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16 March 2020 A study of energy resolution in CPD indirect photon-counting x-ray imaging
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Recently we have reported indirect X-ray photon-counting imaging using CMOS photon detectors (CPD) and have shown its high spatial resolution with MTF of over 0.7 at 10LP/mm [1]. However, at that time its energy resolution potential and the quantum efficiency were totally unknown. Thus, there was a question about whether it can detect relatively low energy X-ray photons, for example around 20keV used for mammography, with sufficient quantum efficiency while eliminating counting errors.

In this study we exposed the CPD test devices to near single energy X-ray photons of 19.5keV adopting a clinical mammography equipment and additional Mo filters, and measured output intensity distributions. We also fitted our intensity distribution model to the results estimating signal yield per keV and parameters for signal variation.

We tested with 2 types of Hamamatsu Photonics CsI(Tl) scintillators. A CPD with J6675-01 scintillator plate has a signal distribution of 76% FWHM, still showing sufficient capability of photon counting with a little loss of actual signals. The signal yield is about 3.6 e-/keV. In the meanwhile, a CPD with J6675 has a superior distribution of 49% FWHM with signal yield of 7.1 e-/keV.

In both types, the dominant factor of the variation is not shot noise or sensor noise but something that is proportional to the signal intensity. This variation factor is possibly attributed to random noise of scintillation intensity, which is about 28% rms in J6675-01 and 17% rms in J6675.
Conference Presentation
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Toshiyuki Nishihara, Hiroyasu Baba, Oichi Kumagai, Takashi Izawa, and Norimitsu Shinohara "A study of energy resolution in CPD indirect photon-counting x-ray imaging", Proc. SPIE 11312, Medical Imaging 2020: Physics of Medical Imaging, 113120D (16 March 2020);

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