NONINVASIVE DETECTION OF CEREBRAL HYPOPERFUSION AND REVERSIBLE ISCHEMIA FROM REDUCTIONS IN THE MAGNETIC-RESONANCE-IMAGING RELAXATION-TIME, T-2

The hypothesis was tested that hypoperfused brain regions, such as the ischemic penumbra, are detectable by reductions in absolute transvers e relaxation time constant (T-2) using magnetic resonance imaging (MRI ). To accomplish this, temporal evolution of T-2 was measured in sever al models of hypoperfusion and focal cerebral ischemia in the rat at 9 .4 T. Occurrence of acute ischemia was determined through the absolute diffusion constant D-av = 1/3TraceD, while perfusion was assessed by dynamic contrast imaging. Three types of regions at risk of infarction could be distinguished: (1) areas with reduced T-2 (4% to 15%, all fi gures relative to contralateral hemisphere) and normal D-av, correspon ding to hypoperfusion without ischemia; (2) areas with both reduced T- 2 (4% to 12%) and D-av (22% to 49%), corresponding to early hypoperfus ion with ischemia; (3) areas with increased T-2 (2% to 9%) and reduced D-av (28% to 45%), corresponding to irreversible ischemia. In the fir st two groups, perfusion-deficient regions detected by bolus tracking were similar to those with initially reduced T-2. In the third group, bolus tracking showed barely detectable arrival of the tracer in the r egion where D-av was reduced. We conclude that T,reduction in acute is chemia can unambiguously identify regions at risk and potentially disc riminate between reversible and irreversible hypoperfusion and ischemi a.