Digital images are produced by many medical imaging modalities. In general, imaging techniques have tended to compare rather than combine image information. Video and photographic methods have, however, been used to transfer image features from one modality to another (Higa et al, 1983; Morin & Sirr, 1984) and it has become practicable to transfer digital image data between devices. Hence it is now feasible to investigate the digital combination of images which display data in the same plane of interest. This should be particularly worthwhile if one modality provides complementary information, either by providing features which the other modality lacks or by confirming image features derived from a different physiological measurement. Transmission computed tomography (CT) and emission computed tomography (ECT) images are particularly suited for combination as the former provides anatomical detail whilst the latter provides functional information. The good edge features of the CT images should allow the correlation of images and improve the quantitation of the ECT studies. This paper describes the methods used and the problems found both in transferring digital image data between systems and in presenting the combined information on a single image data processor.

The potential and problems of the superimposition of medical images from different techniques is addressed. Examples of current work and methods of combining images from nuclear medicine and radiology are presented, with discussion of their usefulness.