Effects of the reference tissue setting on the parametric image of 11C-WAY 100635

C-WAY 100635 has been used widely in the PET research field to measure 5-HT1A receptors in the living human brain. Reference tissue model analysis, in which the cerebellum was used as the reference tissue, was employed in clinical studies. However, the reported binding potentials varied greatly among the reports. In this study, regions of interest (ROIs) of the cerebellum for C-WAY 100635 were determined in five different approaches for six healthy male volunteers, and the effects on binding potential parametric images were compared.

First, ROIs on the cerebellar cortex were decided based on information from magnetic resonance imaging (MRI), and then divided into upper and lower parts. They were then refined according to coregistered positron emission tomography (PET) image information, generating upper-refined and lower-refined parts. In a different approach, circular ROIs were decided upon, based on the PET images, and the areas under the curve (AUCs) of five ROIs were compared. Parametric images of binding potentials were calculated with the five ROIs as the reference, and compared with each other.

The AUCs of the lower-refined parts were the lowest among the first four ROIs. The ratio of the AUCs between the upper part and lower-refined part was 1.54+/-0.17, that between the upper-refined part and lower-refined part was 1.27+/-0.21, that between the lower part and lower-refined part was 1.04+/-0.02, and that between the circular ROIs and lower-refined part was 1.01+/-0.04. The differences in AUCs among the five cerebellar parts led to significant differences in the binding potential parametric images.

The binding potential parametric images of C-WAY100635 could vary significantly according to the different methods of establishing ROIs, using the cerebellum as the reference tissue, because radioactivity in the cerebellum for C-WAY100635 is very low.