광자계수기반 검출기를 이용한 스펙트럴 X-선 영상기법들의 개선

Dept. of Radiological Science/박사The overall aim of this dissertation was to improve spectral X-ray imaging methods with photon-counting detectors. First, we proposed a thickness-calibration-based (TCB) flat-field (FF) correction and energy-weighting imaging with the photon-counting detector. We experimentally demonstrated that the TCB FF correction method, which uses various calibration data points, is useful to correct the non-uniformity of photon-counting detectors that suffer from the non-linearity induced by the count rate limitation of the detectors. TCB FF correction method is able to liaise with the TCB energy-weighting method. The TCB energy-weighting method requires the calibration data using a set of calibration objects with different thicknesses. In this dissertation, aluminum (Al) sheets were chosen as the calibration object. Then, using the calibration, the equivalent Al thicknesses corresponding to the unknown target objects can be found, and TCB energy-weighting factors can be calculated using the equivalent Al thicknesses and the theoretical linear attenuation coefficients of Al. Simulation and experimental studies were performed to validate the feasibility of the TCB energy-weighting method. The results showed that the TCB energy-weighting method improves the signal-difference-to-noise ratio (SDNR) compared to the counting mode without needing to know the properties of the target objects in advance. Second, we proposed the application of optimized low-energy X-ray filters to K-edge imaging with a photon-counting detector to reduce the radiation dose induced by low-energy X-ray photons. We used Al filters with different thicknesses as low-energy X-ray filters and investigated iodine K-edge imagines with an energy bin of 34-48 keV. We experimentally demonstrated that use of an optimal low-energy X-ray filter thickness, which was determined by maximizing the FOM, reduces the radiation dose significantly while maintaining the image quality in K-edge imaging. For example, the best figure of merit (FOM) value at 50 kVp was obtained with 8 mm Al filter, which results in 58% reduction in entrance surface air kerma (ESAK).In conclusion, the results of this dissertation showed the potential feasibility of the TCB FF correction and energy-weighting imaging method and an optimized low-energy X-ray filter with the prototype photon-counting detector and simple phantom. We expect that photon-counting detectors will continue to improve, and the proposed methods will be adapted in many areas of clinical X-ray imaging.restrictio