You probably learned at some point that everything, all of matter, is composed of atoms. These atoms, in turn, are composed of electrons orbiting a central nucleus. The nucleus is composed of protons and neutrons, and how many protons the nucleus has determines what we call the atom – hydrogen or oxygen or carbon, and so on. In many types of atoms – especially simple hydrogen, a single proton – the nucleus has both a magnetic moment and angular momentum. These two properties are used in Nuclear Magnetic Resonance Imaging, NMRI or MRI.
Let’s think about how this works. Your body has a whole lot of water, H2O (two hydrogen atoms and one oxygen atom), in it, and therefore a whole lot of hydrogen nuclei. To take a Magnetic Resonance Image (also called an MRI), we put whatever we want to image (perhaps your body) inside a large magnet. Once inside, because of their magnetic moment a slight majority of the hydrogen nuclei will try to align themselves with the magnetic field created by the magnet. However, because they also have angular momentum they will actually end up precessing around the magnetic field like a spinning top. How large the precession frequency is – how fast the nuclei will precess about the field – depends on how strong the field is. The stronger the magnetic field, the bigger the precession frequency.
Now, what we want to image is not only inside a magnet, but is also inside what is essentially a coil of wire. The precessing spins, which cause their own much smaller magnetic field, can be pulled down into the plane of the coil, where they cause a voltage to appear in the coil that allows us to “see” them. In fact, we can use this voltage to make a plot (or “spectrum”) that essentially shows the number of spins at every precession frequency. If we really had a perfect magnet, all the spins would be at the same frequency, and our spectrum would just have one peak. The taller it was, the more hydrogen spins in the object.
But that wouldn’t give us an image. What we need to do is give the spins in different parts of your body (or whatever else we are imaging) different precession frequencies. And we have to do it carefully, so that we know what frequency corresponds to what position. If we do this, then our spectrum will have a peak everywhere hydrogen (water) exists, and the height of each peak will depend on how dense the water is at each position, which can indicate what sort of tissue is there. And that is exactly what we do. The “buzzing” sound you may have noticed when an MRI is being performed is the activation of “gradient coils” that cause a magnetic field gradient – that is, they make the magnetic field vary over the span of what is being imaged in a particular way so we can form an image.
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This above was a basic, and classical, explanation of how MRI works, appropriate for a Helium article and most readers. If you’d like to learn more, pick up a text on nuclear magnetic resonance.
