Monday, March 31, 2008

Planning MRI suites

The purpose of this post is to share some basic information about MRI units, so you will have an understanding of its principles of operation. It is a crucial role for a medical equipment planner, or anyone responsible for the medical equipment planning of a healthcare project to understand the operation of an MRI unit.

First, the MRI is not an x-ray device. While it is often found in departments called x-ray, radiology, or imaging, it is actually a "listening" device. Much like a video camera can capture an image of a person, the MRI unit captures an image of the body. But instead of light, it uses very faint radio-frequency waves. Here is an excellent video explaining how an MRI works:

MRI stands for magnetic resonance imaging. This is a very accurate acronym for how an MRI works. First, the "M": Magnets are measured by their field strength in tesla. The most common MRI units in US hospitals seem to be 1.5 tesla, but they range from .3 to 3.5 depending upon the application. The "R" resonance, refers to the RF signal emitted by the hydrogen atoms. Finally, "I" is for Imaging. Much more friendly terminology than oto/ophthalmoscope or sphygmomanometer.

So now that you know just enough to be dangerous, let's look at the planning criteria:

1) The room is shielded. Some folks mistakenly think the room is shielded to protect people outside of the room, as is the case with Radiation Therapy and X-ray rooms. The MRI is actually being shielded to ensure an accurate image. The shielding is usually a thin screen of copper. The purpose is to screen OUT any ambient RF signals from other sources. Think of every other electro-magnetic device in the area as offering the potential to create static on the image. The MRI room is shielded to keep out all other RF signals except those being generated by the patients body. This will ensure a clear image.

2) The magnet must be kept cool. Cryogens (Extremely cold liquids) are used to keep the magnet cool. If the magnet ever overheats, the cryogens heat up and expand, creating tremendous pressure. (Same as when a radiator overheats). MRI units are vented to the building exterior in the event the unit overheats, causing the cryogen to do an emergency pressure release. (This is a called a "quench".) The following video is more sensational than educational, but is does show the rapid venting occur. If the unit had been properly installed in the facility, the venting would have been directed safely through duct work to the building exterior. In this instance, it was outside being vented for transport and when it "quenched" it blew the insulation material inside the cabinet everywhere. Very surprising to the folks, but nobody was injured.

3) The magnet is very powerful. While the MRI doesn't produce anything dangerous to humans, the introduction of anything ferrous (Containing iron) is of great concern. Anything small, such as a staple, nail or screw could penetrate the skin of a patient if he or she were between the object and the magnet. Being inside of the MRI tube is certainly a bad place to be if a metal object is being accelerated toward the magnet. Large objects also represent a significant threat. Oxygen cylinders, IV poles, chairs, etc. can cause great harm if slammed into the patient. (See video)

4) The magnet is very heavy. Many range from 12,000 to 20,000 pounds. This means that floors must be reinforced. And not just the room, but also the route they traverse to arrive at the room. Usually a roof hatch or a knock-out wall allows a crane to set the MRI into place. Make sure you do not damage the flooring, or more importantly the building structure if the unit must be rolled into place.

5) Ferrous metal is bad for the image. The magnet field is defined by gauss lines that emanate from the unit in concentric rings. Any movement of a metal object within these fields will affect the image quality. Make sure to study the manufacturers site planning guide and ensure that vehicles (Is the MRI located at an exterior wall?), Elevators (Is the MRI located near an elevator or other vertical transport shaft?) and other moving or transported metal items are outside of the recommended zones.

If you can provide a room that is properly shielded from outside RF "static", provide a hard-ducted vent in the event your MRI should "quench", keep all metal objects out of the room, and, of course, prevent any damage to the floors or structure from its weight, you are freed to focus on the more aesthetic pursuits of lighting, music and patient comfort.

Do you have any suggestions or clarifications? Maybe an interesting picture or video? Please submit your insights and I will supplement this post for others to learn from.

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