Tag Archives: MRI

Imaging

More On MRI And External Fixators

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I’ve covered the problem of performing MRI on patients with external fixators. This is typically a problem that arises in head-injured patients with extremity or pelvic fixators for concomitant fractures.

MRI is an indispensable tool for the evaluation of head, spine, and soft tissue trauma. However, a great deal of effort is required to ensure that any patient scheduled for this test is “MRI compatible.” The fear is that any retained metallic fragments may move or heat up once the magnets are activated.

But what about trauma patients with external fixators? That is one big hunk of metal inserted deep into your patient. There are three major concerns:

  • Is the material ferromagnetic? If so, it will move when the magnets are activated and may cause internal injury. These days, many fixator sets are not ferromagnetic, avoiding this problem.
  • Can currents be induced in the material, causing heating? This is not much of a problem for small, isolated objects. However, external fixators are configured so that current loops can be created. The fluctuating magnetic fields can induce currents that, in turn, will heat the surrounding tissue. And thinner materials (narrow pins) result in more current and heating.
  • Will the metal degrade image quality?

Thankfully, there is a continuing trickle of evidence that is accumulating to give us some guidance. One paper from 2017 described a retrospective case series from four trauma centers. The authors performed MRIs on 38 patients with 44 external fixators. The adverse events they monitored for were catastrophic hardware pullout, thermal injury to the skin, field distortions that impaired the images, and damage to the magnet casing.

Twelve patients with 13 external fixators had MIR performed with the hardware inside the MRI bore, and 27 patients had the study with the fixator outside the bore. Most MRIs were performed to evaluate the cervical spine. There were no adverse events.

A recent Massachusetts General Hospital study involved a larger group (97 patients with 110 fixators). The fixators were located on the ankles, knee, femur, and pelvis. Most were performed on a 1.5T MRI, although a few were done on a 3T machine. Again, most scans were performed for head or cervical spine evaluation. Two of the 97 studies were terminated early due to patient discomfort. In both cases, the frame was outside the MRI bore.

The biggest challenge in our clinical practice is that there is no standard ex-fix configuration. Our orthopedic colleagues get to unleash their creativity, trying to devise the appropriate architecture to hold bones together so they can heal properly. This makes developing standardized guidelines regarding what can and can’t go into the scanner difficult.

We do know from clinical simulation studies that heating is influenced by ex-fix configuration. Increasing pin depth (thicker extremities) and closer pin spacing produces smaller temperature rises. For example, pins placed in a 15cm bar at a depth of 11cm produced a temperature rise of 2 degrees, but pins placed along a 30cm bar at a depth of 2cm showed a rise of 6 degrees.

However, a growing body of literature shows that the heating effects are relatively small and get smaller as the distance from the magnet increases. And non-ferromagnetic materials move very little, if at all, and do not interfere with the image. So as long as nonferromagnetic materials are used, the patients are probably safe as long as basic principles are adhered to:

  • Other diagnostic options should be considered and/or exhausted prior to using MRI.
  • Informed consent must be obtained, explaining that the potential risks are not completely understood.
  • The fixator must be tested with a handheld magnet so that all ferromagnetic components can be identified and removed.
  • All traction bows must be removed.
  • Ice bags or cooling packs should be placed at all skin-pin interfaces.
  • The external fixator should remain at least 7cm outside the bore at all times, if possible. If any portion must be inside the bore, monitoring efforts should be stepped up even more.

Bottom line: MRI of patients with external fixators can be safely accomplished. Consult your radiologists and physicists to develop a policy that is specific to the scanners used at your hospital. 

References:

  1. Magnetic Resonance Imaging of Trauma Patients Treated With Contemporary External Fixation Devices: A Multicenter Case Series. Journal of Orthopaedic Trauma, 31 (11), e375-e380. doi: 10.1097/BOT.0000000000000954.
  2. Magnetic Resonance Imaging of Trauma Patients Treated With Contemporary External Fixation Devices: A Multicenter Case Series. J Orthop Trauma. 2017 Nov;31(11):e375-e380. doi: 10.1097/BOT.0000000000000954. PMID: 28827510.

 

Imaging

MRI And External Fixators

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MRI is an indispensable tool for evaluation of spine and soft tissue trauma. However, a great deal of effort was be made to ensure that any patient scheduled for this test is “MRI compatible.” The fear is that any retained metallic fragments may move or heat up once the magnets are activated.

But what about trauma patients with external fixators? That is one big hunk of metal that is inserted deep into your patient. There are three major concerns:

  • Is the material ferromagnetic? If so, it will move when the magnets are activated and may cause internal injury. These days, there are many fixator sets that are not ferromagnetic, avoiding this problem.
  • Can currents be induced in the material, causing heating? This is not much of a problem for small, isolated objects. However, external fixators are configured in such a way that loops are created. The fluctuating magnetic fields can induce currents that in turn will heat the surrounding tissue. And thinner materials (narrow pins) result in more current and more heating.
  • Will the metal degrade image quality?

The biggest challenge is that there is no standard ex-fix configuration. Our orthopaedic colleagues get to unleash their creativity trying to devise the appropriate architecture to hold bones together so they can heal properly. This makes it difficult to develop standardized guidelines regarding what can and can’t go into the scanner.

However, there is a growing body of literature showing that the heating effects are relatively small, and get smaller as the distance from the magnet increases. And non-ferromagnetic materials move very little, if at all, and do not interfere with the image. So as long as nonferromagnetic materials are used, the patients are probably safe as long as basic principles are adhered to:

  • Other diagnostic options should be exhausted prior to using MRI.
  • Informed consent must be obtained, explaining that the potential risks are not completely understood.
  • The fixator must be tested with a handheld magnet so that all ferromagnetic components can be identified and removed.
  • All traction bows must be removed.
  • Ice bags are placed at all skin-pin interfaces.
  • The external fixator must remain at least 7cm outside the bore at all times.

Bottom line: MRI of patients with external fixators can be safely accomplished. Consult your radiologists and physicists to develop a policy that is specific to the scanners used at your hospital. 

Related posts:

Imaging

Best of EAST #3: Spine MRI Usage After EAST Guidelines

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In 2015, EAST published their practice guidelines for spine clearance in the obtunded blunt trauma patient. Click here to view them. They stated that a high-quality CT scan can be used to remove (clear) the cervical collar in these patients. This avoids the use of the expensive and personnel-intensive MRI clearance.

The group at UCSF used the NTDB to review the use of MRI in such patients over an 11 year period. They focused on comatose patients (GCS < 8) with an AIS head > 3 and intubation for more than 72 hours. They used logistic regression to equalize confounders while examining the use of MRI over time, before and after the guidelines were published.

Here are the factoids:

  • More than 75,000 patients from 530 trauma centers were included
  • Patients who were older, Hispanic, uninsured, or involved in a car crash were less likely to undergo spinal MRI
  • Level I centers were more likely to use MRI for clearance than Level II centers
  • Patients evaluated after release of the practice guidelines were 1.7x more likely to undergo MRI for spine clearance (!!)

The authors concluded that spinal MRI use has been increasing since 2007 despite publication of the EAST guideline.

My comments: To me, this indicates one of the following:

  1. Nobody reads the EAST guidelines, or
  2. Trauma programs believe that they alone are able to figure out what is right, and everyone else is wrong

I suspect that it is #2. For some reason, trauma programs insist on doing it their own way despite what decent evidence shows. I think that this represents a defense mechanism to minimize the cognitive dissonance that comes with defying what is published in the literature.

I always encourage programs to borrow/steal what is already out there when crafting their own practice guidelines. Someone else has already done the work, why not take advantage of it? Typically, it’s just an excuse to continue doing things the way they’ve always been done.

This incessant reinventing the wheel becomes tiresome. And for once, I don’t have many questions or suggestions for the authors. Their evidence is pretty well laid out. 

My questions for the author / presenter are:

  1. Do you use MRI for spine clearance in your obtunded blunt trauma patients? And if so, WHY?
  2. Why do you think there are demographic and trauma center level disparities? Is it the teaching environment? Something else?

To everyone else, I say “get over yourself and read the literature!”

Reference: Assessing the e3ffect of the EAST guideline on utilization of spinal MRI in the obtunded adult blunt trauma patient over time. EAST 2021, Paper 7.

General, Imaging

What? Still Using MRI For Cervical Spine Clearance?

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Cervical spine clearance as evolved considerably over the years. First, there were five views of the spine using plain radiography. Then there were three. Then we moved to CT scan with clinical clearance. And currently, many institutions are relying only on CT.

But MRI has been used as an adjunct for quite some time. Initially, it was the tie breaker in patients who had equivocal CT findings, and for a while it was used for clearance in obtunded patients. And thanks to conflicting literature and disparate studies, the occasional usage became more frequent.

The group at Cedars-Sinai Medical Center in Los Angeles  noted that the percentage of patients undergoing MRI for cervical spine evaluation at their center slowly slowly crept up from 0.9% to 5.6% over a 10 year period. They designed a study to analyze the utility of this practice and inform their future practice.

Here are the factoids:

  • Over 9,000 patients had cervical spine CT during the 10-year study period; 513 (5.6%) were positive
  • Of the 513 CT-positive patients, 290 (56%) underwent an MRI. This showed:
    • Confirmation of the major injury in 250
    • Minor injury in 40
    • Clinically significant injury was seen in only 2 which was no surprise since they both had neurologic deficits
  • Of the 8,588 CT-negative patients, only 9 had clinically significant findings and 8 of them had neurologic deficits

Bottom line: So what have we learned here? First, MRI usage at Cedars-Sinai increased over time but was really not that useful. The main use was for imaging obtunded patients or those with an obvious neurologic deficit.

More than half of patients with positive CT scans also underwent MRI. If a major injury was seen on CT, MRI confirmed it. But if the CT findings were minor, none of the MRIs added any clinically significant findings in the absence of a neurologic deficit.

And what about MRI after negative CT? In the absence of a deficit, only one had a clinically significant finding (which only required a brace).

This study shows the wisdom of monitoring “how we do it.” There is sometimes some creepage away from what the literature shows is the best practice. The best way to remedy this is to do a good study, just like the authors did. They saw a slow change in practice, investigated it, and found that there was no good clinical reason for it. This gives the trauma program the ammunition to squelch the unwelcome behavior and return the clinicians to best practices.

Reference: Is MRI becoming the new CT for cervical spine clearance? Trends in MRI utilization at a Level I trauma center. J Tra publish ahead of print, DOI: 10.1097/TA.0000000000002752, 2020.

Imaging

Are You Still Using MRI To Clear The Cervical Spine?

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There is a fairly robust  amount of data that shows that, properly performed, the cervical spine can be cleared using a high quality CT read by a highly skilled radiologist. This is true even for obtunded patients. Pooled data suggest that the miss rate in this group is only 0.017%. And MRI is not perfect either, missing significant ligamentous injury in a small number of patients.

But it seems that some trauma professionals are still using MRI in some cases despite this data. The latest study on MRI focuses on the cost-effectiveness of the technique. The authors selected patients with GCS < 13 to be their obtunded group, which is probably a bit high. Nevertheless, they used a fairly sophisticated (meaning hard to understand) modeling-based decision analysis using a computerized simulation. This allowed them to compare different clearance strategies without performing large randomized clinical trials.

The authors considered MRI vs no MRI, false results, collar use and complications, MRI use with cost and complications, and the worst-case scenario of tetraplegia. Here is a flow chart of the scenarios considered. (Courtesy JAMA Surgery)

Here are the factoids:

  • The mean cost for followup vs no followup was $14K vs $1K, with no increase in quality adjusted life years (QALY)
  • No followup was the better strategy when the negative predictive value of CT was high (>98%), when the risk of an unstable injury treated with a collar turning into a permanent deficit was >25%, or if the chance of a missed injury becoming a permanent deficit was >58%
  • No followup MRI was the better strategy in all 10,000 iterations of the simulation

Bottom line: Yes, this is a fairly heavy computer simulation. But the reality is that we will never be able to design a large enough study to critically evaluate this issue and have it pass any IRB review. So it’s probably as good as it will ever get. It’s time to stop wasting money and putting obtunded patients in harm’s way by locking them into a relatively inaccessible MRI scanner for 30 minutes just to confirm the CT. Or keeping a collar until until the skin breaks down.

Here is a copy of the practice guideline we use for clearing all cervical spines, obtunded or not. Yes, there is some weirdness with soft collars, which mainly serve as a reminder to re-examine the patient at some point. But note the scan technique and requirement that it be read by a neuroradiologist for final clearance.

Related link:

Reference: Cost-effectiveness of Magnetic Resonance Imaging in Cervical
Clearance of Obtunded Blunt Trauma After a Normal
Computed Tomographic Finding