Hung Phi Do1, Andrew J Yoon2, Michael W Fong2, Farhood Saremi3, Mark L Barr4, and Krishna S Nayak5
1Department of Physics and Astronomy, University of Southern California, Los Angeles, CA, United States, 2Department of Medicine, Divison of Cardiology, Keck School of Medicine of USC, University of Southern California, Los Angeles, CA, United States, 3Department of Radiology, Keck School of Medicine of USC, University of Southern California, Los Angeles, CA, United States, 4Department of Cardiothoracic Surgery, Keck School of Medicine of USC, University of Southern California, Los Angeles, CA, United States, 5Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA, United States
Synopsis
Double-gating in myocardial ASL allows for variations
in the post-labeling delay in order to ensure that both labeling and imaging occur
in the same cardiac phase. Originally proposed by Poncelet et al. in 1999, this
was believed to provide insensitivity to heart rate variation. Despite this,
most groups have utilized single-gating with a fixed post-labeling delay for
pairs of control and tagged images, since this allows for simpler quantification
of myocardial blood flow. In this study, we demonstrate that the double-gating
is indeed more robust to heart rate variation compared to single-gating for
myocardial ASL, based on experiments in healthy volunteers and heart transplant
recipients.Background
Myocardial arterial
spin labeling (ASL) is a non-contrast perfusion imaging method that is capable
of quantifying regional myocardial blood flow (MBF) and flow reserve.
1,2 It is
compatible with stress testing and can identify angiographically significant
coronary artery disease (CAD).
3 ASL has low signal-to-noise ratio (SNR) resulting
in high variation in in MBF measurement (called physiological noise, PN). One
of the known dominant sources of PN is cardiac motion.
4 This study
demonstrates that double-gated ASL
5 significantly reduces PN in the presence
of heart rate (HR) variation, and provides increased temporal SNR (tSNR=MBF/PN)
compared to the more widely used single-gated ASL approach.
1-4Methods
Our Institutional
Review Board approved the study protocol, and all subjects provided written
informed consent. Ten healthy subjects (n=10, 3F/7M, age 23-34) and 3 heart
transplant recipients (n=3, 3M, age 26-63) were enrolled in the study. All
experiments were performed at 3T (Signa Excite HDxt, GE Healthcare). At rest, single-gating
and double-gating flow-sensitive alternating inversion recovery (FAIR) ASL were
performed for 3 min each (1 5-second breath-hold for baseline and labeling check,
and 6 12-second breath-holds for control and tagged image pairs). The order of
the two scans was randomized. Imaging parameters were: FOV=180-280 mm, slice
thickness = 10 mm, flip angle = 50º, matrix size = 96x96, and GRAPPA
factor =1.6.6 HR was recorded in all scans, and HR variation was defined as the
average absolute difference between the instantaneous HR when control and
tagged images were acquired.
Left
ventricular myocardium was manually segmented for global and per-segment (6
segments) analysis. Single-gated MBF was calculated as previously described.4
Double-gated MBF was quantified using the same equation with interpolated
signal difference from T1 curve fits as previously described.5 Both
single-gated and double-gated PN were calculated identically, as the standard
deviation of 6 measurements from 6 breath-holds.1 Paired Student’s t-test was
used to assess statistical difference between two methods. P-values < 0.05 were
considered statistically significant. Results are reported as mean ± SD.
Results
Figure 1 illustrates
the advantage of double-gating compared to single-gating when HR variation >
4 bpm. Double-gating was insensitive to HR variation (PN did not depend on HR
variation). In contrast, single-gating PN increased with HR variation.
The data was divided
into two sub-groups: (1) low HR variation (n=7, HRV ≤ 4 bpm) and (2) high HR
variation (n=6, HRV > 4 bpm). Table 1
compares double-gating and single-gating from the two sub-groups. Double-gating
provided a statistically significant increase in tSNR compared to single-gating
in both groups. Double-gating PN was significantly reduced in the high HR
variation group (p < 0.001).
Figure 2 compares
global MBF and PN. No significant
difference was found in measured MBF between the two methods (p = 0.65).
Double-gating shows significant reduction in PN compared to single-gating (p =
0.04). Group mean ± SD of per-segment MBF, PN, and tSNR from the two methods from
all subjects are listed in Table 2. Double-gating
demonstrates significant reduction in per-segment PN (p<0.001) and increase
in per-segment tSNR (p=0.003) compared to the reference single-gating without statistically
significant difference in measured MBF between the two methods (p=0.90).
Conclusions
This study demonstrates
that double-gating myocardial ASL is robust with respect to HR variation compared
to single-gating, with the same scan time. This is expected to be valuable for
stress testing (physiological or pharmacologic stress).
Acknowledgements
L.K. Whittier
Foundation #0003457-00001; American Heart Association 13GRNT13850012; Wallace
H. Coulter Foundation Clinical Translational Research Award.References
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4. Do HP et al., J Cardiovasc Magn Reson. 2014; 16:15.
5. Poncelet BP et
al., Magn Reson Med. 1999; 41:510-19.
6. Jao TR et al., Proc. SCMR 2016
(accepted)