For how long is brain tissue salvageable? Imaging-based evidence.

During the acute phase shortly after the onset of an ischemic stroke, tissue in the penumbra surrounding an infarct receives sufficient blood flow to survive, but not enough to function. As time passes, neurons in this penumbra die. Imaging techniques have given valuable information about the length of time that brain cells can survive under these ischemic conditions. 15O positron-emission tomography (PET) scanning gives information about perfusion of tissue, its oxygen consumption, and its oxygen extraction fraction. Tissue in the penumbra has a reduced blood flow, near normal oxygen consumption, but markedly raised oxygen extraction fraction. With the use of a set of rigorous criteria, PET scanning has provided evidence that, in a fraction of the patients, a penumbra of viable, potentially salvageable neurons exists for at least 7 hours, and possibly for as long as 16 hours, after the onset of ischemic stroke, whereas in others the infarct reaches its maximal extent only a few hours after clinical onset. Recent developments in magnetic resonance imaging (MRI) technology, especially diffusion-weighted and perfusion-weighted imaging (DWI and PWI), also have enabled potentially salvageable penumbral tissue to be identified in patients who have suffered ischemic strokes. The typical signature of salvageable tissue is that it has a PWI-DWI mismatch. This type of MRI evidence shows that there may be salvageable tissue as late as 24 hours after the onset of symptoms.