There are lots of meaning of PET but normally it means Positron Emission Tomography which tells how your body is functioning. It can find out the body chemistry by just one scan.
Jan, wish I didn't know this from first hand experience but I do. You are injected with a sugary liquid containing a small amount of a radioactive material. In just a short time the sugar and ra material accumulate in your body where there is any type of irritation. The PET scan is very detailed and is darned sobering when they can show you a 3d picture of your internals with many of them lit up like a Christmas tree.