MRI safety: leaded pacemakers vs leadless pacemakers

There is a recent post concerning MRI safety for a leaded pacemaker. This led me to think about the MRI safety for leadless pacemakers, which I currently have a pair implanted in me.

When a current is flowing in a conductor that is placed in a magnetic field, a force is exerted on the conductor. This force is proportional to the “length” of the conductor. When a conductor is placed in an alternating magnetic field, an alternating voltage is generated across the conductor. This voltage is proportional to the length of the conductor. In a close circuit, this ac voltage produced an ac current. Both force and ac current are proportional to the length of the conductor.

“MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated,….” (I assume the radio frequency current is first applied to the magnet then the magnetic field is transmitted to the current.) (https://www.nibib.nih.gov/science-education/science-topics/magnetic-resonance-imaging-mri)

To my knowledge, the MRI machines currently have either 3 or 1.5  Tesla of magnet. The 3T has the higher resolution than the 1.5T. The radio frequency used for the 1.5T is 63.9MHz whereas for 3T is 127.8MHz. (https://mriquestions.com/radiofrequency-waves.html)

Thus, when a patient with a pacemaker or pacemakers is placed in the MRI’s magnet, both the pacemakers and leads(a conductor) will be surrounded by a static as well as a radio frequency magnetic fields.

Modern pm generator cases are made of non-magnetic material so they are not affected by the magnetic fields.  So it is the two long leads that will suffer the magnetic affects: the static force and the induced radio frequency current, both are proportional to the length of the leads. For the force that is acting on the lead, the current in the leads are the pacing currents which are typically tens of microampere, which are very small. The induced radio frequency current (I don’t know how large it is) will heat up the lead; this current may also penetrate slightly into the muscle because the tip of the lead is inserted into the heart muscle, potentially heating up that area of muscle.

This effects are concerning enough that the radiologist typically won’t use the 3T machine on pacemaker patients and sometimes avoid taking too many pictures that will expose the patient to the magnetic field longer. See for example, Mayo Clinic:

“The MRI pulse sequences are determined by the radiologist and the physicist. "All the exams take place on a 1.5-tesla magnet, and the specific absorption rate is limited to 1.5 W/kg for a maximum of 30 minutes," according to Joel P. Felmlee, Ph.D., a radiation physicist at Mayo Clinic in Minnesota. "If pre-scan sequences demonstrate heart rate synchronization to the transmission-receive interval of the radiofrequency coil, the scan is discontinued." (https://www.mayoclinic.org/medical-professionals/cardiovascular-diseases/news/new-protocols-allow-for-mri-in-selected-pacemaker-patients/mac-20430571_)

Now let’s look at the leadless, the length of the tallest pacemakers are shorter than 2 inches. (In comparison, the length of the lead for the leaded ventricle pm is about 1 foot.) Within the 2 inches, the volume mostly is occupied by the battery. The longest conductor is,  from the tip of the pacemaker to the circuit board, perhaps less than 1 cm.

So the conductor length of a leadless pm is only 3% of the length of  lead of a leaded pm. So even the 3T MRI machine may not be a problem. This indeed seems to be the case. See below:

“Advantages of MRI and leadless pacemaker compatibility

Leadless pacemakers have significantly improved the relationship between pacemakers and MRI, with the main advantage being the absence of leads and their small size. Micra and Nanostim leadless pacemakers allow patients to undergo body scans in 1.5-, and 1.5- and 3-tesla MRI machines, respectively. Additionally, many of the adverse events that patients may experience with standard transvenous pacemakers are rare, nonexistent and/or clinically insignificant. (Data on the Nanostim device is inadequate.)”(https://www.magellanhealthcare.com/documents/2022/02/advanced-imaging-digest-vol-2-iss-2.pdf/)


6 Comments

I'll be blunt: you have this all wrong

by crustyg - 2022-11-06 18:19:42

The fixed magnetic field of an MRI imaging system is NOT the issue for PMs and leads: yes, if you're foolish enough to put a ferro-magnetic object into a 1.5T or 3T field then it will rip out of whatever was holding it.  This is why you aren't allowed to take metal objects into the MRI suite unless they are proven to not be ferro-magnetic.

The real issue for PMs and leads with MRI imaging relates to two other things:

1 The additional, pulsed magnetic field overlaid on the fixed and very even field of the main magnet - this is the 'exploration' magnetic field that provides a magnetic gradient without which there would be no spatial information.  The setup and collapse of this field induces a current in any conductor within the field, and this includes the pacing wires.  The induced current in the leads can easily produce a lack of pacing during the scan which is why the PM has to be set to a fixed pacing mode during the scan.

2 The RF field which excites the protons and which, when it turns off, allows the protons to emit a tiny 'squeak' of RF energy, the frequency of which is a function of the magnetic field that the proton is in - a magnetic gradient allows the 'squeak' to map to a given location, so providing spatial information. The setup and collapse of this RF field also induces a current in any conductor, BUT this RF field is created many times per second, so a lot of current is produced and this can easily produce significant heating of the PM leads.  MRI-safe leads are designed to minimise the inductive pickup from this RF field and so reduce heating effects.

As the PM industry has shown, it's perfectly possible to design leads and boxes that are safe for MRI scans even of the chest area.  For scans in other areas, head+neck, abdomen+lower limbs most of the concerns are academic.  The normal operational currents flowing in the leads and PM itself are tiny, so the effect of putting a conductor carrying a current into a powerful magnetic field is negligible.

If I were really sceptical I'd be wondering if your post was just a disguised advertisement for leadless PMs.

A litttle more information

by AgentX86 - 2022-11-06 18:46:16

A typical MRI has a static field of 1.5T (15,000G) to 3.0T (30,000G) (larger ones on the way).  A rare-earth magnet can be 2,000 to 5,000G. A fridge magnet is about 100G. The gradient (RF) field is about 150-300G. IOW, really small comparitively. These, other than the MRI gradient field, are static fields.  Only a moving field can induce a current (thus heat) in a wire.

An induction cooktop, by comparison, might be .002G, so the gradient field in an MRI is quite significant.

AIUI, the issue is the leads.  In addition to a possible heating issue, unupolar leads will create an electrical gradient which might cause over-sensing. Bipolar leads should mitigate this (one of my leads is unipolar).

I think the big difference between MRI conditional and non-compliant is 20% EMI hardeneing, 40% testing, and 40% FDA paperwork.

I forgot to add my comments... I wouldn't have a leadless for three reasons (other than I couldn't).  I don't think the technology is mature yet. I don't think retrieval has been demonstrated sufficiently. I'm not confident of the indirect sensing (from the ventricle).

Sorry I got it all wrong

by brady - 2022-11-06 19:29:53

Crustyg, you are an expert on magnetism and I have gained more understanding from your post and thank  you for pointing out my mistake. I participate in this forum to learn as I am new to pacemaker. I am not connected to any company nor their advertisement.
 

Since my posts have created misunderstandings. To avoid future misunderstandings, I will quit this forum and will not participate in it anymore.

This is my last post and I sincerely enjoyed the replies I have received and the knowledgeable I have learned from this forum. 

If you think quitting will make me feel guilty, you're wrong

by crustyg - 2022-11-07 04:43:13

Hi Brady.  No-one knows everything, and I'm included in that list.  One of the challenges of Medicine is that some of what we used to 'know' turns out to be incorrect.  One of the best lectures I ever attended was in my first week at Med School: the nice lady said that 'half of what's in those expensive textbooks you've just bought is wrong, and sadly, probably half of what we're about to teach you will turn out to be wrong.'  Many textbooks in the 1970s were simply the expert contributors' personal opinions.  Evidence-based medicine has done a lot since to improve this, *but* the increasing level of academic dishonesty and downright fraud has undermined it.

I suggest you stay, read, think and ask questions - which is how we test our understanding.  If you do stay, please fill in your Member Profile.

It's your choice.  You don't have to be a Member to read the posts here - only to contribute.  All of this site is indexed by the major search engines.

MRI ok's.

by PacedNRunning - 2022-11-07 17:47:36

I work as a nurse and walk right up to the door threshold of the MRI room and nothing. The issue is noise from the magnetic field and the pacemaker not pacing appropriately. They put it in MRI mode which will pace you 100% to ignore all outside stimulus during the MRI.  OR they can turn if off. Which ever method is safe for you they will do. But the actually MRI machine will not pull your device out or have a magnetic attraction.  It's more for noise and signals/interference picked up by the machine.  

Keep researching

by Lavender - 2022-11-15 08:39:11

I admire your quest for knowledge and desire in sharing of information. Keep searching and learning and don't be too upset as corrections come along. Sometimes we can be over enthusiastic in our journey-for whatever reason. 
 

We are many parts but all one body. 

You know you're wired when...

You make store alarms beep.

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