MRI: A Guided Tour
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You’re made up mostly of water, which means a large number of the atoms inside your body are hydrogen atoms. This turns out to be quite fortuitous, because hydrogen atoms happen to be built in such a way that they react dependably to the forces they will be subjected to inside this scanner. The first of these, as you by now know, is a main magnetic field. The second will be pulses of radio waves. But before we talk about those radio waves, let’s get better acquainted with the H of H2O.
In the nucleus of every hydrogen atom is a positively-charged proton that spins (or precesses, scientifically speaking) around an axis, much in the same way as a child’s top. This spinning generates its own tiny magnetic field, giving the proton its own north and south poles. Now, the nuclei of other atoms spin, too, but for a number of reasons (including, as we’ve mentioned, their sheer quantity), MRI is generally interested only in hydrogen atoms.
Under normal circumstances, these hydrogen protons spin about willy-nilly, on randomly oriented axes, as in the depiction below (more or less), showing hydrogen atoms before the MRI’s magnetic field is turned on.
However, when these atoms are placed in a more powerful magnetic field, it’s as if a drill sergeant blew a whistle: the protons line up at attention. Specifically, the axes realign with the more powerful magnetic field: Half of them face in the direction of the field, the other half in the opposite direction. In the Java applet above, click in the little box to turn the magnetic field on, and watch what happens. See the RF Pulse button? Don't touch that dial! We'll get to that in a minute.
PHYSICS FACTOID: In recent years, "open MRIs" have become increasingly available, offering more room for claustrophobic and large patients. Some radiologists caution the images may not be as good, as the scanners often use weaker magnets.
Did we say half of the protons line up one way, and half the other? Well, not exactly. More precisely, a few more atoms (represented by the little blue guys) line up with the magnetic field in the low-energy configuration (north-south north-south) than in the north-north south-south configuration, which requires a bit more energy. (When we say “few,” we mean, in an MRI powered by a 1.5 tesla magnet, a measly 9 out of 2 million protons!) Those few “leftover” protons (the ones not cancelled out by a proton lined up in the opposite direction) are the ones your MRI scanner will be using. Think of these protons as the wallflowers left on the sidelines at a school dance after everyone else has paired up.
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