Some technical aspects of magnetic stimulation coil design with the ferromagnetic effect

Shielded Cone Coil Array for Non-Invasive Deep Brain Magnetic Stimulation

Non-invasive deep brain stimulation using transcranial magnetic stimulation is a promising technique for treating several neurological disorders, such as Alzheimer's and Parkinson's diseases. However, the currently used coils do not demonstrate the required stimulation performance in deep regions of the brain, such as the hippocampus, due to the rapid decay of the field inside the head. This study proposes an array that uses the cone coil method for deep stimulation. This study investigates the impact of magnetic core and shielding on field strength, focality, decay rate, and safety. The coil's size and shape effects on the electric field distribution in deep brain areas are also examined. The finite element method is used to calculate the induced electric field in a realistic human head model. The simulation results indicate that the magnetic core and shielding increase the electric field intensity and enhance focality but do not improve the field decay rate. However, the decay rate can be reduced by increasing the coil size at the expense of focality. By adopting an optimum cone structure, the proposed five-coil array reduces the electric field attenuation rate to reach the stimulation threshold in deep regions while keeping all other regions within safety limits. In vitro and in vivo experimental results using a head phantom and a dead pig's head validate the simulated results and confirm that the proposed design is a reliable and efficient candidate for non-invasive deep brain magnetic stimulation.

Simulation Study to Improve Focalization of a Figure Eight Coil by Using a Conductive Shield Plate and a Ferromagnetic Block

A new method to improve the focalization and efficiency of the Figure of Eight (FOE) coil in rTMS is discussed in this paper. In order to decrease the half width of the distribution curve (HWDC), as well to increase the ratio of positive peak value to negative peak value (RPN) of the induced electric field, a shield plate with a window and a ferromagnetic block are assumed to enhance the positive peak value of the induced electrical field. The shield is made of highly conductive copper, and the block is made of highly permeable soft magnetic ferrite. A computer simulation is conducted on ANSYS® software to conduct the finite element analysis (FEA). Two comparing coefficients were set up to optimize the sizes of the shield window and the block. Simulation results show that a shield with a 60 mm × 30 mm sized window, together with a block 40 mm thick, can decrease the focal area of a FOE coil by 46.7%, while increasing the RPN by 135.9%. The block enhances the peak value of the electrical field induced by a shield-FOE by 8.4%. A real human head model was occupied in this paper to further verify our method.

Experimental study to improve the focalization of a figure-eight coil of rTMS by using a highly conductive and highly permeable medium

A method to improve the focalization of the repetitive transcranial magnetic stimulation figure-eight coil in a magnetic stimulation is presented in this paper. For the purpose of reducing the half width of the distribution curve, while improving the ratio of positive to negative electric field, a shield plate with a window and a magnetic conductor were adopted. The shield plate, which was made of highly conductive copper, focused the magnetic field into a smaller area. The magnetic inductor, which was made of highly permeable soft magnetic ferrite, strengthened the magnetic field. A group of experiments was conducted to validate the focalizing effect. Experimental results showed that the negative peak and the half width of the distribution curve reduced by using the shield plate and the magnetic conductor. Especially for to the Magstim 70 mm double coil, when the shield window was 30 × 60 mm, the ratio of positive to negative electric field could be increased 109%, while the half width of the distribution curve could be reduced about 55%.

Circuit and coil design for in-vitro magnetic neural stimulation systems

Magnetic stimulation of neural tissue is an attractive technology because neural excitation may be affected without requiring implantation of electrodes. Pulsed discharge circuits are typically implemented for clinical magnetic stimulation systems. However, pulsed discharge systems can confound in-vitro experimentation. As an alternative to pulsed discharge circuits, we present a circuit to deliver asymmetric current pulses for generation of the magnetic field. We scaled the system down by using ferrite cores for the excitation coil. The scaled system allows observation using electrophysiological techniques and preparations not commonly used for investigation of magnetic stimulation. The design was refined using a comprehensive set of design equations. Circuit modeling and simulation demonstrate that the proposed system is effective for stimulating neural tissue with electric-field gradients generated by time-varying magnetic fields. System performance is verified through electrical test.

High permeability cores to optimize the stimulation of deeply located brain regions using transcranial magnetic stimulation

Efficient stimulation of deeply located brain regions with transcranial magnetic stimulation (TMS) poses many challenges, arising from the fact that the induced field decays rapidly and becomes less focal with depth. We propose a new method to improve the efficiency of TMS of deep brain regions that combines high permeability cores, to increase focality and field intensity, with a coil specifically designed to induce a field that decays slowly with increasing depth. The performance of the proposed design was investigated using the finite element method to determine the total electric field induced by this coil/core arrangement on a realistically shaped homogeneous head model. The calculations show that the inclusion of the cores increases the field's magnitude by as much as 25% while also decreasing the field's decay with depth along specific directions. The focality, as measured by the area where the field's norm is greater than 1/sq.rt.2 of its maximum value, is also improved by as much as 15% with some core arrangements. The coil's inductance is not significantly increased by the cores. These results show that the presence of the cores might make this specially designed coil even more suited for the effective stimulation of deep brain regions.

In vitro magnetic stimulation of unmyelinated nerves

Magnetic stimulation of neural tissue is an intriguing technology because excitation may be affected without a direct interface between the stimulator and the tissue. Current methods of magnetic stimulation use large air core coils, limiting the size of the neural preparations available for study. We use ferrite cores to reduce the stimulation area. Scaling allows the use of unmyelinated neural preparations with a range of space and length constants. Results are shown using two well studied neural preparations under varying test conditions.

High-permeability core coils for transcranial magnetic stimulation of deep brain regions

Stimulation of deep brain regions with Transcranial Magnetic Stimulation (TMS) may have an important role as a therapy to treat chemical dependency and depression. The coils traditionally used in TMS, however, have a poor performance in stimulating deep neurons. In this work we study the usage of high permeability cores combined with coils specifically designed to induce fields that decay slowly with depth. By using the finite elements method we show that the use of such cores increases the field's magnitude, decreases its decay rate and improves its focality. Such improvements make these high permeability core coils more suited to stimulate deep brain regions.