Optical distortion due to geomagnetism in quantitative angiography

Modern X-ray systems exhibit distortion effects due to the influence of the geomagnetic field on the electron rays inside the image intensifier. In parallel orientation of the image intensifier axis to the geomagnetic field, a characteristic S-shaped distortion arises. This distortion can produce errors in the evaluation of angiograms. Miscalculations may arise in determination of the spatial orientation of the vessel segment from biplane projections, as well as in wall-motion analysis of the left ventricle. Using the central part of the image intensifier only, the distortion can be completely corrected by readjusting the coordinate systems on the monitor by means of small lead markers fixed on the entrance field. In contrast to pin-cushion distortion, geomagnetic distortion is dependent on the spatial orientation of the image intensifier relative to the geomagnetic field, and thus varies with different projections. Therefore, corrections for geometrical distortions are insufficient as long as the pixel-transformation algorithm fails to consider the spatial orientation of the image intensifier