1. An X-ray system for generating X-ray images, which system comprises: an X-ray source that provides X-ray image data; a data processing unit in which said X-ray data is fed; said data processing unit configured to carry out the processing steps of: forming at least one measurement field from a plurality of image points in the X-ray image data, wherein the measurement field is positioned surrounding a region of interest; detecting motion of the region of interest; reforming the at least one measurement field based on the motion detected; and controlling the X-ray source based on information extracted from the measurement field; and a display unit for visualizing the X-ray image data processed by the data processing unit.