Cerebral blood flow measured by stable xenon enhanced computerized tomography compared to H215O positron emission tomography

Background and aims: Both the vasodilatory effect of stable xenon and the limited blood brain barrier permeability of H215O on cerebral blood flow (CBF) measured by CT and PET, respectively, have been extensively studied with a general consensus that PET CBF is the gold standard for comparison (1). Methods: We measured CBF by stable xenon-CT (Xe/CT) (28% xenon in 40% oxygen for 4.5 min) and within 24 hrs, by dynamic PET with H215O in 12 symptomatic stroke patients which allowed comparison of the CBF values using both Methods: in the major cerebral vascular territories, white matter, and basal ganglia in 4 axial slices in each patient. Results: The linear regression relationship between PET CBF and Xe/CT CBF was described by the equation: PET CBF = 0.608 (Xe/CT CBF) + 3.204, R=0.693, P<0.001, N=144) showing that Xe/CT CBF overstated CBF relative to PET CBF by 40%. However, Herscovitch et al (2) reported that paired CBF measurements by PET using H215O and 11C-butanol showed a linear regression relationship described by the equation: Butanol CBF = 1.237 (H215O CBF) -0.830, R = 0.989, N=18, P<0.0001. Thus, H215O CBF understated 11C-butanol CBF by 24%. Correcting for the 24% understatement of PET H215O CBF, PET CBF understates Xe/CT CBF by approximately 27% (Fig.1). Conclusions: In light of studies showing good correlation between Xe/CT CBF and CBF by the Kety-Schmidt nitrous oxide method (3) and iodoantipyrine (4), it is likely that the difference between PET CBF and Xe/CT CBF is due to a combination of partial volume effects, water permeability surface area product and some flow activation by xenon.