Rapid monitoring of diffusion, DC potential, and blood oxygenation changesduring global ischemia - Effects of hypoglycemia, hyperglycemia, and TTX

Background and Purpose-The increasing interest in diffusion-weighted MRI (M RI) for diagnosis and monitoring of acute stroke in humans calls for a soun d understanding of the underlying mechanisms of this image contrast in acut e cerebral ischemia. The present study aimed to show that a rapid decrease in brain-water apparent diffusion coefficient (ADC) occurs coincident with anoxic depolarization and that this change is delayed by hyperglycemia and sodium channel blockade but accelerated by hypoglycemia, Methods-Rats were divided into groups: normoglycemic, hypoglycemic, and hyp erglycemic, and those given local tetrodotoxin (TTX) application. Cardiac a rrest was effected by intravenous KCl injection during serial high-speed di ffusion and blood oxygenation-sensitive gradient-recalled echo MRI. Brain D C potential was recorded simultaneously. Serial ADC maps were calculated fr om the diffusion-weighted data and fitted to a model function to measure th e delay between cardiac arrest and rapid ADC decrease. Results-The time of anoxic depolarization indicated by DC change agreed wel l with the rapid drop in ADC in all groups; both were accelerated with hypo glycemia and delayed by hyperglycemia. A more gradual ADC decline occurred before anoxic depolarization, which was more pronounced in hyperglycemic an imals and less pronounced in hypoglycemic animals. Rapid drop in ADC was al so delayed by local TTX application. Changes in gradient-recalled echo imag e intensity were not significantly different among groups. Conclusions-While much of the ADC decrease in ischemia occurs during anoxic depolarization, significant but gradual ADC changes occur earlier that may not be due to a massive loss in ion homeostasis.