This Quarter in Physics History
July 1977: MRI uses fundamental physics for clinical diagnosis.
An image of a brain
from a MRI exam
Isidor Isaac Rabi was born on July 29, 1898 in Rymanow, Austria. In 1899 his family moved to New York, where they lived in poverty in the Lower East Side before moving to Brooklyn in 1907. Rabi’s parents were Orthodox Jews, and though Rabi never practiced religion as an adult, he was always influenced by his religious upbringing. He felt that doing good physics was “walking the path of God.”
Rabi graduated from Cornell University in 1919 with a degree in chemistry. But he wasn’t really captivated by chemistry, and spent three years not doing much of anything before deciding to go to graduate school in physics at Cornell. After finishing his PhD in 1927, Rabi went to Europe, where he spent time working with the giants of quantum mechanics, including Sommerfeld, Bohr, Pauli, Stern, and Heisenberg.
Rabi was fascinated by quantum ideas, especially the Stern–Gerlach experiment. Otto Stern and Walther Gerlach had sent a thin beam of silver atoms through a non-uniform strong magnetic field, and observed that the beam separated into two distinct sub-beams, the atoms in the beam having been deflected slightly according to the direction of their magnetic moments.
When Rabi returned to the United States in 1929, he took a teaching position at Columbia University. After spending two years searching for a problem that interested him, in 1931 Rabi set up his molecular beam lab and took up the problem of determining the nuclear spin and associated magnetic moment of sodium. The nuclear magnetic moment, much smaller than that of the electron, was difficult to determine precisely. Rabi and Gregory Breit figured out how to modify the classic Stern-Gerlach apparatus to find the nuclear spin of sodium.
Rabi, who was often viewed as lazy, was always impatient with routine experimental techniques and data analysis. He liked to say he wanted an answer at the end of the day, and was driven to design clever, clean experimental methods, methods that brought him “nearer to God.”
Throughout the 1930s, Rabi improved the molecular beam method and used it to gather increasingly accurate values for the nuclear spin of atoms, including hydrogen and deuterium. The work culminated in the magnetic resonance method which is the basis for magnetic resonance imaging.
Magnetic moments tend to align either parallel or antiparallel to an external magnetic field, and tend to behave somewhat like tops, precessing about the direction of the magnetic field, with a frequency that depends on the magnetic field strength and the atom’s nuclear magnetic moment. In 1937 Rabi predicted that the magnetic moments of nuclei in these experiments could be induced to flip their orientation if they absorbed energy from an electromagnetic wave of the right frequency.
They would also emit this amount of energy in falling back to the lower energy orientation. Rabi would be able to detect this transition from one state to the other. He called his method molecular beam magnetic resonance.
Rabi and his team modified the molecular beam apparatus so the beam was also exposed to a radio frequency signal as it traveled through the magnetic field. Tuning either the external magnetic field or the radio frequency can produce resonance. They observed the first magnetic resonance absorption in 1938, with beam of lithium chloride molecules. Rabi was enthralled by the flopping of the magnetic moment, and the group held a party to celebrate the achievement. Each atom or molecule has a characteristic pattern of resonance frequencies. Rabi detected a series of resonances in different molecules that could be used to identify the type of atom or molecule and give more detail into molecular structure.
After World War II broke out, Rabi left his molecular beam laboratory and went on to become Associate Director of the MIT Radiation Laboratory. He was awarded the Nobel Prize in 1944, for his resonance method for recording the magnetic properties of atomic nuclei.
In 1946 Edward Purcell and Felix Bloch independently found a way to study the magnetic resonance properties of atoms and molecules in solids and liquids, instead of individual atoms or molecules as in Rabi’s molecular beam method. Later, nuclear magnetic resonance was further developed into the imaging technique that is now commonly used for medical diagnosis. The first images were produced in the early 1970s, and the first live human subject was imaged in 1977. MRI machines became commercially available in the 1980s, and are now commonly used for imaging internal body structures, especially soft tissues like the brain.
Shortly before he died in January of 1988, Rabi was imaged in an MRI machine. “It was eerie. I saw myself in that machine,” he said. “I never thought my work would come to this.”
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