FIG 7. Average CBF values of gray matter with both CT and PET against varying VPE threshold values in subject 1. Without VPE (plotted at the most right-hand side, threshold of 104 mL/100 g), the CT-CBF value is overestimated. As the VPE threshold decreases, CT-CBF values decrease and approach the corresponding PET values. * indicates a statistically significant difference (P < .01)