The invention relates to an apparatus for determining a control protocol for controlling a C-arm system comprising a radiation source. The control protocol comprises a sequence of roll angles and a sequence of propeller angles. The apparatus comprises a tilt plane providing unit for providing a tilt plane tilted with respect to a vertical plane, and a control protocol determining unit for determining a control protocol by determining the sequence of roll angles and the sequence of propeller angles such that the radiation source follows a trajectory that comprises a circular component in the tilt plane and that allows the C-arm system to acquire projection data for image reconstruction. The apparatus allows to acquire computed images, for instance tomographic images, with an improved quality and to reduce radiation in radiation-sensitive areas.
