Simultaneous PET/SPECT imaging with the small animal scanner YAP-(S)PET

To exploit the YAP-(S)PET II scanner intrinsic capability of both PET and SPECT imaging, we have implemented the simultaneous PET/SPECT dual imaging modality. Two opposing heads, equipped with collimator and set in anticoincidence, independently acquire single events (SPECT mode), the other pair of heads detects coincidence events (PET mode). During the simultaneous PET/SPECT acquisition, both a high energy (PET) and a low energy (SPECT) radiotracer are placed inside the Field of View (FOV). Since the thickness of the collimator (2 cm) is optimized to stop only low energy photons, the down-scatter from 511 keV photons seriously affects the single photon acquisition. It is therefore necessary to perform a subtraction procedure before SPECT data analysis. The subtraction procedure is based on a multi-energy window method. The background underneath the 99mTc photo-peak region can then be approximated as a fraction (f) of the counts measured in an energy window close to the photo-peak but containing only Compton events. Good results were obtained for simultaneous imaging of two 5 mm syringes containing 99mTc and 18F with a SPECT/PET radiotracers ratio 10:1.