As the field strength rises to 7 Tesla and above, this talk will assess the risks of scanning patients with implants and surgeries at ultra-high-field and review what more needs to be done.
In the past decade, an increasing number of MRI research sites have installed MRI scanners operating at a field strength of 7 Tesla or above. With clinical vendors now actively pursuing a clinical market for these scanners, particularly for MSK and neuro applications, questions on the safety of scanning people with implants at ultra-high field is increasing in its importance.
When scanning people with implants, the obvious additional risk is from the increased static magnetic field. Implants that display any degree of ferromagnetism may experience stronger forces that would be experienced by that implant at lower field strength. However, also of concern are the effects of RF heating. The half-wavelength in tissue of 300 MHz RF (the resonant frequency at 7 Tesla) is 5.5 cm, making it much closer to the size of many implants. The higher frequency doesn’t inherently make an implant more likely to heat up, but it does mean that it is not straightforward to interpolate results from lower field strengths.
Much scanning at ultra-high field has currently been done on healthy subjects, or in clinical research studies where the risk-benefit ratio is heavily skewed towards minimising risk. A recent analysis on scanning done in our facility showed how, as the age of the participant rose, the number of potential contraindications for ultra-high field MRI increased, with many of these relating to surgical clips being used where the level of ferromagnetism is unknown.
There are relatively small number of implants that have been tested at 7 Tesla. These include clips, stents, orthopaedic and dental implants. Whilst many show no contra-indication to MRI there are others that demonstrate deflection, torque or heating.
For the future, there will need to be an increase in testing of implants to determine what is safe, and hopefully computer modelling approaches will start to give us confidence in the safety of implants. In addition, when use of ultra-high-field scanners for clinical care becomes well established then it may become acceptable to take some small additional risks as the risk-benefit ratio is different.
Kangarlu, A. & Shellock, F. G. Aneurysm clips: Evaluation of magnetic field interactions with an 8.0 T MR system. J. Magn. Reson. Imaging 12, 107–111 (2000).
Santoro, D. et al. Detailing Radio Frequency Heating Induced by Coronary Stents: A 7.0 Tesla Magnetic Resonance Study. PLoS One 7, (2012).
van Rijn, G. A., Mourik, J. E. M., Teeuwisse, W. M., Luyten, G. P. M. & Webb, A. G. Magnetic resonance compatibility of intraocular lenses measured at 7 Tesla. Investig. Ophthalmol. Vis. Sci. 53, 3449–3453 (2012).
Kraff, O. et al. MR safety assessment of potential RF heating from cranial fixation plates at 7 T. Med. Phys. 40, 42302 (2013).
Sammet, C. L. et al. RF-related heating assessment of extracranial neurosurgical implants at 7T. Magn. Reson. Imaging 31, 1029–1034 (2013).
Dula, A. N., Virostko, J. & Shellock, F. G. Assessment of MRI issues at 7 T for 28 implants and other objects. Am. J. Roentgenol. 202, 401–405 (2014).
Wezel, J., Kooij, B. J. & Webb, A. G. Assessing the MR compatibility of dental retainer wires at 7 tesla. Magn. Reson. Med. 72, 1191–1198 (2014).
Feng, D. X. et al. Evaluation of 39 medical implants at 7.0 T. Br. J. Radiol. 88, 20150633 (2015). Noureddine, Y. et al. Experience with magnetic resonance imaging of human subjects with passive implants and tattoos at 7 T: a retrospective study. Magn. Reson. Mater. Physics, Biol. Med. 28, 577–590 (2015).
Winter, L. et al. On the RF heating of coronary stents at 7.0 Tesla MRI. Magn. Reson. Med. 74, 999–1010 (2015).