An apparatus and a related method of processing a 2D projection image (110a-b) taken of a tubular structure comprising two or more tubes. One of the tubes branches off from the other at a sidewall opening. The apparatus is configured to estimate a position of the sidewall opening. The estimation is based on a segmentation of the one or more projection images. A marker for the estimated position of the sidewall opening can be displayed overlaid on the projection image.
