Quantification of Regional Myocardial Oxygenation by Magnetic Resonance Imaging: Validation with Positron Emission Tomography

Adequate oxygenation is fundamental to myocardial health. The balance of oxygen supply (myocardial blood flow [MBF]) and oxygen demand (myocardial oxygen consumption [MVO2]) is the central pathological tenet of myocardial ischemia. Conversely, luxuriant MVO2 compared to level of cardiac work is indicative of reduced cardiac efficiency, a common finding in most forms of heart failure.1 Therefore, accurate measurements of the key components of myocardial oxygenation, including MBF, MVO2, and fractional oxygen extraction (OEF), are needed to better understand the pathophysiology of these various disease processes. This knowledge may lead to new diagnostic strategies and could facilitate the evaluation of therapies designed to improve oxygen supply/demand imbalances or improve energy transduction. To date, positron emission tomography (PET) has been used to provide these measurements, with 15O-water to measure MBF,2 and 11C-acetate to determine MVO2.3 However, ionizing radiation, long scan times, limited availability, requirement of an on-site cyclotron, and high costs have limited PET’s widespread use for these purposes. Because of the physical half-life of the radiotracers and concerns about radiation exposure, measurements are typically limited to only two time points, such as rest and low-level catecholamine stress. Additionally, PET cannot assess myocardial blood volume (MBV), another important parameter of myocardial oxygen supply.4 In contrast, magnetic resonance imaging (MRI) does not use ionizing radiation, requires relatively short scan times, is more readily available and at lower cost, and is relatively fast and reproducible. Thus, it is optimal for serial assessments of myocardial oxygenation. We have developed a variety of MRI techniques for evaluating MBF5–6, MBV7, OEF8–9, and MVO2 10 through a combination of first-pass perfusion and blood-oxygen-level-dependent (BOLD) methods. In this study, we utilized these techniques to evaluate the regional changes in myocardial perfusion and oxygenation induced by an acute coronary artery stenosis at rest and during pharmacologic hyperemia. The MRI measurements were validated against those obtained by PET imaging. Dipyridamole and dobutamine were used in dogs with or without moderate to severe coronary artery stenosis to induce a wide range of myocardial perfusion and oxygenation changes.