Myocardial arterial spin labeling (ASL) is an emerging technique for the assessment of myocardial perfusion. ASL does not require any contrast agents, and therefore can be used in patients with renal failure. We recently demonstrated feasibility of myocardial ASL . In this study, we applied myocardial ASL to measure increased MBF during adenosine infusion.
Eleven patients were recruited, among those scheduled for routine cardiac MRI. All experiments were performed on a GE Signa 3 T scanner. Myocardial ASL measurements were obtained from a single mid short-axis slice, using flow-sensitive alternating inversion recovery (FAIR) tagging and balanced steady-state free precession (SSFP) imaging . In each patient, one or two scans were performed at rest, and one scan was performed during adenosine infusion (dosage: 0.14 mg/kg/min). All patients also underwent first-pass perfusion imaging before adenosine infusion stopped (Table 1).
Table 1. Excerpt from the imaging protocol
CMR first-pass images were read by two experienced cardiologists, and resulted in seven of the eleven subjects classified as "normal", based on having no visible perfusion defects. Fig. 1 contains the average MBF measurements of six myocardial segments (each segment size = 1.5 ± 1.0 cm3) from ASL in these "normal" subjects. The average MBF across subjects was 1.09 ± 0.53 ml/g/min at rest and 3.75 ± 1.06 ml/g/min during adenosine infusion, showing 4.97 ± 4.64 times increase. The increase in MBF was statistically significant (P < 0.001, t-test).
Figure 1. MBF measurements using ASL and heart rates (HR, in bpm) at rest and during adenosine infusion, from "normal" patients.
The adenosine-induced MBF increase in normal myocardium has been documented to be 4.00 ± 1.10 times based on 15O-H2O PET . The present study has demonstrated that myocardial ASL is able to capture these dramatic MBF changes. This study is on-going, and as we obtain more subjects with angiographically-significant disease and/or perfusion defects on first-pass imaging, we will be able to evaluate the ability of rest-stress myocardial ASL to detect perfusion defects. Fig. 2 contains segment-based MBF measurements using ASL and coronary angiogram from our first patient with single-vessel disease. Lowered MBF increase on anterior wall may be due to total occlusion of left anterior descending (LAD) artery.
Figure 2. Left: MBF measurements using ASL for each myocardial segment. Right: Coronary angiogram with total LAD occlusion (arrow) from the same patient.
MRM. 2009, in press.