This research is CZT Compton PET for maximizing detection efficiency and improving in image quality. First, detection efficiency and angular uncertainty is calculated. Second, image is reconstructed by simple backprojection and MLEM. After reconstructing, image is evaluated by three methods. First s...
This research is CZT Compton PET for maximizing detection efficiency and improving in image quality. First, detection efficiency and angular uncertainty is calculated. Second, image is reconstructed by simple backprojection and MLEM. After reconstructing, image is evaluated by three methods. First study is about maximization of the efficiency for PET by using Compton event. Multiple interactions in positron emission tomography (PET) using scintillators are generally treated as noise events because each interacted position and energy of the multiple interactions cannot be obtained individually and the sequence of multiple scattering is not fully known. Therefore, the first interaction position, which is the crucial information for a PET image reconstruction, cannot be determined correctly. However, in the case of a pixelized semiconductor detector, such as CdZnTe, each specific position and energy information of multiple interactions can be obtained. The emission of two 511 keV radiations in PET, if one radiation deposits all the energy in one position (photoelectric effect) and the other radiation undergoes Compton scattering followed by the photoelectric effect, the sequence of Compton scattering followed by the photoelectric effect can be determined using the Compton scattering formula. Therefore, the correct position of Compton scattering can be determined, and the Compton scattering effect, which is discarded in conventional PET systems can be recovered. The PET system in this study, which was simulated using GATE 5.0 code, was composed of 20mm x 10 mm x 10 mm CdZnTe detectors consisting of 1mm x 0.5mm x 2.5 mm pixels. The angular uncertainties caused by Doppler broadening, pixelization effect and energy broadening were estimated and compared. The pixelized effect was the main factor in increasing the angular uncertainty and was strongly dependent on the distance between the 1st and 2nd interaction positions. The effect of energy broadening to an angular resolution less than expected and that of Doppler broadening was minimal. The number of Compton events was double that of the photoelectric effect assuming full energy absorption. Since both the photoelectric effect and Compton scattering are utilized, the detection efficiency of this new PET system can be improved greatly. The angular resolution is strongly dependent on the pixelization effectand hence a finer pixel size and proper distance between the 1st and 2nd interaction position were needed to optimize the performance of CZT Compton PET. The total number of effective events detected by CZT Compton PET was 2.75 times higher than that by the same system using the photoelectric effect only. Second study is about the image evaluation for CZT Compton PET using simple backprojection and MLEM (Maximum Likelihood Expectation Maximization) method. Three methods were used to evaluate the CZT Compton PET compared with conventional PET using photoelectric effect only. The FWHMs (Full Width Half Maximum) and maximum counts of point source images reconstructed by simple backprojection were calculated for comparison. Using a MLEM method, the relative standard deviation and FWHM were calculated at each iteration to evaluate the modalities quantitatively. For a 3D source phantom, simple backprojection and MLEM method were applied to each modality and the reconstructed images were compared with each other by visual inspection. All three evaluation methods prove that the performance of the Compton PET is significantly higher than that of conventional PET.
This research is CZT Compton PET for maximizing detection efficiency and improving in image quality. First, detection efficiency and angular uncertainty is calculated. Second, image is reconstructed by simple backprojection and MLEM. After reconstructing, image is evaluated by three methods. First study is about maximization of the efficiency for PET by using Compton event. Multiple interactions in positron emission tomography (PET) using scintillators are generally treated as noise events because each interacted position and energy of the multiple interactions cannot be obtained individually and the sequence of multiple scattering is not fully known. Therefore, the first interaction position, which is the crucial information for a PET image reconstruction, cannot be determined correctly. However, in the case of a pixelized semiconductor detector, such as CdZnTe, each specific position and energy information of multiple interactions can be obtained. The emission of two 511 keV radiations in PET, if one radiation deposits all the energy in one position (photoelectric effect) and the other radiation undergoes Compton scattering followed by the photoelectric effect, the sequence of Compton scattering followed by the photoelectric effect can be determined using the Compton scattering formula. Therefore, the correct position of Compton scattering can be determined, and the Compton scattering effect, which is discarded in conventional PET systems can be recovered. The PET system in this study, which was simulated using GATE 5.0 code, was composed of 20mm x 10 mm x 10 mm CdZnTe detectors consisting of 1mm x 0.5mm x 2.5 mm pixels. The angular uncertainties caused by Doppler broadening, pixelization effect and energy broadening were estimated and compared. The pixelized effect was the main factor in increasing the angular uncertainty and was strongly dependent on the distance between the 1st and 2nd interaction positions. The effect of energy broadening to an angular resolution less than expected and that of Doppler broadening was minimal. The number of Compton events was double that of the photoelectric effect assuming full energy absorption. Since both the photoelectric effect and Compton scattering are utilized, the detection efficiency of this new PET system can be improved greatly. The angular resolution is strongly dependent on the pixelization effectand hence a finer pixel size and proper distance between the 1st and 2nd interaction position were needed to optimize the performance of CZT Compton PET. The total number of effective events detected by CZT Compton PET was 2.75 times higher than that by the same system using the photoelectric effect only. Second study is about the image evaluation for CZT Compton PET using simple backprojection and MLEM (Maximum Likelihood Expectation Maximization) method. Three methods were used to evaluate the CZT Compton PET compared with conventional PET using photoelectric effect only. The FWHMs (Full Width Half Maximum) and maximum counts of point source images reconstructed by simple backprojection were calculated for comparison. Using a MLEM method, the relative standard deviation and FWHM were calculated at each iteration to evaluate the modalities quantitatively. For a 3D source phantom, simple backprojection and MLEM method were applied to each modality and the reconstructed images were compared with each other by visual inspection. All three evaluation methods prove that the performance of the Compton PET is significantly higher than that of conventional PET.
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