Method of determining the position of a medical instrument

The invention relates to a method and an arrangement for determining the position of a medical instrument (13) partially inserted into an examination object in a three-dimensional image data set (CT) of the examination subject. On the one hand to achieve the highest possible accuracy of the position determination and on the other hand to keep the required effort as low as possible, in particular to save expensive registration steps before an intervention, the invention proposes at the same time for the detection of X-ray images (I.r,) to detect their spatial positions and the spatial position of a medical instrument (6) used, then a spatial association between an X-ray image (Ir) and the three-dimensional image data set (CT), and to use them appropriately to convert the spatial position of the medical instrument (16) to a position relative to the three-dimensional image data set (CT). As a result, images can be created which contain image information captured both preoperatively and intraoperatively and in which the current position of the medical instrument is shown