Medical imaging using magnetized rare gases was invented in 1994 to provide a way to study diseases of the lung, heart, and brain that depend on the flow of gas and blood through the vital organs. This marriage of AMO science and medical imaging grew out of the basic study of the atomic nucleus.
But how did this connection between AMO physics and medical imaging come about? It started with some basic research on helium by Luis Alvarez at the University of California, Berkeley, in the 1940s. He discovered a form of helium gas, 3He, that behaves like a tiny bar magnet. 3He is abundant in the universe but exceedingly rare on Earth—the world’s supply amounts to only about 1 ton. There is more 3He in the rocks on the Moon’s surface than on our planet. In recent years, there has been a steady supply of the gas from an unusual source: Although 3He is not radioactive, it is produced from the radioactive decay of tritium, the form of heavy hydrogen present inside nuclear warheads.
Because 3He is a gas, its atoms are randomly oriented at room temperature, so the total magnetism in a bottle of it is essentially zero. However, scientists studying the basic physics of atomic collisions perfected an optical pumping scheme that uses polarized laser light to align most of the 3He with their magnets pointing in the same direction. This creates a very unusual material: a gas that acts like a bar magnet. It was later recognized that this new “breathable magnet” might have marvelous properties for imaging the lung and the circulatory system. But many technical obstacles had to be overcome before this promise could be demonstrated in the 1990s, when the first magnetic images were obtained using this technique. Now, only a few years later, breathable magnets are producing dramatic images like the ones shown on the opening page of this chapter. Tiny features in the lung, brain, and circulatory system are visible for the first time.