1. In a PET scanner, a method for processing coincidence data, the method comprising: associating a selected portion of the coincidence data with one of a plurality of subspaces of a coincidence space; deriving, from the coincidence data associated with a subspace, a contribution to be used in forming an image; and weighting the contribution by an amount that depends on a likelihood that the coincidence data associated with that subspace includes data indicative of trues.