A method and an image processing system for the evaluation of projection images generated by an X-ray imaging system, wherein the images may show different instruments of a given set of interventional instruments like catheters or guide wires. The instruments are equipped with markers such that their configuration is characteristic of the corresponding instrument. Preferably three markers are arranged on a straight line, the ratio of the distances between them being characteristic for the corresponding instrument. The image processing system may then identify the instruments present in a given projection and provide functionalities for a user that correspond to said instruments. Moreover, the system may be used to locate an instrument of interest in a projection image if the marker configuration of that instrument is known a priori.