Feasibility study of using hybrid collimation for nuclear environmental imaging

This paper presents a feasibility study of a gamma-ray imager using a hybrid collimation (HC) scheme. This detector is based on the use of a multiple pinhole collimator and a position sensitive scintillation detector with Anger logic readout. A pixelated semiconductor detector, located between the collimator and the scintillation detector, is used as a scattering detector. For gamma-rays scattered in the first detector and then stopped in the second detector, an image can be built up based on the joint probability of their passing through the collimator and falling into a broadened conical surface, defined by the detected Compton scattering event. Since these events have a much smaller angular uncertainty, they provide more information per photon compared with using solely the mechanical or electronic collimation. Therefore, the overall image quality can be improved. This feasibility study used a theoretical approach based on analyzing the resolution-variance tradeoff in images reconstructed using maximum a posteriori (MAP) algorithms. The effect of the detector configuration, Doppler broadening, the energy resolution of the scattering detector and mechanical aperture design are studied. The results showed that the combined collimation leads to a significant improvement in image quality at energies below 300 keV. However, due to the mask penetration, the performance of such a detector configuration is worse than a standard Compton camera above this energy.