More about magnetism and MRI's

[micky]

More about magnetism and MRI's

It came up here wrt my planned MRI that there seemed to be things not made out of iron that were subject to magnets. By coincidence I came across this.

From Wikip ...

Ferrites Ferrites are ceramic compounds of the transition metals with oxygen, which are ferromagnetic but nonconductive. Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese compounds.

So I guess all these compounds and maybe more (that aren't used commericialy or not in transformers) would be magnetic like iron, but not containing iron, and to top it off, nonconductive.

What a complicated world we live in, where there are general rules and exceptions to the rules.

Nowadays, people are most likely to see ferrite cores as the small cylinders near the end of electric wires, such as USB cables, power supply adapter cables, etc.

"hey have a low coercivity and are called "soft ferrites" to distinguish them from "hard ferrites", which have a high coercivity and are used to make ferrite magnets. The low coercivity means the material's magnetization can easily reverse direction without dissipating much energy (hysteresis losses), while the material's high resistivity prevents eddy currents in the core, another source of energy loss. The most common soft ferrites are:

Manganese-zinc ferrite (MnZn, with the formula MnaZn(1-a)Fe2O4). MnZn have higher permeability and saturation levels than NiZn. Nickel-zinc ferrite (NiZn, with the formula NiaZn(1-a)Fe2O4). NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz. "

[harry]

Magnetic fields are also associated with any electric current.

[clare]

Ferrites DO have iron in them. Look at the formulae you posted. The Manganeze Zinc Ferrites have Fe204, as do the Nickel Zinc Ferrites.

In effect they are a ceramic iron alloy

[gregz]

Such as those coils in the MRI.

Greg

[gregz]

Even iron transformer plates, are plates glued together to avoid conduction.

Greg.

[nestork]

OK, I'll admit I'm having trouble with those formulae you posted.

I understand the character "a" to be a variable, but the formula doesn't make sense.

If you presume "a" is any number above one, the term (1-a) becomes a negative number, suggesting you have fewer than zero zinc atoms in the ferrite.

If you presume "a" is one, the term (1-a) becomes zero, and you have no zinc atoms in the ferrite at all. It therefore consists entirely of manganese and rust OR nickel and rust.

And, if "a" is any number less than one, you have zero, or fewer than zero manganese or nickel atoms in your ferrite.

I'm certainly not a chemist, but that "a" character needs some explanation.

[micky]

I just copied and pasted. I wanted to get MnZn and NiZn, and didn't want to ship the rest of each sentnece. I don't know what a is, but I have a feelling that the a in Mna in the first line should not even be there.

After all, there is no a in the other one, NiZn. .....no, wait, there is in the secone one after the word forumula. Okay, now I have less than no idea what a means.

Indded it does.

[micky]

You're right. I gues I meant to say that there is iron in things we don't think of as being made of iron.

[micky]

Just for the record, I was told the MRI machines were magnetic even when they are off.. That's why I think they won't let me in the room just to look at a machine with no one in it, until I've undressed. They let me look at it through a window.

[Kurt Ullman]

Yep. That is because it is never off, it is just in standby. From my MRI safety lectures in the past, IIRC, to turn it all the way off there is an emergency "quenching" that stops the superconducting magnets from conducting but it take a while, and a whole bunch of money, to bring it back online.

[George]

The "whole bunch of money" is because the emergency shutdown procedure vents the liquid helium coolant outside.

[krw]

..and a bunch more.

[micky]

Thanks. Someone at the MRI office said they were permanent magnets, but I suspect she was just guessing. Probably never had to do the emegency shutdown.

[krw]

Well, they are "permanent" in that they don't use energy when they're sitting there. They sure suck it down ramping up and then, of course, have to shed it on shut-down.

[nestork]

Mickey:

Why don't you just tell the people at the hospital where you're going to have your MRI done about your concerns. They may be able to give you some idea of how to test to see if the MRI's magnetic field will affect you BEFORE they sit you onto the table and slide you into the machine.

[micky]

I did. Although it was an "Imaging Clinic", not a hospital. The front desk at the orthopedist had detailed flyers for each of two chains of such clinics here. .

You don't have to raise the subject. They do. There's a questionnaire about whether you've ever worked with metal, and around

30 questions about what metal things you have in your body. There are loads of things that surgeons lieave in people's bodies, on purpose. Pins in their bones, staples, stents, blood vessel clips, even things in people's heads. I'd heard of many of them, but not all.

But I don't have any of that, and the only question was, metal filings in my eye. Both places asked the same question, and they were both vague, imo. When I said I was just a hobbyist, maybe 3 hours of grinding total in the last 30 years, the first place said, Don't worry. It's for people who work with a grinder or sheers or a saw for a living, 40 hours a week.

Couldn't take it at the first place because of claustrophobia. The second place had a machine with a larger bore (but still closed. Doctor insisted on closed, not open, because the image is much better) And the moment I started to ask the same question to the woman on the phone at the second place, she interrupted and said, You need an orbital X-ray. The x-ray machine didn't move, so I don't know what orbit they're talking about, but I think it's my eyeballs themselves

Yes, that's it.

The orbits are the cavities in the skull where the eyes are found, and maybe they are also the eyes. .

My eyes don't have any metal in them, andt it was worth one x-ray to find out. And I'll pay closer attention from now on so I won't need another x-ray.

The second clinic depended on earplugs, which the tech said did a better job of keeping the noise I heard down, and yet I could still har him. Not good enough, and I frieaked out in about 5 minutes. I thought all these places had headphones with music and even maybe radio stations. I should have asked about that.

[Wes Groleau]

Look up the words:

ferromagnetic

Paramagnetic

Diamagnetic