A system for estimating a position in an x-ray projection image. The position corresponds to a projected probe position of an intravascular probe at a time of acquiring data from the intravascular probe. Marker detector for identifying in the x-ray projection image a plurality of projected positions of markers (302). The markers are located at predetermined distances along an acquisition trajectory of the intravascular probe. Interpolator for interpolating the projected positions to obtain the projected probe position on the acquisition trajectory. The projected probe position corresponds to a location of the intravascular probe at a time of acquiring "the data, in dependence on a distance along the acquisition trajectory between the intravascular probe at the time of acquiring the data and at least one of the markers. Indicator for indicating in an angiographic projection image the projected probe position.