Metallic implants/prosthesis – are they MRI safe?

28th October 2018, Dr Chee L Khoo

Metallic objects, implants or prosthesis can render magnetic resonance imaging (MRI) unsafe or affect the quality of the images. Our patients who have metallic joint prosthesis in-situ not uncommonly require further imaging. This can pose a bit of a challenge as many of the same patient population who require the joint replacement are the same population of patient that require further imaging. For example, a patient who broke their neck of femur and underwent total hip replacement now has severe sciatica. That happened to one of my younger patient recently and I pondered over whether we could organise an MRI of her lumbo-sacral spine.

Safety

Ferromagnetic objects can experience strong forces that originate from the static magnetic field. The forces are strongest in regions near the magnet where the field strength changes rapidly over a small distance. Unfortunately, the distance over which forces can change from negligible to strong enough to project an object can be as small as tens of centimetres. Further, some metals can heat up when subjected to magnetic forces. The most common example is guide wires.

Image distortion

The presence of metallic objects can cause substantial artifacts in images including signal loss, failure of fat suppression, geometric distortion and signal pile-up. Signal loss can largely be corrected by the use of spin echoes, while some distortion can be reduced by the choice of scan parameters. Displacement artifacts that cause geometric distortion, signal loss and pile-up can be corrected with a variety of methods. These tricks are pretty much used by most radiology practices and are available on most scanners.

Our friends at Spectrum Medical Imaging are experts with their MRI machines. I rang them to double check whether my patient was suitable for an MRI of her spine. Her injuries were work related and with just one phone call, not only were they able to reassure me that the MRI is safe and the images will not be distorted. They then handled the rest of the process with the insurer.

Spectrum Medical Imaging have produced a very easy to read reference guide here on what is safe and what is not.

Reference

Brian Hargreaves, Pauline W. Worters, Kim Butts Pauly et al. Metal Induced Artifacts in MRI. AJR Am J Roentgenol. 2011 September ; 197(3): 547–555. doi:10.2214/AJR.11.7364.