Simulation of depth of interaction effects for pinhole SPECT

Pinhole imaging is receiving attention as a method to enable SPECT of small animals. However, gamma rays passing through the pinhole aperture and entering the detector at an oblique angle can cause parallax errors due to the depth of interaction (DOI) of the incident gamma ray in the absorber layer of the detector. This can cause a displacement and blurring of the position coordinates recorded by the detector, resulting in an error known as the "DOI effect". The DOI effect for projection data can be analytically calculated, but the error due to the DOI effect in reconstructed SPECT images is difficult to predict analytically due to the complexity of reconstruction algorithms. Therefore, we used an analytic model of the DOI effect in the projection data to numerically simulate the acquisition of point sources to determine the point spread function at different points in the object. The resulting projection data were reconstructed using a Feldkamp algorithm, from which point spread functions were extracted to quantify loss in spatial resolution due to the DOI effect. The simulations were performed for 140-keV gamma rays and a 6 mm thick CsI scintillator with a reconstruction volume of (40 mm)³. The simulation was repeated with a 10 mm thick high purity germanium (HPGe) detector. These detectors and parameters are representative of a system that can be used for small animal imaging. From these calculations, we estimate tomographic reconstruction contributes a resolution loss of 0.32-0.63 mm FWHM in plane and 0.21-0.42 mm FWHM out of plane of the detector. For the CsI scintillator, the DOI effect contributes a maximum resolution loss of 0.5 mm FWHM in plane and 0.3 mm FWHM out of plane. With the HPGe detector, the DOI effect contributes a spatial resolution loss of 0.6 mm FWHM in plane and 0.5 mm out of plane.