A New York Times science Q&A by the nonpareil C. Claiborne Ray gave a clear, succinct explanation of something many non-radiologist doctors don't know.
Here's the piece.
- Magnetic Metals
Q. I have a titanium screw in my skull for a bone-anchored hearing aid. Does this mean I cannot have an M.R.I.? What elements make something attractive to a magnet?
A. You can still have magnetic resonance imaging with a titanium implant.
In fact, titanium is used for many medical implants because it does not interfere with CT scans and M.R.I.s the way steel implants can, implant manufacturers say.
Titanium may obscure specific areas of an image, but it is not affected by even strong magnetic fields.
Not all metallic elements are attracted to magnets, and most orthopedic implants are made of nonmagnetic alloys.
The four strongly magnetic elements are iron, nickel, cobalt, and gadolinium.
The strength of magnetic attraction depends on the way moving electrons are aligned in the atoms of a substance, sometimes forming minuscule internal magnets.
In some substances, the tiny magnetic fields cancel one another out or are always randomly oriented.
In materials attracted to magnets, the internal magnets within the material line up with the magnetic field of the magnet.
That gives the material a magnetic field, and it is then pulled toward the magnet.
The fields vary with temperature, becoming stronger as it grows colder and disappearing at high enough temperatures.
In the four ferromagnetic elements, the internal magnetic fields can stay aligned even at room temperature.
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Some disasters in the MRI suite involve anesthesiology equipment such as ferromagnetic steel cylinders containing compressed gas (the dark object in the photo below)
which become bomb-like projectiles once the MRI machine is turned on.
The only thing I’d change in what you have written is the last sentence. MRI machines, so long as they are functional, will attract ferrous metal toward the centre of the bore. Most people think the machine is only dangerous in this way when it is actively collecting images (you can tell because the machine makes loud banging noises when it is collecting images). This is not true. You should never walk into the MRI room with your cell phone, wearing your watch, or with your wallet holding your credit cards. Unless the magnet is quenched, it is “on”.
ReplyDeleteThe other thing that sometimes causes issues in the scanner are certain tattoos. Some inks have metals in them and can cause a burning sensation and heating up of the area. In these cases, if an MRI is absolutely necessary and the tattoo is far from the body part being scanned, then a wet paper towel can be placed on the tattoo to absorb any heat. Lastly, welders and metal smiths should avoid going into an MRI scanner, it’s possible they have small metal shavings in their eyes and the strength of the magnet could cause grave damage.