PET kinetic modeling of 13N-ammonia from sinograms in rats

Positron emission tomography (PET) image reconstruction methods inherently introduce inaccuracies in images, especially in dynamic acquisitions, where the frame length is short and/or when the count rate is low. In order to provide an alternate approach to deal with this data analysis problem in PET kinetic modeling, we extracted the kinetic parameters directly from the sinograms. The present work describes modeling ¹³N-ammonia in the sinograms obtained in rats with the Sherbrooke small animal PET scanner. Rats were injected with ¹³N-ammonia and a set of dynamic scans was acquired for a total of 20 min. Each element of the dynamic sinograms was decomposed into its basis functions using a spectral analysis technique in conjunction with ¹³N-ammonia kinetic model. Components that seemed to be representative of homogeneous structures were obtained as well as the kinetic model parameters. Structures were assumed to be associated with a small range of frequencies and were individually reconstructed on that basis. The resulting images were apparently compartments of blood, nonmetabolized, and metabolized ammonia, which were clearly resolved. Moreover, there were two blood components with time activity curves that showed a difference in the occurrence of their peaks. In conclusion, modeling in the sinogram domain allows one to extract a specific component associated with its major kinetic components and to obtain images that may be representative of kinetic parameters.