FIG 2. Vascular maps and CT-CBF images obtained in subject 1 at various levels of VPE threshold. On the vascular maps, gray matter is displayed in green, and white matter in dark blue. The separation between gray matter and white matter was performed on the basis of the pixel attenuation on nonenhanced scans. Vascular pixels are expressed in red. The extent of the vascular area increases as the level of VPE threshold decreases. VPE threshold is indicated below the CT-CBF images. If the VPE threshold is very small (such as 5 mL/100 g), part of the brain parenchyma also disappears from the CT-CBF image.