Evaluating the effects of the detectors thickness variations on image quality in Compton imaging

The Compton camera has been developed to estimate high-energy gamma-ray sources distribution. The advantages of this modality over SPECT system are larger field of view, higher sensitivity, and wider energy range. All of the mentioned properties of this device caused to have special applications for use in nuclear medicine and especially hadron therapy. Image reconstruction in the Compton camera are more complex in comparison with conventional imaging systems. In this way, different methods based on the iteration method have been developed to reconstruct the images. In fact, image reconstruction in Compton camera is performed by using conical projections and cone surfaces which gamma rays are probably emitted from them. Finally, cone surfaces projected into the image matrix. The Compton scattering camera consists of two detectors, the detector closest to the source for the occurrence and recording of the Compton scattering in this detector, and the second detector located behind the primary detector (farther from the source) designed to absorbing  the scattered photons (by the first detector). Position, energy and interaction time are stored by both detectors. Opening angle and the apex of the Compton cone can be calculated by using the amount of energy deposition in the two detectors and the location of the interactions. By having cone characteristics, cone surface is projected into the image space which allows to estimate the values of the traversed voxels. In this way, 3-D distribution of the source could be acquired by single-shot and without collimator. In this paper, the results of the Compton camera simulation in the GATE code are presented along with the written reconstruction algorithm. Then the effects of the detector thickness variations are evaluated by using a simulated phantom consisting of four radioactive spheres with different diameters. Image reconstruction was performed using LM-MLEM algorithm, which is based on the iterative method, in MATLAB software. The results of the image analysis show that the characteristic of the detector in the Compton camera along with iteration number have a strong impact on image quality.