Isocenter calibration transforms cardiac structures in digitized biplane angiograms to absolute dimensions, calculating their radiological magnification and video transformation. Since a scaling device is not required, isocenter calibration yields to more accurate measurements than the widely used reference object calibration. Both isocenter methods reported so far, regarding geometrically inaccurate x-ray gantries, yield to different and complex computational formulas. Since these formulas are hard to understand, isocenter calibration is less widely used. To facilitate the implementation of the isocenter calibration, the basic formulas for accurate x-ray gantries are derived. Shifting virtually one x-ray system onto the other, basic isocenter calibration is derived geometrically in three simple steps. The radiological magnification of an object is illustrated as a ratio of planes. The calculation of all parameters entering the computations is demonstrated geometrically, by use of the isocenter of the x-ray gantry. The derivation gives a clear idea of isocenter calibration. It is simple to derive and facilitates the understanding of the error regarding developments. When geometrical inaccuracies vanish, all formulas become equivalent. However, even if the inaccuracies increase, all methods provide nearly identical results, indicating the robustness of isocenter calibration.
