Demonstrating strength of magnetism in MRI machine

MRI

by docklock - 2023-10-10 17:28:21

Wow!   Impressive video.  Surprised the MRI doesn't rip PM/or whatever out of your chest.  I know "most" PM are OK thru MRI -- but still gotta marvel at the demo.  

MRI and ferromagnetic materials must not mix...

by crustyg - 2023-10-10 18:26:22

..because if your PM/ICD were ferromagnetic (iron or iron-like) the device would actually rip out of your chest, or pin you to the machine.  If you have ferromagnetic clips on an aneurysm, the magnetic field of the MRI magnet would rip it off the artery with probably fatal consequences.  Lots of aneurysm clips are/were made of stainless steel: try putting a magnet next to cheap s/steel cutlery - it sticks.  'Good' s/steel is not ferromagnetic.  Modern PM/ICD metal cases are titanium, and not ferromagnetic.

Happily, nothing in a PM or ICD that's MRI safe (more about this below) is ferromagnetic.

Everyone thinks that it's the intense magnetic field of the MRI (usually between 1.5 and 4 Tesla these days), that's the issue with PM/ICDs and MRIs, but it's not.

MRI works by using pulses of RadioFrequency energy to tweak the protons in the nuclei of the atoms that we're made of, whilst held in an intense magnetic field, and as the RF excitation is stopped, each proton releases a tiny squeak of energy whose frequency is related to the strength of the magnetic field where each proton is positioned.  The MRI magnetic (a super-conducting electromagnet) generates a very even field across the hole that you slide in and out of.  The cage that they put over you is a key part of the other component of the system: those bang/bang, buzz/buzz noises are the RF excitation and the current flowing through the coils in that cage that produce a magnetic gradient across the hole in the machine.

Now, the protons are producing different squeaks that relate to their position across the hole - which allows the computer to work out where they are, and generate a picture from the signal.

The RF field turning on and off tends to create signals in the PM/ICD leads that swamp the normal sensing of your device.  Old style leads actually heat up due to the induced currents being produced, and this is a key reason why it's the combination of device + leads that has to be MRI-safe - called MRI conditional as it's only 'safe' if the PM/ICD is put into a safe mode during the scan.

As the PM/ICD can't sense the natural beats of your heart whilst in MRI-safe mode, the device has to be reprogrammed to something simple, like a fixed, non-sensing rate for the duration of the scan.  This has a small risk with it, which is why they are so careful to monitor your ECG and pulseox O2sat during the scan.

Hope that helps.

Magnetism

by piglet22 - 2023-10-11 07:07:50

The mysterious force that powers so much of what we do.

I won't attempt to explain it. It's come a long way since lodestones.

The power of even small rare earth magnets is phenomenal and the big ones can be extremely dangerous. Don't get your fingers between a large magnet and a lump of steel.

crustyg is right about stainless steel. Another subject to steer clear of.

Rightly or wrongly, I judge SS by it's reaction to magnetism. Good grades are often only weakly attracted. I have a drawer of SS cutlery and all but one soup spoon are attracted to a magnet. Therefore, I assume that this is better grade of SS.

There's a lot of cheap SS out there, so pick your items with care. If you don't then don't be surprised if your new kitchen sink starts to grow rusty pin prick holes.

Another thing to try is any coinage you have. UK coinage has gone cheap too.

The "copper" coins like old pennies, farthings, three pence pieces were made of decent copper alloys and were non-magnetic.

The new ones are cheapskate plated steel, magnetic and only fit for making washers.

PS Lurker

Your bio was throwing an error this morning

heat

by Tracey_E - 2023-10-11 08:37:18

Crusty's excellent explanation is spot on. The heat has always been the risk, not the magnet.

I was told, to my shock, recently that I can have an mri with my 25+ year old leads if it was dire circumstances. It could possibly change my pacing thresholds permanently, but if it was THAT important to have an MRI, my ep said it could be done. 

Tracy

by piglet22 - 2023-10-11 11:08:26

The problem is that you have something fitted 25-years ago, state of the art, then something else comes along that renders it not so good.

Video killed the radio star as they sang.

Realistically, it must be nigh on impossible to design that far ahead, future-proofing while technology develops exponentially.

Maybe nano robots will be able to scurry around down narrow veins and arteries and carefully remove leads out of harms way.