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  1. Letters to Health Care Providers

Update Regarding Risk of Tissue Overheating Due to Inaccurate Magnetic Resonance Thermometry

November 8, 2018

The FDA is providing additional recommendations to help protect patient safety and mitigate the risks of tissue overheating because of potentially inaccurate magnetic resonance (MR) thermometry readings displayed during the use of magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) devices. In our April 2018 letter to health care providers, the FDA provided preliminary information about possible risks of tissue overheating and underestimation of thermal damage associated with various MR thermometry parameters such as voxel size (measurement of the image resolution or detail) and MR image acquisition time.

The FDA continues to work with the two device manufacturers that market MRgLITT devices: Medtronic with its Visualase Thermal Therapy System and Monteris Medical with its NeuroBlate System, to encourage safe use of MR thermometry. Both manufacturers have provided updated information with suggested procedural techniques to reduce unintended thermal damage for health care providers to consider.

Based on our latest analysis, the FDA is summarizing recommendations from the manufacturers as well as reemphasizing recommendations from our April communication to help mitigate the risks of potential tissue overheating.


The FDA recommends that health care providers be aware and review the new information from the MRgLITT device manufacturers:

  • In response to Medtronic's recall related to MR thermometry, Medtronic issued three advisory letters dated June 1, June 16, and August 21, to provide health care providers with detailed mitigation strategies. Contact Medtronic with any questions.
  • The FDA reviewed and cleared a premarket submission for a redesigned NeuroBlate System from Monteris Medical that includes updated labeling regarding the use of MR thermometry to aid in predicting thermal damage. Contact Monteris Medical for information about availability of the redesigned system or other questions.

The FDA continues to reiterate our earlier recommendations from April 2018 and is providing a *new recommendation in the table below:

Actions During Procedure Medtronic Visualase Thermal Therapy System Monteris Medical NeuroBlate System
Set the peak temperature to mitigate risk of tissue overheating Keep the temperature below 90 degrees Celsius right outside(or immediately adjacent to) the laser fiber's image artifact. Keep the temperature below 85 degrees Celsius at 4-6 millimeters (3rd voxel) away from the fiber surface.
Mitigate the risk of thermal damage to critical structures Set the low temperature targets on nearby critical structures to 43 degrees Celsius or less. Follow the outer perimeter or isothermal contour line at the Cumulative Equivalent Minutes at 43 degrees Celsius (CEM43) =2 minutes or less.
Minimize the difference between the actual and predicted thermal damage

Consider heating the target tissue slowly to reduce the potential for inaccurate MR thermometry readings. Additionally, heating the tissue slowly may lessen unanticipated thermal spread.

*Keep the cooling system running throughout thermal monitoring, including when the laser is on and after it is shut off, to bring the tissue next to the fiber back to baseline temperature within 120 seconds post laser delivery.

The FDA will keep the public informed as any significant new information or recommendations become available.

Additional Resources

Previous Letter

4/24/2018 Risk of Tissue Overheating Due to Inaccurate Magnetic Resonance Thermometry: Letter to Health Care Providers

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