Calibration method of projection coordinate system for X-ray cone-beam laminography scanning system

Computed laminography (CL) is different from conventional computed tomography(CT) for its specialization in testing plate-type structures, such as integrated circuit boards (ICB), multilayer printing circuit boards (MPCB), and ball grid arrays (BGA). It adopts a different scanning way to successfully produce cross-sectional images of the plate-type objects, while the conventional CT cannot be used. According to the CL reconstruction algorithm, precise determination of the origin of the projection coordinate system is the first step during cone-beam CL scanning system calibration. But unfortunately, for the practical CL scanning system, it is impossible to measure the position of the X-ray focus projection by direct means. A new method to determine the X-ray focus projection coordinates with high accuracy is proposed. Firstly the angle between the central X-ray and the axis of rotation is set to zero by rotating the X-ray source and detector synchronously. Acquiring digital radiography (DR) images of several spherical objects at two geometrical magnification ratio positions in the cone X-ray beam, the two DR images are combined to one image, on the base of which image processing methods are employed to get the center points of each projection of the spherical object. And every two-positioned center points of the same spherical object determine a line, so all couples of projection centers of all the spherical objects construct a group of lines, mathematically an over-determined equation set. After solving the over-determined equation, the X-ray focus projection coordinates are finally obtained. The experimental results prove that the accuracy of this method can satisfy the requirements of the practical scanning system, meanwhile it is feasible to realize. (C) 2012 Published by Elsevier Ltd.