MRI: A Guided Tour
Table of Contents
The Wonder of Waves
Now that the magnet has gotten the hydrogen protons lined up at attention, the scanner is ready to subject them to the next step, the one that will result in an actual signal.
You’ve probably been wondering about that coil the technologist placed under your back. No, that’s not one of the main magnetic coils (those are all inside the cylinder, beyond your view). Essentially, it’s a radio transceiver, also called an RF coil, which can communicate with your hydrogen atoms via radio frequency (RF) waves. These waves are close in frequency to those of your favorite FM station. In fact, the room in which the MRI scanner is located is probably shielded so that the local easy listening station doesn't interfere with your images.
PHYSICS FACTOID: The vast benefits of MRI have not gone unnoticed by the folks in Oslo who dole out Nobel Prizes. In 2003, Paul C. Lauterbur and Peter Mansfield were jointly awarded the Nobel Prize in Physiology or Medicine for their discoveries related to the technology.
Your technologist is using that coil to send RF pulses at your spine. They are precisely timed (taking into account the type of tissue targeted and the fact that just the hydrogen atoms are of interest) to achieve the effect we’re about to describe. This, by the way, explains the “Resonance” part of MRI (told you we’d get to that!).
Remember those “unmatched” hydrogen protons – the ones (still blue, in our applet below) left hanging without a partner after the magnetic field caused them all to jump into alignment? Well, those protons absorb the energy of the RF pulses, which causes them to flip on their axes – still in line with the magnetic field, but now in the opposite direction, in the high-energy configuration. Now you may click the RF Pulse button in the applet to see how that works (make sure the magnetic field is still on). See how the unmatched protons flip as the RF pulses (denoted in red) are turned on?
It’s impressive enough that scientists figured out exactly how to make those hydrogen protons do that (the frequency needed is called the Larmor frequency). Now, the real magic happens: When the RF pulse stops, the protons release that absorbed energy, return to their previous alignments and, in so doing, emit a signal back to the coil (also in red, in the opposite direction). You may have noticed that has already happened in the applet. If you missed it, hit the “RF Pulse” button again for another look.
The signal gets turned into an electric current, which the scanner digitizes. The lower the water content in an area, the fewer hydrogen protons there will be emitting signals back to the RF coils. Different types of MRIs display this data differently, but in any case you get a variety of shades of grey that reflect the different densities. In some scans, the weaker the signal, the darker that part of the image will be. So bone will be fairly dark, while fat will be light.
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