Introduction

Radiofrequency (RF) tissue heating around medical implants is a prominent safety risk, restricting accessibility to MRI^1,2. Currently, there are no MR-conditional epicardial cardiac implantable electronic devices (CIEDs), which disproportionally affects the pediatric population who are the main recipients of epicardial CIEDs. Current methods for predicting RF heating of active devices outlined in ISO/TS 10974, particularly the transfer function (TF) method, do not specify the approach for bipolar epicardial leads which include lead bifurcation³. Recent studies have developed TF methods for parallel wires, but bifurcating leads have yet to be studied^4,5. Here, we present the first successful application of the TF concept for evaluating RF heating of a CIED with a bipolar epicardial lead.

Methods

The TF concept was used to model a 35-cm bipolar epicardial lead (Medtronic CapSure® EPI 4968) connected to a Medtronic Azure™ XT DR MRI SureScan implantable pulse generator (IPG). We measured, calibrated, and validated the TFs following Clause 8 of ISO/TS 10974³ and determined them based on the reciprocity method⁶. The transmit probe was constructed using a rg402 semirigid coaxial cable to create a monopole antenna that excited the electrode (Figure 1A)⁷. The receive probe was constructed from a rg402 semirigid coaxial cable wound around a 4C65 ferrite torus. The probes were connected to a two-port vector network analyzer (VNA 5063, Keysight Technologies) where the complex S-parameter S₂₁ was measured at 5 mm intervals along the lead while submerged in a boxed phantom filled with saline (σ=0.5 S/m, 𝜀_r=80).

Cumulative transfer function: During excitation of the cathode, the cumulative TF, TF_C,total, was measured by adding two terms. First, the transfer function TF_C was measured as S₂₁ as the receive probe moved along the lead from the proximal end to the branch leading to the cathode (Figure 1B). Then, the coupling effect due to the presence of the anodal branch was measured as S₂₁ when the receive probe moved along the anodal branch while still exciting the cathode (TF_CA). The total temperature increase (ΔT) at the cathode was estimated using the following equation:
$$\triangle T_{C,total} = C \vert \int_{0}^{z=L} TF_{C}(z) E_{tan,C}(z)dz + \int_{z_{1, bifurcation}}^{z_{1}=L} TF_{CA}(z_{1}) E_{tan,CA}(z_{1})dz_{1}\mid^{2}$$

To assess the effect of coupling, this process was repeated three times with different separation distances between the two electrodes: 35, 65, and 90 mm (Figure 1C). The reciprocal was performed during excitation of the anode resulting in 6 total TFs.

Validation: To calibrate and validate the cumulative TFs, we performed experiments with the CIED in a phantom filled with polyacrylic acid (PAA) gel representing average tissue (σ=0.50 S/m, 𝜀_r=88.96). The CIED lead was routed along 24 different configurations amounting to 48 total trajectories (Figure 2). The ΔT was measured in the PAA around the electrodes using fiber optic probes (Osensa) (Figure 3A-C). All experiments were performed in a 1.5T Siemens Aera MRI scanner with RF exposure from a T1-TSE sequence (TE/TR=7.3/897 ms, TA=280 s, FA=133^o, B₁⁺_rms=4.1µT). Electromagnetic simulations (ANSYS Electronic Desktop 2021 R1 HFSS) were performed to determine the incident electric field distribution inside the phantom (Figure 3D). The complex tangential component of the incident electric field (E_tan) was calculated along each trajectory and its corresponding opposing branch. For each TF, the calibration factor, C, was calculated based on the temperature measurement of one arbitrary trajectory. Following calibration, we applied the calibrated TFs to predict RF heating for the remaining trajectories.

Results

Figure 4 shows the magnitude and phase of the TFs. Figure 5 shows the measured and TF-predicted ΔT at the anode and cathode. The cumulative TFs demonstrated high fidelity in predicting ΔT at both the anode and cathode across separation distances (correlation coefficient, r=0.90-0.97, p<0.05 for all cases). Interestingly, the anodal and cathodal TFs were asymmetrical, indicating the need to measure at least two cumulative TFs to predict RF heating at both electrode ends of a bipolar lead.

Discussion

We presented the first results of measured, calibrated, and validated cumulative TFs for predicting RF heating of a CIED with a bipolar epicardial lead. Our results show that the TFs are asymmetrical for predicting RF heating at the anode versus the cathode. There is also a coupling effect between the conductor branches which necessitates the cumulative TF model as the induced current along one branch may be coupled to the other branch as previous studies have suggested^4,5. Our results demonstrate good agreement between the experimentally measured and the TF-predicted ΔT across 48 trajectories. Future work includes further investigation of the effect of other separation distances and applying the TFs for predicting RF heating of clinically relevant trajectories in heterogeneous human body models.

Acknowledgements

This work was supported by NIH grant R01EB034377.