EXTENDED 3D CT METHOD FOR THE INSPECTION OF LARGE COMPONENTS

Conventional X-ray 3D computed tomography requires the object to be fully enclosed by the X-ray cone in the horizontal direction. Furthermore, 3D reconstruction algorithms require a 360° rotation of the object. These limitations do not allow the scan of large parts in many cases or lead to a low spatial resolution. In this article we present a new CT system which allows besides conventional 3D CT the scan of large objects by an extended reconstruction method. Region of interest scanning can be performed to achieve a high resolution even in large parts. Examples will be given from the inspection of welds in large objects. The potential for future applications is shown. Introduction: The acceptance of computed tomography (CT) methods for industrial applications has been significantly increasing during the past years. Especially direct 3D CT systems have been becoming a standard inspection tool for flaw detection as well as for geometrical analysis. State-of-the-art 3D scanners perform fast volumetric scanning and deliver voxel data with an isotropic spatial resolution up to a few microns [1]. These scanners allow the reconstruction of three dimensional structures with a single rotation (Fig. 1). The scan procedure is briefly described as follows: A conical beam from an X-ray source penetrates the investigated object. The attenuated radiation is measured by a large area detector. In order to irradiate the object from all sides, the object rotates in the X-ray cone. During rotation a set of projections is measured and stored. The set of projections is then used to reconstruct the 3D structure of the object. Fig. 1: Principle scheme of 3D cone-beam tomography In spite if the wide range of applications, the conventional CT reconstruction methods [2,3,4,5] show the following limitations: Requirements of the conventional CT reconstruction method: Limitations for the application: Object must be fully enclosed horizontally by the X-ray cone Limited object size 360° rotation of the object Limited object size Sufficient SNR in all projections Limited material penetration length All projections need to be within the dynamic range of the detector Limited asymmetries and density variations Table 1: Limitations of conventional CT systems Table 1 shows that conventional CT systems limit the application to objects with small to medium size, which need to be smaller than the size of the X-ray detector. Since a sufficient SNR is required in all projections, the quality of the reconstruction is primarily determined by the “worst case” projections, i.e. the SNR of the projections with the highest penetration length. Therefore, the system limitations do not allow to scan large objects as well as flat objects. If the requirements are not met, artefacts will deteriorate the result. Another significant limitation of the conventional CT methods is the spatial and the density resolution. In Table 2 the limitations are summarized. Resolution of the conventional CT reconstruction method: Limitations for the application: Spatial resolution ≈ object diameter / horizontal detector pixels Limited spatial resolution Density resolution = f (dynamic range of detector, dynamic range of object, system linearity) Limited density resolution Table 2: Resolution limitations of conventional CT systems The spatial resolution of conventional high quality state-of-the-art CT scanners is determined approximately by the diameter of a cylinder which fully encloses the object divided by the number of horizontal detector pixels. Hence, the spatial resolution depends primarily on the object size. Therefore, small details in large objects can often not be visualised. Due to the system requirements described in Table 1, the object needs a full rotation in the X-ray cone. In this case the combination of the detector dynamic range as well as the object dynamic range determine the density resolution. System non-linearities further reduce the density resolution. Results: The above mentioned description of conventional CT methods show severe limitations with regard to possible applications and the achievable resolutions. Many industrial requirements cannot be met by CT because of these limitations. For this reason we developed an extended reconstruction method which overcomes the limitations in two ways: Region of interest (ROI) CT CT Scan with limited angle The region of interest CT overcomes the requirement that the object must be fully enclosed horizontally by the X-ray cone. Therefore, large objects can be scanned and the spatial resolution can be increased. The CT scan with limited angle allows to zoom in small areas of large objects. ROI CT and limited angle CT are so far known to produce severe artefacts. However, our approach of reconstructing the data minimize the artefacts to a level which is well acceptable in industrial applications. The result is a 3D reconstruction of sections of large objects with a high resolution. In the following some examples are given to demonstrate the potential of the extended reconstruction method. The examples are reject parts which show some defects. Some of the defects were artificially created in order to demonstrate the recognisability of the inspection method. Fig. 2 shows a part of a car body. The maximum dimension of the object is approximately 800 mm which is too large for most of the conventional CT scanners. Therefore, we performed our new ROI CT method on the section as indicated in Fig. 2.