For a detector consisting of a phosphor screen and a photodiode array made by complementary metal-oxidesemiconductor
(CMOS) process, we have experimentally re-investigated the long-term stability of the signal and noise
characteristics as a function of the accumulated dose at the entrance surface of the detector in addition to the previous
study [IEEE Trans. Nucl. Sci. 56(3) 1121 (2009)]. The irradiation and analysis were more systematically performed. We
report the aging effect in image quality in terms of dark pixel signal, dynamic range, modulation-transfer function (MTF),
and noise-power spectrum (NPS). Unlike the previous study, the electronic noise was dominantly increased with the total
dose and the other statistical and structural noise sources were nearly independent on the cumulative dose. Similarly, the
increase of dark pixel signal and the related noise gradually reduces the dynamic range as the total dose increases. While
MTF was almost insensitive to the total dose, degradation in NPS was observed. Therefore, preprocessing without
properly updated offset and gain images would underestimate the detective quantum efficiency when performing quality
control of a detector in the field. Restoration of degraded dark signals due to aging is demonstrated by annealing the aged
detector with thermal activation energy. This study provides a motivation that the periodic monitoring of the imagequality
degradation is of great importance for the long-term and healthy use of digital x-ray imaging detectors.
We are developing pixel-structured scintillators for the eventual purpose of high-resolution and high-sensitivity x-ray
imaging applications. The pixel-structured scintillators were fabricated by filling Gd2O2S:Tb phosphor powder into the
silicon micro-well arrays by using a simple sedimentation method. The micro-well arrays having a depth of 180 μm were
fabricated by deep reactive ion etching of silicon wafers. To enhance the optical gain and the Swank noise factor, we
applied reflectance at the inside wall surfaces. Two different
inside-surface treatments were applied; 0.2-μm-thick
titanium which has 70% reflectance and 1-μm-thick silicon dioxide which was grown by thermal oxidation. The imaging
performance was evaluated in terms of modulation-transfer function (MTF), noise-power spectrum (NPS), and detective
quantum efficiency (DQE). Compared with the commercial phosphor screen as a reference, much enhanced MTF results
were measured. However, very low values of the system gain due to trapping of the generated optical photons at the wall
surfaces give rise to the poorer DQE performance rather than that of the reference detector. The theoretical cascaded
model analysis estimates much improved DQE performances with improved design parameters, such as higher
reflectance of 90% at the wall surfaces.
KEYWORDS: Sensors, Signal detection, Amplifiers, Photodetectors, Semiconductors, X-rays, Photonics systems, Signal generators, Digital x-ray imaging, Signal processing
We are developing a theoretical model to describe signal pulses from a detector-amplifier system operated in photoncounting
mode. In the model, we include incomplete signal generation due to the charge trapping within a semiconductor
detector as well as to the ballistic deficit caused by insufficient charge integration time. This model can be utilized for
the characterization of detector material properties such as the mobility and the lifetime, as well as the optimization of
operation conditions such as the applied bias voltages and the charge integration time. The model was experimentally
verified with the measurement of charge collection efficiency of a planar cadmium zinc telluride detector with respect to
the applied bias voltage and the charge integration time. We expect that the developed model will be helpful for the
design of photon-counting detectors.
We investigated the potential use of CMOS (complementary
metal-oxide-semiconductor) imaging detectors with a pixel
pitch of 48 μm for mammography. Fundamental imaging characteristics were evaluated in terms of modulation-transfer
function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). The magnitudes of various
image noise sources, such as optical photons, direct x rays unattenuated and scattered x rays from the scintillator, and
additive electronic noise, were measured and analyzed. For the analysis of the measurement results, we applied a model
describing the signal and noise transfer based on the cascaded
linear-systems approach. The direct x-ray was very
harmful to the detector noise performance with white noise characteristics in the spatial frequency domain, and which
significantly degraded the spatial-frequency-dependent DQE at higher frequencies. Although the use of a fiber-optic
plate (FOP) reduces the detector sensitivity and the MTF performance, it enhances the DQE performance by preventing
the direct x-ray photons from the absorption within the photodiode array.
We have investigated the optical properties of Gd2O2S: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. SiO2) 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.
We exploit the development of a pixel-structured scintillator that would match the readout pixel array, such as a
photodiode array. This combination may become an indirect-conversion detector having high x-ray sensitivity without
sacrificing the inherent resolving power defined by the pixel geometry of the photodiodes, because the scintillation
material has a relatively high atomic number and density compared with the photoconductors, and the pixel-structured
design may provide a band-limited modulation-transfer function (MTF) characteristic even with a thicker scintillator. For
the realization of pixel-structured scintillators, two-dimensional (2D) array of pixel-structured wells with a depth of 100μm was prepared by using a deep reactive ion etching (DRIE) process on a silicon wafer. Then, Gd2O2S:Tb phosphor
powders with organic binders were filled within the well array by using a sedimentation method. Three different pixel
designs of 50, 100 and 200 μm with a wall (or septum) thickness of 10 μm were considered. Each sample size was 20 × 30 mm2 considering intra-oral imaging. The samples were coupled to the CMOS photodiode array with a pixel pitch of 48 μm and the imaging performances were evaluated in terms of MTF, NPS (noise-power spectrum) and DQE (detective quantum efficiency) at intra-oral imaging conditions. From the measurement results, the sensitivities of the samples with 50, 100 and 200 μm pitch designs were about 12, 25 and 41% of that of the reference commercial phosphor screen (MinR-2000). Hence the DQE performances at 0.2 lp/mm were about 3.7, 9.6, 22.7% of the reference screen. According to the Monte Carlo simulations, the lower sensitivity was due to the loss of optical photons in silicon walls. However, the MTF performance was mainly determined by the designed pixel apertures. If we make pixel-structured scintillators with a deeper depth and provide reflectance on walls, much enhanced DQE performance is expected.
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