Transcranial magnetic stimulation: design of a high current magnetic pulse generator with custom coil for the application on small animals

Abstract

Transcranial Magnetic Stimulation (TMS) is a non-invasive and outpatient treatment for Major Depressive Disorder (MDD)[1]. It has also proven to produce beneficial effects for other neurological and psychiatric disorders including: Parkinson's Disease (PD), Obsessive Compulsive Disorder (OCD), Anxiety, Post-Traumatic Stress Disorder (PTSD) and Schizophrenia [2]–[6]. Recently, researchers have shown interest in investigating the effect of TMS in neurological disorders which have not been explored by utilizing animal studies [7]. Animal studies are required to test the efficacy and safety of the treatments before using the new treatment in human subjects. Furthermore, animal studies can reduce research cost and speed up the development of new TMS treatment procedures. That is why it is important to have flexible and tunable hardware/coil solutions that allow tenability of power, current, and the shape of the signals needed for experimentation. TMS is based on the principal of Faraday's law of induction, in which time-varying magnetic fields are used to induce an electric field in the brain. Substantial research has been published related to the improvement of coil designs, using head models and clinical studies. However, few publications are available on TMS stimulators and even less on TMS stimulators for use on small animals such as rodents and mice. Current commercial TMS stimulators are very expensive and they have limited usage due to the coil's unique geometry and size. Perhaps an important contrasting is that commercial coils are large enough to stimulate the entire body of the small animal, so studies needed to focally stimulate part of the brain are impossible. Hence, in this article a novel design for a small size TMS stimulator has been proposed. TMS stimulator has been designed for the stimulation of small animals, namely mice and other rodents. This new design was created to focally stimulate parts of rodent brain instead of stimulating their whole body when a commercial coil is used. This will provide researchers an opportunity to choose small TMS coils since commercial TMS stimulators do not support small inductive loads [8]. Mice brains are very small in size when compared with human brains that consist of approximately 2 cm 2 surface area, whereas human brains have about 100 times more surface area [9]. This small area allows the distance between the coil and the surface of the brain to be small hence the field needed to stimulate the brain to initiate action potential is about 0.1Tesla [7]. This means the circuit designed has to be capable of handling current more than 300 A. In addition, the proposed design provides additional flexibility, so that the coil can be changed to match the required specification of other coil designs for different locations of stimulation. Figure 1 shows the TMS stimulator and a coil positioned on a mice model.