In my new job I am learning to work on Medical Imaging equipment.
The assignment du jour was to exchange a defective “cold head” on a magnetic resonance imaging machine in St. Petersburg. Neat. Now I must digress into some potentially boring techno-geek stuff.
The very large magnet in an MRI machine is an electro-magnet. In Jr. High science you’ll remember wrapping wire around a nail and hooking the arrangement to a battery. Then, quickly before your battery died or the wires got too hot, you could pick up some paper clips with the nail.
An MRI machine is sort of like that. Except the magnet is a super-conductor, cooled with liquid helium. Miles and miles of very fine wire, with 450 amps of current running through it at a little under 1 volt. Because it’s a superconductor, once this magnet is turned on, it cannot easily be turned off. So it stays on, hopefully, from the time the MRI machine is installed until they replace it with a new one. For years. And the magnet is strong enough to suck an office chair or oxygen tank across the room and into the magnet core, where you will need a crew of well trained gorillas and a “come-along” to get it out again. Very big magnet.
Now there are two helium based cryogenic systems at work here. One is the large vessel that contains the magnet. This is the simple system. It’s just a big barrel full of very cold liquid helium and a big magnet in there with it.
The other is like your air conditioner, which takes heat away from the vessel and dumps it someplace else. It does this by making a very cold place on the magnet vessel, (the barrel) so cold that the helium in the vessel condenses to a liquid and then drips onto the magnet. The part of the system that makes all this cold transfer into the barrel is called…..the cold head. (What else?)
The cold head has moving parts and as such will eventually wear out and need replacement. Thus my trip to St. Petersburg.
Now the fun part, and trying to get back to my point after all that techno-geek explanatory mumbo.
After two hours of careful preparation, we are now ready to extract the cold head from the machine. As the cold head assembly begins to emerge, there is a flood of thick fog, like dry ice on steroids. I’m ready for this. More of the assembly emerges, and it is instantly covered in ice drawn from water vapor from the air. I’m not surprised.
Now the cold head assembly is completely clear of the machine and is suspended in air by the other engineer who is holding it. Fog is rolling everywhere, ice is forming so fast you can watch it grow. And there is a liquid cascading down the assembly like water, running off the end in a stream, but evaporating before it has time to hit the floor.
This mystery fluid is the nitrogen and oxygen being pulled from the air in the room, condensing into liquid on the cold copper on the assembly and running off like water.
This I was not at all prepared to see.
And that is by far the coolest thing I personally have ever seen.
Attached image is the de-installed cold head...after more than an hour it was still covered in ice.