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A software tool to measure the geometric distortion in x-ray image systems

A software tool is presented to measure the geometric distortion in images obtained with X-ray systems that provides a more objective method than the usual measurements over the image of a phantom with usual rulers. In a first step, this software has been applied to mammography images and makes use of the grid included into the CDMAM phantom (University Hospital Nijmegen). For digital images, this software tool automatically locates the grid crossing points and obtains a set of corners (up to 237) that are used by the program to determine 6 different squares, at top, bottom, left, right and central positions. The sixth square is the largest that can be fitted in the grid (widest possible square). The distortion is calculated as ((length of left diagonal - length of right diagonal)/ length of left diagonal) (%) for the six positions. The algorithm error is of the order of 0.3%. The method might be applied to other radiological systems without any major changes to adjust the program code to other phantoms. In this work a set of measurements for 54 CDMAM images, acquired in 11 different mammography systems from 6 manufacturers are presented. We can conclude that the distortion of all equipments is smaller than the recommendations for maximum distortions in primary displays (2%)