A method of producing a resultant object data set is presented in which a reference object data set and a contrast object data set containing extrinsic contrast information are both provided. Both object data sets contain information describing the same part of the body. Extrinsic contrast information from the contrast object data set is segmented to produce a segmented data set and this data is dilated to produce a dilated data set. A mask is then created from the dilated data set and the reference object data set is filtered by the mask to produce a resultant object data set. Sometimes a further method is found to be useful, in which information which occurs in both object data sets is segmented from the reference object data set to produce a data set which is used to create a further mask for filtering.