A psychiatrist is a physician who specializes in psychiatry. A psychiatrist specializes in the diagnosis and treatment of mental disorders. Psychiatrists are medical doctors who must evaluate patients to determine whether or not their symptoms are the result of a physical illness, a combination of physical and mental, or a strictly psychiatric one. As part of the clinical assessment process they may employ a mental status examination, a physical examination, brain imaging such as computerized tomography (CT/CAT scan), magnetic resonance imaging (MRI), and positron emission tomography (PET) scanning, and blood testing. Psychiatrists prescribe medicine, and may also use psychotherapy, although the vast majority do medical management and refer to a psychologist or another specialized therapist for weekly to bi-monthly psychotherapy.