Posted by: Thixia | April 9, 2008

MRI 3 of 14

To understand how MRI works, let’s start by focusing on the “magnetic” in MRI. The biggest and most important component in an MRI system is the magnet. The magnet in an MRI system is rated using a unit of measure known as a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). The magnets in use today in MRI are in the 0.5-tesla to 2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater than 2 tesla have not been approved for use in medical imaging, though much more powerful magnets — up to 60 tesla — are used in research. Compared with the Earth’s 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are.

 


Photo courtesy NASA
In this MRI scan, you can clearly see the shattered fragments of a human wrist broken from a fall.

 

Numbers like that help provide an intellectual understanding of the magnetic strength, but everyday examples are also helpful. The MRI suite can be a very dangerous place if strict precautions are not observed. Metal objects can become dangerous projectiles if they are taken into the scan room. For example, paperclips, pens, keys, scissors, hemostats, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet (where the patient is placed) at very high speeds, posing a threat to everyone in the room. Credit cards, bank cards and anything else with magnetic encoding will be erased by most MRI systems.

The magnetic force exerted on an object increases exponentially as it nears the magnet. Imagine standing 15 feet (4.6 m) away from the magnet with a large pipe wrench in your hand. You might feel a slight pull. Take a couple of steps closer and that pull is much stronger. When you get to within 3 feet (1 meter) of the magnet, the wrench likely is pulled from your grasp. The more mass an object has, the more dangerous it can be — the force with which it is attracted to the magnet is much stronger. Mop buckets, vacuum cleaners, IV poles, oxygen tanks, patient stretchers, heart monitors and countless other objects have all been pulled into the magnetic fields of MRI machines. The largest object I know of being pulled into a magnet is a fully loaded pallet jack (see below). Smaller objects can usually be pulled free of the magnet by hand. Large ones may have to be pulled away with a winch, or the magnetic field may even have to be shut down.

 

 

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