Acoustically transparent skull and method of testing thereof

Abstract

This invention provides an acoustically transparent skull based on the transfer matrix method. The skull includes a matching layer, a coupling layer, and a biomimetic skull. The biomimetic skull matrix has acoustic windows and a stepped groove design for easy and quick assembly, disassembly, and repositioning. The matching layer module is made of epoxy resin-nano-alumina composite material, and its thickness is designed according to a quarter wavelength of the target frequency band. This effectively optimizes acoustic impedance matching and reduces energy loss, becoming a key breakthrough in improving the transmission efficiency of transcranial ultrasound and expanding its application in non-invasive medical fields. This invention focuses on the field of biomechanics and medicine, addressing the problem of low transmission efficiency in transcranial ultrasound technology caused by differences in acoustic impedance of the skull. It can be used to compare the sound transmission effects of single or multiple matching layers at different frequencies. In the future, ultrasound can be more widely used for diagnosis, treatment, neuromodulation, and health / mental status monitoring of astronauts.

Description

Technical Field

[0001] This invention relates to the field of transcranial ultrasound technology, specifically an experimental method for the design, fabrication, and acoustic performance verification of an acoustically transparent skull. Background Technology

[0002] Cancer is a major killer of human health, and in many countries it is a leading cause of premature death and reduced life expectancy. However, current cancer treatments, such as chemotherapy, surgery, and immunotherapy, while killing cancer cells, also cause significant damage to healthy cells, resulting in severe side effects and low cure rates. Mechanomedicine is a newly emerging interdisciplinary field at the medical-engineering level. It utilizes mechanical measurement, mechanical loading, or interventional force transduction to solve medical problems, leading to new medical technologies such as mechanoimmunology, mechanodiagnosis, and mechanotherapy. Theoretical predictions of the mechanical behavior and functional regulation of functional materials and devices under the influence of external fields such as force, electricity, and magnetism are being conducted on radio frequency wires.

[0003] With advancements in aviation technology, modern aircraft systems are constantly being updated and developed, gradually achieving mechanization and intelligence. This has led to flight missions being increasingly concentrated on a small number of pilots. While this reduces physical labor to some extent, it also results in pilots facing greater physiological and psychological burdens. Factors such as night flights and cross-time zone flights further disrupt pilots' biological clocks, leading to sleep deprivation and consequently, severe pilot fatigue. Pilot fatigue not only impairs pilots' operational abilities but can also affect their attention, cognitive abilities, and even induce flight illusions, thereby causing serious flight accidents. Therefore, effectively detecting, warning, and intervening in pilots' mental state has become one of the key means of ensuring flight safety.

[0004] Transcranial ultrasound (TCU) technology, due to its non-invasive and low-cost advantages, is being widely applied in fields such as brain imaging, tumor treatment, neuromodulation, and mental status monitoring. TCU holds immense potential by directing ultrasound waves to specific areas of the brain to intervene in or modulate pathological tissues. However, the acoustic impedance differences within the skull are the biggest challenge facing this technology. When ultrasound waves pass through the skull, most of the energy is reflected or absorbed, limiting effective transmission. While this phenomenon cannot be completely avoided, reflection loss can be significantly reduced by selecting a suitable medium to match the acoustic impedance between the ultrasound transducer and the tissue, thereby enhancing the transmission effect of ultrasound waves. The material and parameters of the medium need to be selected experimentally.

[0005] To address this, we propose an acoustically transparent skull material designed to counteract the high impedance of the skull, optimize ultrasound propagation efficiency within the skull, and maximize the therapeutic and monitoring effects of ultrasound. This technological advancement will enable broader future applications of ultrasound in pilot mental state monitoring, health surveillance, and other neuromodulation fields. Summary of the Invention

[0006] To address the problems of existing transcranial ultrasound technology, this invention provides an experimental method for the design, fabrication, and acoustic performance verification of an acoustically transparent skull. By using the transfer matrix method, the matching layer is optimized in terms of parameters and structure. At a selected frequency, optimal acoustic impedance band matching of the PEEK bionic skull is achieved, and focal distortion caused by interface reflection interference is significantly suppressed. This provides key experimental evidence for promoting the clinical and aerospace applications of transcranial ultrasound technology.

[0007] The present invention provides an acoustically transparent skull, comprising a coupling layer, a matching layer, and a biomimetic skull.

[0008] The bionic skull substrate is preferably made of PEEK material and formed by 3D printing. Its shape simulates the partial or whole skull structure of an adult skull. The predetermined sound beam incident area of ​​the bionic skull is prefabricated with a planar or weakly curved acoustic window that matches the beam size of the ultrasonic transducer. The acoustic window is processed into a stepped groove structure along the thickness direction to form an installation step on the outside of the skull.

[0009] The matching layer is a uniformly thick circular sheet cast and polished from an epoxy resin-nano alumina composite material. The outer diameter of each matching layer sheet is either interference-fitted or clearance-fitted to the mounting steps of the bionic skull. After insertion, axial compression and radial positioning are achieved through a connecting and fixing structure. Assuming the acoustic impedance of the coupling layer is Z1 and the acoustic impedance of the bionic skull is Z2, then the acoustic impedance of the matching layer is Z... m satisfy The thickness d of the matching layer is one-quarter of the working frequency band of the cranial ultrasound standardized verification sample.

[0010] The coupling layer is an ultrasound coupling gel or degassed water. When the acoustically transparent skull is connected to the ultrasound transducer, the coupling layer fills the space between the outer matching layer and the ultrasound transducer.

[0011] In a further improvement, the connection and fixing structure includes an annular pressure frame or threaded pressure ring set on the outside of the bionic skull and several fastening screws that cooperate with it. The inner edge of the pressure frame applies a uniform clamping force to the matching layer module, and the outer edge is connected and fixed to the pre-embedded copper nut or threaded hole of the bionic skull substrate through a threaded hole, so as to realize the quick loading and unloading of the matching layer module on the outside of the skull.

[0012] Further improvements include the matching layer plane or the weakly curved sheet structure.

[0013] The present invention also provides an experimental method for acoustically transparent skull, comprising the following steps: 1) Fabrication of biomimetic skull matrix: PEEK material is used to form the matrix by 3D printing, and its shape simulates the partial or whole skull structure of an adult skull; 2) Preparation of mounting steps: A planar or weakly curved "acoustic window" matching the beam size of the ultrasonic transducer is prefabricated in the predetermined sound beam incident area of ​​the biomimetic skull substrate. The acoustic window is processed into a stepped groove structure along the thickness direction to form a mounting step on the outside of the skull. 3) Preparation of matching layer: A uniformly thick circular sheet is cast and polished using epoxy resin-nano alumina composite material. The sheet is then fitted with the mounting steps on the bionic skull substrate with an interference or clearance fit. After insertion, axial compression and radial limiting are achieved through a connecting and fixing structure. The thickness of the prepared matching layer is one-quarter of the working frequency band of the cranial ultrasound standardized verification sample. 4) Connecting the ultrasonic transducer: The ultrasonic transducer is coaxially connected to the positioning hole on the reference plane on the outside of the bionic skull base through an external clamp positioning bracket. The ultrasonic transducer and the matching layer are filled with ultrasonic coupling gel or degassed water to form a connection structure of ultrasonic transducer-matching layer-skull-target point, so as to realize the rapid replacement and position reuse of matching layer modules with different λ / 4 thicknesses, single or double layers on the same bionic skull. 5) Verification of acoustic performance of ex vivo skull: The bionic skull sample was placed in a saline environment to simulate in vivo conditions. The ultrasonic transmittance was measured in three cases: no matching layer, with a single matching layer and with a double matching layer. The results were then compared with the COMSOL simulation results.

[0014] Further improvements are made by using irregularly shaped spherical alumina powder with a particle size of 1 μm and EPO-TEK301 epoxy resin to prepare the matching layer in step 3), employing a casting process. The specific method is as follows: 3.1) Add the corresponding mass fraction of alumina powder to the epoxy resin matrix that has been degassed, and stir thoroughly with a glass stirring rod until a fine white mixture is formed. Then place it in a vacuum chamber for 30 minutes to remove the bubbles. 3.2) Pour the defoamed mixture into the mold, remove any new air bubbles generated during the casting process, and centrifuge at 3500 r / min for 5 min. Place the centrifuged mixture into a constant temperature oven and cure it at 60℃. 3.3) Use sandpaper to polish the surface of the cured sample to ensure that its thickness is uniform.

[0015] Further improvements include step 3), where the operating frequency band of the standardized transcranial ultrasound verification sample is 300kHz and 700kHz.

[0016] Further improvements were made to the acoustic performance verification process of the ex vivo skull described in step 5), specifically as follows: a frequency sweep was used, covering frequencies from 0.5 to 1.0 MHz, and a hydrophone was used to receive the transmitted sound pressure signal. The sound intensity transmittance at each frequency point was calculated and compared with the COMSOL simulation results for verification. Simultaneously, an infrared thermal imager was used to monitor the temperature rise of the skull surface with thermocouples to ensure that the temperature change during the experiment was controlled within a safe range, with a temperature difference ΔT ≤ 2℃.

[0017] The beneficial effects of this invention are as follows: 1. This invention introduces the classical single-layer acoustic impedance matching theory into the design of transcranial acoustic verification prototypes by using the transmission matrix method and COMSOL simulation optimization approach. Using PEEK bionic skull as the substrate, the matching layer thickness is determined according to the quarter-wavelength principle around the target operating frequency band, thereby achieving narrowband optimal matching of PEEK bionic skull in the selected frequency band and significantly suppressing focal distortion caused by interface reflection interference.

[0018] 2. There is no obvious air gap between the matching layer and the PEEK skull, and repeated positioning is achieved. The matching layer module can be quickly loaded and unloaded on the outside of the skull through the connection and fixing structure. The matching layer module can be quickly replaced and reused on the same bionic skull.

[0019] 3. Through the design of an ex vivo skull experiment, the aim is to counteract the strong impedance problem caused by the skull, optimize the propagation efficiency of ultrasound waves in the skull, maximize the therapeutic and monitoring effects of ultrasound, verify the theoretical assumptions of acoustic matching layers, provide key experimental evidence for promoting the clinical and aviation applications of transcranial ultrasound technology, and make it possible for the wider application of ultrasound in the future for pilot mental state detection, health monitoring and other neuromodulation fields. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of a biomimetic skull structure. Figure 2 This is a schematic diagram of the process for preparing the matching layer using the centrifugation method; Figure 3 A schematic diagram of the experimental platform for measuring the sound velocity and sound attenuation coefficient of the matching layer; Figure 4 This is a schematic diagram of a bionic skull device. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides an acoustically transparent skull verification prototype with an integrated matching layer and a PEEK bionic skull, and the matching layer is modularly replaceable. Based on the transfer matrix method and COMSOL simulation optimization approach, the classic single-layer acoustic impedance matching theory is introduced into the design of the transcranial acoustic verification prototype. Using the PEEK bionic skull as the substrate, the matching layer thickness is determined according to the quarter-wavelength (λ / 4) principle around the target operating frequency band, such as 300kHz and 700kHz: for water / gel medium (acoustic impedance Z1) — matching layer (acoustic impedance Z... m —The PEEK skull (acoustic impedance Z2) is a three-layer structure. Ignoring absorption, the transfer matrix can be used to deduce that when the matching layer satisfies the characteristic impedance:

[0024] It also satisfies the phase condition. (c) m When the sound velocity in the matching layer is denoted as f0 (where f is the design center frequency), that is... The incident wave and the first reflected wave are approximately out of phase and superimposed at the water-side interface, causing the total reflection coefficient to approach zero and the transmitted sound intensity to reach a maximum value. This achieves narrowband optimal matching of the PEEK bionic skull in the selected frequency band and significantly suppresses the focal distortion caused by interface reflection interference.

[0025] Based on the above principles, the matching layer module uses an epoxy resin matrix mixed with inorganic fillers such as alumina of different proportions, particle sizes, and types to create various sheets with adjustable characteristic impedance. First, material parameters are inferred through sound velocity and impedance tests. Then, standard thickness series for different frequency bands such as 300kHz and 700kHz are determined according to the λ / 4 condition. A complete model of "piezoelectric sheet—matching layer—PEEK skull—target point" is established in COMSOL to compare the two cases: no matching layer and sheet with a thickness deviating from λ / 4. Simultaneously, this invention fabricates the matching layer as a planar or weakly curved sheet structure. The structure utilizes positioning and fixing structures such as slots, pressure frames, and threaded pressure rings on the outer side of the PEEK bionic skull to enable rapid loading, unloading, and reuse of single or multiple matching layers on the outer side of the skull. This allows the matching layer modules to be managed with standard dimensions and codes, enabling rapid replacement of λ / 4 matching layer combinations with different formulations, sound velocities, and thicknesses on the same bionic skull sample. It can also reserve mounting holes for temperature / pressure / sound field probes, facilitating experiments in various scenarios such as transcranial ultrasound transmittance, focusing morphology, sound field distribution, and energy deposition distribution in coupling gel or water bath environments. By integrating an adjustable matching layer precisely designed according to the target operating frequency band λ / 4 principle with a PEEK bionic skull into a single, detachable, and reusable physical prototype, this invention forms a standardized experimental platform for transcranial ultrasound that differs from existing exponential gradient flexible matching layers without relying on real skull samples. It can significantly improve the comparability and repeatability of experiments across frequency bands, formulations, and teams while ensuring ease of processing and assembly, reducing trial production costs and ethical barriers, and facilitating subsequent expansion to multi-frequency band and multi-mode transcranial acoustic applications by changing the matching layer formulation, thickness, and number of layers.

[0026] One specific embodiment of the present invention is as follows: The acoustically transparent skull consists of three parts: a bionic skull base, a replaceable matching layer module, and a connection and fixation structure for positioning and sealing. The specific structure and connection method of each part are as follows: The biomimetic skull matrix is ​​preferably made of PEEK material and formed by 3D printing. Its shape simulates the local structure of an adult skull, such as... Figure 1 As shown, a planar or weakly curved "acoustic window" matching the beam size of the ultrasonic transducer is prefabricated in the predetermined sound beam incident area. The acoustic window is processed into a stepped groove structure along the thickness direction, forming a first mounting step and a second mounting step on the outer side of the skull to accommodate a single or multiple matching layer modules.

[0027] Bionic skull devices such as Figure 4As shown. The matching layer module is an independent and detachable structure, cast and polished into a uniformly thick circular sheet using epoxy resin-nano alumina composite material. Its thickness is designed according to the principle of one-quarter wavelength (λ / 4) corresponding to the sound velocity of the target working frequency band. Standardized sheets of various thicknesses can be prepared according to different center frequencies. The outer diameter of each matching layer sheet is interference-fitted or clearance-fitted with the stepped groove of the acoustic window of the bionic skull substrate. After insertion, axial compression and radial limiting are achieved through the connecting and fixing structure, thereby ensuring that there is no obvious air gap between the matching layer and the PEEK skull and achieving repeatable positioning. The connecting and fixing structure may include an annular pressure frame, a threaded pressure ring, and several fastening screws set on the outside of the bionic skull. The inner edge of the pressure frame applies a uniform pressure force to the matching layer module, and the outer edge is connected and fixed to the threaded hole of the bionic skull substrate through the threaded hole, realizing the rapid loading and unloading of the matching layer module on the outside of the skull.

[0028] In the above process, the epoxy resin-nano aluminum powder composite material for the matching layer is prepared using the following casting process: In this experiment, irregularly shaped spherical alumina powder with a particle size of 1 μm and imported EPO-TEK301 epoxy resin were selected for the preparation of the alumina-epoxy resin matching layer, and a casting process was used. In the experiment, various matching layer samples with alumina powder mass fractions of 25%, 50%, 75%, 100%, 150%, and 200% were prepared. The specific preparation process is as follows: First, the corresponding mass fraction of alumina powder was added to the degassed epoxy resin matrix, and the mixture was thoroughly stirred with a glass stirring rod until a fine white mixture was formed. Then, the mixture was placed in a vacuum chamber for 30 minutes to remove air bubbles. Next, the degassed mixture was poured into a mold, and new air bubbles generated during casting were removed. The mixture was then centrifuged at 3500 r / min for 5 minutes. The centrifuged mixture, after separation, was placed in a constant temperature oven and cured at 60℃. Finally, the surface of the cured sample was sanded to ensure uniform thickness. Based on the simulation results, the thicknesses of the different component matching layer samples prepared using the above method were fabricated for ultrasonic testing at 300 kHz and 700 kHz. For these samples, their density was first calculated using physical methods, and then their acoustic properties were tested using the time-of-flight method.

[0029] The matching layer epoxy resin-nano aluminum powder composite material can also be prepared by centrifugation. The centrifugation preparation process is as follows: Figure 2As shown, specifically, the raw materials used in the centrifugation method for preparing the ultrasonic matching layer are basically the same as those used in the casting method. The matrix is ​​epoxy resin E-51, the curing agent is epoxy resin amine curing agent W93, and the fillers are alumina powder with a particle size of 5μm and tungsten powder with a particle size of 2μm. The main instruments required for preparation are: electronic balance, vacuum pump, centrifuge, constant temperature chamber, and cutting machine. The reason for choosing alumina powder with a particle size of 5μm instead of alumina powder with a particle size of 2.5μm as in the casting method is that experiments have shown that alumina powder with a particle size of 2.5μm does not easily form a gradient under centrifugation, resulting in poor experimental results.

[0030] Matrix preparation: Mix epoxy resin E51 and epoxy curing agent W93 at a mass ratio of 4:1 and stir until homogeneous. Record the mass of epoxy resin and curing agent added. Adding filler: Gradually add the calculated amount of alumina powder or tungsten powder to the matrix while continuously stirring until the filler powder is evenly distributed in the matrix. Here, when a large amount of powder is added and the mixture is quite viscous, the sample can be placed in a vacuum chamber for stirring, which will ensure a more thorough next step.

[0031] Vacuum treatment: Place the polymer in a bowl-shaped container and then place it in a vacuum chamber to remove air bubbles. Centrifugation: Pour the vacuum-treated, filler-containing matrix into centrifuge tubes, avoiding the introduction of air bubbles during this process. Then, place the centrifuge tubes in a centrifuge with a leveling rotor and centrifuge for several minutes before removing them.

[0032] Curing: Place the centrifuge tubes in an incubator for an extended period until the sample is completely cured. Finally, remove the matching layer from the centrifuge tubes.

[0033] Slicing: The cylindrical sample is typically sliced ​​for measurement. Finally, the sample surface must be thoroughly wiped clean. Due to the presence of metal powders such as copper and tungsten, the sample is prone to rusting if moisture is present on the surface after prolonged storage. The sample must also be wiped clean after each measurement in water.

[0034] After the layered sample is prepared, its key acoustic parameters need to be measured: sound velocity, density, and attenuation coefficient, etc., in order to calculate the acoustic impedance and evaluate its matching effect. The testing steps generally include: Density determination: A solid volume measurement and mass ratio method is used. The sample mass (m) is accurately measured, and its volume (V) is measured simultaneously. Dimensions can be obtained using calipers, and the density is calculated. .

[0035] Sound velocity test: The propagation velocity of ultrasonic waves in a sample is measured using the pulse transmission method. First, the thickness of the sample, with its upper and lower surfaces flat and parallel, is measured. Then, the transmitting and receiving probes are aligned and attached to both sides of the sample, and a coupling agent is applied to reduce loss. Next, the transmitting probe emits an ultrasonic pulse, and the receiving probe receives and records the time difference of the pulse passing through the sample. The velocity of sound in the sample can be expressed by the formula The calculation is performed, where d is the sample thickness. To improve accuracy, multiple points on the sample are typically measured and the average value is taken in the experiment.

[0036] Acoustic impedance calculation: Based on the measured sound velocity c and density ρ, calculate the acoustic impedance Z = cρ. This impedance value should be close to the design target to ensure matching with the impedance of water and skull.

[0037] Attenuation coefficient test: The absorption attenuation coefficient of the sample is calculated by measuring the pulse attenuation using the pulse transmission method. A common method is to compare the ultrasonic transmission intensity between samples of different thicknesses and calculate the attenuation coefficient using an exponential attenuation model.

[0038] Formula for calculating sound attenuation coefficient:

[0039] In the formula: h1 is the transmitted amplitude, h2 is the received amplitude, and L is the sample length.

[0040] After testing, data such as sound velocity, density, acoustic impedance, and attenuation characteristics of the sample will be obtained. By comparing the measured acoustic impedance with the design value, it can be verified whether the preparation process meets expectations. For further verification of anti-reflection performance, the sample can be placed in a water bath with a transducer to measure its transmission intensity curve; however, the focus of this study is on the determination of the acoustic parameters of the sample itself.

[0041] The method for conducting experiments using this invention is as follows: 1. Construction of the ultrasonic transmission experimental platform: Depend on Figure 3 An ultrasonic platform is constructed by placing a transparent water tank in the designated position on the experimental stage and injecting an appropriate amount of pure water, ensuring the water is homogeneous and free of obvious air bubbles to provide a stable acoustic environment for ultrasonic propagation. The power amplifier and ultrasonic generator are placed in a stable area above the water tank, with parameters meeting experimental requirements and a stable power supply ensured. The ultrasonic transducer and matching layer material are attached to the surface of the PEEK bionic skull using clamps. Another clamp is used to position a hydrophone to receive the signal along the sound wave propagation path. The signal is then amplified and transmitted to an oscilloscope. The oscilloscope waveform is observed. If the signal amplitude is unstable or waveform distortion occurs, the coupling state between the probe and the water body, as well as the alignment accuracy of the transmitter and receiver probes, must be checked. Adjustments are made repeatedly until the signal is stable and the waveform meets expectations.

[0042] 2. Experimental design of isolated skull: Transcranial ultrasound (TCU) technology holds immense potential for brain neuromodulation and monitoring, but its application is limited by severe energy attenuation and waveform distortion caused by the high acoustic impedance of the skull. Although the acoustic matching layer theory offers a possibility for improving ultrasound transmittance, its effectiveness, stability, and safety in real-world biological environments have not yet been systematically verified. This study aims to verify the acoustic matching layer theory through experiments at both in vitro and in vivo levels, providing crucial experimental evidence for advancing the clinical and aerospace applications of TCU technology.

[0043] 3. Verification of acoustic properties of ex vivo skull: A biomimetic skull sample was placed in a saline environment to simulate in vivo conditions. Ultrasonic transmittance was measured sequentially under three conditions: no matching layer, with a single matching layer, and with a double matching layer. A frequency sweep was used, covering frequencies from 0.5 to 1.0 MHz. Transmitted sound pressure signals were received using a hydrophone, and the acoustic transmittance at each frequency point was calculated and compared with COMSOL simulation results for verification. Simultaneously, an infrared thermal imager with thermocouples was used to monitor the temperature rise of the skull surface, ensuring that temperature changes were controlled within a safe range during the experiment, with a temperature difference ΔT ≤ 2℃.

[0044] The ultimate application of acoustically transparent skull materials is in the aerospace and medical fields. When applied to aerospace, the acoustically transparent skull provided by this invention needs to be validated in simulated aerospace environments, such as extreme environments with high noise, high vibration, high risk, and high pressure differentials. Research will investigate the improved ultrasound transmission effect of acoustically transparent skull materials at different ultrasonic frequencies using different materials, and conduct a comprehensive evaluation in conjunction with other physiological signals (such as electroencephalography and eye movements) to provide design guidance for optimizing material design.

[0045] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on its differences from other embodiments. In particular, for the device embodiments, the above descriptions are merely preferred embodiments of the present invention. Since they are fundamentally similar to the method embodiments, the descriptions are relatively simple, and relevant parts can be referred to the descriptions of the method embodiments. The above descriptions are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations and substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention, without departing from the principle of the present invention, should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An acoustically transparent skull, characterized in that: Includes a matching layer, a coupling layer, and a biomimetic skull; The bionic skull substrate is preferably made of PEEK material and formed by 3D printing, and its shape simulates the partial or whole skull structure of an adult skull. The predetermined sound beam incident area of ​​the bionic skull is prefabricated with a planar or weakly curved acoustic window that matches the beam size of the ultrasonic transducer. The acoustic window is processed into a stepped groove structure along the thickness direction to form an installation step on the outside of the skull. Assuming the acoustic impedance of the coupling layer filled when the bionic skull is connected to the external ultrasonic transducer is Z1, and the acoustic impedance of the bionic skull is Z2, then the acoustic impedance of the matching layer is Z... m satisfy The thickness d of the matching layer is one-quarter of the working frequency band of the cranial ultrasound standardized verification sample. The matching layer is a uniformly thick circular sheet made by centrifugation and polishing of epoxy resin-nano alumina composite material. The outer diameter of each matching layer sheet is interference-fitted or clearance-fitted with the installation step of the bionic skull. After insertion, axial compression and radial limiting are achieved through the connection and fixing structure. The coupling layer is an ultrasound coupling gel or degassed water, which is filled between the matching layer and the ultrasound transducer when the acoustically transparent skull is connected to the ultrasound transducer.

2. The acoustically transparent skull according to claim 1, characterized in that: The connection and fixing structure includes an annular pressure frame or threaded pressure ring set on the outside of the bionic skull and several fastening screws that cooperate with it. The inner edge of the pressure frame applies a uniform clamping force to the matching layer module, and the outer edge is connected and fixed to the pre-embedded copper nut or threaded hole of the bionic skull substrate through a threaded hole, so as to realize the quick loading and unloading of the matching layer module on the outside of the skull.

3. The acoustically transparent skull according to claim 1, characterized in that: The matching layer plane or weakly curved sheet structure.

4. An experimental method for acoustically transparent skull, characterized in that, Includes the following steps: 1) Fabrication of biomimetic skull matrix: PEEK material is used to form the matrix by 3D printing, and its shape simulates the partial or whole skull structure of an adult skull; 2) Preparation of mounting steps: A planar or weakly curved acoustic window matching the beam size of the ultrasonic transducer is prefabricated in the predetermined acoustic beam incident area of ​​the biomimetic skull substrate. The acoustic window is processed into a stepped groove structure along the thickness direction to form a mounting step on the outside of the skull. 3) Preparation of matching layer: A uniformly thick circular sheet is made by centrifuging and casting epoxy resin-nano alumina composite material and then grinding it. The sheet is then fitted with the mounting steps on the bionic skull substrate with an interference or clearance fit. After insertion, axial compression and radial limiting are achieved through the connecting and fixing structure. The thickness of the prepared matching layer is one-quarter of the working frequency wavelength of the cranial ultrasound standardized verification sample. 4) Connecting the ultrasonic transducer: The ultrasonic transducer is coaxially connected to the positioning hole on the reference plane on the outside of the bionic skull base through the external clamp positioning bracket. The ultrasonic transducer and the matching layer are filled with ultrasonic coupling gel or degassed water to form a connection structure of ultrasonic transducer-matching layer-skull-target point. This enables the rapid replacement and repositioning of one-quarter of the working frequency wavelength of different cranial ultrasound standardized verification samples, single or multi-layer matching layer modules on the same bionic skull. 5) Verification of the acoustic performance of the bionic skull: The bionic skull sample was placed in a physiological saline environment to simulate in vivo conditions. The ultrasonic transmittance was measured in three cases: no matching layer, single matching layer, and double matching layer. The results were then compared with the COMSOL simulation results for verification.

5. The experimental method for acoustically transparent skull according to claim 4, characterized in that: Step 3) The matching layer is prepared using irregular spherical alumina powder with a particle size of 1 μm and EPO-TEK301 epoxy resin by centrifugation. The specific method is as follows: 3.1) Add the corresponding mass fraction of alumina powder to the epoxy resin matrix that has been degassed, and stir thoroughly with a glass stirring rod until a fine white mixture is formed. Then place it in a vacuum chamber for 30 minutes to remove the bubbles. 3.2) Pour the defoamed mixture into the mold, remove any new air bubbles generated during the casting process, and centrifuge at 3500 r / min for 5 min. Place the centrifuged mixture into a constant temperature oven and cure it at 60℃. 3.3) Use sandpaper to polish the surface of the cured sample to ensure that its thickness is uniform.

6. The experimental method for acoustically transparent skull according to claim 4 or 5, characterized in that: Step 3) The operating frequencies of the standardized transcranial ultrasound verification sample are 300 kHz and 700 kHz.

7. The experimental method for acoustically transparent skull according to claim 4, characterized in that: Step 5) describes the acoustic performance verification process of the ex vivo skull, specifically as follows: a frequency sweep is used, covering frequencies from 0.1 to 1.0 MHz. A hydrophone is used to receive the transmitted sound pressure signal, and the sound intensity transmittance at each frequency point is calculated and compared with the COMSOL simulation results for verification. Simultaneously, an infrared thermal imager or thermocouple is used to monitor the temperature rise on the skull surface to ensure that the temperature change during the experiment is controlled within a safe range, with a temperature difference ΔT ≤ 2 ℃.

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