Direct-conversion Flat-panel Detector Research Articles

The Fabrication and Characterization of Direct Conversion Flat Panel X-ray Imager with TlBr Film

The novel direct conversion flat panel detectors (FPDs) utilizing thallium bromide (TlBr) films are developed and characterized in this study. Thin TlBr films, deposited using an evaporation technique, achieved thicknesses of 10 μm and 50 μm. A pixel sensor array, with a pixel size of 235 × 235 μm2 and a sensitive area of 29.61 × 39.48 mm2, was developed using low-temperature polysilicon thin-film transistor (LTPS TFT) technology as the initial study to explore performance characteristics of developed imaging system. The TlBr film was characterized based on the current-voltage (I-V) profile, showing a resistivity of 2.78 × 101⁰ Ω·cm. The LTPS TFT showed a leakage current of about 10-13 A, while the noise of TlBr FPD system was around 104 e- rms, primarily limited by the readout system. The detector system was characterized using radiation from an X-ray tube with a voltage in the range of 30 kV to 100 kV and a current of 2 mA, involving an X-ray test chart as the object with the processing time of 18.15 to 18.30 ms per frame. Based on the contrast and contrast-to-noise ratio (CNR) values, the 10 μm and 50 μm TlBr FPD clearly captured the image in operation between 30 kV and 100 kV. The 50 μm TlBr FPD demonstrated improved sensitivity, showing pixel values with 10 times higher intensity and enhanced line pair detail at 30 kV X-ray operation. The 10 μm TlBr achieved spatial resolution of 267 μm full width at half maximum (FWHM), while the 50 μm TlBr achieved 250 μm FWHM. In the X-ray imaging demonstration, the shape and structure of anchovy was clearly distinguished. The TlBr film FPD presents promising potential for a room temperature X-ray imager.

PRINCIPLES &FUNCTIONS OF CONVENTIONALS & DIGITAL X-RAYS IMAGING SYSTEMS IN DIAGNOSIS

Digital radiography (DR) is an advanced form of x-ray inspection which produces a digital radiographic image instantly on a computer. This technique uses x-ray sensitive plates to capture data during object examination, which is immediately transferred to a computer without the use of an intermediate cassette. The incident x-ray radiation is converted into an equivalent electric charge and then to a digital image through a detector sensor. Compared to other imaging devices, flat panel detectors, also known as digital detector arrays (DDAs) provide high quality digital images. They can have better signal-to-noise ratio and improved dynamic range, which, in turn, provides high sensitivity for radiographic applications. Flat panel detectors work on two different approaches, namely, indirect conversion and direct conversion. Indirect conversion flat panel detectors have a scintillator layer which converts x-ray photons to photons of visible light and utilise a photo diode matrix of amorphous silicon to subsequently convert the light photons into an electrical charge. This charge is proportional to the number and energy of x-ray photons interacting with the detector pixel and therefore the amount and density of material that has absorbed the x-rays. Direct conversion flat panel detectors use a photo conductor like amorphous selenium (a-Se) or Cadmium telluride (Cd-Te) on a multi-micro electrode plate, providing the greatest sharpness and resolution. The information on both types of detectors is read by thin film transistors. In the direct conversion process, when x-ray photons impact over the photo conductor, like amorphous Selenium, they are directly converted to electronic signals which are amplified and digitised. As there is no scintillator, lateral spread of light photons is absent here, ensuring a sharper image. This differentiates it from indirect construction.

Engineering of a Blocking Layer Structure for Low-Lag Operation of the a-PbO-Based X-Ray Detector

Direct conversion flat panel detectors are of great significance to the field of medical X-ray imaging since they offer imaging performance and diagnostic capabilities not achievable with other methods. Currently, mammographic direct conversion detectors employ a layer of amorphous selenium (a-Se) photoconductor. Although its properties ideally fit the requirements of mammography, where “soft” X-rays are used, a-Se cannot be used in high-energy X-ray procedures. To extend the diagnostic capabilities of the direct conversion detectors, amorphous lead oxide (a-PbO) is proposed as an alternative photoconductor. It is a high effective atomic number material and thus has a higher X-ray stopping power over the wide X-ray energy range. a-PbO is, therefore, a suitable candidate for applications in radiography, fluoroscopy, and digital tomosynthesis. Here, we report on the development of a blocking structure with a polyimide (PI) layer needed to maintain low dark current at high electric fields. We demonstrate that a 1- $\mu \text{m}$ -thick PI blocking layer allows the operation of the detector at strong electric fields (≥10 V/ $\mu \text{m}$ ) while suppressing the dark current to an innocuous level (< 1 pA/mm2). It also improves temporal performance by reducing signal lag. No ghosting effect was observed at exposure rates up to 1 R/s; however, at high radiation levels, the detector’s sensitivity degraded. This degradation is not permanent as the detector restores its original sensitivity after several hours of rest in the dark without bias applied.

A Novel Amorphous Selenium Avalanche Detector Structure for Low Dose Medical X-Ray Imaging

A novel amorphous selenium (a-Se) avalanche detector structure for low dose direct-conversion flat-panel X-ray detector is proposed. The proposed structure contains blocking layers to reduce carrier injection from metal electrodes and hole trapping layer to separate X-ray absorption layer from avalanche gain region. The feasibility of the structure for avalanche gain with negligible avalanche noise is studied by using the semiconductor module of COMSOL multiphysics together with a cascaded linear system. The model considers carrier injection from electrodes and charge carrier transport through various layers of multilayer a-Se structure in order to analyze the transient and steady-state electric field distribution across the detector. A cascaded linear system model that includes reabsorption of ${K}$ -fluorescent X-rays, carrier trapping in bulk and trapping layer, and avalanche multiplication of charge carrier is used to calculate the frequency-dependent detective quantum efficiency [DQE( ${f}$ )] and modulation transfer function (MTF) of the proposed structure. The avalanche gain enhances the signal strength and improves the DQE( ${f}$ ) by overcoming the effect of electronic noise at low X-ray doses. The structure is applied for breast tomosynthesis and observed that the proposed structure offers the required avalanche gain to ensure quantum noise limited operation at reduced exposures.

Performance evaluation of a direct-conversion flat-panel detector system in imaging and quality assurance for a high-dose-rate 192Ir source

In high-dose-rate (HDR) brachytherapy, a direct-conversion flat-panel detector (d-FPD) clearly depicts a 192Ir source without image halation, even under the emission of high-energy gamma rays. However, it was unknown why iridium is visible when using a d-FPD. The purpose of this study was to clarify the reasons for visibility of the source core based on physical imaging characteristics, including the modulation transfer functions (MTF), noise power spectral (NPS), contrast transfer functions, and linearity of d-FPD to high-energy gamma rays. The acquired data included: x-rays, [X]; gamma rays, [γ]; dual rays (X + γ), [D], and subtracted data for depicting the source ([D] − [γ]). In the quality assurance (QA) test for the positional accuracy of a source core, the coordinates of each dwelling point were compared between the planned and actual source core positions using a CT/MR-compatible ovoid applicator and a Fletcher–Williamson applicator. The profile curves of [X] and ([D] − [γ]) matched well on MTF and NPS. The contrast resolutions of [D] and [X] were equivalent. A strongly positive linear correlation was found between the output data of [γ] and source strength (r2 > 0.99). With regard to the accuracy of the source core position, the largest coordinate difference (3D distance) was noted at the maximum curvature of the CT/MR-compatible ovoid and Fletcher–Williamson applicators, showing 1.74 ± 0.02 mm and 1.01 ± 0.01 mm, respectively. A d-FPD system provides high-quality images of a source, even when high-energy gamma rays are emitted to the detector, and positional accuracy tests with clinical applicators are useful in identifying source positions (source movements) within the applicator for QA.

Optimization of dual-energy subtraction chest radiography by use of a direct-conversion flat-panel detector system.

We aimed to optimize the exposure conditions in the acquisition of soft-tissue images using dual-energy subtraction chest radiography with a direct-conversion flat-panel detector system. Two separate chest images were acquired at high- and low-energy exposures with standard or thick chest phantoms. The high-energy exposure was fixed at 120kVp with the use of an auto-exposure control technique. For the low-energy exposure, the tube voltages and entrance surface doses ranged 40-80 kVp and 20-100% of the dose required for high-energy exposure, respectively. Further, a repetitive processing algorithm was used for reduction of the image noise generated by the subtraction process. Seven radiology technicians ranked soft-tissue images, and these results were analyzed using the normalized-rank method. Images acquired at 60kVp were of acceptable quality regardless of the entrance surface dose and phantom size. Using a repetitive processing algorithm, the minimum acceptable doses were reduced from 75 to 40% for the standard phantom and to 50% for the thick phantom. We determined that the optimum low-energy exposure was 60kVp at 50% of the dose required for the high-energy exposure. This allowed the simultaneous acquisition of standard radiographs and soft-tissue images at 1.5 times the dose required for a standard radiograph, which is significantly lower than the values reported previously.

Investigation of image lag and modulation transfer function in fluoroscopy images obtained with a dynamic flat-panel detector

Digital imaging with a dynamic flat-panel detector (FPD) is commonly used in clinical practice. However, several factors reduce the accuracy of target tracking in fluoroscopic imaging, including image lag and blurring. There have been several reports focusing on the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) in different types of FPD. However, there have been no studies comparing image lag and MTF properties in dynamic images obtained with indirect- and direct-conversion FPDs. We investigated the image lag and MTF under several imaging conditions in fluoroscopic images obtained with an indirect-conversion and a direct-conversion FPD system. The measurements of image lag and MTF were obtained under several conditions according to IEC 62220-1-3 standards. We examined whether the image lag and MTF were influenced by the dose level and target movement speed. Indirect-conversion FPD showed dependence on the dose level, which was not observed for direct-conversion FPD. Furthermore, there were large differences in MTF between images of static and moving plate with indirect-conversion FPD in comparison to the differences observed with direct-conversion FPD. These results will be useful for the determination of imaging conditions for target tracking and other types of dynamic imaging.

Simulation system for understanding the lag effect in fluoroscopic images

Real-time tumor tracking in external radiotherapy can be achieved by diagnostic (kV) X-ray imaging with a dynamic flat-panel detector (FPD). It is crucial to understand the effects of image lag for real-time tumor tracking. Our purpose in this study was to develop a lag simulation system based on the image lag properties of an FPD system. Image lag properties were measured on flat-field images both in direct- and indirect-conversion dynamic FPDs. A moving target with image lag was simulated based on the lag properties in all combinations of FPD types, imaging rates, exposure doses, and target speeds, and then compared with actual moving targets for investigation of the reproducibility of image lag. Image lag was simulated successfully and agreed well with the actual lag as well as with the predicted effect. In the indirect-conversion FPD, a higher dose caused greater image lag on images. In contrast, there were no significant differences among dose levels in a direct-conversion FPD. There were no relationships between target speed and amount of image blurring in either type of FPD. The maximum contour blurring and the rate of increase in pixel value due to image lag were 1.1mm and 10.0%, respectively, in all combinations of imaging parameters examined in this study. Blurred boundaries and changes in pixel value due to image lag were estimated under various imaging conditions with use of the simulation system. Our system would be helpful for a better understanding of the effects of image lag in fluoroscopic images.

Feasibility of Direct-Conversion Flat Panel Detector (FPD) System as Quality Assurance (QA) Tool for 192-Ir Source of High-dose-rate (HDR) Remote Afterloading Unit: Evaluation of Positional and Temporal Accuracy

The direct-conversion FPD has made it possible to confirm the localization and configuration of a 192-Ir radioactive source due to its wide dynamic range and high spatial resolution, and absence of image halation (presented at ASTRO 2011, # 3438). The purpose of this study was to evaluate the accuracy of FPD for positioning and time duration (dwell time) of a 192-Ir source, and its feasibility as a QA tool for the 192-Ir source. Background and Purpose

Comparison of anti-scatter grids for digital imaging with use of a direct-conversion flat-panel detector

Our purpose in this study was to establish a selection standard for anti-scatter grids for a direct conversion flat-panel detector (FPD) system. As indices for grid evaluation, we calculated the selectivity, Bucky factor, and the signal-to-noise ratio improvement factor (SIF) by measuring rates of scatter transmission, primary transmission, and total transmission (based on the digitally displayed measurement values of the FPD system), using 4 acrylic phantoms of different thicknesses. The results showed that the SIF was less than 1.0 when the phantom thickness was 5 cm. When the phantom thickness was 25 cm and the grid ratio was 16:1, the SIF was 1.505 and 1.518 (maximum value) at 90 and 120 kV, respectively. Compared with the grid ratio of 12:1, the SIF at the grid ratio 16:1 was improved by 6.1% at 90 kV, and by 7.0% at 120 kV. In a direct-conversion FPD system, the grid ratio of 16:1 is considered adequate for eliminating the scattered-radiation effect when much scattered radiation is present, such as with a thick imaged object or a high X-ray tube voltage.

Potential of Direct Conversion Flat Panel Detector (FPD) as a New Quality Assurance (QA) Device for 192-Ir High Dose Rate (HDR) Remote Afterloading Units

Potential of Direct Conversion Flat Panel Detector (FPD) as a New Quality Assurance (QA) Device for 192-Ir High Dose Rate (HDR) Remote Afterloading Units

直接変換型ＦＰＤを搭載したディジタルＸ線透視撮影システムの画質特性と視覚評価の対応

The purpose of this study was to examine the correlation between the basic imaging properties of two digital radiographic X-ray systems with a direct conversion flat-panel detector and their image qualities, which were evaluated by the observer in hard copy and soft copy studies. The subjective image quality was evaluated and compared in terms of the low-contrast detectability and image sharpness in the two digital radiographic X-ray systems. We applied the radiographs of a contrast detail phantom to the evaluation of low-contrast detectability and analyzed the contrast detail diagrams. Finally, low-contrast detectability was evaluated by the image quality figure (IQF) calculated from the contrast detail diagrams. Also, the subjective image sharpness of human dry bones of two systems was examined and evaluated by the normalized-rank method. The results indicated that System A tended to provide superior subjective image quality compared to System B in both observer studies. We also found high correlations between IQFs and basic imaging properties, such as the noise power spectrum (NPS) and the noise equivalent quantum (NEQ). In conclusion, the low-contrast detectability of the two digital radiographic X-ray systems with a direct conversion flat-panel detector corresponded to the NPS and the NEQ in both outputs (soft copy and hard copy). On the other hand, the subjective image sharpness of human dry bones was affected by their noise properties.

直接変換型ＦＰＤを搭載したディジタルＸ線透視撮影システムの基本的な画質特性の比較

The purpose of this study was to evaluate the basic imaging properties of the two latest digital radiographic X-ray systems, namely, the DynaDirect Winscope 6000 (abbr. as System A, Toshiba Medical Systems) and the Sonialvision Safire Multi (abbr. as System B, Shimadzu Corp.). These systems were based on a direct-conversion flat panel detector (FPD) of amorphous selenium. The basic imaging properties of the two systems were evaluated by measuring characteristic curves, presampled modulation transfer functions (MTFs), and noise power spectra (NPS) using DICOM images to which no resampling was performed with a matrix size of 2048x2048. In addition, noise equivalent quanta (NEQ) and detective quantum efficiency (DQE) calculated from the result of the basic imaging properties were evaluated. The characteristic curves of the two systems showed quite high linearity. The MTFs of the two systems indicated high-resolution properties, as is well known to be an advantage of the direct conversion FPD system. However, the NPS of System A showed better performance than System B under the same exposures. Therefore, the DQE of System A was higher than that of System B at all spatial frequencies.

X‐ray imaging using avalanche multiplication in amorphous selenium: Investigation of depth dependent avalanche noise

The past decade has seen the swift development of the flat-panel detector (FPD), also known as the active matrix flat-panel imager, for digital radiography. This new technology is applicable to other modalities, such as fluoroscopy, which require the acquisition of multiple images, but could benefit from some improvements. In such applications where more than one image is acquired less radiation is available to form each image and amplifier noise becomes a serious problem. Avalanche multiplication in amorphous selenium (a-Se) can provide the necessary amplification prior to read out so as to reduce the effect of electronic noise of the FPD. However, in direct conversion detectors avalanche multiplication can lead to a new source of gain fluctuation noise called depth dependent avalanche noise. A theoretical model was developed to understand depth dependent avalanche noise. Experiments were performed on a direct imaging system implementing avalanche multiplication in a layer of a-Se to validate the theory. For parameters appropriate for a diagnostic imaging FPD for fluoroscopy the detective quantum efficiency (DQE) was found to drop by as much as 50% with increasing electric field, as predicted by the theoretical model. This drop in DQE can be eliminated by separating the collection and avalanche regions. For example by having a region of low electric field where x rays are absorbed and converted into charge that then drifts into a region of high electric field where the x-ray generated charge undergoes avalanche multiplication. This means quantum noise limited direct conversion FPD for low exposure imaging techniques are a possibility.

ＦＰＤシステムにおけるリサンプリングが基本的な画質特性の測定に及ぼす影響

We investigated the effect on image data resampling in an evaluation of the basic imaging properties for a digital radiographic system based on a flat panel detector (FPD). One of the latest digital radiographic systems was used in this study. This system was based on a direct-conversion FPD of amorphous selenium. The basic imaging properties of the system were evaluated by measuring characteristic curve, presampled modulation transfer function (MTF), and Wiener spectrum (WS) using DICOM image with a matrix size of 2048 x 2048. The evaluations were performed under two conditions because matrix size automatically changes according to the selection of imaging size. One of the conditions was a different matrix size between image data acquired on the FPD and the output image (DICOM image for which resampling was performed). The other condition was that these matrices be the same size (DICOM image with no resampling performed). Resampling did not affect the characteristic curves. However, MTF and the WS obtained from the resampled data were different from those of the one not resampled, which is considered to be the "inherent" basic imaging properties, and this phenomenon was remarkable, especially in terms of the MTFs. Our study indicates that the effect on resampling should not be disregarded in evaluating the basic imaging properties of digital radiographic systems. Therefore, it is mandatory to use DICOM images for which no resampling was performed in order to evaluate the inherent basic imaging properties for digital radiographic systems.

Comparison of different commercial FFDM units by means of physical characterization and contrast‐detail analysis

The purpose of this study was to perform a complete evaluation of three pieces of clinical digital mammography equipment. Image quality was assessed by performing physical characterization and contrast-detail (CD) analysis. We considered three different FFDM systems: a computed radiography unit (Fuji "FCR 5000 MA") and two flat-panel units, the indirect conversion a-Si based GE "Senographe 2000D" and the direct conversion a-Si based IMS "Giotto Image MD." The physical characterization was estimated by measuring the MTF, NNPS, and DQE of the detectors with no antiscatter grid and over the clinical range of exposures. The CD analysis was performed using a CDMAM 3.4 phantom and custom software designed for automatic computation of the contrast-detail curves. The physical characterization of the three digital systems confirms the excellent MTF properties of the direct conversion flat-panel detector (FPD). We performed a relative standard deviation (RSD) analysis, for investigating the different components of the noise presented by the three systems. It turned out that the two FPDs show a significant additive component, whereas for the CR system the statistical noise is dominant. The multiplicative factor is a minor constituent for all the systems. The two FPDs demonstrate better DQE, with respect to the CR system, for exposures higher than 70 microGy. The CD analysis indicated that the three systems are not statistically different for detail objects with a diameter greater than 0.3 mm. However, the IMS system showed a statistically significant different response for details smaller than 0.3 mm. In this case, the poor response of the a-Se detector could be attributed to its high-frequency noise characteristics, since its MTF, NEQ, and DQE are not inferior to those of the other systems. The CD results were independent of exposure level, within the investigated clinical range. We observed slight variations in the CD results, due to the changes in the visualization parameters (window/level and magnification factor). This suggests that radiologists would benefit from viewing images using varied window/level and magnification.

Compatibility of interventional x-ray and magnetic resonance imaging: Feasibility of a closed bore XMR (CBXMR) system

A next-generation interventional guidance system is proposed that will enable intraprocedural access to both x-ray and magnetic resonance imaging (MRI) modalities. This closed bore XMR (CBXMR) system is comprised of a conventional radiographic rotating anode x-ray tube and a direct conversion flat panel detector on a rotating gantry positioned adjacent to the bore of a 1.5 T MRI. To assess the feasibility of such a system, we have investigated the degree of compatibility between the x-ray components and the MRI. For /-->B(ext)/ < 200 G the effect on the radiographic tube motor was negligible regardless of the orientation of -->B(ext) with respect to the motor axis of rotation--the frequency of anode rotation remained within 6% of the 3400 rpm frequency measured at 0 G. For /-->B(ext)/ >2400 G the anode slowed down to below 2400 rpm at all orientations. At intermediate B(ext), the frequency of rotation varied between 2400 and 3200 rpm, showing a strong dependence on orientation, with -->B(ext) perpendicular to the tube axis having a much stronger effect on the rotation frequency than -->B(ext) parallel to the tube axis. In contrast to the effect of -->B(ext) on the induction motor, parallel -->B(ext) had a stronger detrimental effect on the cathode-anode electron beam, whose path was at 16 degrees to the tube axis, than the perpendicular -->B(ext). Parallel -->B(ext) of several hundred Gauss had a defocusing effect on the x-ray focal spot. -->B(ext) perpendicular to the electron beam shifted the beam without significant defocusing. We have determined that the direct conversion flat panel detector (FPD) technology is not intrinsically sensitive to -->B(ext), and that the modifications required to make the proposed FPDs MRI compatible are minimal. The homogeneity of the MRI signal in the normal field of view was not significantly degraded by the presence of these x-ray components in the vicinity of the MRI bore entrance.

Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium

Direct flat-panel detectors using amorphous selenium (a-Se) x-ray photoconductors are gaining wide-spread clinical use. The goal of our investigation is to understand the physical mechanisms responsible for ghosting, i.e., x-ray induced change in sensitivity that results in image persistence, so that the knowledge can be used to consistently minimize ghosting artifacts in a-Se flat-panel detectors. In this paper we will discuss the effect on x-ray sensitivity of charge trapping in a-Se, which is the dominant source for ghosting in a-Se flat-panel detectors. Our approach is to correlate ghosting in electroded a-Se detectors with the trapped charge concentration measured by the "time-of-flight" (TOF) method. All measurements were performed as a function of radiation exposure X of up to approximately 20 R at electric field strength's of E(Se)=5 and 10 V/microm. The results showed that the x-ray sensitivity decreased as a function of X and the amount of ghosting decreased with increasing E(Se). The shape of the TOF curves changed as a result of irradiation in a manner indicating trapped electrons in the bulk of a-Se. The density of trapped electrons n(t) increases as a function of X. A method was developed to determine the values of n(t) in the bulk of a-Se from the TOF measurements, and to predict the corresponding change in x-ray sensitivity. Our results showed that a recombination coefficient consistent with that predicted by Langevin produced good agreement between calculated and measured x-ray sensitivity changes. Thus it can be concluded that the trapping of electrons in the bulk of a-Se and their subsequent recombination with x-ray generated free holes is the dominant mechanism for ghosting in a-Se.

X-ray-induced recombination effects in a-Se-based x-ray photoconductors used in direct conversion x-ray sensors

Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient Cr between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the concentration of x-ray-induced negative centers increases almost linearly with the x-ray exposure. As a corollary, in terms of fundamental physics of amorphous semiconductors, we can also conclude that the influence of potential fluctuations in the noncrystalline structure in shielding a charged center in a-Se is relatively small.

Review X-ray photoconductors and stabilized a-Se for direct conversion digital flat-panel X-ray image-detectors

Recently developed direct conversion flat-panel X-ray image detectors provide not only potentially superior images but also enable a simple and convenient means of achieving digital radiography. The present flat-panel sensors use stabilized a-Se as an X-ray photoconductor to convert absorbed X-ray photons to collectable charge carriers. The present paper outlines the basics of the direct conversion flat-panel X-ray detector, formulates and reviews the required X-ray photoconductor properties and examines to what extent stabilized amorphous selenium (a-Se) fulfills these requirements and how it compares with other potential X-ray photoconductors that have been proposed. Charge transport and the electron–hole pair creation energy in stabilized a-Se are reviewed within the context of our present knowledge. Current materials problems and future potential developments are also critically discussed.