1. A method of determining a start position for a 2D to 3D image registration, the method comprising: a) obtaining characteristic feature sets characterising one or more registration features imaged in a plurality of synthetic 2D images, the plurality of synthetic 2D images having been generated from a 3D image data set, the synthetic 2D images containing the one or more registration features having been imaged according to a plurality of respective viewing parameters b) obtaining a 2D image to be registered with the 3D image data set; c) applying the characteristic feature sets to the obtained 2D image to locate one or more registration features therein; and d) determining which of the one or more characteristic features sets locate the one or more registration features in the obtained 2D image; wherein at least the viewing parameters relating to the synthetic image corresponding to the determined characteristic features sets provide information relating to a start position for a subsequent registration of the obtained 2D image to the 3D image data set.