Safety for Human MR Scanners at 7T

Abstract

After introduction of the first human 7 tesla (7T) system in 1999, 7T MR systems have been employed as one of the most advanced platforms for human MR research for more than 20 years. Currently, two 7T MR models are approved for clinical use in the U.S.A. The approval facilitated introduction of the 7T system, summing up to around 100 worldwide. The approval in Japan is much awaited. As a clinical MR scanner, the 7T MR system is drawing attention in terms of safety.Several large-sized studies on bioeffects have been reported for vertigo, dizziness, motion disturbances, nausea, and others. Such effects might also be found in MR workers and researchers. Frequency and severity of reported bioeffects will be presented and discussed, including their variances. The high resonance frequency and shorter RF wavelength of 7T increase the concern about the safety. Homogeneous RF pulse excitation is difficult even for the brain, and a multi-channel parallel transmit (pTx) system is considered mandatory. However, pTx may create a hot spot, which makes the estimation of specific absorption rate (SAR) to be difficult. The stronger magnetic field of 7T causes a large force of displacement and heating on metallic implants or devices, and the scan of patients with them should not be conducted at 7T. However, there are some opinions that such patients might be scanned even at 7T, if certain criteria are met. This article provides a brief review on the effect of the static magnetic field on humans (MR subjects, workers, and researchers) and neurons, in addition to scan sound, SAR, and metal implants and devices. Understanding and avoiding adverse effects will contribute to the reduction in safety risks and the prevention of incidents.

7‐Tesla Magnetic Resonance Imaging Scanning in Deep Brain Stimulation for Parkinson's Disease: Improving Visualization of the Dorsolateral Subthalamic Nucleus

“…Studies have reported a possible increase in bio‐effects and adverse events related to radiofrequency‐induced tissue heating and metallic implants compared to 3 T MR‐scans 19,20 . High field related side effects were not reported in this study (dizziness, motion disturbances, nausea) 19,20 .…”

“…Studies have reported a possible increase in bio‐effects and adverse events related to radiofrequency‐induced tissue heating and metallic implants compared to 3 T MR‐scans 19,20 . High field related side effects were not reported in this study (dizziness, motion disturbances, nausea) 19,20 . In the 450 7 T MR‐scans we have performed to date, we rarely encountered non‐compatible implants (all patients undergo safety screening with a questionnaire and magnetometer) or severe side effects.…”

“…19,20 High field related side effects were not reported in this study (dizziness, motion disturbances, nausea). 19,20 In the 450 7 T MR-scans we have performed to date, we rarely encountered non-compatible implants (all patients undergo safety screening with a questionnaire and magnetometer) or severe side effects. We are convinced that 7 T MRI in DBS will enable a safe, impactful and long-term line of clinical application and research.…”

Application of 7T MRS to High-Grade Gliomas

“…159 Although there is growing literature on the safety of surgical implants at higher field strengths, [160][161][162][163] there are not yet any definitive guidelines for imaging patients with nonmagnetic standard titanium plates placed during HGG resections, which has limited the clinical integration of 7T MRS thus far. 164 It is important to perform additional testing and develop explicit guidance on whether surgical implants are safe at higher fields. 165 The increased use of 7T MRS to scan patients in this category would allow longitudinal evaluations of patients with HGG at different phases of treatment, including postoperative follow-ups during radiation therapy, which have largely been confined to lower field strengths but ought to be expanded more widely to higher field strengths.…”

Neuroimaging at 7 Tesla: a pictorial narrative review

“…This review highlights the advantages of 7T neuroimaging for anatomical visualization, presents a brief review of 7T neurochemical measurements and technical issues, and summarizes the principal difficulties of 7T imaging. We recommend other reviews for information on 7T-related safety issues (13)(14)(15)(16)(17)(18).…”