Transcranial magnetic stimulation coil based on magnetic negative metamaterial and method thereof

Abstract

The invention discloses a transcranial magnetic stimulation coil based on a magnetic negative metamaterial and a method of the transcranial magnetic stimulation coil. The transcranial magnetic stimulation coil comprises a flat plate magnetic core located above an 8-shaped coil, the 8-shaped coil and the resonance type magnetic negative metamaterial located below the 8-shaped coil. On the basis of the 8-shaped coil, the flat plate magnetic core with high magnetic conductivity and the electromagnetic metamaterial with negative equivalent magnetic conductivity are introduced at the same time, and the magnetic field in the space is regulated and controlled by the flat plate magnetic core and the electromagnetic metamaterial on the basis of not changing the driving source and the 8-shaped coil winding structure; and the stimulation intensity and the stimulation precision of the stimulation coil on the brain model are greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of transcranial magnetic stimulation, and in particular to a transcranial magnetic stimulation coil based on magnetic negative metamaterial and a method for achieving optimal stimulation intensity thereof. Background Art

[0002] Transcranial magnetic stimulation (TMS) is a non-invasive, painless and green treatment method. This technology generates a time-varying magnetic field by applying a pulsed current to the stimulation coil. This magnetic field can penetrate the skull without attenuation and stimulate the brain nerves, achieving neuroregulation by controlling the excitation and inhibition of nerve cells.

[0003] The transcranial magnetic stimulation coil is a crucial device in transcranial magnetic stimulation technology, and its performance even directly determines the effect of the entire stimulation. Existing stimulation coils have the following defects: in order to achieve a higher stimulation intensity, a high-intensity stimulation current drive is required; the three indicators of stimulation intensity, stimulation depth and stimulation accuracy are difficult to improve at the same time.

[0004] Among the existing methods for improving transcranial magnetic coils: the method of improving stimulation performance by designing the coil structure often makes the coil winding too complicated, which is difficult to process and use; the method of improving the stimulation accuracy by combining different coils to make the magnetic field superimposed in phase or offset in reverse phase requires driving multiple coils at the same time and controlling the current direction of each coil, which makes the driving circuit complicated; the method of introducing high magnetic permeability core materials for magnetic focusing to improve the stimulation intensity and accuracy, the shape and placement of the core materials will have a great influence on the stimulation performance. In general designs, the core materials are placed above the coil, which can improve the stimulation intensity to a certain extent, but the improvement is limited. When the core material with a central window is placed below the coil, it will play a certain magnetic shielding role. Although it can improve the stimulation accuracy, it will also cause the stimulation intensity to decrease. In addition, when a special core shape is used, it is also required to redesign the outer shell of the coil, and the heavy weight of the core material will also bring inconvenience to medical staff.

[0005] In addition, although magnetic-negative metamaterials have unique electromagnetic response characteristics that do not exist in nature, their properties such as evanescent wave amplification and negative refractive index can well meet the needs of high stimulation intensity and high stimulation accuracy in transcranial magnetic stimulation. However, most of the existing research is on metamaterial structure design at MHz, GHz or higher frequency bands, and there are few magnetic-negative metamaterial designs within the kHz frequency band. Therefore, it is impossible to directly apply existing magnetic-negative metamaterials to the field of transcranial magnetic stimulation. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a transcranial magnetic stimulation coil based on magnetic negative metamaterials and a method for achieving its optimal stimulation intensity. By simultaneously introducing magnetic negative metamaterials and high magnetic permeability planar magnetic cores to regulate the magnetic field in space, the stimulation intensity and stimulation accuracy of the coil are improved without changing the complexity of the coil itself and the driving circuit.

[0007] To achieve the above-mentioned purpose, the technical solution provided by the present invention is: a transcranial magnetic stimulation coil based on magnetic negative metamaterial, comprising a planar magnetic core, an 8-shaped coil and a magnetic negative metamaterial; the planar magnetic core is located above the 8-shaped coil, and is used to concentrate and reflect the magnetic field above the 8-shaped coil; the magnetic negative metamaterial is located below the 8-shaped coil, and is used to converge the magnetic field below the 8-shaped coil; the magnetic negative metamaterial is a resonant electromagnetic metamaterial with an n×n planar periodic arrangement, where n is the number of units arranged periodically in each row or column, and each unit is a multi-layer square spiral structure, where the layers are connected to each other through metal vias, and the winding direction of the odd-numbered layers is mirror-symmetric with respect to the winding direction of the even-numbered layers, so that the flow direction of the induced current in each layer is consistent, thereby ensuring that the distributed inductance of each layer of the square spiral structure is connected in series to form a large equivalent inductance; the square spiral structure The width of each turn of the structure gradually decreases from the outer circle to the inner circle, and the turn width is reduced in the inner circle where the magnetic field intensity is the strongest, which helps to reduce losses and improve the Q value; the square spiral structure introduces interdigital capacitors in the outer circle with a large turn width. Due to the uneven charge distribution in the interdigital capacitors, a voltage difference can be formed to store electrical energy, which is equivalent to introducing a large capacitor; the top and bottom layers of each unit are loaded with chip capacitors and chip inductors respectively, and large capacitors and inductors are additionally introduced while ensuring that the unit size remains unchanged; the magnetic negative metamaterial has a large inductance value and a large capacitance value as a whole, which can enable it to work in the kHz frequency band required for transcranial magnetic stimulation and generate negative equivalent magnetic permeability. The evanescent wave amplification and negative refractive index characteristics of the negative equivalent magnetic permeability are used to achieve regulation of the magnetic field in space, thereby enabling the brain model to obtain higher intensity and more precise stimulation.

[0008] Furthermore, the planar magnetic core is made of a material with high magnetic permeability and high saturation magnetization intensity; the area of ​​the planar magnetic core is larger than the central tangent part of the figure-8 coil and is much smaller than the area of ​​the magnetically negative metamaterial.

[0009] Furthermore, the substrate of the magnetic negative metamaterial is made of a material with a high dielectric constant.

[0010] Furthermore, the planar magnetic core is located 4 to 10 mm above the figure-8-shaped coil, and the figure-8-shaped coil is located 4 to 10 mm above the magnetically negative metamaterial.

[0011] The present invention also provides a method for achieving the optimal stimulation intensity of the transcranial magnetic stimulation coil based on the magnetic negative metamaterial, comprising the following steps: 1) Load periodic boundary conditions to solve the unit resonance frequency of the magnetic negative metamaterial, and adjust the size of the patch capacitor or patch inductor to make the unit resonance frequency of the magnetic negative metamaterial lower than the frequency required for transcranial magnetic stimulation, ensuring that the magnetic negative metamaterial can provide negative equivalent permeability within the required frequency; 2) Solve the induced electric field distribution of the figure-8 coil with a planar magnetic core but no magnetic negative metamaterial in the brain model, and record the peak induced electric field intensity at the cortex in the brain model as E0; 3) The magnetically negative metamaterials are periodically arranged and placed under the figure-8 coil to solve the induced electric field distribution in the brain model, and the peak induced electric field intensity at the cortex of the brain model is recorded as E1; 4) Compare the size of E0 and E1: If E0 ≥ E1, it is necessary to gradually reduce the size of the patch capacitor or patch inductor, and solve the peak induced electric field strength of the 8-shaped coil with a planar magnetic core and a magnetically negative metamaterial at the cortex of the brain model again until the induced electric field strength no longer increases, and the optimal induced electric field strength is obtained, which is recorded as E2; If E1>E0, it cannot be ensured that the performance is optimal at this time, so it is necessary to slightly increase the size of the patch capacitor or patch inductor, and solve the peak induced electric field strength of the 8-shaped coil with a planar magnetic core and a magnetically negative metamaterial at the cortex of the brain model again. If the induced electric field strength increases, continue to gradually increase the size of the patch capacitor or patch inductor until the induced electric field strength no longer increases, and obtain the optimal induced electric field strength E2; if the induced electric field strength decreases, adjust the value of the patch capacitor or patch inductor to be smaller than the original value, and solve the peak induced electric field strength of the 8-shaped coil with a planar magnetic core and a magnetically negative metamaterial at the cortex of the brain model again. At this time, if the induced electric field strength still decreases, the value of E1 is set as the optimal induced electric field strength E2. If the induced electric field strength increases, continue to gradually reduce the size of the patch capacitor or patch inductor until the induced electric field strength no longer increases, and obtain the optimal induced electric field strength E2; 5) After the optimal induced electric field strength E2 is obtained at the cortex of the brain model, the peak value of the induced electric field strength is recorded as the stimulation intensity.

[0012] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. Compared with the common figure-8 coil, the present invention improves the stimulation accuracy and greatly increases the stimulation intensity without changing the complexity of the driving circuit and coil. The increase in stimulation intensity means that after the introduction of the planar magnetic core and the magnetic negative metamaterial at the same time, the current intensity of the driving circuit can be appropriately reduced, alleviating the problems of high heat and high noise caused by the high current of the coil.

[0013] 2. Compared with the stimulation coil that only introduces high magnetic permeability magnetic core material, the present invention adds a flat magnetic core above the coil and a magnetic negative metamaterial below the coil, which can fully utilize the space above and below the coil. Both can simultaneously improve the stimulation intensity and stimulation accuracy of the coil, making the improvement in stimulation intensity and stimulation accuracy more significant.

[0014] 3. Compared with the existing magnetic negative metamaterials, the present invention reduces the resonant frequency to the kHz band and has a higher Q value through lamination, introduction of lumped elements, interdigitated structure, gradient turn width design and other methods.

[0015] 4. The method for realizing the optimal stimulation intensity proposed in the present invention takes into account the phenomenon that the stimulation intensity of the brain model is related to the frequency after the introduction of magnetic negative metamaterials in the field of transcranial magnetism. This method adjusts the idea of ​​lumped elements to relatively simply move the optimal stimulation intensity to a frequency point consistent with the driving current frequency.

[0016] 5. In the implementation method proposed by the present invention, the planar magnetic core and the magnetically negative metamaterial are both designed as thin flat plates with light weight, and can be easily integrated into the existing commercial figure-8 coil without the need to redesign the complex coil housing.

[0017] 6. In the implementation method proposed by the present invention, the method of simultaneously introducing the magnetic negative metamaterial and the magnetic core does not have too many requirements on the coil itself. Theoretically, as long as the relative positions of the magnetic negative metamaterial, the magnetic core and the coil are properly set, the implementation method can be applied to other coils such as V-shaped coils, Sinky coils, Halo coils, etc., and has strong compatibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 3D structural diagram of a transcranial magnetic stimulation coil and a double-layer ball head model based on magnetically negative metamaterials in an embodiment of the present invention.

[0019] Figure 2 This is a front view of a transcranial magnetic stimulation coil and a double-layer ball head model based on magnetic negative metamaterials in an embodiment of the present invention.

[0020] Figure 3 3D structure diagram of a four-layer magnetic negative metamaterial unit in an embodiment of the present invention.

[0021] Figure 4 4 is a diagram of the first layer winding structure of a four-layer magnetic negative metamaterial unit in an embodiment of the present invention.

[0022] Figure 5 4 is a diagram of the second layer winding structure of the four-layer magnetic negative metamaterial unit in an embodiment of the present invention.

[0023] Figure 6 This is a diagram of the third layer winding structure of the four-layer magnetic negative metamaterial unit in an embodiment of the present invention.

[0024] Figure 7 4 is a structural diagram of the fourth layer winding of the four-layer magnetically negative metamaterial unit in an embodiment of the present invention.

[0025] Figure 8 This is a simulation result diagram of the equivalent magnetic permeability of a four-layer magnetically negative metamaterial unit in an embodiment of the present invention.

[0026] Fig. 9 This is a flow chart of a method for achieving optimal stimulation intensity of a transcranial magnetic stimulation coil based on magnetically negative metamaterials in an embodiment of the present invention.

[0027] Fig.10 1 is a comparison diagram of the induced electric field distribution on the measurement line corresponding to the embodiments of the present invention when there is no planar magnetic core and magnetically negative metamaterial, with a planar magnetic core but no magnetically negative metamaterial, and with a planar magnetic core and magnetically negative metamaterial.

[0028] Fig.11 This is a comparison diagram of the corresponding stimulation performances when there is no planar magnetic core and magnetically negative metamaterial, with a planar magnetic core but no magnetically negative metamaterial, and with a planar magnetic core and magnetically negative metamaterial in an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0030] Figure 1 and Figure 2 A transcranial magnetic stimulation coil based on magnetic negative metamaterials is provided in this embodiment, and a double-layer ball head model used to test the stimulation performance. The transcranial magnetic stimulation coil includes a planar magnetic core 1, an 8-shaped coil 2 and a magnetic negative metamaterial 3; the planar magnetic core 1 is located above the 8-shaped coil 2, and is used to concentrate and reflect the magnetic field above the 8-shaped coil 2; the magnetic negative metamaterial 3 is located below the 8-shaped coil 2, and is used to converge the magnetic field below the 8-shaped coil 2; the inner layer of the double-layer ball head model with a radius of 70 mm represents the cortical layer, and the electromagnetic parameters of the gray matter of the brain are selected: conductivity 0.10696, relative dielectric constant 26640. The outer layer with a thickness of 15 mm represents the cerebral scalp, and the electromagnetic parameters of the scalp are selected: conductivity 0.31061, relative dielectric constant 25809. Although the structure of the double-layer ball head model is simple, it is sufficient to provide standardized conditions for evaluating the performance of different coil models.

[0031] In this embodiment, the evaluation criteria for stimulation performance are as follows: the maximum induced electric field strength at the cerebral cortex is used to evaluate the stimulation intensity. In order to produce effective stimulation, the stimulation intensity only needs to reach a certain threshold. However, since the stimulation intensity varies linearly with the driving current intensity, the greater the stimulation intensity, the lower the driving current intensity can be, alleviating the problems of high heating and high noise of the coil. It can be seen that the greater the stimulation intensity, the better; the stimulation accuracy is evaluated by the electric field distribution of the induced electric field along the measuring line, and the value of the stimulation accuracy is quantified by calculating the volume within the cerebral cortex that is greater than half of the maximum induced electric field strength, where the measuring line is 10 mm below the cortex. In order to reduce stimulation to non-target areas of the patient, the smaller the stimulation accuracy, the better.

[0032] In this embodiment, the magnetic negative metamaterial 3 is 5 mm above the brain model, the figure-8-shaped coil 2 is 4 mm above the magnetic negative metamaterial 3 , and the planar magnetic core 1 is 4 mm above the figure-8-shaped coil 2 .

[0033] In this embodiment, the 8-shaped coil 2 uses coil parameters provided by Magstim, with an outer radius of 43.5 mm, an inner radius of 28 mm, a driving current of 5 kA, and a current frequency of 2.5 kHz. Since the current frequency is 2.5 kHz, the frequency of change of the magnetic field in space is also 2.5 kHz, so the magnetic negative metamaterial designed in the embodiment needs to provide a negative equivalent magnetic permeability within this frequency point.

[0034] In this embodiment, the parameters of the planar magnetic core 1 are 40 mm×40 mm×5 mm, and the material selected is iron with a relative magnetic permeability of 4000. Theoretically, the introduction of high magnetic permeability materials can change the spatial distribution of the magnetic field, making the magnetic field more concentrated in the required area, thereby improving the stimulation accuracy. At the same time, since there is no ideal magnetic conductor in nature, the introduction of magnetic core materials will only increase the stimulation intensity to a certain extent.

[0035] In some optional embodiments, the size of the figure-8 coil 2 is not limited, such as a diameter of 20 mm, 90 mm, etc.; the shape of the coil is not limited, such as circular, rectangular, elliptical, regular polygon, etc.; the cross-sectional shape is not limited, such as circular, rectangular, multi-strand wire, etc.; the implementation structure is not limited, such as a figure-8 coil with overlapping centers, a figure-8 coil with both sides offset upward or downward by a certain angle, etc.

[0036] In some optional embodiments, the material of the planar magnetic core 1 may be a magnetic material with a higher relative magnetic permeability such as Permalloy.

[0037] Since magnetic negative metamaterials are resonant electromagnetic metamaterials, the premise for the realization of electromagnetic metamaterials is that the unit structure must be much smaller than the wavelength of the working electromagnetic wave. This premise can be fully met in the frequency band required in the field of transcranial magnetic stimulation. At the same time, because transcranial magnetic stimulation only involves magnetic coupling, it is only necessary to design the magnetic permeability to be a negative number. However, in order to ensure resonance within the frequency band required for transcranial magnetic stimulation, the unit structure needs to have a large inductance and capacitance, so the unit structure needs to be designed.

[0038] Figures 3 to 7 The three-dimensional structure diagram of the magnetic negative metamaterial unit and the structure diagram of each layer in this embodiment. In this embodiment, the unit of the magnetic negative metamaterial adopts a 4-layer square spiral structure, in which: the first layer and the second layer are connected by metal vias 8, the second layer and the third layer are connected by metal vias 9, the third layer and the fourth layer are connected by metal vias 10, and the fourth layer and the first layer are connected by metal vias 7. At the same time, the first layer and the third layer are wound clockwise, and the second layer and the fourth layer are wound counterclockwise, so as to ensure that the induced current flows in the same direction in each layer, so that the distributed inductance of each layer of the square spiral structure is connected in series; the first layer and the fourth layer are loaded with patch capacitors 4 and patch inductors 5 respectively, and the lumped elements are added to make It is necessary to introduce additional large capacitors and inductors while ensuring that the unit volume remains unchanged; in the unit of the magnetic negative metamaterial, the distance between the square spiral structure and the edge is controlled, and the positive unit inductance and the negative mutual inductance between the units are balanced to obtain a larger overall inductance; a gradient turn width design is adopted in each layer, and a smaller turn width is adopted in the center where the magnetic field intensity is the strongest, which can reduce losses and increase the Q value; each layer introduces interdigital capacitors in the outer ring with a larger turn width. Due to the uneven charge distribution in the interdigital capacitor, a voltage difference can be formed to store electrical energy, which is equivalent to introducing a larger interdigital capacitor. Through these designs, the total inductance and total capacitance of the unit structure are large, and it can resonate in the kHz frequency band.

[0039] In this embodiment, the magnetic negative metamaterial adopts a 4×4 planar periodic arrangement structure, and the unit size of the magnetic negative metamaterial is: 30 mm×30 mm×1 mm, with 15 turns of winding, the turn width varies from 0.4 mm to 0.1 mm, the turn pitch is 0.4 mm, the lumped capacitor 4 and the lumped inductor 5 are 39 μF and 56 μH respectively; the spiral structure adopts high conductivity metal material, and the dielectric substrate 6 adopts FR-4 epoxy glass fiber board.

[0040] like Figure 8As shown in the figure, it is the equivalent magnetic permeability of the magnetic negative metamaterial obtained by the loading periodic boundary simulation in this embodiment. It can be seen that the real part of the equivalent magnetic permeability is negative and the imaginary part is small in the frequency band near 2.5 kHz. Since the wave of the coil in the near field behaves as an evanescent wave, and since the equivalent magnetic permeability is negative at this time, its refractive index is always greater than 1, so the magnetic negative metamaterial can amplify the evanescent wave; at the same time, the negative magnetic permeability can make the incident wave focus twice, and it has the characteristics of a perfect lens. In theory, the artificial magnetic negative metamaterial can improve the magnetic field strength and improve the magnetic field distribution, thereby improving the induced electric field strength and improving the induced electric field distribution, and improving the stimulation intensity and stimulation accuracy.

[0041] Furthermore, the time-varying magnetic field emitted by the coil into space diffuses to both sides above and below it, and only the magnetic field below the coil can stimulate the brain model. Therefore, the present invention adds a high-permeability flat magnetic core above the coil to change the magnetic resistance in the space, and reflects the magnetic field above the coil to the bottom by converging magnetic flux lines and reflecting, so that the magnetic field below is enhanced to a certain extent. At the same time, a small-area design can only reflect part of the magnetic field, making the reflected magnetic field more concentrated; adding magnetic negative metamaterials below the coil, through its amplification of evanescent waves and the characteristics of perfect lenses, also enhances the magnetic field below the coil, and at the same time, the magnetic field becomes more concentrated after negative refraction. Theoretically, compared with ordinary figure-8 coils and figure-8 coils with magnetic materials added only on the top, the present invention can simultaneously utilize the space above and below the coil, and converge the magnetic field diffused by the coil into the space more to the brain model, achieving higher intensity and higher precision stimulation.

[0042] In some optional embodiments, a periodic arrangement of more units can be used, such as 6×6, 8×8, etc. Since only when the number of units is large enough, the infinite periodic arrangement result obtained by loading the periodic boundary can be well matched with the finite periodic arrangement result in practice, but the required resonant frequency in this example is very low and the corresponding working wavelength is very large, so the theoretical number of units required is more than one hundred, otherwise the resonant frequency will shift to a certain extent. However, in order to ensure less simulation time, a smaller number of units have to be used. In addition, although negative permeability has the characteristic of evanescent wave amplification, only when the real part of the equivalent permeability is negative and large and the imaginary part is small, the negative refractive ability of the unit is strong and the loss is small, the amplification effect will be most obvious, so the amplification amplitude is related to the frequency at this time, and there will be an optimal frequency point with the largest enhancement amplitude. Therefore, it is necessary to move the frequency point with the largest enhancement amplitude to the same frequency as the driving current frequency of the transcranial magnetic stimulation coil (2.5 kHz) so that the performance of the stimulation coil at this frequency point can reach the expected level. To this end, the present invention also provides a method for realizing the optimal stimulation intensity of a transcranial magnetic stimulation coil based on a magnetic negative metamaterial to achieve effective frequency movement, such as Fig. 9 As shown, the method comprises the following steps: 1) Load periodic boundary conditions to solve the unit resonance frequency of the magnetic negative metamaterial, and adjust the size of the patch capacitor or patch inductor to make the unit resonance frequency of the magnetic negative metamaterial slightly lower than the frequency required for transcranial magnetic stimulation, ensuring that the magnetic negative metamaterial can provide negative equivalent permeability within the required frequency; 2) Solve the induced electric field distribution of the figure-8 coil with a planar magnetic core but no magnetic negative metamaterial in the brain model, and record the peak induced electric field intensity at the cortex in the brain model as E0; 3) The magnetically negative metamaterials are periodically arranged and placed under the figure-8 coil to solve the induced electric field distribution in the brain model, and the peak induced electric field intensity at the cortex of the brain model is recorded as E1; 4) Compare the size of E0 and E1: If E0 ≥ E1, it is necessary to gradually reduce the size of the patch capacitor or patch inductor, and solve the peak induced electric field intensity of the 8-shaped coil 2 with the planar magnetic core 1 and the magnetic negative metamaterial 3 at the cortex of the brain model again until the induced electric field intensity no longer increases, and the optimal induced electric field intensity is obtained, which is recorded as E2; If E1>E0, it cannot be ensured that the performance is optimal at this time, so it is necessary to slightly increase the size of the patch capacitor or patch inductor, and solve the peak induced electric field strength of the 8-shaped coil 2 with the planar magnetic core 1 and the magnetic negative metamaterial 3 at the cortex of the brain model again. If the induced electric field strength increases, continue to gradually increase the size of the patch capacitor or patch inductor until the induced electric field strength no longer increases, and obtain the optimal induced electric field strength E2; if the induced electric field strength decreases, adjust the value of the patch capacitor or patch inductor to be slightly smaller than the original value, and solve the peak induced electric field strength of the 8-shaped coil 2 with the planar magnetic core 1 and the magnetic negative metamaterial 3 at the cortex of the brain model again. At this time, if the induced electric field strength still decreases, the value of E1 is set as the optimal induced electric field strength E2. If the induced electric field strength increases, continue to gradually reduce the size of the patch capacitor or patch inductor until the induced electric field strength no longer increases, and obtain the optimal induced electric field strength E2; 5) After the optimal induced electric field strength E2 is obtained at the cortex of the brain model, the peak value of the induced electric field strength is recorded as the stimulation intensity.

[0043] In this embodiment, the frequency point with the largest stimulation intensity increase is adjusted to 2.5 kHz through the optimal stimulation intensity implementation method. According to this method, the frequency point can also be moved to other common driving frequencies of the transcranial magnetic stimulation coil, such as 3 kHz, 5 kHz, etc. In practice, the capacitance and inductance can be fine-tuned by connecting lumped elements in series and parallel.

[0044] Fig.10 and Fig.11The following are the comparison diagrams of the normalized electric field distribution on the corresponding measurement lines at 2.5 kHz for the embodiment without planar magnetic core and magnetic negative metamaterial, with planar magnetic core but without magnetic negative metamaterial, with planar magnetic core and magnetic negative metamaterial, and the corresponding stimulation performance comparison diagram after adjusting the lumped element. Fig.10 It can be seen from the figure that after the introduction of the planar magnetic core and the magnetically negative metamaterial at this frequency point, the distribution of the induced electric field along the measurement line is more concentrated, indicating that it can achieve more precise stimulation. Fig.11 It can be seen that the addition of magnetic negative metamaterials to this frequency point greatly improves the magnitude of the stimulation intensity, and the improvement is more significant than that of adding only magnetic core materials, with an improvement of more than two times. According to the Biot-Savart law, the magnetic field strength is proportional to the magnitude of the driving current, so the stimulation intensity is also proportional to the magnitude of the driving current. The two-fold increase in stimulation intensity means that even if the coil driving current intensity in the present invention is reduced by two times, the same intensity of stimulation as the ordinary figure-8 coil can still be achieved. The substantial reduction in the driving current intensity can alleviate the high heating and high noise problems of the coil under high current. At the same time, it can also be seen from the volume occupied by more than half of the maximum induced electric field intensity of the cortex that the addition of magnetic negative metamaterials reduces the impact on non-target areas in the brain, achieving more precise stimulation.

[0045] The above simulation results show that the transcranial magnetic stimulation coil based on magnetic-negative metamaterials and the method for achieving the optimal stimulation intensity provided by the present invention can meet the expected purpose. Without changing the complexity of the driving circuit and the coil, the high magnetic permeability flat magnetic core and the magnetic-negative metamaterial with negative equivalent magnetic permeability are used to regulate the magnetic field in space, thereby greatly improving the stimulation intensity of the stimulation coil on the brain model, and the stimulation accuracy is also improved to a certain extent, which is worthy of promotion.

[0046] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A transcranial magnetic stimulation coil based on magnetic negative metamaterials, characterized in that: The invention comprises a planar magnetic core (1), an 8-shaped coil (2) and a magnetically negative metamaterial (3); the planar magnetic core (1) is located above the 8-shaped coil (2) and is used to concentrate and reflect the magnetic field above the 8-shaped coil (2); the magnetically negative metamaterial (3) is located below the 8-shaped coil (2) and is used to converge the magnetic field below the 8-shaped coil (2); the magnetically negative metamaterial (3) is a resonant electromagnetic metamaterial arranged periodically in an n×n plane, where n is the number of units arranged periodically in each row or column, and each unit is a multi-layer square spiral structure, where the layers are connected to each other through metal vias, and the winding direction of the odd-numbered layers is mirror-symmetric with respect to the winding direction of the even-numbered layers, so that the flow direction of the induced current in each layer is consistent, thereby ensuring that the distributed inductances of the square spiral structures in each layer are connected in series to form a large equivalent inductance; the width of each turn of the square spiral structure The degree of the magnetic field gradually decreases from the outer circle to the inner circle, and the turn width is reduced in the inner circle where the magnetic field intensity is the strongest, which helps to reduce losses and improve the Q value; the square spiral structure introduces an interdigital capacitor in the outer circle with a large turn width. Due to the uneven charge distribution in the interdigital capacitor, a voltage difference can be formed to store electrical energy, which is equivalent to introducing a large capacitor; the top and bottom layers of each unit are loaded with a patch capacitor and a patch inductor respectively, while ensuring that the unit size remains unchanged, additional large capacitors and inductors are introduced; the magnetic negative metamaterial (3) has a large inductance value and a large capacitance value as a whole, which can enable it to work in the kHz frequency band required for transcranial magnetic stimulation, generate a negative equivalent magnetic permeability, and realize the regulation of the magnetic field in space through the evanescent wave amplification and negative refractive index characteristics of the negative equivalent magnetic permeability, thereby enabling the brain model to obtain higher intensity and more precise stimulation.

2. The transcranial magnetic stimulation coil based on magnetic negative metamaterial according to claim 1, characterized in that: The planar magnetic core (1) is made of a material with high magnetic permeability and high saturation magnetization intensity; the area of ​​the planar magnetic core (1) is larger than the central tangent portion of the figure-8 coil (2) and is much smaller than the area of ​​the magnetically negative metamaterial (3).

3. The transcranial magnetic stimulation coil based on magnetic negative metamaterial according to claim 1, characterized in that: The substrate of the magnetic negative metamaterial (3) is made of a material with a high dielectric constant.

4. The transcranial magnetic stimulation coil based on magnetic negative metamaterial according to claim 1, characterized in that: The flat magnetic core (1) is located 4 to 10 mm above the figure-8-shaped coil (2), and the figure-8-shaped coil (2) is located 4 to 10 mm above the magnetically negative metamaterial (3).

5. The method for achieving optimal stimulation intensity of a transcranial magnetic stimulation coil based on magnetic negative metamaterials according to any one of claims 1 to 4, characterized in that: The following steps are involved: 1) Loading periodic boundary conditions to solve the unit resonance frequency of the magnetic negative metamaterial (3), and by adjusting the size of the patch capacitor or the patch inductor, the unit resonance frequency of the magnetic negative metamaterial (3) is made lower than the frequency required for transcranial magnetic stimulation, ensuring that the magnetic negative metamaterial (3) can provide negative equivalent magnetic permeability within the required frequency; 2) Solve the induced electric field distribution of the figure-8 coil (2) with a planar magnetic core (1) but without magnetic negative metamaterial (3) in the brain model, and record the peak induced electric field intensity at the cortex in the brain model as E0; 3) The magnetic negative metamaterial (3) is periodically arranged and placed under the figure-8 coil (2), and the induced electric field distribution of the figure-8 coil (2) with the planar magnetic core (1) and the magnetic negative metamaterial (3) in the brain model is solved, and the peak induced electric field intensity at the cortex in the brain model is recorded as E1; 4) Compare the size of E0 and E1: If E0 ≥ E1, it is necessary to gradually reduce the size of the patch capacitor or patch inductor, and solve the peak induced electric field intensity of the 8-shaped coil (2) with the planar magnetic core (1) and the magnetic negative metamaterial (3) at the cortex of the brain model again until the induced electric field intensity no longer increases, and obtain the optimal induced electric field intensity, which is recorded as E2; If E1> E0, it is impossible to ensure that the performance is optimal at this time, so it is necessary to first increase the size of the patch capacitor or patch inductor, and solve the peak induced electric field intensity of the 8-shaped coil (2) with the planar magnetic core (1) and the magnetic negative metamaterial (3) at the brain model cortex again. If the induced electric field intensity increases, continue to gradually increase the size of the patch capacitor or patch inductor until the induced electric field intensity no longer increases, and obtain the optimal induced electric field intensity E2; if the induced electric field intensity decreases, adjust the value of the patch capacitor or patch inductor to be smaller than the original value, and solve the peak induced electric field intensity of the 8-shaped coil (2) with the planar magnetic core (1) and the magnetic negative metamaterial (3) at the brain model cortex again. If the induced electric field intensity still decreases at this time, the value of E1 is set as the optimal induced electric field intensity E2. If the induced electric field intensity increases, continue to gradually reduce the size of the patch capacitor or patch inductor until the induced electric field intensity no longer increases, and obtain the optimal induced electric field intensity E2; 5) After the optimal induced electric field strength E2 is obtained at the cortex of the brain model, the peak value of the induced electric field strength is recorded as the stimulation intensity.

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