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, HgI(2) 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.

Experimental characterization of a direct conversion amorphous selenium detector with thicker conversion layer for dual-energy contrast-enhanced breast imaging.

PURPOSE: Dual-energy contrast-enhanced imaging is being investigated as a tool to identify and localize angiogenesis in the breast, a possible indicator of malignant tumors. This imaging technique requires that x-ray images are acquired at energies above the k-shell binding energy of an appropriate radiocontrast agent. Iodinated contrast agents are commonly used for vascular imaging, and require x-ray energies greater than 33 keV. Conventional direct conversion amorphous selenium (a-Se) flat-panel imagers for digital mammography show suboptimal absorption efficiencies at these higher energies. METHODS: We use spatial-frequency domain image quality metrics to evaluate the performance of a prototype direct conversion flat-panel imager with a thicker a-Se layer, specifically fabricated for dual-energy contrast-enhanced breast imaging. Imaging performance was evaluated in a prototype digital breast tomosynthesis (DBT) system. The spatial resolution, noise characteristics, detective quantum efficiency, and temporal performance of the detector were evaluated for dual-energy imaging for both conventional full-field digital mammography (FFDM) and DBT. RESULTS: The zero-frequency detective quantum efficiency of the prototype detector is improved by approximately 20% over the conventional detector for higher energy beams required for imaging with iodinated contrast agents. The effect of oblique entry of x-rays on spatial resolution does increase with increasing photoconductor thickness, specifically for the most oblique views of a DBT scan. Degradation of spatial resolution due to focal spot motion was also observed. Temporal performance was found to be comparable to conventional mammographic detectors. CONCLUSIONS: Increasing the a-Se thickness in direct conversion flat-panel imagers results in better performance for dual-energy contrast-enhanced breast imaging. The reduction in spatial resolution due to oblique entry of x-rays is appreciable in the most extreme clinically relevant cases, but may not profoundly affect reconstructed images due to the algorithms and filters employed. Degradation to projection domain spatial resolution is thus outweighed by the improvement in detective quantum efficiency for high-energy x-rays.

Development of solid-state avalanche amorphous selenium for medical imaging.

PURPOSE: Active 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. METHODS: The 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. RESULTS: An 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. CONCLUSIONS: We 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.