PET scans have been around for a long time and most people think of them in relationship to the diagnosis of cancer and tumors. But they are also used in diagnosing brain injuries. They are different than an x-ray or an MRI because they are not showing the structure of the brain itself. What they do is show the function of the brain.
A PET scan does involve the use of a medium, a contrast material that is inserted into the patient. When structures of the brain are damaged they don't take up or absorb this medium as the healthy tissue would do. This then shows up on the image and helps the doctor confirm that certain areas of the brain are not absorbing this dye which indicates that there is damage to those areas. So, a PET scan is a very useful tool in not showing the structure but the but the function.