Photoconductors for medical imaging detectors: materials issues and device designs

平板有源矩阵成像仪（FPAMI）是医学成像中最成功的数字x射线探测器，如乳房x线照相术和普通x射线照相术。FPAMIs通常是基于荧光粉（间接转换）或基于光导体（直接转换，目前是基于非晶硒，a-Se）。基于砷硒的探测器比间接转换探测器产生更清晰的图像。虽然FPAMIs在降低辐射剂量方面具有很高的潜力，但它们的工作原理与基本量子噪声水平相差甚远。量子噪声限制探测器的剂量水平可以比目前可用的fpami使用的剂量水平低得多。近年来，有机钙钛矿（如多晶甲基铵三碘化铅）光导体已显示出良好的潜力，可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。它的x射线灵敏度几乎比a-Se高一个数量级。该有机钙钛矿可采用印刷方法大面积均匀沉积。该方法比制备CsI或a-Se所需的高温工艺更便宜，并且应该降低FPAMI系统的成本。然而，有机钙钛矿x射线探测器要想实现商业化，必须克服一些科学难题，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。在高电荷信号的a- se层中，利用载流子倍增（如雪崩过程）也可以实现非常低剂量的医用x射线成像，从而提高信噪比。

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.