ANALYTIC ISOCENTER CALIBRATION - A NEW APPROACH FOR ACCURATE X-RAY GANTRIES

Isocenter calibration transforms cardiac structures in digitized bipla ne angiograms to absolute dimensions, calculating their radiological m agnification and video transformation. Since a scaling device is not r equired, isocenter calibration yields to more accurate measurements th an the widely used reference object calibration. Both isocenter method s reported so far, regarding geometrically inaccurate x-ray gantries, yield to different and complex computational formulas. Since these for mulas are hard to understand, isocenter calibration is less widely use d. To facilitate the implementation of the isocenter calibration, the basic formulas for accurate x-ray gantries are derived. Shifting virtu ally one x-ray system onto the other, basic isocenter calibration is d erived geometrically in three simple steps. The radiological magnifica tion of an object is illustrated as a ratio of planes. The calculation of all parameters entering the computations is demonstrated geometric ally, by use of the isocenter of the x-ray gantry. The derivation give s a clear idea of isocenter calibration. It is simple to derive and fa cilitates the understanding of the error regarding developments. When geometrical inaccuracies vanish, all formulas become equivalent. Howev er, even if the inaccuracies increase, all methods provide nearly iden tical results, indicating the robustness of isocenter calibration.