Detecting Myocardial Ischemia at Rest with Cardiac Phase-Resolved BOLD MRI: Early Findings

Introduction: Vasodilatory stress is the standard paradigm for probing myocardial oxygenation (O2) changes due to coronary artery stenosis on the basis of BOLD MRI (1-3). However, since vasodilation is typically achieved with provocative stress, approaches that can identify the presence of stenosis on the basis of microvascular alterations at rest are highly desirable. It is known that myocardial blood volume (MBV) varies throughout the cardiac cycle; MBV increases during diastole and decreases during systole (4,5). It has also been shown that changes in MBV lead to increased O2 extraction by cardiomyocytes (6). Thus, MBV and O2 are expected to vary at different parts of the cardiac cycle. In particular, in diastole, it is expected that MBV and O2 extraction are maximal, while in systole, MBV and O2 extraction are minimal. In addition, as MBV increases, even at a stable level of O2, the number of deoxygenated hemoglobin molecules within a voxel increases, causing a proportionate elevation in the local magnetic field inhomogeneities (7). Moreover, with increasing grade of stenosis, the MBV in the myocardial territory supplied by a stenotic artery increases in systole (8-11). Thus, the relative MBV and O2 changes between systole and diastole are expected to be different between myocardial territories supplied by healthy and stenotic coronary arteries. Moreover, it is also known that T1 of myocardium is dependent on MBV and that the apparent T2 is dependent on blood O2. Since SSFP signals are approximately T2/T1 weighted, it is hypothesized that cardiac phase-resolved BOLD SSFP (CP-BOLD) (12) signal intensities at systole and diastole may reflect changes in MBV and blood O2. In addition, since stenosis leads to an increase in systolic MBV and is accompanied by a reduction in blood O2, it is hypothesized that systolic and diastolic CP-BOLD signal intensities may be used to detect the ischemic territories at resting states. These hypotheses were tested with simulations and canine experiments.