In an Article by von Schulthess GK, Steinert HC, Hany TF in Radiology 2006 Feb;238(2):405-22.Integrated PET/CT: Current Applications and Future Directions, The authors discuss the rationale behind and role of PET/CT in oncological imaging with PET giving the functional information and CT adding the anatomical edge.
"For the past 5 years, combined positron emission tomography (PET) and computed tomography (CT), or PET/CT, has grown because the PET portion provides information that is very different from that obtainable with other imaging modalities. However, the paucity of anatomic landmarks on PET images makes a consistent "hardware fusion" to anatomic cross-sectional data extremely useful. Clinical experience indicates a single direction: Addition of CT to PET improves specificity foremost, but also sensitivity, and the addition of PET to CT adds sensitivity and specificity in tumor imaging. Thus, PET/CT is a more accurate test than either of its individual components and is probably also better than side-by-side viewing of images from both modalities."