When conductive leads are exposed to a radiofrequency field, induced currents are generated along the leads (2
). The effects of such induced currents can be seen in two perspectives: (a) potential safety issues related to local increase of electric-field deposition at the interface between the EEG electrode and the human head, historically measured in terms of specific absorption rate (17
) and (b) effects on electromagnetic-field distribution related to the EM scattering of the leads; i.e., the EEG leads cannot be considered proper “RF-transparent” measurement probes. In this perspective, changes in the EMF due to the EEG leads can be considered as “noise” with respect to the measurements of EEG for RF-field exposure of a specific antenna. Schmid et al.
) presented a new exposure system for studies requiring EEG recording on human subjects exposed to GSM900-and UMTS-equivalent sources. Although the study showed minimal interference when EEG leads were placed orthogonally to the source, the study raised potential issues for different lead orientations or numbers of EEG electrodes/leads. Since the amount of current induced depends on the frequency and the specific geometry of the leads with respect to the source, estimation of such currents can be cumbersome for complex geometries, such as conductive leads placed on the head of human subjects during electroencephalography recordings. In such cases, an anatomically precise computational model can be useful to evaluate the interactions between the leads and the RF-field source. Several MRI-based human head models are now available (15
). The model used in this study is characterized by a 1-mm3
isotropic spatial resolution and 29 anatomical structures, which allowed the modeling with geometrical accuracy the thin structures of interests for the problem of EEG leads, such as epidermis, where EEG electrodes are connected to the head, and bone marrow, where potential local increases in SAR are possible (21
Previous studies used peak 1-g average and 10-g average SAR to evaluate changes in SAR with EEG leads exposed an RF field generated by mobile phones (10
). The regulatory limit for localized exposure to an RF field generated by mobile phones set by the Federal Communications Commission (FCC) is 1.6 W/kg for peak 1-g average SAR (39
). The limit set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) (41
) and the American National Standard Institute (ANSI) and IEEE (42
) is 2 W/kg for peak 10-g average SAR. It has been shown that the temperature rise in tissue correlates well with an average tissue mass of 10 g (43
). This study showed that the peak 1-g average and peak 10-g average SARs with mobile phones were below the corresponding exposure limits. Thus, given the relatively low input power of mobile phones as opposed, for example, to the RF coil used in MRI (21
), the conductive EEG leads are not expected to generate excessive temperature rises during exposure to the RF field of mobile phones.
The whole-head average SAR and 10-g average SAR did not vary significantly (i.e., less than 10%) in the different cases considered. The peak 1-g average SAR varied less than 10% with dipole and mobile phone, while a twofold change compared to the case of no leads was observed with Set 1 and the patch antenna. The results of the study confirmed the peculiar local behavior of RF power in the presence of conductive leads (45
), with changes occurring in volumes of a few mm3
rather than over the whole head. While relatively coarse parameters such as whole-head average SAR, 10-g average SAR or even 1-g average SAR did not show significant changes in the RF-field power with and without leads, a more comprehensive volumetric assessment of changes in RF power over the entire head showed significant local increases of RF-field energy absorption in anatomical structures underneath the EEG electrodes.
The resolution of the numerical model used in this study is one of the highest among the anthropomorphic numerical models for RF-field dosimetry (15
); this allowed a 1-mm3
-resolution volumetric assessment of RF-field power. Numerical errors related to staircasing of small structures may still be present (8
), and a proper validation of these results with geometrically matched experimental measurements may be needed. The SARs found in this study were in line with previous published work. The simulations with the dipole and mobile phone were in line with canonical results.3
The peak 10-g average SAR with the patch antenna and the head model without leads was 1.2 W/kg (1 W of net input power), similar to the value of 1.02 W/kg simulated and 0.92 W/kg measured with an antenna of same dimensions and at the same distance from the head reported in ref. (10
). Simulations with a mobile phone at 915 MHz and a head model without leads resulted in a peak 10-g average of 1.77 W/kg (1 W of net input power) compared to 3.4 W/kg in ref. (15
). Since the study in ref. (15
) was performed with the SAM phantom, for a more direct comparison, simulations with an electrically homogeneous model (σ = 0.97 S/m, εr
= 55.6, ρ = 1040 kg m−3
) based on the same head model used in this study were performed with the mobile phone at 915 MHz. The peak 10-g average SAR for this homogeneous model was 3.18 W/kg (1 W of net input power), within 10% of the value in ref. (15
Our simulations showed that the presence of conductive EEG leads combined with specific configurations of an RF-field source created a reduction of the RF field (i.e., “shielding effect”) in anatomical structures near the EEG electrodes, similar to that reported in refs. (21
) and (25
). In particular, the value of Pi
was less than one in the epidermis or bone marrow with the dipole at 915 MHz. The shielding effect was greater with the patch antenna. The results of this study also confirm that the leads with higher resistivity (ρ = 0.01 Ωm), used in previous studies to minimize interactions with other RF-field sources (35
), helped reduce the currents induced in the leads and the related local SAR on the epidermis and surrounding structures.