Geometric image correction and iso-center calibration at oblique biplane angiographic views

In the X-ray image intensifier TV system there are geometric image distortions which can be subdivided into pincushion distortions, S-shaped warping, image twisting, and shifting. These errors depend partly on the direction of the geomagnetic field relative to the image intensifier axis and are to be corrected for each view specifically. For caudal or cranial views there is the additional problem that in biplane images the slices of an object are projected on oblique lines. Based on four small bronze washers marking a centered square on each image intensifier screen, a dewarping program automatically calculates a nonlinear polynomial transformation. Then, all points of the image or the study are dewarped and centered accordingly. At the same time, the geometric calibration factors are derived. From the registered viewing angles, those angles are calculated by which both images have to be rotated for biplane assignment. Both optimal biplane visualization and the basis for further analyses are achieved