Welch, an expert in synthetic chemistry, has been a leader for more than 30 years in the development of synthetic imaging agents that have allowed doctors to use positron emission tomography (PET) to diagnose an increasingly wide variety of disorders. He is also head of the Radiochemistry Institute at the Mallinckrodt Institute of Radiology. Welch is the current principal investigator of Washington University's longest continuously renewed NIH grant, "Cyclotron Produced Isotopes in Biology and Medicine," now in its 45th year. The grant incarnation of the grant is dedicated to the development of new imaging agents that can help scientists better understand and diagnose the heart. The same grant supplied the funding that allowed WUSTL researcher Michel Ter-Pogossian to lead the development of the first PET scanner at the School of Medicine in the 1970s. Welch succeeded Ter-Pogossian as principal investigator in 1984.