fig 4.

Positron emission tomographic and CT scans.

A, Example of subjects with normal (left) and increased (right, white arrows) oxygen extraction fraction on positron emission tomographic sections at the level of the centrum semiovale. Left and right on positron emission tomographic scans are reversed relative to CT or MR convention. Note the uniformity of oxygen extraction fraction values across the image on the left, compared with the hemispheric increase in oxygen extraction fraction seen on the image on the right. Oxygen extraction fraction values in the centrum semiovale are not quantitatively or qualitatively greater than those in the more superficial regions (white arrows).

B, Corresponding CT scan of patient 30 (Table 1). This patient presented with two episodes of profound left-sided weakness that occurred 10 days apart. These symptoms had nearly completely resolved except for a mild pronator drift at the time of the CT and positron emission tomographic examinations. Angiography showed complete occlusion of the right internal carotid artery at its origin and moderate 25% to 50% stenosis of the contralateral carotid siphon.