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

X射线成像探测器依赖于X射线量子到电子信号的间接或直接转换。在直接法中，X射线量子被光电导体吸收，光电导体直接沉积在成像电子设备上，充当X射线到电荷的换能器。X射线吸收产生电子空穴对，它们随后被偏置场分开以产生信号。电荷在电场的作用下移动，这样就有可能在不显著降低分辨率的情况下产生具有较厚探测器层的图像。转换过程的效率可以比间接方法高出十倍，因此可以将空间分辨率和剂量效率提高到所需的最低辐射曝光量。只有当具有适当的x射线量子效率、时间性能、x射线-电荷转换增益和良好的传输特性的合适的光导体被开发出来时，这才是可能的。 最近，我在莱克黑德大学的研究小组合成了一种用于直接转换X射线成像系统的新的光导体材料，称为非晶态氧化铅(a-PbO)。A-PBO在射线照相和荧光成像方面具有巨大的应用潜力。它是一种合适的高原子序数材料，与非晶态硒(a-Se)相比具有明显的优势，目前非晶态Se是直接转换X射线探测器中唯一可行的X射线光导体，例如：由于它具有更高的原子序数，因此它具有比a-Se更高的X射线探测效率，从而允许以更薄的层有效地吸收更高能量的X射线，(2)它具有更低的电子-空穴对产生能，因此具有更高的X射线-电荷转换增益。这有可能将医用X射线所需的辐射剂量降低到基本的量子和空间分辨率极限。 尽管前景看好，但a-PbO层仍处于实验阶段。现在需要更多关于a-PbO光导体的工作，以确保制造具有直接沉积在成像阵列上的光导层的大面积探测器的技术可行性，以优化PbO技术，并增强a-PbO作为X射线-电荷转换器的性能。 NSERC Discovery计划的目标有两个：对一种用于X射线成像系统的新型光导体材料，即无定形氧化铅进行全面的材料科学研究，并开发基于这种材料的X射线探测器的功能原型，用于透视和射线照相应用。新的a-PBO技术将在低剂量率下达到高探测量子效率的理论极限，允许探测器在低曝光时受到x射线量子噪声的限制。这反过来将允许减少获得图像所需的入射X射线剂量，从而提高诊断和图像引导介入手术的整体安全性。

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.