Friday, March 16, 2007

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?

8 comments:

Miss Kitty said...

You couldn't get anyone to mimic vomiting inside the MRI room? No beer cans lying around? They're aluminum...

faded said...

I have a friend who is the director of radiology at the hosptial center where I live. He tells a story about the hospital's mobile MRI unit. The unit is set up and ready to work. Somehow a person carrying a metal folding chair walked in the space with the magnet. The chair leapt out of the peron's hand and pinned a nurse to the side of the unit. They had to quench the magnet to get the nurse free. The nurse was not injured.

Subsquent MRI investigation of the chair carrier's head showed the complete absence of a brain.

The Wandering Author said...

Pixie, it is pretty cool. However, since you brought Tesla into it... there are few things as cool as a Tesla Coil. Have you ever heard of the giant structure, Wardenclyffe, where he was going to build a Tesla coil / building - indeed, started to do so? Just imagine if that had worked out, the precautions that would have been needed! :-D

(I once tried to build a Tesla Coil - only the plans I had required an 811A vacuum tube, which hasn't been made since roughly the 1920s and is about the size of a gallon milk jug. Never could find one in usable condition. Which meant I wasted hours of work hand winding a transformer for it. LOL!)

BaxterWatch said...

ha.
You got confused. -273 is KELVIN for abs. zero. You mean - 460 for F.

very interesting to hear the architect talking about engineering and medicine. Scary. Because I know YOU know what you do, but what about bubba-mcbuilder who built the hospital here in east bumble?

Mile High Pixie said...

OMG Bax! I totally remembered that -273 K is absolute zero Friday night as I was sdrifting off to sleep and resolved to fix it the next day...and promptly forgot when we went to the mountains. Derp!

I checked at work today: the actual temp of the cryogen spitting out during a quench is -240 degrees F.

As for building a hospital, you get a contractor who's done several before. Not just any Joe Schmoe gets to build them. Not on my watch.

Anonymous said...

You guys are both wrong! Absolute zero is not -273K, (Kelvin), it's -273C, (Celsius). Absolute zero is 0 degrees Kelvin (or rather in correct scientific terms Zero Kelvins). And it is close, but not quite -460F (Fahrenheit).

ElTob said...

Older magnet systems used dual-cryogen cooling systems, with liquid nitrogen as the 'warmer' of the two as something of a pre-cooling outer jacket, and liquid helium is what the conductive coils of the MRI system are immersed in.

The cooling technology of the MRI systems has improved dramatically in the past several years, and today nearly all clinical MRI systems just use liquid helium. Liquid helium has a boiling point of about 4 degrees Kelvin. That's the temperature of the escaping gas in a quench.

And you mentioned the fatal oxygen tank accident and how MRI suite designs had gotten much safer since then. Well, in the US, January of 2010 was the introduction of the very first new building code design requirements for MRI safety, which are still in the patchwork state-by-state adoption phase. These don't impose any sorts of safety requirements on the thousands of existing MRI suites, which have been virtually unregulated for safety.

And lastly, you also described MRI projectile accidents, like the folding chair. Out of embarrassment these are frequently kept out of the press, but these accidents happen all the time. If you're interested in seeing a (very small) sample of my collection of photos, you might find http://mrimetaldetector.com/blog/2009/05/fmd-dont-we-have-screening-protocols-for-that/ interesting.

Michael Bergin said...

Thank you very much for your article.