Demonstration of Velocity Selective Myocardial Arterial Spin Labeling
Terrence Jao1 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

Synopsis

Arterial spin labeled CMR is a non-contrast myocardial perfusion imaging technique capable of assessing coronary artery disease. A limitation of current methods is potential underestimation of blood flow to myocardial segments that have coronary arterial transit time longer than 1 R-R, which are found in regions with significant collateral development from chronic myocardial ischemia. In this work, we demonstrate the feasibility of a velocity selective labeling scheme for ASL-CMR that is insensitive to arterial transit time.

Introduction

Arterial spin labeling of the heart has been shown to estimate myocardial perfusion and perfusion reserve for coronary artery disease assessment.1 However, current spatial labeling methods suffer from transit delay effects when imaging is extended to more than a single slice. Velocity selective (VS) labeling is a promising alternative that does not suffer from transit delay effects.2

Methods

Images were acquired using a 3T GE Signa Excite HD scanner with an 8-channel cardiac coil in 12 healthy volunteers. Myocardial ASL measurements were made at a single short axis slice using both VSASL and conventional flow alternating inversion recovery (FAIR) ASL as a reference.1,2 Figure 1 shows the VS pulse, which consists of a symmetric BIR4 for reduced eddy current sensitivity3 and bipolar gradients to prevent spatial signal modulation in static tissue.4 The VS labeling pulse was executed during mid-diastole when coronary blood flow is high (>15-40 cm/s)5 and myocardial velocity is low (< 2 cm/s)6 to increase labeling efficiency and avoid spurious myocardial tagging. The cutoff velocity was 10 cm/s and applied in the through-slice direction. An additional triple inversion recovery background suppression preparation was used to null myocardial T1s between 1250 ms and 1450 ms. Each scan consisted of 6 breath-held labeled/control image pairs. In a single volunteer, an additional FAIR experiment was performed with a thick inversion slab to simulate whole heart coverage with increased transit delay. Myocardial blood flow (MBF), physiological noise (PN), and temporal SNR (TSNR = MBF/PN) were measured within the left ventricular myocardium ROI.

Results and Discusssion

Figure 2 illustrates successful VS labeling of blood within the right coronary artery in two subjects. VSASL performance in individual subjects are summarized in Table 3. In 3 subjects, VSASL performance was poor (TSNR < 1) while in the other 9, VSASL performed well with results comparable to the FAIR reference scan. In the poor performers, MBF and PN measurements for VSASL and FAIR were -0.13 ± 1.35 ml/g/min and 1.86 ± 0.42 ml/g/min respectively while in good performers, they were 2.13 ± 0.48 ml/g/min and 1.97 ± 0.38 ml/g/min. We speculate that the high PN in poor performers is from inconsistent pulse performance, possibly due to variations in B1 and off-resonance, but have yet to test this. Poor performance may also be from spurious labeling of myocardium, which can be further reduced by more consistent background suppression. Low TSNR will be addressed by further sequence improvements that explore different cutoff velocities and velocity labeling directions.

In the volunteer in whom we performed FAIR with a thicker inversion slab (FAIR-TS), MBF and PN measurements from VSASL, FAIR, and FAIR-TS were 1.38 ± 0.58 ml/g/min, 0.88 ± 0.37 ml/g/min, and -0.04 ± 0.24 ml/g/min respectively. FAIR-TS suffered from a large transit delay and was unable to estimate MBF while VSASL did not suffer from transit delay effects.

Conclusion

VS labeling has several important advantages over spatial labeling sequences, notably its insensitivity to transit delay and its compatibility with whole heart coverage. We have demonstrated the feasibility of VS labeling of coronary blood and demonstrated that VSASL is sensitive to myocardial perfusion. Performance of VSASL was comparable to FAIR in 75% of the subjects scanned. In the other 25% of subjects, resolving the inconsistent performance is still a work in progress.

Funding

American Heart Association 13GRNT13850012; Wallace H. Coulter Foundation Clinical Translational Research Award.

Acknowledgements

No acknowledgement found.

References

[1] Zun. et. al. MRM 2009; 62(4): 975-83

[2] Wong. et. al. MRM 2006;55(6)1334-41

[3] Guo et. al. MRM. 2015; 73(3):1085-94

[4] Fang et. al. MRM 2009; 62(6):1523-32

[5] Anderson et. al. Circ. 2000; 102(1):48-54

[6] Gorcsan III et. al. Am. Heart T. 1996; 131(6):1203-13

Figures

Figure 1. A. the Velocity selective labeling pulse is made using a symmetric BIR4 for reduced eddy current sensitivity and bipolar gradients to prevent spatial signal modulation in static tissue. B. the Velocity selective ASL pulse sequence is composed of VS labeling during diastole followed by triple inversion recovery background suppression and b-SSFP imaging.

Figure 2 Right coronary arterial blood (red) is saturated by VS labeling at mid-diastole with a velocity cutoff of 10 cm/s in two subjects. T2 weighting from labeling decreased signal by 16.4 ± 0.1% in left ventricular myocardium. Left: Gz “off” (T2 weighting only) Right: Gz “on” (T2 weighting + velocity selective saturation).

Figure 3: Global MBF, PN, and TSNR are reported for each volunteer. 3 subjects had TSNR < 1 and were categorized as poor performers.

Figure 4. MBF from FAIR, FAIR-TS, and VSASL in a single volunteer. FAIR uses a 3 cm thick inversion slab while FAIR-TS uses a 10 cm thick inversion slab to simulate whole heart coverage. Both FAIR and VSASL were able to measure MBF. FAIR-TS was unable to measure MBF in the lateral, posterior, and septal walls (red arrows) because the thickened inversion slab imparts a large transit delay.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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