Similarity and robustness of PET and SPECT binding parameters for benzodiazepine receptors

The single photon emission computed tomography (SPECT) radiotracer [I-123]i omazenil is used to assess benzodiazepine receptor binding parameters. Thes e measurements are relative indices of benzodiazepine receptor concentratio n (B'(max)). To evaluate the ability of such indices in accurately accessin g the B'(max) the authors compared them with absolute values of B'(max), me asured using positron emission tomography (PET). The authors performed SPEC T, PET, and magnetic resonance imaging (MRI) studies on a group composed of seven subjects. For SPECT studies, the authors administered a single injec tion of [I-123]iomazenil and estimated the total and specific distribution volumes (DVT SPECT, DVS SPECT) and the binding potential (BP) using unconst rained (BPSPECT) and constrained (BPC SPECT) compartmental models. For PET studies, the authors used a multiinjection approach with [C-11]flumazenil a nd unlabeled flumazenil to estimate absolute values of receptor concentrati on, B'(max) and some other binding parameters. The authors studied the corr elation of different binding parameters with B'(max). To study the robustne ss of the binding parameter measurements at the pixel level, the authors ap plied a wavelet-based filter to improve signal-to-noise ratio of time-conce ntration curves, and the calculated kinetic parameters were used to build u p parametric images. For PET data, the B'(max) and the DVPET were highly co rrelated (r = 0.988). This confirms that it is possible to use the DVPET to access benzodiazepine receptor density. For SPECT data, the correlation be tween DVSPECT estimated using a two- and three-compartment model was also h igh(r = 0.999). The DVT SPECT and B'(max) parameters estimated with a const rained three-compartment model or the DVT"(SPECT) parameter estimated with a two-compartment model were also highly correlated to the B'(max) paramete r estimated with PET. Finally, the robustness of the binding parameters all owed the authors to build pixel-by-pixel parametric images using SPECT data .