Medical system comprising an X-ray tube unit catheter 302 and an X-ray tube unit including a miniaturised X-ray tube 301, wherein the X-ray tube unit is adapted to be inserted into the X-ray tube unit catheter in order to generate X-ray radiation at a treatment position in a vessel within a human or animal body.The X-ray tube is provided with a distal electrical pole and a proximal electrical pole. The proximal pole is connected via an insulated electrical conductor 305 to an external power source. The distal pole is connectable to a conducting means 304 at the inner surface of the catheter wall via distal connecting means 303. The conducting means has a predetermined length extending in the longitudinal direction of the catheter and is connected via an insulated electrical conductor to the external power source. During treatment the X-ray tube unit is adapted to be moved in relation to the X-ray tube unit catheter, wherein the distal pole is electrically connected to the conducting means.