High-Flux Multi-Spectral X-Ray Imaging with Energy-Sensitive CZT Detectors

传统的医用X射线成像系统配备有：i)安装在快速旋转的机架上的强大的x射线管，其产生以广谱能量为特征的多色辐射；以及ii)x射线探测器，其记录透过患者身体的所有x射线的总能量。X射线束穿过患者身体时的衰减取决于光子能量，这种能量依赖于不同的材料、组织和元素。因此，每个探测到的光子的能量包含关于被扫描物体的元素组成的额外的有价值的信息。目前的X射线探测器大多对这种光谱信息不敏感，因为它们的信号输出与沉积在探测器有源区内的总能量成正比，而具有能量辨别能力的探测器可以通过记录所有这些不同的能量光子并将它们排列到各自的光谱箱中，为加强对这种额外光谱信息的利用提供解决方案。直接转换型半导体探测器具有高灵敏度、高阻挡能力、高空间分辨率和优良的能量分辨率等优点，在光谱和成像领域中已成为占主导地位的固态室温探测器。最近，人们对将CZT探测器用于下一代高通量多能量X射线成像系统的兴趣与日俱增，尤其是计算机断层扫描(CT)和3DX射线乳腺成像。这两种成像模式都有共同的目标，即进行定量测量的能力，因此，在低患者剂量下增强诊断能力。然而，这些应用需要非常快的数据采集，因此，需要能够有效地在高光子通量下工作的探测器，几乎每平方毫米每秒1亿光子。用于高通量光子计数多光谱X射线成像的能量敏感CZT探测器阵列的设计和制造带来了重大的技术挑战和问题，这是本方案研究的重点。该项目的主要目标是：1.用于高通量多能X射线成像的光子计数CZT探测器的创新设计规范；以及2.基于优化的CZT探测器用于多能X射线光谱CT和3D乳腺成像的概念验证光子计数光谱X射线成像系统的最佳架构。这项工作的主要目的是研究光子计数、能量识别的CZT探测器能够在多大程度上克服基本性能限制，进行定量成像，揭示额外的光谱信息，从而改进传统的X射线医学成像。如果能量信息与强度一起记录，x射线医学成像模式可以通过软组织分化、材料分解、肿瘤特征、靶标量化以及开发针对疾病的靶向造影剂和药物来提高诊断准确性。后者可以在显著降低辐射剂量的情况下提高低对比度分辨率和整体图像质量，并以较低的成本获得更好的诊断性能。光谱x射线成像可以成为一种重要的成像技术，结合高空间分辨率成像提供特定材料的定量信息，从而导致x射线医学诊断的范式转变。

Conventional medical x-ray imaging systems are equipped with: i) a powerful x-ray tube, mounted on a fast rotating gantry, which generates polychromatic radiation characterised by a broad spectrum of energies, and ii) an x-ray detector, which records the total energy of all the x-rays that transmitted through the body of the patient. The attenuation of the x-ray beam, as it passes through the patient's body, depends on the photon energy and this energy dependence is different for different materials, tissues and elements. Therefore, the energy of each detected photon contains additional valuable information about the elemental composition of the scanned object. The current x-ray detectors are mostly insensitive to this spectral information, because their signal output is proportional to the total energy deposited within the active area of the detector, while a detector with energy-discrimination capabilities can provide the solution for enhanced exploitation of this additional spectral information by recording all these different energy photons and arranging them into respective spectral bins. Direct conversion CdZnTe (CZT) semiconductor detectors with high sensitivity, high stopping power, high spatial resolution and excellent energy resolution have emerged as the dominant solid-state room temperature detectors in a wide range of spectroscopic and imaging applications. Most recently, there has been a growing interest in using the CZT detectors for the next generation of high-flux multi-energy x-ray imaging systems, with a particular emphasis on Computed Tomography (CT) and 3D x-ray breast imaging. Both these imaging modalities have common goals, the ability to make quantitative measurements, and therefore, the enhancement of diagnostic capability at low patient doses. However, these applications require very fast data acquisition, and hence, there is a need for detectors that can efficiently operate at a high photon flux, almost 100 million photons per second per square millimetre. The design and fabrication of energy-sensitive CZT detector arrays for high-flux photon-counting multi-spectral x-ray imaging pose significant technological challenges and issues, which are the focus of the investigations of this proposal. The main objectives of this project are: 1. the innovative design specifications of a photon-counting CZT detector for high-flux multi-energy x-ray imaging; and 2. the optimum architecture of a proof-of-concept photon-counting spectral x-ray imaging system based on optimised CZT detectors for multi-energy x-ray spectral CT and 3D x-ray breast imaging. The main motivation of this work is to investigate to which extent photon-counting, energy discriminating CZT detectors are capable of overcoming fundamental performance limits and carrying out quantitative imaging revealing the additional spectral information, and therefore, improving conventional x-ray medical imaging. If energy information is recorded alongside the intensity, x-ray medical imaging modalities could increase diagnostic accuracy through soft-tissue differentiation, material decomposition, tumour characterisation, target quantification and development of disease-specific targeted contrast agents and drugs. The latter could improve low-contrast resolution and overall image quality at significantly reduced radiation doses and lead to superior diagnostic performance with lower cost. Spectral x-ray imaging can become an important imaging technique providing material-specific quantitative information in combination with high spatial resolution imaging, and therefore, leading to a paradigm shift in x-ray medical diagnostics.