Detail of the Week: Mechanical Magnetism

One of the benefits of designing hospitals is that by learning what goes into a hospital, it demystifies the equipment and processes. Especially the radiology, or imaging, department--understanding how the equipment works makes them less scary. Historically, the MRI (magnetic resonance imaging) is the freakiest. That's the machine Linda Blair is stuck in in The Exorcist; it's the machine that goes BAM-BAM-BAM-BAM! while she has a needle in her neck and they're trying to figure out if there's something wrong with her brain.

Well, MRIs (or MRs, as Wheatlands' head of radiology calls them) have come a long way since 1972. They boom faster but quieter now, and as they pass repeatedly over you, they make different sounds. What MRs do is align all the cells in your body in the same direction and use that alignment to take pictures of you in slices. The magnet is always on, so there's a perimeter into which no ferrous (iron-containing) objects can be taken. Here's a plan of an MR:

An MR's magnet strength is measured in teslas (as in Nikolai Tesla, who discovered radio frequencies and essentially created radios). Two kinds of MRs are made: 1.5T MRs and 3.0T MRs. This is a 1.5 T MR; usually, 3.0T MRs are used in teaching facilities. (They're not that useful because the software hasn't caught up to them yet.) You can kinda see light outlines with labels of 70 Gauss out to 1 Gauss around the MR. The power of a magnetic field is measured in gauss (pronounced gowss), and these lines show how strong the magentic field is. The RF (radiofrequency) shielding in the walls protects objects outside from the super-badass magnet inside this room (as opposed to the lead shielding in a CT scan or X-ray room, which protects people outside from the radiation inside). See, people have actually died from ferrous items flying in an MR room. Several years ago, a child was undergoing an MRI when a nurse not familiar with MRI procedures brought a portable oxygen tank into the room. The tank flew across the room, hit the child in the head, and killed the child. Lawsuits ensued and better design ideas and clinical procedures followed.

This is a special door with RF shielding in it. It's four feet wide so you can roll a stretcher in it (an aluminum stretcher, of course) and it's really heavy because of all the shielding in it. It also opens out of the room instead of into the MR room. This is in case of a quench. See, the magnet that runs all the time needs to stay cool, so there are two gases in the compartment with the magnet that stay compressed to keep it cool. (When the gases uncompress, which happens regularly, a pump recompresses them. If you ever hear an MR machine "chirping", it's that pump.) Very very rarely, something terrible happens and those gases uncompress all at once. If they do, a pipe running from the magnet compartment to outside the building funnels the now-solidified ice crystals (at nearly absolute zero, which is -273 degrees F, if I recall correctly) of the gases at a couple hundred miles an hour outside to a safe, clear area. However, this explosion of gases (called a quench) also makes the air pressure in the MR room really high. If the door opens into the room, you can't get in to get the patient, and they're trapped in the room with a million-dollar piece of equipment that's kinda having a meltdown. But if the door opens out like this one, you can get in and get the person out.

This is looking at the RF shielding in the wall before they finished the door jamb. The wide, silver things on each side are metal studs holding up the wall, and the reddish-copper curvy thing in the middle is, well, copper shielding. To the right of the copper is a layer of another shielding material, the composition of which escapes me right now.

Panels with copper and other shielding materials are laid on the floor and ceiling, again to protect the magent from iron in the soil, rebar in the concrete floor slab, etc.

Here's the inside of the MR room, shielding in place. The lines you see over the silverish inside of the shielding are aluminum studs, to which the contractor will affix drywall. Sheet vinyl flooring will be installed over the floor shielding. An acoustical lay-in ceiling will be hung to cover the shielding above.

Okay, one more photo and I'll leave you alone for the weekend. The above is pretty blurry, since there were no indoor ceiling lights in the building at the time. This is the window from the control room into the MR room. The reflection is partially caused by a screen in the window, which looks like a really dense window screen. Truth be known, the glass in the window is only to protect the screen. It provides no magnetic protection whatsoever. Cool, huh?

Edited: Faithful commenter and engineer BaxterWatch reminded me that absolute zero is actually -273 Kelvin, which is about -460 degrees F. The temperature of the cryogen from an MR quench is about -240 degrees F. Still pretty cold, but not as cold as absolute zero. What makes me a bigger dork, that I know what abosulte zero is, or that I'd correct myself about its actual value?