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

X射线成像检测器依赖于X射线量子到电子信号的间接或直接转换。在直接方法中，X射线量子被吸收在光电导体中，光电导体直接沉积在成像电子器件上并充当X射线到电荷换能器。 X射线吸收产生电子空穴对，其随后被偏置场分离以产生信号。电荷通过电场移动，使得可以创建具有厚检测器层的图像，而不会显著损失分辨率。转换过程的效率可以比间接方法高出10倍，从而可以将空间分辨率和剂量效率提高到最低所需的辐射曝光。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传输特性的正确光电导体，这才是可能的。 最近，我在湖首大学的研究小组合成了一种用于直接转换X射线成像系统的新光电导体材料，称为非晶氧化铅（a-PbO）。 a-PbO在射线照相和荧光成像中具有巨大的应用潜力。这是一个合适的高Z具有明显优于非晶硒（a-Se）的优点的非晶硒（原子序数）材料是目前在直接转换X射线检测器中唯一商业上可行的X射线光电导体，例如：由于其较高的原子序数，其具有比a-Se更高的X射线检测效率，因此允许用较薄的层有效吸收较高能量的X射线，（2）它具有较低的电子-空穴对产生能量，因此具有较高的X射线-电荷转换增益。 这有可能将医疗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。 虽然非常有前途，但a-PbO层仍处于实验阶段。现在需要在a-PbO光电导体上进行更多的工作，以确保生产具有直接沉积在成像阵列上的光电导层的大面积检测器的技术可行性，以优化PbO技术并增强a-PbO作为X射线-电荷转换器的性能。 该NSERC Discovery计划的目标有两个方面：对用于X射线成像系统的新型光电导体材料（即非晶氧化铅）进行全面的材料科学研究，并开发基于该材料的X射线探测器的功能原型，用于荧光透视和射线照相应用。新的a-PbO技术将在低剂量率下实现高探测量子效率的理论极限，从而允许探测器在低曝光下受到X射线量子噪声限制。这又将允许减少获取图像的入射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.