Olivier Tousignant

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

Evaluation of the imaging properties of an amorphous selenium-based flat panel detector for digital fluoroscopy

The imaging performance of an amorphous selenium (a-Se) flat-panel detector for digital fluoroscopy was experimentally evaluated using the spatial frequency dependent modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). These parameters were investigated at beam qualities and exposures within the range typical of gastrointestinal fluoroscopic imaging (approximately 0.1 - 10 microR, 75 kV). The investigation does not take into consideration the detector cover, which in clinical use will lower the DQE measured here by its percent attenuation. The MTF was found to be less than the expected aperture response and the NPS was not white which together indicate presampling blurring. The cause of this blurring was attributed to charge trapping at the interface between two different layers of the a-Se. The effect on the DQE was also consistent with presampling blur, which reduces the aliasing in the NPS and thereby reduces the spatial frequency dependence of the DQE. (The DQE was independent of spatial frequency from 0.12 to 0.73 mm(-1) due to antialiasing of the NPS.) Moreover, the first zero of the measured MTF and the aperture response appeared at the same spatial frequency (6.66 mm(-1) for a pixel of 150 microm). Hence, the geometric fill factor (77%) was increased to an effective fill factor of 99 +/- 1%. A large scale ( approximately 32 pixels) correlation in the noise due to the configuration of the readout electronics caused increased noise power in the gate line NPS at low spatial frequency (< 0.1 mm(-1)). The DQE (f = 0) was exposure independent over a large range of exposures but became exposure dependent at low exposures due to the electronic noise.

Dark current in multilayer amorphous selenium x-ray imaging detectors

A theoretical model for describing the bias-dependent transient behavior of dark current in multilayer (n-i-p) amorphous selenium (a-Se) detectors has been developed. The transient dark currents in these detectors are measured and are compared to the proposed dark current model. It has been found that the dark current is mainly controlled by Schottky emission of holes from the metal/a-Se contact. The initial and steady state dark currents are mainly controlled by the barrier height and the trap centers in the n layer, respectively.

Progress report on the performance of real-time selenium flat-panel detectors for direct x-ray imaging

Real time flat panel detectors based on amorphous selenium (a-Se) have demonstrated to be the most advanced technology for direct conversion X-ray imaging in various medical applications. In continuation of real time detector development, ANRAD Corporation introduce in this paper a large size 14 inches X 14 inches active area detector built with an amorphous selenium (a-Se) converter coated on a TFT array. This new detector is a scaled up version of the 9 inches X 9 inches presented last year based on a TFT array with 150 um x 150 um pixel and a 1000 mm thick a-Se PIN structure operated at 10V/um. DQE(f=0) measurements were performed in low dose range and demonstrated to be in agreement with a linear model including 2500e of electronic noise. It is also shown that the spatial resolution (MTF) could be controlled by selenium coating process and can almost reach the theoretical limit defined by the pixel pitch. Finally, the first 14 inches X 14 inches chest image is presented.

Dark current in multilayer stabilized amorphous selenium based photoconductive x-ray detectors

We report on experimental results which show that the dark current in n-i-p structured, amorphous selenium films is independent of i-layer thickness in samples with consistently thick blocking layers. We have observed, however, a strong dependence on the n-layer thickness and positive contact metal chosen. These results indicate that the dominant source of the dark current is carrier injection from the contacts and any contribution from carriers thermally generated in the bulk of the photoconductive layer is negligible. This conclusion is supported by a description of the dark current transients at different applied fields by a model which assumes only carrier emission over a Schottky barrier. This model also predicts that while hole injection is initially dominant, some time after the application of the bias, electron injection may become the dominant source of dark current.

Performance of a real-time selenium-based x-ray detector for fluoroscopy

As amorphous selenium based flat panel detectors gain more interest for direct, real-time x-ray imaging, we report in this paper the performance of such a detector by ANRAD Corporation. This new detector is based on a 1536 X 1536 array of amorphous silicon TFT pixels coupled with a 1000 micrometers selenium converter biased at 10 V/micrometers . Each 150 micrometers X 150 micrometers pixel is made of a thin film transistor, a storage capacitor and a collecting electrode having a geometrical fill factor of 77% and an effective fill factor of nearby 100%.

Development of solid‐state avalanche amorphous selenium for medical imaging

PURPOSEnActive matrix flat panel imagers (AMFPI) have limited performance in low dose applications due to the electronic noise of the thin film transistor (TFT) array. A uniform layer of avalanche amorphous selenium (a-Se) called high gain avalanche rushing photoconductor (HARP) allows for signal amplification prior to readout from the TFT array, largely eliminating the effects of the electronic noise. The authors report preliminary avalanche gain measurements from the first HARP structure developed for direct deposition onto a TFT array.nnnMETHODSnThe HARP structure is fabricated on a glass substrate in the form of p-i-n, i.e., the electron blocking layer (p) followed by an intrinsic (i) a-Se layer and finally the hole blocking layer (n). All deposition procedures are scalable to large area detectors. Integrated charge is measured from pulsed optical excitation incident on the top electrode (as would in an indirect AMFPI) under continuous high voltage bias. Avalanche gain measurements were obtained from samples fabricated simultaneously at different locations in the evaporator to evaluate performance uniformity across large area.nnnRESULTSnAn avalanche gain of up to 80 was obtained, which showed field dependence consistent with previous measurements from n-i-p HARP structures established for vacuum tubes. Measurements from multiple samples demonstrate the spatial uniformity of performance using large area deposition methods. Finally, the results were highly reproducible during the time course of the entire study.nnnCONCLUSIONSnWe present promising avalanche gain measurement results from a novel HARP structure that can be deposited onto a TFT array. This is a crucial step toward the practical feasibility of AMFPI with avalanche gain, enabling quantum noise limited performance down to a single x-ray photon per pixel.

Spatial and temporal image characteristics of a real-time large area a-Se x-ray detector

Large area, real-time, amorphous selenium (a-Se) based Flat Panel Detectors (FPD) were recently equipped with low noise front end electronics. In full resolution, 14”x14” detectors (FPD14) and 9”x9” detectors (FPD9) show an electronic noise of 1400 electrons. To evaluate the positive impact of such low noise on image quality, a dedicated report on spatial characteristics (MTF, NPS and DQE) covering the low dose range from 0.6 μR to 12 μR per frame, will be presented in the first section of this paper. For one RQA5 beam quality, DQE corrected for lag extrapolated at zero spatial frequency was equal to 0.6 for quantum noise limited exposure and equal to 0.4 for 0.6 μR. Almost no difference was found between 1x1 and 2x2 resolution mode giving the opportunity to 1x1 fluoroscopy. Recent advances to reduce image temporal artifacts such as lag and ghost will make the second part of this paper. It is demonstrated that the most significant contribution to detector lag is coming from the PIN selenium structure. Above electric field of 10 V/μm charges release from traps following one x-ray exposure could not explain selenium lag. Active ghost correction based on deep trapped charge recombination was developed giving good preliminary results in showing no residual ghost for a high dose rate of 33 mR/min.

Imaging performance of an amorphous selenium flat-panel detector for digital fluoroscopy

The imaging performance of a 34.5 x 34.5 cm2 direct conversion flat-panel detector with a 1 mm thick amorphous selenium layer was measured over the fluoroscopic exposure range (0.56 - 10.8 μR/frame). The pixels measured 300 x 300 μm. Measurements of the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE) were made. By comparing the MTF to the sinc function the measured effective fill factor of the active matrix was determined to be almost 100%. The electronic noise of the active matrix was measured and found to be 3800 electrons. The DQE(f) was found to be better than the expected sinc2 function. This was due to the presence of a pre-sampling blur identified as charge trapping at an interface in the a-Se layers. At the highest exposure investigated, the DQE(0) was found to be less than the quantum efficiency and the difference was ascribed to a combination of the electronic noise, a small drop in sensitivity due to the charge trapping blur, and incomplete charge collection.

Investigation of Ghosting Recovery Mechanisms in Selenium X-ray Detector Structures for Mammography

The ghosting recovery mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field . A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, detrapping, structural relaxation and electric field dependent electron-hole pair creation), and effects of charge injection from the metal contacts. Carrier trapping in both charged and neutral defect states has been considered in the model. It has been found that the X-ray induced deep trap centers are charged defects. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During the reverse bias, a huge number of carriers are injected from the metal contacts, and fill the existing trap centers. This results in an abrupt recovery of the relative sensitivity. However, the relative sensitivity slightly decreases with time after this abrupt recovery due to the release of the trapped electrons as well as the long recovery time of the induced trap centers. The theoretical model shows a very good agreement with the experimental results.