Lea Alhilali, MD Profile picture
Jan 5, 2023 22 tweets 10 min read Read on X
1/Nothing strikes fear into the heart of a radiologist like the question,“Is it safe to do an MRI on this pt w/an implanted device?”

Never fear again! Here’s a #tweetorial on how to navigate implanted devices & #MRI
#medtwitter #meded #radtwitter #radres #neurotwitter #neurorad
2/MRI & CT are like nuclear & coal power, respectively. Everyone knows CT is worse for you & usually MRI is very safe & better for your body

But like nuclear power, when things go bad in MRI, they can go horribly wrong. Flying chairs into the magnet wrong. So, people are afraid
3/The trouble is from the magnetic attractive forces. There are 3 ways these attractions can wreak havoc. First is translation. Magnet literally pulls an object, like a chair, towards itself. This is the strongest attraction—like two lovers who literally can’t stay apart.
4/Second is torque or rotation. This is when the force isn’t strong enough to pull the object away, but enough to make it wiggle or turn a bit.

It’s like an attraction that isn’t enough to make you run, but enough to make you turn your head & look.
5/Last is the sneakiest way the magnet damages—heat. Radiofrequency (RF) waves deposit heat, like other waves, such as microwaves. This causes internal heating w/o any movement.

It’s like the hot passion you feel deep inside for your lover, regardless of any physical contact
6/All of these effects stem from the fact that the MR is just a giant magnet & its exerts forces on objects in the magnetic field.

Since these effects are from a magnet, it makes sense that metal objects would be the most affected—as metals can be magnetized.
7/But not all metals are affected the same by the magnetic field. We all know that metals like nickel & iron are very attracted to magnets, while other metals like calcium are not.

More affected objects will feel more force in the MRI & are more likely to move/cause damage.
8/We classify implants by how likely they’ll move in the MR field. MR unsafe devices are highly magnetic & could fly into the MRI & thus are banned. MR safe means no metal or magnetic properties, completely unaffected. MR conditional is in between, some attraction, but not strong
9/How do we know which metals are unsafe & which are possibly safe?

There are two main types of magnetic metals.

Ferromagnetic metals are very magnetic. I remember this b/c ferro sound like ferocious, & so they are ferociously magnetic.

These are MR unsafe.
10/Four main ferromagnetic metals exist: iron, nickel, cobalt, & steel. Remember this by remembering a dashing, some might say magnetic, Knight. He wears wrought IRON armor, holds a strong STEEL sword, & rides a bolting colt (COBALT). He’s a poor mercenary, so he’s paid w/NICKELs
11/While ferromagnetic metals are MR unsafe, their alloys are not. Adding other metals can counteract the magnetism or transform it into a completely new metal that isn’t magnetic.

Most medical devices are these alloys. You really only see true ferromagnetic metals in shrapnel
12/While ferromagnetic objects are strongly magnetic, paramagnetic objects are only weakly magnetic.

I remember this b/c they are PARamagnetic & PAR in golf means just average, nothing really special.

So there is no special or strong magnetism in these metals.
13/Paramagnetic objects are MR conditional. They have the potential to cause tissue damage by torque objects or heating objects. This risk must be weighed against the benefit of getting an MRI
14/Torque can be a problem.

However, if the device is in anything w/motion (vessel w/flowing blood, beating heart, moving bones), torque from physiologic motion is stronger than any from the magnet.

So if it stays in place w/natural forces, it won’t be moved by the magnet.
15/They say you should wait 6 weeks after any implanted device before scanning, to let scar tissue form to further anchor the device.

While this is ideal, it isn’t really necessary—b/c if the physiologic forces haven’t dislodged it yet, neither will the magnet.
16/But what if the paramagnetic device isn’t in a location where there is motion to test it? What if it’s in the kidney? Is it still safe? It probably is, b/c the magnetic forces are weak. Check the manufacturer recommendations to see how much magnetic force you can use & be safe
17/Paramagnetic objects can heat up. Even w/low magnetism, you get heating—& it’s hard to predict b/c the heat amount depends on the patient, scan parameters, etc

So every pt w/a device should get a squeezy ball to squeeze if they feel heating—to stop the scan before any damage
18/A special problem for heating is 1 dimensional (1D) wires.

These collect RF energy like an old TV antenna & concentrate the energy at their tip—leading to high risk of burns at the tip.

So any device with a 1D wire needs a special protocol to prevent overheating
19/RF pulses not only heat, they also can interfere w/electronics of devices—like jamming radio signals.

This can lead to device malfunction or even delivery of incorrect signals that can cause arrhythmias.

Special care must be taken & devices should be checked after scanning
20/As a result, scanning protocols for devices w/1D leads (pacers, DBS) are very strict & require oversight. Even then, there is hesitancy to scan 1D leads w/high risk of heating (abandoned leads, temporary leads)
21/So there are 4 questions to ask yourself to determine if an device is safe:

Is it:
(1) ferromagnetic?
(2) a 1D lead?
(3) a device w/vulnerable electronics?

If not, it usually safe to scan using the protocol recommended by the manufacturer.
22/The quick & dirty method: Is it a ferromagnetic knight? Is it an old TV w/electronics or antenna? If not, then scan carefully w/manufacturer’s recs.

Now you know the secret of safe MRI scanning w/implants. Hopefully this tweetorial has been a white knight to your rescue!

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Sep 15
1/Time is brain!

So you don’t have time to struggle w/that stroke alert head CT.

Here’s a thread to help you with the CT findings in acute stroke! Image
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(2) exclude other pathologies mimicking acute stroke. But you can also see other findings to help diagnosis a stroke. Image
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In first 12 hrs, the most common imaging finding is…a normal head CT

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Do you?

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Here’s thread w/all you need to know! Image
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(1) T2 bright changes (indicating edema, Modic 1)
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Are you hungry for a way to classify these patients?

Donut you worry!

Here’s a short thread to help you remember the modified Fisher scale for classifying subarachnoid hemorrhage. Image
2/Just think of the brain as a donut. Like a donut, it’s a bunch of stuff around a hole in the middle.

Ventricles are the hole in the middle of the brain just like there’s a hole in the middle of the dough in a donut.

Just don’t quote me to your neuroanatomy professor…. Image
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Sep 8
1/Talk about twisting your back!

Do spine vascular lesions make your brain feel as tangled as the dilated vessels you see?

Want some more information on malformations?

Here’s a thread on spine vascular anatomy to give you durable knowledge on dural arteriovenous fistulas (dAVF)Image
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The spine is LONG—to get blood from the top to the bottom is like going through the length of a marathon course Image
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When you run a marathon, you replenish yourself at aid/water stations along the way so you can make it all the way through.

Same w/spinal arterial vasculature—it needs to be replenished on the way down. Image
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Sep 3
1/Does the work up for dizziness make your head spin?

Wondering what to look for on an MR for dizziness

This month’s @theAJNR SCANtastic will tell you all you need about imaging Meniere’s disease!

ajnr.org/content/46/8/1…Image
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So it is important to try to differentiate

Meniere’s is a common cause & we can help diagnose it w/imaging! Image
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Inside the membranous labyrinth is endolymph Image
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1/They say form follows function!

Brain MRI anatomy is best understood in terms of both form & function.

Here’s a short thread to help you to remember important functional brain anatomy--so you truly can clinically correlate! Image
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3/It is also easy to recognize on imaging. It looks like a big thumb pointing straight up out of the brain. I always look for that thumbs up when I am looking for the superior frontal gyrus (SFG) Image
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