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Design and characterization of the multi-energy monochromatic X-ray beam in X-ray imaging systems

Other Titles
 엑스선 영상 시스템에서 필터를 이용한 다중에너지 단일화 엑스선 빔의 설계 
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Dept. of Radiological Science/박사
Multi-energy X-ray imaging (or spectral imaging) is widely used in medical, industrial, and security fields. In the medical field, multi-energy X-ray imaging systems are suitable for contrast enhancement of lesions, quantitative analysis of specific materials, and functional imaging of the human body. Therefore, the dual-energy (DE) system was widely adopted for use in clinical examinations by operating dual-source, dual-layer detectors, and fast kV-switching. Recently, a photon-counting detector has been developed that can obtain multiple pieces of information about an object by discriminating between the detected photon energies of the X-rays from broad energy band by the application of specific integrated circuits (ASIC). Quasi-monochromatic beam can be generated by using Bragg diffraction and filter design for multi-energy X-ray imaging. The aim of this dissertation is to develop a triple-energy (TE) monochromatic X-ray beam with filter designed to separate three materials, and the results of an image acquired with the proposed TE monochromatic X-ray beam were compared to an image obtained with the photon-counting method through both simulation and experimental measurement.Various monochromatic X-ray beams, having filter materials (Al, Cu, I, Ba, Ce, Gd, Er, and W) with K-edge energy, were generated with a charge-integrating detector by simulation based on empirical models. An appropriate filter thickness was decided through comparison between the mean energy of a filtered beam and the K-edge energy of the filter. Quantitative indices such as mean energy ratio, contrast variation ratio, and exposure efficiency were estimated for each monochromatic beam using Monte Carlo simulation. The mean energy of each filter material was characterized with respect to increasing the tube potential due to the K-edge energy of the filter. The values of mean energy ratio of the filtered beam were below that of the result without a filter for all filter materials in a phantom study. This means that the filtered X-ray beam is monochromatic, thereby maintaining minimal beam hardening by the K-edge filter. Filtered X-ray beams obtained with I, Ba, and Ce were of a higher contrast than an unfiltered X-ray beam, in accordance with tube potential. In exposure efficiency, the filtered beams using I, Ba, Ce, and Gd filters outperformed the unfiltered X-ray beams at same tube potential.The TE monochromatic X-ray beams were generated by I, Ba, and Gd filters at 50, 60, and 70 kV from the simulation results, respectively. The spectra of the simulated TE monochromatic beams were compared to the experimental results obtained with the photon-counting detector. The results indicate that the energy peaks of the simulated spectra were well matched to those of experimental spectra. The thickness density map that was acquired with TE monochromatic beams was compared to that obtained with photon-counting method for both the simulation and experiment. In the simulation results, the thickness map obtained by using TE monochromatic beams were estimated to 1.00, 1.00, and 0.99 cm for iodine, aluminum, and PMMA, respectively, when the true values of the thickness density were 1.00 cm for each. In the simulation results of the photon-counting method, the thickness density maps of iodine, aluminum, and PMMA were 1.00, 0.96, and 1.07 cm, respectively. The thickness density maps of iodine, aluminum, and PMMA obtained with TE monochromatic beams were compared with the photon-counting method. The resultant thickness densities of iodine, aluminum, and PMMA were 0.57, 0.52, and 1.99 cm by the TE monochromatic method when the true values of the thickness density were 0.50, 0.50, and 2.00 cm for iodine, aluminum, and PMMA, respectively. In the photon-counting method, the thickness densities of iodine, aluminum, and PMMA were 0.50, 0.51, and 2.05, respectively.In this paper, we proved that TE monochromatic X-ray beams are a reliable design with tube voltages and additional filters for triple-energy imaging. The proposed additional filtration has proven its feasibility as the imaging method with a high accuracy of material thickness over the three materials, and this method can be used in the multi-energy X-ray imaging technique for medical imaging.
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