## 4 Discussion  
Based on ISO/TS 10974:2018, an assessment of the enhanced tissue heating due to coupling between leads of close proximity should be performed, but a specific method for this process is not defined. In this study, we performed numerical simulations to estimate the effect of the coupling between two leads on the absorbed power using two identical generic implants of uncapped wires at different distances and tip position offsets. In particular, the full-wave simulations for different configurations of the wire pairs resulted in lower values of the power deposited at the tips compared to the case of the single wire. Moreover, the induced power became lower as the distance between the leads became shorter, and the deviation from the single wire case could become higher than 3 dB. Furthermore, considering the standard Tier 3 process for the assessment of the deposited power at the tips of the coupled implants, using the TF of each wire to assess the deposited power separately resulted in a more than 6 dB overestimation. It should be noted that the findings of this study, with regard to the low induced power at the tips of the pair of generic implants compared to the single implant, are not representative of all positioning configurations and types of implants. In the study in [44], it was also observed that the coupling between parallel, closely spaced leads had lower induced heating compared to a single lead, and the smaller spacing between the leads resulted in lower lead tip heating. On the other hand, other studies indicated that the  
Figure 11: The SAR distribution at the wires placed along realistic paths. Figure 10: The SAR distribution at the electrodes of the pair of realistic multi-electrode leads (**a**) and the deposited power at each electrode as assessed by the full-wave simulations, the standard Tier 3 process and the modified Tier 3 approach (**b**). presence of a second lead modified the heating profile and could have an unpredictable effect on the induced heating depending not only on the relative placement of the implants but also on the lead structure and the incident electric fields [28; 45]. Therefore, there is a need for a systematic method to evaluate the induced heating profile caused by a pair of coupled (due to proximity) implant leads during MRI. To calculate the deposited power at the tips of the coupled leads, we propose a numerical technique based on the Tier 3 approach with a standard uncertainty lower than 1 dB. Initially, the power deposition at the tip of the lead was estimated and then it was converted to temperature increase. We verified this method using electric fields with low correlation among them for different relative placements between the wires, and the resulting deviation between the values from the full-wave simulations and this modified Tier 3 approach was within the confidence interval. The introduced methodology is based on the additive model of [31], which is an experimental process performed on a parallel couple of cables with constant distance between them. However, our method advances this model, developing an estimation methodology of the coupling factors between the leads and the tangential field along their paths, allowing the calculation of the deposited power for realistic in vivo implant placements and not only for parallel placement of the leads. The proposed numerical methodology can be performed for various configurations of implant leads, dividing the total process into three discrete steps:  
* Excite each lead alone with an iso-electric field and determine the profile of the scattered electric field at the trajectory of the other lead. * Calculate the TF of each lead in the absence of the second lead. Then, placing the leads in positions similar to those inside the patient, excite each lead separately and acquire the TF of both the excited and the unexcited lead.