Geometrical calibration of a PET-scanner-based multi-pinhole SPECT using LYSO background radiation

In previous work, we have developed a cost-effective small-animal SPECT imaging system by adding a multi-pinhole cylinder collimator into an animal PET scanner. To achieve high-quality reconstructed SPECT image, accurate geometrical calibration of the collimator is critical. The pinhole position in PET-scanner´s coordinate system (PSC) could be factorized into two parts: pinhole position in collimator´s coordinate system (fixed, only need one measurement) and collimator position in PSC (would vary after times of use after times of use). In this paper, we propose a novel geometrical calibration method for the second part based on the coincidental beta-gamma events of LYSO background radiation of the PET scanner. To obtain sufficient conditions for describing the collimator in PSC, a mark ball was added to one end of the collimator and imaged with collimator by LYSO background radiation simultaneously. With the reconstructed image of the collimator and the add-on mark ball, the axis of the collimator and the center of the mark ball were derived using least-squares fitting and centroid calculation, individually. Monte-Carlo simulation studies with five groups of different positions of collimator were implemented and geometrical parameters were calibrated. The proposed method was implied on a real system. Both simulation and experiment studies show that the proposed method could achieve high accuracy for parameter calibration. In conclusion, it is a practical and efficient way to perform SPECT collimator calibration by using PET scanner´s LYSO background radiation in the studied PET/SPECT system.