Acoustic Fresnel metamaterial with non-uniform thickness matching and transmission enhancing capability

Abstract

The invention discloses an acoustic Fresnel metamaterial with non-uniform thickness matching and transmission enhancing capabilities, which comprises a Fresnel layer and a coupling layer, the Fresnel layer is composed of a plurality of Fresnel unit cells with different heights and inclination angles, and the coupling layer is used for tightly coupling the Fresnel layer and a skull; the Fresnel unit cell is designed based on the thickness and curvature of different areas of the skull, and phase modulation and path control of ultrasonic waves are achieved by adjusting the height and the inclination angle of the unit cell. According to the invention, the ultrasonic wave can form an efficient and accurate focus area after penetrating through the skull, so that non-invasive nerve stimulation or accurate thermal injury treatment can be carried out on the brain cortex. Acoustic phase distortion and energy attenuation caused by nonuniformity of the skull can be effectively reduced, the treatment precision and efficiency are improved, and the method has the advantages of being easy and convenient to operate, high in adaptability and high in clinical application potential and is suitable for popularization and application of transcranial ultrasonic nerve regulation and related brain disease treatment.

Description

Technical Field

[0001] The invention relates to an acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability, belonging to the field of acoustic materials. Background Art

[0002] Transcranial Ultrasound Therapy (TUT), as a non-invasive neuromodulation technology, shows new potential for treating neurological diseases by directly stimulating the cerebral cortex with ultrasound. TUT has many advantages, such as high spatial resolution, controllable depth penetration, and non-invasiveness. However, in actual applications, there is a significant acoustic impedance mismatch between the skull and the ultrasound transducer, which seriously hinders the effective penetration of ultrasound. In addition, the uneven thickness of the skull introduces spatial phase distortion. Especially in high-frequency (1MHz-3MHz) ultrasound therapy, the treatment focus may deviate from the target area, affecting the treatment effect and safety.

[0003] In the prior art, there have been designs of acoustic metamaterials to match the acoustic impedance of the skull, such as a transcranial acoustic soft ultrasound gel material and its preparation method and application (patent number: CN201910813534.9), a double-negative acoustic metamaterial based on Mie resonance for transcranial ultrasound imaging (patent number: CN202010044301.X), etc. However, these methods are mainly aimed at lower high-frequency (<0.3MHz) applications and are difficult to meet the needs of high-frequency TUT. In addition, most existing metamaterials show poor adaptability when dealing with uneven skull thickness. Traditional homogeneous acoustic lenses and metamaterials have difficulty coping with the complex non-uniform thickness and curvature changes of the skull, resulting in significant energy scattering and phase distortion of sound waves during penetration, reducing the efficiency and accuracy of ultrasound therapy. Summary of the Invention

[0004] The purpose of the present invention is to provide an acoustic Fresnel metamaterial (AFM) with non-uniform thickness matching and enhanced transmission capability for transcranial ultrasound therapy. The AFM can effectively match the acoustic impedance of the skull and flexibly compensate for changes in skull thickness and curvature, thereby achieving self-focusing transmission of the ultrasound beam, thereby improving the efficiency and accuracy of ultrasound therapy.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] In one aspect, the present invention provides an acoustic Fresnel metamaterial having non-uniform thickness matching and enhanced transmission capability, comprising:

[0007] An acoustic Fresnel metamaterial with the ability to adapt to non-uniform thickness, characterized by comprising:

[0008] The Fresnel layer is composed of multiple Fresnel cells with different heights and tilt angles;

[0009] a coupling layer, used for tightly coupling the Fresnel layer to the skull;

[0010] The Fresnel cells are designed based on the thickness and curvature of different skull regions. This design involves adjusting the radial dimensions of the Fresnel cells in the acoustic wave incident plane, changing the equivalent volume fraction of the cells and the coupling layer within that plane, thereby optimizing the equivalent impedance Z to equal the acoustic impedance Z1 of the skull surface at the corresponding thickness, thereby adapting to the regional characteristics of different skull thicknesses. Adjusting the height and tilt angle of the cells adjusts the path of the sound wave to adapt to the varying curvature of different skull regions, thereby achieving phase modulation and path control of the ultrasound wave, directing the path of the sound wave to the focal location of treatment. Furthermore, the acoustic path difference between different Fresnel cells should be an integer multiple of the wavelength.

[0011] Furthermore, the Fresnel layer is made of a rigid high acoustic impedance material and is staggered with a flexible low acoustic impedance coupling material, so that the Fresnel layer has tunable equivalent acoustic impedance and equivalent sound velocity.

[0012] Furthermore, the material acoustic impedance of the Fresnel layer is smaller than the piezoelectric material acoustic impedance of the transducer, but larger than the biological tissue acoustic impedance; the material impedance of the Fresnel layer ranges from 2.5 MRayl to 4 MRayl.

[0013] Furthermore, each wedge of the Fresnel layer is a Fresnel unit cell. Assume that the thickest thickness of a unit cell structure along the arrangement direction is d, and the unit cell material satisfies d<λ / 2, where λ is the wavelength of ultrasound.

[0014] Furthermore, the coupling layer is made of a flexible low-acoustic-impedance material, and its equivalent acoustic impedance is calculated based on the volume fraction ratio of the interleaved portion of the Fresnel layer and the coupling layer.

[0015] Furthermore, the acoustic impedance of the coupling layer is 0.5 Mrayl-1.5 Mrayl, and is compounded with the AFM layer in a volume fraction ratio to form an equivalent impedance and an equivalent longitudinal wave speed, thereby achieving acoustic impedance compensation for the skull.

[0016] Furthermore, the height and tilt angle of the Fresnel cell are dynamically adjusted according to changes in skull thickness and curvature to achieve precise focusing of ultrasound waves.

[0017] Furthermore, the tilt angle of each Fresnel cell relative to the skull is used to measure the change in its acoustic path using the prism formula:

[0018]

[0019] Where α is the refraction angle of the prism, δ is the deflection angle of the outgoing sound beam relative to the incident sound beam, and c m is the sound velocity of the medium outside the lens, c is the sound velocity of the lens material, I1 and I2 are the incident angles of the incident and exit interfaces respectively, I ′ 1.I ′ 2 are the incidence angles of the incident and exit interfaces respectively;

[0020] After passing through the skull, the sound wave is distorted again, and its acoustic path can be measured by Snell's law of acoustics between two layers of media:

[0021] c1sinθ1=c2sinθ2

[0022] Among them, θ1 is the incident angle of the sound wave in the skull relative to the normal line of the skull interface; θ2 is the refraction angle of the sound wave behind the skull relative to the normal line of the skull interface; c1 and c2 are the sound speeds of the skull and brain tissue behind the skull, respectively.

[0023] Assume that the inclination angle of the skull relative to the incident direction of the sound wave is γ, the distance of the skull area from the focal position in the direction of sound wave propagation is l1, and the distance from the focal position in the normal direction of sound wave propagation is l2, then

[0024] l1 tan(θ2+γ)=l2

[0025] This completes the correction of the acoustic path.

[0026] Furthermore, let the total path of the sound wave through the i-th AFM cell be d i , the wavelength of the sound wave is λ, which should satisfy

[0027] d i+1 -d i =nλ

[0028] The acoustic phase correction can be completed.

[0029] On the other hand, the present invention provides a transcranial ultrasound therapy device based on acoustic Fresnel metamaterials, which includes an ultrasonic transducer, an AFM layer and a coupling layer, wherein the ultrasonic transducer is used to emit high-frequency ultrasonic waves, the AFM layer is used to achieve phase compensation and energy focusing of the incident ultrasonic waves, and the coupling layer is tightly adhered to the surface of the skull to improve acoustic matching and reduce wave energy loss.

[0030] In another aspect, the present invention provides an application of an acoustic Fresnel metamaterial with non-uniform thickness adaptability in transcranial ultrasound therapy.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) By adjusting the height and tilt angle of the Fresnel cell, it can adapt to different skull thicknesses and curvatures, significantly improving the transmission efficiency of ultrasound in the skull.

[0033] (2) The wavelength-scale spacing design of the Fresnel cells ensures precise adaptation to small changes in skull thickness during sound wave propagation, reducing energy scattering and phase distortion.

[0034] (3) The Fresnel layer can be made of polyetheretherketone (PEEK) material and the coupling layer can be made of acrylamide material, which provides good acoustic impedance matching and mechanical properties, and enhances the stability and durability of the metamaterial.

[0035] (4) The application of flexible connecting materials enhances the coupling stability between the Fresnel cell and the skull, ensuring the reliability and consistency of the metamaterial in long-term use. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the structure of an acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to the present invention.

[0037] Figure 2 This is a schematic diagram of the acoustic field simulation behind the skull on the axially symmetrical plane with the transducer plus AFM.

[0038] Figure 3 This is a schematic diagram of the acoustic field simulation behind the skull on the axisymmetric plane without AFM.

[0039] Markings in the figure are: 1-ultrasound transducer, 2-Fresnel layer (AFM layer), 3-coupling layer, 4-skull, 5-brain area. DETAILED DESCRIPTION

[0040] The following is a detailed description of the specific implementation of the present invention in conjunction with the accompanying drawings and the technical solutions involved in the embodiments of the present invention. It should be understood that the embodiments described herein are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0041] See also Figure 1 (Schematic cross-sectional view) shows the structure of an ultrasound therapy system using an acoustic Fresnel metamaterial (AFM) to achieve skull penetration and focusing. In this embodiment, the system includes an ultrasonic transducer 1, an AFM layer 2, a coupling layer 3, a skull 4, and a target brain region 5. The following describes each component and how it works together in detail.

[0042] 1. Ultrasonic transducer 1:

[0043] Ultrasonic transducer 1 is the ultrasonic energy input source of the present invention. It can be a circular or array transducer, with the transmission frequency, power, and pulse parameters tuned by an external control system. This transducer uses a frequency band commonly used in medical ultrasound (e.g., 1 MHz-3 MHz) to accommodate the needs of transcranial ultrasonic neuromodulation. When activated, transducer 1 converts high-frequency electrical signals into ultrasonic waves, transmitting the ultrasonic energy into AFM layer 2.

[0044] 2. Acoustic Fresnel metamaterial layer 2 and coupling layer 3:

[0045] The AFM layer 2, the core innovative structure of this invention, is composed of a series of metamaterial units arranged in a Fresnel structure. These units are constructed from solid materials (such as polyetheretherketone) with an impedance of 2.5-4 MRayl, using different structures and distributions. The unit spacing of the AFM layer 2 is designed to be on the scale of the acoustic wavelength, and the phase distribution of the incident acoustic wave is dynamically modulated by varying the unit height, tilt, and volume fraction.

[0046] Unlike traditional homogeneous acoustic lenses, the AFM layer 2 can adapt to changes in the local thickness and curvature of the skull 4, achieving phase compensation and energy integration of the local sound field by precisely adjusting the acoustic impedance and sound velocity of different regions. Specifically, the AFM layer changes the radial distribution of the Fresnel cell on the plane of sound wave incidence, regulates its equivalent volume fraction with the coupling layer, and adjusts the equivalent impedance of the bottom of the AFM layer to match the acoustic impedance of the skull surface to match skull regions of different thicknesses. In addition, by precisely designing the height and tilt angle of the cell, the ultrasonic phase can be effectively modulated and the propagation path can be guided to adapt to different changes in skull curvature. By adjusting the unit structure of the AFM layer 2, the coherent phase of the sound wave is reorganized and the energy is concentrated during the process of penetrating the skull 4, thereby forming a stable and precise acoustic focus behind the skull.

[0047] A coupling layer 3 is provided between the AFM layer 2 and the skull 4. The layer can be made of a material with a sound velocity and density close to that of water (such as acrylamide hydrogel). The coupling layer 3 can also have a flexible and conformable function, so that the AFM layer 2 maintains good acoustic coupling and mechanical stability when in contact with the irregular skull surface. At the same time, the role of the coupling layer 3 is to adapt to the AFM layer 2 and generate an equivalent impedance for impedance matching. The equivalent impedance Z and the equivalent longitudinal wave sound velocity v generated by the two at the wavelength scale are l for:

[0048]

[0049]

[0050] Where n is the volume fraction ratio, ρ and ρ ′ are the densities of the AFM layer and the coupling layer, cij = ∫ ...

[0051]

[0052] In order to manipulate the height of different AFM cells, assuming that the reflected echo of the brain tissue is small enough (infinite domain), the acoustic impedance of the skull surface can be expressed as:

[0053]

[0054] Among them, z s is the acoustic impedance of the skull surface, z b is the acoustic impedance of the skull, β b =k b dcosθ, where k b is the wave number of the sound wave propagating in the skull, d is the thickness of the skull, and cosθ is the cosine of the angle between the wave number direction and the normal direction of the skull surface.

[0055] At this time, the most suitable impedance Z1 satisfies the equation:

[0056]

[0057] Therefore, just control

[0058] Z=Z1

[0059] The AFM layer can achieve acoustic impedance matching for skulls of different thicknesses.

[0060] 3. Skull 4:

[0061] The uneven thickness of the skull significantly affects the sound field. Conventional transcranial ultrasound often fails to achieve efficient focusing due to strong reflections, phase mismatch, and energy attenuation caused by the skull. The present invention utilizes adaptive phase compensation and acoustic impedance matching strategies within the AFM layer 2 to reshape the sound pressure amplitude and phase distribution after the sound wave passes through the skull 4, enhancing transmission efficiency and reducing energy consumption and scattering in the skull and surrounding tissues.

[0062] Target Brain Area 5:

[0063] Behind the skull 4 is the target brain region 5, where the sound waves are focused. Through the system design of the present invention, after the sound waves pass through the AFM layer 2, the coupling layer 3, and the skull 4, their energy is precisely focused at a specific depth and location in the brain tissue.

[0064] Implementation process:

[0065] The use process of the present invention includes:

[0066] An external control system calculates and precisely processes the parameters of each unit in the AFM layer 2 (height, tilt angle, and composite material ratio) based on the thickness distribution and curvature data of the skull 4. By calculating the acoustic impedance Z1 of skull surfaces of varying thicknesses, the equivalent volume fraction n of the AFM cells and coupling layer is adjusted so that the equivalent acoustic impedance Z at the bottom of the AFM layer 2 equals Z1, achieving impedance matching for different skull thickness regions. The curvature and height of the AFM cells are adjusted, taking into account the tilt angle of the skull, to account for variations in the curvature of various skull regions.

[0067] The ultrasonic transducer 1 is tightly coupled to the AFM layer 2 , and a coupling layer 3 is provided thereunder to fit the outer surface of the skull 4 .

[0068] The ultrasonic transducer 1 is activated to emit ultrasonic pulses of specific frequency and power. The sound waves pass through the AFM layer 2 and the coupling layer 3 through the skull 4 and focus on the target brain area 5.

[0069] In summary, the AFM layer 2 proposed in the embodiment of the present invention realizes acoustic phase control and energy concentration in the complex transmission environment of the coupling layer 3 and the skull 4, effectively overcoming the sound field distortion and focusing problems in traditional transcranial ultrasound transmission. Figure 2 and Figure 3 As shown in the figure, the addition of the acoustic Fresnel metamaterial increased the sound pressure at the focal point of the intracranial sound field by approximately 9 times. This invention provides an optimized high-frequency transcranial ultrasound transmission method that significantly improves the transmission efficiency and focusing accuracy of acoustic energy, and has broad technical application potential.

[0070] The above illustrates and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not in any way limit the scope of protection of the present invention. Any technical solution obtained by equivalent substitution or other methods falls within the scope of protection of the present invention. Any portion not covered by the present invention is equivalent to or can be implemented using existing technology.

Claims

1. An acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability, characterized in that: include: The Fresnel layer is composed of multiple Fresnel cells with different heights and tilt angles; A coupling layer, used for tightly coupling the Fresnel layer with the skull; The Fresnel cell is designed based on the thickness and curvature of different areas of the skull; the design includes: by adjusting the radial size of the Fresnel cell in the sound wave incident plane, changing the equivalent volume fraction of the cell and the coupling layer in the plane, thereby optimizing the equivalent impedance Z equal to the acoustic impedance ratio Z1 of the skull surface of corresponding thickness, thereby adapting to the regional characteristics of different skull thicknesses; and adjusting the height and tilt angle of the cell to achieve phase modulation and path control of the ultrasound wave, so that the path of the sound wave points to the focal position that needs to be treated; and the acoustic path difference between different Fresnel cells is an integer multiple of the wavelength.

2. The acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1, characterized in that: The Fresnel layer is made of a rigid high acoustic impedance material and is staggered with a flexible low acoustic impedance coupling material, so that the Fresnel layer has tunable equivalent acoustic impedance and equivalent sound velocity.

3. The acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1, characterized in that: The material acoustic impedance of the Fresnel layer is smaller than the piezoelectric material acoustic impedance of the transducer, but larger than the biological tissue acoustic impedance; the material impedance range of the Fresnel layer is 2MRayl-6MRayl.

4. The acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1, characterized in that: Each circle of wedges of the Fresnel layer is a Fresnel unit cell. Assume that the thickest thickness of a unit cell structure along the arrangement direction is d, and the unit cell material satisfies d<λ / 2, where λ is the wavelength of the ultrasonic wave.

5. The acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1, characterized in that: The coupling layer is made of a flexible low-acoustic impedance material, and its equivalent acoustic impedance Z is calculated based on the volume fraction ratio of the interlaced parts of the Fresnel layer and the coupling layer, so that the equivalent acoustic impedance Z is equal to the acoustic impedance ratio Z1 of the skull surface of corresponding thickness, thereby achieving acoustic impedance matching of different thickness regions of the skull.

6. The acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1 or 5, characterized in that: The acoustic impedance of the coupling layer is 0.5 Mrayl-1.5 Mrayl.

7. An acoustic Fresnel metamaterial with non-uniform thickness matching and enhanced transmission capability according to claim 1 or 5, characterized in that: The height and tilt angle of the Fresnel cell are adjusted according to the thickness and curvature of the skull to achieve precise focusing of the ultrasound.

8. A transcranial ultrasound treatment device based on the acoustic Fresnel metamaterial as claimed in claim 1, characterized in that: It includes an ultrasonic transducer, an AFM layer and a coupling layer, wherein the ultrasonic transducer is used to emit high-frequency ultrasonic waves, the AFM layer is used to achieve phase compensation and energy focusing of the incident ultrasonic waves, and the coupling layer is tightly attached to the surface of the skull to improve acoustic matching and reduce wave energy loss.

Images