL method² in four healthy volunteers (2M/2F, age 24-30Y). Previously published VS-ASL sequence with background suppression¹⁹ was used for myocardial VS-ASL and three velocity selective labeling pulses were used for comparison: 1) BIR8 VS-saturation (VS-Orig)¹⁹, 2) VS-Prop (FOVv = 140 cm/s), and 3) VS-Prop (FOVv = 80 cm/s) . Imaging was performed using balanced steady state free precession with previously published settings¹⁹.
Images were reconstructed using a custom implementation of GRAPPA²⁰. Left ventricular myocardium was segmented in all images using semi-automatic segmentation, with previously published settings²¹. Global MP and PN were calculated for VS-ASL and FAIR as described by Jao et al.¹⁹ and Zun et al.², respectively. For the VS-prop pulses calculation of MP was updated by; 1) dividing the signal (C-L) by 2 to account for inversion of flowing spins and 2) adjusting the T2-weighting term to reflect the duration of VS-Prop pulses.
Statistical equivalence of MP measurements between different methods was established using a two one-sided test (TOST) at a difference of 0.3 ml/g/min. Statistical difference in PN between methods was established using a paired T-test.

Results

Figure 1 illustrates the proposed pulse (FOVv = 140 cm/s). Figure 2 illustrates reduced labeling of myocardium and improved labeling efficiency of blood after optimization. Figure 3 demonstrates the Fourier-relationship between b1 = 1 velocity profile and sub-pulse envelope. Figure 4 shows MP and PN measured using 4 ASL experiments (3 VS/1 FAIR) across 4 subjects. Figure 5 depicts a reduction in spurious labeling of moving myocardium in VS-Prop pulse compared to VS-Orig pulse.

Discussion

This work demonstrates how FT-based velocity-selective RF pulses can be tailored for a specific application (myocardial ASL). Low-field applications and brain applications will likely require optimization over smaller range of off-resonance and b1 variations. Body applications with less motion will relax constraints on the stop-band of a tailored pulse. Other VS applications may benefit from consideration of T1 and T2 relaxation.
Statistical testing is limited by analysis across only 4 subjects. MP measurements across methods presented in this study were physiologically reasonable but not statistically equivalent and statistical significance could be realized with inclusion of more datasets.
This study, like the previous study of myocardial VSASL (19), lacks experimental verification of insensitivity to transit delay. Theoretically, the proposed VS pulses are designed to label coronary blood and reduce transit delay effects. The extent to which transit delay is reduced is unknown. Future work is needed to experimentally verify insensitivity to transit delay.

Conclusion

We have demonstrated reduced physiological noise in myocardial VSASL with a specially tailored velocity-selective inversion pulse. This technique gives comparable MP measurements as FAIR ASL, making it a suitable candidate for multi-slice myocardial ASL and for myocardial ASL in patients with slow coronary flow.

Acknowledgements

We thank Jia Guo and Eric Wong for helpful discussions. We gratefully acknowledge funding from the National Institutes of Health (R01-HL130494, PI: Nayak).

References

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3. Jao TR, Nayak KS. Demonstration of velocity selective myocardial arterial spin labeling perfusion imaging in humans. MRM. 2018;80:272–278.

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Figures

Figure 1: The proposed VSI pulse is designed using a FT-VS labeling pulse train to reduce spurious labeling of moving myocardium and achieve better labeling efficiency. 9 sub-pulses (blue) achieve velocity-selective inversion. Hard refocusing pulses (green) utilize MLEV phase cycling to reduce sensitivity to off-resonance. Bipolar gradients (red) between paired refocusing pulses achieve velocity encoding while removing potential for VS-stripe artifact. A 0.25 ms delay follows each gradient pair to reduce sensitivity to eddy currents.

Figure 2: Bloch simulations of the proposed VS pulse with FOVv =140 cm/s for coronary 10≤v≤130 cm/s (left) and for myocardial velocities |v|≤3 cm/s before (top) and after (bottom) optimization of sub-pulse amplitudes. Bloch simulations are shown for Δf=0 and b1=0.5-1 as well as b1=1 and |Δf|≤125 Hz. The VS pulse achieves better labeling efficiency of flowing signal (green regions) after optimization, especially for different off-resonance values. In addition, it can be seen that the VS pulse allows for less spurious labeling of moving myocardium (purple regions) after optimization.

Figure 3: On-resonant, b1=1 velocity profile (top) and corresponding kv-encoding (blue) for the proposed VS pulse with FOVv=140 cm/s before (black) and after (blue) sub-pulse optimization. The kv-encoding is shaped as a windowed sinc, which leads to a slice-like selection of velocities. After optimization, the transition band of the velocity profile narrows, improving labeling efficiency of coronary velocities above ±10 cm/s. The velocity profile also flattens around v=±3 cm/s, reducing the potential to spuriously label moving myocardium in stable-diastole.

Figure 4: Myocardial perfusion (MP) and physiological noise (PN) across four healthy subjects (S1-S4) using FAIR-ASL and VS-ASL with the original or proposed VS pulses. MP is within a physiologically reasonable range (1-3 ml/g/min), and PN is lower in subjects 1 and 2 with VS-Prop compared to VS-Orig. TOST with a difference of 0.3 ml/g/min and p=0.05 indicated MP measured with FAIR and VS-Prop (140 cm/s FOVv) were statistically equivalent. A t-test with p=0.05 indicated FAIR and VS-Prop (140 cm/s FOVv) had lower PN compared to the original VS pulse (2.5x).

Figure 5: Myocardial perfusion (MP) is shown in subject 1 using the original (VS-Orig) and proposed (VS-Prop) pulse. The Vs-Orig pulse has very high MP (>4 ml/g/min) in the lateral wall, indicating possible spurious labeling of moving myocardium. Comparatively, VS-prop has more consistent MP across the septal and lateral walls, indicating reduction in spurious labeling of moving myocardium.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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