1. A method for generating a solid model of a heart in a predetermined phase of a periodic pumping movement of the heart, comprising: observing an electrocardiogram signal which exhibits consecutive RR intervals by an EKG trigger during a single C-arm rotation of a C-arm X-ray unit; generating digital X-ray images in each of the RR intervals by the C-arm X-ray unit with the EKG trigger as soon as a time difference between a current phase and a predetermined phase is less than a predetermined value; determining a parameter influencing a geometry of the C-arm X-ray unit for each of the X-ray images; and generating the solid model from the X-ray images, wherein at least one X-ray image is selected from the each of the RR intervals, wherein a projection data set is determined for the selected X-ray image for generating the solid model by the parameter, and wherein a projection matrix is provided for the selected X-ray image, wherein a provisional solid model is calculated based on the projection matrices by backprojection, wherein a shape of an object mapped in the provisional solid model is compared with a target shape of the object, wherein the projection matrix is altered iteratively and the provisional solid model is recalculated until the shape matches the target shape or at least a difference between the shape and the target shape is smaller than a predetermined value, and wherein the altered projection matrix is used as the projection data set.