As I started telling in my last column, I recently dealt with a kidney stone.
Kidney stones aren’t fun. Fortunately, the doctors hunted mine down, trapped it and killed it (well, removed it). I was sore for a few weeks, but I am pretty much back to normal now.
One method used to find the stone was the old reliable X-ray. X-ray imaging has been around for about a century. The downside, as most people know, is that too much exposure to X-rays is dangerous.
So doctors often turn to other methods. In general, such methods are referred to as “remote sensing” because they can image, or “sense,” things without touching them.
One type of remote sensing is Nuclear Magnetic Resonance Imaging — abbreviated as NMRI, or more commonly just as MRI.
(The reason the “N” often is missing is because the word “nuclear” apparently scares some folks. But NMRI has nothing to do with nuclear radioactivity.)
The nucleus of the hydrogen atom — as part of water, you’re made of quite a lot of them — is commonly an individual proton, which has a property called “spin” by physicists. Spinning charged particles, like protons, create magnetic fields. Exposing such atoms to a strong external magnetic field can change the energy state of the proton, and these atoms can emit low-energy electromagnetic waves (again, not harmful) that can be detected.
For this to work, the external magnetic field must be fairly uniform (smooth) and strong. This is why, if you’ve ever had an MRI, you were placed inside a large cylindrical tube. The strength of the magnetic field is at least 1½ teslas. For comparison, the Earth’s magnetic field is typically about 5/10,000 of a tesla.
MRI scans are generally safe, as long as the patient doesn’t have any magnet-susceptible materials inside oneself. However, MRI scans can take a long time (many minutes) and don’t always possess the resolution (detail) that doctors need. So another option is the CT (or “CAT”) scan.
CT (Computed Tomography) scans are X-rays taken to the next level. Instead of a single burst of X-rays that are detected by a single detector, a CT sends out several beams of X-rays to be viewed by a set of detectors that rotate around the patient’s body.
This innovation allows the doctors to build up images in layers, and with the associated software, see very fine details. CT scans are very quick (sometimes as quick as a few seconds), especially compared to MRIs.
The disadvantage of CT is the patient’s body is exposed to X-rays, and so CT is not nearly as safe as MRI (there is a small, but measurable, cancer rate). Doctors use either MRI or CT, depending upon circumstances.
Apparently, my stone was not only evil, but sneaky to boot. Throw in ultrasound, and I had four different types of scans performed before the end.
Next: How remote sensing uncovered a truly ancient crime of nature. See you then!
Christopher Sirola is an associate professor in the Department of Physics and Astronomy at USM. Contact him at email@example.com.