Industrial Computed Tomography（abbr. ICT）can clearly, accurately and visually display the internal structure, composition, material and defects of workpieces by two-dimensional cross-section image or three-dimensional image without damage to workpieces. ICT technology is considered to be the best non-destructive testing and non-destructive evaluation technology.
ICT technology integrates with multidiscipline, such as nuclear physics, microelectronics, photonics technology, instrument, precision machinery and control technology, computer image processing and pattern recognition, etc. An ICT system is a technology-intensive high-tech product.
ICT technology is widely used in many fields, such as automotive, material, railway, aerospace, aviation, military industry and national defence. It provides important technical means for the successful launch of space vehicle and spacecraft, development of aero-engine, weapon system testing, analysis of geological structure, railway vehicle acceleration overload safety, oil reserves prediction, and mechanical product quality judgment, etc.
Inspection range: It refers to the objects to be inspected by an industrial CT (abbr. ICT) system, such as Max. penetrable thickness of steel, Max. turning diameter of workpiece, Max. height or length of workpiece, Max. weight of workpiece, etc.
Scanning mode: TR (Translate and Rotate) scanning mode and RO (Rotate Only) scanning mode. RO scanning mode has higher scanning efficiency, but scanning size of workpiece is limited by size of detector. Efficiency of TR scanning mode is about 10%~20% of efficiency of RO scanning mode, but TR is good for the inspect of big turning diameter workpieces. In addition, a CT system usually has function of digital radiography (DR).
Scanning time:time required for scanning a typical tomography (1024 x 1024 image matrix).
Image reconstruction time: time required for image reconstruction. Because of high speed in running of modern computers, reconstructed image can be displayed quickly, and reconstruction time is less than 3s.
Resolution: it is a core performance index for ICT system, and it includes spatial resolution and density resolution.
Spatial resolution: the ability to identify the minimum structural details from CT image.
Relationship between spatial resolution and density resolution: when radiation dose is fixed, spatial resolution and density resolution are contradictory. To improve the spatial resolution will decrease the density resolution, and vice versa.
For a general ICT system, both spatial resolution and density resolution are the core performance indexes. For a high measurement accuracy ICT system, there are two more core performance indexes as follows.
Geometric measurement precision: absolute error between real size of workpiece and size of workpiece which is measured from CT image.
Density measurement precision: relative error between real density of workpiece and density value of workpiece which is measured from CT image.
When choosing an ICT system, it is needed to consider following factors:
--- Technical requirements, and making a balance between budgets and costs.
--- How to assess technical specifications of an industrial CT system.
--- Credit of products supplier and after-sale service.
Users’ technical requirement is the base point of a whole ICT product, mainly includes following aspects:
--- Workpiece: size, shape, weight, composition materials.
--- Basic functions of ICT system: CT, DR, real-time imaging, radiography
--- Inspection targets: defect (porosity, inclusion, looseness, flaw, debonding), internal structure, assembly validity, workpiece measurement, or 3D reconstruction
--- According to the inspection targets, technical requirements should be put forward. If internal defect of workpiece is inspected, detection sensitivity of different defects should be put forward. If inside dimension of workpiece is inspected, detection sensitivity and detection precision should be put forward.
--- Workpiece quantity or inspection speed: inspection speed is related to scanning time, image reconstruction time, and adjusting slice position time and replacing samples time.
--- Special requirements: explosionprotection, necessary fixture, and special requirement for touching detected objects, e.g., biological samples or some positions of workpiece shouldn’t be touched or clamped.
--- System operation and inspection results: User interface, output format of inspection results, special image display and image post-processing, special data processing software, document management and test report form.
--- Radioactive dose and other security problems: it should comply with the relevant national and industry standards or regulations.
--- Operating conditions: site requirement for environments temperature, humidity, power capacity and stability, etc.
In generally, there are two types of X-ray sources, X-ray tube and electron linear accelerator. Both of them are called electronic radiation generator. The common advantage of electronic radiation generator is not able to generate X-ray when powered off, and the intrinsic security is very helpful for industrial site usage.
The biggest advantages of isotope radiation source are the simplicity of energy spectrum, low power consumption, small volume and simple structure. The disadvantage of isotope radiation source is lowintensity. In order to improve the intensity of radiation source, the volume of the radiation source must be increased, which will lead to increase of focus size and reduce of resolution. Therefore, isotope radiation source is seldom used in industrial CT systems.
--- X-ray tube
Peak value of X-ray energy can be adjusted, and it ranges from a few kVto 450kV.
--- Electron linear accelerator
Peak value of X-ray energy is no-adjustable in general, and it ranges from 1 MeV to 16 MeV. Higher peak value of X-ray energy can be reached but it is only used for experiments.
In application, selecting X-ray sources mainly depends on the wall thickness of inspected workpieces. Cost also needs to be considered.
There are two types of detectors for ICT systems, linear array detector and flat panel detector.
--- Linear Array Detector
Advantages: Because length of the scintillation along X-ray beaming direction are not restricted, most of incident X-ray photons are captured, which can help to improve detection efficiency.Especially in high-energy situation, photon capture time can be shortened. X-ray crosstalk is reduced because each scintillator is independent, and there is tungsten or other heavy metal spacers between scintillators.
Disadvantages: Pixel size can not be made too small, and pixel pitch is bigger than 0.1mm. What’s more, the price of linear array detector is higher. Usually, linear array detector can be used in both high-energy and low-energy ICT systems.
--- Flat Panel Detector
Compared with linear array detector, flat panel detector has high X-ray utilization rate. It is suitable for DR imaging, and is able to reach real-time or quasi-real-time dynamic imaging. Moreover, flat panel detector is also fit for three-dimensional direct imaging.
Limited by structure of flat panel detector, there are some disadvantages, such as low X-ray detection efficiency, difficulty in elimination of scattering and crosstalk, less dynamic range and poor application in high energy range.
Generally, if X-ray energy is lower than 4MeV, flat panel detector can be chosen. But in high-energy situation and only structure is need to be detected instead of defect, flat panel detector can also be used, just image accuracy is not high.