Flat-panel X-ray Imaging Research Articles

Evaluation of organic perovskite photoconductors for direct conversion X-ray imaging detectors

Enhancing the sensitivity of a direct conversion flat panel X-ray imaging detector with minimum manufacturing cost has been a major dream for long decades. This criterion has been recently addressed by the usage of MAPbX3 (MA is CH3NH3 and X is a halogen atom such as Cl, I, or Br) perovskite in X-ray imaging detectors. Though MAPbI3 has shown large area deposition capability and good X-ray sensitivity, it has to fulfil other criteria such as low dark current, high spatial resolution and high signal to noise transfer capabilities. This paper evaluates the imaging performances such as X-ray sensitivity, detective quantum efficiency (DQE) and modulation transfer function (MTF) of organic perovskites (e.g., MAPbI3 and MAPbBr3) with comparison to amorphous selenium (a-Se). These perovskite materials have slightly higher linear attenuation coefficients than a-Se and the expected X-ray sensitivity of these two perovskite photoconductors are higher than a-Se. The mechanisms of the dark current and photocurrent gain in MAPbI3 detector are also investigated. The MAPbI3 detector shows some photocurrent gain, which is due to the enhanced electron injection under X-ray illumination. The expected theoretical zero spatial frequency DQE of the MAPbI3 detectors is similar to that of a-Se while the MAPbBr3 detector establishes a better DQE than a-Se. The expected MTF of the MAPbBr3 detectors is similar to that of a-Se while the MAPbI3 shows worse resolution than a-Se. Based upon our theoretical investigation, we believe that the organic perovskite can find its state of the art in near future if rigorous research for improving the charge carrier transport properties and optimizing its detector structure for low dark current were to be made.

X-ray performance of a wafer-scale CMOS flat panel imager for applications in medical imaging and nondestructive testing

This paper presents a wafer-scale complementary metal-oxide semiconductor (CMOS)-based X-ray flat panel detector for medical imaging and nondestructive testing applications. In this study, our proposed X-ray CMOS flat panel imager has been fabricated by using a 0.35µm 1-poly/4-metal CMOS process. The pixel size is 100µm×100µm and the pixel array format is 1200×1200 pixels, which provide a field-of-view (FOV) of 120mm×120mm. The 14.3-bit extended counting analog-to digital converter (ADC) with built-in binning mode was used to reduce the area and simultaneously improve the image resolution. The different screens such as thallium-doped CsI (CsI:Tl) and terbium gadolinium oxysulfide (Gd2O2S:Tb) scintillators were used as conversion materials for X-rays to visible light photons. The X-ray imaging performance such as X-ray sensitivity as a function of X-ray exposure dose, spatial resolution, image lag and X-ray images of various objects were measured under practical medical and industrial application conditions. This paper results demonstrate that our prototype CMOS-based X-ray flat panel imager has the significant potential for medical imaging and non-destructive testing (NDT) applications with high-resolution and high speed rate.

Theoretical and Monte Carlo optimization of a stacked three-layer flat-panel x-ray imager for applications in multi-spectral medical imaging.

We propose a new design of a stacked three-layer flat-panel x-ray detector for dual-energy (DE) imaging. Each layer consists of its own scintillator of individual thickness and an underlying thin-film-transistor-based flat-panel. Three images are obtained simultaneously in the detector during the same x-ray exposure, thereby eliminating any motion artifacts. The detector operation is two-fold: a conventional radiography image can be obtained by combining all three layers' images, while a DE subtraction image can be obtained from the front and back layers' images, where the middle layer acts as a mid-filter that helps achieve spectral separation. We proceed to optimize the detector parameters for two sample imaging tasks that could particularly benefit from this new detector by obtaining the best possible signal to noise ratio per root entrance exposure using well-established theoretical models adapted to fit our new design. These results are compared to a conventional DE temporal subtraction detector and a single-shot DE subtraction detector with a copper mid-filter, both of which underwent the same theoretical optimization. The findings are then validated using advanced Monte Carlo simulations for all optimized detector setups. Given the performance expected from initial results and the recent decrease in price for digital x-ray detectors, the simplicity of the three-layer stacked imager approach appears promising to usher in a new generation of multi-spectral digital x-ray diagnostics.

Flat Panel X-ray Imaging of LuAG:Ce,Mg Ceramic Scintillators

(Lu, Mg, Ce)(3)Al5O12 (LuAG: Ce, Mg) transparent ceramic scintillators (LuAG: 0.3at% Ce, 0.2at% Mg) with high optical and scintillation efficiency were fabricated by solid-state reaction method combined with vacuum sintering. The ceramic scintillator with dimension of 20 mm x 20 mm x 0.05 mm was packed with graphite substrate of 25 mm x 25 mm x 4 mm by graphite powders contained silica gel. Laser cutting of cubic array with dimension of 50 mu m x 50 mu m and distance of 10 mu m was performed. Flat panel X-ray imaging based on as prepaired ceramics was demonstrated. The images were good quality with a high resolution. The quality of the photos were evaluated by line-pair pattern method and knife edge method, respectively. MTF value of 17.5% was at 10 lp/mm for the ceramic scintillators after laser cutting, while resolution of 9 lp/mm achieved at 10% MTF by knife edge measurement. Based on above data, LuAG:Ce,Mg ceramics are proved to be a promising scintillator in flat panel X-ray imaging.

(Invited) Flat-Panel X-ray Image Sensor Using Thin Film Transistors and Field Emitter Arrays

Active matrix flat panel imagers (AMFPI) have been commercialized for a wide range of x-ray imaging applications. They have demonstrated superior imaging performance except in fluoroscopy, where the electronic noise degrades the image quality, making it inferior to x-ray imaging intensifier (XRII). In this paper we will present two FPI approaches under investigation to overcome the electronic noise limitation. They use an amorphous selenium layer with programmable avalanche gain to detect light generated from an x-ray scintillator upon x-ray absorption. Two charge readout methods are being investigated: a thin-film transistor (TFT) array; and a field emitter array (FEA). The amorphous selenium (a-Se) avalanche photoconductor is called HARP (high-gain avalanche rushing photoconductor). The avalanche gain of HARP depends on both a-Se thickness and applied electric field ESe. At ESe of > 80 V/μm, the avalanche gain can enhance the signal at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing ESe to < 70 V/μm, thus ensuring a wide dynamic range without burdening the readout electronics. The potential x-ray imaging performance of both FPI approaches, especially the aspect of programmable gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure in fluoroscopy, have been investigated.

A novel technique for image-guided local heart irradiation in the rat.

In radiotherapy treatment of thoracic, breast and chest wall tumors, the heart may be included (partially or fully) in the radiation field. As a result, patients may develop radiation-induced heart disease (RIHD) several years after exposure to radiation. There are few methods available to prevent or reverse RIHD and the biological mechanisms remain poorly understood. In order to further study the effects of radiation on the heart, we developed a model of local heart irradiation in rats using an image-guided small animal conformal radiation therapy device (SACRTD) developed at our institution. First, Monte Carlo based simulations were used to design an appropriate collimator. EBT-2 films were used to measure relative dosimetry, and the absolute dose rate at the isocenter was measured using the AAPM protocol TG-61. The hearts of adult male Sprague-Dawley rats were irradiated with a total dose of 21 Gy. For this purpose, rats were anesthetized with isoflurane and placed in a custom-made vertical rat holder. Each heart was irradiated with a 3-beam technique (one AP field and 2 lateral fields), with each beam delivering 7 Gy. For each field, the heart was visualized with a digital flat panel X-ray imager and placed at the isocenter of the 1.8 cm diameter beam. In biological analysis of radiation exposure, immunohistochemistry showed γH2Ax foci and nitrotyrosine throughout the irradiated hearts but not in the lungs. Long-term follow-up of animals revealed histopathological manifestations of RIHD, including myocardial degeneration and fibrosis. The results demonstrate that the rat heart irradiation technique using the SACRTD was successful and that surrounding untargeted tissues were spared, making this approach a powerful tool for in vivo radiobiological studies of RIHD. Functional and structural changes in the rat heart after local irradiation are ongoing.

Scintillator high‐gain avalanche rushing photoconductor active‐matrix flat panel imager: Zero‐spatial frequency x‐ray imaging properties of the solid‐state SHARP sensor structure

The authors are investigating the feasibility of a new type of solid-state x-ray imaging sensor with programmable avalanche gain: scintillator high-gain avalanche rushing photoconductor active matrix flat panel imager (SHARP-AMFPI). The purpose of the present work is to investigate the inherent x-ray detection properties of SHARP and demonstrate its wide dynamic range through programmable gain. A distributed resistive layer (DRL) was developed to maintain stable avalanche gain operation in a solid-state HARP. The signal and noise properties of the HARP-DRL for optical photon detection were investigated as a function of avalanche gain both theoretically and experimentally, and the results were compared with HARP tube (with electron beam readout) used in previous investigations of zero spatial frequency performance of SHARP. For this new investigation, a solid-state SHARP x-ray image sensor was formed by direct optical coupling of the HARP-DRL with a structured cesium iodide (CsI) scintillator. The x-ray sensitivity of this sensor was measured as a function of avalanche gain and the results were compared with the sensitivity of HARP-DRL measured optically. The dynamic range of HARP-DRL with variable avalanche gain was investigated for the entire exposure range encountered in radiography∕fluoroscopy (R∕F) applications. The signal from HARP-DRL as a function of electric field showed stable avalanche gain, and the noise associated with the avalanche process agrees well with theory and previous measurements from a HARP tube. This result indicates that when coupled with CsI for x-ray detection, the additional noise associated with avalanche gain in HARP-DRL is negligible. The x-ray sensitivity measurements using the SHARP sensor produced identical avalanche gain dependence on electric field as the optical measurements with HARP-DRL. Adjusting the avalanche multiplication gain in HARP-DRL enabled a very wide dynamic range which encompassed all clinically relevant medical x-ray exposures. This work demonstrates that the HARP-DRL sensor enables the practical implementation of a SHARP solid-state x-ray sensor capable of quantum noise limited operation throughout the entire range of clinically relevant x-ray exposures. This is an important step toward the realization of a SHARP-AMFPI x-ray flat-panel imager.

ACR–AAPM–SIIM Practice Guideline for Digital Radiography

This guideline was developed collaboratively by the American College of Radiology (ACR), the American Association of Physicists in Medicine (AAPM), and the Society for Imaging Informatics in Medicine (SIIM). Increasingly, medical imaging and patient information are being managed using digital data during acquisition, transmission, storage, display, interpretation, and consultation. The management of these data during each of these operations may have an impact on the quality of patient care. “CR” and “DR” are the commonly used terms for digital radiography detectors. CR is the acronym for computed radiography, and DR is an acronym for digital radiography. CR uses a photostimulable storage phosphor that stores the latent image, which is subsequently read out using a stimulating laser beam. It can be easily adapted to a cassette-based system analogous to that used in screen-film (SF) radiography. Historically, the acronym DR has been used to describe a flat-panel digital X-ray imaging system that reads the transmitted X-ray signal immediately after exposure with the detector in place. Generically, the term CR is applied to passive detector systems, while the term DR is applied to active detectors. This guideline is applicable to the practice of digital radiography. It defines motivations, qualifications of personnel, equipment guidelines, data manipulation and management, and quality control (QC) and quality improvement procedures for the use of digital radiography that should result in high-quality radiological patient care. In all cases for which an ACR practice guideline or technical standard exists for the modality being used or the specific examination being performed, that guideline or standard will continue to apply when digital image data management systems are used.

Flexible X-Ray Detector Array Fabricated With Oxide Thin-Film Transistors

A flat-panel flexible X-ray image sensor fabricated with oxide thin-film transistors (TFTs) is described. Bottom-gate GaInZnO TFTs are fabricated on a plastic substrate and integrated with amorphous silicon p-i-n photodiodes, with a maximum process temperature of 170°C . X-ray images obtained with a 160 × 180 pixel array in the indirect detection mode are reported.

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.

Effect of 1/ f noise in integrating sensors and detectors

The authors calculate the variance in the output of an integrating sensor or detector when in the presence of 1/fα noise in the input of the sensor. The calculations are based on mapping the detector onto a linear, time-invariant filter; the approach is general and can be used for any detector that can be so mapped. Formulae for the output variance and signal-to-noise ratio are given for a simple integrating detector and a detector with three different methods of background subtraction, including double sampling, that has two integrations, and triple sampling where the average of two integrations before and after the signal is subtracted from the integration during the signal. The authors consider cases in which α is unity, less than unity and more than unity, given that quite often α is not ideally unity. Also, for the case of an integrating detector that is used to sample a signal, a formula is derived for the expected variance of N samples when the input contains 1/f noise. The authors apply the treatise herein to the input stage of an a-Se based flat panel X-ray image detector and demonstrate that the 1/f noise fluctuations in the dark current of the photoconductor exceeds those because of shot noise.

Integration of flat panel X-ray detector for high resolution diagnostic medical imaging

In these days, flat panel X-ray image detectors have shown their potential for replacing traditional screen-film systems. To detect the X-ray photon energy, there are two main methods known as a direct method and an indirect method. The X-rays are converted immediately into electrical signal with the direct method. The indirect method has two conversion steps: the scintillator absorbs the X-rays and converts them to visible light. And then the visible light is converted to electrical signal (e.g. by photodiodes). In this work, the flat panel digital X-ray image detector based on direct method with a high atomic number material was designed and evaluated. The high atomic number material for X-ray conversion is deposited by a rubbing method with about 300 μm. The rubbing method is similar to the screen printing method. It consists of two elements: the screen and the squeegee. The method uses a proper stiff bar stretched tightly over a frame made of wood or metal. Proper tension is essential for proper laminated structure. The detector prototype has 139 μm pixel pitch, total 1280×1536 pixels and 86% fill factor. Twelve readout ICs are installed on digital X-ray detector and simultaneously operated to reach short readout time. The electronics integrated: the preamplifier to amplify generated signal, the Analog to Digital converter and the source of bias voltage (1 V/μm). The system board and interface use an NI-camera program. Finally, we achieved images from this flat panel X-ray image detector.

Photon and primary electron arithmetics in photoconductors for digital mammography: Monte Carlo simulation studies

Materials like CdTe, CdZnTe, Cd0.8Zn0.2Te and ZnTe are suitable photoconductors for direct conversion digital flat panel x-ray image detectors. The x-ray induced primary electrons inside photoconductor's bulk comprise the initial signal that propagates and forms the final signal (image) on detector's electrodes. An already developed Monte Carlo model that simulates the primary electron generation in photoconducting materials, such as those mentioned above, has been used to study the arithmetics of fluorescent photons, escaping photons and primary electrons in these materials in the mammographic energy range. In this way insights are gained concerning the primitive stage of signal formation that lead to the investigation of the factors that affect the primary electron generation. It was found that for the typical photoconductor thicknesses (300-1000 μm): (i) in all materials and incident energies the escaping of primary photons is negligible and does not influence the number of primary electrons, (ii) the overwhelming majority of escaping photons is backwards escaping fluorescent photons, (iii) the number of primary electrons increases at energies higher than Cd and Te K-edges where the fluorescent photon escaping decreases and their absorption is followed by long atomic deexcitation cascades and (iv) ZnTe has the maximum number of primary electrons produced for energies between 16 and 26 keV while CdTe for higher energies.

Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2

Active matrix, flat-panel x-ray imagers based on a-Si:H thin-film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal-to-noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI2 is of considerable interest by virtue of its low effective work function, high atomic number and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach. Publisher's note: The title was changed from: 'Signal behavior of polycrystalline HgI2 at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers' to 'Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2' 24 hours after initial publication to correct a mistake.

Selenium-based amorphous silicon flat-panel digital X-ray imager for protein crystallography

This work proposes a large-area detector for protein crystallography based on an amorphous silicon (a-Si:H) thin film transistor (TFT) pixel-array backplane and an overlying amorphous selenium (a-Se) photoconductor for direct conversion of incident X-rays into an image charge. To achieve high sensitivity, avalanche multiplication in a-Se is adopted to make the detector sensitive to each incident X-ray. The use of a-Si:H technology enables large-area imaging of protein diffraction patterns at less expense compared to existing charge coupled device (CCD) and imaging plate (IP) detectors. In addition, a theoretical analysis shows that the detector exhibits fast readout speed (readout time <1 s), high dynamic range (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ), high sensitivity (~1 X-ray photon), and high detective quantum efficiency (~0.7), thus validating its suitability for protein crystallography.

Noise Characterization of Polycrystalline Silicon Thin Film Transistors for X-ray Imagers Based on Active Pixel Architectures

An examination of the noise of polycrystalline silicon thin film transistors, in the context of flat panel x-ray imager development, is reported. The study was conducted in the spirit of exploring how the 1/f, shot and thermal noise components of poly-Si TFTs, determined from current noise power spectral density measurements, as well as through calculation, can be used to assist in the development of imagers incorporating pixel amplification circuits based on such transistors.

Comparison of Amorphous Selenium and Mercuric Iodide Film for Diagnostic X-Ray Imaging

In this paper, we present the imaging parameters and compare both mercuric iodide (HgI2) and amorphous selenium (a-Se) films. Using MCNPX code, we designed the film structure and its thickness for the optimized detector in the diagnostic x-ray range. The mercuric iodide film was formed by a wet binder process, while the amorphous selenium film was deposited by physical vapor deposition (PVD). These deposition methods are capable of being scaled up to sizes required in diagnostic imaging applications. The electronic properties are investigated using dark current, x-ray sensitivity and signal to noise ratio (SNR). From our results, the developed HgI2 film as an alternative to a-Se photoconductor, which is in practical use in flat panel x-ray imaging detector, will prove its usefulness in the future design and the optimization for various diagnostic modalities.

Dual-energy cardiac imaging: an image quality and dose comparison for a flat-panel detector and x-ray image intensifier

This study presents a comparison of dual-energy imaging with an x-ray image intensifier and flat-panel detector for cardiac imaging. It also investigates if the wide dynamic range of the flat-panel detector can improve dual-energy image quality while reducing patient dose. Experimental contrast-to-noise (CNR) measurements were carried out in addition to simulation studies. Patient entrance exposure and system tube loading were also recorded. The studied contrast objects were calcium and iodine. System performance was quantified with a figure-of-merit (FOM) defined as the image CNR2 over patient entrance exposure. The range of thickness studied was from 10 to 30 cm of Lucite (PMMA). Detector dose was initially set to 140 nGy (16 µR)/frame. The high-energy 120 kVp beam was filtered by an additional 0.8 mm silver filter. Keeping the same filament current, the kVp for the low-energy beam was adjusted as a function of thickness until 140 nGy was achieved. System performance was found to be similar for both systems, with the x-ray image intensifier performing better at lower thicknesses and the flat-panel detector performing better at higher thicknesses. This requirement of fixed detector entrance exposure was then relaxed and the kVp for the low-energy beam was allowed to vary while the mAs of the x-ray tube remained fixed to study changes in dual-energy image quality, patient dose and FOM with the flat-panel detector. It was found that as the kVp for the low-energy beam was reduced, system performance would rise until reaching a maximum while simultaneously lowering patient exposure. Suggested recommendations for optimal dual-energy imaging implementation are also provided.

A Flat-Panel X-Ray Imaging Module for Medical Imaging and Nondestructive Testing

In this study, we designed a flat-panel digital X-ray imaging module based upon the amorphous silicon (a-Si) technology and tested potential for medical imaging and nondestructive testing. The module employs a commercially available a-Si photosensor array of a 143 μm x 143 μm pixel size and a 42.9 cm x 42.9 cm active area, coupled with a CsI(Tl) scintillator of a 550 μm thickness, and a readout IC board which can be accessed through our home made GUI software. The experimental test was performed to evaluate the system response with exposure, modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE).

Low noise signal-to-noise ratio enhancing readout circuit for current-mediated active pixel sensors

Diagnostic digital fluoroscopic applications continuously expose patients to low doses of x-ray radiation, posing a challenge to both the digital imaging pixel and readout electronics when amplifying small signal x-ray inputs. Traditional switch-based amorphous silicon imaging solutions, for instance, have produced poor signal-to-noise ratios (SNRs) at low exposure levels owing to noise sources from the pixel readout circuitry. Current-mediated amorphous silicon pixels are an improvement over conventional pixel amplifiers with an enhanced SNR across the same low-exposure range, but whose output also becomes nonlinear with increasing dosage. A low-noise SNR enhancing readout circuit has been developed that enhances the charge gain of the current-mediated active pixel sensor (C-APS). The solution takes advantage of the current-mediated approach, primarily integrating the signal input at the desired frequency necessary for large-area imaging, while adding minimal noise to the signal readout. Experimental data indicates that the readout circuit can detect pixel outputs over a large bandwidth suitable for real-time digital diagnostic x-ray fluoroscopy. Results from hardware testing indicate that the minimum achievable C-APS output current that can be discerned at the digital fluoroscopic output from the enhanced SNR readout circuit is 0.341nA. The results serve to highlight the applicability of amorphous silicon current-mediated pixel amplifiers for large-area flat panel x-ray imagers.