Potential application of ¹⁷O MRI to human ischemic stroke

In cerebral ischemia, measurement of cerebral blood flow (CBF) alone is not a sensitive or specific predictor of tissue survival. Measurements of oxygen metabolism, which are directly related to cellular energy metabolism, are better predictors of tissue survival and the best of these is the "oxygen extraction fraction" (OEF). Elevation of OEF in Stage 2 hemodynamic failure, or "misery" perfusion, indicates that prolongation of this state or further reduction in blood flow will lead to failure of oxygen metabolism and cellular necrosis,making it a sensitive and specific biomarker for the "ischemic penumbra" and a predictor of impending cerebral infarction. The methods now used to measure in vivo human cerebral metabolic rate of oxygen (CMRO(2)) and OEF include (15)O-PET and MRI deoxyhemoglobin sensitive techniques (Blood Oxygen Level Dependent, BOLD methods). These methods have practical and fundamental limitations for use in the clinical stroke setting. (17)O-MRI is a method of imaging oxygen metabolism by detecting the tissue water (H(17) (2) O) produced by oxidative metabolism of (17)O(2) gas that can be performed on conventional, clinical MRI scanners using a chemically stable, non-radioactive, MR-detectable isotope of oxygen. It is more logistically applicable to clinical stroke than 15O-PET and more directly quantitative than BOLD MRI. (17)O-MRI promises to provide a direct, quantitative, widely available and clinically practical method for assessing CMRO(2) and OEF for evaluation of human cerebral ischemia.