1. Generation of the AIMD Response Model

The ISO Technical Specification 10974 (ISO/TS 10974) defines the procedures to assess the local power deposition (RF heating) at the electrodes and the voltage/current (EMC) at the device terminals of an active implanted medical device (AIMD). The AIMD model can be determined either experimentally and/or with simulations based on the Tier 3 approach described in ISO/TS 10974. 

In the experimental case, a physical sample of the device under test (DUT) is measured with the  piX system equipped with E1TDSz probe (RF Heating) or RFoF1P4MED probe (EMC). The photonic technology avoids crosstalk of the excitor with the DUT (E1TDSz probe) and allows measurement of voltages without modifying the devices (RFoF1P4MED). 

In numerical evaluation, Sim4Life electromagnetics solver and a CAD model of the DUT are used to simulate the AIMD model. The model is then calibrated to a well-defined exposure condition with a medical implant test system (MITS). 

2. Validation of the AIMD Response Model

The created model needs to be validated with a sufficient set of orthogonal test functions. This involves specific absorption (SAR) or temperature rise (ΔΤ) measurements in RF heating case, or measurements of the current at the terminals and/or the induced voltages inside the device for EMC with RFoF1P4MED. The MITS allows for the fast evaluation of the AIMD model under worst-case incident fields of commercial scanners, but also for specific exposures generated by varying the polarization of the B1-field which is monitored continuously with TDS B1 measurements system.

The integration of the measurement and Sim4Life results is seamless for the user, facilitating comparison and sensitivity and/or uncertainty analysis. It should be noted here that the AIMD model can be established for each operating mode of the AIMD separately.

3. Computation of Incident Field Distributions

Once the AIMD response model is known and validated, the next step is to estimate typical incident field distributions with which the AIMD can be excited when implanted inside a human body (in vivo). The approach described in TS 10974 requires that the risk assessment of RF heating covers a wide range of the population. 

The calculation of the incident electric fields for all the human models and for all the possible clinical exposure scenarios would be a challenging task in terms of human and computational resources. Instead of performing the calculations anew, the user has the option of acquiring MRIxViP, a validated library of the electric field distributions in eight human models placed at different imaging positions (according to TS 10974) inside various birdcage coils, which are carefully selected to represent the majority of commercially available ones. MRIxViP is qualified by the FDA for MRI safety evaluations.

4. Power Deposition and Risk Assessment

The incident field distributions are used to assess power deposition at the distal tip of the AIMD lead when implanted inside the human body. With the IMAnalytics module of Sim4Life, this becomes a simple, reliable and traceable procedure. From the AIMD response model, the incident field distributions and the implants’ routing trajectories, IMAnalytics automatically performs the statistical analysis of power deposition at the tip of the AIMD lead in all possible scenarios.v IMAnalytics and MRIxViP are qualified by the FDA for MRI safety evaluations.

The industry mainly applies two approaches to translate the power deposition into risk assessment. The first one is to use animal experiments by injecting the equivalent power to the electrodes and assessing the response (e.g., change in the pacing threshold) as a function of the deposited power. The other approach is to translate the power deposition into in vivo temperature rise inside the human tissues using the Sim4Life thermal solver, which has been validated both for local and locoregional RF heating in humans.