Simultaneous PET and SPECT dual-tracer imaging with common detector and bed will potentially open the door for many new clinical and preclinical applications, such as two receptor binding studies or perfusion-metabolism differential imaging of neurodegenerative diseases, and can be technically advantageous in certain aspects compared to conventional SPECT dual-tracer or PET dual-tracer imaging.In this feasibility study, PET and SPECT data from concurrently existing Tc-99m and F-18 radiotracers were acquired with the same acquisition hardware setup: a microPET and a collimator inside the gantry. The collimator is a slit-slat type, consists of knife-edge slits formed by Lead plates and septa formed by annular Tungsten sheets to form 2D fan beam geometry. SPECT data were acquired in list-mode with 135-145 keV energy window. With the collimator stayed inside the gantry, PET data were acquired and processed to correct collimator effects. A hot-rod phantom was filled with Tc-99m and F-18 tracers in separated rods so that possible contamination between the PET and SPECT acquisitions can be evaluated. The rods are 0.9-2.3 mm in diameters and separated by roughly twice of the rod diameter. The ratio of activity concentration between the two tracers was close to 1:1. Since there is no corresponding simultaneous data acquisition mode available in the current system, PET and SPECT data were acquired successively with the same hardware setup and phantom position to mimic the simultaneous imaging. The rod images corresponding to PET and SPECT were well separated without severe artifacts. The data corrections, including LSO background and down-scatter from 511 keV photons to the SPECT acquisition were applied to the corresponding images. In addition, F-18 PET images of a phantom acquired with and without collimator reveal slight degradation in image quality due to the attenuation and extra data correction. Initial PET-SPECT dual-tracer mouse images were acquired and fused with mixed Tc-99m MDP and F-18 FDG injection. These studies have demonstrated the feasibility to acquire simultaneous PET-SPECT dual-tracer images with a microPET and a collimator insert.

This work was part of the feasibility study of performing simultaneous PET and SPECT imaging with a microPET scanner. SPECT data were acquired with slit-slat collimation by inserting a slit-slat collimator into microPET's detector ring but were contaminated by 511 keV photons' downscattered events from PET tracer. In this study, contribution of down-scattered events in Tc-99m energy window (135-145 keV) was characterized through Monte Carlo simulation and measurements. With relative ratio of Tc-99m and F-18 activity being 1:1, ratio of recorded events from Tc-99m and F-18 source was 1:18.31. Nevertheless, simulation results revealed that the majority of down-scattering effects occurred in the detector crystals and thus were less object dependent. We investigated two subtraction-based compensation methods. The first method used an auxiliary energy window (AEW) and the second method employed extra shielding measurement (ESM) to acquire downscatter-only data. Simulated and experimental studies showed improved image quality after applying the correction methods. With the activity ratio of SPECT tracer to PET tracer being 5:1, good SPECT images were obtained after correction, which were comparable to those from pure SPECT imaging.

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