What keeps rebar in concrete slabs from being pulled into MRI machines over time?
In: Engineering
Concrete isn’t a thick liquid. Things don’t slowly sink into concrete, and likewise things don’t slowly tug through it when pulled on. Rebar is *very* firmly embedded in the concrete around it, so the risk of it breaking is slim to none unless your installer did something wrong.
Concrete is pretty rigid after it sets and also very strong. Also MRIs are built in special rooms with aluminum or copper-clad walls to limit the magnetic field to inside the room.
Because MRIs are not installed in normal room. MRI rooms have to be specially built.
The walls and any hardware is built with copper or aluminum (or other non-magnetic alloys) and the floors have an extra layer of cement paneling on top of them, all to prevent the MRI from messing with the structure around it.
You don’t need to worry about the MRI machine pulling the rebar through the concrete, concrete is really strong. That said, as others have explained, you don’t use rebar to make an MRI room. Not because the MRI might move the rebar but because the rebar might make the MRI images blurry. That makes the images worthless and that’s the whole purpose of having the machine.
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).
They use fiberglass:
>Floor The floors should be poured slab on grade with fiberglass-impregnated or epoxy-reinforced concrete. Reinforcing bars or corrugated iron sheets should be
avoided if possible, lespecially within the 50 gauss line
[MR lmager Site Planning](https://www.aapm.org/pubs/reports/RPT_20.pdf)
Are MRI machines strong enough to theoretically to do structural damage if the wrong materials were used? I’m genuinely curious.
MRI’s aren’t magic. Magnetism decreases by the square of the distance from the magnet. Combined with as someone else said MRI rooms being specially built, the distance between the MRI and the concrete wall/floor is more than enough to decrease it to equal to or less than the normal stresses it is under.
Just want to add this in to the conversation but I apologise that it isn’t an answer, many have covered it though.
In the UK i think maybe all of our MRI machines are basically outside somewhere (usually near or in a carpark area) but inside a giant trailer/RV-looking thing. [Something like this.](https://www.imgur.com/a/rmpEh7O) Often with a decent amount of space around them and sometimes not. I don’t think I’ve ever seen one inside, although I’m sure there probably is.
Non-ferrous metals are used in construction for buildings that require specific applications.
Stainless steel rebar is often used in bridges as well as MRI buildings.
Places that need non corrosive metals often use non-ferrous metals. Food manufacturing is a prime example of non-ferrous metals being used in production in building materials.
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