1. An apparatus for assisting positioning of an implant on a specific point in a vessel of a patient using a minimally-invasive intervention, comprising: a first interventional instrument with a first set of two x-ray positive markers and a functional element located between the first set of the two x-ray positive markers that is inserted in a region of the specific point of the vessel; a second interventional instrument with a second set of two x-ray positive markers and the implant located between the second set of the two x-ray positive markers that is inserted in the region of the specific point of the vessel; an x-ray device that is adapted to record: a first series of 2D x-ray recordings of the vessel comprising the first interventional instrument by adjusting the x-ray device relative to the patient, a second series of fluoroscopy images of the vessel comprising the second interventional instrument by adjusting the x-ray device relative to the patient; and a computer that is adapted to: define at least two reference lines based on the first set of the two markers by connecting instances of one of the first set of the two markers to each other and instances of the other of the first set of the two markers to each other imaged in the first series of 2D x-ray recordings for restricting the specific point, superimpose the reference lines defined in the first series of the 2D x-ray recordings into the second series of fluoroscopy images.