The outbreak of World War II interrupted work on nuclear magnetic resonance, but the postwar years saw an explosion of progress. In the United States, two groups of physicists independently set out to develop a simpler method for observing the magnetic resonance in the nuclei of molecules in liquids and solids instead of the isolated molecules of Rabi’s experiments. At Harvard University, Edward Purcell, whose group included Henry Torrey and Robert Pound, led the research. At Stanford University, Felix Bloch led a team that included William Hansen and Martin Packard.

Both Purcell and Bloch chose to study the proton --the nucleus of the hydrogen atom (H). Because the hydrogen nucleus is composed of a single proton, it has a significant magnetic moment. Hydrogen would turn out to be the most important element for MRI because of its favorable nuclear properties, nearly universal presence, and abundance in the human body as part of water (H2O). Purcell's group used a two-pound block of paraffin wax as their hydrogen source; Bloch's group used a few drops of water contained in a glass sphere. The two research groups placed the samples in a magnetic field and waited for their nuclei to reach thermal and magnetic equilibrium, a magnetized state in which slightly more of the nuclei are aligned parallel to the external field than antiparallel to it. Then, as Rabi's team had done, the research teams applied radio waves in an effort to get the magnetic moments of the nuclei in the samples to flip. Purcell and Bloch hoped to detect magnetic resonance by observing the energy that precessing nuclei absorbed or gave to the radio frequency field when the resonance condition was satisfied.

In 1945, within three weeks of each other, both groups managed to create the conditions necessary to observe the phenomenon. Their experiments demonstrated what is technically known as nuclear magnetic resonance in condensed matter (now shortened to NMR) as distinguished from Rabi's discovery, molecular beam magnetic resonance. In 1952, Bloch and Purcell shared the Nobel Prize in physics for these experiments.

Research in NMR now leaped ahead. The researchers who surrounded the Purcell and Bloch labs quickly began using NMR spectroscopy to investigate the chemical composition and physical structure of matter. One of the first advances in the course of this work was measuring quantities called relaxation times, T1 and T2. T1 is the time it takes the nuclei in test samples to return to their natural alignment; T2 is the duration of the magnetic signal from the sample. One of Purcell's first graduate students, Nicolaas Bloembergen, who had arrived at Harvard from the Netherlands in 1946, played a key role with Pound and Purcell in this research. Bloembergen was the first researcher to measure relaxation times accurately and, along with Purcell and Pound, also measured how they changed in a variety of liquids and solids. Fortunately for future research and applications, relaxation times could be measured in seconds or fractions of seconds, making NMR a practical research tool.

Bloembergen, Purcell, and Pound published a paper in 1948 that became extremely influential in several branches of physics. The manipulation of relaxation times has provided a powerful method in chemistry and biology for analyzing the structure of molecules--and as other researchers would learn later, is essential for producing the contrast needed to image useful images of tissues in the human body.