[Becky]

CT scans are xrays taken in small "slices" so there are many of the pictures and together they represent the organ or area being viewed.

A Pet/CT is the same thing but you are given an injection of radioactive glucose (a sugar). Theory is that cancer, since it is growing faster than normal cells, takes up this glucose and the special xrays picks up a glow if cancer is there (false positives include injury or infection since your body is rapidly "growing" in those situations - ie: healing). A CT could show a tumor but a PET/CT would tell you if that tumor is alive or dead. Therefore, a PET might be better to track a tumor you know you have but you don't know if the chemo has worked its magic.

An MRI uses no radiation as PET or CT does. It uses magnetic waves. The waves pass through your body (or organ) and make a picture. Some tissues, like the brain, don't xray well. Also, if you have a brain tumor and your doctor is tracking its regression or progression, an MRI is better as you are not radiating the brain over and over again - especially since it is radiation that is the cure for the tumor. MRI is a good method to track some tumors. They work well to diagnose both breast cancer and DCIS but MRI does not work as well as mammograms to uncover calcifications - especially calcifications that might be hiding a small DCIS (therefore, both used together could be best).

PET/CT is probably best for the body (as you will be able to tell if the mass detected is cancer) and MRI for the brain.
I hope this helps