Optimal repetition time for free breathing myocardial arterial spin labeling

Comparison of Myocardial Blood Flow Quantification Models for Double ECG Gating Arterial Spin Labeling MRI: Reproducibility Assessment

BACKGROUND

Arterial spin labeling (ASL) allows non-invasive quantification of myocardial blood flow (MBF). Double-ECG gating (DG) ASL is more robust to heart rate variability than single-ECG gating (SG), but its reproducibility requires further investigation. Moreover, the existence of multiple quantification models hinders its application. Frequency-offset-corrected-inversion (FOCI) pulses provide sharper edge profiles than hyperbolic-secant (HS), which could benefit myocardial ASL.

PURPOSE

To assess the performance of MBF quantification models for DG compared to SG ASL, to evaluate their reproducibility and to compare the effects of HS and FOCI pulses.

STUDY TYPE

Prospective.

SUBJECTS

Sixteen subjects (27 ± 8 years).

FIELD STRENGTH/SEQUENCE

1.5 T/DG and SG flow-sensitive alternating inversion recovery ASL.

ASSESSMENT

Three models for DG MBF quantification were compared using Monte Carlo simulations and in vivo experiments. Two models used a fitting approach (one using only a single label and control image pair per fit, the other using all available image pairs), while the third model used a T1 correction approach. Slice profile simulations were conducted for HS and FOCI pulses with varying B0 and B1. Temporal signal-to-noise ratio (tSNR) was computed for different acquisition/quantification strategies and inversion pulses. The number of images that minimized MBF error was investigated in the model with highest tSNR. Intra and intersession reproducibility were assessed in 10 subjects.

STATISTICAL TESTS

Within-subject coefficient of variation, analysis of variance. P-value <0.05 was considered significant.

RESULTS

MBF was not different across acquisition/quantification strategies (P = 0.27) nor pulses (P = 0.9). DG MBF quantification models exhibited significantly higher tSNR and superior reproducibility, particularly for the fitting model using multiple images (tSNR was 3.46 ± 2.18 in vivo and 3.32 ± 1.16 in simulations, respectively; wsCV = 16%). Reducing the number of ASL pairs to 13/15 did not increase MBF error (minimum = 0.22 mL/g/min).

DATA CONCLUSION

Reproducibility of MBF was better for DG than SG acquisitions, especially when employing a fitting model.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 1.

Improved reproducibility for myocardial ASL: Impact of physiological and acquisition parameters

PURPOSE

To investigate and mitigate the influence of physiological and acquisition-related parameters on myocardial blood flow (MBF) measurements obtained with myocardial Arterial Spin Labeling (myoASL).

METHODS

A Flow-sensitive Alternating Inversion Recovery (FAIR) myoASL sequence with bSSFP and spoiled GRE (spGRE) readout is investigated for MBF quantification. Bloch-equation simulations and phantom experiments were performed to evaluate how variations in acquisition flip angle (FA), acquisition matrix size (AMS), heart rate (HR) and bloodT 1 $$ {\mathrm{T}}_1 $$ relaxation time (T 1 , B $$ {\mathrm{T}}_{1,B} $$ ) affect quantification of myoASL-MBF. In vivo myoASL-images were acquired in nine healthy subjects. A corrected MBF quantification approach was proposed based on subject-specificT 1 , B $$ {\mathrm{T}}_{1,B} $$ values and, for spGRE imaging, subtracting an additional saturation-prepared baseline from the original baseline signal.

RESULTS

Simulated and phantom experiments showed a strong dependence on AMS and FA (R 2 $$ {R}^2 $$ >0.73), which was eliminated in simulations and alleviated in phantom experiments using the proposed saturation-baseline correction in spGRE. Only a very mild HR dependence (R 2 $$ {R}^2 $$ >0.59) was observed which was reduced when calculating MBF with individualT 1 , B $$ {\mathrm{T}}_{1,B} $$ . For corrected spGRE, in vivo mean global spGRE-MBF ranged from 0.54 to 2.59 mL/g/min and was in agreement with previously reported values. Compared to uncorrected spGRE, the intra-subject variability within a measurement (0.60 mL/g/min), between measurements (0.45 mL/g/min), as well as the inter-subject variability (1.29 mL/g/min) were improved by up to 40% and were comparable with conventional bSSFP.

CONCLUSION

Our results show that physiological and acquisition-related factors can lead to spurious changes in myoASL-MBF if not accounted for. Using individualT 1 , B $$ {\mathrm{T}}_{1,B} $$ and a saturation-baseline can reduce these variations in spGRE and improve reproducibility of FAIR-myoASL against acquisition parameters.

Update on state-of-the-art for arterial spin labeling (ASL) human perfusion imaging outside of the brain

This review article provides an overview of developments for arterial spin labeling (ASL) perfusion imaging in the body (i.e., outside of the brain). It is part of a series of review/recommendation papers from the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group. In this review, we focus on specific challenges and developments tailored for ASL in a variety of body locations. After presenting common challenges, organ-specific reviews of challenges and developments are presented, including kidneys, lungs, heart (myocardium), placenta, eye (retina), liver, pancreas, and muscle, which are regions that have seen the most developments outside of the brain. Summaries and recommendations of acquisition parameters (when appropriate) are provided for each organ. We then explore the possibilities for wider adoption of body ASL based on large standardization efforts, as well as the potential opportunities based on recent advances in high/low-field systems and machine-learning. This review seeks to provide an overview of the current state-of-the-art of ASL for applications in the body, highlighting ongoing challenges and solutions that aim to enable more widespread use of the technique in clinical practice.

Non-contrast myocardial perfusion in rest and exercise stress using systolic flow-sensitive alternating inversion recovery

OBJECTIVE

To evaluate systolic flow-sensitive alternating inversion recovery (FAIR) during rest and exercise stress using 2RR (two cardiac cycles) or 1RR intervals between inversion pulse and imaging.

MATERIALS AND METHODS

1RR and 2RR FAIR was implemented on a 3T scanner. Ten healthy subjects were scanned during rest and stress. Stress was performed using an in-bore ergometer. Heart rate, mean myocardial blood flow (MBF) and temporal signal-to-noise ratio (TSNR) were compared using paired t tests.

RESULTS

Mean heart rate during stress was higher than rest for 1RR FAIR (85.8 ± 13.7 bpm vs 63.3 ± 11.1 bpm; p < 0.01) and 2RR FAIR (83.8 ± 14.2 bpm vs 63.1 ± 10.6 bpm; p < 0.01). Mean stress MBF was higher than rest for 1RR FAIR (2.97 ± 0.76 ml/g/min vs 1.43 ± 0.6 ml/g/min; p < 0.01) and 2RR FAIR (2.8 ± 0.96 ml/g/min vs 1.22 ± 0.59 ml/g/min; p < 0.01). Resting mean MBF was higher for 1RR FAIR than 2RR FAIR (p < 0.05), but not during stress. TSNR was lower for stress compared to rest for 1RR FAIR (4.52 ± 2.54 vs 10.12 ± 3.69; p < 0.01) and 2RR FAIR (7.36 ± 3.78 vs 12.41 ± 5.12; p < 0.01). 2RR FAIR TSNR was higher than 1RR FAIR for rest (p < 0.05) and stress (p < 0.001).

DISCUSSION

We have demonstrated feasibility of systolic FAIR in rest and exercise stress. 2RR delay systolic FAIR enables non-contrast perfusion assessment during stress with relatively high TSNR.

Reduction of motion effects in myocardial arterial spin labeling

PURPOSE

To evaluate the accuracy and reproducibility of myocardial blood flow measurements obtained under different breathing strategies and motion correction techniques with arterial spin labeling.

METHODS

A prospective cardiac arterial spin labeling study was performed in 12 volunteers at 3 Tesla. Perfusion images were acquired twice under breath-hold, synchronized-breathing, and free-breathing. Motion detection based on the temporal intensity variation of a myocardial voxel, as well as image registration based on pairwise and groupwise approaches, were applied and evaluated in synthetic and in vivo data. A region of interest was drawn over the mean perfusion-weighted image for quantification. Original breath-hold datasets, analyzed with individual regions of interest for each perfusion-weighted image, were considered as reference values.

RESULTS

Perfusion measurements in the reference breath-hold datasets were in line with those reported in literature. In original datasets, prior to motion correction, myocardial blood flow quantification was significantly overestimated due to contamination of the myocardial perfusion with the high intensity signal of blood pool. These effects were minimized with motion detection or registration. Synthetic data showed that accuracy of the perfusion measurements was higher with the use of registration, in particular after the pairwise approach, which probed to be more robust to motion.

CONCLUSION

Satisfactory results were obtained for the free-breathing strategy after pairwise registration, with higher accuracy and robustness (in synthetic datasets) and higher intrasession reproducibility together with lower myocardial blood flow variability across subjects (in in vivo datasets). Breath-hold and synchronized-breathing after motion correction provided similar results, but these breathing strategies can be difficult to perform by patients.

Quantification of Myocardial Perfusion With Vasodilation Using Arterial Spin Labeling at 1.5T

BACKGROUND

Myocardial perfusion is evaluated in first-pass MRI using a gadolinium-based contrast agent, which limits its repeatability and restricts its use in patients with abnormal kidney function. Arterial spin labeling (ASL) is a promising technique for measuring myocardial perfusion without contrast injection. The ratio of stress to rest perfusion, termed myocardial perfusion reserve (MPR), is an indicator of the severity of stenosis in patients with coronary artery disease (CAD).

PURPOSE

To quantify perfusion increases with pharmacological vasodilation, explore MPR differences between segments with and without perfusion defects, and examine the correlations between quantitative ASL and semiquantitative first-pass measurements.

STUDY TYPE

Prospective.

SUBJECTS

Sixteen patients with suspected CAD: 10 classified as "healthy," having normal perfusion on first-pass and no enhancement on late gadolinium enhancement (LGE), and six as "nonhealthy," having hypoperfused segments including ischemic and infarcted.

FIELD STRENGTH/SEQUENCE

Flow-sensitive alternating inversion recovery (FAIR) rest-stress cardiac ASL with balanced steady-state free precession (bSSFP), rest-stress first-pass imaging using gradient-echo and LGE using a phase-sensitive inversion-recovery bSSFP at 1.5T.

ASSESSMENT

For healthy subjects, rest-stress perfusion data were compared in global, coronary artery territory, and segment regions of interest (ROIs). A segmental MPR comparison was performed between normal segments from healthy subjects and abnormal segments from nonhealthy subjects. Correlations between ASL and first-pass parameters were explored.

STATISTICAL TESTS

Wilcoxon-signed-rank test, nonparametric factorial analysis of variance (ANOVA), and Pearson's/Spearman's correlations.

RESULTS

Perfusion increases were significant globally (P = 0.005), per coronary artery territory (P = 0.015), and per segment (P = 0.03 for all segments in ASL and first-pass, except anteroseptal in ASL P = 0.04). MPR differences between normal and abnormal segments were significant (P = 0.0028: ASL, P = 0.033: first-pass). ASL and first-pass measurements were correlated (MPR: r = 0.64, P = 0.008 and perfusion: rho = 0.47, P = 0.007).

DATA CONCLUSION

This study demonstrates the feasibility of ASL to detect hyperemia, the potential to differentiate segments with and without perfusion defects, and significant correlations between ASL and semiquantitative first-pass.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.