Temperature Control at DBS Electrodes Using Heat Sinks: Experimentally Validated FEM Model of DBS Lead Architecture

Abstract

There is a growing interest in the use of Deep Brain Stimulation (DBS) for the treatment of medically refractory movement disorders and other neurological and psychiatric conditions. The extent of temperature increases around DBS electrodes during normal operation (joule heating and increased metabolic activity) or magnetic coupling (e.g., MRI) remain poorly understood, and methods to mitigate temperature increases are actively investigated. Indeed, brain function is especially sensitive to the changes in temperature including neuronal activity, metabolic functions, blood-brain barrier integrity, molecular stability, and viability. We developed technology to control tissue heating near DBS leads by modifying the thermal properties of lead materials. A micro-thermocouple was used to measure the temperature near DBS electrodes immersed in a saline bath. 3387 and 3389 Leads were energized using Medtronic DBS stimulators. The RMS of the driving voltage was monitored. Peak steady-state temperature was determined under different RMS values. A micro-positioning system was used, which allowed the generation of temperature field map. We developed and solved a finite element method (FEM) bio-heat transfer model of DBS incorporating realistic DBS lead architecture. The model was first validated using the experimental results (by matching saline thermal conductivity and electrical conductivity) and was then applied to develop methods to control temperature rises in the brain using heat-sink technology. Experimental measurements are consistent with theoretical predictions including: 1) Peak temperature increases directly with the RMS square of the applied voltage, such that different waveforms with the same RMS induce the same peak temperature rise; 2) Peak temperatures increases with contact proximity such the maximal temperature rise was observed using adjacent contacts of lead 3389; 3) Temperature decayed over ∼2 mm distance away from energized contacts. FEM results demonstrated the central role of lead materials (material properties and geometry) in controlling temperature rise by conducting heat: namely by acting as passive heat sinks. We report that the relatively high thermal conductivity of exiting DBS lead wiring affects the temperature field, indicating the importance of detailed lead architecture. We then demonstrate how modifying lead design to optimize heat conduction can effectively control temperature increases; the manifest advantages of this approach over complimentary heat-mitigation technologies is that heat-sink controls include: 1) insensitive to the mechanisms of heating (e.g., nature of magnetic coupling); 2) does not interfere with device efficacy (e.g., the electric fields induced in the tissue during stimulation are unaffected); and 3) can be practically implemented in a broad range of implanted devices (cardiac/neuro-prothethics, pumps...) without modifying device operation or implant procedure.