Assessment of brain tissue viability in acute ischemic stroke by BOLD MRI

The introduction of new neuroprotective treatment strategies for acute stroke patients has provided a requirement for neuroimaging methods capable of identifying salvageable tissue in acute stroke patients. Substantial positron emission tomography evidence points to the fact that a peri-infarct zone with blood flow of 20-45% of normal, metabolic rate of oxygen of >35% of normal and oxygen extraction ratio (OER) of >0.7 are indices of tissue at risk of infarction, yet with potential for recovery. The sensitivity of T(2) to blood oxygen level dependent (BOLD) effects allows the mismatch between oxygen delivery and consumption in the brain to be imaged. Previous evidence from animal models of cerebral hypoperfusion and ischemic stroke strongly suggest that T(2) BOLD MRI highlights viable and salvageable brain regions. The Hahn-echo T(2) and diffusion show distinct flow thresholds in the rat brain so that the former parameter probes areas with high OER and the latter genuine ischemia. In the flow-compromised tissue showing negative T(2) BOLD, substantial residual perfusion is evident as revealed by bolus-tracking perfusion MRI, in agreement with the idea that tissue metabolic viability must be preserved for expression of BOLD. It is concluded that BOLD MRI may have potential for the assessment of tissue viability in acute ischemic stroke.