The invention relates to a process for the preparation of a contrast medium which process comprises: i) obtaining a composition comprising a lipid membrane in a high axial ratio conformation; ii) contacting said composition with an agent serving to release the lipid membrane from the high axial ratio conformation; and iii) agitating said composition in the presence of a biotolerable contrast generator or precursor whereby to form contrast generator or precursor containing lipid-membraned vesicles and to contrast media obtained thereby and to methods of imaging utilising said media.
