Lens distortion in optically coupled digital x-ray imaging

The objectives of this research are to analyze geometrical distortions intr oduced by relay lenses in optically coupled digital x-ray imaging systems a nd to introduce an algorithm to correct such distortions. Methods: The radi al and tangential errors introduced by a relay lens in digital x-ray imagin g were experimentally measured, using a lens-coupled CCD (charge coupled de vice) prototype. An algorithm was introduced to correct these distortions. Based on an x-ray image of a standard calibration grid, the algorithm first identified the location of the optical axis, then corrected the radial and tangential distortions using polynomial transformation technique. Results: Lens distortions were classified and both radial and tangential distortion s introduced by lenses were corrected using polynomial transformation. For the specific lens-CCD prototype investigated, the mean positional error cau sed by the relay lens was reduced by the correction algorithm from about ei ght pixels (0.69 mm) to less than 1.8 pixels (0.15 mm). Our investigation a lso shows that the fourth order of polynomial for the correction algorithm provided the best correction result. Conclusions: Lens distortions should b e considered in position-dependant, quantitative x-ray imaging and such dis tortions can be minimized in CCD x-ray imaging by appropriate algorithm, as demonstrated in this paper. (C) 2000 American Association of Physicists in Medicine.