It is not often you see photos from inside the MRI room. Why not? Reason: The magnetic field is strong enough to pull any camera or phone right out of your hand. Even if you can hold onto it, there is a good chance that the magnetic pull can ruin your device. How strong is the magnetic pull you ask? Well this particular MRI is at a field strength of 1.5Tesla. Tesla is unit of magnetism. To provide you with a bit of perspective, a common refrigerator magnet is around 5mT. So this magnet is 300 times stronger than what is holding your bills up on your fridge! Another tidbit: Earth’s magnetic field (at the equator) is around 30microT. So this MRI is around 50,000 times stronger than Earth! Wow!
So you may be wondering, “Where can I find a magnet that strong to put on my fridge?” Well you aren’t going to find a rock or something that will have quite that much magnetic pull. The way this magnet works is on the principles of electromagnetism and superconductivity. Remember in science class when you wrapped a couple wires around a nail, hooked the other end of the wire to a battery, and the nail could pick up paperclips? Well the magnet is a big loop of coiled wire that has electrical current running through it, but instead of having a big battery hooked to it at all times, we cool the wire down so much that there is no resistance. This means electricity can just keep traveling around and around, creating the magnetic field. To get the coil cold enough to allow it this superconductive property, we use the coldest liquid we can think of: Helium. Helium has a boiling point of 4K. That is -452F or -269C, a mere 4 degrees above absolute zero. By filling the magnet vessel with this super-mega-ultra cold liquid helium, we can add the electric current to the windings once and just let it ride. Even if the power goes out the MRI is still a magnet, although the other systems won’t be operational.
So how did I get this picture?!
Well things don’t always work perfectly. If they did I wouldn’t have a job! What happens if somehow some heat gets into the magnet vessel? All the helium rapidly boils off, now the windings have a resistance and the electrical current gets eaten up, effectively turning the magnet off. We call this quenching. What can cause heat to get in there you ask? Ice. It is cold to you and I, but to a big tub filled with -269C helium, a 0C ice cube is like being in the center of hell.
This is a picture of me after the magnet spontaneously quenched one night. No magnetic field was present, hence the awesome snapshot. I have a copper tube with one end hooked up to a helium gas tank. I am blowing the gas into the top of the magnet vessel to hopefully loosen up some of that ice so it will fly out the top. De-icing, we call it. The smoke-like stuff is actually the helium boiling out since I have the lid off. When the ice flies out it just looks like a little bit of snow shooting out. Gloves for protection. That ish is cold!
And because you are wondering, yes I totally sounded like a chipmunk while this was going on. :)