Measurement of the Ischemic Penumbra With MRI: It’s About Time
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The idea that there exist 2 ischemic thresholds in the pathogenesis of cerebral infarction came from seminal microelectrode studies of the baboon cortex the late 1970s1,2 that measured the effects of progressive reductions in cerebral blood flow (CBF). These studies described a level of CBF reduction that led to cessation of cortical evoked responses in the absence of terminal increases in extracellular potassium or reductions in pH and a yet lower level of CBF reduction, at which occurred large increases in extracellular potassium and reductions in pH indicative of failure of membrane ion homeostasis and cell death.
Derived from animals too few and results too variable to specify a precise threshold, the insight nonetheless emerged that there were 2 levels of ischemia, one for tissue dysfunction without destruction and a lower one for irreversible cell injury. The metaphor of the ischemic penumbra was coined to describe this intermediate zone of ischemia between functionally normal and dead brain tissue. Restoration of normal CBF to the penumbral …