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.Index Terms-Cascaded system analysis, image quality, noise power spectrum, radiography.
We developed a computer-aided defect inspection system based on computed tomography ͑CT͒. The system consists of a homemade small cone-beam CT ͑CBCT͒ system and a graphical toolbox, which is used to extract a computer-aided design ͑CAD͒ model from the CT data. In the small CBCT system, the x-ray imaging detector is based on a complementary metal-oxide-semiconductor photodiode array in conjunction with a scintillator. Imaging performance of the detector was evaluated in terms of modulation-transfer function, noise-power spectrum, and detective quantum efficiency. The tomographic imaging performance of the small CBCT system was evaluated in terms of signal-to-noise ratio and contrast-to-noise ratio. The graphical toolbox to support defect inspection incorporates various functional tools such as volume rendering, segmentation, triangular-mesh data generation, and data reduction. All the tools have been integrated in a graphical-user interface form. The developed system can provide rapid visual inspection as well as quantitative evaluation of defects by comparing the extracted CAD file with the original file, if available, of an object. The performance of the developed system is demonstrated with experimental CT volume data.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.