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 . 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.
CMOS photon detectors (CPDs) are recently proposed photon sensing devices utilizing the latest CMOS image sensor (CIS) technologies . CPDs are non-electron-multiplying devices, whose pixels have a fully depleted photo diode and have a readout noise of sub electron RMS even at room temperature. Using a 15μm pixel CPD test device coupled to a CsI(Tl) scintillator, we successfully obtained photon-count X-ray images. A Hamamatsu Photonics scintillator with 150μm CsI(Tl) layer coupled to fiber optic plate (FOP) of 3mm thickness is diced in dry condition and directly glued to the sensor surface. X-ray photons are injected from an X-ray tube with accelerating voltage of 30kV and 45kV using W target. Each X-ray photon creates a scintillation light spot in the captured images, where the injected position and photon energy are determined by integrating multiple pixel outputs at that spot. X-ray energy distributions were obtained at both 30kV and 45kV with reasonable differences. Modulated transfer function (MTF) of over 0.7 at 10LP/mm was achieved by mapping injected positions at 30kV. Photon-count images for slanted-edge MTF measurements as well as 10LP/mm of X-ray test chart were achieved. Those photon-count images were compared with conventional energy integrating images obtained with the same sensing device. Both image types confirmed superior resolution with photon-counting. This indirect X-ray photon counting technique using CPD has a potential of getting critical X-ray information for medical applications by achieving accurate injected positions of X-ray photons and their energies simultaneously.