I've had 40+ CT scans and three PET scans. CT scans are essentially multiple X-rays orchestrated by computer that give nice images of your organs and, to a lesser extent, bones. Drinking scan gack before the scan, and getting injected with contrast improves the images, like adjusting the contrast and brightness controls of your TV set (oops, just dated myself).
With PET scans, they inject you with radioactive sugar water and see where it collects. Cancer cells are more active than normal cells and suck it up, becoming hot spots on the PET scan.
If you know your anatomy, you can glance at a CT scan and see everything that should, or shouldn't, be there. Of the three types of scans (CT, MRI, PET), CT is cheapest. Hence CT scans are most commonly used.
PET scans are the most expensive, hence they're infrequently used. Insurance companies will balk at paying for them, but usually cave when their denial is appealed. Also, GISTs are sometimes so slow growing they won't show up on PET scans. All three of my PET scans showed nothing, yet my GIST was clearly visible on CT scans.
This is a cool one-minute video that shows the difference between CT and PET, and what a CT/PET combo looks like: http://www.webmd.com/video/ct-pet-scans
My local onc wants to do a FUSION SCAN. He says it's a COMBINATION pet scan and CT scan without the contrast.
FUSION SCANS are the kind of scans my local Onc has been giving me. I have had 3 now. They are certainly easy in that you don't have to drink any awful goop. But they do stress that the evening before the scan you have a high protein diet, and no exercise. They come and inject you with the radioactive stuff, then place a warm blanket on you, so you lie still in a big comfy recliner for about 45 minutes to an hour, then they take you in and do the scan which lasts approximately 25 minutes. It is important to drink a lot of fluid after to flush it out of your system.
My local guy insists these are the best. However, the specialist - Dr. Trent uses the CT with contrast. The thing about the PET/CT that is beneficial is that it shows actual tumor activity, where the plain CT scan can only give the size and shape. At least that is my limited understanding of it all. It seems to me the PET/CT gives more info, but the specialists don't use it on a regular ongoing basis for some reason. I have been told that the PET is especially useful for initial diagnosis. But have been told in the past that, they aren't necessary to do each time. I guess it depends who you talk with and what they are looking for.