Experienced patients know some of the differences between “CAT scans” and “MRIs.” CAT scans are generally fast and easy, but you don’t want to have them repeatedly, because they use x-rays. MRIs are often not as pleasant; you are asked a lot of questions, and then they slide you into a very narrow tube, where you’ll stay for about an hour—assuming you don’t scream “Get me out of here!” before the scan is finished. Whether a CAT scan or MRI is prescribed depends on what your doctor is looking for. Both are expensive.
Perhaps the best way to understand the MRI machine is to compare it with the x-ray machine and CAT scan machine.
A traditional x-ray picture is made by aiming parallel x-ray beams at your body. A piece of film records the amount of x-ray energy that makes it through to the other side. In other words, the image on an x-ray film is a shadow.
A CAT scan also uses x-rays, but with a major twist. Pretend that your body is at the center of a big wheel with spokes. Now imagine that half of the spokes are x-ray beams heading towards your body and the other half of the spokes are the x-ray beams that passed through your body and are heading towards detectors distributed around the edge of the wheel. And yes, the wheel rotates.
Unlike a traditional x-ray, a CAT scan image is not a shadow. Because the x-ray beams rotate around your body, it’s possible to calculate the relative density of specific points inside your body. The result is a finely detailed image of a cross-section of tissue. Repeat the process for adjacent tissue slices, and a three-dimensional image can be constructed.
MRI uses magnetic fields and radio signals instead of x-rays. The first thing you must understand is that the nuclei of many (but not all) types of atoms behave like little magnets. When you get an MRI, you are placed in a very strong magnetic field that causes the little magnets (nuclei) to line up with each other. Actually, just over half line up face forward, and just under half line up face backward. Now, if you turn on a relatively weak radio signal at the right frequency, then it will make a bunch of the little magnets flip around; turn the radio signal off again, and the nuclei gradually return to equilibrium.
What good is this? You may recall from high school physics that thrusting a magnet in and out of a coil of wire produces an electric current in the coil. Similarly, if you can make the nuclei of many atoms swing around in unison, they will produce a signal that can be detected by a coil.
Now that we have a signal, is there any way to extract more information? Yes. One thing I didn’t mention is that the frequency of the radio signal can be set to resonate with a specific type of atom. The most popular choice is hydrogen, because the body is largely composed of water. Another thing I didn’t mention is that if we superimpose a gradient magnetic field on the very strong magnetic field, then there will be slight differences in field strength at each point in the body. We can use these differences to localize the data.
OK, now we can detect the strength and location of the hydrogen signal. Luckily, the signal also varies depending upon the composition of the surrounding tissue. Plus, what I just described is the T1 signal; there is also a related T2 signal. (My book explains more fully how MRI works and how it was developed.)
The original Star Trek television series featured a portable “Tricorder” that could quickly scan a patient—human or alien—providing detailed, on-the-spot diagnosis. A portable device employing MRI’s basic operating principles and using a planet’s ambient magnetic field is not only conceivable, it’s already available for applications such as emergency medicine. Nor is the incredible wealth of data and sophisticated analysis depicted on Star Trek inconceivable. We have only just scratched the surface in exploiting MRI.
Next time: The Nuclear Option
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