Cardiac ¹⁷O MRI: Toward direct quantification of myocardial oxygen consumption

A new ¹⁷O-labeled blood contrast agent was injected intravenously in control dogs. Electrocardiogram (ECG)-triggered myocardial T_1ρ imaging was performed to obtain spin-locking T _1ρ-weighted myocardial signals for the detection of resultant metabolite H₂¹⁷O water in the heart. Bolus and slow injection methods of various doses of the ¹⁷O-labeled and ¹⁶O-labeled agents were carried out in order to evaluate the sensitivity of this method and determine the optimal injection method. Bolus injection provided approximately 1% signal reduction, whereas slow injection with larger amount of agent yielded 11.9 ± 0.6% signal reduction. Myocardial oxygen consumption rate was determined by a technique to quantify cerebral oxygenation consumption rate previously developed in ¹⁷O brain studies. With either injection method, myocardial oxygen consumption rate at rest was 5.0 - 5.6 μmol/g/min. Therefore, it appears feasible to detect metabolically generated H₂¹⁷O water in vivo in the heart, using the ¹⁷O-labeled blood tracer. Myocardial oxygen consumption rate can then be quantified in vivo, which may open new doors for the assessment of myocardial metabolism.