Quantitative assessment of cerebral blood volume by single-photon emission computed tomography. We implemented a technique for measuring regional cerebral blood volume using single-photon emission computed tomography and in vivo technetium-99m-labeled red blood cells and then evaluated it in nine normal human volunteers (controls) and seven patients with bilateral occlusion or severe stenosis of the internal carotid artery. We also measured regional cerebral blood flow using single-photon emission computed tomography and intravenous xenon-133 in the same subjects. We studied regional cerebral blood flow, regional cerebral blood volume, and their ratio before and after the intravenous injection of 1 g acetazolamide. Mean +/- SD baseline regional cerebral blood volume was higher in the patients than in the controls (4.1 +/- 0.6 versus 3.2 +/- 0.3 ml/100 g, p less than 0.01), and mean +/- SD baseline regional cerebral blood flow was lower in the patients than in the controls (40.5 +/- 11 versus 55.6 +/- 11 ml/100 g/min, p less than 0.05). Acetazolamide induced similar mean +/- SD increases in regional cerebral blood volume in both the controls and the patients (0.3 +/- 0.1 and 0.3 +/- 0.2 ml/100 g), while the mean +/- SD regional cerebral blood flow reactivity was significantly less in the patients than in the controls (12.6 +/- 7.6 versus 24.5 +/- 9.6 ml/100 g/min, p less than 0.05). Our study shows that single-photon emission computed tomography can provide quantitative estimates of both regional cerebral blood volume and regional cerebral blood flow in humans. 