The relationship between the apparent diffusion coefficient measured by magnetic resonance imaging, anoxic depolarization, and glutamate efflux during experimental cerebral ischemia

A reduction in the apparent diffusion coefficient (ADC) of water measured b y magnetic resonance imaging (MRI) has been shown to occur early after cere brovascular occlusion. This change may be a useful indicator of brain tissu e adversely affected by inadequate blood supply. The objective of this stud y was to test the hypothesis that loss of membrane ion homeostasis and depo larization can occur simultaneously with the drop in ADC. Also investigated was whether elevation of extracellular glutamate ([GLU](e)) would occur be fore ADC changes. High-speed MRI of the trace of the diffusion tensor (15-s econd time resolution) was combined with simultaneous recording of the extr acellular direct current (DC) potential and on-line [GLU](e) from the stria tum of the anesthetized rat. After a control period, data were acquired dur ing remote middle cerebral artery occlusion fur 60 minutes, followed by 30 minutes of reperfusion, and cardiac arrest-induced global ischemia. After e ither focal or global ischemia, the ADC was reduced by 10 to 25% before ano xic depolarization occurred. After either insult, the time for half the max imum change in ADC was significantly shorter than the corresponding DC pote ntial parameter (P < 0.05). The [GLU](e) remained at low levels during the entire period of varying ADC and DC potential and did not peak until much l ater after either ischemic insult. This study demonstrates that ADC changes can occur before membrane depolarization and that high [GLU](e) has no inv olvement in the early rapid ADC decrease.