<human>:  What is the difference between a PET scan and an MRI scan?
<bot>: A PET scan is a type of nuclear medicine imaging. It uses radioactive tracers to visualize metabolic activity in the body. An MRI scan is a type of medical imaging that uses magnetic fields to visualize the brain. It is used to diagnose and treat a variety of conditions, including brain tumors, stroke, and Alzheimer's disease.