Cool Facts About MRI Scanners
I spent a summer working for General Electric Medical Systems in Australia installing MRI machines for various hospitals and radiology practises. Here are some facts and stories that I learnt during my work experience.
- Metal Kills:
If you ever get a MRI scan done on you, you will be asked to remove all metallic objects from your person before entering the room. The reason for this is that the magnetic field is so strong that it will literally rip magnetic metallic items from you.
- Cheap Husband:
There is a story of a woman who went for a MRI scan and was asked by the radiographer to remove her earings. She assured the radiographer that they were solid 18k gold earings from her husband and therefore not magnetic. Unbeknownst to her, the earings were only gold plated and so when she entered the MRI scanner, the earings tore from her ear lobes and stuck to the scanner - that would have been one unpopular husband that night I'm guessing!
- Metal Flakes:
One question they will ask you is if you have metal flakes in your eyes (a common problem with metal workers). If you do and you go into an MRI machines, your eyeball could literally tear itself to shreds when the metal flakes try to leave your eye for the scanner!
To achieve accurate results, the magnetic field should be as parallel as possible. It is for this reason that the MRI scanner is a long tube into which patients are slid. When the patient is inside the scanner (ie inside the tube) they are surrounded by a huge superconductor.
To keep the superconductor functioning, it is immersed in liquid nitrogen. Most of the bulk of the MRI machine is taken up with insulating and containing the liquid nitrogen.
- Emergency Vent:
If you look at an MRI machines, it will have a tube connected at the top of the machine and going up into the roof. The purpose of this tube is in case the cooling unit in the scanner breaks down - which means that the liquid nitrogen would start to heat up and boil - boiling liquid nitrogen rapidly expands - and this tube is the emergency vent.
- No Radio Please!:
Because the MRI scanner is very sensitive, all external radio waves must be eliminated. To do this, the entire room is lined with copper sheeting. To get wires and stuff into the room, pipes of a certain length are used to tunnel through the wall. These pipes are of a length which will cause radio waves to superimpose on themselves and therefore cancel themselves out. If you take a radio into a MRI room and shut the door, you will get zero radio reception!
- Blinding Copper:
One of the MRI machines that I helped install had to be lowered into the hospital via the roof. We couldn't work from about 11am to 2pm while the sun was overhead because the glare reflecting from the copper lined walls was blinding!
- Physics Lesson:
One of the most impressive demonstrations of the strength of the magnetic field in the MRI scanner was when one of the techs grabbed a piece of non-magnetic metal and placed it on edge inside the MRI scanner. He then let go. Instead of the piece of metal falling flat straight away (as you would expect), it very slowly keeled over. It occured in a slow motion kind of speed. The reason this occurred was simple physics. A conductor moving in a magnetic field will generate an electric current which in turn generates a magentic field which will try to oppose its motion.