Sporadic diffraction and absorption volumetric X-ray imaging

该项目将通过揭示埋在三维扫描中的材料的真实性质，为X射线成像带来令人兴奋的进步。传统X射线吸收成像的主要局限性在于成像信号是衰减系数的函数，这几乎不能告诉我们被检查物体的化学或晶体结构。然而，很好地理解，如果收集衍射通量而不是透射X射线，则可以使用与常规计算机断层摄影（CT）类似的算法来重建切片图像。通过测量衍射X射线的能量或波长以及相关的衍射角，可以计算晶体学参数，从而确定样品的物相。科学家和工程师通常使用衍射仪测量精心制备的样品的衍射通量。通常，这种“分子指纹”过程使用相对较软的辐射和较长的检查时间，这两者对于安全性和体内诊断成像都是不切实际的。尽管几十年来做出了重大努力，但几乎没有证据表明在实验室环境中实现的“金标准”特异性和灵敏度在时间关键的商业可行的三维成像技术中得以实现。例如，自20世纪90年代初以来，安全检查行业已经认识到X射线衍射作为“黄金标准”探针的潜力。该领域的挑战包括在行李安全扫描中识别隐藏在日常物品中的粉末、液体、气溶胶和凝胶。现有的CT光谱扫描仪在处理自制爆炸物时存在一定的局限性，衍射辐射的主要局限性在于其信号往往比入射光弱几个数量级。这种基本限制导致长的检查时间，即每点测量数分钟或数小时，这通常对于成像是不切实际的。我们以前已经证明了一个焦点构造几何（FCG）的方法，空心或锥形壳光束产生高强度的图案或焦散线的衍射通量从样品。明亮的焦散线可以进行高速测量，可以进行去卷积以形成深度分辨的截面图像。我们的新方法使空间特征远小于直径的询问光束被准确地解决在重建图像。为了与标准的计算机断层扫描保持一致，FCG断层扫描在吸收和衍射方面都使用类似的重建原理。在本项目中，我们建议通过对FCG吸收/衍射信号进行零星采样，将X射线测量的总次数和X射线剂量减少90%以上。我们使用一个国家的最先进的平板X射线源与多个X射线发射点光耦合到能量分辨探测器。我们把发射器的阵列作为一个虚拟的或空间偏移的线性阵列（SOLA），独立于发射器点之间的最小间隔（由发射器物理限制），并实现零星采样，以尽量减少测量之间的串扰。我们期望我们的方法能够以相同的扫描速率收集衍射和吸收信号，以实现深度分辨的材料特定成像。我们的方法的成功演示将建立一个平台技术可扩展的X射线能量和检测空间。这项工作将使英国在安全和诊断成像领域保持在这些独特和令人兴奋的科学发展的前沿。

This project will bring exciting advances to X-ray imaging by revealing the true nature of materials buried in 3-dimensional scans. The main limitation of conventional X-ray absorption imaging is that the image forming signals are a function of the attenuation coefficient, which tells us almost nothing about the chemical or crystallographic structure of the object under inspection. However, it is well understood that if diffracted flux, rather than the transmitted X-rays, is collected then slice images may be reconstructed using similar algorithms to conventional computed tomography (CT). The measurement of the energy or wavelength of the diffracted X-rays together with their associated diffraction angles enables the calculation of crystallographic parameters to identify, for example, the material phase of a sample.Scientists and engineers routinely measure diffracted flux from carefully prepared samples in instruments called diffractometers. Typically, this 'molecular fingerprinting' process uses relatively soft radiation and long inspection times of which both are impractical for security and in vivo diagnostic imaging. Despite significant efforts over the decades, there is little evidence of the 'gold standard' specificity and sensitivity achieved in laboratory settings being realised in time critical, commercially viable 3-dimensional imaging technologies. For example, the security screening industry has recognised the potential for X-ray diffraction as a 'gold standard' probe since the early 1990s. The challenge in this sector includes identifying powders, liquids, aerosols, and gels buried amongst the clutter of everyday objects in security scans of luggage. State-of-the-art CT spectroscopic scanners are limited fundamentally and are unable to deal adequately with homemade explosives.A main limitation of using diffracted radiation is that the signals are often orders of magnitude weaker in comparison with the primary incident beam. This fundamental limitation leads to long inspection times i.e. minutes or hours per point measurement, which in general is impractical for imaging. We have previously demonstrated a focal construct geometry (FCG) method where a hollow or conical shell beam produces high-intensity patterns or caustics in the diffracted flux from a sample. The bright caustics enable high-speed measurements that can be deconvoluted to form depth-resolved sectional images. Our novel method enables spatial features much smaller than the diameter of the interrogating beam to be resolved accurately in the reconstructed images. In keeping with standard computed tomography, FCG tomography in absorption and diffraction both use similar reconstruction principles.In this project, we propose reducing the total number of X-ray measurements and X-ray dose by more than 90% by applying sporadic sampling to FCG absorption/diffraction signals. We use a state-of-the-art flat panel X-ray source with multiple X-ray emission points optically coupled to energy resolving detectors. We treat the array of emitters as a virtual or spatially offset linear array (SOLA) to implement sporadic sampling independently of the minimum separation between emitter points (limited by the emitter physics) and to minimise crosstalk between measurements. We expect our method to enable the collection of diffraction and absorption signals at the same scan rate to realise depth-resolved material specific imaging. A successful demonstration of our method would establish a platform technology scalable in both X-ray energy and inspection space. This work will maintain the UK at the forefront of these unique and exciting scientific developments in security and diagnostic imaging.