Syllabus

The FDA regulates several aspects of MRI in the US: nerve stimulation, B0 field, acoustic noise, and SAR from RF; of these only PNS has no inherent relationship to B0. In this talk I will touch on eight aspects of SHF MR safety, based in part on our work at 10.5T: 1) Results of cognitive, vestibular, and physiological testing in humans at 10.5T; 2) Pre-clinical results of mouse exposure to 16.4T; 3) Validation of electromagnetic RF coil models; 4) Acoustic noise measurements; 5) B0 projectile risks; 6) Implants at UHF; 7) Occupational exposure; 8) Practical considerations.

Human subjects at 10.5T
Our 10.5T operates under an investigational device exemption (IDE) from the FDA. Our initial protocol enrolled 40 subjects in a two-part study; 26 subjects completed the protocol which included two magnet visits and extensive cognitive, vestibular, and physiological measurements before, during, and after each magnet exposure. Initial results (1) showed that cognitive performance was not compromised at isocenter, subjects experienced increased eye movements at isocenter, and small changes in vital signs were observed but there was no field-induced increase in blood pressure. Based on these results, we have revised our protocol to allow continued surveillance while increasing researcher access to our volunteer pool.

Mouse exposure to 16.4T
Tkac, et al. (2) report on the neurochemical and behavioral effects of chronic exposure to 16.4T in mice. Mice were at 16.4t for 3 hours twice per week for 4 or 8 weeks. Hippocampal neurochemical profile, cognitive, and basic motor functions were not impaired from the exposure, but they did observe changes in motor coordination and balance, especially a tight circling movement pattern in the Morris water maze. Some changes persisted for weeks after exposure to 16.4T but not to 10.5T. While mice do not translate directly to humans, this work suggests that vestibular testing will be critical as we develop >10.5T MR systems for human use.

Coil validation
Increasing B0 does not inherently lead to significant risk with respect to specific absorption rate (SAR); i.e., at 10.5T we are within the non-significant risk SAR limits for normal or first level mode for body and head studies. The shorter wavelength can lead to greater concern with localized hot spots and may be subject to increasing scrutiny by local or federal regulatory bodies. We have pursued numerous methods of validating electromagnetic simulations of our RF coils, including MR thermometry (3) and fiber optic heating studies (4). An overview of one possible safety testing process is given in Hoffmann, et al. (5).

Acoustic noise measurements
Increasing B0 means increasing mechanical forces on the gradient coil which increases acoustic noise. We have measured peak loudness and A-weighted noise level for a large variety of acquisitions at 3T, 7T, and 10.5T, including EPI and DTI with various resolutions and orientations. We have found 7T is about 5dB louder than 3T, and 10.5T is 10dB louder than 7T for body gradient coils. They are all well within FDA regulations for acoustic noise exposure, but head gradient measurements on our 7T system suggest head gradients at 10.5T should be carefully characterized for acoustic noise exposure.

B0 projectile risks
Naturally the higher B0 field brings increased risk of projectiles, and with magnetic objects the force and acceleration will be higher and potentially more destructive. Some types of stainless steel are magnetic, and even the non-magnetic alloys can become slightly magnetic if they are worked without a careful annealing process afterward. Items that appear non-magnetic at 7T have proven to be ferrous at 10.5T. In addition, custom-built SHF magnets may be passively shielded, increasing the extent of the fringe field into nearby portions of the building.

Implants at UHF
Very few medical implants have safety information for even 7T safety, for example, see Fagan, et al. (6) or ACR (7). We are compiling our implant approval data from the past 6 years for our 3T and 7T systems.

Occupational exposure
Occupational exposure by research or medical staff is a concern even at 7T, with some studies showing negative impacts on cognitive or manual control (8-10) and postural sway (11). For SHF research magnets, the exposure to research staff is non-negligible. Anecdotal reports of occupational exposure at 10.5T span the range of those experienced at our 7Ts, with some researchers feeling no ill effects, while others report dizziness, fatigue, nausea, metallic taste, phosphenes, or headache, all of which are transient.

Practical considerations
The stronger fringe field and occupational effects do create their own safety concerns for working around the magnet, especially when ladder work is involved and dizziness could be an issue. We require any new equipment or tool to be tested for projectile risk by our hardware/engineering group. Lenz’s forces are quite strong, which can also pose problems simply maneuvering non-magnetic equipment in the fringe field.

Acknowledgements

I thank Greg Metzger and Yigitcan Eryaman for helpful discussions.