1. A device for providing a basis for registering a three-dimensional model of an anatomical region with two-dimensional projection images, comprising: a 3D imaging system ( 10 ) for deriving a three-dimensional model of an anatomical region at a phase of a periodic signal; a 2D imaging system ( 11 ) for deriving two-dimensional projection images of a region which overlaps with the anatomical region reconstructed by the 3D imaging system; a pre-selector ( 15 ) for selecting a subset of projection images out of the derived two-dimensional projection images, which are closest to the phase; and a selector ( 16 ) for selecting a reference image for the registration from the subset of pre-selected projection images by choosing the projection image with the most contrast.