Extensive research efforts since the 1980s have resulted in the practical introduction of active-matrix flat-panel imagers (AMFPIs) to numerous medical x-ray applications in this decade. These include imaging procedures involving cone beam computed tomography (CBCT). While AMFPIs offer many advantages compared to traditional film-screen and x-ray image intensifier systems (XRIIs), the technology nevertheless suffers from several significant limitations. AMFPI image quality degrades at low exposures so that, for example, it cannot match the image quality of XRIIs across the entire fluoroscopic exposure range. Secondly, AMFPIs are subject to image artifacts, originating from the trapping of charge in amorphous materials in the arrays. Such artifacts are particularly prominent when fluoroscopic images are acquired shortly after a large radiographic exposure - a phenomenon called ghosting. Finally, the maximum achievable frame rates of AMFPIs are restrictive. Research leading up to this proposal has identified an innovative, highly promising strategy for overcoming these limitations, involving substitution of the amorphous silicon thin-film transistors (a-Si:H TFTs), used in most conventional AMFPIs, with polycrystalline silicon (poly-Si) TFTs. This allows creation of considerably more sophisticated arrays with in-pixel amplifiers - referred to as an active pixel (AP) architecture. Coupled with the incorporation of novel a-Si:H photodiode structures that are compatible with these more complex pixel circuits, poly-Si AP arrays would overcome the various limitations listed above, while preserving the many favorable properties of conventional AMFPIs. The objectives of the research focus on the development of a series of increasingly higher performance, small area, prototype arrays that exhibit these desirable properties. The objectives are: (1) Development of prototypes with progressively better performance (higher detective quantum efficiency, lower charge trapping effects, higher frame rates) - involving iterative design, fabrication and evaluation of increasingly sophisticated AP prototypes. (2) Quantitative modeling (involving cascaded systems analysis and detailed circuit simulations) to provide guidance in array design and assist in prototype evaluation. (3) Detailed characterization of the properties of individual poly-Si TFTs and other test circuits to support the circuit simulation activities and to provide guidance in improving array performance through improvements to fabrication techniques. (4) Creation of the various tools (mathematical, software, firmware and hardware) required to accomplish the above objectives. The successful conclusion of this research will result in the creation of a technology that offers image quality limited only by the fundamental properties of X rays and x-ray converters, reduces artifacts and increases frame rates. Ultimately, this will improve image quality and/or reduce dose for fluoroscopic procedures, as well as facilitate advanced clinical applications including breast and chest tomosynthesis, and CBCT for breast and angiographic procedures.
The practical application of the novel x-ray imaging technology to be developed in the proposed research will offer significant enhancement of imaging capabilities, ultimately improving patient care in a wide variety of ways. For example, compared to existing x-ray technologies, the new technology will facilitate the realization of higher quality images at very low doses (helping to minimize dose to the patient in fluoroscopic procedures) and enable the visualization of smaller and/or lower contrast features (assisting in the identification of suspicious objects in mammographic examinations). Moreover, it is strongly anticipated that the new technology will enable advanced applications (involving tomosynthesis or cone beam computed tomography techniques for chest and breast imaging) that require rapid acquisition of multiple, high quality images at relatively low doses per image in order to produce three dimensional anatomical views.
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