Amorphous Silicon Flat-panel Detector Research Articles

High-energy and thermal-neutron imaging and modeling with an amorphous silicon flat-panel detector

The Los Alamos Neutron Science Center (LANSCE) operates two spallation neutron sources dedicated to research in materials science, condensed-matter physics, and fundamental and applied nuclear physics. Prior to 1995, all thermal neutron radiography at Los Alamos was done on a beam port attached to the Omega West reactor, a small 8 MW research reactor used primarily for radioisotope production and prompt and delayed neutron activation analysis. After the closure of this facility, two largely independent radiography development efforts were begun at LANSCE using moderated cold and thermal neutrons from the Target-1 source and high-energy neutrons from the Target-4 source. Investigations with cold and thermal neutrons employed a neutron converter and film, a scintillation screen and CCD camera system, and a new high-resolution amorphous silicon (a-Si) flat-panel detector system. Recent work with high-energy neutrons ( E n>1 MeV) has involved storage-phosphor image plates. Some comparison high-energy images were obtained with both image plates and the a-Si panel and showed equivalent image quality for approximately equal exposure times.

Characterization of a 14 in /spl times/ 17 in flat panel detector based on ion shower doped a-Si : H P-I-N diodes

In recent years, it has become technically and economically feasible to use solid-state detector technology to display, store and transfer X-ray images. In this paper, we report the performance of a 33 cm /spl times/ 41 cm amorphous silicon flat panel detector (FPD) based on an ion shower doped P-I-N photodiode/thin-film transistor (TFT) array. The p-layer of a diode is formed by an ion shower doping method instead of the conventional plasma-enhanced chemical vapor deposition method. Measurements of X-ray imaging performances are reported for general imaging metrics such as modulation transfer function, noise power spectrum and detective quantum efficiency. The presampling MTF was found to be 0.51 and 0.26 at 1 lp/mm and 2 lp/mm. The measured DQE at 1 lp/mm and 2 lp/mm was 0.35 and 0.17.

Dose Reduction in Patients Undergoing Chest Imaging: Digital Amorphous Silicon Flat-Panel Detector Radiography Versus Conventional Film-Screen Radiography and Phosphor-Based Computed Radiography

We sought to compare the radiation dose delivered to patients undergoing clinical chest imaging on a full-field digital amorphous silicon flat-panel detector radiography system with the doses delivered by a state-of-the-art conventional film-screen radiography system and a storage phosphor-based computed radiography system. Image quality was evaluated to ensure that the potential reduction in radiation dose did not result in decreased image acuity. SUBJECTS AND METHODS. Three groups of 100 patients each were examined using the amorphous silicon flat-panel detector, film-screen, or computed radiography systems. All patient groups were matched for body mass index, sex, and age. To measure the entrance skin dose, we attached 24 calibrated thermoluminescent dosimeters to every patient. The calculation of the effective dose, which represents the risk of late radiation-induced effects, was based on measurements on an anthropomorphic phantom. Image quality of all three systems was evaluated by five experienced radiologists, using the European Quality Criteria for Chest Radiology. In addition, a contrast-detail phantom study was set up to assess the low-contrast detection of all three systems. The amorphous silicon flat-panel detector radiography system allowed an important and significant reduction in both entrance skin dose and effective dose compared with the film-screen radiography (x 2.7 decrease) or computed radiography (x 1.7 decrease) system. In addition, image quality produced by the amorphous silicon flat-panel detector radiography system was significantly better than the image quality produced by the film-screen or computed radiography systems, confirming that the dose reduction was not detrimental to image quality. The introduction of digital flat-panel radiography systems based on amorphous silicon and cesium iodide is an important step forward in chest imaging that offers improved image quality combined with a significant reduction in the patient radiation dose.

Performance of a 41 x 41 cm2 amorphous silicon flat panel x-ray detector designed for angiographic and R&F imaging applications.

We measured the physical imaging performance of a 41 x 41 cm2 amorphous silicon flat panel detector designed for angiographic and R&F imaging applications using methods from the emerging IEC standard for the measurement of detective quantum efficiency (DQE) in digital radiographic detectors. Measurements on 12 production detectors demonstrate consistent performance. The mean DQE at the detector center is about 0.77 at zero frequency and 0.27 at the Nyquist frequency (2.5 cycles/mm) when measured with a 7 mm of Al HVL spectrum at about 3.6 microGy. The mean MTF at the center of the detector for this spectrum is 0.24 at the Nyquist frequency. For radiographic operation all 2048 x 2048 detector elements are read out individually. For fluoroscopy, the detector operates in two 30 frame per second modes: either the center 1024 x 1024 detector elements are read out or the entire detector is read out with 2 x 2 pixel binning. A model was developed to predict differences in performance between the modes, and measurements demonstrate agreement with the model. Lag was measured using a quasi-equilibrium exposure method and was found to be 0.044 in the first frame and less than 0.007 after 1 s. We demonstrated that it is possible to use the lag data to correct for temporal correlation in images when measuring DQE with a fluoroscopic imaging technique. Measurements as a function of position on the detector demonstrate a high degree of uniformity. We also characterized dependences on spectrum, exposure level, and direction. Finally, we measured the DQE of a current state of the art image intensifier/CCD system using the same method as for the flat panel. We found the image intensifier system to have lower DQE than the flat panel at high exposure levels and approximately equivalent DQE at fluoroscopic levels.

Construction and characterization of an amorphous silicon flat-panel detector based on ion-shower doping process

In this paper, we introduce a new 36×43cm2 amorphous silicon flat-panel detector for digital radiography. A prototype flat-panel detector was fabricated using a p–i–n photodiode/thin-film transistor (TFT) array. The main difference of this flat panel detector to the similar general flat-panel detectors is p–i–n photodiode fabrication method. The p-layer of diode is formed using an ion shower doping method instead of the conventional PECVD method to increase the quality of array. The diode shows a leakage current of 2pA/mm2 at −5V and dark current uniformity of the detector is 2.5%. The modulation transfer function (MTF) of the detector is 0.41 at 2lp/mm.

Chest radiography: optimization of X-ray spectrum for cesium iodide-amorphous silicon flat-panel detector.

To ascertain the optimum x-ray spectrum for chest radiography with a cesium iodide-amorphous silicon flat-panel detector. End points for optimization included the ratio of tissue contrast to bone contrast and a figure of merit (FOM) equal to the square of the signal-to-noise ratio of tissue divided by incident exposure to the patient. Studies were conducted with both computer spectrum modeling and experimental measurement in narrow-beam and full-field exposure conditions for four tissue thicknesses (8-32 cm). Three parameters that affect spectra were considered: the atomic number (Z) of filter material (Z = 13, 26, 29, 42, 50, 56, 64, 74, and 82), kilovoltage (from 50 to 150 kVp), and filter thickness (from 0.25 to 2.00 half-value layer [HVL]). Computer modeling and narrow-beam experimental data showed similar trends for the full range of parameters evaluated. Spectrum model results showed that copper filtration at 120 kVp or more was optimum for FOM. The ratio of contrasts showed a trend to be higher with higher kilovoltage and only a minor variation with filter material. Full-field experimental results, which reflect the added contribution of x-ray scatter, differed in magnitude but not trends from the narrow-beam data in all cases except the ratio of contrasts in the mediastinum. The best performance overall, including both FOM and ratio of contrasts, was at 120 kVp with 1-HVL copper filtration (0.2 mm). With this beam spectrum and an increase in tube output (ie, milliampere seconds) of about 50%, a chest radiograph can be obtained with image quality approximately equal to that with a conventional spectrum but with about 25% less patient exposure.

Radiation Dose Reduction in Chest Radiography using a Flat-Panel Amorphous Silicon Detector

AIM: The aim of this study was to evaluate the image quality and the potential for radiation dose reduction with a digital flat-panel amorphous silicon detector radiography system. MATERIAL AND METHODS: Using flat-panel technology, radiographs of an anthropomorphic thorax phantom were taken with a range of technical parameters (125kV, 200mA and 5, 4, 3.2, 2, 1, 0.5, and 0.25mAs) which were equivalent to a radiation dose of 332, 263, 209, 127, 58.7, 29, and 14μGy, respectively. These images were compared to radiographs obtained by a conventional film-screen radiography system at 125kV, 200mA and 5mAs (equivalent to 252μGy) which served as reference. Three observers evaluated independently the visibility of simulated rounded lesions and anatomical structures, comparing printed films from the flat-panel amorphous silicon detector and conventional x-ray system films. RESULTS: With flat-panel technology, the visibility of rounded lesions and normal anatomical structures at 5, 4, and 3.2mAs was superior compared to the conventional film-screen radiography system. (P⩽0.0001). At 2mAs, improvement was only marginal (P=0.19). At 1.0, 0.5 and 0.25mAs, the visibility of simulated rounded lesions was worse (P⩽0.004). Comparing fine lung parenchymal structures, the flat-panel amorphous silicon detector showed improvement for all exposure levels down to 2mAs and equality at 1mAs. CONCLUSION: Compared to a conventional x-ray film system, the flat-panel amorphous silicon detector demonstrated improved image quality and the possibility for a reduction of the radiation dose by 50% without loss in image quality. Hosch, W. P. etal. (2002). Clinical Radiology57, 902–907.

Leakage current of amorphous silicon p-i-n diodes made by ion shower doping

In this letter, we report the leakage current of amorphous silicon (a-Si:H) p-i-n photodiodes, of which the p layer is formed by ion shower doping. The ion shower doping technique has an advantage over plasma-enhanced chemical vapor deposition (PECVD) in the fabrication of a large-area amorphous silicon flat-panel detector. The leakage current of the ion shower diodes shows a better uniformity within a 30 cm×40 cm substrate than that of the PECVD diodes. However, it shows a higher leakage current of 2–3 pA/mm2 at −5 V. This high current originates from the high injection current at the p-i junction.

Alignment of a volumetric tomography system.

A test-bed system has been developed for imaging phantoms with tomosynthesis and volumetric computed tomography. This system incorporates an amorphous silicon flat panel detector on a movable gantry and a computer-controlled rotational positioning stage. In this paper, an analysis of the sensitivity of reconstructed images to geometrical misalignment is presented. Application of this method to circular digital tomosynthesis is examined, with spatial resolution in the focal plane as the criterion for evaluating the effect of misalignment. A software-based method is presented for correcting data for imperfect system alignment prior to image reconstruction. Experimental results yield reconstructed images with spatial resolution approaching the theoretical limit based on detector pixel size and accounting for data interpolation.