Lawrence J. Crowther

Transcranial magnetic stimulation: Improved coil design for deep brain investigation

This paper reports on a design for a coil for transcranial magnetic stimulation. The design shows potential for improving the penetration depth of the magnetic field, allowing stimulation of subcortical structures within the brain. The magnetic and induced electric fields in the human head have been calculated with finite element electromagnetic modeling software and compared with empirical measurements. Results show that the coil design used gives improved penetration depth, but also indicates the likelihood of stimulation of additional tissue resulting from the spatial distribution of the magnetic field.

New Designs for Deep Brain Transcranial Magnetic Stimulation

New applications for transcranial magnetic stimulation are developing rapidly for both diagnostic and therapeutic purposes. Therefore, so is the demand for improved performance, particularly in terms of the ability to stimulate deeper regions of the brain and to do so selectively. The coil designs that are used presently are limited in their ability to stimulate the brain at depth and with high spatial focality. Consequently, any improvement in coil performance would have a significant impact in extending the usefulness of TMS in both clinical applications and academic research studies. New and improved coil designs have been developed, modeled, and tested as a result of this work. A large magnetizing coil, 300 mm in diameter and compatible with a commercial TMS system, has been constructed to determine its feasibility for use as a deep brain stimulator. This coil, used in a Helmholtz configuration, can produce 105 V/m at the surface of the head and 93 V/m at a depth of 15.2 mm compared to a single turn 60 mm coil which produces 82.6 V/m at the surface and only 15 V/m at 15.2 mm. The results of this work have suggested directions that could be pursued in order to further improve the coil designs.

Transcranial magnetic stimulation of mouse brain using high-resolution anatomical models

Transcranial magnetic stimulation (TMS) offers the possibility of non-invasive treatment of brain disorders in humans. Studies on animals can allow rapid progress of the research including exploring a variety of different treatment conditions. Numerical calculations using animal models are needed to help design suitable TMS coils for use in animal experiments, in particular, to estimate the electric field induced in animal brains. In this paper, we have implemented a high-resolution anatomical MRI-derived mouse model consisting of 50 tissue types to accurately calculate induced electric field in the mouse brain. Magnetic field measurements have been performed on the surface of the coil and compared with the calculations in order to validate the calculated magnetic and induced electric fields in the brain. Results show how the induced electric field is distributed in a mouse brain and allow investigation of how this could be improved for TMS studies using mice. The findings have important implications in f...

Developments in deep brain stimulation using time dependent magnetic fields

The effect of head model complexity upon the strength of field in different brain regions for transcranial magnetic stimulation (TMS) has been investigated. Experimental measurements were used to verify the validity of magnetic field calculations and induced electric field calculations for three 3D human head models of varying complexity. Results show the inability for simplified head models to accurately determine the site of high fields that lead to neuronal stimulation and highlight the necessity for realistic head modeling for TMS applications.

Deep brain transcranial magnetic stimulation using variable “Halo coil” system

Transcranial Magnetic Stimulation has the potential to treat various neurological disorders non-invasively and safely. The “Halo coil” configuration can stimulate deeper regions of the brain with lower surface to deep-brain field ratio compared to other coil configurations. The existing “Halo coil” configuration is fixed and is limited in varying the site of stimulation in the brain. We have developed a new system based on the current “Halo coil” design along with a graphical user interface system that enables the larger coil to rotate along the transverse plane. The new system can also enable vertical movement of larger coil. Thus, this adjustable “Halo coil” configuration can stimulate different regions of the brain by adjusting the position and orientation of the larger coil on the head. We have calculated magnetic and electric fields inside a MRI-derived heterogeneous head model for various positions and orientations of the coil. We have also investigated the mechanical and thermal stability of the adjustable “Halo coil” configuration for various positions and orientations of the coil to ensure safe operation of the system.

Effect of Anatomical Brain Development on Induced Electric Fields During Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is increasingly becoming an established method for the treatment of drug-resistant major depressive disorder. It is also widely used in cognitive neuroscience and psychology to map brain function and modulate brain activity. The majority of studies utilizing TMS are performed with adult patients, but the method may also be of use for a variety of purposes for pediatric patients. To study the safety implications of performing TMS on patients of various ages, the induced electric field in the brain has been calculated with numerical methods with a variety of anatomically realistic human body models of different ages. The results of this paper show that large differences in induced electric field occur in patients of different ages and consideration of these differences must be taken into account to achieve the desired neurological effect.

Realistically Modeled Transcranial Magnetic Stimulation Coils for Lorentz Force and Stress Calculations During MRI

Transcranial magnetic stimulation (TMS) uses transient magnetic field to activate brain regions by inducing an electric field across and an electric current through neurons. Functional magnetic resonance imaging (fMRI) allows the measurement of brain activity making it a potentially useful tool in combination with TMS to analyze the sites of stimulation within the brain. TMS typically utilizes a high current pulse up to 8 kA at approximately 2.5 kHz to induce an electric field inside the brain sufficient for neural stimulation. Lorentz forces are created on the TMS coil and are increased in the presence of a high external magnetic field from the MRI magnet. This study implements a realistic coil model developed from X-ray images of a commercial figure-of-eight TMS coil. The Lorentz forces and stress profile inside the current carrying material of this realistically modeled coil are presented. Results show that the maximum Lorentz force density is significantly larger than previously calculated, by a factor of more than 3.

Thermal and Mechanical Analysis of Novel Transcranial Magnetic Stimulation Coil for Mice

Transcranial magnetic stimulation (TMS) has potential to treat various neurological disorders noninvasively and safely. There has been significant work on coil designs for use on the human brain; however, there are fewer reports on the coil design for small animal brains, such as mice. Such work is essential to validate TMS treatment procedures on animals prior to clinical trials. We report thermal and mechanical analysis of a new small-animal coil system designed to produce focused electric fields resulting in more selective deep-brain stimulation. Thermal and magnetic force analyses conducted at experimental TMS operating conditions are used to determine the mechanical stability of the new coil system. Low magnetic linear attraction and rotational forces suggest mechanical stability of the coil. Small temperature increase over a simulated 60 s TMS therapy session indicates that the coil system operates within safe temperature limits. This coil configuration can be used on mice to stimulate selective regions of the brain to study various neurological disorders, such as Parkinsons disease.

Focused and deep brain magnetic stimulation using new coil design in mice

Deep brain transcranial magnetic stimulation (TMS) offers promising treatment for neurological disorders that originate from deeper regions of the brain, such as Parkinsons disease. Coils designed for the human head need significant redesigning to stimulate selective regions of the mouse brain for advanced TMS therapy analysis. We report a focused and deep brain TMS coil for mice that is based on a two coil configuration similar to the “Halo coil”. A heterogeneous MRI derived head model of mouse was used to obtain an electric field of about 150 V/m in selective deeper regions of the brain. Focality of stimulation was quantified using the ratio of half value volume to half value of depth of electric field. A prototype of the final coil design was fabricated and characterized to compare simulated and physical magnetic field profiles.

Calculation of Lorentz Forces on Coils for Transcranial Magnetic Stimulation During Magnetic Resonance Imaging

New applications for transcranial magnetic stimulation (TMS) are rapidly being developed for diagnostic and therapeutic purposes but are restricted by a number of technical limitations. The ability to reliably perform TMS concurrently with functional magnetic resonance imaging (fMRI) would be a valuable tool for researchers. However the forces experienced by TMS coils during normal operation due to large transient magnetic fields which are produced, are problematic. When operated within a large external field, such as in an fMRI scanner these forces can result in mechanical failure of the coils. This paper presents calculations of these forces to form a basis from which coil failure can be prevented.