Dr. Kessler is a neuroradiologist and nuclear medicine physician whose research centers on cerebral neurotransmission and energy metabolism and their relationship to psychiatric and neurological disorders. He initiated the positron emission tomography (PET) program at the National Institutes of Health and served as its first director, conducting numerous studies on cerebral metabolic changes in schizophrenia and affective disorders. Dr. Kessler began developing radioligands (radioactive binding molecules) to study dopamine D2 receptors in schizophrenia. This resulted in the definition of a new class of radioligands that could be used with PET or with SPECT, another neuroimaging technology, to study these receptors. His team applies these methods to studying attention-deficit disorder, schizophrenia and drug abuse.