Simultaneous PET/SPECT imaging with shared detector on an existing preclinical PET scanner

Objectives The objective is to develop a method to perform simultaneous PET / SPECT imaging on an existing PET scanner.

Methods This study was based on a prototype animal PET/CT scanner. The PET ring detector was optimized to measure both PET photons at 511 keV and SPECT Tc-99m photons at 140 keV. To acquire SPECT data, a concentric multi-pinhole collimator was inserted into PET ring detector. The wall thickness of the tungsten collimator was chosen to be 3.5 mm to allow reasonable penetration percentage of PET photons. In this way, PET coincidence events and SPECT single events could be acquired simultaneously. PET image was reconstructed with a list-mode OS-EM iterative reconstruction algorithm. A triple-energy-window(TEW) method was applied on SPECT data to correcting for down-scatter contamination from PET tracer. A multi-pinhole OS-EM iterative reconstruction algorithm was used in SPECT reconstruction with analytically derived system matrix. Stand-alone PET and SPECT phantom studies were performed to evaluate imaging performance. Simultaneous PET/SPECT imaging feasibility was demonstrated with an F-18 / Tc-99m dual tracer phantom study.

Results The 1.35 mm hot rods were distinguishable in stand-alone PET and SPECT hot rod phantom images. When the collimator was inserted into PET detector ring, the coincidence sensitivity dropped by 80%, but was still 50 times higher than SPECT, therefore in the same acquisition time duration, a relatively high SPECT to PET tracer ratio could be used. No prominent artifact was observed in PET image with the existence of SPECT collimator, even without attenuation correction. PET down scattered photons caused severe artifacts in SPECT image, which was effectively corrected with TEW down-scatter correction method. Reasonable PET/SPECT dual-tracer image was generated with 11:1 Tc-99m to F-18 tracer ratio.

Conclusions The proposed simultaneous PET/SPECT imaging method was demonstrated to be feasible in the evaluated animal PET/CT scanner. It is reproducible for existing PET systems to enrich their functionalities with low additional cost.

Research Support The research was supported by the National Natural Science Foundation of China (No. 11375096).