FIG 1. Positron emission tomographic studies show reduced cerebral blood flow (left image) and increased oxygen extraction (middle image) in the cerebral hemisphere distal to an occluded carotid artery. When autoregulatory vasodilation is insufficient to maintain normal cerebral blood flow (CBF, far-left image), CBF will fall (white arrows). The brain, however, can increase the amount of oxygen extracted from the blood (oxygen extraction fraction) (middle image, white arrows), to maintain normal oxygen metabolism and function (cerebral rate of metabolism for oxygen [CMRO₂], far-right image). FIG 2. Regions of interest for superficial (black boxes) and deep white matter (black circles) oxygen extraction fraction measurements. Each region was actually a 15-mm sphere with its center in the center of the box or circle depicted in the figure. All positron emission tomographic data were converted to Talairach and Tournoux atlas space for the reproducible placement of these regions (18). The y coordinates for each coronal slice are shown beneath