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Improved Velocity-Selective Labeling Pulses for Myocardial ASL
Vanessa Landes1, Terrence R. Jao2, Ahsan Javed3, and Krishna S. Nayak3

1Biomedical Engineering, University of Southern California, Los Angeles, CA, United States, 2Keck School of Medicine, University of Southern California, Los Angeles, CA, United States, 3Electrical Engineering, University of Southern California, Los Angeles, CA, United States

Synopsis

Velocity selective ASL is an exciting option for myocardial perfusion imaging as it does not require any contrast agents and is insensitive to coronary arterial transit times. Feasibility in humans was recently demonstrated with performance primarily limited by 1) spurious labeling of moving myocardium, and 2) low labeling efficiency. We present improvements to the velocity selective labeling pulse that overcome these limitations, leveraging recent developments in velocity-selective MRA. Specifically, we use Fourier Velocity Encoding to reduce spurious labeling of moving myocardium and use inversion to increase labeling efficiency.

Purpose

To develop and test a velocity selective (VS) labeling pulse for improved myocardial VS arterial spin labeling (ASL) perfusion imaging by addressing two current limitations: 1) spurious labeling of moving myocardium and 2) low labeling efficiency [1]. We overcome these by using Fourier Velocity-Encoded design and inversion instead of saturation labeling.

Methods

VS pulse design

The proposed myocardial VS ASL labeling pulse is based on a pulse used for a non-contrast magnetic resonance angiography and brain VS ASL by Qin et al [2,3] with four modifications. 1)The shape of the sub-pulse train was changed to a maximum-phase sinc with TBW=2 to reduce the transition width of the velocity selection profile and increase pass-band uniformity of inversion (static and near-static myocardium) [4]. 2) Inversion pulses were replaced with composite inversion pulses (90x-180y-90x) to reduce sensitivity to B1 inhomogeneity. 3) Velocity encoding gradients were turned off instead of inverted in control acquisitions to reduce sensitivity to B1 and B0 inhomogeneity. 4) The B1 scale was designed for a nominal peak B1 of 0.10G and played at an actual peak of 0.13G to tolerate a B1 variation of 0.7-1 in myocardium [5].

Figure 1 shows the proposed pulse with a velocity cutoff of 6 cm/s. Bloch simulations of the label-control difference signal as a function of velocity were performed under a range of B1 scales and off-resonance.

Spurious labeling experiments

The Qin pulse and the proposed pulse were evaluated in-vivo to determine whether the proposed pulse reduces spurious labeling of moving myocardium compared to the Qin pulse. Experiments were performed on 2 healthy volunteers (M, 28 and 31 years) under a protocol approved by our Institutional Review Board.

VS labeling experiments were performed by playing a VS prepulse in mid-diastole, waiting one heartbeat for labeled blood to flow into the myocardium, and acquiring bSSFP images of the mid short-axis. Images were acquired after control and after label VS prepulses for velocity cutoff (Vc) values of 3, 6, 9, 12, 18, 24, 48, and 100 cm/s, where Vc is defined as the full-width at half the maximum range of the simulated velocity profile on-resonance with a B1 scale of 1. Two image pairs were acquired per Vc. Vc was adjusted by changing the amplitudes of bipolar encoding gradients and pulse duration was kept constant.

ASL signal was evaluated in the blood pool to confirm the presence of velocity selective excitation and across the myocardium to evaluate spurious labeling.

Results and Discussion

Figure 2 shows Bloch simulations of label - control signal as a function of velocity under different B1 scales and off-resonance conditions. The proposed pulse is robust to B1 variation over the extreme ranges anticipated in the heart. Label-control signal difference of the proposed pulse is flatter between ±2 cm/s, the expected range of longitudinal velocity in the myocardium during diastole [4]. This signal difference also has a more uniform stop-band region for |Vc|>10 cm/s, indicating improved labeling efficiency.

Figure 3 shows mean label signal in the blood pool. Label signal decreased as a function of Vc and plateaued around 24 cm/s while control signal was independent of Vc.

Figure 4 shows the ASL signal in the myocardium. ASL signal decreases with increasing Vc for both pulses and approaches 0, as expected. ASL signal is lower for Vc<6 cm/s in the proposed pulse, indicating reduced spurious labeling, and flatter for Vc>6 cm/s, corresponding with consistent labeling of fresh inflowing blood into the myocardium. ASL signal is higher in the lateral wall for low Vc, as the lateral wall moves at a higher velocity.

Conclusion

We have adapted the pulse designed by Qin et al for myocardial ASL to increase labeling efficiency and reduce spurious labeling of moving myocardium. We demonstrate reduction in spurious labeling of moving myocardium compared to the Qin pulse. Future work will integrate and test this pulse with myocardial VS ASL.

Acknowledgements

Grant support: NIH #R01-HL130494

References

[1] Jao TR, Nayak KS. Demonstration of velocity selective myocardial arterial spin labeling perfusion imaging in humans. Magn. Reson. Med. 2018; 80:272–278.

[2] Qin Q, Shin T, Schar M, Guo H, Chen H, Qiao Y. Velocity-selective magnetization-prepared non-contrast-enhanced cerebral MR angiography at 3 Tesla: Improved immunity to B0/B1 inhomogeneity. Magn. Reson. Med. 2016;75:1232–1241.

[3] Qin Q, van Zijl PCM. Velocity-selective-inversion prepared arterial spin labeling. Magn. Reson. Med. 2016;76:1136–1148.

[4] Codreanu I, Robson MD, Golding SJ, Jung BA, Clarke K, Holloway CJ. Longitudinally and circumferentially directed movements of the left ventricle studied by cardiovascular magnetic resonance phase contrast velocity mapping. J. Cardiovasc. Magn. Reson. 2010;12:48.

[5] Sung K, Nayak KS. Measurement and characterization of RF nonuniformity over the heart at 3T using body coil transmission. J. Magn. Reson. Imaging 2008;27:643–648.

Figures

Figure 1: Proposed velocity-selective labeling pulse. This pulse was designed to be insensitive over a range of ±100 Hz off-resonance and 0.7-1.3 B1 scale, and provide uniform inversion of myocardium during stable diastole (longitudinal velocity ±2 cm/s).

Figure 2: Bloch simulations of the label - control difference signal as a function of Vc in myocardium using (A) Qin’s pulse and (B) the proposed pulse as well as in blood using (C) Qin’s pulse (D) the proposed pulse. Pulses were simulated across a range of B1 and off-resonant frequencies. Difference signal between ±5 cm/s in the myocardium is highlighted by blue boxes to represent potential spurious labeling of slow-moving myocardium. Difference signal for high velocities <-10 and >10 cm/s in blood is highlighted by green boxes to demonstrate labeling efficiency.

Figure 3: ASL signal in the blood pool. ASL signal reduces as velocity cutoff increases for both the Qin pulse and Proposed pulse, as expected. ASL signal begins to plateau around Vc=24 cm/sec, indicating flow velocities in the blood pool. ASL signal approaches 0 for high Vc, as expected.

Figure 4: ASL signal across the entire myocardium (left), septum (middle), and lateral wall (right) for both the Qin pulse (Qin) and the proposed pulse (Prop). ASL signal decreases with increasing Vc, indicating reduced spurious labeling, and approaches 0, as expected. ASL signal is higher in the lateral wall for low Vc, as the lateral wall moves at a higher velocity.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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