1. Method to determine the projection geometry of an x-ray system comprising: generating, with the x-ray system, an x-ray image of an object introduced into a patient; determining, from the x-ray image, a first quantitative measurement of a characteristic dimension of the imaged object in the x-ray image; determining a second quantitative measurement of the characteristic dimension from real geometry data of the object; and determining the projection geometry using the first and second quantitative measurements.