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Ho Kyung Kim,Min Kook Cho,Achterkirchen, T.,Wonho Lee IEEE 2009 IEEE transactions on nuclear science Vol.56 No.3
<P>We report the observation of performance degradation in a detector consisting of a phosphor screen and a CMOS (complementary metal-oxide-semiconductor) photodiode array under the continuous irradiation of 45-kVp x-rays. The performance was assessed in terms of dark pixel signal, dynamic range, modulation-transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). From the measurement results, it has been observed that the increase of dark pixel signal and the related noise gradually reduces the dynamic range as the cumulative input exposure to the detector increases. Severe degradation in NPS was observed, which gives rise to reduction in DQE as the cumulative input exposure increases. With carefully updated offset and gain correction, however, we can overcome the detrimental effects of increased dark current on NPS and DQE.</P>
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.
Seungman Yun,Ho Kyung Kim,Chang Hwy Lim,Min Kook Cho,Achterkirchen, T.,Cunningham, I. IEEE 2009 IEEE transactions on nuclear science Vol.56 No.3
<P>We report the measurement results of signal and noise characteristics induced by the direct x-rays in an indirect-conversion CMOS photodiode array detector. In order to isolate the signal and noise due to the direct x-rays from those due to the optical photons, we inserted a light-absorbing blackout material between a phosphor screen and the photodiode array. From the images irradiated with and without the blackout paper, the signal and noise characteristics due to the optical photons emitted from a phosphor screen were estimated. For the analysis of the measurements, we have developed a model describing the signal and noise transfers based on the cascaded linear-systems approach. The measured results show the direct x-ray is very harmful to the detector performances, such noise power spectrum (NPS) and signal-to-noise ratio (SNR). However, from the theoretical estimation, the degradation of NPS and SNR would not be due to the directly absorbed x-ray photons, but we believe that other sources, such as Compton and photoelectric scattered rays from a scintillator, a photodiode passivation layer or bulk substrate, are main causes.</P>