To make interventional instruments such as catheters more easily identifiable in X-ray images, the catheters are provided with marking elements which can be recognized in the X-ray image. Examples of marking elements are sphere-shaped and ring-shaped marking elements, the ring-shaped marking elements being able to identify the catheters in the manner of a barcode and so being able to make different catheters distinguishable from one another in the X-ray image.