Photoconductors for medical imaging detectors: materials issues and device designs

平板有源矩阵成像仪是目前最成功的用于医学成像的数字X射线探测器，如乳房X光照相和普通X光照相。FAMI通常是基于荧光粉(间接转换)或基于光导体(直接转换，目前是非晶态的Se，a-Se)。基于a-Se的探测器产生比间接转换探测器清晰得多的图像。尽管FAMI具有很高的降低辐射剂量的潜力，但它们的工作距离基本量子噪声水平很远。量子噪声受限探测器的剂量水平可以比目前可用的FAMI使用的剂量水平低得多。近年来，有机钙钛矿(如多晶三碘甲铵铅)光电导体在低剂量、低成本的医用X射线成像探测器中显示出良好的应用潜力。它的X射线灵敏度几乎比a-Se好一个数量级。使用印刷方法可以大面积均匀地沉积有机钙钛矿。这种方法比制备CsI或a-Se所需的高温工艺便宜，应能降低FPAMI系统的成本。然而，有机钙钛矿型X射线探测器不能完全商业化，除非克服一些科学挑战，如获得低暗电流、高分辨率、无图像滞后和重影以及材料和器件的长期稳定性。利用a-Se层中的载流子倍增(例如雪崩过程)在很高的电场下获得更高的电荷信号，也可以实现极低剂量的医用X射线成像，这可以提高信噪比。申请者的研究计划将努力开发使用有机钙钛矿和雪崩硒的低剂量和低成本的医用X射线成像探测器。研究目标包括：1)研究有机钙钛矿光电导体和a-Se中电荷载流子的产生和输运机制；2)研究合适的多层钙钛矿探测器结构，使其具有极低的暗电流、高分辨率和可忽略的图像滞后和重影；3)研究载流子通过各层的传输机制，从而确定a-Se雪崩X射线探测器的多层结构的最佳设计；以及4)长期稳定性的研究。在这项提议中，新型有机钙钛矿型X射线探测器的开发将对医学成像产生重大影响。它可以大大降低透视和普通X射线摄影中使用的辐射剂量。它还将为新型低成本有机钙钛矿型大面积X射线传感器打开大门，并推动其他X射线成像技术的创新，如计算机断层扫描。这一建议的另一个创新之处是设计了多层a-Se雪崩探测器结构，用于极低剂量和高分辨率的直接转换X射线成像。这项研究将有益于加拿大在医学数字成像方面的研发努力。

The flat-panel active matrix imagers (FPAMI) are the most successful digital X-ray detectors for medical imaging such as mammography and general radiography. The FPAMIs are generally either phosphor based (indirect conversion) or photoconductor based (direct conversion, presently amorphous selenium, a-Se, based). The a-Se based detectors produce much sharper images than the indirect conversion detectors. Although the FPAMIs have high potential to reduce the radiation dose, they operate far from the fundamental quantum-noise level. The dose levels at quantum-noise limited detectors can be much lower than that used by currently available FPAMIs. Recently, the organic perovskite (e.g., polycrystalline methylammonium lead triiodide) photoconductors have shown good potential to be used in direct conversion detectors for low dose and lost cost medical X-ray imaging detectors. Its X-ray sensitivity is almost an order better than a-Se. The organic perovskite can be uniformly deposited over large area using a printing method. This method is cheaper than the high temperature-process needed to prepare CsI or a-Se, and should reduce the cost of FPAMI system. However, organic perovskite X-ray detectors can't be commercialized unitil overcoming few scientific challenges such as obtaining low dark current, high resolution, free of image lag and ghosting, and long-term material and device stability. Very low-dose medical X-ray imaging could also be achieved by utilizing carrier multiplication (e.g., avalanche process) at a very high electric field in a-Se layer for higher charge signal, which could improve the signal to noise ratio. The applicant's research program will endeavor to develop low-dose and low-cost medical X-ray imaging detectors using organic perovskites and avalanche a-Se. The research objective includes 1) investigation of charge carrier generation and transport mechanisms in organic perovskites photoconductors and a-Se, 2) investigation of appropriate multilayer perovskite detector structure that shows very low dark current together with high resolution and negligible image lag and ghosting, 3) Investigation of carrier transport mechanisms through various layers and hence determination of the optimum design of multilayer structure for a-Se avalanche X-ray detectors, and 4) study of long term stability. The development of novel organic perovskite X-ray detectors in this proposal can make a big difference to medical imaging. It can greatly lower the radiation dose used in fluoroscopy and general X-ray radiography. It will also open the door to a new class of low-cost organic perovskite large-area X-ray sensors and fuel innovation in other X-ray imaging techniques, such as computed tomography. Another novelty of this proposal is to design multilayer a-Se avalanche detector structure for very low dose and high resolution direct conversion X-ray imaging. This research will be beneficial to Canada's R&D efforts in medical digital imaging.