Two Flavors of Ischemia...

That's right. Brain ischemia comes in two flavors...more or less.

Focal brain ischemia is your basic stroke. In focal ischemia, something has occluded a cerebral vessel, like a blood clot, gas bubble, or the wire you let go of when you tried to hit that guy's internal jugular but cannulated his carotid instead. Such an arterial occlusion results in brain ischemia distal to the site of the obstruction, characterized by a central core of dense ischemia (which is almost certainly going to die) and a penumbra of less-densely ischemic tissue (which may be salvageable).

This is an ex-rat. Section of rat brain subjected to focal ischemia stroke. Pale area on the left indicates necrotic brain. Immediately after the occlusion that led to the stroke, much of this area may have been salvageable (penumbra). But it's too late now.