Quantitative ¹⁷O MRSI of myocardial oxygen metabolic rate, blood flow, and oxygen extraction fraction under normal and high workload conditions

Purpose: The heart is a highly aerobic organ consuming most of the oxygen the body in supporting heart function. Quantitative imaging of myocardial oxygen metabolism and perfusion is essential for studying cardiac physiopathology in vivo. Here, we report a new imaging method that can simultaneously assess myocardial oxygen metabolism and blood flow in the rat heart. Methods: This novel method is based on the ¹⁷O-MRSI combined with brief inhalation of ¹⁷O-isotope labeled oxygen gas for quantitative imaging of myocardial metabolic rate of oxygen consumption (MVO₂), myocardial blood flow (MBF), and oxygen extraction fraction (OEF). We demonstrate this imaging method under basal and high workload conditions in rat hearts at 9.4 T. Results: We show that this ¹⁷O MRSI–based approach can directly measure and image MVO₂ (1.35–4.06 μmol/g/min), MBF (0.49–1.38 mL/g/min), and OEF (0.33–0.44) in the heart of anesthetized rat under basal and high workload (21.6 × 10³–56.7 × 10³ mmHg • bpm) conditions. Under high workload condition, MVO₂ and MBF values in healthy rats approximately doubled, whereas OEF remained unchanged, indicating a strong coupling between myocardial oxygen metabolic demand and supply through blood perfusion. Conclusion: The ¹⁷O-MRSI method has been used to simultaneously image the myocardial metabolic rate of oxygen consumption, blood flow, and oxygen extraction fraction in small animal hearts, which are sensitive to the physiological changes induced by high workload. This approach could provide comprehensive measures that are critical for studying myocardial function in normal and diseased states and has a potential for translation.