The present invention relates to image processing device (5) for generating a time series of 3D volume images showing the blood flow in a vascular tree of an object (40), based on a first series of X-ray projection images (D) of the object acquired from different projection directions and a second and a third series of X-ray projections images (E, F) of the object acquired alternately at a first or second fixed projection plane, respectively, during inflow of contrast agent into the vascular tree of the object. In order to allow image reconstruction of unambiguous structures in the object of interest, and, in particular, the generation of 3D volume images showing the blood flow in a vascular tree of an object even if that vascular tree comprises non-tubular structures, such as aneurysms, an image processing device is proposed comprising: a reconstruction unit (51) for reconstruction of a 3D volume image of said object from said first series of X-ray projection images (D), a segmentation unit (52) for segmentation of the vessel tree from said 3D volume image, —a forward projection unit (54) for forward projection of the segmented vessel tree onto said first and projection plane (R1, R2), respectively, and a mapping unit (55) for mapping of the image values of pixels of the vessel tree in said second and third, respectively, series of X-ray projection images onto corresponding voxels of said 3D volume image to obtain said time series of 3D volume images showing the blood flow in the vascular tree of the object.