Sporadic diffraction and absorption volumetric X-ray imaging

该项目将通过揭示隐藏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。传统的x射线吸收成像的主要限制是成像信号是衰减系数的函数，它几乎不能告诉我们被检查物体的化学或晶体结构。然而，众所周知，如果收集衍射通量而不是透射x射线，则可以使用与传统计算机断层扫描（CT）类似的算法重建切片图像。对衍射x射线的能量或波长及其相关衍射角的测量，使晶体学参数的计算能够识别，例如，样品的物质相。科学家和工程师经常用衍射仪测量精心准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巨大的努力，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、商业上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，安检行业已经认识到x射线衍射作为“黄金标准”探针的潜力。这一领域的挑战包括在行李安检中识别埋在杂乱的日常物品中的粉末、液体、气雾剂和凝胶。最先进的CT光谱扫描仪从根本上是有限的，无法充分处理自制爆炸物。使用衍射辐射的一个主要限制是，与主入射光束相比，信号通常要弱几个数量级。这一基本限制导致检测时间长，即每点测量分钟或小时，这通常是不切实际的成像。我们之前已经演示了一种聚焦构造几何（FCG）方法，其中空心或锥形壳体光束在样品的衍射通量中产生高强度图案或焦散。明亮的焦散使高速测量能够反卷积以形成深度分辨率的截面图像。我们的新方法能够在重建图像中精确地分辨出比询问光束直径小得多的空间特征。与标准计算机断层扫描保持一致，吸收和衍射中的FCG断层扫描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。我们使用最先进的平板x射线源与多个x射线发射点光学耦合到能量分辨探测器。我们将发射器阵列视为虚拟或空间偏移线性阵列（SOLA），以实现独立于发射器点之间最小分离（受发射器物理限制）的零星采样，并最大限度地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，以实现深度分辨材料的特定成像。我们的方法的成功演示将建立一个在x射线能量和检测空间可扩展的平台技术。这项工作将使英国在安全和诊断成像方面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.