Comments on "Ensuring safety of implanted devices under MRI using reversed polarization".

In the work titled ‘‘Ensuring Safety of Implanted Devices Under MRI Using Reversed RF Polarization,’’ the authors propose a method based on reversed polarization, which can be used as a prescan technique for patients with implants. In their work, to visualize qualitatively the induced radiofrequency (RF) currents on a metallic lead inside the patient, they propose to reverse the RF polarization for the transmit and/or the receive procedure. They performed simulations and experiments to investigate the performance of this technique and correctly identified the dangerous coupling currents that may cause risk to the patients. They formulated the RF field that is generated by an induced current on a straight wire, and they calculated its effect on the image intensity in both the forward and the reversed polarization cases. They compared these analytical test results with the experiments and showed that the results accurately match. They claim that the coupling currents can be visualized with better sensitivity when reversed polarization is used. According to the authors, the transmit polarization reversal provides better background suppression, making it easier to distinguish signal variation in the vicinity of the lead from variations due to anatomy. With polarization reversal, they claim that better wire-signal accuracy is provided. Although using reversed polarization to visualize the coupling currents is an innovative idea and could enhance the safety of patients with implants who are undergoing MRI, with this letter we would like to note that there may be certain situations where the proposed method may not work efficiently to accurately visualize the induced currents. In these specific cases, the RF safety of the patient could still be under question. In the Discussion section of the paper (p. 831), the authors state that using reversed polarization in transmit mode exposes the wire to an electric field distribution that is not necessarily equivalent to the forward case with asymmetrical loads. The authors also state that this problem should be investigated further. For the implant modeled as a spherical perfect electric conductor with a helical bare wire, shown in Fig. 1, the tangential electric field on the wire has a constant magnitude and a linear phase variation (1). In Fig. 2, the phase of the incident tangential electric field on the wire is plotted for the forward and the reversed excitations. The results are obtained from EM simulations made by using the commercial software FEKO (EM Software & Systems-S.A, Stellenbosch, South Africa). Although the magnitudes of the electric fields are the same, the phase variations are different for the two different excitation methods. An asymmetrical current distribution is obtained on the wire with the forward and the reversed excitations, as shown in Fig. 3. The induced current near the free end of the wire for the reversed excitation is significantly less than the induced current obtained with the forward excitation, as can be seen in the figure. Note that the local specific absorption rate (SAR) at the tip of the wire is proportional to the square of the tip current, as mentioned in the work titled ‘‘Ensuring Safety of Implanted Devices Under MRI Using Reversed RF Polarization.’’ Because of these results, a 8.5-times higher SAR is expected at the tip due to the forward excitation, as compared to the reversed excitation. To verify these results experimentally, we made phantom experiments with the helical copper wire, as shown in Fig. 1. The phantom was prepared with commercially available gel (Dr. Oetker Jello, Izmir, Turkey).