The invention relates to a positions determination apparatus for determining positions, at which an interventional instrument (3) is located, in a projection image. An input unit (16) allows a user to indicate the position of a tip (23) of the interventional instrument (3) in the projection image, wherein the positions, at which the interventional instrument is located, in the projection image are determined based on the projection image, the indicated tip position and a three-dimensional representation of the interventional instrument, which is defined by its position and shape as determined by a tracking device like an OSS device. By using the projection image, the indicated tip position and the representation the positions in the projection image, at which the interventional instrument is located, can be determined very accurately. Based on these determined positions the projection device and the tracking device can be very accurately registered to each other.