What keeps rebar in concrete slabs from being pulled into MRI machines over time?

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What keeps rebar in concrete slabs from being pulled into MRI machines over time?

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Anonymous 0 Comments

MRI manufacturers won’t allow structural metals within certain distances of the isocenter of the magnet, with the distances being different depending on the weight of the metal and the strength of the magnet. Last time I designed a 3 tesla MRI exam room, we couldn’t use steel rebar within about 6′ of the isocenter. We ended up using fiberglass rebar in a 12″ thick concrete slab. Because the ceiling was within 6′ of the isocenter too, we had to use aluminum ceiling grid. Smaller objects can still be pulled slowly into the magnet though, so our rule of thumb was no magnetizable metal beyond the plane of the walls. So the drywall wall inside of the rf (radio frequency) shielding was normal metal studs, and the drywall was held up using steel screws, but anything inboard of that had to be stainless steel or other safe materials. To finish answering your question, beyond the 6′ (or whatever distance the particular MRI manufacturer requires) the MRI machine simply can’t pull the rebar hard enough to damage the floor.

Another comment mentioned shielding, the rf shielding doesn’t affect the magnet’s strength at all, it’s only there because the MRI machine reads faint radio waves coming off your body during the exam and so we need to keep radio waves from entering the room. We can use magnetic shielding in the form of dozens of thin steel plates screwed to the inside of the walls or under the floor, but magnetic shielding is time consuming to engineer, extremely expensive, and the above distance recommendations work fine with no added cost involved. I’ve only ever seen magnetic shielding used once in a situation where there was a very sensitive piece of electronic equipment in the next room over from the MRI exam room so magnetic shielding was used on that one wall to keep the equipment from being affected (and no that equipment couldn’t just be moved which would have been so much easier).

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