Life Safety in MRI Design[edit]

What is an MRI?[edit]

Unlike an X-ray or a CT scan, an MRI uses no radiation to detect the properties of any soft tissue inside a body. Instead it uses an extremely strong magnetic field that literally aligns all of the bodies atoms in a single direction and then uses radio pulses to disturb those atoms position. It then makes readings from those changes in the atoms in order to interpret what type of cells it is looking at. The machine itself has to create a tremendously strong magnetic field. In order to do this the machine is typically super cooled with cryogens in order to improve the computer functions of the machine. A basic distinction in the room is that these rooms are not shielded like X-ray rooms, which typically require lead or concrete. Instead these rooms are completely shielded 4 radio frequencies also referred to as RF. This lining is all six sides of the box including the floor and it prevents any outside radio frequencies or anomalies from affecting the computer's ability to read and interpret the soft tissues of the body.

Zones of Safety[edit]

The largest concern of these types of rooms and that is the extreme magnetic force that the machines produce. As mentioned above the machines have to generate a pretty extreme amount of magnetic force in order to affect the polarity of a person's cells. Although magnetic fields do not affect the body adversely like radiation does, their force on metal cannot be understated. Probably the most common risk in an MRI is making sure that no person steps into a room with any kind of metal on their person. Things like keys, belt buckles, even the tiny bra straps if they are not secured can be old towards the center of the machine. The force is typically minor away from the center also known as the bore, however the force increases exponentially the closer you get to the center of the machine. Often a piece of metal can be pulled with great force before a person is really aware that the metal is being dragged into the machine. The worst documented examples of this risk has occurred when someone was in the machine and then suddenly a large metal object such as a metal wheelchair or a welding torch canister was sucked into the machine and crushed the person while they were inside the bore. Another example of the risk from the magnetic force comes to people with pacemakers. Because these are small machines that are inserted into a person's body they are easy to overlook however they have metal working parts in them and although the parts are not heavy enough to be pulled out of someone's body, the magnet can close them shut and in the case of a pacemaker would stop it and consequently stop a person's heart without them having any warning.

The most direct way to safeguard the public from these machines is to create security zones to help distance any unsuspecting public from the dangers of the machine. The American College of Radiology issued a white paper on MR safety and outlines four basic zones surrounding any MRI suite.

These zones are easy to imagine in a two-dimensional world however must be examined above and below the machine. As these machines are often installed inside a hospital they tend to be on the lower levels and not necessarily in buildings outside on their own. Therefore it's just as crucial for a designer to understand that these magnetic forces can be exerted on a basement below an MRI or even on a floor above an MRI or quite often to the outside of the building where it's easy to forget that someone might be walking around the outside of a building. And these cases you cannot easily implement the ACR zones above. A designer may have to resort to installing what is referred to as passive shielding. A passive shield is basically a layer of metal sheeting that could be applied on the side where any of these magnetic forces might affect the public either outside above or below the machine. For this we typically look for the five gauss line which has been recognized as the maximum field that could be exerted on the public. Any force 5 gauss or higher must be shielded with either a passive shield or a way to ensure that no public could accidentally walk into this field.

Cryogen Consideration[edit]

One last consideration is the cryogen gas used to cool the machine. This works much like refrigeration lines to your air conditioner Though they are often extremely cold to help produce the superconductor temperatures. Although these gases are contained within the machines that cool the MRI, consideration should be given in the event that there is a cryogen leak inside the MRI. In this particular example the room becomes pressurized with these gases (which by the way would suffocate a person inside the room). In part because the rooms are so tight to shield the radiofrequency signals, the rooms can become so pressurized that the door pushing into the room cannot be opened because of the pressure against it. It is for this reason that MRI doors should open outward to avoid this dangerous condition.