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 have investigated the optical properties of Gd 2 O 2 S:Tb granular phosphor screens for the use in indirect-conversion detectors by using the Monte Carlo method. For the optical model of the phosphor screen, it was regarded as a weak absorbing medium in which scattering is caused by refraction at boundaries between the phosphor grains and organic binders. For the estimation of the light collection efficiency, we included thin passivation (e.g. SiO 2 ) and Si layers as a photodiode in the Monte Carlo geometry only because the optical photons which escape from the phosphor screen exit and towards the Si layer can contribute to signals. In addition, optical coupling materials (e.g., optical fluids), which are practically used in the indirect-conversion detector, were considered. In the Monte Carlo simulations, various design parameters of the phosphor screen were considered such as the refractive index of an optical coupler and passivation layer, a reflection coefficient at the screen backing, and the thickness of an optical coupler. According to the simulation results, the optical coupler played a great role both in light collection efficiency and point-spread function (PSF). The maximum light collection efficiency was achieved when the refractive index of the optical coupler matched to either that of the phosphor screen or that of the photodiode. Moreover, the matched refractive index provided a lesser light spread in the resulting images. The simulation method and result can provide guidelines for a better design of indirect-conversion detectors based on a photodiode array coupled to a phosphor screen.
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We have developed a sandwich-like multilayer detector capable of imaging a large field of view for the purpose of dental computed tomography. Two detectors forming the sandwich detector are based on indirect-conversion flat-panel detector technology. This development employed a relatively thin Gd 2 O 2 S : Tb scintillator for the front-detector scintillator and a relatively thick CsI:Tl for the rear one. On the other hand, the same photodiode arrays based on complementary metal-oxide-semiconductor active pixel technology were used for both detector layers. While the substrate of the rear flat-panel detector was a conventional printed-circuit board (PCB), the front detector was realized onto a flexible PCB, which is then electrically connected to a separate corresponding PCB placed under the rear detector PCB. Imaging performance of each detector layer in the sandwich detector was evaluated in terms of large-area or zero-frequency signal and noise, modulation-transfer function, noise-power spectrum, and detective quantum efficiency. Compared to previous designs, the current design showed higher signal-to-noise ratio performances for which the scintillators used were mainly responsible. Demonstration dual-energy images obtained for a postmortem mouse supported the quantitative measurements. K: Detector design and construction technologies and materials; X-ray detectors; X-ray radiography and digital radiography (DR)
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