Electric field measurements in preclinical MRI at 11.7 T and 7 T for experimental SAR comparison

SAR assessment is a major concern in MRI. The energy absorbed by tissues increases quadratically with the static magnetic field; therefore, ultra-high field (>= 7 T) systems require careful dosimetry to exploit their potential. The objectives are to validate the use of electric-field probe for SAR assessment for high-field MRI, and to study the advantages and drawbacks of E-field measurements. The experiments were performed at 7 and 11.7 T on preclinical systems in a phantom with calibrated dielectric properties. Absolute values of the E-field were measured according to position inside a birdcage coil and electrical conductivity, local temperature increase were simultaneously evaluated with operating RF frequency, as well as the re-positioning precision through five repetitive measurements. Results yielded a 14.8 +/- 0.36 W/kg SAR near the coil's capacitors compared to 6.8 +/- 0.17 W/kg estimated at the center of the coil. The temperature rise was nevertheless higher in the center likely due to heat transfer effects. The SAR measured in similar conditions was 5.2 times higher at 11.7 T than at 7 T. The probes induced no visible artefact, and the test to estimate the reproducibility of positioning the sensors granted a low 2.3 % coefficient of variation. Measuring both the cause (E-field) and the effect (temperature rise) yielded different information, both useful in the context of EM simulation validation.