1. A C-arm x-ray imaging apparatus ( 100 ) for processing a 2D projection image ( 110 a - c ) taken of a tubular structure comprising a first tube ( 205 ) and a second tube ( 210 ), the second tube branching off from the first tube at an opening ( 212 ) in a sidewall of the first tube, the image taken whilst a contrast agent carrying fluid is residing in the tubular structure, the imaging apparatus comprising: an input unit; a processing unit ( 420 ); and an output unit ( 410 ); the input unit configured to select an initial projection image ( 110 b ) taken along a projection axis, the first tube and the second tube, when viewed along the projection axis, overlapping to so form an overlap region in the initial projection image, the processing unit comprising a segmenter ( 430 ) configured to segment the overlap region in the initial projection image, the processing unit being configured to establish, based on the segmentation of the initial projection image, an estimate for the position in the projection image of the sidewall opening the output unit configured to display a marker for the estimated sidewall opening position thereby providing a visual representation for the position in the initial projection image of the sidewall opening, wherein an intensity detector ( 440 ) configured to detect inside or at a boundary of the overlap region an intensity drop, thereby establishing the position where the drop occurs as the estimate for the position of the first tube's sidewall opening.