Evaluation of MR-Derived Cerebral Oxygen Metabolic Index in Experimental Hyperoxic Hypercapnia, Hypoxia, and Ischemia

Abstract

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 (O 2 Sat MRv ) with blood gas oximetry measurements in the jugular vein (O 2 Sat JV ) and superior sagittal sinus (O 2 Sat SSS ). 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 O 2 Sat MRv and O 2 Sat JV /O 2 Sat SSS 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 O 2 Sat MRv 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.