Model constrained x-ray tomographic reconstruction, a blueprint for better images

X射线成像技术越来越多地用于专门的无损检测和测量应用，特别是在高价值和安全/性能关键行业，如国防或航空航天。目前的层析成像方法可以成像详细的内部几何图形，需要从被调查对象周围均匀分布的角度拍摄X射线图像。然而，问题的出现是因为1)X射线穿透深度有限，2)不同材料对X射线的吸收程度不同。因此，较大的物体，如较大的面板或组件，其材料的X射线吸收曲线变化很大，因此通常不能使用这些技术成像，即使X射线可以在某些方向和某些材料中穿透。由于高纵横比或高吸收金属结构的存在，目前用传统的X射线成像技术无法对航空航天结构中遇到的许多复合材料部件进行检测。然而，由于它们的安全关键功能，这些部件经常需要使用非破坏性检测方法定期检查内部缺陷。然而，目前的无损检测方法往往不适用。例如，超声波检测往往不能应用于许多先进的复合材料，而热成像技术的深度能力有限。对于这种复合材料，如果要在未来的航空航天设计中充分利用这些材料的潜力，基于x射线的技术仍然是为数不多的替代技术之一。在目前的一个项目的基础上，我们正在开发X射线成像硬件，使我们能够用非传统的扫描轨迹扫描平板碳纤维结构，目前的提议旨在开发新的计算方法，从我们的新系统进行的X射线测量中恢复三维X射线吸收轮廓。目前使用传统方法的实验表明，现有的X射线层析重建算法在不能从所有方向扫描组件的扫描设置中执行得很差。这是由于系统的基本物理限制，无法获取足够的信息来进行良好的重建。只有在重建中引入更多的信息，才能克服这一基本限制。对于几乎所有现代制造的部件，幸运的是，这些信息都可以以详细的CAD图纸的形式获得，我们建议使用这些信息来克服X射线层析重建问题。1)设计了一种利用CAD图纸作为先验信息的重建算法，以直接估计物体边界。2)允许估计的表面接近于CAD模型预测的表面，但不一定与CAD模型预测的表面相同。3)只有在估计了对象边界后，才使用第二个重建步骤来估计X射线吸收的空间分布。4)使用多重网格方法来提高方法的计算效率为了设计更轻、更便宜和更高效的飞机，许多部件需要变得更轻、更便宜，同时仍然满足严格的性能标准。这通常可以使用现代复合材料来实现。由于这些部件的使用是安全关键的，如果要使用这些部件，则需要可靠的检查方法。我们的发展将使这一点成为可能。

X-ray imaging technology is increasingly used for specialised non-destructive testing and measurement applications, especially in high value and safety/performance critical industries, such as defence or aerospace. Current tomographic methods, which can image detailed internal geometries, require x-ray images to be taken from angles evenly spaced around the object under investigation. However, problems arise because 1) x-ray penetration depth is limited and 2) different materials absorb x-rays in varying amounts. Thus, large objects such as larger panels or assemblies with materials with widely varying x-ray absorption profiles can thus often not be imaged with these techniques, even if x-ray penetration is possible in some directions and through some materials. The inspection of many composite components encountered in aerospace structures is not currently feasible with traditional x-ray imaging techniques, either due to their high aspect ratio or due to the presence of highly absorbing metal structures. Yet owing to their safety critical function, such components often need to be inspected regularly for internal defects using non-destructive testing methods. Current non-destructive inspection approaches, however, are often not applicable. For example, ultrasound testing can often not be applied to many advanced composite materials whilst thermography has limited depth capability. For such composite materials, x-ray based techniques remain one of the few alternatives if these materials are to be used to their full potential in future aerospace design. Building on a current project in which we are developing x-ray imaging hardware that allows us to scan flat panel carbon fibre structures with unconventional scan trajectories, the current proposal aims at the development of new computational methods to recover the three dimensional x-ray absorption profile from the x-ray measurements made with our new system. Current experiments using traditional methods have shown that available x-ray tomography reconstruction algorithms perform poorly in scan settings where a component cannot be scanned from all directions. This is due to a fundamental physical limitation of the system, which is just not able to acquire enough information to allow good reconstructions. This fundamental limit can only be overcome if we can introduce additional information into the reconstruction.For nearly all modern manufactured components, such information is luckily available in the form of detailed CAD drawings and it is this information we propose to use to overcome the x-ray tomographic reconstruction problem.In particular, we will address the following problems. 1) Design a reconstruction algorithm that uses CAD drawings as prior information in order to directly estimate object boundaries. 2) Allow the estimated surface to be close to, but not necessarily identical to the surfaces predicted by the CAD model.3) Only once object boundaries have been estimated, use a second reconstruction step to estimate the spatial distribution of x-ray absorption.4) Use a multigrid approach to increase computational efficiency of the methodTo design lighter, cheaper and more efficient airplanes, many components need to become lighter and cheaper whilst still satisfying strict performance standards. This can often be achieved using modern composite materials. As the use of these components is safety critical, reliable inspection methods are required if these components are to be used. Our developments will enable this.