RF-related heating assessment of extracranial neurosurgical implants at 7T

Purpose: The purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neurosurgical implants in 7-T magnetic resonance imaging (MRI).

Materials and methods: Experiments were performed using a 7-T MR system equipped with a transmit/receive RF head coil. Four commonly used titanium neurosurgical implants were studied using a test procedure adapted from the American Society for Testing and Materials Standard F2182-11a. Implants (n=4) were tested with an MRI turbo spin echo pulse sequence designed to achieve maximum RF exposure [specific absorption rate (SAR) level=9.9W/kg], which was further validated by performing calorimetry. Maximum temperature increases near each implant's surface were measured using fiberoptic temperature probes in a gelled-saline-filled phantom that mimicked the conductive properties of soft tissue. Measurement results were compared to literature data for patient safety.

Results: The highest achievable phantom averaged SAR was determined by calorimetry to be 2.0±0.1W/kg due to the highly conservative SAR estimation model used by this 7-T MR system. The maximum temperature increase at this SAR level was below 1.0°C for all extracranial neurosurgical implants that underwent testing.

Conclusion: The findings indicated that RF-related heating under the conditions used in this investigation is not a significant safety concern for patients with the particular extracranial neurosurgical implants evaluated in this study.