A method, and a system when implementing a method, of reducing artefacts in image creation in 4D cone beam computed tomography (4DCBCT) images, the method comprising the steps of: (a) performing a 4DCBCT scan of a target patient including a series of spaced apart projections through the target patient, with each projection having an associated estimated or measured respiratory state； (b) initially dividing the series of projections into a corresponding series of respiratory bins, with each respiratory bin having projections substantially from a portion of a cyclic respiratory state； and (c) optimising the projections at the bounds of each respiratory bin so as to improve an image quality measure of the images.
