Photoconductors for medical imaging detectors: materials issues and device designs

平板有源矩阵成像器（FPAMI）是用于医学成像（如乳腺X射线摄影和普通X射线摄影）的最成功的数字X射线探测器。FPAMI通常是基于磷光体的（间接转换）或基于光电导体的（直接转换，目前基于无定形硒，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。虽然FPAMI有很大的潜力，以减少辐射剂量，他们的工作远离基本的量子噪声水平。量子噪声限制探测器的剂量水平可以比目前可用的FPAMI所使用的剂量水平低得多。最近，有机钙钛矿（例如，多晶甲基铵三碘化铅）光电导体已经显示出用于低剂量和低成本医用X射线成像探测器的直接转换探测器的良好潜力。它的X射线灵敏度几乎比a-Se高一个数量级。有机钙钛矿可以使用印刷方法在大面积上均匀沉积。这种方法比制备CsI或a-Se所需的高温工艺便宜，并且应该降低FPAMI系统的成本。然而，有机钙钛矿X射线探测器不能商业化，除非克服一些科学挑战，例如获得低暗电流，高分辨率，无图像滞后和重影，以及长期材料和器件稳定性。非常低剂量的医学X射线成像也可以通过利用载波倍增（例如，雪崩过程），用于更高的电荷信号，这可以提高信噪比。 申请人的研究计划将努力开发使用有机钙钛矿和雪崩a-Se的低剂量和低成本医用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中的电荷载流子产生和传输机制，2）研究适当的多层钙钛矿探测器结构，其显示非常低的暗电流以及高分辨率和可忽略的图像滞后和重影，3）研究了载流子在不同层中的输运机理，从而确定了多层膜结构的最佳设计。硒雪崩X射线探测器，和4）长期稳定性的研究。 该提案中新型有机钙钛矿X射线探测器的开发可以对医学成像产生重大影响。它可以大大降低透视和普通X射线摄影的辐射剂量。它还将为新型低成本有机钙钛矿大面积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。该方案的另一个新颖之处是设计用于极低剂量和高分辨率直接转换X射线成像的多层a-Se雪崩探测器结构。这项研究将有利于加拿大在医学数字成像方面的研发工作。

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.