Simply Physics has a wonderful page of photos depicting objects which have accidentally become stuck in MRI scanners because of the pull of the powerful magnets.
These include chairs, floor cleaners, oxygen tanks and bits of medical equipment.
They even have a video of a team of people trying to extract a swivel chair from an MRI machine using straps and some large pieces of wood for leverage.
The magnets in fMRI scanners are usually at least 1.5 tesla in strength and most are now 3 tesla.
The strong magnets cause all the hydrogen atoms in the human body to align. A radio pulse is then sent which knocks the atoms out of alignment. The amount of non-alignment caused by this will differ depending on the tissue density.
As the atoms are knocked out of alignment and re-align they return a radio pulse. This can be measured, and because the returned pulse is related to tissue density, the signal can be computed into a ‘3D map’ of the tissue.
Hence the name, Magnetic Resonance Imaging – magnets align the atoms, the atoms resonate with the radio pulse and the image is computed from the pattern of pulses.
The strong magnets means bringing certain metals into the scanning room can be dangerous.
If you go for an MRI scan you’ll be asked to remove all metal from your body and you’ll be interviewed to make sure you have no metal implants in you.
If loose metal objects enter the room they can fly towards the magnet causing injury to anyone in the way.
Unfortunately, people can sometimes wander unaware into the room with unsuitable objects.
This probably accounts for why so many of the objects stuck inside the scanners on the Simply Physics page are cleaning equipment, as cleaners have wandered in not realising the risks.
Link to Simply Physics gallery of MRI mishaps.