Lead Oxide (PbO) x-ray-to-charge transducer for direct conversion medical imaging detectors

x射线成像探测器依靠x射线量子到电子信号的间接或直接转换。在直接方法中，x射线量子被吸收在光导体中，光导体直接沉积在成像电子器件上，并充当x射线电荷转换器。x射线吸收产生电子空穴对，它们随后被偏置场分离以产生信号。电荷在电场的作用下移动，使得有可能产生具有较厚的探测器层的图像，而不会有明显的分辨率损失。转换过程的效率可达间接方法的十倍，从而有可能将空间分辨率和剂量效率提高到最低所需的辐射暴露。只有开发出具有适当的x射线量子效率、时间性能、x射线-电荷转换增益和良好输运特性的合适的光导体，才有可能实现这一目标。

X-ray imaging detectors rely on indirect or direct conversion of x-ray quanta to an electron signal. In the direct method, the x-ray quanta are absorbed in a photoconductor which is deposited directly on imaging electronics and acts as an x-ray-to-charge transducer. X-ray absorption creates electron hole pairs, which are subsequently separated by a bias field to generate a signal. The charges are moved by an electric field making it possible to create an image with a thick detector layer without significant loss of resolution. The conversion process can be up to ten times more efficient than for the indirect method, thus making it possible to improve both spatial resolution and dose efficiency down to the lowest required radiation exposure. This is only possible if the right photoconductor with an appropriate x-ray quantum efficiency, temporal performance, x-ray-to-charge conversion gain and good transport properties can be developed. Recently, my research group at Lakehead University synthesized a new photoconductor material for direct conversion x-ray imaging systems called, amorphous Lead Oxide (a-PbO). a-PbO has enormous potential for applications in radiographic and fluoroscopic imaging. It is a suitable high-Z (atomic number) material with distinct advantages over amorphous selenium (a-Se) currently the only commercially viable x-ray photoconductor in direct conversion x-ray detectors such as: it has higher x-ray detection efficiency than a-Se due to its higher atomic number, and thus permits effective absorption of higher energy x-rays with a thinner layer, (2) it has lower electron-hole pair creation energy, and hence a higher x-ray-to-charge conversion gain. This has a potential to reduce the radiation dose needed for medical X-rays to the fundamental quantum and spatial resolution limits. Although very promising, a-PbO layers are still experimental. More work on a-PbO photoconductor is now needed to assure technical feasibility to produce large area detectors with a photoconductive layer deposited directly on the imaging array, to optimize PbO technology and to enhance a-PbO properties as an x-ray-to-charge convertor. The objectives of this NSERC Discovery program are two-fold: to undertake comprehensive material science research into a new photoconductor material for x-ray imaging systems, namely amorphous Lead Oxide, and to develop functional prototypes of x-ray detectors based on this material for fluoroscopic and radiographic applications. The new a-PbO technology will achieve the theoretical limit of high detective quantum efficiency at low dose rates allowing the detector to be x-ray quantum noise limited at the low exposures. This in turn will allow the reduction of the incident x-ray dose to acquire an image and, thereby, to improve the overall safety of diagnostic and image-guided interventional procedures.