The Powerful Magnets of an MRI Machine
Don't Ignore it's PowerBy Wayne Hunter
People often underestimate the power of an MRI magnet. I once worked for a healthcare manufacturing company that produced MRI machines and was periodically astonished at the consequences of ignoring its power.
We had a picture in the manufacturing lobby that showed a metal toolbox in the bore of an MRI magnet with a metal chain sticking straight out both ends of the magnet. The toolbox was understandable— workmen often don't realize how strong the 2 Tesla magnets for the MRI system really are. Even a full toolbox at about 10 feet from the end of the magnet can get picked up off the floor and sucked straight into the magnet. Needless to say, tools and toolboxes inside the magnet are not uncommon, but a chain? That I couldn't figure out.
I asked the field engineers if anyone knew what had happened. Eventually I talked with the service person who was assigned to the installation. The room where the magnet was being installed was still under construction and the magnet had not been powered up for a couple weeks after it was installed. Many of the workmen would pass by the magnet as a shortcut to the job site.
Once the magnet was activated, yellow and black tape was stuck to the floor and plastic warning signs saying "NO METAL BEYOND THIS POINT," were hung. As one workman was walking to the job site, toolbox in hand, he walked inside of the taped area. Before he knew it the toolbox was yanked into the MRI.
He tried to pull the toolbox out by himself but couldn't overpower the strength of the magnet, so he recruited a few others to help. None of them could get a good grip on the toolbox, so the workman retrieved a rope from his truck. They tied the rope to the toolbox and pulled without any luck. The magnet was so strong that it broke the rope in two.
Determined to retrieve his toolbox, the workman went back to his truck and returned with chain in hand but before he could even get near the magnet, it started pulling the chain too. His firm grip onto the chain wouldn't stop it from being pulled straight through the magnet and then out the other side! It stuck through the magnet after it had become centered inside the magnetic field.
In order to retrieve the toolbox and chain, the magnet had to be powered down. This magnet was a helium cooled superconducting electromagnet and in order for it to power down, all of the helium had to be removed. $4000 worth of helium later, the workman got his toolbox and chain back. He learned a valuable lesson though and after that; he was seen walking as far away from the magnet as possible.
Another time we got an MRI magnet back from a customer for repair that looked like someone had taken a sledgehammer to one end. My curiosity was spiked and after some investigation I discovered that the magnet had been used at Churchill Downs horse racing track. The purpose of this particular MRI machine was to perform scans of the knees and ankles of racehorses. Of course the horses were sedated prior to being scanned.
Apparently the sedation had worn off a little too early on one horse, and it woke up during the scan and began kicking the magnet to pieces. Though it wasn't exactly what you would consider "normal wear and tear," the manufacturer decided to replace the magnet at no charge, and we used it to advertise the strength of our equipment.
One last memorable experience with an MRI magnet involved a transportable MRI that was installed inside of a semi-truck trailer. The hospital had leased a transportable unit while they were constructing a new scan room and installing a new MRI system. The truck was parked in a back parking lot so that cars and other metallic vehicles wouldn't park nearby. After about a month in operation, we received a call from the hospital saying the scans were coming back unreadable, so a service engineer went to the site and recalibrated the unit. After a few weeks he was called back again to recalibrate the same unit. Finally, after a couple more times of doing this, the service engineer could not get the MRI machine to calibrate.
While he sat in the control room trying to figure out what could be causing this, he heard a knock on the side of the trailer. Though he thought that the knock was strange, he continued thinking through the possibilities of what could be causing the problem. Suddenly there were several more knocks on the side of the trailer.
He decided to go outside to see what was causing the noise. As he walked around the side of the trailer, a group of children next to the trailer started running away. Puzzled, the service engineer looked at the side of the trailer and saw layers of iron and other metal objects stuck. He finally realized what was preventing the unit from calibrating. There was so much metal clinging to the trailer that it had actually caused the magnetic field of the magnet to be pulled far enough off the centerline.
The service engineer went to the nearest hardware store and bought a push broom. He took the hospital technicians around to the side of the truck, and he swept all the metal off of the trailer and had the hospital maintenance bring a truck to remove it all. After he calibrated the unit, he added a sign to the control console saying, "SWEEP SIDES OF TRAILER DAILY!" He jokingly told the hospital technicians that he was surprised the trailer hadn't tilted sideways from all of the iron stuck to it, and they all had a good laugh.
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Wayne Hunter graduated with a BSEET from Metropolitan State University in Denver Colorado in 1973. He now lives in Kemblesville, Pennsylvania. He worked as a radio and TV repairman, a commercial radio engineer, and then became an Electromagnetic Compatibility (EMC) Engineer for over 30 years. He helped design and test instruments for Apollo spacecraft, the space shuttles, computers, medical diagnostic electronics, and chemical analysis test equipment. He was a product regulatory engineer and helped draft international EMC test and certification standards for Industrial, Scientific, and Medical (ISM) equipment. He retired in 2010 and has been a Jameco customer since his retirement.