I learned this from one of my PhD students' field notes (She was observing doctors' meetings at a neurology dept.): MRIs are pretty much a standard test nowadays for anything neurological. When they can't figure something out, they tend to say "Hmm. Have we done an MRI yet?" The MRI shows lots of interesting stuff. The results look like an x-ray, but where an x-ray mostly shows bone vs. soft tissue, the MRI differentiates between all kinds of tissues with varying densities and water content. So an MRI can differentiate between bone, muscle, nerves, veins, and so on. That makes it like a "shotgun" exam -- it "hits" just about anything in front of it. Tumor, yes, but also stuff as banal as a pinched nerve or a bruise. Depending, of course, on how close together they take the images, which are like "slices" that they can then patch together for a 3D-view. Close together they're more likely to catch even teensy stuff, further apart and they might miss that pinched nerve.

They can also retune the whole machine to show physiochemical processes, like lactic acid accumulating as you work a muscle. But that does take re-setting the machine, so it's either the tissue differentiation view or the physiochemical view in any given exam.

The magnet is noisy, kind of like a car backfiring. The machine is also kinda clanky as it revs up. But with ear plugs, and maybe some Xanax, the noise shouldn't be a problem. The machine looks like a big tube with a gurney that slides through it. You lie on the gurney, as comfey as they can make it (but it is narrow and confining, so they can never get it to the level of a 5* hotel bed). They slide the gurney stepwise through the machine, taking image after image "slice" by "slice" (metaphorically, of course. there are no sharp edges to this thing, so no actual slicing). Then they slide you out and it's over. Each image can take a few seconds, and they don't want you to move, so they tell you when to expect the noisy bits that you have to lie still for and when you have a wriggle break.

The physics of it all are pretty fascinating too. I'm sure there are actual physicists on the board who can tell me I've got this wrong, but I think it has to do with hydrogen atoms, the way they spin and the way the line up all parallel to one another under the influence of certain combinations of magnetism and radio waves. Somehow, the machine can tell whether they're lined up or relaxed and whether they're spinning one direction or another. It can also tell how long it takes them to line up and relax as the radio waves (or is it the magnet?) are switched on and off, and the difference in these times has to do with the density of the tissue. So where an x-ray machine is sending radioactive particles through you at a film, the MRI is only using magnetic and radio frequency waves. In other words, it's safer than the more familiar (and therefore less scary?) x-ray machine.