MRI and non-ferrous metals

[Derek Smith]

I went for an MRI scan. I removed all metal objects but forgot to remove an 18 carrot ring.

During the scan I could occasionally feel what felt like my ring (no childish jokes) vibrate slightly. It went in phases.

I thought the ring might be adulterated, but I put it on a strong magnet, one from an HDD and there was no response. It would even slide off it easily and at the same rate as silver.

I know an 18c ring is not pure but there's unlikely to be any iron in a ring from a reputable dealer. They'd mix it with too much copper or silver. Even if there was some iron, it would have been attracted to the magnet.

So why did the ring react in the scanner?

[Simpo Two]

I read that a few times and thought 'Blimey that's tricky, I'll post it on the PH Science forum'.

Dang...

(though as you're a writer I do hope 'carrot' was intentional; if not you should donate al your earnings to charity lol)

[CubanPete]

If you move your hand, i.e. the ring, it will induce Eddie currents, which create a resistive force to stop it rotating.


It was probably your hand vibrating and the ring wanted to stay still!


We took a bit ally billet into an MRI scanner (just to see...). You could move it linearly, but not rotate it. Even though you understand the physics, it was very strange.

[Mr2Mike]

You have introduced a shorted turn into a rapidly changing magnetic field, ao a current will be induced into the ring. This current will produce its own magnetic field which will oppose the one inducing the current.

Im a little surprised the ring didn't heat up noticeably.

[Derek Smith]

Thanks for that guys.

It gave me something to think about in the tube.

As for my hand vibrating, the slide moved back and forth a few times.

Without going into too much detail, it was a prostrate exam and my hands were in that region so I assume the force was strong with my bits, or in that general area.

You find out something new all the time.

(I discovered I suffer from a degree of claustrophobia.)

[Mr2Mike]

Derek Smith said:

(I discovered I suffer from a degree of claustrophobia.)

You have my sympathies. I had an MRI scan for a damaged shoulder last year and not only did I discover claustrophobia, I also can't stand being too hot and MRI scanners are akin to slow cookers. My shoulder was wedged into a plastic fixture to stop it moving and it was painfully uncomfortable after 5 minutes, took a lot of will power to keep from pressing the stop button by the end!

[annodomini2]

In a powerful enough magnetic field everything is affected:

https://www.youtube.com/watch?v=uY8btfJZ9Z8

[Derek Smith]

Mr2Mike said:

Derek Smith said:

(I discovered I suffer from a degree of claustrophobia.)

You have my sympathies. I had an MRI scan for a damaged shoulder last year and not only did I discover claustrophobia, I also can't stand being too hot and MRI scanners are akin to slow cookers. My shoulder was wedged into a plastic fixture to stop it moving and it was painfully uncomfortable after 5 minutes, took a lot of will power to keep from pressing the stop button by the end!

I feel your pain. Literally so.

It was remarkable. I was 64 for my first and I had no idea I'd break out in a sweat and have to close my eyes. 35 minutes for me. The second time I asked for a fan and even then it was an effort. I could not get a thumb between my nose and the top of the machine. Just thinking about it now makes me cringe.

I've been asked a number of times why I was scared when there's no danger. I point out that I have claustrophobia, an irrational fear. They then say, 'Yes, but why have you got it.'

What's so bloody difficult to understand about the word irrational? One should be allowed to kick them.

I have annual checks on a progressive back problem, eventually to be 6 monthly, and then I had high PSA. I've going to have something stuck up my fundament with pins sticking out of it. They say they need another scan to check they are doing it in the right place. I'd be happy for them to stab me all over.

The vibrating ring (that made two) was a welcome distraction.

As you say, it takes a lot of will power not to press the bulb. That I take away from the horrid machine with a degree of satisfaction, if not pride.

[Djtemeka]

I found my ring (pooper) got slightly warmer. I’ve had 2-3 mri in the last 12 months and always notice it

[Mr2Mike]

Derek Smith said:

I feel your pain. Literally so.

It was remarkable. I was 64 for my first and I had no idea I'd break out in a sweat and have to close my eyes. 35 minutes for me. The second time I asked for a fan and even then it was an effort. I could not get a thumb between my nose and the top of the machine. Just thinking about it now makes me cringe.

That all sounds horrifyingly familiar, mine took about the same time. It surprised me just how loud they are, even with the hearing protection.

[turbobloke]

There are a few hosptials and similar locations across the country e.g. Birmingham where you can undergo MRI scanning in a seated (upright) position and can avoid the discomfort of claustrophobia. At least that's the position (no pun intended) according to the local MRI centre.

Cmon Derek first carrots now you're prostrate (again no pun intended!) for your prostate scan - hope it all goes well for you and your ring

[CAPP0]

I'm mildly claustrophobic too; for example, if I'm working under a car, I can be fine for ages but then suddenly I will HAVE to get out for a few minutes. Then I can go back under. I could not in a million years do this caving lark.

Anyway, I've had several MRIs but only one where I had to be completely inside it. They offered me a range of CDs to listen to so I picked one I knew well (Foo Fighters, if that matters!). They told me I'd be in for about 40 minutes. By the time track 8 came on, I figured that at roughly 5 minutes a track, I must be nearly there, but as it was it took everything I had not to press the button near the end. One trick I almost got right, I closed my eyes before I went in and vowed not to open them again until I was out. I failed for about 2 seconds about 30 minutes into the scan and opening my eyes and looking up, confirming what I already knew which was that I couldn't have moved if I had wanted to, was just hideous.

[V8LM]

annodomini2 said:

In a powerful enough magnetic field everything is affected:

https://www.youtube.com/watch?v=uY8btfJZ9Z8

This is what was probably happening to your gold ring. Gold is diamagnetic - creates its own magnetic field to oppose an applied one - and so is repelled by a magnetic field. The high field in the MRI coupled with acoustic vibration will cause the ring to vibrate (and warm?)

Edited by V8LM on Thursday 2nd August 07:23

[HaiKarate]

Had no idea they took so long. No wonder waiting lists are as long as they are. Whilst on the subject of MRI and in the science forum, how do these things work?

Although I realise electricity and magnetism are linked, I’ve never really understood magnetism as a force.

[Dog Star]

Derek Smith said:

I could not get a thumb between my nose and the top of the machine. Just thinking about it now makes me cringe.

Jesus! I think I'd just freak - I am feeling a little odd just thinking about this. The odd thing is that when I was young (I'm 50 now) I never had this fear of tiny spaces - in fact I'd go caving and was slim enough to get through the 9 inch "cheese press" in Derbyshire (with a couple of hundred metres of rock on top of me).

I'm not sure how I'd manage in an MRI scanner - can you be sedated? I remember a few years ago being given a lift by my mate with my OH and getting into the back of his 1990 Porsche (968?). I lasted literally seconds before I flipped - no way was that happening. He had to do two trips.

[WatchfulEye]

Derek Smith said:

I went for an MRI scan. I removed all metal objects but forgot to remove an 18 carrot ring.

During the scan I could occasionally feel what felt like my ring (no childish jokes) vibrate slightly. It went in phases.

So why did the ring react in the scanner?

There are 3 major magnetic fields in the scanner.
1. The static magnetic field (from the giant magnet). This does not change, and is used to align the spin axes atomic nuclei which form the image.
2. Magnetic field gradients. These distort the main magnetic field in a controlled manner, and are used to select which row of pixels to read. These must be switched on and off as each row of pixels is read out (10- 200 times per second).
3. The rotating magnetic field. This is used to rotate atomic nuclei so that their precession can produce an electromagnetic signal which can be detected. This rotates at a frequency of (typically 62 MHz for a scanner with 1.5 Tesla static field; or 124 MHz for a 3.0 Tesla scanner, and proportional frequencies for those rare scanners with odd-ball static fields).

In general, the static field won't affect non-ferromagnetic metals, but it can pull or torque ferromagnetic metals (iron, some stainless steels, etc.).

The rotating magnetic field is well into the radio frequency range, so won't cause any vibration, but can generate currents in metal conductors or metal loops. This can cause electrical heating of metal, especially if it is a metal loop. Serious burns can occur if metal is allowed too close to the edge of the scanner bore because this is where the rotating magnetic field comes from, and the rotating field is much stronger at the edge, resulting in much stronger induced currents. There is a recent case from the US, where someone had an MRI scan with a metal ECG electrode left on, and due to moderate obesity, that part of the body was wedged against the scanner wall, resulting in heating sufficient to cause 2rd degree burns.

The magnetic gradients result in large and rapid changes to the magnetic field strength in the scanner. This rapidly changing magnetic field results in induction of eddy current pulses in metal objects, which will result in a force on the metal object, resulting in vibration, and to some extent, some heating - although the heating contribution is usually less than the RF field. The gradient electromagnets are remarkable devices taking thousands of Amps of current, and even the cables running from the amplifier to the coils, need to be rigidly fixed, because the magnetic forces are so strong that the cables may be damaged from the vibration caused by the current.

Some other people have mentioned warm feelings. In MRI, the rotating field (radio frequency) will get absorbed in the body via a variety of mechanisms as heat. The field is generated by a powerful radio transmitter (capable of producing 40-70 kW pulse power). This can result in substantial heating of normal body tissue. There is a legal limit, and this is built into the scanner firmware, of a heating effect (so called specific absorption rate or SAR) of 4W/kg of tissue - that's a heating effect close to that of a decent workout at the gym. Most scan protocols don't get anywhere close to this SAR level, but some do (especially, if high detail is required in things like brain, joints or spine), and the heating effect can be really noticeable, especially if the scan is long or multiple body parts.

Edited by WatchfulEye on Thursday 30th August 20:05

[julianm]

Interesting stuff & I hope you are all OK Derek. I`d like to know what causes the loud clanking noises in the MRI process - really took me by surprise!

[WatchfulEye]

julianm said:

Interesting stuff & I hope you are all OK Derek. I`d like to know what causes the loud clanking noises in the MRI process - really took me by surprise!

The noises come from the interaction of the magnetic fields 1 and 2 in my answer above.

You have (what is effectively a permanent magnet), and then powerful controllable electromagnets next to it.

This combination, is very similar to that of a loudspeaker. The only difference is that in a loudspeaker, the controllable electromagnets are allowed to move freely and move a diaphragm.

However, in an MRI scanner, the magnets are so strong, that the forces exerted are enormous. Even though they are firmly mounted, they experience strong vibration as a result of forces. Some scanners mitigate the noise by placing the electromagents in a vacuum chamber, but sound still escapes from the mountings.

So, effectively every time a row of pixels is read out, you get a clunking sound. To reduce scan time, many scanners aim for 50 or more lines per second, so this becomes loud buzzing. Some ultra-fast imaging techniques, may read 500 lines a second (essentially capturing an full 128x128 image in about 0.25 second), and these produce beeping sounds.

The latest scanners often have noise mitigation software. Instead of reading out rows of pixels, which requires jerky control of the magnets producing lots of vibrations, it is possible to steer the pixel read-out gently in smooth spiralling curves, which means smooth forces of the magnets and much less vibration. There are some limitations with these techniques and they may require scan parameter changes because they can affect the image, so not all sites will use them or be familiar with them. However, they can be extremely effective, and allow some parts of the scan to be performed with no audible noise (other than the background ventilation/refrigeration).

[julianm]

Thanks a lot for the explanation I think I get it! I had some background in Chemical analysis with NMR spectroscopy but I am decades out of date.
We would discuss spin spin decoupling & similar - it was interesting being on the fringe of those techniques as a lowly user. As far as I know these days you can just let the machines suck in a sample, have yourself a coffee & read out the results!
When I heard about the first MRI machines I was fascinated.
Where will the next advance leave us I wonder?

[Derek Smith]

julianm said:

Thanks a lot for the explanation I think I get it! I had some background in Chemical analysis with NMR spectroscopy but I am decades out of date.
We would discuss spin spin decoupling & similar - it was interesting being on the fringe of those techniques as a lowly user. As far as I know these days you can just let the machines suck in a sample, have yourself a coffee & read out the results!
When I heard about the first MRI machines I was fascinated.
Where will the next advance leave us I wonder?

Hopefully in a wider space.

This thread's been great. Thanks, guys.