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선형시스템 전달이론을 이용한 간접변환방식 디지털 래디오그라피 디텍터의 신호 및 잡음 분석
윤승만,임창휘,한종철,조옥라,김정민,김호경,Yun, Seung-Man,Lim, Chang-Hwy,Han, Jong-Chul,Joe, Ok-La,Kim, Jung-Min,Kim, Ho-Kyung 한국의학물리학회 2010 의학물리 Vol.21 No.3
간접변환방식 CMOS (complementary metal-oxide-semiconductor) 엑스레이 디텍터 시스템의 성능 분석 및 개선을 위하여 공간주파수에 따른 DQE (detective quantum efficiency)를 모델링 하였다. 모델의 검증을 위하여 마모그라피 W/Al 선질에 대한 modulation-transfer function (MTF), noise-power spectrum (NPS)를 측정하고 이로부터 DQE를 계산하였으며, 모델과 측정된 DQE는 전체 공간주파수 영역에서 서로 잘 일치함을 확인하였다. 검증된 모델을 이용하여 형광스크린 양자효율 및 MTF, Swank 잡음, 포토다이오드 양자효율 등 CMOS 디텍터 시스템의 DQE 성능에 영향을 미칠 수 있는 다양한 디자인 파라미터의 역할을 살펴보았다. 엑스레이 디텍터 시스템의 신호 및 잡음 분석에 대해 이와 같은 선형시스템 전달을 이용한 이론적인 접근법은 이미 개발된 의료영상시스템을 이해할 수 있는 유용한 도구일 뿐만 아니라 새로운 디텍터 개발 및 최적화를 위한 도구로 활용될 수 있을 것이다. For the use of Indirect-conversion CMOS (complementary metal-oxide-semiconductor) detectors for digital x-ray radiography and their better designs, we have theoretically evaluated the spatial-frequency-dependent detective quantum efficiency (DQE) using the cascaded linear-systems transfer theory. In order to validate the developed model, the DQE was experimentally determined by the measured modulation-transfer function (MTF) and noise-power spectrum, and the estimated incident x-ray fluence under the mammography beam quality of W/Al. From the comparison between the theoretical and experimental DQEs, the overall tendencies were well agreed. Based on the developed model, we have investigated the DQEs values with respect to various design parameters of the CMOS x-ray detector such as phosphor quantum efficiency, Swank noise, photodiode quantum efficiency and the MTF of various scintillator screens. This theoretical approach is very useful tool for the understanding of the developed imaging systems as well as helpful for the better design or optimization for new development.
윤승만,한종철,Okla Joe,고종수,김영수,김호경 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.60 No.3
The development of a pixel-structured scintillator has recently been exploited to confine optical transport onto a corresponding photodiode pixel. This conceptual design of scintillators may provide high detection efficiency while preserving high spatial resolution in digital X-ray imaging detector systems. The sedimentation approach is one method to fill gadolinium-oxysulfide phosphors into a pixel-structured micro-well array. To fully understand the X-ray imaging characteristics of a pixel-structured design with sedimented gadolinium-oxysulfide phosphors, the imaging performance of gadolinium-oxysulfide phosphor layers made by using sedimentation should be characterized in advance. We have fabricated gadolinium-oxysulfide phosphor screens by using a simple sedimentation method. In order to characterize the imaging performances of the developed phosphor screens, we overlaid them onto a photodiode array with a pixel pitch of 48 microns and thus completed indirect-conversion X-ray imaging detectors. The imaging performance of the detector was investigated in terms of the modulation-transfer function (MTF), the noise-power spectrum, and the detective quantum efficiency (DQE). The results were compared with those of detectors employing commercial phosphor screens. Although the developed screen was about 1.5 times thicker than the commercial one, the X-ray sensitivity was comparable to that of the commercial phosphor. The MTF performance was worse than that of the commercial screen, and that MTF performance also governed the DQE performance of the detector over the entire spatial-frequency band. If the sedimented phosphors are pixel-structured with micro-well arrays, however, the MTF performance can be defined by using only the pixel size. Therefore, a design to enhance the X-ray sensitivity for the sedimentation method should be considered.
Graphene Oxide induced Crystallization in Polymer-Graphene Oxide Composite
윤승한,김소연 한국고분자학회 2021 한국고분자학회 학술대회 연구논문 초록집 Vol.46 No.2
Graphene oxide (GO) is known to promote crystallization kinetics and change the crystalline morphology for semicrystalline polymers. Previous studies reported that the large specific surface area of GO can provide nucleation sites, which enhances the polymer crystallization on GO reducing the nucleation barrier. However, the crystallization kinetics can be more complex such that the crystallization can be enhanced or suppressed depending on the cooling rate or the GO content. In addition, the GO-polymer interactions at the GO interface can play an important role in changing crystallization kinetics. In this regard, the polymer crystallization with GO is investigated considering the cooling rate and GO content focusing on the GO-polymer interaction employing various experimental methods including polarized optical microscopy, differential scanning calorimetry, and small and wide-angle x-ray scatterings.