The invention relates to the assistance of the navigation of a catheter (1) in a vessel (2). A sequence of cross-sectional images of the portion of the vessel that is of interest is first obtained with the help of an intravascular ultrasound (IVUS) probe (3) and stored as a roadmap of the vessel. A cross-sectional image (10) that is obtained at the current position of the IVUS probe (3) can then be sorted to the position on the roadmap that is the best fit. A model (3′) of the probe, and a model (11′) of the instrument (a stent (11), for example) coupled to the probe, can be shown on a display (6) at the corresponding position on the roadmap.