1. An x-ray system comprising: an x-ray source; an image detector; a monitor; at least one input device configured to provide coordinates relative to an x-ray image displayed on said monitor; a controller connected with said at least one input device, said controller configured to determine at least one Region of Interest (ROI) on said displayed image based on coordinates provided by said at least one input device; a collimator configured to expose different areas of a patient to different radiation levels according to said determined at least one ROI; and an image processing unit connected with said detector and said monitor, said image processing unit configured to process an image captured through said collimator by correcting at least one image part outside said determined at least one ROI according to previously acquired data comprising at least part of said at least one image part that was included in at least one previous ROI, using a tone-correction function.