Jacobs, Paul

AbstractBackgroundThe imaging of patients with implanted electrically‐conductive devices via magnetic resonance imaging at ultra‐high fields is hampered by uncertainties relating to the potential for inducing tissue heating adjacent to the implant due to coupling of energy from the incident electromagnetic field into the implant. Existing data in the peer‐reviewed literature of comparisons across field strengths of tissue heating and its surrogate, the specific absorption rate (SAR), is scarce and contradictory, leading to further doubts pertaining to the safety of imaging patients with such devices.PurposeThe radiofrequency‐induced SAR adjacent to orthopedic screws of varying length and at frequencies of 64 to 498 MHz was investigated via full‐wave electromagnetic simulations, to provide an accurate comparison of SAR across MRI field strengths.MethodsDipole antennas were used for RF transmission to achieve a uniform electric field tangential to the screws located 120 mm above the antenna midpoints, embedded in a bone‐mimicking material. The input power to the antennas was constrained to achieve the following targets without the screw present: (i) E = 100 V/m, (ii) B1+ = 2 μT, and (iii) global‐average‐SAR = 3.2 W/kg. Simulations were performed with a spatial resolution of 0.2 mm in the volume surrounding the screws, resulting in 76–137 MCells, noting the maximum 1 g‐averaged SAR value in each case. Simulations were repeated at 128 and 297 MHz for screws embedded in muscle tissue.ResultsThe peak SAR, occurring at the resonant screw length, substantially increased as the frequency decreased when the input power to the dipole antenna was constrained to achieve constant electric field in background tissue at the screws’ locations. A similar pattern was observed when constraining input power to achieve constant B1+ and global‐average‐SAR. The dielectric properties of the tissue in which the screws were embedded dominated the SAR comparisons between 297 and 128 MHz.ConclusionsThe study design allowed for a direct comparison to be performed of SAR across frequencies and implant lengths without the confounding effect of variable incident electric field. Lower frequencies produced substantially larger SAR values for implants approaching the resonant length for the worst‐case uniform incident electric field along the screws’ length. The data may inform risk‐benefit assessments for imaging patients with orthopedic implants at the new clinical field strength of 7 Tesla.

PurposeA 7T magnetic resonance thermometry (MRT) technique was developed to validate the conversion factor between the system‐measured transmitted radiofrequency (RF) power into a home‐built RF wrist coil with the system‐predicted SAR value. The conversion factor for a new RF coil developed for ultra high magnetic field MRI systems is used to ensure that regulatory limits on RF energy deposition in tissue, specifically the local 10g‐averaged specific absorption rate (SAR10g), are not exceeded. MRT can be used to validate this factor by ensuring that MRT‐measured SAR values do not exceed those predicted by the system.MethodsA 14‐cm diameter high‐pass birdcage RF coil was built to image the wrist at 7T. A high spatial and temporal resolution dual‐echo gradient echo MRT technique, incorporating quasi‐simultaneous RF‐induced heating and temperature change measurements using the proton resonance frequency method, was developed. The technique allowed for high‐temperature resolution measurements (~±0.1°C) to be performed every 20 s over a 4‐min heating period, with high spatial resolution (2.56 mm3 voxel size) and avoiding phase discontinuities arising from severe magnetic susceptibility‐induced B0 inhomogeneities. Magnetic resonance thermometry was performed on a phantom made from polyvinylpyrrolidone to mimic the dielectric properties of muscle tissue at 297.2 MHz. Temperature changes measured with MRT and four fiber optic temperature sensors embedded in the phantom were compared. Electromagnetic simulations of the coil and phantom were developed and validated via comparison of simulated and measured B1+ maps in the phantom. The position of maximum SAR within the coil was determined from simulations, and MRT was performed within a wrist‐sized piece of meat positioned at that SAR hotspot location. MRT‐measured and system‐predicted SAR values for the phantom and meat were compared.ResultsTemperature change measurements from MRT matched closely to those from the fiber optic temperature sensors. The simulations were validated via close correlation between the simulated and MRT‐measured B1+ and SAR maps. Using a coil conversion factor of 2 kg−1, MRT‐measured point‐SAR values did not exceed the system‐predicted SAR10g in either the uniform phantom or in the piece of meat mimicking the wrist located at the SAR hotspot location.ConclusionsA highly accurate MRT technique with high spatial and temporal resolution was developed. This technique can be used to ensure that system‐predicted SAR values are not exceeded in practice, thereby providing independent validation of SAR levels delivered by a newly built RF wrist coil. The MRT technique is readily generalizable to perform safety evaluations for other RF coils at 7T.