A theoretical model for describing the bias-dependent transient behavior of dark current in multilayer amorphous
selenium (a-Se) detectors has been developed by solving the trapping rate equations and Poisson's equation in the a-Se
layer. The transient dark currents in these detectors are measured and the proposed dark model is compared with the
measured data. The model shows a very good agreement with the experimental results. It has been found that the dark
current is mainly controlled by the Schottky emission of holes from the metal/a-Se contact. The space charge build-up
due to the hole injection and trapping in the blocking layer reduces the internal field at the metal/a-Se interface of
positive side and thus the dark current eventually is limited by the space charge. It has been found that the electric fields
at the metal contacts reduce to 20-30% of the applied field (applied voltage/thickness). The comparison of the model
with the experimental data estimates some important properties (e.g., trap center concentrations, space charges, and
effective barrier heights) of the blocking layers of the multilayer detectors. The dependence of the X-ray sensitivity of
multilayer a-Se X-ray imaging detectors on repeated X-ray exposures is studied by considering accumulated trapped
charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron-hole pair
creation), the carrier transport in the blocking layers, X-ray induced metastable deep trap center generations, and the
effects of dark current. We simultaneously solve the continuity equations for both holes and electrons, trapping rate
equations, and the Poisson's equation across the photoconductor for a step X-ray exposure by the Backward Euler finite
difference method. The theoretical model shows a very good agreement with the experimental relative sensitivity versus
cumulative X-ray exposure characteristics. The electric field distribution across the multilayer detector and the dark
current density under repeated exposures are also estimated.
A prototype breast tomosynthesis system has been developed, allowing a total angular view of ±25°. The detector used in this system is an amorphous selenium direct-conversion digital flat-panel detector suitable for digital tomosynthesis. The system is equipped with various readout sequences to allow the investigation of different tomosynthetic data acquisition modes. In this paper, we will present basic physical properties -- such as MTF, NPS, and DQE -- measured for the full resolution mode and a binned readout mode of the detector. From the measured projections, slices are reconstructed employing a special version of filtered backprojection algorithm. In a phantom study, we compare binned and full resolution acquisition modes with respect to image quality. Under the condition of same dose, we investigate the impact of the number of views on artifacts. Finally, we show tomosynthesis images reconstructed from first clinical data.
In this paper, two new selenium large area detectors are introduced. The first detector is a 24x30cm detector suitable for full-field digital mammography applications. The second detector is a new 43x43cm detector for general radiographic applications. Both detectors are capable of static and dynamic imaging, and are quantum noise limited over the exposure ranges intended for their typical use. For static imaging applications, ghost and lag were compared on both detectors, and no measurable artefacts were reported. On dynamic imaging sequences, lag was shown to be significant on both detectors, and a method for reducing the artifact due to lag was presented.
The advent of digital detectors will enable several advanced imaging applications to be used in the fight against breast cancer. For example, dynamic imaging applications such as tomosynthesis, contrast enhanced and dual energy mammography have demonstrated promising results. In this paper, we will assess the suitability of this detector for these advanced applications. MTF and DQE measurements were performed on a selenium FFDM detector to assess image quality. Ghosting properties of a digital detector are also an important factor, since it can strongly degrade image quality. In this paper, we will also report on the ghosting characteristics of the selenium detector, using typical exposures envisioned to be used in tomosynthesis exams. The physical mechanisms that create ghost images will be discussed and will be quantified.
A selenium-based flat-panel direct converter detector suitable for digital mammography was developed. The detector is based on a TFT-array with a resolution of 2816 X 2048 pixels, and a pixel pitch of 85 micrometers . Although the geometric fill factor for each pixel is around 70% , the effective fill factor for the detector is closer to 88% due to internal electric field shaping within the selenium layer. A selenium multilayer p-i-n structure of 200 micrometers was deposited onto the array by selectively doping the regions near each contact to produce unipolar conducting blocking layers. This structure absorbs more than 95% of a typical mammography beam.
The purpose of this paper is to analyze the image quality of a selenium-based flat panel detector suited for digital interventional mammography. To characterize the image quality, the DQE was measured at various x-ray exposures. The results indicate that when the detector is quantum noise limited, the DQE is independent of the exposure. A measurement of the quantum detection efficiency of 90% indicates that an electrostatic field shaping effect within the selenium layer gives a greater collection efficiency than that predicted simply by the geometric fill factor of each pixel collection electrode. Measurements were also conducted to determine the relative strength of ghost images on the detector. An image of a high contrast object using an exposure of 183 mR was acquired, followed by a low exposure 6 mR flat field image. No visual indication of a ghost could be found in the latter image even after appropriate windowing and leveling of the image was performed. A subjective comparison of image quality between film/screen and the detector was conducted by acquiring images of the ACR phantom under various exposure conditions. The digital images were printed on film using optimally adjusted LUT's. The resulting images were randomly presented to 15 non-trained observers, who assessed a score for each image. The comparison results show that the image quality obtained with the digital detector is superior to the images acquired with film/screen.
In this paper, we report measurements from a prototype 1024 X 1024 selenium-based flat panel detector suited for interventional digital mammography applications. This detector is based on an amorphous silicon TFT array, with a pixel pitch of 85 micrometer and a fill factor of 70%. A 200 micrometer layer of amorphous selenium is used to directly convert the incident x-rays into electrical charges. The detector electronics, TFT array, and selenium converter structure are designed to operate at a frame rate of 10 images per second. Experimentally, this detector yields an x-ray sensitivity of nearly 290 electrons/absorbed x-ray nearly 100% absorption of x-rays at a beam energy of 18 keV, a high spatial resolution (limited only by the pixel pitch up to the Nyquist limit), and quantum-noise limited operation down to the lowest exposures currently investigated. Images from the ACR phantom and contrast detail phantom reveal all embedded targets in the phantoms, which indicates the potential of this technology for digital mammography.