Brain scan images produced by positron emision tomography (PET) show differences between an adult with Attention deficit Hyperactivity Disorder (ADHD) (right) and an adult free of the disease (left).
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Source:
Alan Zametkin, M.D.
Section on Clinical Brain Imaging
Laboratory of Cerebral Metabolism
Division of Intramural Research Programs, NIMH
1990

For more information on ADHD see http://www.nimh.nih.gov/publicat/adhdmenu.cfm.