A friend of mine was a technician at Johns Hopkins Hospital in Baltimore, and he had been asked to reduce the power-line interference bedeviling an electrophysiology experiment conducted in a Faraday cage made of copper window-screen material. It soon emerged that the space was lit by multiple large institutional 4-tube fluorescent lamps directly overhead. With SCR dimmers. Unhh. No wonder it's so noisy, even inside the shield cage - probably have hundreds of volts of hash riding in on the power lines, and/or through small imperfections in the shield. Solution was to replace the overhead fluorescent lamps with incandescent lamps dimmed by a variac. Blessed silence. The researchers were very happy.
During the 1990's, the local hospital was blessed with a large donation for a permanent MRI installation. At the same time, the ER was moving to new quarters. The logistical details were a little like repairing a train wreck while the train was still in motion. The MRI machinery moved twice, and the ER moved three times during the construction. My contributions were breaking the credit card machine, protecting the ER admitting computer (which I maintained), and troubleshooting strange electronic anomalies apparently resulting from the moves.
One such anomaly was that during the first move of the MRI machine, every computer within about a 20ft radius would reboot when the machine was in operation. Ok, shielding problem, I thought. There is allegedly copper in the walls for RF shielding. Something like this: Nobody wanted to climb into the overhead to be sure, so I was volunteered. There was no copper because the room was not intended to be the machines permanent home. The temporary solution was to shutdown all the computers in the area when the MRI machine was operating. There was a slight warning beep over the PA giving about 5 minutes warning and a different beep signaling all clear. I recommended brass wire mesh shielding stapled to the walls, but that was vetoed by the aesthetics committee.
Unfortunately, the computer I was maintaining was on the opposite side of the wall from the MRI machine. There was no magnetic shielding in the wall. It was initially difficult to justify relocating the computers. That became much easier after it destroyed two computers, one printer, a badge printer, a few telephone instruments, numerous electronic watches, several pagers, and some medical electronics. Since it was going to be several months before the MRI machine was moved again, it was decided to install a large sheet of MuMetal on the computer side of the wall. To make it less industrial looking, someone was decided to have a mural painted on one side. Hospital policy required competitive bidding on such artwork, which delayed the installation until after the MRI had been moved again. The hospital now has an office with one magnetically shielded wall.
RF interference also works in the other direction, where RFI from nearby devices affects the quality of the MRI image. Without proper shielding, it's just asking for problems. Since the local radio club meets at the hospital, I had to ask them to not transmit in the hospital parking. I also snooped around the hospital grounds looking for sources of RFI. It turned out that the electronic ballasts in the newly constructed sections of the hospital were the worst offenders. So, for about 3 months, the MRI machine was operated with most of the room lights turned off and only one fixture running a non-dimming conventional ballast left on. There were some truly amazing rationalizations offered to the doctors and patients as to why the lights needed to be turned off, none of which were even close to the real reason. I also identified a few other sources, which were dealt with by the maintenance staff.
The other major interference sources were the cell sites on the roof. Old photos: The antennas were eventually relocated away from the MRI machinery and the roof was shielded with welded wire mesh.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Been there, done that. I never heard of rf interference with a smaller animal MRI unit. The walls of a large research building were made using aluminum backed drywall, as well as another one built later. I was the one who initiated having shielding, because the main culprit was a analog tv tower close by. Video buzz getting into things and many other services on that tower. Building engineer specified construction technique.
I later noticed the animal MRI unit, placed between floors 11 and 13 . I walked with a string and magnet above it, and had it swing around. I figured any vibration in electrical equipment, or wiring could induce noise. They came up with the idea of wielding thick steel plates on floor, and halfway up the wall, in the lab. The costs involved didn't go over too well with management. I would have kept my mouth shut, thinking later, but perhaps medical research made some headway.
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