The hypothesis was tested that hypoperfused brain regions, such as the ischemic penumbra, are detectable by reductions in absolute transverse relaxation time constant (T₂) using magnetic resonance imaging (MRI). To accomplish this, temporal evolution of T₂ was measured in several 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 = 1/3Trace [double overline] D, while perfusion was assessed by dynamic contrast imaging. Three types of regions at risk of infarction could be distinguished: (1) areas with reduced T₂ (4% to 15%, all figures relative to contralateral hemisphere) and normal D_av, corresponding to hypoperfusion without ischemia; (2) areas with both reduced T₂ (4% to 12%) and D_av (22% to 49%), corresponding to early hypoperfusion with ischemia; (3) areas with increased T₂ (2% to 9%) and reduced D_av (28% to 45%), corresponding to irreversible ischemia. In the first two groups, perfusion-deficient regions detected by bolus tracking were similar to those with initially reduced T₂. In the third group, bolus tracking showed barely detectable arrival of the tracer in the region where D_av was reduced. We conclude that T₂ reduction in acute ischemia can unambiguously identify regions at risk and potentially discriminate between reversible and irreversible hypoperfusion and ischemia.

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Noninvasive Detection of Cerebral Hypoperfusion and Reversible Ischemia from Reductions in the Magnetic Resonance Imaging Relaxation Time,T 2