Design and calibration of electric field probes in the range 10 - 120 MHz

Abstract

In view of potential thermal hazards, there is a need to determine the specific absorption rate (SAR) distributions associated with radiofrequency coils used in magnetic resonance imaging (MRI) (typically 10-120 MHz). Electric field (E-field) distributions in tissue-equivalent phantoms may be determined using a probe comprising a dipole antenna and a detector. The geometry of the dipole dictates the sensitivity of the device, thus two designs are discussed in this paper. Both probes are compact, have a spatial resolution of 2.5 cm3, operate at MR frequencies and have a response independent of the dielectric characteristics of the phantom material. Calibration of these probes requires a system capable of producing a known E-field both in air and in a tissue-like medium at frequencies between 10 and 120 MHz. Transverse electromagnetic wave (TEM) cells answering these specifications are described and the calibration procedure outlined. Accurately calibrated E-field probes can make field measurements in phantoms which can be used to verify predictions from numerical models. These numerical techniques may then be used to predict E-fields, and hence SAR, in patients.

Electromagnetic and thermal modeling of SAR and temperature fields in tissue due to an RF decoupling coil

“…Results (E-field magnitude, SAR) using these standardized models were compared with other FDTD codes and other computational methods as part of the European Cooperation in the field of Scientific and Technical Research (COST) programme (31,32). Verification of the code for applications in clinical hyperthermia has also been made (33) and recently we have further verified the use of this code by comparing numerically predicted E-fields due to a rectangular coil in a phantom experiment with those measured using a calibrated minimally perturbing E-field probe in an identical experiment (24,34).…”

Electric field measurements and computational modeling at ultrahigh-field MRI

“…This makes the task of quantification of RF heating (SARs and temperature rises) and its correlation with the electric field more difficult as precise measurement techniques, and rigorous [16] computational methods are required for their evaluation. As MRI RF induced SARs and power deposition in tissues have been reported by many authors [9,[17][18][19][20] using computational electromagnetics, these calculations/simulations along with electric field [16,21,22] and RF heating measurements [23,24] have confirmed temperature changes in biological samples during MRI scans. In this context, the fluorooptic techniques, for instance, have shown that they are capable of relatively accurate temperature measurements [25].…”

“…1. A dipole antenna based probe [22] was built and optimized for measuring the E 1 field intensities at 340 MHz. Using this E 1 field probe, one Cartesian component of the E 1 field distribution was mapped inside the cylindrical phantom loaded in the coil and inside the empty coil.…”

“…In this work, a probe developed by Taylor et al [21,22] was built and used to measure the transverse electric field (E 1 field) distributions within an empty 8-T (340 MHz) RF head coil and within a saline water phantom loaded in the same coil. Simulations of the same coil and loadings were also performed using an in-house finite difference time domain (FDTD) method [15,26,27].…”

Accuracy of electrical field measurement using the flexible Schottky diode sheet at 433 MHz

“…Although electrical field probes have been studied over a long period, only one group reported on the sensitivity of their diode electrical field probe in a 'biological' equivalent tissue. Taylor et al 8 calibrated their E-field probe in a water filled TEM cell. They used a rectangular dipole probe with a BAT-17 surface mount diode and tested it in the frequency range of 10-120 MHz using field strengths from 10-120 V/ m. From the values reported, a sensitivity of 0.54 (V/m)/(V/m) could be calculated which was more or less independent of the frequency in the range of 20-120 MHz.…”