Simulation, measurement, and design of multi-electrode spinal cord stimulation Leads with a focus on MRI safeness and stimuli induced artefacts

Abstract

The electromagnetic (EM) and thermodynamic (TD) models for a Spinal Cord Stimulator (SCS) presented in this work, permit on one hand to analyse the impact of electrical stimuli patterns on artefacts and on the other hand to analyse the impact of RF exposure during MRI scans on tissue heating. Historically, many models have focused on single electrode Leads whereas this work focuses on multi-electrode Leads. Testing of Leads and IPGs for Magnetic Resonance Imaging (MRI) compliance is a tedious, time-consuming and expensive process. A table-top RF test bed has been conceived and calibrated to systematically investigate MRI compliance for various RF exposure scenarios. In addition, a table-top test bed has been conceived to determine the propagation constant of electromagnetic (EM) waves along Leads embedded in electrolytes. A thin low permittivity layer around the multi-lumen conduct (MLC) has been added to the EM model to solve the discrepancy between measured and initially calculated propagation constants. Further, several new measures to minimise tissue heating caused by Leads during MRI scans are presented. The work compares EM and TD simulation results obtained from finite element method (FEM) solvers with measurement results obtained from various test beds that have been conceived or adapted. Despite some discrepancies between simulated and measured results the EM and thermodynamic models permit to investigate the impact of stimuli patterns on artefacts and the impact of RF exposure on tissue heating. The results show that worst case tissue heating does not necessarily occur at the distal end electrode in multi-electrode Leads and that RF exposure scenarios have an impact on locations where RF power is dissipated at the distal end. Simulations and measurements show that Lead wire routing anomalies, i.e. changes in RF coupling between Lead wires along a multi-lumen conduct, have a significant impact on locations where RF power is dissipated at the distal end. The measures proposed to minimise tissue heating have proved their effectiveness in both the table-top RF test bed and in a 3 Tesla MRI scanner. Finally, artefacts captured by recording electrodes in multi-electrode Leads can be significantly reduced if the input impedance of the recording amplifier is low during stimuli on therapeutic electrodes.