KEYWORDS: Sensors, Modulation transfer functions, Selenium, X-rays, X-ray detectors, Digital mammography, Image acquisition, Digital breast tomosynthesis, Digital imaging, Signal detection
The purpose of this work is to report the performance of an amorphous selenium (a-Se) based flat-panel x-ray imager under development for application in digital breast tomosynthesis. This detector is designed to perform both in the conventional Full Field Digital Mammography (FFDM) mode and the tomosynthesis mode. The large area 24 x 29 cm detector achieves rapid image acquisition rates of up to 4 frames per second with minimal trapped charge induced effects such as ghost or lag images of previously acquired objects.
In this work, a new a-Se/TFT detector layer structure is evaluated. The design uses a top conductive layer in direct contact with the a-Se x-ray detection layer. The simple structure has few layers and minimal hole and electron trapping effects. Prototype detectors were built to investigate the basic image performance of this new a-Se/TFT detector. Image signal generation, image ghosting, image lag, and detector DQE were studied.
For digital mammography applications, the residual image ghosting was less than 1% at 30 seconds elapsed time. DQE, measured at a field of 5.15 V/um, showed significantly higher values over previously reported data, especially at low exposure levels. For digital breast tomosynthesis, the image lag at dynamic readout rate was < 0.6 % at 0.5-second elapsed time. A prototype tomosynthesis system is being developed utilizing this new a-Se/TFT detector.
Amorphous selenium direct-conversion x-ray detectors have been used successfully for full field digital mammography (FFDM) and digital radiography (DR). Such detectors characteristically exhibit high spatial resolution and conversion efficiency that is a function of the applied electric field. About 50 electron volts of photon energy are required to generate each electron-hole pair in a typical amorphous selenium x-ray conversion layer biased at 10 volts per micron. At FFDM and DR imaging x-ray energies each absorbed photon can generate only about 250 to 1000 electron-hole pairs. Medical imaging applications must therefore employ low noise thin film transistor (TFT) arrays and charge integration amplifiers to achieve high signal-to-noise ratio (SNR) and detective quantum efficiency (DQE). To assure quantum-noise limited imaging results with the lowest practical x-ray exposure dose, it is desirable to include
an additional low-noise gain stage in the x-ray conversion layer. We have proposed and studied a new structure for an amorphous selenium detector that employs an internal biased gain grid to cause avalanche-gain within the x-ray conversion layer. An amplification of at least 10X can be achieved without introducing excessive noise. Quantum-limited image detection should then be attainable for even very low exposures.
DirectRay direct-conversion digital x-ray-imaging detectors using selenium exhibit high sensitivity and resolution to x-ray energies just below the K-fluorescence edge compared to energies just above. Detector sensitivity and self-protected dynamic range can be manipulated by modifying dielectric thickness and selenium electric field. Replacing the dielectric layer with a charge-transport layer (CTL) allows a faster cycle time, lower residual-image charge, improved signal to noise ratio, and better operating stability. The new CTL structure allows fast multi-frame imaging, enabling applications such as dual-energy subtraction, tomosynthesis, and dynamic imaging (fluoroscopy).
We describe a high-resolution digital x-ray detector suitable for producing high quality mammographic images. The detector consists of an array of 3584 by 4096 pixels on 70 micrometer centers covering an area of 25 cm by 29 cm. The conversion layer of the detector is 250 micrometer thick amorphous selenium. Each pixel of the array consists of a storage capacitor for collecting x-ray signals and an amorphous silicon switching transistor. The signal is read out by custom high-speed, low-noise electronics. The integration of this detector with a mammographic x-ray system and acquisition console is described, as well as algorithms for calibration of the full system. We review characterization of the imaging performance of our system based on quantitative analyses of MTF and DQE data, and compare experimental results with theoretical calculations. We compare the performance of our direct conversion system with that of screen/film analog systems and indirect conversion digital detectors, such as LORAD's CsI/CCD detector, operated under similar conditions. MTF degradation mechanisms and system noise sources and their effect on DQE are discussed. We review qualitative aspects of image quality from our detector and present preliminary observer performance characteristics on clinical studies run with our system.
Direct conversion of x-ray energy into electrical charge has been extensively developed into imaging products in the past few years. Applications include general radiography, mammography, x-ray crystallography, portal imaging, and non-destructive testing. Direct methods avoid intermediate conversion of x-rays into light prior to generating a measurable electrical charge. This eliminates light scattering effects on image sharpness, allowing detectors to be designed to the limit of the theoretical modulation transfer function for a discrete-pixel sensor. Working exposure range can be customized by adjusting bias and thickness of sensor layers in coordination with readout-electronics specifications. Mature amorphous selenium technology and recent progress on high-quality Thin-Film Transistor (TFT) arrays for computer displays have allowed development of practical large-area high-resolution flat-panel x-ray imaging systems. A variety of design optimizations enable direct-conversion technology to satisfy a wide range of applications.
This paper will describe details of and results for a frequency-dependent filtered gain calibration technique that optimizes DQE, yet does not reduce MTF performance which is important to both systems.
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