A method of acquiring an X-ray image of a region of interest of an object as well as a corresponding X-ray image acquisition device are described. A semitransparent X-ray transmitting device 11 comprised in a collimator 9 is positioned within a beam 35 of irradiating X-rays thereby at least partly shielding regions 17 on a detector surface 15 of a detector 5. After acquiring an X-ray image by detecting X-rays transmitted through the object 13, the position of the semitransparent X-ray transmitting device 11 is automatically adjusted based on image information included in the at least partly shielded region 17 on the detector surface 15. Taking into account X-ray absorption properties of the semitransparent X-ray transmitting device 11, a virtual image may be calculated within an analyzing unit 23 of a control device 7 wherein the virtual image corresponds to an X-ray image as if there was no X-ray absorption within the collimator 9. From this virtual image, a feature detection unit 21 may derive a region of interest and an automatic wedge positioning device 25 may position the wedges of the semitransparent X-ray transmitting device 11 of the collimator 9 in order to continuously follow a moving region of interest.
