Electro‐optic probe for real‐time assessments of RF electric field produced in an MRI scanner: Feasibility tests at 3 and 4.7 T

Abstract

During magnetic resonance imaging (MRI) examinations, the average specific absorption rate (SAR) of the whole body is calculated as an index of global energy deposition in biological tissue without taking into account the presence of metallic implants or conductive materials. However, this global SAR calculation is not sufficient to ensure patient safety and a local SAR measurement should be carried out. Several measurement techniques have already been used to evaluate the local SAR, in particular electric field (E-field) probes, but the accuracy of the measurements and the resolutions (spatial and temporal) depend strongly on the measurement method/probe. This work presents an MR-compatible, subcentimeter probe based on an electro-optic (EO) principle enabling a real-time measurement of the local E-field during MRI scans. The experiments using these probes were performed on two different MR systems (preclinical and clinical) having different static magnetic field strengths and with different volume coil geometries. The E-field was measured with unloaded (in air) and loaded volume coils in order to assess the sensing characteristics of the optical probe. The results show an excellent linearity between the measured E-field and the radiofrequency (RF) magnetic field in both experimental conditions. Moreover, the distribution of the E-field throughout the volume coil was experimentally determined and was in good agreement with numerical simulations. Finally, we demonstrate through our measurements that the E-field depends strongly on the dielectric properties of the medium.

A simple head‐sized phantom for realistic static and radiofrequency characterization at high fields

“…Advantages of the demonstrated RF validation procedure compared to other approaches such as using near field probes include the simplicity of the experiment as well as the realistic experimental conditions to which the RF coil is exposed. Methods using near‐field probes typically require a mechanical positioning system to scan the interior of the phantom, which is difficult to realize within the MR scanner and in ellipsoidal phantom designs.…”

Parallel transmission medical implant safety testbed: Real‐time mitigation of RF induced tip heating using time‐domain E‐field sensors

“…The optical receiver converts the optical back to an RF signal that is fed to one channel of the receiver card for digitization. E‐field probes were successfully implemented previously in validation and implant safety related experiments 42‐45 . The H ‐field sensor operates similarly using a loop instead of a dipole.…”

“…E-field probes were successfully implemented previously in validation and implant safety related experiments. [42][43][44][45] The H-field sensor operates similarly using a loop instead of a dipole. A 90° rotation of the H-field sensor is needed to measure H x and H y and calculate B + 1 .…”

Valid Exposure Protocols Needed in Magnetic Resonance Imaging Genotoxic Research

“…For more detailed information, see, for instance, Kim and Kim [2017], Frankel et al [2018], andMcRobbie et al [2017]. There are also electric fields present in the MRI environment [Kangarlu et al, 2007;Park et al, 2015;Saniour et al, 2018]. These are not well characterized today, but definitely need more attention.…”