[chrisy]

Hi Pink

They all measure different things. And it's a heckuva alphabet soup.

CT is more like an xray, and can show areas of different density than the surrounding tissue. So a tumor or scar tissue or necrotic (dead tumor) would appear different than the surrounding area and "show up" on a CT scan.
PET measures metabolic activity - specifically the uptake of a radioactive glucose (sugar) contrast. Since cancer cells are "sugar feeders", they will light up on a PT scan because they are actively sucking up the contrast. PT also can give false positives because inflammation or tissue repair (like after an injury or working out) will also have a lot of metabolic activity.
MRI measures the reflection of magnetic pulses. Again, different types of tissue reflects the pulse differently.

Real world example of how these come into play, from my experience:
I had liver tumor identified on CT scan. During the course of chemo, I was monitored with CT scans which showed some reduction, but may also have been dead/dying tumor. Eventually the CT scan was completely clear, NED. Oops, not completely clear! There were/are 2 tiny spots which were there all along and still are, they are thought NOT to be cancer.

Fast forward 2 years, CT scan shows new areas of concern in liver and spine. But it is not clear that either of these are cancer. So more tests - bone scan which I didn't mention but is sort of like a PT for bones. Showed no cancer there. PT scan showed that the areas of interest in the liver (this is what they called them!) did have increased glucose uptake, so they were determined to be cancer. I now get full body PT scans which thank God show stable liver lesions and ALSO show a ton of activity in my brain. BUT WAIT! Your brain cells are also sugar feeders! So that is ok! If I want to check out my brain some more, I need to get an MRI. A CT scan can sometimes show something, but MRI is best for the brain

Like I said, quite the alphabet soup - not to mention radioactive merry go round. They should choose which type of test based on what info they need. Sometimes, as Andi mentioned, they will go with a cheaper (but less conclusive) test in hopes they can rule out problems, which they sometimes can, but if something shows up, they need to follow up with the more precise test.

Hope this helps. Also hope you never need any of them! They're really fun, but you probably have better things to do with your time!