EEG's are usually standard. PET scans are not even though it looks like they give a good amount of information about brain activity (with relation to thoughts, electrical activity). I don't think PET scans are used because of costs and insurance companies or maybe the FDA has not approved them for diagnostic treatment.

Some disorders have links to genetics. I think OCD patients have extra activity in the frontal lobe. Schizophrenia and other mood disorders point to problems in the hippocampus (which functions include memory, learning, and emotion). I'm sure you can find video of PET scan use and mental disorders on youtube or somewhere else.