Research & Development
Controllable Pulse Parameter TMS: We have developed a new class of efficient transcranial magnetic stimulation (TMS) devices that allow adjustment of the magnetic stimulus parameters over a wide range. This degree of flexibility was not possible with existing technology. Pulse shaping is accomplished by employing high-power insulated-gate bipolar transistors (IGBTs) and high-energy storage capacitors, as well as snubber and control circuits, to enable switching of current up to 7 kA, with pulse widths ranging from microseconds to hundreds of microseconds. cTMS generates nearly rectangular pulses that use less electrical energy and cause at most four times less coil heating than conventional devices. cTMS devices could enable research trials and clinical applications not previously possible due to technical limitations. For instance, cTMS could be used to non-invasively characterize changes in neuronal membrane properties associated with brain pathology or pharmacological interventions. In addition, cTMS can produce briefer magnetic pulses than conventional TMS, which could reduce unpleasant scalp sensation. This has the advantage of making the treatment more tolerable and providing more effective blinding for randomized clinical trials.
The design of TMS coils determines the electric field pattern that is induced in the brain. Depending on the stimulation target, designs with various degrees of focality and field penetration depth could be desirable. We are interested in coil designs for deep brain TMS, which are particularly challenging due to the rapid decay of electric field away from the coil. In the process of coil development, we use finite element modeling (FEM) of electromagnetic fields which allows fast optimization of designs before a hardware prototype is constructed.
We use finite element modeling to characterize the electric and magnetic fields induced in the brain by various transcranial stimulation modalities including transcranial magnetic stimulation (TMS), electroconvulsive therapy (ECT), magnetic seizure therapy (MST), transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial static magnetic stimulation (tSMS).
Stimulation & Imaging
We successfully set up state-of-the-art systems at Columbia University that allow functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) to be carried out simultaneously with TMS and are currently implementing such systems at Duke. We are interested in the effect of baseline brain activity, sensed by fMRI and EEG, on the outcome of TMS. We are also interested in synchronizing TMS application to various aspects of endogenous brain activity sensed by EEG.
Many clinical and research studies list implanted metallic hardware as an absolution contraindication to transcranial brain stimulation, since the safety of the dual-device usage has not been established. The strong magnetic field emitted by TMS/MST coils and the electrical current from ECT may induce large voltages in nearby wires and electronic devices, interfering with their function and potentially damaging the implanted system components. Further, interactions between the stimulation devices may result in significant, undesirable stimulation effects, and even in neural injury. As more patients receive electronic implants for a variety of neural disorders, and as the use of transcranial brain stimulation devices grows, it is of great importance to evaluate the safety of the interactions between intracranial devices and transcranially applied electric and magnetic stimulators. Safety concerns currently limit the access of subjects with intracranial electronic devices to therapies involving transcranial stimulation technology, which may preclude them from obtaining appropriate medical treatments. Gaining better understanding of the interactions between transcranial and implanted stimulation devices will demarcate significant safety risks from benign interactions, and will provide recommendations for reducing risk, thus enhancing the patient’s therapeutic options. Further, if transcranial brain stimulation can be safely applied in patients with intracranial implants, this may lead to novel synergistic dual-device therapies.
Mechanisms & Dosing
Transcranial electric and magnetic stimulation are widely used as research tools and in clinical applications, but many questions remain about their underlying mechanisms and how to choose the stimulation parameters (dose) for optimal outcome. In BSEL and through collaborations, we explore the effect of various stimulus parameters on the stimulation outcome in pre-clinical models, healthy volunteers, and patients, deploying some of the technology and analysis developed by BSEL.
Estimation of response thresholds and other response parameters is widely used in various TMS paradigms and is critical to the reliability of experimental results as well as the safety and efficacy of therapeutic application. Existent methods are time consuming and lack rigorous characterization of their accuracy. We aim to develop fast, practical, and accurate approaches to response parameter estimation with transcranial magnetic stimulation (TMS) and other brain stimulation modalities.
We conduct research in power electronics with applications to both magnetic brain stimulation as well as electrical energy conversion. Our current focus is on modular multilevel converters, including the modular multilevel series parallel converter (M2SPC) invented by Dr. Goetz and colleagues. In this project, we collaborate with Dr. Srdjan Lukic from NC State University.