To avoid arterial blood sampling and complicated analyses in 15O-gas PET studies, we evaluated a noninvasive technique using the count-based method for measuring asymmetric increases in oxygen extraction fraction (OEF) in cerebrovascular disease.
Methods: Eighteen patients (mean age +/- SD, 61 +/- 16 y) with atherothrombotic large-cerebral-artery disease were studied for the measurement of hemodynamic parameters using the 15O-gas steady-state method with inhalation of 15O2, C15O2, and C15O. All patients also underwent H2(15)O PET with the bolus injection method. Count-based ratio images of 15O2/C15O2 and (15)O2/H2(15)O were calculated, and asymmetry indices (AIs) were obtained (cbOEF(SS)-AI and cbOEF(BO)-AI, respectively) using regions of interest drawn bilaterally on the cerebral cortices. These AIs were compared with the AIs of absolute OEF (qOEF-AI) and with those after cerebral blood volume (CBV) correction. A contribution factor for this correction was defined as a variable alpha, and the effect of the correction was evaluated.
Results: cbOEF(SS)-AI underestimated qOEF-AI significantly, especially with a greater AI (P < 0.05). cbOEF(BO)-AI linearly correlated well with qOEF-AI. CBV correction improved the slopes of regression lines between qOEF-AI and cbOEF(SS)-AI, and the optimal alpha was defined as 0.5. On the other hand, cbOEF(BO)-AI fairly estimated qOEF-AI without CBV correction. Correlation between qOEF-AI and cbOEF(BO)-AI was adversely affected, and the mean bias was increased, with a greater alpha.
Conclusion: cbOEF(BO)-AI can fairly estimate the AI of OEF without CBV correction, whereas cbOEF(SS)-AI might require CBV correction for better estimation. The examination time and stress to patients would be reduced with the count-based method because it is noninvasive.