Radiopaque endocardial markers implanted by catheter are a useful tool for investigating cardiac function because one can follow the motion to a specific element to myocardium in three dimension. This motion is computed by recording each marker's X-ray shadow with a biplane radiographic system. It is necessary to identify which marker is which on both views to complete this computation. Rotation of the heart with respect of the X-ray system can change the pattern of markers so dramatically that one cannot identify each marker's shadow with confidence. We describe a method to predict where each marker's shadow will be on the X-ray images after the heart rotates, permitting identification of each marker in biplane radiographs, even after extreme changes in the heart's orientation.