Purpose: To compare magnetic displacement forces on 3T and 7T scanners for a range of implant devices at on-axis locations where the forces are greatest, and to extrapolate forces to the off-axis locations given precise knowledge of the magnetic fields of the scanners.
Methods: The displacement force for ten devices was measured on 3T (Prisma, Siemens) and 7T (Terra, Siemens) MRI scanners using the standardized ASTM set-up. Devices were suspended from a string of negligible weight. Deflection angles were measured on-axis at: (a) entrances to 3T and 7T bores, and (b) position of the maximum spatial field gradient (SFG). A 3-axis teslameter was used to map the static and SFG magnetic fields to determine the maximum values.
Results: Three devices tested herein deflected <45° at 3T, but at 7T deflection angles >45° were found, and hence these devices could be unsafe at 7T. Deflection angles were also found to be larger when extrapolated to the location of maximum SFG off-axis at 7T, and hence measurements made on-axis following the ASTM methodology could mislabel a device as having no dangerous displacement force if the 45° threshold lay between these on- and off-axis values.
Conclusion: Accurately measuring the deflection angles at these off-axis locations is extremely difficult and prone to substantial errors, but a knowledge of the field profile within the bore allows for extrapolation to worst-case locations. The MR community would benefit if more detailed field maps were made available by manufacturers. This would be useful in situations where a risk-benefit analysis for a patient were to make a difference to a scan/no scan scenario based on limiting exposure of the implanted device to a particular SFG.