Last week I went on a radiology field trip to check out some of the fascinating machines we have at Gritman Medical Center here in Moscow.
One of the radiologists there, Jody Polly, provided the tour.
She was extremely helpful and sweet. She’s especially excited about the new technology they have for doing mammograms and detecting osteoporoses. Being the ever curious aspiring neurologist though, I asked her to take me to see the CT scanner and MRI first.
Here’s Dr. Rizenhour sitting at the computer deck for the computerized tomography (CT) scanner.
CT scanners are very similar to traditional x-rays in that they make use of electromagnetic radiation to produce images of the inside of the body. An internal "shadow" is projected by the differences in how much X-ray can pass through each tissue type. Rather than use a photographic plate to collect the X-rays though, the CT scanner detects the X-rays electronically.
The CT machine looks a bit like a big, delicious doughnut. The patient must lie on a platform which moves through the hole as it produces images of the body. The X-ray tube and detectors are positioned in the doughnut and can circle the body as it moves through the machine.
The CT scanner operates at 16 slices per second. These visual "slices" of the body can then be put together by the computer to produce a very detailed image of the area of interest.
Next I toured the magnetic resonance imaging (MRI) facilities. Here is a picture of the MRI computer deck. Of course, I was not allowed to go into the MRI chamber as it was in use.
There I spoke with Milt (yes, just Milt, even his nametag says "Milt") about their MRI.
"People are often scared when they come in here," he said, "They think the machine is going to suck up all their jewelry, but I just tell them it only takes the precious gems."
Metals can actually become projectiles in the presence of such a strong magnet (1.5 teslas at Gritman), so patients are required to wear a hospital gown.
"With that in mind, can you tell me what MRI stands for?" he says smiling at me.
"Magnetic Resonance Imaging?" I say.
"Nope." He smiles again, "'Milt's Retirement Investment.'"
Aside from joshing patients, Milt knows all about MRIs and told me a few things (most of which I had to re-look up by today).
Basically, an MRI works by detecting the presence of hydrogen atoms (mainly those found in water) in relation to the magnetic field produced by the huge MRI magnet. Hydrogen atoms are easily detected because they have a single proton and a large magnetic momentum.
The nucleus of each atom spins (processes) on an arbitrary axis and when exposed to the magnetic field of the MRI, the spin of the hydrogen atoms will line up in the direction of the magnetic field. If a patient is lying on their back, for example, some of the hydrogen protons will be aligned toward their feet and some toward their head.
Because they can either align up or down with the magnetic field, many of the hydrogen’s spins will cancel each other out. Since there are so many hydrogen atoms in the body though (billions!) there are still plenty of un-canceled signals available for the MRI machine to detect.
To detect these newly aligned hydrogen protons, a coil in the MRI emits a radio frequency (RF) pulse that causes those hydrogen atoms that have not been canceled by their oppositely aligned comrades to absorb the energy required to make them spin with the magnetic field. These atoms begin to spin in a new direction with the emitted RF pulse.
When the RF pulse is turned off, the hydrogen protons can start realigning themselves with the magnetic field and release their newly stored energy. This is the special moment when they emit a signal that the coil can pick up and interpret. They also release heat energy at this time.
"It will warm you up a bit," says Milt, "It really depends on what we're imaging. A knee will be barely be noticeable, but a full torso exam might get you sweating."
The signal from these protons can be interpreted by the computer with use of Fourier transform (there's that word again) to create an image the radiologist can understand.
Each tissue responds to the field in different ways according to its hydrogen content, so you get a very amazingly accurate view of the inside of the body without harmful side effects. It really is an amazing machine!
Here is an animation of MRI slices I collected from wikipedia: