NMR for lay people
September 30, 1999[Note: I originally wrote this for my non-scientist parents as an attempt to explain the basics of nuclear magnetic resonance.]
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Figure 1. The subject, played here by postdoctoral fellow Kenneth Marro, with whom I share an office, lies supine prior to the experiment. The generic physician-professor, portrayed by former U.W. Radiology secretary Jason Lingel, checks the patient’s footwear and finds his faux Birkenstocks to be acceptable. The big white box behind Lingel contains an enormously powerful magnet. The thin plastic table on which the subject rests slides into the magnet so as to bring the patient into the position where the magnetic field is the strongest — the so-called “sweet spot.” |
| Figure 2. “Tell me about your mother….” Dr. Lingel asks Marro a few final questions in preparation for the scan and assures him that the experimental procedure will be safe and painless. NMR is a popular clinical and research tool precisely for this reason: the experience is not at all unpleasant for the subject unless he/she is (a) claustrophic (in which case being trapped in the center of the magnet will make him/her uneasy), (b) wearing or carrying metallic clothes or items (in which case the metal objects get sucked into the sweet spot of the magnet), or (c) carrying credit cards (in which case they are erased). |  |
 Figure 3. The NMR session complete, a relieved Marro thanks the doctor for taking such good care of him. |
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But wait — what went on between Figures 2 and 3? Let’s back up a little. All matter is made up of atoms, right? Right. Your body is no exception; it contains billions of different atoms — carbon atoms, nitrogen atoms, hydrogen atoms, phosphorus atoms, calcium atoms, oxygen atoms, etc. — which are linked together in lots of interesting ways. Also recall that all atoms have a nucleus. These nuclei are very responsive to electromagnetic fields, and we can excite them by exposing them to electromagnetic radiation.
What do we mean when we say that a nucleus is “excited”? We mean it spins around its axis (like the earth, but about 10 trillion times faster) in a different way than a “relaxed” or “unexcited” nucleus does. That is, by subjecting a nucleus to electromagnetic fields, we can make it spin in a unique, predictable way. A related point is that each nucleus spins in a slightly different way, depending on what kind of nucleus it is. An excited carbon nucleus spins differently than, say, an excited nitrogen nucleus; furthermore, a carbon nucleus attached to an oxygen spins differently than a carbon nucleus which is only attached to hydrogen.
One more thing: when these nuclei spin, they actually create electromagnetic fields of their own, which can be detected by sensitive recording equipment. Again, the type of field created depends on the type of nucleus; an excited, spinning carbon nucleus will create different fields than an excited, spinning nitrogen nucleus. Therefore, if we analyze these newly created fields, we can figure out which nuclei created them!
So, to review, we excite the body’s nuclei with electromagnetic fields, causing them to spin in certain predictable patterns. These excited, spinning nuclei then create their own electromagnetic fields which we can measure. These latter fields will vary according to the nuclei that we excited, so once we know what the fields are, we can identify the nuclei that gave rise to them. We can say, for instance, “Aha! These electromagnetic fields clearly were produced by a bunch of hydrogen nuclei.”
And we don’t need to stop there. If we scrutinize the data a bit more closely, we may deduce that these fields were produced specifically by the hydrogen nuclei in water, since the hydrogen in water creates different fields than, say, the hydrogen in fat. In addition, the strength of the fields tells us how much of this type of nucleus there is, and thus how much water there is. A weak field would mean that there aren’t many of these hydrogen-in-water nuclei present; a strong field would mean that they are abundant. Thus, NMR is a tool for measuring the amount of something, like water, in your body. NMR can produce wonderful pictures of your brain, for example, just by measuring the amount of water in each area, since different parts of your brain contain different amounts of water.
Our research group doesn’t study water in the brain; we study various other molecules, such as lactic acid, in muscles. However, the general strategy is the same. We can use NMR to measure how much lactic acid there is in a muscle and whether the amount of lactic acid is changing over time.
This little NMR tutorial may have raised more questions than it has answered, but I hope it has proven at least mildly educational.
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