A medical imaging system having an X-ray image acquisition device, a plurality of proximity sensors located at components of the X-ray image acquisition device and a processing unit is able to prevent unintended X-ray radiation to an object in a field of irradiation. For this purpose, the processing unit is configured for determining temporal differences of distance information delivered by the proximity sensors, for generating reference distance information based on acquired distance information with an X-ray source, an X-ray detector and the patient support in fixed positions, for filtering out reference distance information from the acquired distance information and for generating a signal in case temporal differences are determined exceeding a predetermined threshold.