## 4 Discussion  
In previous publications, it has been shown that many factors affect the heating of an implant tip, namely the implant's position inside the human body, the imaging landmark and the type of coil system. Therefore, in order to find simple and practical methods to control the induced RF heating close to the CIEDs, the influence of these factors on the induced tangential electric field along cardiac leads was estimated numerically. As the safety guidelines offer the option to cap either the global SAR or the B1+RMS, which are both automatically calculated and reported by the scanners, we conducted a systematic comparative study of these impact factors on the induced electric field considering the limitations of IEC 60601-2-33 [36]. As was shown in previous works, the body positioning inside the scanner affects the distribution of the tangential electric field along the leads and therefore the absorbed energy at the implant tip, determining the neck and the thorax imaging landmarks as the worst imaging zone. In particular, assuming constant B1+RMS, at both frequencies, the peak values were obtained at the level of the heart (LD3) and at the high part of the chest (LD4), followed by the landmarks at the low part of the chest and the neck. On the other hand, considering constant global SAR, higher W values were estimated for most of the human bodies at the landmark of the neck (LD2). Therefore, although the central position of an implant is suggested to generate the maximum heating [8], the consideration of the limits could affect this. For both limitations, the pelvis imaging position could be characterized as a safety zone for the cardiac leads since the calculated W values were decreased by more than 10 dB compared to the maximum, ensuring highly reduced deposition of the energy. This was also confirmed in previous studies [14, 25, 26, 48]. Furthermore, it was noticed that the obtained reduction in the electric field removing the imaging position from the worst-case landmark depended on the dimensions of the coil. In particular, the length of the birdcage coil seemed to influence the induced electric field  
subsequently to lower electric field, which is in line with previous work [46]. On the other hand, the constant B1\({}^{+}\)RMS value generates higher electric field at higher resonant frequency. This is in agreement with previous studies that compared the RF heating of DBS implants across 1.5 T and 3 T coils [30; 47]. The difference in the resonant frequency of the coil implies an increase in the induced potential on a lead by 1.7 to 5.7 dB at 128 MHz in comparison to 64 MHz for the same B1\({}^{+}\). The obtained increase is close to the value reported in previous studies, where the SAR rose quadratically with B0, increasing by 6 dB [49]. Nevertheless, it is important to note that the estimated factor characterizes the contribution to the final RF-induced power deposition of the electric field only and not to the TF, which also varies by changing the applied frequency and the electric length, which is a key parameter for the coupling mechanism between the coil and the implant. In summary, the consideration of limiting SAR led to a different distribution of the W values in comparison to those of limiting the B1\({}^{+}\)RMS. The reason is that the higher resonant frequency, the imaging position close to the thorax and the longer coils generally tend to generate higher induced local SAR, and thus the electric field was scaled to lower values to maintain compliance with the limiting values. This appears to be in accordance with a previous study [26]. Therefore, this numerical analysis of the induced electric field along cardiac implants under different imaging scenarios revealed conflicts among different versions of the IEC 60601-2-33 standard. Specifically, in the 2015 edition, the determination of normal mode (NM) and first-level controlled mode of operation for a magnetic resonance imaging (MRI) system introduced the FPO:B (Fixed Parameter Option: Basic) choice.