Targeted Subcortical Nerve Recruitment by Controlling the Waveform of Electrical Stimulation for MRI-guided Surgical Ablation

MRI-guided surgical ablation has been used as a novel technique to achieve minimally invasive glioma resection. In this surgery, low-power thermal energy is supplied via a laser probe, which is inserted into the patient's brain to coagulate the tumor region. To position the laser probe precisely, intraoperative MRI is performed. However, MRI images cannot provide sufficient details regarding the fiber tracts. In order for surgeons to know the spatial distribution of subcortical nerve tracts and improve safety during surgery, we propose the use of subcortical mapping to support MRI-guided surgical ablation. Conventional electrical stimulation using biphasic rectangular pulses has poor selectivity in nerve recruitment, and precise spatial distribution of nerve fibers in the subcortical region is difficult to obtain. In this study, we conducted computer simulations to determine which waveform of stimulation results in better spatial selectivity. We used a multi-layer volume conductor model of the human skull and neocortex, in which a mathematical model of a human myelinated nerve fiber was embedded. We numerically calculated the nerve responses to extracellular stimulation provided by an electrode attached to the laser probe inserted into the white matter. We evaluated the distance and diameter selectivity of a solitary test pulse alone and that of conditioning stimuli consisting of a triangular waveform and burst pulses followed by the test pulse. The results showed that the distance and diameter selectivity are markedly improved when using conditioning stimuli prior to the test pulse. A mechanism for improvement of selectivity can be explained based on the nonlinear dynamic properties at the axonal node. These results suggest that the novel conditioning waveform has the potential to provide detailed information of nerve fiber tracts to surgeons during MRI-guided surgical ablation.