Compartmental analysis of washout effect in rat brain: in-beam OpenPET measurement using a 11C beam

In-beam positron emission tomography (PET) is expected to enable visualization of a dose verification using positron emitters (beta+ decay). For accurate dose verification, correction of the washout of the positron emitters should be made. In addition, the quantitative washout rate has a potential usefulness as a diagnostic index, but modeling for this has not been studied yet. In this paper, therefore, we applied compartment analyses to in-beam PET data acquired by our small OpenPET prototype, which has a physically opened field-of-view (FOV) between two detector rings. A rat brain was located at the FOV and was irradiated by a C-11 beam. Time activity curves of the irradiated field were measured immediately after the irradiations, and the washout rate was obtained based on two models: the two-washout model (medium decay, k(2m); slow decay, k(2s)) developed in a study of rabbit irradiation; and the two-compartment model used in nuclear medicine, where efflux from tissue to blood (k(2)), influx (k(3)) and efflux (k(4)) from the first to second compartments in tissue were evaluated. The observed k(2m) and k(2s) were 0.34 and 0.005 min(-1), respectively, which was consistent with the rabbit study. Also k(2m) was close to the washout rate in cerebral blood flow (CBF) measurements by dynamic PET with O-15-water, while, k(2), k(3), and k(4) were 0.16, 0.15 and 0.007 min(-1). Our present work suggested the dynamics of C-11 might be relevant to CBF or permeability of a molecule containing C-11 atoms might be regulated by a transporter because the k(2) was relatively low compared with a simple diffusion tracer.