Photoconductors for radiation imaging detectors: materials issues and device designs

点击翻译按钮获取中文摘要

There is a huge demand for lowering irradiation dose in various medical radiation imaging modalities especially in general X-ray radiography and real-time radiation imaging. After last two decades of extensive research, amorphous selenium (a-Se) based direct conversion flat-panel digital X-ray detector (the incident X-rays directly generate charge carriers in the photoconductor layer) is recently commercialized for digital mammography. The a-Se detector is not perfect and the main drawback of the conventional a-Se detector under normal operation is its low sensitivity compared to other potential photoconductors such as polycrystalline lead oxide or mercuric iodide. Low sensitivity gives low signal to noise ratio in low-dose imaging and thus severely affects the diagnostic features of the image. Very low-dose medical X-ray imaging could be achieved by; (1) utilizing avalanche multiplication process at a very high electric field in a-Se layer for higher charge signal, and/or (2) replacing a-Se by other potential photoconductors that can provide higher collected charge. Low-cost X-ray imaging can be obtained by utilizing organic photoconductors in X-ray detectors. The research on avalanche a-Se solid state imaging detector is in a very premature level; it needs extensive research works to clearly understand the fundamental underlying physics of carrier generation, multiplication, and transport mechanisms, and to optimize the detector design by examining imaging performances. The PI's research program will endeavor to develop low-dose and/or low-cost medical X-ray/optical imaging detectors by extensively investigating charge carrier generation, transport, multiplication, and noise creation mechanisms in photoconductors, developing mathematical models for analyzing imaging detector performances such as dark current, sensitivity, DQE (detective quantum efficiency), and MTF (modulation transfer function) as a function of field, temperature, detector structure, X-ray induced effects, excess noise, and hence optimizing the detector designs. The proposed work is vital to understand the fundamental physics of X-ray/optical detector operations and photoconductor properties, and to identify the important factors that limit the detector performances, which can ultimately lead to finding the efficient materials and designs for low-cost and efficient radiation imaging detectors for various digital medical imaging applications. This research will advance scientific knowledge in high field transport mechanisms in amorphous and polycrystalline photoconductors. The mathematical models can be used as design tools by researchers in academia and industries. The proposed work has excellent scope for original and fundamental research for graduate students, which will advance fundamental scientific knowledge and transfer it to the next generation. This research will be beneficial to Canada’s R&D efforts in medical digital imaging, and the digital radiation image detector community (e.g., Analogic and Hologic).