MRI imaging of external fixators

Hi,

Interesting discussion.

We haven’t had any issues with metal implants over two decades of my practice.

Ex fixes have a potential problem of skin burns. Most have not been MR compatible in our set-up and we havent scanned any for that reason.

Recently, there was a skin burn issue from yoga pants (they have copper material in garment.

At UTSW, we have taken the safer approach of not scanning any ex-fixes till date.

Avneesh Chhabra, M.D. M.B.A.

Professor Radiology & Orthopedic Surgery

Chief, Division of Musculoskeletal Radiology

UT Southwestern Medical Center, Dallas, Tx

5373 Harry Hines Blvd.

Dallas, Tx-75390-9178

Office: 214-648-2122

avneesh.chhabra@utsouthwestern.edu<mailto:avneesh.chhabra@utsouthwestern.edu>

http://www.utsouthwestern.edu<http://www.utsouthwestern.edu/>​

________________________________
Sent: Tuesday, September 14, 2021 7:13 PM
Cc: OCAD <ocad-msk@googlegroups.com>; Jessica M. Sin <Jessica.M.Sin@hitchcock.org>

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Here is an updated version on heating of implants (by example of different hip arthroplasty constructs) at 1.5T and 3.0T when using metal artifact reduction pulse sequences: clinical-mri.com/wp-content/uploads/2021/04/Heating_of_Hip_Arthroplasty_Implants_During_Metal.5.pdf

Our results show that the chance of heating was actually lower at 3.0T than 1.5T for different hip implants, which is likely due to implant resonance / antenna effects due to the longer RF wavelength of 1.5T. Hence, SAR can be a misleading predictor of heating.

The results also shows that the risk of heating may be less for CS SEMAC or SEMAC than for high BW TSE with dependence on the implant type, shape, alloy, etc. Since SEMAC usually takes much longer than FSE/TSE, the imparted energy can be lower per unit time (= lower SAR/RMS). Afforded by the high receiver BW, high BW FSE can be run with very short echo spacing, long ETLs, minimized TR, and high flip angles and hence may be most aggressive. If one were wanting to heat an orthopedic implant, that’s a good way to attempt it experimentally.

There are many other factors that play into heating of implants, including positioning relative the B0 and B1, which this paper shows with MR conditional needles during interventional MRI: clinical-mri.com/wp-content/uploads/2020/06/Needle_Heating_During_Interventional_Magnetic.10.pdf

Here is another paper where very high temperatures > 100 degrees were induced by MRI of cryo ablation probes oriented in parallel to B0: pubmed.ncbi.nlm.nih.gov/33663924/

Based on our experience, short and small implants, such as knee (without stems) and ankle arthroplasty implants and short screws are least likely to heat. Substantial and very rapid heating can occur with implant component lengths between 15-25 cm, e.g, ExFix pins.

Anecdotal reports and evidence may only apply to a specific protocol, scanner type, patient position, shimming techniques, coil setup, etc.

If it’s really clinically necessary, a best practice approach is to use the field strength were RF wave length and implant lengths are least likely to match (0.2T to 3.0T) with regard to an antenna effect, use lowest possible flip angles (non-linear relationship to imparted RF energy) < 130 degrees, use low ETLs / limit refocusing pulses, use extra long TRs rather than minimize TE, put 1-2 min breaks inbetween each sequences, use highest intra-bore vent setting, constantly observe patient in direct sight, check verbally after every sequence, and stop immediately if patient starts moving as this might be a sign of pain/burn happening. Practically, it seems to come down to the indication justifying the risk and if / how good the testing of the local MRI protocol has been done to estimate safety to a reasonable degree. ________________________________ Sent from my iPhone ________________________________ Thanks everyone for your personal experiences. Comments below. Attached 2 excellent articles including Tim Mosher's. "We do this all of the time. Safe at 1.5T and minimal artifact, especially with MAR technique and sequences. No problems with heating." "We do MRI on pts with ex fix not infrequently. No difference in protocol, but we have had patients complain about pain and have to come out of the magnet. Some do fine.These are usually trauma patients with things like knee dislocations (so looking at what ligaments are injured) and the ex fixs are often quite fresh." "We do MRIs with MRI conditional Exfix occasionally. It's a little easier to deal with if the surgeon is willing to remove the rods before the MRI. Most of the patients have pathologic femur fractures and need MRI for preop planning although I'm pretty sure we have done these for trauma as well. The images turn out fine." "As long as you are sure that absolutely nothing is made of steel or an alloy containing iron/steel, imaging around ex-fix hardware should be pretty safe. We routinely image around non-ferromagnetic implants in the body such as spine hardware and joint arthroplasties... even while using high-SAR sequences such as STIR. I don't think I have ever encountered a situation where a metallic implant was heated sufficiently to cause injury to a patient. May be worth using MAR sequences such as MAVRIC or SEMAC if the joint of interest is the one being ex-fixed... monitor the patient for heating using these sequences since they are VERY high SAR, particularly at 3T. If the area of imaging is far from the ex-fix device, the risk is probably minimal/zero since hardware way off isocenter won't be significantly exposed to the excitation pulses. Also, make sure your techs don't use first-level mode. The one thing I would be careful about is the process of bringing the patient into/out of the scanner. If you are using a 60cm bore, the hardware itself could hit the side of the scanner and be displaced. Additionally, induced currents in the ex-fix hardware could produce small forces on it as the patient is moved into/out of the scanner, but this shouldn't be a problem if they are moved into the bore slowly on the table." onlinelibrary.wiley.com/doi/epdf/10.1002/mrm.1910070302
sci-hub.se/https://doi.org/10.1002/mrm.1910070302

Hi OCAD friends,

Has anyone done MRI of external fixators, example below ? We’d love to hear from your experience.

Thanks
Bao

On Mon, Sep 13, 2021

“MRI compatible” external fixator devices

http://www.strykermeded.com/media/1599/hoffmann-ii-mri-wallchart-for-radiologists.pdf