3-D X-Ray Computed Tomography

Though well established in medicine as a technique of inspection, x-ray computed tomography (CT) is just beginning to have an impact on nondestructive evaluation. A recent review [1] emphasizes the potential of the technology. An aspect which is apparent from the description of most current systems is that they are being applied to inspection of objects which tend to be locally uniform in the axial direction, e.g., electrical and telephone poles, trees, and rocket motors. The CT systems used are basically adaptations of medical CT technology, and reconstruct slices of an object one at a time. Under the hypothesis that single- or few-slice CT examination will not suffice for certain NDE problems, we have been developing a tomographic system which performs direct three-dimensional reconstruction, in the sense that the transaxial slice is not the fundamental unit of reconstruction. This is accomplished by collecting a set of 2D projection images and performing a reconstruction directly into a 3D array, using an algorithm developed for the purpose. The result differs from a set of spaced conventional slices primarily in the respect that our spatial resolution in the axial direction is substantially as good as that in transaxial planes. Our developmental system is currently limited by beam energy and physical size to examination of small objects; because of its scale it has better absolute resolution (approx. 0.1 mm) than is generally found in CT equipment. The principles employed are quite general and, given appropriately energetic radiation and a 2D array of detectors, could be applied to the inspection of much larger objects. The system will be described, and example reconstructions will be used to illustrate salient aspects of the methodology.