Lens distortion in optically coupled digital x-ray imaging

The objectives of this research are to analyze geometrical distortions introduced by relay lenses in optically coupled digital x-ray imaging systems and to introduce an algorithm to correct such distortions. Methods: The radial and tangential errors introduced by a relay lens in digital x-ray imaging were experimentally measured, using a lens-coupled CCD (charge coupled device) 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 distortions introduced by lenses were corrected using polynomial transformation. For the specific lens-CCD prototype investigated, the mean positional error caused by the relay lens was reduced by the correction algorithm from about eight pixels (0.69 mm) to less than 1.8 pixels (0.15 mm). Our investigation also shows that the fourth order of polynomial for the correction algorithm provided the best correction result. Conclusions: Lens distortions should be considered in position-dependant, quantitative x-ray imaging and such distortions can be minimized in CCD x-ray imaging by appropriate algorithm, as demonstrated in this paper. (C) 2000 American Association of Physicists in Medicine.