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业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形状，需要从被调查物体周围均匀间隔的角度拍摄x射线图像。然而，问题出现了，因为1)x射线穿透深度有限，2)不同的材料吸收的x射线量不同。因此，大型物体，如较大的面板或材料的组件，其x射线吸收剖面变化很大，因此通常不能用这些技术成像，即使x射线在某些方向和通过某些材料是可能的。航空结构中遇到的许多复合材料部件的检测目前用传统的x射线成像技术是不可行的，要么是由于它们的高纵横比，要么是由于高吸收金属结构的存在。然而，由于其安全关键功能，这些组件通常需要使用无损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，超声波测试通常不能应用于许多先进的复合材料，而热成像的深度能力有限。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方案之一。基于我们目前正在开发的x射线成像硬件项目，该项目允许我们使用非常规的扫描轨迹扫描平板碳纤维结构，目前的提案旨在开发新的计算方法，以从使用我们的新系统进行的x射线测量中恢复三维x射线吸收剖面。目前使用传统方法的实验表明，在不能从所有方向扫描组件的扫描设置中，可用的x射线断层扫描重建算法表现不佳。这是由于系统的基本物理限制，它无法获得足够的信息来进行良好的重建。只有在重建过程中引入额外的信息，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，这些信息以详细的CAD图纸的形式提供，我们建议使用这些信息来克服x射线层析成像重建问题。重点解决以下问题：1)设计一种以CAD图纸为先验信息的重建算法，直接估计物体边界。2)允许估计的表面接近，但不一定与CAD模型预测的表面相同。3)只有估算出目标边界后，才能使用第二步重建来估算x射线吸收的空间分布。为了设计更轻、更便宜、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。这通常可以通过使用现代复合材料来实现。由于这些组件的使用对安全至关重要，因此如果要使用这些组件，则需要可靠的检查方法。我们的发展将使之成为可能。

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.