7T MR Thermometry technique for validation of system-predicted SAR with a home-built radiofrequency wrist coil

A 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 (SAR_10g ), 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.

A 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.

Temperature 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 SAR_10g in either the uniform phantom or in the piece of meat mimicking the wrist located at the SAR hotspot location.

A 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.