Modeling 13N-ammonia from projections in rat-PET studies

Positron emission tomography (PET) image reconstruction techniques introduce inaccuracies in images, especially in dynamic acquisitions where the frame length is short and/or when the radioactivity emission is low. To avoid these inaccuracies in PET kinetic modeling, kinetic parameters can directly be obtained from the sinograms. The work describes modeling ¹³N-ammonia in the sinograms obtained in rats with the Sherbrooke small animal PET scanner. Rats of nearly 250 g were injected approximately 5 mCi of ¹³N-ammonia during 30 sec and scans were acquired in 20 min. Each bin of the dynamic sinograms was decomposed in its basis functions using spectral analysis technique in conjunction with the ¹³N-ammonia kinetic model. Components of homogeneous structures were obtained as well as the kinetic model parameters. Structures were identified according to their frequencies and they were individually reconstructed. Compartments of blood, non-metabolized and metabolized ammonia were clearly separated. Moreover, blood components having different shapes were discriminated and their time activity curves showed a delay in the occurrence of their peaks. In conclusion, modeling in the sinogram domain allows one to extract a specific component and to obtain images of kinetic parameters.