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박하령,윤한빈,한종철,감수화,김호경 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.5
The development of photon-counting pixel detectors for spectral x-ray imaging is in its prime. The optimization of pixel geometry is crucial for obtaining a reliable spectral response in each pixel element with the least distortion due to fundamental x-ray interactions and electrical properties. We investigated the fundamental electrical properties in semiconductor pixel detectors with respect to normalized pixel geometries: the aspect ratio (the length ratio between pixel pitch and detector thickness) and the fill factor (the area ratio between the pixel electrode and pixel extent). For various geometries, we performed three-dimensional field simulations using a commercial finite element analysis code, and obtained the potential and field distributions. Based on weighting potential distributions, we calculated the net charge buildup for various interaction positions within the boundary of pixel, where a single electron-hole pair starts to drift, assuming no charge trapping and diffusion. For upper limit performance, a small aspect-ratio and fill-factor design would be preferable for fast and reliable signal measurements. The results and methodology from this study can offer insight into the optimal design of pixel geometries in photon-counting pixel detectors.
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Characterization of On-site Digital Mammography Systems: Direct Versus Indirect Conversion Detectors
윤한빈,한종철,윤성만,감수화,조승룡,김호경 한국물리학회 2015 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.66 No.12
We investigated the performances of two digital mammography systems. The systems use a cesium-iodide (CsI) scintillator and an amorphous selenium (a-Se) photoconductor for X-ray detection and are installed in the same hospital. As physical metrics, we measured the modulationtransfer function (MTF), the noise-power spectrum (NPS), and the detective quantum efficiency (DQE). In addition, we analyzed the contrast-detail performances of the two systems by using a commercial contrast-detail phantom. The CsI-based indirect conversion detector provided better MTF and DQE performances than the a-Se-based direct conversion detector whereas the former provided a poorer NPS performance than the latter. These results are explained by the fact that the CsI-based detector used an MTF restoration preprocessing algorithm. The a-Se-based detector showed better contrast-detail performance than the CsI-based detector. We believe that the highfrequency noise characteristic of a detector is more responsible for the visibility of small details than its spatial-resolution performance.
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한종철,윤성만,윤한빈,감수화,조승룡,Thorsten G. Achterkirchen,김호경 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.5
Active pixel design using the complementary metal-oxide-semiconductor (CMOS) process is acompelling solution for use in X-ray imaging detectors because of its excellent electronic noisecharacteristics. We have investigated the imaging performance of a CMOS active pixel photodiodearray coupled to a granular phosphor through a fiber-optic faceplate for mammographic applications. The imaging performance included the modulation-transfer function (MTF), noise-power spectrum(NPS), and detective quantum efficiency (DQE). Because we observed a nonlinear detector responseat low exposures, we used the linearization method for the analysis of the DQE. The linearizationmethod uses the images obtained at detector input, which are converted from those obtained atdetector output by using the inverse of the detector response. Compared to the conventionalmethod, the linearization method provided almost the same MTF and a slightly lower normalizedNPS. However, the difference between the DQE results obtained by using the two methods wassignificant. We claim that the conventional DQE analysis of a detector having a nonlinear responsecharacteristic can yield wrong results. Under the standard mammographic imaging condition, weobtained a DQE performance that was competitive with the performances of conventional flat-panelmammography detectors. We believe that the CMOS detector investigated in this study can besuccessfully used for mammography.
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Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging
한종철,김호경,김동원,윤승만,윤한빈,감수화,Jesse Tanguay,Ian A. Cunningham 한국물리학회 2014 Current Applied Physics Vol.14 No.12
We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-tonoise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern.
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