Numerical calculation of the radiofrequency transfer function of cochlear implants for assessing deposited power in MRI.

The medical imaging of patients with a cochlear implant inside a Magnetic Resonance Imaging (MRI) scanner carries the risk of power deposition in the tissues at the tip of the implant lead, which may result in their heating. In order to assess this risk, ISO/TS 10 974 (2018) describes a methodology (Tier 3 approach) whereby a radiofrequency electrical model for the implant lead in the form of a transfer function is constructed. The construction of the transfer function takes place by assuming that a homogenous medium surrounds the implant, whereas, in reality, implants can traverse various tissues of different electrical properties. The results show that the use of a High Permittivity Medium (HPM) overestimates the Tier 3 calculated deposited power by almost 6 dB, whereas a Low Permittivity Medium (LPM) underestimates it by 9 dB, compared to the in vivo power deposition in three virtual human models, obtained following the Tier 4 approach of ISO/TS 10 974(2018). Since the Tier 3 approach requires less computational resources compared to Tier 4, we suggest its modification with the use of two media (mixed media approach), where implant is immersed. By carefully choosing the media electrical properties, it is possible to calculate power deposition values at the lead tip that differ less than 1 dB from the in vivo ones.