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Union University Department of Physics

The Science Guys

Science Guys > February 2003

February 2003

What is nuclear magnetic resonance (NMR), and how does it work?

A holy grail of medicine is diagnosing illnesses without the trauma of cutting you open. The discovery of x-rays contributed to this pursuit, and now Nuclear Magnetic Resonance (NMR) has yielded another noninvasive tool for the physician. NMR is another name for Magnetic Resonance Imaging (MRI), now used by hospitals around the world.

The word "nuclear" sends people into conniptions because the word is associated with nuclear bombs and radioactivity. However, this technology has nothing to do with ionizing radiation and there is no radioactivity involved. Nonetheless, the negative connotations of the word "nuclear" compelled NMR technologists to change the name from nuclear magnetic resonance to magnetic resonance imaging (MRI). This seems to relieve people's fear they might become a three-eared beast with bilious green eyes due to nuclear radiation!

NMR instruments use very strong magnetic fields. If you have a pacemaker, you shouldn’t wander too near an MRI imager. And you’d better remove your credit cards or else the magnetic fields will wipe them clean. Hotel concierges find out that an NMR researcher’s credit card often won’t work. Keep anything magnetic away from NMR machines.

Nuclei of atoms have magnetic properties. Nuclei can act like little magnets and orient themselves in a magnetic field. Then, if you point a source of radio waves at these nuclei, the nuclei absorb the energy associated with certain radio waves in a process known as "resonance." The machine controls what type of atom is absorbing the waves. Therefore, an NMR instrument has a huge magnet, a radio wave generator, and a console to collect and analyze data.

NMR is a powerful tool in science because the chemical environment surrounding the nuclei also affects how the nuclei absorb radio waves. This means that different molecules absorb radio waves in different ways, and one can reconstruct the molecular structure of those chemicals by studying the NMR "spectrum of absorbed waves" (the fingerprint for the compound). Today NMR is probably the world's most versatile tool for understanding the structures of chemicals. One can even use NMR to understand the rates of chemical processes, because the NMR experiments occur on timescales of thousandths of a second. Thus, NMR has proven to be an indispensable tool to scientists.

NMR complements x-ray technology. X-rays do not interact well with soft tissue but do with bone, which contains calcium and little water. Conversely, the element which yields the very strongest magnetic resonance is hydrogen. There are two atoms of hydrogen in every water molecule. That means that MRI shows up soft tissue, which contains a lot of water, better than bone. And since the chemical environment of the hydrogen atoms can vary in each type tissue, each different tissue can appear differently in an MRI scan. Thanks to NMR, our organs are now open to public scrutiny but without poking, prodding, or perforating.