We have developed a novel sandwich-style single-shot (single-kV) detector by stacking two indirect-conversion flat-panel detectors for preclinical mouse imaging. In the sandwich detector structure, extra noise due to the direct x-ray absorption in photodiode arrays is inevitable. We develop a simple cascaded linear-systems model to describe signal and noise propagation in the flat-panel sandwich detector considering direct x-ray interactions. The noise-power spectrum (NPS) and detective quantum efficiency (DQE) obtained from the front and rear detectors are analyzed by using the cascaded-systems model. The NPS induced by the absorption of direct x-ray photons that are unattenuated within the photodiode layers is white in the spatial-frequency domain like the additive readout noise characteristic; hence that is harmful to the DQE at higher spatial frequencies at which the number of secondary quanta lessens. The model developed in this study will be useful for determining the optimal imaging techniques with sandwich detectors and their optimal design.
We revisit the doubly-layered sandwich detector configuration for single-shot dual-energy x-ray imaging. In order to understand its proper operation, we investigated the contrast-to-noise performance in terms of the x-ray beam setup using the Monte Carlo methods. Using a pair of active photodiode arrays coupled to phosphor screens, we have built a sandwich detector. For better spectral separation between the projection images obtained from the front and rear detectors during a single x-ray exposure, we inserted a copper sheet between two detectors. We have successfully obtained soft tissue- and bone-enhanced images for a postmortem mouse with the developed sandwich detector using weighted logarithmic subtraction, and the image quality was comparable to those achieved by the conventional kVp-switching technique. Although some problems to be mitigated for the optimal and practical use, for example, the scatter effect and image registration, are still left, the performance of the sandwich detector for single-shot dual-energy x-ray imaging is promising. We expect that the active sandwich detector will provide motion-artifact-free dual-energy images with a reasonable image quality.
Detectors for computed tomography (CT) typically consist of scintillator and photodiode arrays which are coupled using optical glue. Therefore, the leakage of optical photons generated in a scintillator block to neighboring pixel photodiodes through the optical glue layer is inevitable. Passivation layers to protect the silicon photodiode as well as the silicon layer itself, which is inactive to the optical photons, are another causes for the leakage. This optical crosstalk reduces image sharpness, and eventually will blur CT images. We have quantitatively investigated the optical crosstalk in CT detectors using the Monte Carlo technique. We performed the optical Monte Carlo simulations for various thicknesses of optical components in a 129 × 129 CT detector array. We obtained the coordinates of optical photons hitting the user-defined detection plane. From the coordinate information, we calculated the collection efficiency at the detection plane and the collection efficiency at the single pixel located just below the scintillator in which the optical photons were generated. Difference between the two quantities provided the optical crosstalk. In addition, using the coordinate information, we calculated point-spread functions as well as modulation-transfer functions from which we estimated the effective aperture due to the optical photon spreading. The optical crosstalk was most severely affected by the thickness of photodiode passivation layer. The effective aperture due to the optical crosstalk was about 110% of the detector pixel aperture for a 0.1 mm-thick passivation layer, and this signal blur was appeared as a relative error of about 3-4% in mismatches between CT images with and without the optical crosstalk. The detailed simulation results are shown and will be very useful for the design of CT detectors.
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 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 Gd<sub>2</sub>O<sub>2</sub>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<sub>2</sub>) 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.