Neuroimaging studies have contributed greatly to our understanding of Alzheimer's disease and other dementias. Computed tomography and magnetic resonance imaging reveal brain structure and aid in the diagnostic evaluation of patients with cognitive impairment. Functional neuroimaging studies use positron emission tomography, single-photon emission computed tomography, and other methods to measure regional cerebral activity, including metabolic rate, blood flow, and neuroreceptor density. Functional neuroimaging results can be useful clinically and have also been used in a variety of research applications to examine physiologic variables in neuropsychiatric illnesses.