I've had a PET/CT scan to rule out Castleman's disease as the cause of the persistent swelling in my lymph nodes.  The contrast is radioactive, which is not the healthiest thing (it's creepy to see them take a vial out of a lead canister and then inject it into your vein!), but it didn't bother my mast cells at all.  Piece of cake.  

The scan gives very detailed information about the metabolism of your cells.  It's great for tracking neoplastic growth.  So... if your doctor is recommending one it's probably for a very good reason.

Heather