MRI

[Enzo]

I had to have an MRI scan today, never done that.

MRI machines are a huge torus they stick you in, and it has MIGHTY magnetic fields and somehow reads your body's reaction to view your innards. I must say I am glad it was just my leg, because if I had to go all the way into it, I;d still be screaming and running off into the night. Talk about claustrophobia.

I got to keep my trousers, after I emptied the pockets, apparently it can see through denim. I asked them if it were true that an MRI machine can suck up a lawnmower if it gets too close. Oh yes. For some reason, they didn't make me remove my belt. They said it might tug on the buckle, but don;t worry, it won;t hurt anything.

After three hours in it, I was more than ready to be out of it. They gave me ear plugs but it is really loud anyway. And it has mightly magnetic fields, and darned if it didn't tug on my belt buckle. In fact as they slid me out of the gadget, my belt was trying to stay on the machine - very strange in my mind.

As I lay there for three hours, unable to move, no way I could listen to music, I got to thinking about the systems inside the MRI. I imagine some very large currents must be flowing. I wondered just what sorts of components are controlling those currents. Giant triacs? SCRs? MOSFETs? 6L6s? I haven't looked anything up yet. But most of the time in the machine right above my face was a huge General Electric logo. MAybe I'll write to them for info.


And now 12 hours later, my back is sore from the machine tugging hard upwards on my belt buckle, that pulled the belt up into my back the whole time. I'd complain about it to the wife, but she'd just say something like "Oh, so you can now hurt yourself just lying there?"

[myfoot]

Yeah I never considered myself claustrophobic till I laid in one of those the 1st time.I've had 2 for my back and one for my shoulder (so far) and I'm pretty sure my right knee will be next. Getting older isn't all it's cracked up to be but it's better than the alternative

[Sweetfinger]

Your buckle wanted to stay where it was because of eddy currents induced in the metal. Poke around on the Yootoobs for amusing MRI machine vids like this one demonstrating eddy currents in an Aluminum slab:

[Steve Conner]

There are three lots of power electronics in there:

The main magnet, a giant superconducting coil bathed in liquid helium. Probably charged with hundreds of amps by a 3-phase SCR power supply for all I know. It's superconducting, so you charge it up, short the terminals together and forget about it.

The "gradient coils" superimpose a smaller time-varying magnetic field on the field from the main magnet. In older machines, they were driven by racks of Crown audio amps. Copley Controls eventually brought out a range of class-D power amps designed for driving MRI gradient coils, with up to 300kW peak output at 2100V. Gradient Power Amplifier Systems for MRI - Analogic

Last of all, there is a big RF power amplifier and antenna system that zaps you with radar-like pulses at 130-something MHz, and a radar-like receiver that listens to the echoes.

How does all this stuff fit together? The magnetic field causes the atoms in your body to resonate at a RF frequency that depends on the field strength. So with the RF system tuned to a given frequency, it will see a return signal from the spot in your body where the magnetic field strength is just right for that frequency, and nowhere else. The gradient coils move that spot around to build up a complete scan.

[Jazz P Bass]

Thump, thump, thump, thump, thump.........

[Enzo]

SUperconductor? Like Leonard Bernstein?

NO kidding? I did see a large - like 1 to 18 inch diameter - sort of duct snaking from the thing out through the wall. Appeared to be soft insulation all around that. Must be the liquid helium. (In my previous shop location, there was a store at the end of the block with a window sign that read, "We sell huilum baloons." Not sure what "huilum" might be, but it sounds OK.)


300kw at 2100v? Wow, just thinking about service. Guy brings in his MRI for me to fix. As usual, forgets to bring in his own liquid helium tank. And what am I going to use for a dummy load.

Thanks guys, all I know is there was some serious power going around in there.

[woodyc]

There are three lots of power electronics in there...

Actually I don't think the RF is all that much power. And its not echoes like radar. The RF pulses flip proton spins into a higher energy but short-lived state. The same frequency is re-emitted as they decay before the next pulse. The RF is sent and received by wire loops with a fairly small number of turns, i.e. 1, in a simple Helmholtz or similar arrangement.

But good summary Steve. Do you work on this stuff? I have occasionally.

[Enzo]

I suppose when superconducting magnets are trying to pull my belt off me, the RF can be a minor issue.


I don't think, as an aside, that radar uses all that much power. I seem to recall the radar like they use for weather transmits about 100mw. I could be wrong. The beam only has to be strong enough to reflect back to a radio receiver.

[Regis]

I had all kinds of tests getting ready for my tumor surgery, an MRI being one of them. I had to go all the way in since it was in my torso.

I did ok for a while, thinking about what I was going to have for lunch, about the hot nurse I just saw in the hall, etc. About half way through I just freaked out. I kicked and banged and the guy stopped the test and pulled me out. I felt foolish but I couldn't help it. He said it was not abnormal and they have to sedate some people before doing the test.

When he put me back in the machine it died and he couldn't get it started. He called a tech and they decided it couldn't be fixed in a short time so the operator had to call a car to take us to another building on the hospital campus with an MRI machine. That machine was of a different design and was not as claustrophobic.

When we were leaving the door to the generator room for the MRI machine was open and the tech was in there looking it over. I swear it looked like the engine room of the starship Enterprise: Cabling 8-10 inches thick, racks of gear, massive heat sinks, etc.

[Enzo]

Yeah, in the room with it, it is just this huge donut in the middle of the floor. I could only imagine the support stuff that had to be on the other side of the wall.

[Mark Hammer]

You have to wonder about all those neuropsychological studies where they stick people in an MRI and instruct them to engage in various intellectual tasks while brian activity is measured.

"Okay, we're gonna stick you in a tightly enclosed space, that's uncomfortable and really really noisy...and we want you to just relax and add up these 5-digit numbers in your head, or say as many words as you can think of that rhyme with 'orchestrate' until you can't think of any more."

[Jazz P Bass]

Thought I would throw this out there: http://www.howstuffworks.com/mri.htm

[Enzo]

Oh, cool.