Incident electric field on implanted lead vs. source position and field polarization
Elena Lucano1,2, Micaela Liberti2, Gonzalo G Mendoza1, Tom Lloyd3, Francesca Apollonio2, Steve Wedan3, Wolfgang Kainz1, and Leonardo M Angelone1

1Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, Silver Spring, MD, United States, 2Department of Information Engineering, Electronics and Telecommunications, Univerisity of Rome "Sapienza", Rome, Italy, 3Imricor Medical Systems, Burnsville, MN, United States

Synopsis

We aim to generate a quantitative method for RF-safety of patients with partially implanted leads at 64 MHz. Within this aim, the position of the RF feeding sources and the orientation of the polarization is often unknown, as it is the quantitative effect of such variables on the induced currents on the leads. The Electric field profile was studied by means of simulations and measurements with a coil loaded with a phantom, and simulations with an anatomical human model. Changes of up to 40% of E-field magnitude were observed. Future work is needed to develop a systematic exposure procedure.

Purpose

Commercial 1.5T MRI scanners used in clinical applications are typically driven with a dual port excitation for which the position of the feeding sources and the orientation of the polarization (i.e., clockwise CW vs. counterclockwise CCW) with respect to the patient is often unknown. For partially implanted leads (e.g., interventional cardiac ablation catheters), the radiofrequency (RF) induced heating of tissue near the lead tip depends on the magnitude and phase of the tangential electric field component ($$$ \overrightarrow{E}_{tan}$$$) along the lead. This study aims to answer the question on how position of feeding sources and field polarization affect $$$ \overrightarrow{E}_{tan}$$$. The study was based on electromagnetic (EM) measurements and simulations inside an ellipsoidal phantom and on simulations with an anatomical human body model.

Methods

The MITS1.5 for 1.5 Tesla RF Safety Evaluation (ZurichMedTech) was used for the measurements. The system consists of a high-pass 64 MHz birdcage body coil [1]. The coil was driven at two ports (I and Q in Figure1a) physically shifted at 90o. The EM fields were measured with the DASY52NEO robotic measurement system (SPEAG) [2]. The computational model was geometrically equivalent to the physical coil (Figure 1b) and it was implemented with Sim4Life software (ZurichMedTech). Both coil and shield were modeled as perfect electric conductors. Four different positions of the feed points were studied (Figure 1d): starting from the one implemented in the physical coil (“left” position) the pair of sources were turned 90o. Feeding phase was imposed such to have opposite quadrature setup (i.e., 0o-90o CW and 90o-0o CCW polarization). Both the physical and the numerical coil was loaded with a superellipse-shaped phantom (Figure 1b). Three different planes inside the phantom were measured for comparison. The measured and numerical dataset were normalized to the same magnetic field magnitude for a location inside the phantom. Additionally, the coil model and source configuration were used with an anatomical human female model (“Ella” Figure1c [3]). The $$$ \overrightarrow{E}_{tan}$$$ profile was studied along the extraction lines shown in Figures 3a and 4a. The extraction line in the phantom was along a device mount track for evaluation of a partially inserted catheter, whereas in the “Ella” model was based on a typical path of MR-interventional ablation catheters. Data with “Ella” were normalized to a whole body average SAR of 2 W/kg.

Results

Figure 2 shows the electric field magnitude for the physical vs the numerical model in one plane of the phantom. The results with the sources in “left” and same polarization (CW) of the physical coil were closer to measurements with a 15.3 % Symmetric Mean Absolute Percent Error (SMAPE)[4] . When using a CCW polarization the field distribution was mirrored with respect to the central line compared to CW. When comparing CW vs. CCW in the phantom (Figure 3b) the highest difference of $$$\parallel \overrightarrow{E}_{tan}\parallel$$$ was found around the exit point (x=1579 in Figure 3a), with up to 14 % higher $$$\parallel \overrightarrow{E}_{tan}\parallel$$$ in the left and top position. Spikes in the profile were due to the pins of the device mount track made of plexiglass. With respect to the $$$\angle \overrightarrow{E}_{tan}$$$ inside the phantom, the top and right positions showed variations less than 0.5 rad, and the left and bottom positions changed by a mean of 4.5 rad and 3 rad, respectively; conversely, outside the phantom the profile of the phase was the opposite with a change of up to 3.5 rad for top/right and less than 1.3 rad for left/bottom. In the “Ella” model (Figure 4b) differences of up to 40% were observed (e.g., $$$\parallel \overrightarrow{E}_{tan}\parallel$$$ 40 % less with CCW vs. CW near the exit point, x=786 in Figure 4b). Additionally, when changing sources positions the phase had qualitatively comparable trends, but differences in values up to 4.8 rad (CW).

Discussion and Conclusions

Accurate modeling of both the magnetic and electric fields is important when evaluating RF-induced heating medical devices that are partially implanted or have external components in contact with the body. Information about field polarization and source position with respect to the patient undergoing an MRI scan can be essential in the determination of RF safety. Future work is needed to develop a systematic exposure procedure for RF heating evaluations of partially implanted leads in phantoms. These procedures must be able to mimic worst-case exposure in the patient.

The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health and Human Services.

Acknowledgements

This project was supported in part by a Cooperative Research and Development Agreement between Imricor Medical Systems and the Center for Devices and Radiological Health (CDRH), FDA, and in part by the Office of Women’s Health, U.S. FDA.

References

[1] Lucano et al., ISMRM (2014), 4903

[2] SPEAG, Zurich, Switzerland

[3] Christ , et al., The Virtual Family—development of surface-based anatomical models of two adults and two children for dosimetric simulations. Physics in Medicine and Biology, 2010. 55(2): p. N23

[4] J. S. Armstrong, Long-Range Forecasting: From Crystal Ball to Computer: Wiley 1985.

Figures

Figure 1: (a) MITS1.5 physical coil loaded with the superellipse-shaped phantom. (b-c) 3D view of the computational model loaded with phantom and Ella model. (d) schematic representation of feeding sources positions used for the numerical model.

Figure 2 Electric field magnitude measured and simulated for one plane inside the superellipse-shaped phantom. Mean values (µ) are reported below each figure for the field distribution and the SMAPE between measurements and simulations. SMAPE equation is reported in the left side of the figure.

Figure 3: Extraction line for Etan calculation along the mount track of the superellipse-shaped phantom. For the spatial resolution used, the total length of the line was composed of 1776 points. b) Results of Etan for different port positions, magnitude and phase imposing a CW (top) and CCW (bottom) polarization.

Figure 4: Extraction line for Etan calculation inside Ella’s vein starting from the hearth. For the spatial resolution used, the total length of the line was composed of 1157 points. b) Results of Etan for different port positions, magnitude and phase imposing a CW (top) and CCW (bottom) polarization.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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