"The PET scan is like the satellite photo which shows the severe weather front moving through a particular area. We often superimpose the PET scan on top of the CT scan and therefore the CT scans is like the outline of the cities and states so one can see where the severe weather is currently and where it is heading. In a PET/CT scan the CT provides the structure to be evaluated and the PET scan provides the function. In most cases the function of a disease, especially cancer, precedes many of the structural changes. (use link below for weather map analogy) "