Nuclear magnetic resonance

Nuclear magnetic resonance was inevitable by 1946 because three separate stockpiles had accumulated: quantum mechanics had matured enough to predict nuclear spin behavior, radar programs had perfected microwave generation at precise frequencies, and a generation of physicists trained in molecular beam techniques needed peacetime problems. When Felix Bloch at Stanford and Edward Purcell at Harvard independently demonstrated NMR in condensed matter in January 1946—Bloch using water, Purcell using paraffin—they weren't making a discovery. They were opening a door that quantum mechanics had built and radar technology had unlocked.

The first prerequisite arrived in 1922 when Otto Stern and Walther Gerlach deflected silver atoms through an inhomogeneous magnetic field, proving atomic angular momentum is quantized. By 1933, Stern measured the proton's magnetic moment. Isidor Rabi at Columbia built on this work; his 1938 paper described NMR in molecular beams. But Rabi's method worked only for isolated atoms. The crucial missing piece was technology for generating and detecting radio frequencies with precision.

World War II created the adjacent possible. The MIT Radiation Laboratory became radar's epicenter. Rabi served as associate director. Edward Purcell led the advanced developments group. Felix Bloch worked on the Manhattan Project. By 1945, these physicists possessed equipment that hadn't existed seven years earlier: precision electromagnets, stable RF oscillators, sensitive receivers.

In December 1945, Purcell's team placed paraffin between magnet poles and detected energy absorption when the field matched the RF frequency. About one month later, Bloch's group detected the signal from rotating nuclear magnetization in water. Both teams published simultaneously in the January 1946 Physical Review and shared the 1952 Nobel Prize.

Within a decade, chemists recognized that nuclear resonance frequencies shift based on molecular environment—the "chemical shift." NMR became chemistry's premier structural tool. The leap to medical imaging came in 1971 when Raymond Damadian published that tumors exhibit prolonged relaxation times. Paul Lauterbur conceived magnetic field gradients for spatial encoding; on July 3, 1977, Damadian's team completed the first human MRI scan. Today, 100-150 million MRI scans are performed annually worldwide.