The invention relates to a calibration apparatus for calibrating a system for introducing an influencing element like a radiation source into an object, particularly for calibrating a brachytherapy system. First and second images show a longish introduction device (12) like a catheter and a tracking device (16) like an electromagnetically trackable guidewire inserted into the introduction device as far as possible, and the introduction device and a calibration element (46) having the same dimensions as the influencing element and being inserted into the introduction device as far as possible. A spatial relation between the tracking device and the calibration element is determined based on the images for calibrating the system. Knowing this spatial relation allows accurately determining an influencing plan like a brachytherapy treatment plan and accurately positioning the influencing element in accordance with the influencing plan, which in turn allows for a more accurate influencing of the object.
