Evaluation of MR-derived cerebral oxygen mjetabolic index in experimental hyperoxic hypercapnia, gypoxia, and ischemia

BACKGROUND AND PURPOSE-: A noninvasive MRI method to measure cerebral oxygen metabolism has the potential to assess tissue viability during cerebral ischemia. The purposes of this study were to validate MR oxygenation measurements across a wide range of global cerebral oxygenation and to examine the spatiotemporal evolution of oxygen metabolism during focal middle cerebral artery occlusion in rats. METHODS-: A group of rats (n=28) under normal, hyperoxic hypercapnia and hypoxia were studied to compare MR-measured cerebral oxygen saturation (O2SatMRv) with blood gas oximetry measurements in the jugular vein (O2SatJV) and superior sagittal sinus (O2SatSSS). In a separate group of rats (n=31), MR-measured cerebral oxygen metabolic index (MR-COMI) was acquired at multiple time points during middle cerebral artery occlusion. Histogram analysis was performed on the normalized MR-COMI (rMR-COMI) to examine evolution of oxygen metabolism during acute ischemia. RESULTS-: Highly linear relationships were obtained between O2SatMRv and O2SatJV/O2SatSSS in rats under global cerebral oxygenation alterations. In the focal ischemia study, rMR-COMI values were significantly lower within the areas of eventual infarction than other regions. Moreover, the rMR-COMI values within the ischemic territory decreased with time, concomitant with an increase in the number of voxels with severely impaired oxygen metabolism. CONCLUSION-: Accurate estimates of O2SatMRv can be obtained across a broad and physiologically relevant range of cerebral oxygenation. Furthermore, this method demonstrates a dynamic alteration of cerebral oxygen metabolism during acute ischemia in rats.