Let's start by breaking down the American College of Radiology's white paper on MRI safety guidelines. Can you explain the four basic zones surrounding an MRI suite?
“You can picture these zones as layers of protection. Zone I encompasses areas freely accessible to the public, like the entrance, where the magnet poses no threat. Zone II is the buffer between public spaces and the more restricted areas, like the reception and screening rooms. Zone III restricts access with coded doors, allowing only approved personnel and screened patients. Zone IV is the room where the magnet is located. Access to Zone IV should only be possible by passing through Zone III. Zone IV is also designed so that the walls of magnet room contain the five 0.5 mT line (or 5 Gauss) line of the fringe field of the magnet."
How does this translate into a three-dimensional consideration?
“MRI's magnetic forces extend beyond the machine's footprint. 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."
Can you elaborate on the challenges designers face in implementing these MRI safety measures?
“While it's relatively simple to envision these safety zones on a flat plane, the challenge arises when you introduce height and depth. Designers need to consider the magnetic forces on different levels and the surroundings. It's easy to overlook the fact that someone might be walking around the exterior of the building, potentially exposed to magnetic fields. Passive shielding becomes a crucial tool in addressing these concerns.”
How does passive shielding work, and when is it necessary?
“Passive shielding acts as a protective barrier against magnetic forces. When the potential impact zone extends beyond the immediate surroundings of the MRI, designers will strategically apply metal sheeting to counteract these forces. It becomes necessary when the safety of individuals outside, above, or below the MRI is at risk. 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."
Any other considerations on MRI safety measures?
“Yes, 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 RF signals, the rooms can become so pressurized that the door pushing into the room cannot be opened because of the pressure against it. It's for this reason that MRI doors should open outward to avoid this dangerous condition.”