1. A method for operating an image system ( 10 ) of an imaging medical examination device ( 1 ), the image system ( 10 ) having a reception unit ( 11 ) for receiving a plurality of signals arising at different locations, and a display unit ( 19 ) for the imaging representation of pixels, the pixels each being assigned at least one signal, characterized in that an event of the undisturbed operation of the medical examination device ( 1 ) automatically triggers a defect determination ( 63 ) for determining a defective pixel possibly present in the image.