[en] Gas detectors have definite advantages over other types of detectors: large-sensitive area, flexible geometry, low cost, radiation resistance etc. With these merits as well as low density they are very attractive for tracking devices in high-energy physics experiments and other applications. Nevertheless they suffer from some problems: dead zone effect, electrical breakdown, and angular dependence. In two-dimensional gas avalanche pixel detector, the dead zone effect is most predominant because of the drift field perturbation by the readout bus. It is found that the conductive coating onto the substrate is the best method to minimize this effect when the ratio between the surface coating conductivity and the substrate bulk conductivity is greater than 0.1 in unit of centimeter. Otherwise, avoiding the serious dead zone effect, and simultaneously allowing good spatial resolution and large avalanche signal, the 100 μm-pitch detector is optimum. In order to avoid the permanent damage of electrical breakdown due to sparking, the charge preamplification method is most desirable because it can share the avalanche gain to the preamplifying structure and the electrodes of detector so it reduces the operation voltage. However, the conventional gas electron multiplier (GEM) as the preamplifying structure shows an unstable signal output as a function of time, which is due to the charging effect. A new GEM having very steep wall sides and high-aspect ratio has been made by the LIGA process, and which can lead to stable signal output and larger avalanche signal. First experimental results show a large avalanche gain (lower limit to 3x10³), and a good time stability (∼ 2% during 30 min). A columnar cesium iodide (CsI) layer was initially intended as a dominant ionization source in gas avalanche microdetectors to overcome the angular dependence of the detector performance upon the incident radiation. However, based upon the results from the electric field simulation and experiments, it is revealed that the electric field does not penetrate sufficiently in the space between CsI columns to produce avalanches. Therefore there is no improvement with this. As an alternative structure, the LIGA technique has been proposed, because the micro-hole array made by this method can define the electric fields by introducing electrodes between the top and the bottom sides. From the feasibility test calculation, the detection efficiency is about 95% and the improvement of spatial resolution is a factor of 5 compared with a conventional microstrip detector accounting for 30 .deg. angle of the incident radiation

[en] The hydrogenated amorphous silicon (a-Si:H) holds good promise for radiation detection from its inherent merits over crystalline counterpart. For the application to alpha spectroscopy, the induced charge collection in a-Si:H pin detector diodes was simulated based on a relevant non-uniform charge generation model. From the simulation, the diode requires an intrinsic layer of thickness ∼ 10μm or larger. Also the input equivalent noise charge (ENC) as well as the reverse current for the sample diodes was measured and analyzed into three sources; shot noise, flicker or 1/f noise, and thermal noise from the contact resistance. By comparing the measured ENC with the calculated signal charge, the signal-to-noise ratio (S/N) for the sample diodes was estimated as a function of operational parameters; the applied reverse bias and the shaping time of Gaussian-pulse shaper. The analysis of signal and noise is useful for the optimum design of a-Si:H pin diodes for various charged particle radiation spectroscopy

[en] Linear analysis is on image quality(Fourier-based image quality), detector performance(Cascaded linear-systems theory on flat-panel detectors) and system performance(Dual-energy CBCT). Requirements and linearity, shift invariance and stationary noise. Dose-independent if only if the additive noise can be ignored. Additive noise is harmful to DQE at high frequencies where the number of secondary quanta lessens. Unfortunately, detectors and systems are neither LSI nor stationary. Nevertheless, the linear analysis(with reasonable assumptions) is useful to understand the working principle, and it can describe the actual performance in some limited extents

[en] The projection of three-dimensional (3D) human body on a two-dimensional (2D) radiograph results in the superimposition of normal tissue that can obscure abnormalities and in some common cases be misread as abnormalities. To reduce or eliminate this effect, 3D depth-discrimination techniques such as computed tomography can be used. Another method for improving conspicuity of abnormalities is an energy discrimination technique such as dual-energy imaging (DEI). The DEI discriminates, or enhances, material content (e.g. bone or soft tissue) within a 2D radiograph by combining images obtained at separte low and high energies. A commercial DEI system uses the fast kilovoltage (kVp) switching technique, which acquires low and highkVp projections in successive x-ray exposure. To obtain better quality in DE images, a large energy separation between the low and high-kVp setups is typically used for chest (e.g. 60/120 kVp). The optimal CsI thickness for dual-energy chest imaging has been theoretically investigated by evaluating prewhitening observer model detectability indexes. To evaluate the PW and PWE detectability indexes, dual-energy fluence and MTF have reviewed compared to the conventional descriptions

[en] To reduce the patient dose, several approaches such as spectral imaging using photon counting detectors and statistical image reconstruction, are being considered. Although image-reconstruction algorithms may significantly enhance image quality in reconstructed images with low dose, true signal-to-noise properties are mainly determined by image quality in projections. We are developing an analytical simulation platform describing projections to investigate how quantum-interaction physics in each component configuring CT systems affect image quality in projections. This simulator will be very useful for an improved design or optimization of CT systems in economy as well as the development of novel image-reconstruction algorithms. In this study, we present the progress of development of the simulation platform with an emphasis on the theoretical framework describing the generation of projection data. We have prepared the analytical simulation platform describing projections in computed tomography systems. The remained further study before the meeting includes the following: Each stage in the cascaded signal-transfer model for obtaining projections will be validated by the Monte Carlo simulations. We will build up energy-dependent scatter and pixel-crosstalk kernels, and show their effects on image quality in projections and reconstructed images. We will investigate the effects of projections obtained from various imaging conditions and system (or detector) operation parameters on reconstructed images. It is challenging to include the interaction physics due to photon-counting detectors into the simulation platform. Detailed descriptions of the simulator will be presented with discussions on its performance and limitation as well as Monte Carlo validations. Computational cost will also be addressed in detail. The proposed method in this study is simple and can be used conveniently in lab environment

[en] The dual-energy imaging has been extensively studied as a technique for distinction of lesions by suppressing anatomical background. The basis material decomposition is a technique for describing the linear attenuation coefficient of an arbitrary material using the linear attenuation coefficients of the basis materials. An arbitrary material can be expressed as a linear combination of known basis material information. The simulation images are obtained with 90 kVp and 0.5 mm-Cu intermediate filter thickness. Figure 3 shows the sandwich detector signal as a function of Al and PMMA thicknesses obtained using least-square regression. It can be seen that the signal decreases with increasing Al and PMMA thicknesses, and it can be confirmed that it is linearly regressed according to the combination of thickness. Figure 4 shows the Al and PMMA images obtained using material decomposition algorithm in dual-shot (60 kVp / 90 kVp) and single-shot (90 kVp) conditions. The material decomposition image using single shot shows a little more noise than the material decomposition image using dual shot.

[en] The location and statistics of secondary carriers are directly related to the absorbed x-ray energy. Even under monochromatic exposure conditions, there will be a distribution of absorbed energies due to the statistical nature of x-ray interactions. At diagnostic energies (10-120 keV), escape of fluorescent x rays following a photoelectric absorption and Compton-scattered x rays are primary sources of absorbed energy dispersion. This increases both variability in deposited x-ray energy and image noise in both energy-integrating and photon-counting x-ray detectors. Accurate measurement of incident photon energy is particularly important in photon-counting techniques such as K-edge imaging. Therefore, it is necessary to model and understand the effects of x-ray interaction physics on the absorbed energy distribution (AED) which describes the expected distribution of absorbed energy as a function of incident photon energy. This energy dispersion determines the energy resolution of spectroscopic systems and affects image quality in radiographic systems through the Swank factor and detective quantum efficiency. We believe understanding the AED will give insight to the fundamental performance limitations of both conventional and novel photon-counting x-ray imaging detectors

[en] While the front FPD(Flat-Panel Detectors) measures relatively low energy, the rear one measures relatively high energy because of x-ray beam hardening through the front FPD. Onto the same CMOS photodiode platform, thus, we placed a thicker scintillator in the rear FPD than the front one to achieve high quantum efficiency with the relatively higher-energy x-ray spectrum. An intermediate copper (Cu) filter can be used to further increase spectral separation between the two FPD measurements, which may provide a better contrast-to-noise performance in the subtracted images. We also observed that the single-shot method showed better SNR at higher spatial frequencies (e.g. edge regions and bone details) than the double-shot method. The reason can be explained by the inherent 'unsharp masking' effect of the sandwich detector; the rear FPD with a thicker scintillator provides a blurrier image than the front FPD, hence subtraction of the two images enhances edges in the resultant image. Bone-enhanced tomographic images have been obtained using dual-energy sandwich detectors for a postmortem mouse phantom, and they outperformed the tomographic images obtained from the conventional detectors (i.e. the front and rear flat-panel detectors constituting the sandwich detectors) for bone details. Although use of an intermediate filter, which was placed between the front and rear flat-panel detectors, resulted in less residual soft tissues in the reconstructed bone-enhanced images, it degraded the visual image quality of bone details because of increased noise

[en] For the fabrication of a Gas Electron Multiplier (GEM), a novel technique, LIGA process or deepetch X-ray lithography, is introduced. The LIGA process has advantages in design flexibility, such as in thickness and pitch, and allows increased path length for gas avalanche development. There is also a reduced internal detector capacitance due to the large aspect ratio. In addition, since polymethamethylcrylate (PMMA) insulating substrate of the LIGA-processed device has a relatively low surface resistivity (10¹⁴ Ω/□) compared with Kapton of the conventional GEM (10¹⁶ Ω/□), it is expected that the gain decrease due to event rate, and gain instabilities, will be less. Initial measurements of the LIGA-processed device yield a very promising performance such as the avalanche gas gain of ∼6,000. With improved or optimized design parameters in thickness, pitch, and circular hole, it is expected that the overall performances will be much enhanced

[en] In the direct conversion detector, the signal is transferred by the flow of charges generated when the interaction occurs in the photoconductor by incident energy. In this moment, if the charges generated are trapped by various factors while in constant time they travel to the electrodes, the loss of signal and addition of noise happen. This incomplete charge collection results in the degradation of the detector performance. So it is very important to investigate how the incomplete charge collection affects the performance of detector depending on different photoconductors to choose the best material and design the detector system. In this study, we investigated how the performance of the detector system is degraded due to incomplete charge collection in various photoconductors in terms of detective quantum efficiency