Positron emission tomography

Positron emission tomography emerged from the collision of nuclear physics and medical imaging, creating a technology that could watch the living brain think. When Michel Ter-Pogossian, Michael Phelps, and Edward Hoffman built the first practical PET scanner at Washington University in 1975, they transformed positron-emitting isotopes from physics curiosities into windows on human metabolism.

The adjacent possible had accumulated through decades of nuclear research. Physicists had understood positron-electron annihilation since the 1930s—when a positron meets an electron, both vanish, producing two gamma rays traveling in opposite directions. This 'coincidence detection' principle meant that if a patient could be injected with positron-emitting tracers, and detectors could identify these paired gamma rays, the scanner could locate exactly where the tracer had accumulated.

Early attempts at positron imaging began in the 1950s at Massachusetts General Hospital, where Gordon Brownell and William Sweet explored brain tumor detection. But the technology remained crude—single planes of detection, poor resolution, limited clinical utility. The breakthrough required advances in computing, scintillation detection, and radiochemistry to converge.

Ter-Pogossian, a nuclear physicist at Washington University's Mallinckrodt Institute of Radiology, assembled the team that would solve these problems. Phelps and Hoffman, then assistant professors, designed a hexagonal array of sodium iodide detectors surrounding the patient. Heavy side shielding minimized interference. Sophisticated electronics distinguished true coincidences from random noise. The result was PETT (Positron Emission Transaxial Tomograph), producing cross-sectional images of unprecedented clarity.

The critical enabling chemistry came in 1976 when Alfred Wolf's team at Brookhaven National Laboratory synthesized fluorodeoxyglucose labeled with fluorine-18 (FDG). Glucose is the brain's primary fuel; FDG concentrates wherever metabolism is active. The first FDG-PET brain images at UCLA in 1977 revealed what no imaging technique had shown before: the living brain at work, thinking, processing, diseased or healthy.

Path dependence shaped PET's trajectory. When Phelps and Hoffman moved to UCLA and collaborated with EG&G ORTEC in Oak Ridge, they produced the ECAT (Emission Computerized Axial Tomograph)—the first commercial PET scanner. The ECAT II became the dominant platform of the late 1970s, establishing standards that influenced subsequent designs.

The cascade was transformative for oncology, neurology, and cardiology. Cancer cells consume glucose voraciously; PET scans reveal tumors invisible to anatomical imaging. Alzheimer's disease, epileptic foci, and cardiac viability assessments all became PET applications. The 1998 combination of PET with CT scanning (PET-CT) created hybrid imaging that remains standard in 2026—each scan combining metabolic function with anatomical precision.