7T MR Safety

Abstract

Magnetic resonance imaging and spectroscopy (MRI/MRS) at 7T represents an exciting advance in MR technology, with intriguing possibilities to enhance image spatial, spectral, and contrast resolution. To ensure the safe use of this technology while still harnessing its potential, clinical staff and researchers need to be cognizant of some safety concerns arising from the increased magnetic field strength and higher Larmor frequency. The higher static magnetic fields give rise to enhanced transient bioeffects and an increased risk of adverse incidents related to electrically conductive implants. Many technical challenges remain and the continuing rapid pace of development of 7T MRI/MRS is likely to present further challenges to ensuring safety of this technology in the years ahead. The recent regulatory clearance for clinical diagnostic imaging at 7T will likely increase the installed base of 7T systems, particularly in hospital environments with little prior ultrahigh-field MR experience. Informed risk/benefit analyses will be required, particularly where implant manufacturer-published 7T safety guidelines for implants are unavailable. On behalf of the International Society for Magnetic Resonance in Medicine, the aim of this article is to provide a reference document to assist institutions developing local institutional policies and procedures that are specific to the safe operation of 7T MRI/MRS. Details of current 7T technology and the physics underpinning its functionality are reviewed, with the aim of supporting efforts to expand the use of 7T MRI/MRS in both research and clinical environments. Current gaps in knowledge are also identified, where additional research and development are required. Level of Evidence 5 Technical Efficacy 2 J. MAGN. RESON. IMAGING 2021;53:333-346.

Keywords: 7Tesla MRI/MRS safety; SAR; implants; numerical simulations; parallel transmission; radiofrequency coils.

FIGURE 1:

Comparison of the magnetic fringe field for actively-shielded 3T (Prisma, Siemens Medical Systems, Erlangen, Germany) and 7T (Terra, Siemens Medical Systems) systems, demonstrating the similar spatial extent of the static field and spatial field gradients in the area outside of the magnet bore.

FIGURE 2:

Schematic illustration of basic RF coil structures and instantaneous field patterns for fundamental coil structures when empty: (a) birdcage coil, (b) loop coil, and (c) dipole antenna. Black lines indicate conductor paths, blue lines indicate the direction of the B₁ field, and red lines indicate the direction of the electric field. The field pattern of the birdcage coil rotates about the coil’s central axis while that of the loop and dipole would oscillate at the Larmor frequency.

FIGURE 3:

Simulated representations of instantaneous field patterns for fundamental coil structures when loaded with a subject model. (a) Sagittal electric field pattern for birdcage coil. (b) Coronal electric field pattern for loop coil. (c) Sagittal electric field pattern for dipole antenna. (d) Transverse B₁-field pattern for birdcage coil. (e) Transverse B₁-field pattern for loop coil. (f) Transverse B₁-field pattern for dipole antenna.

FIGURE 4:

Commercially available RF coils for UHF MRI. (a) 2-channel birdcage transmit/receive coil with 32-channel receive array (Nova Medical, Wilmington, MA). (b) Birdcage transmit/receive wrist coil (Rapid Biomedical, Rimpar, Germany). (c) 16-channel transmit/receive cardiac loop coil array (MRI.Tools, Berlin, Germany)

FIGURE 5:

Illustration of three simulated B₁⁺ field and corresponding 10-g averaged local SAR distributions for an 8-channel transmit/receive head coil array using Finite Difference Time Domain (Sim4Life, ZMT, Switzerland). The phase settings of the individual drive channels determine the resulting B₁⁺ field patterns; shown here are three distinct distributions resulting from a quadrature drive and two particular RF shim settings, although many more can be realized. The black squares indicate the focal region where B₁⁺ fields are maximized. The corresponding local SAR distributions were calculated for a 1% duty cycle. RF phase settings can be used to steer the B₁⁺ field distributions, but this also influences the local SAR distribution and therefore the potential level of tissue heating.