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

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 method

To 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.

Beyond academia, our proposal is expected to have major impact on several realms, including Economy, Society, People and Knowledge.

The proposed technique is an enabling technology that will allow advanced composite materials to be used in aerospace manufacturing more widely. Economic impact will come through lighter, more fuel efficient aircraft, reduced operational costs, simplification of manufacturing logistics, use of high throughput manufacturing inspection, reduced material wastage, rapid inspection and sentencing of rotorcraft and improved metrological inspection during manufacturing. Economic impact will thus be through improvements to aerospace design, build, testing and overhaul procedures. The aerospace industry contributes over £24 billion per year to the UK economy and is one of the key UK advanced manufacturing industries. The x-ray based non-destructive inspection tools developed here will enable the routine use of advanced composite materials in performance- and safety-critical applications. Over the next 5-10 years, advanced x-ray imaging tools will thus lead to safer, quieter and more fuel efficient air transport. Over the next 50 years, this will benefit several other key UK industries including defence and energy, which also increasingly rely on components with complex shapes manufactured using advanced materials.

Additional economic benefits will come from manufacturers of industrial x-ray scanners such as Nikon metrology Ltd., the main UK manufacturer for industrial measurement x-ray scanners. Effective exploitation will be further aided through close collaboration with key industrial stakeholders, including Rolls Royce and Nikon metrology.

Impact on Society will largely be a consequence of new technologies. The routine use of advanced composite materials in performance and safety critical applications will, in the next 50 years, enable new technologies for transport, energy, defence and healthcare. For example, global warming is caused by greenhouse gas emission, 26% of which can be attributed to energy supply and 13% to transport. Significant impact can thus come through the use of advanced materials that enable the design and manufacture of more fuel-efficient airplane components. Advanced design will furthermore have significant societal benefits through a reduction of noise.

Impact on People will come through training of different stakeholders. This will include industrial partners as well as the PI, CoIs and postdocs directly funded through this grant. The PI will be able to develop into a leader in his field, will have the opportunity to further strengthen his work with industrial partners and will gain valuable academic skills in the running of a complex, long-term project. The postdocs will learn a range of academic skills, including the ability to work with a range of partners, both academic and industrial. Academic x-ray users will gain valuable knowledge in advanced reconstruction methods whilst knowledge transfer to industry will benefit industrial partners.

Impact on Knowledge will result directly from a better understanding of x-ray image reconstruction. The proposal is also part of a concerted effort to establish unique world leading expertise in this field and will inform related projects. Additionally, many of the tools will be useful more widely for XCT reconstruction and so will benefit other application areas. For example, XCT is now one of the main imaging tools in a wide range of scientific areas so that advances will contribute to knowledge in fields from biology to material science.