An exponential gradient flexible matching layer for matching the skull, its preparation method and application

Abstract

This invention discloses an exponentially gradient flexible matching layer for matching the skull. The matching layer includes several conical structures, and along the height direction of the conical structures, the acoustic impedance gradually changes exponentially from one end of the matching layer to the other. Alternatively, the matching layer includes a mold base and several conical structures disposed on the mold base. This invention also discloses a method for preparing the exponentially gradient flexible matching layer for matching the skull and its application in the preparation of products for cranial ultrasound imaging and high-intensity focused ultrasound therapy. The matching layer provided by this invention exhibits excellent acoustic intensity transmission performance when applied to cranial ultrasound imaging and high-intensity focused ultrasound therapy.

Description

Technical Field

[0001] This invention belongs to the field of acoustic impedance matching materials technology, and specifically relates to an exponential gradient flexible matching layer for matching the skull, its preparation method and application. Background Technology

[0002] Ultrasound is a sound wave with a frequency higher than 20kHz. It has good directionality and strong reflection ability, making it easy to obtain concentrated sound energy. It travels farther in water than in air and can be used for ranging, speed measurement, cleaning, welding, lithotripsy, sterilization, and disinfection. As the application range of ultrasound continues to expand, increasingly higher requirements are being placed on the measurement accuracy, measurement range, and ultrasonic power of ultrasound.

[0003] In ultrasonic testing and biomedical ultrasonic engineering, impedance mismatch between a high acoustic impedance piezoelectric layer and a low acoustic impedance working medium can cause a large loss of acoustic energy. When ultrasound penetrates the skull, a huge potential barrier is formed at the skull interface due to acoustic impedance mismatch, causing most of the sound wave energy to be reflected by the skull, which seriously affects the accuracy of the test.

[0004] Currently, commonly used matching layer technology involves adding several layers of matching layer material with a gradient distribution of characteristic impedance. This results in a gradual change in acoustic impedance from one end of the matching layer to the other, thus widening the transducer bandwidth. For example, Chinese Patent Publication No. CA114273193A discloses a matching layer with a uniformly gradient change in acoustic impedance and an ultrasonic transducer. The matching layer with a uniformly gradient change in acoustic impedance includes a shell, a conical structure, and a filling material. One or more conical structures are disposed within the shell, and the filling material fills the gap between the conical structures and the shell. Along the height direction of the conical structures, the acoustic impedance gradually changes from one end of the matching layer to the other. For example, Chinese Patent Publication No. 110270493A discloses an ultrasonic transducer, an acoustic impedance matching layer, and its preparation method. The acoustic impedance matching layer of this invention includes magnetic particles coated with a low-density material layer and an adhesive. The magnetic particles coated with the low-density material layer are gradient-distributed in the adhesive according to their different acoustic impedances, resulting in a gradual change in acoustic impedance from one end of the matching layer to the other.

[0005] However, the following problems still exist: impedance mismatch still exists between the matching layers, causing energy loss; the frequency of high-frequency ultrasonic transducers is high, the required matching layer thickness is small, the processing accuracy is difficult to control, and the cost is high; the emitted pulse wave contains multiple frequency components, and the theoretical thickness of the resulting matching layer is different. A fixed matching layer thickness will cause different loss of sound waves of different frequency components, affecting signal integrity. Summary of the Invention

[0006] The purpose of this invention is to provide an exponential gradient flexible matching layer for matching the skull, its preparation method and application, which has good acoustic intensity transmission performance when applied to cranial ultrasound imaging and high-intensity focused ultrasound therapy.

[0007] This invention provides the following technical solution:

[0008] An exponentially gradient-variable flexible matching layer for matching the skull, the matching layer comprising several conical structures, wherein the acoustic impedance varies exponentially from one end of the matching layer to the other along the height direction of the conical structures.

[0009] The bottom of the cone-shaped structure faces the skull.

[0010] In this invention, multiple conical structures are arranged alternately or closely. When arranged closely, the acoustic impedance exhibits a more pronounced e-exponential gradient distribution from one end of the matching layer to the other. The density of the conical structures is determined by the magnitude of the desired acoustic impedance gradient; the smaller the acoustic impedance gradient, the denser the conical structure distribution.

[0011] The present invention also provides another exponential gradient flexible matching layer for matching skull, the matching layer comprising a mold base and a plurality of the aforementioned conical structures disposed on the mold base.

[0012] The conical structure is positioned within the mold material such that its bottom is connected to the bottom of the mold base. This design provides strong structural stability and ease of fabrication.

[0013] In this invention, the cross-section of the mold base is circular.

[0014] In the invention, the conical structure and the mold base are an integral structure.

[0015] In this invention, the conical structure has two modes: taking the plane containing the upper surface of the skull as the xy plane, the conical structure is obtained by stretching a segment of the exponential function e and its symmetrical figure about the y-axis in the z-direction by a certain length (or understood as stretching a segment of the exponential function e and its closed figure enclosed by its projections in the x and y directions and its symmetrical figure about the y-axis in the z-direction by a certain length); or a three-dimensional figure obtained by rotating a segment of the exponential function e around the y-direction axis by one revolution (or understood as rotating a segment of the exponential function e and its closed figure enclosed by its projections in the x and y directions by the y-direction axis by one revolution).

[0016] The conical structure contains a filling material. Preferably, the filling material is a material with an acoustic impedance close to that of the skull, meaning it is between 80% and 120% of the acoustic impedance of the skull. The acoustic impedance of the skull is 7.5 μm.

[0017] Preferably, the cross-sectional size of the conical structure varies exponentially along its height direction.

[0018] Preferably, in another exponentially gradient flexible matching layer for matching the skull provided by the present invention, the material of the mold substrate is agarose gel. In the present invention, the mold material should be a material with an acoustic impedance similar to that of the medium to be tested. For example, the medium to be tested is generally water, and the acoustic impedance of water is 1.5 mRayl. Therefore, the conical structure can be selected from agarose gel with an acoustic impedance almost equal to 1.5 mRayl.

[0019] Since the acoustic impedance of the mold material is almost the same as that of water, in underwater testing applications, the mold material can be peeled off after the filling material solidifies, which allows the filling material to fit the skull better without affecting the matching performance.

[0020] Preferably, the conical structure satisfies:

[0021] y = A + Be Cx

[0022] Where A+B=Z1, Where Z1 is the required low acoustic impedance, Z2 is the required high acoustic impedance, and d is the height of the conical structure. Typically, A = 0. In this invention, the low acoustic impedance is the acoustic impedance of water, and the high acoustic impedance is the acoustic impedance of the skull. Without considering absorption, the gradient matching layer satisfying this structure has the least reflection of ultrasound waves in the direction of sound propagation, thus its transmission performance is better. The calculation process is relatively complex, and the steps involving the main coefficients are as follows:

[0023] The characteristic impedance of the dielectric is:

[0024]

[0025] Where p represents sound pressure, v consists of waves propagating in both directions, ρ is the density of the material, and c is the speed of sound of ultrasound propagating in the material.

[0026] For non-uniform layers, they are typically divided into 30 incremental layers, where for the i-th layer, the parameter α is calculated. i As shown below, where d represents the thickness.

[0027] α i =(1 / 2d i )ln[z(x i+1 ) / z(x i )]

[0028] For a matching layer with an exponentially varying acoustic impedance, its energy reflection coefficient E R for:

[0029]

[0030] Where f represents the frequency of the ultrasound.

[0031] Define the matching layer thickness as equal to half the wavelength. Calculations show that waves with frequencies higher than half the wavelength will be transmitted through the e-exponentially graded matching layer with a reflection loss of less than 10%.

[0032] Preferably, the conical structure satisfies: y = 0.44 + 0.47e -1.83x .

[0033] The present invention also provides a method for preparing the above-mentioned exponential gradient flexible matching layer for matching the skull, the method comprising:

[0034] (1) Design and fabrication of a conical structure: A conical structure mold with edges defined by a specific e-exponential function was fabricated using a 3D printer;

[0035] (2) Using agarose gel for molding: Prepare an agarose solution and pour it onto a conical mold. After curing, remove the conical mold to complete the molding process and obtain the agarose mold base and the conical structure set on the agarose mold base.

[0036] (3) Injecting filler material: Fill the cone-shaped structure on the agarose mold base with filler material, and use the whole as a matching layer after curing; or remove the agarose mold base after curing and use the cone-shaped structure peeled off separately as a matching layer.

[0037] The mold base has an open end after casting, which facilitates the filling material into the conical structure and allows the filling material to directly contact the skull during underwater testing of the matching layer.

[0038] The present invention also provides an application of the above-mentioned exponential gradient flexible matching layer for matching the skull in the preparation of products for cranial ultrasound imaging and high-intensity focused ultrasound therapy.

[0039] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0040] (1) Compared with the traditional uniform matching layer, the filling material of the matching layer has a uniform density and the acoustic impedance is distributed in an e-exponential gradient. The acoustic impedance of the matching layer changes from the acoustic impedance of the skull to the acoustic impedance of the working medium in an e-exponential gradient.

[0041] (2) The acoustic impedance change gradient of the matching layer in this invention can be adjusted by the size, shape and density of the cone structure, so as to meet different acoustic impedance change gradient requirements.

[0042] (3) In the application scenario of underwater testing, the mold base can be peeled off after the filling material solidifies to improve the flexibility of the matching layer, so that the filling material fits the skull better, reducing the sound energy loss caused by poor fit, and without affecting the matching performance. Attached Figure Description

[0043] Figure 1 Two ways to realize the conical structure: (a) a one-dimensional structural unit obtained by stretching the e-exponential curve, and (b) a three-dimensional structural unit obtained by rotating the e-exponential curve.

[0044] Figure 2 This is a schematic diagram of the conical structure of the matching layer in Example 1, where the acoustic impedance varies exponentially with an e-index gradient.

[0045] Figure 3 This is a three-dimensional structural diagram of the matching layer in Example 1, where the acoustic impedance varies according to an e-exponential gradient.

[0046] Figure 4 The following are physical images of the matching layer with acoustic impedance varying according to the e-exponential gradient in Example 1: (a) a 3D printed cone structure, (b) a cone structure obtained by molding with agarose gel, (c) a cone structure obtained by molding with agarose gel, and (d) a cone structure obtained by cutting open the matching layer after injecting filler material.

[0047] Figure 5 The image shows the actual cone-shaped matching layer obtained by removing the agarose molding portion and peeling it off separately in Example 1. The acoustic impedance of the bottom part is consistent with that of the skull, which does not affect practical application. It can be retained or removed.

[0048] Figure 6 This is a schematic diagram of the simulation using Comsol and the underwater test.

[0049] Figure 7 The results of simulations using Comsol and MATLAB are as follows: (a) The acoustic intensity transmission and reflection coefficients obtained after simulation in Comsol and corresponding FFT processing using MATLAB in Example 1; (b) The acoustic intensity transmission and reflection coefficients obtained after simulation in Comsol and corresponding FFT processing using MATLAB in the traditional matching layer.

[0050] Figure 8 The results of simulation calculations of Example 1 and the linear tapered gradient matching layer in Comsol and MATLAB are as follows: (a) The structure of Example 1 in Comsol simulation; (b) The acoustic intensity transmission and reflection coefficients obtained after corresponding FFT processing in MATLAB; (c) The structure of the traditional matching layer in Comsol simulation; (d) The acoustic intensity transmission and reflection coefficients obtained after corresponding FFT processing in MATLAB. Detailed Implementation

[0051] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. Here, the embodiments and descriptions of the present invention are used to explain the invention, but are not intended to limit the invention.

[0052] The matching layer proposed in this invention, with acoustic impedance varying exponentially (e), has an agarose gel as its mold substrate and a filling material obtained by uniformly mixing and curing tungsten powder and polydimethylsiloxane (PDMS). The specific preparation method is as follows:

[0053] (1) Design and fabrication of the conical structure. The conical structure with edges defined by a specific exponential function e was drawn using AutoCAD and then fabricated using a 3D printer.

[0054] In this context, the plane containing the upper surface of the skull is taken as the xy-plane, and the conical structure is obtained by stretching a segment of an exponential function e and its symmetrical figure about the y-axis by a certain length in the z-direction, as shown below. Figure 1 As shown in (a); or a three-dimensional figure obtained by rotating a segment of the exponential function e around the y-axis, as shown in Figure (a); Figure 1 As shown in (b) of the diagram.

[0055] (2) Casting using agarose gel. Prepare a 3% agarose solution and heat it to 105°C while stirring. After observing that the solution is clear and transparent, cool it to 60°C and continue stirring for about 30 minutes until the bubbles in the solution disappear. Pour the solution onto the cone-shaped structure obtained in (1) and let it stand at room temperature for about 3 hours until the solution becomes gel-like and completely solidifies. Remove the cone-shaped structure mold from (1) to complete the casting.

[0056] (3) Injecting filler material. Mix 2µm tungsten powder particles, PDMS and curing agent uniformly at a mass ratio of 20:10:1, and then fill the cone-shaped structure of the agarose mold in (2). Let it stand at room temperature for about 12 hours until the filler material is cured.

[0057] It should be noted that the directional indication in the embodiments of the present invention is only used to explain the relative positional relationship and movement of the components in a specific posture. When the specific posture changes, the directional indication will also change accordingly.

[0058] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixing," etc., should be interpreted broadly. For example, "fixing" can mean a fixed connection, a detachable connection, or an integral part; it can mean a direct connection or an indirect connection through an intermediate medium, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0059] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0060] Example

[0061] like Figure 2 to- Figure 5 The matching layer with acoustic impedance varying exponentially according to the e-index provided in this embodiment includes a conical structure 1, a filling material 2, and a mold base 3. Multiple conical structures 1 are provided in the mold base 3, and the filling material 2 is filled in the conical structures 1. Along the height direction of the conical structure 1, the acoustic impedance is distributed in a gradient from one end to the other end of the matching layer.

[0062] In this embodiment, the conical structure 1 satisfies: y = 0.44 + 0.47e -1.83x .

[0063] In this embodiment, the filler material 2 is obtained by uniformly mixing tungsten powder particles, PDMS and curing agent in a mass ratio of 30:10:1. The acoustic impedance of tungsten powder is 54 mrayl. The mold material is agarose gel with an acoustic impedance of 1.5 mrayl. The test medium is water, and the acoustic impedance of water is 1.5 mrayl. Thus, the acoustic impedance gradient changes from 30 mrayl to 1.5 mrayl.

[0064] A simulation was performed in the time domain using Comsol Multiphysics software. The simulation diagram is shown below. Figure 6 The obtained sound intensity transmission and reflection coefficient Figure 7 As shown in (a), the sound intensity transmission coefficient remains in a high range between 0.5MHz and 2.5MHz, indicating that this gradient matching layer has better sound intensity transmission performance than the traditional matching layer.

[0065] Comparative Example 1

[0066] Comparative Example 1 uses a conventional 1 / 4 wavelength matching layer. Its upper layer material is the same as filler material 2, consisting of tungsten powder particles, PDMS, and curing agent uniformly mixed in a mass ratio of 20:10:1, with a thickness of approximately 3 mm. The lower layer material is the same as the mold substrate 3, which is agarose gel. The matching layer material is thus obtained.

[0067] The acoustic intensity transmission and reflection coefficients were obtained by performing a simulation in the time domain using Comsol Multiphysics software, as shown below. Figure 7In (b), although the sound intensity transmission performance is very good at its matched 2MHz, the sound intensity transmission performance at other frequencies is significantly worse than that of the e-index graded matching layer.

[0068] Comparative Example 2

[0069] Comparative Example 2 uses a linearly tapered, gradually changing matching layer, and its structural cross-section is as follows: Figure 8 As shown in (b) above, the acoustic intensity transmission and reflection coefficients obtained after MATLB processing are as follows: Figure 8 As shown in (d) in the figure, under the condition that all other conditions are exactly the same, although Comparative Example 2 has good sound intensity transmission performance at its matched 2MHz, its matching bandwidth is significantly less than that of the e-index gradient matching layer, and its sound intensity transmission performance in the 0-2MHz range is also slightly lower than that of the Example.

[0070] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An exponentially gradient-varying flexible matching layer for cranial ultrasound imaging, characterized in that, The matching layer includes several conical structures, and the acoustic impedance is distributed in an e-exponential gradient from one end to the other along the height direction of the conical structures. The conical structure is obtained by stretching a segment of the exponential function e and its symmetrical figure about the y-axis in the z-direction by a certain length; or by rotating a segment of the exponential function e around the y-axis to obtain a three-dimensional figure. The conical structure satisfies: y = 0.44 + 0.47e -1.83x ; The matching layer includes a mold base and several conical structures disposed on the mold base; the material of the mold base is agarose gel.

2. The exponential gradient-varying flexible matching layer for cranial ultrasound imaging according to claim 1, characterized in that, The conical structure is filled with a filling material, which is a material with an acoustic impedance close to that of the skull. The acoustic impedance being close to that of the skull is between 80% and 120% of the acoustic impedance of the skull.

3. The exponential gradient-variable flexible matching layer for cranial ultrasound imaging according to claim 1, characterized in that, Along the height of the conical structure, the size of its cross-section changes exponentially.

4. A method for preparing the exponentially gradient-varying flexible matching layer for cranial ultrasound imaging as described in claim 1, characterized in that, The method includes: (1) Design and fabrication of a conical structure: A conical structure mold with edges that are specific e-exponential functions was fabricated using a 3D printer; (2) Using agarose gel for molding: Prepare an agarose solution and pour it onto a conical mold. After curing, remove the conical mold to complete the molding process and obtain the agarose mold base and the conical structure set on the agarose mold base; (3) Injecting filler material: Fill the cone-shaped structure on the agarose mold base with filler material, and use the whole as a matching layer after curing; or remove the agarose mold base after curing and use the cone-shaped structure peeled off separately as a matching layer.

5. The application of the exponential gradient-variable flexible matching layer of claim 1 in the preparation of products for cranial ultrasound imaging.

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