Transfer Functions of a Spinal Cord Stimulation System in Mixed Media and Homogeneous Media for Estimation of RF Heating during MRI Scans

There is a risk for patients with spinal cord stimulation (SCS) implants that MRI-RF induced heating may cause harm to spinal cord (SC) or spinal nerves. The ISO/TS 10974 Clause 8, Tier 3 based evaluation of RF heating has been commonly used for active implantable manufactures through regulatory body world-wide. Generating TFs for the lead and SCS system is an essential component in Tier 3. The objective of this paper is to investigate the effects of media on transfer functions (TFs) and dissipated power near the distal electrode along clinical pathways of SCS implants. The simulation results indicate that SCS TF measurement and validation should be performed in HCM (0.47 S/m) since it has the closest match with SC mixed media. With the SC mixed media model, the compound tissue effect from tissues around the system was seen and TF follows that of the HCM TFs closely. These trends were confirmed through power calculation of TF equation by integrals of electrical fields along clinical SCS implant pathways in an adult male model, with a 2%-4% underestimation from HCM while LCM overestimates by 72%- 74%. This study indicates appropriate section of media for deriving TFs could lead to more clinically relevant RF heating predictions during MRI scans.