A medical mystery, part 2

The MRI? It was a pretty good experience, I guess. I attribute this largely to the MRI tech, who was a large man of extraordinarily good humor and perhaps of Polynesian descent (just a guess). As he cheerfully readied my pillow, merrily adjusted the machine, and jocundly fitted me with earplugs, he plied me with ease-inducing yet non-intrusive small talk. He was also pleased to explain -- in remarkably concise terms, I thought -- just how the machine worked. Let me try to pass on that explanation, based on this conversation and skimming the Wikipedia article as a follow up. As I understand it, the MRI machine generates a powerful magnetic field that causes the nuclei of all the hydrogen atoms in our bodies to align themselves in a certain way. (The majority of the atoms in our molecular makeup are hydrogen, which makes sense if you think about it.) Then, another magnetic field is applied to the body at the same time, which causes the hydrogen nuclei to realign themselves in a different direction, in the process absorbing a tiny bit of energy from the second field.

The second field is then turned off, which causes the nuclei to revert back to their original aligned position. Analogy: think of introducing and removing the second field as being like pushing the plastic bristles of a hairbrush down with your hand and then letting go. When this happens (when they snap back upright, so to speak) the nuclei release the bit of energy that had held them back. This energy released is in the form of an electromagnetic wave and its release is detected by the imaging device.

However, the hydrogen atomic nuclei within different types of tissue revert back to their original aligned position at different speeds. Thus, the energy is released at different rates, depending on the molecular makeup of the body stuff in question; bone will differ from muscle, for example. That difference in the rate of energy release is measurable, which allows the imaging device to distinguish between different types of tissue in the cross section of the body it is targeting. These differences, when combined with data concerning position, combine to create an image. At least, that's how I understand it. Isn't that incredible?

Anyway. So I lay face down on this big tray with my injured hand outstretched in front of me and held immobile in a sort of comfortable, cushioned clamp. As the tray slowly whirred forward into the tube, the MRI tech told me that the process could take anywhere from 20 to 30 minutes. "Just go to sleep if you can, man," he said helpfully. "I mean, I'm so used to doing it by this point I fall asleep right away in one of those things." This was the last thing I heard before the scan began, so I had lots of time to think about it while I lay down in the machine, listening to its raspy, oscillating whumps through my earplugs. Wait, why would an MRI tech be more used to being in an MRI machine than anyone else? Are he and the other techs MRIing themselves on slow days for the fuck of it? I could see it happening. It's a slippery slope. Sprained my ankle playing basketball yesterday. At least I think it's sprained -- oh hell, there's an MRI machine right here that I know how to operate, might as well check it out and see. Weeks pass. This sore throat just won't go away, and I'm running low on lozenges. Ho ho, throat, let's see what Mr. MRI has to say about you! And before long: See, I told you. You really can see the difference right here, in the duodenum -- the all beef franks totally digest differently than the turkey dogs. I'm worried about him.

Has anyone ever had an MRI done? Also, is there anyone who knows more about this topic than me who can tell me whether this description is correct?