WO2021051652A1

WO2021051652A1 - Tunable cylindrical metasurface device for magnetic resonance imaging and preparation method

Info

Publication number: WO2021051652A1
Application number: PCT/CN2019/121273
Authority: WO
Inventors: 赵乾; 池中海; 孟永钢
Original assignee: 清华大学
Priority date: 2019-09-19
Filing date: 2020-05-13
Publication date: 2021-03-25
Also published as: CN112932442A; CN110638453A; CN110638453B

Abstract

Disclosed are a tunable cylindrical metasurface device for magnetic resonance imaging and a preparation method. The device comprises: a printed circuit board, a variable capacitor, annular guide pieces, and a cylindrical support. The printed circuit board comprises a dielectric plate and a first electrode and second electrodes respectively disposed at the front and the back of the dielectric plate, and orthographic projections of the second electrodes on the dielectric plate are located at two ends of an orthographic projection of the first electrode on the dielectric plate, so as to form a parallel-plate capacitor; the variable capacitor is connected in parallel to the parallel-plate capacitor; a first annular guide piece and a second annular guide piece are provided at two ends of the device, and the first annular guide piece and the second annular guide piece are respectively connected to the second electrodes at two ends; the cylindrical support is used for enabling the printed circuit board to be stably and regularly arranged in a circumferential array and generating an imageable area. The device is designed to better conform to the human body form, and can be suitable for MRI detection of objects to be detected having different load effects.

Description

Tunable cylindrical superstructure surface device for nuclear magnetic resonance imaging and preparation method

Cross-references to related applications

This application claims the priority of the Chinese Patent Application No. "201910887862.3" filed by Tsinghua University on September 19, 2019 with the title of "Tunable Cylindrical Metasurface Device for Nuclear Magnetic Resonance Imaging and Preparation Method".

Technical field

The invention relates to the technical field of nuclear magnetic resonance imaging, in particular to a tunable cylindrical metasurface device for nuclear magnetic resonance imaging and a preparation method.

Background technique

MRI (Magnetic Resonance Imaging) is a non-invasive detection method and an important basic diagnostic technology in the fields of medicine, biology, and neuroscience. The signal strength transmitted by traditional MRI equipment mainly depends on the strength of the static magnetic field B _0. The use of a high magnetic field or even an ultra-high magnetic field system can improve the signal-to-noise ratio, resolution and shorten the scanning time of the image. However, the increase in the intensity of the static magnetic field will bring about the following three problems: 1) the non-uniformity of the radio frequency (RF) field increases, and the difficulty of tuning increases; 2) the heat production of human tissues increases, which brings safety hazards, and patients are also prone to dizziness and vertigo. Adverse reactions such as vomiting: 3) The purchase cost has increased significantly, which is a burden for most small-scale hospitals. Therefore, how to use the smallest possible static magnetic field strength while achieving high imaging quality has become a crucial issue in MRI technology.

In response to the above problems, researchers have proposed a variety of solutions. The first is the radio frequency coil optimization method, which greatly promotes the improvement of the resolution and scanning speed of the detector in MRI. Studies have shown that the use of parallel imaging can reduce the scanning time, and the use of multi-channel coils can achieve better imaging quality and a larger detection area. However, the development of this scheme has been relatively complete so far, and considering the optimization of the coil, the MRI system needs to be redesigned, which brings a lot of inconvenience to practical applications. The second is to use special contrast agents to enhance the local magnetic field, such as rare earth magnetic atoms or magnetic nanoparticles. Since the contrast agent needs to be taken orally or injected into human tissues or organs, there are potential side effects and even life-threatening, so it is not the most ideal solution. The third is to increase the strength of the radio frequency magnetic field and reduce the specific absorption rate by introducing a plate or column-shaped dielectric resonator with a high dielectric constant in the MRI to achieve the effect of improving the imaging resolution and reducing the signal-to-noise ratio. The method is a new trend that can effectively improve the characteristics of MRI.

The appearance of metasurfaces (materials) provides a novel and more effective method for improving the quality and efficiency of MRI imaging. The metasurface has special properties that many natural materials do not have. The interaction between electromagnetic waves and metal or dielectric elements on the metastructure surface and the coupling effect between the elements can be used to achieve the effect of electromagnetic wave propagation path and electromagnetic field strength distribution. control. Its working principle is to use the electromagnetic resonance of its structural unit to achieve anisotropic and gradient distribution or even negative electromagnetic parameters, and through the design of the geometric size, shape and dielectric constant of the metastructure surface, it can realize the The resonance of the frequency point is enhanced. Metasurfaces have potential application prospects in the design and manufacture of devices and equipment that manipulate electromagnetic waves, and MRI is one of the important application areas.

There are three main indicators for evaluating the quality of MRI images, namely resolution, signal-to-noise ratio and contrast. The higher the image resolution, the more details that can be displayed; the higher the signal-to-noise ratio, the clearer the image. A high signal-to-noise ratio can support higher resolution or shorten the scanning time; contrast is to distinguish normal tissues from diseased tissues Important basis. Metasurfaces can improve the image signal-to-noise ratio by changing the magnetic field distribution in the NMR system.

However, the uniformity of the magnetic field distribution and the frequency adjustment mechanism of the metasurface devices proposed at present still need to be improved.

Summary of the invention

The present invention was completed based on the inventor's following findings:

The inventor found that in the current nuclear magnetic resonance system, the use of planar metasurface devices has the defect of poor uniformity of magnetic field distribution. As shown in Figure 1, Figure 1 shows the near magnetic field distribution of the planar metasurface, where the x direction represents the direction along the first electrode, and the y direction represents the direction perpendicular to x in a plane 30 mm from the metasurface. The z direction represents the height direction perpendicular to the metasurface. It can be seen that in the y direction and the z direction, the uniformity of the magnetic field is relatively poor, especially in the z direction, the magnetic field strength decays rapidly as the height increases. The inhomogeneity of the magnetic field will lead to different signal-to-noise ratio enhancement multiples in different parts, so that the image contrast will change to a certain extent, and it will affect the identification of the lesion in severe cases. The rapid attenuation of the magnetic field strength in the height direction will result in insufficient penetration depth of the metastructure surface, that is, insufficient depth that can be enhanced, and no enhancement effect on the deeper parts of the human body.

In addition, the resonance frequency of the metasurface is easily affected by the loading effect of the tested sample, especially under high field (≥3T) conditions, which will cause the metasurface to deviate from its optimal working state and reduce the enhancement effect. For this reason, the frequency adjustable design of the metasurface is very necessary. At present, the method of adjusting frequency reported in the literature is mainly to adjust the length of the metal wire, which makes the structure design more complicated.

The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

To this end, an object of the present invention is to provide a cylindrical metasurface device for use in an MRI system. The design of the device is more in line with the shape of the human body. The surface is closer to the inspected part, and a better enhancement effect is obtained; in addition, the resonance frequency of the cylindrical metasurface device is tunable, and can be applied to MRI inspection of test objects with different load effects.

Another object of the present invention is to provide a method for preparing a cylindrical superstructure surface device used in a nuclear magnetic resonance imaging system.

In order to achieve the above objective, an embodiment of the present invention proposes a cylindrical metasurface device for an MRI system, including: a printed circuit board, the printed circuit board includes a dielectric board and a dielectric board located on the dielectric board. The first electrode and the second electrode on the front and back sides, and the orthographic projection of the second electrode on the dielectric plate is located at both ends of the orthographic projection of the first electrode on the dielectric plate to form a parallel plate capacitor Variable capacitor, the variable capacitor is connected in parallel with the parallel plate capacitor; a first ring guide and a second ring guide, the first and second ring guides are arranged in the device two The first ring-shaped guide piece and the second ring-shaped guide piece are respectively connected to the second electrode at both ends, so that the structure capacitor formed by the first electrode and the second electrode is connected in series; The bracket is used for stably and regularly arranging the printed circuit board in a circular array and generating an imageable area.

According to the Biot-Savart theorem, the cylindrical metasurface device used in the nuclear magnetic resonance imaging system of the embodiment of the present invention has a good magnetic field uniformity in the central area of the metasurface, and can uniformly improve the signal-to-noise of the area. And the structure is cylindrical, and the arms, legs, etc. of the human body are also cylindrical. This design is more in line with the shape of the human body. On the one hand, it can save space and on the other hand, it can also make the superstructure surface distance from the detected part Closer, better enhancement effect is obtained; in addition, the resonance frequency of the cylindrical metasurface device is tunable, and can be applied to MRI detection of measured objects with different load effects.

In addition, the cylindrical metasurface device used in the nuclear magnetic resonance imaging system according to the above-mentioned embodiment of the present invention may also have the following additional technical features:

Further, in an embodiment of the present invention, the first electrode and the second electrode are both made of conductive non-magnetic materials.

Further, in an embodiment of the present invention, the conductive non-magnetic material includes one or more of copper, gold, and silver.

Further, in an embodiment of the present invention, the dielectric plate has a preset thickness and dielectric constant to serve as the dielectric of the structural capacitor, so that the metasurface has a target resonant frequency.

Further, in an embodiment of the present invention, a variable capacitor is connected in parallel with the parallel plate capacitor formed by the first electrode and the second electrode to adjust the equivalent capacitance of the device and adjust the resonant frequency To the target resonant frequency.

Further, in an embodiment of the present invention, after the first annular guide piece and the second annular guide piece are connected to the second electrode, the ends of the guide piece are connected to make the entire device isotropic.

Further, in an embodiment of the present invention, the initial resonance frequency of the metasurface device is obtained from the operating frequency of the nuclear magnetic resonance imaging system.

Further, in an embodiment of the present invention, the dielectric constant and thickness of the dielectric plate and the length of the second electrode are determined according to the initial resonance frequency of the metasurface device and the diameter of the cylinder.

In order to achieve the above objective, another embodiment of the present invention proposes a method for preparing a cylindrical metasurface device for an MRI system, which includes the following steps: determining the working frequency of the MRI system The initial resonance frequency of the metasurface device; according to the initial resonance frequency of the metasurface device and the designed cylindrical diameter, the method of numerical simulation is used to determine the dielectric constant and thickness of the dielectric plate and the second electrode Length; fabricate the printed circuit board and the cylindrical support according to the dielectric constant and thickness of the dielectric board and the length of the second electrode; arrange the printed circuit board regularly around the cylindrical support and use Two conductive sheets connect the second electrodes at both ends respectively, and the conductive sheets are connected end to end; the variable capacitors are connected in parallel at both ends of a pair of the first electrode and the second electrode by welding to prepare a cylindrical superstructure Surface devices.

According to the Biot-Savart theorem according to the method for preparing the cylindrical metasurface device for the nuclear magnetic resonance imaging system of the embodiment of the present invention, the central area of the metasurface has good magnetic field uniformity and can uniformly increase the area. The signal-to-noise ratio; and the structure is cylindrical, and the arms, legs, etc. of the human body are also cylindrical. This design is more in line with the shape of the human body. On the one hand, it can save space and on the other hand, it can also make the superstructure surface distance The detected part is closer, and better enhancement effect is obtained; in addition, the resonance frequency of the cylindrical metasurface device is tunable, and can be applied to MRI detection of objects under test with different load effects.

In addition, the method for manufacturing the cylindrical metasurface device for the nuclear magnetic resonance imaging system according to the above-mentioned embodiment of the present invention may also have the following additional technical features:

Further, in an embodiment of the present invention, the initial resonance frequency is 3-5 MHz higher than the operating frequency of the nuclear magnetic resonance imaging system.

The additional aspects and advantages of the present invention will be partially given in the following description, and some will become obvious from the following description, or be understood through the practice of the present invention.

Description of the drawings

The above and/or additional aspects and advantages of the present invention will become obvious and easy to understand from the following description of the embodiments in conjunction with the accompanying drawings, in which:

Figure 1 is a magnetic field distribution diagram of a planar metasurface device in the prior art;

2 is a schematic structural diagram of a cylindrical metasurface device used in an MRI system according to an embodiment of the present invention;

Figure 3 is a schematic diagram of a printed circuit board according to an embodiment of the present invention;

4 is a schematic diagram of the front side (top) and the back side (bottom) of a printed circuit board according to an embodiment of the present invention;

Fig. 5 is a magnetic field distribution diagram of a cylindrical metasurface device according to an embodiment of the present invention;

6 is a flowchart of a method for manufacturing a cylindrical metasurface device for an MRI system according to an embodiment of the present invention;

7 is a schematic diagram of the resonance frequency of the cylindrical metasurface device according to the embodiment of the present invention under different second electrode lengths obtained by electromagnetic simulation software according to the embodiment of the present invention;

FIG. 8 is a schematic diagram of the resonance frequency of the cylindrical metasurface device according to the embodiment of the present invention under different second electrode lengths obtained by the vector network analyzer and the coil measurement according to the embodiment of the present invention;

Fig. 9 is a nuclear magnetic resonance image of an isolated organism pig’s hoofs under the effect of (right) and without (left) embodiment cylindrical superstructure surface enhancement under the same other conditions according to an embodiment of the present invention.

Description of reference signs:

100: printed circuit board; 110: dielectric board; 120: first electrode; 130: second electrode; 200: ring-shaped conductive sheet; 300: cylindrical support; 310: fixed sheet on the cylindrical support; 400: variable capacitor .

detailed description

The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, in which the same or similar reference numerals indicate the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the drawings are exemplary, and are intended to explain the present invention, but should not be construed as limiting the present invention.

Before introducing the cylindrical metasurface device used in the nuclear magnetic resonance imaging system and the preparation method of the embodiment of the present invention, in order to facilitate understanding, the following briefly describes the metasurface device according to the embodiment of the present invention:

As before, most of the meta-surfaces currently used are planar, with the defects of uneven field distribution and small penetration depth. Specifically, Figure 1 shows the near magnetic field distribution of the planar metasurface, where the x direction represents the direction along the first electrode 120, the y direction represents the direction perpendicular to x in a plane 30 mm from the metasurface, and the z direction Indicates the height direction. It can be seen that in the y direction and the z direction, the uniformity of the magnetic field is relatively poor, especially in the z direction, the magnetic field strength decays rapidly as the height increases. The inhomogeneity of the magnetic field will lead to different signal-to-noise ratio enhancement multiples in different parts, so that the image contrast will change to a certain extent, and it will affect the identification of the lesion in severe cases. The rapid attenuation of the magnetic field strength in the height direction will result in insufficient penetration depth of the metastructure surface, that is, insufficient depth that can be enhanced, and no enhancement effect on the deeper parts of the human body.

In order to solve the defects in the prior art, the embodiment of the present invention proposes a cylindrical superstructure surface device used in an MRI system and a preparation method thereof. The following describes the tunable cylindrical metasurface device for nuclear magnetic resonance imaging and the preparation method proposed by the embodiments of the present invention with reference to the drawings. First, the cylinder for the nuclear magnetic resonance imaging system proposed by the embodiments of the present invention will be described with reference to the accompanying drawings. Type metasurface device.

Fig. 2 is a schematic structural diagram of a cylindrical metasurface device used in a nuclear magnetic resonance imaging system according to an embodiment of the present invention.

As shown in Fig. 2, the cylindrical metasurface device 10 used in the MRI system includes: a printed circuit board 100, a conductive sheet 200, a cylindrical support 300, and a variable capacitor 400 (not specifically identified in the figure).

3 and 4, the printed circuit board 100 includes a dielectric board 110 and a first electrode 120 and a second electrode 130 located on the front and back of the dielectric board 110, and the second electrode 130 is on the dielectric board 110 The orthographic projection of is located at both ends of the orthographic projection of the first electrode 120 on the dielectric plate 110 to form a parallel plate capacitor. The variable capacitor 400 is connected in parallel with the parallel plate capacitor. The shaped conductive piece 200 includes a first annular guide piece and a second annular guide piece. The first annular guide piece and the second annular guide piece are arranged at both ends of the device 10, and the first annular guide piece and the second annular guide piece are connected to the two The second electrode 130 at the end is connected, so that the structural capacitance formed by the first electrode 120 and the second electrode 130 is connected in series. The cylindrical support 3000 is used for stably and regularly arranging the printed circuit board 100 in a circular array and generating an imageable area.

It should be noted that, as shown in Figures and 2 to 4, the printed circuit board 100 is arranged in a circular array; the variable capacitor 400 is welded to one end of the printed circuit board 100, and the variable capacitor 400 is connected to the first electrode 120 and the first electrode 120 Parallel plate capacitors formed by two electrodes 130 are connected in parallel to adjust the equivalent capacitance of the device and adjust the resonant frequency to the target resonant frequency; after the first ring-shaped guide plate and the second ring-shaped guide plate are connected to the second electrode, the ends of the guide plate are connected The entire device is isotropic and the magnetic field is more uniform; the cylindrical bracket 300 has the function of supporting and fixing the printed circuit board, and the cylindrical bracket 300 can make the printed circuit board 100 stable and regular under the action of the fixing plate 310. It is arranged in a circular array and can provide a certain imageable area.

Specifically, the cylindrical superstructure surface is composed of 12 printed circuit boards 100 arranged in a circular array. The front and back sides of the printed circuit board 100 are respectively provided with a first electrode 120 and a second electrode 130 to form a parallel plate capacitor. A variable capacitor 400 is soldered to one end of one of the printed circuit boards 100, and the variable capacitor 400 is connected in parallel with the parallel plate capacitor. A ring-shaped conductive sheet 200 is respectively provided at both ends of the second electrode 130 for connecting the parallel plate capacitor, and the conductive ring is connected end to end, so that the entire structure is isotropic and the uniformity of the magnetic field is improved. In addition, the metasurface also includes a cylindrical bracket 300 for fixing the printed circuit board 100. Therefore, according to the Biot-Savart theorem, the central region of the metasurface has good magnetic field uniformity, and the signal-to-noise ratio of this region can be improved uniformly. Figure 5 shows the magnetic field distribution of a cylindrical metasurface with an inner diameter of 100mm in an embodiment, where the z direction is the axial direction of the cylindrical metasurface, and the x and y directions are the radial directions. It can be seen that the cylindrical superstructure The uniformity of the magnetic field in the central area of the surface is significantly better than that of the planar metasurface. And the structure is cylindrical, and the arms, legs, etc. of the human body are also cylindrical. This design is more in line with the shape of the human body. On the one hand, it can save space and on the other hand, it can also make the metastructure surface closer to the inspected part. , Get a better enhancement effect. In addition, by adjusting the capacitance value of the variable capacitor 400, the resonant frequency of the entire metasurface can be adjusted. The frequency tunability of the metasurface makes it suitable for MRI detection of tested objects with different load effects.

Hereinafter, according to specific embodiments of the present invention, each structure of the metasurface device will be described in detail:

According to the embodiment of the present invention, in conjunction with FIGS. 2 to 4, the metasurface device of the embodiment of the present invention is composed of 12 printed circuit boards 100 arranged in a circular array. The front and back sides of the printed circuit board 100 are respectively provided with first electrodes 120 and the second electrode 130, the two electrodes and the dielectric plate 110 in the middle form a parallel plate capacitor (structural capacitance). In other words, the LC circuit is formed on the cylindrical metasurface, and the magnetic field and electric field distribution of the cylindrical metasurface can be controlled by the LC resonance effect. When the metasurface is in resonance, the electromagnetic field on the surface of the printed circuit board will be large. The amplitude is enhanced, and the magnetic field is mainly distributed in the area between the two second electrodes 130, and the electric field is mainly distributed at both ends of the first electrode 120 on the printed circuit board. Therefore, the area between the two second electrodes 130 is used as a detection area. Area, while avoiding the electric field at both ends, reducing the influence of heat on detection.

Constructing a capacitor (capacitance formed by two electrodes) in a metasurface device helps to reduce the length of the detection area of the metasurface device. Specifically, as shown in FIGS. 2 to 4, the first electrode 120 and the second electrode 130 constitute a parallel plate capacitor, the length of the first electrode 120 is L ₁ , the length of the second electrode 130 is D, and the effective length of the first electrode 120 L ₂ is equal to the difference between the length of the first electrode 120 and the length of the two second electrodes 130, that is, L ₂ =L ₁ -2D. For a single printed circuit board, the resonance frequency has the following relationship with the effective length and the capacitance of the parallel plate capacitor:

Among them, λ is the electromagnetic wave wavelength at resonance, L ₂ is the effective length of the first electrode, W is the wave impedance of the first electrode, X is the capacitive reactance of the parallel plate capacitor, C is the capacitance of the parallel plate capacitor, and ω is the angular frequency , Ε ₀ is the vacuum dielectric constant, ε is the relative dielectric constant, S is the facing area of the two plates of the parallel plate capacitor, and d is the thickness of the dielectric.

When the capacitance C is 0, that is, when there is no capacitance in the metasurface device, the capacitive reactance X approaches -∞. Substitute formula (2) into formula (1),

Approaches 0, then

Approaching 0, we get that L ₂ approaches

In other words, when there is no capacitance in the metasurface device, the length of the detection area is

The length of the detection area is too large. When the capacitance is formed in the metasurface device, the length of the detection area is less than

Thus, forming a capacitor in a metasurface device can reduce the size of the structure.

According to the embodiment of the present invention, referring to FIG. 2, a plurality of printed circuit boards 100 are evenly distributed around the cylindrical support 300. Therefore, according to Biot-Savart theorem, a relatively uniform magnetic field can be generated inside the cylindrical metasurface. The number of printed circuit boards 100 is not particularly limited, as long as the metasurface device can generate a uniform magnetic field without generating excessive heat. Those skilled in the art can design according to the specific size of the printed circuit board. In this embodiment, the number of printed circuit boards is 12.

Further, in an embodiment of the present invention, both the first electrode 120 and the second electrode 130 are made of conductive non-magnetic materials, wherein the conductive non-magnetic materials include one or more of copper, gold, and silver. The dielectric plate 110 has a preset thickness and dielectric constant to serve as the dielectric of the structural capacitor, so that the metasurface has a target resonant frequency. Among them, the preset thickness can be understood as requiring the dielectric plate to have a certain thickness, and those skilled in the art can set a specific thickness according to actual conditions, which is not specifically limited here.

Specifically, the first electrode 120 and the second electrode 130 may both be made of conductive non-magnetic materials. Specifically, the material constituting the first electrode 120 and the second electrode 130 may include at least one of copper, gold, and silver. . The first electrode and the second electrode formed of the above-mentioned materials have good electrical conductivity and are easy to be processed and manufactured. In addition, the above-mentioned materials are non-magnetic materials and are suitable for nuclear magnetic resonance imaging systems.

The constituent material of the dielectric plate 110 between the first electrode and the second electrode is not particularly limited, as long as it is an insulating material and can form a parallel plate capacitor with the two electrodes, and those skilled in the art can design according to specific conditions. For example, according to an embodiment of the present invention, the dielectric plate 110 may be a glass fiber epoxy resin plate. According to an embodiment of the present invention, the thickness of the dielectric plate 110 may be 0.4-1.2 mm. Therefore, the metasurface device has the characteristics of thinner thickness and easy manufacture. According to specific embodiments of the present invention, the thickness of the dielectric plate 110 may be 0.6 mm, 0.8 mm, or 1.0 mm.

Further, the shape of the cylindrical support 300 is not particularly limited, as long as the magnetic field distribution is improved, such as an ellipse type, an arc type, and the like. In the embodiment of the present invention, the cylindrical stent is made by a 3D printing method, and the inside is a cylindrical cavity with a diameter of 94 mm, and the outside is a regular twelve prism with a wall thickness of 3 mm.

The number of variable capacitors and the range of capacitance values are not particularly limited, as long as the equivalent capacitance of the entire metasurface can be changed. Different people and different parts of the human body (arms and legs, etc.) have different loading effects, and have different effects on the resonance frequency of the metastructure surface. However, for common tests, this variation range is within 1MHz. In this embodiment, a variable capacitor is used, and its adjustment range is 1-10 pF. From the simulation results, it can be seen that the resonant frequency adjustment range of the variable capacitor in this range is 0.2-1 MHz.

According to an embodiment of the present invention, the initial resonance frequency of the metasurface device is 3-5 MHz higher than the operating frequency of the nuclear magnetic resonance imaging system. After the metasurface device is put into the MRI system, the resonance frequency of the metasurface device will be reduced, and after the object under test is added to the metasurface, the resonance frequency of the metasurface will be further reduced under the influence of the load effect. Therefore, the initial resonance frequency of the metasurface device is designed to be higher than the working frequency of the MRI system by a certain frequency margin, that is, 3-5MHz.

In summary, according to the Biot-Savart theorem, the cylindrical metasurface device for the nuclear magnetic resonance imaging system proposed in the embodiment of the present invention has a good magnetic field uniformity in the center area of the metasurface, and can uniformly improve the magnetic field. The signal-to-noise ratio of the area; and the structure is cylindrical, and the arms, legs, etc. of the human body are also cylindrical. This design is more in line with the human body shape. On the one hand, it can save space and on the other hand, it can also make a superstructure surface The distance to the inspected part is closer, and a better enhancement effect is obtained; in addition, the resonance frequency of the cylindrical metasurface device is tunable, and can be applied to MRI inspection of test objects with different load effects.

Next, the method for preparing the cylindrical metasurface device for the nuclear magnetic resonance imaging system according to the embodiment of the present invention will be described with reference to the accompanying drawings.

Fig. 6 is a flowchart of a method for manufacturing a cylindrical metasurface device for an MRI system according to an embodiment of the present invention.

As shown in Fig. 6, the preparation method of the cylindrical metasurface device used in the nuclear magnetic resonance imaging system includes the following steps:

In step S601, the initial resonance frequency of the metasurface device is determined according to the operating frequency of the nuclear magnetic resonance imaging system.

It is understandable that, considering the shielding effect of the MRI system on the metasurface, the initial resonance frequency of the metasurface device is 3-5MHz higher than the operating frequency of the MRI system.

Specifically, the working frequency of the nuclear magnetic resonance imaging system is determined by the main magnetic field in the nuclear magnetic resonance imaging system, and the calculation formula for the working frequency of the nuclear magnetic resonance imaging system is as follows:

f=γB ₀ (4)

Among them, f is the operating frequency of the nuclear magnetic resonance imaging system, γ is the gyromagnetic ratio, the γ value of hydrogen protons is 42.58MHz/T, and B ₀ is the main magnetic field intensity of the nuclear magnetic resonance imaging system.

For example, for an MRI system with a main magnetic field strength B ₀ of 1.5T, its working frequency f is 63.87 MHz, for _{an MRI system with a main magnetic field strength B 0} of 3T, its working frequency f is 127.74 MHz, for the main magnetic field An MRI system with an intensity B ₀ of 7T has an operating frequency f of 298.06 MHz.

According to the embodiment of the present invention, the initial resonance frequency of the metasurface device is 3-5MHz higher than the working frequency of the nuclear magnetic resonance imaging system. Therefore, after the main magnetic field intensity of the nuclear magnetic resonance imaging system is determined, the metasurface device can be determined The initial resonant frequency. For example, according to a specific embodiment of the present invention, for _{an MRI system with a main magnetic field strength B 0} of 3T, the initial resonance frequency f of the metasurface device may be 67 MHz, and for an MRI system _{with a main magnetic field strength B 0 of 3T} , The initial resonance frequency f of the metasurface device can be 132MHz. For _{an MRI system with a main magnetic field strength B 0} of 7T, the initial resonance frequency f of the metasurface device can be 302 MHz.

In step S602, according to the initial resonance frequency of the metasurface device and the designed cylinder diameter, the method of numerical simulation is used to determine the dielectric constant and thickness of the dielectric plate and the length of the second electrode.

Specifically, the dielectric board may be a glass fiber epoxy board, the dielectric constant of the glass fiber epoxy resin is 4.2-4.7, and the thickness of the dielectric board may be 0.4-1.2 mm. As a result, the finally formed metasurface device can have a thinner thickness, and the use function of the metasurface device can be realized.

According to the embodiment of the present invention, the dielectric constant, thickness, length of the first electrode of the dielectric plate, and the initial resonance frequency of the metasurface device are input on the electromagnetic simulation software, and the length of the second electrode is adjusted to make the metasurface device If the resonant frequency is equal to its initial resonant frequency, the length of the second electrode can be obtained.

In step S603, a printed circuit board and a cylindrical support are fabricated according to the dielectric constant and thickness of the dielectric board and the length of the second electrode.

Specifically, the widths of the first electrode and the second electrode are simulated according to the electromagnetic simulation software, and the first electrode and the second electrode are respectively arranged on the front and the back of the glass fiber epoxy resin board to obtain a printed circuit board with structural capacitance. The positional relationship between the first electrode and the second electrode has been described in detail above, and will not be repeated here.

In step S604, the printed circuit boards are regularly arranged around the cylindrical support, and two conductive sheets are used to respectively connect the second electrodes at both ends, and the conductive sheets themselves are connected end to end.

It can be understood that two pieces of conductive material are used to connect the second electrodes at both ends of the printed circuit board, and the conductive material itself is a closed loop, that is, connected end to end, making the cylindrical metasurface isotropic and increasing the uniformity of the magnetic field. In this embodiment, conductive tape is used to connect the second electrode.

In step S605, a welding method is used to connect variable capacitors in parallel at both ends of a pair of first electrodes and second electrodes to prepare a cylindrical metasurface device.

It is understandable that a pair of the first electrode and the second electrode are selected, and the variable capacitor is connected in parallel at both ends by welding, that is, a uniform circular array of the printed circuit board is placed around the cylindrical support and the variable capacitor is welded. The above-mentioned metasurface can be obtained.

Specifically, the second electrode faces outward and the first electrode faces inward, which facilitates subsequent connection of the second electrodes together and soldering a variable capacitor to one end of one of the printed circuit boards. It should be noted that either direction of the first electrode and the second electrode is ok. Although the resonant frequency of different placement methods will be different, the enhancement principle is the same.

According to the embodiment of the present invention, the parameters of each structure of the metasurface device are obtained by electromagnetic simulation. In order to ensure the accuracy of the use of the metasurface device, the electromagnetic simulation software, vector network analyzer and loop antenna are used to test the superstructure. Structure the resonant frequency of the surface device under different capacitance values. Specifically, referring to Figures 7 and 8, for the cylindrical metasurfaces with the second electrode lengths of 9.7mm, 9.5mm and 9.3mm respectively, the resonance frequencies obtained by the simulation are 132.5MHz, 133.9MHz and 134.9MHz, respectively. The resonant frequencies measured by the vector network analyzer and the coil are 132.6MHz, 133.7MHz, and 134.8MHz, respectively. It can be seen that the experimental and simulation results are consistent.

The solutions of the present invention are described below through specific examples. It should be noted that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the examples, it shall be carried out in accordance with the techniques or conditions described in the literature in the field or in accordance with the product specification. The instruments used without the manufacturer's indication are all conventional products that can be purchased commercially. The specific embodiments are as follows:

The metasurface device is composed of 12 printed circuit boards, two pieces of conductive tape and a cylindrical support. _{The length L 1} of the first electrode of the printed circuit board is 200 mm, the length D of the second electrode is 9 mm, the dielectric plate is a glass fiber epoxy resin plate, the dielectric constant is 4.2-4.7, and the thickness is 0.8 mm. The first electrode on the front side of the dielectric plate and the second electrode on the back side of the dielectric plate form a series of parallel plate capacitors, and the dielectric plate serves as the dielectric of the parallel plate capacitor. The inside of the cylindrical stent is a cylindrical cavity with a diameter of 94mm, and the outside is a regular twelve prism with a side length of 26mm and a height of 200mm. Twelve printed circuit boards are evenly distributed on the periphery of the cylindrical support. There are 12 structural capacitors at both ends, and the second electrode is connected with conductive tape to connect the structural capacitors at both ends, and the conductive tapes are connected end to end. As a result, a cylindrical superstructure surface device with uniform magnetic field distribution, large skin depth and conforming to the shape of the human body is constructed.

The GE Discovery 750 3.0T MRI imaging system of Beijing Tsinghua Chang Gung Memorial Hospital was used to test the image enhancement performance of a cylindrical superstructure surface of the embodiment. The experimental setup is as follows: body coil transmission, 12-channel head coil reception, and the test sequence is a gradient echo sequence. The experimental results are shown in Figure 9. It can be seen that under the same conditions, the use of cylindrical metasurfaces can increase the signal-to-noise ratio by more than 3 times, the image is clearer, and has good uniformity.

It should be noted that the foregoing explanation of the embodiment of the cylindrical metasurface device used in the nuclear magnetic resonance imaging system is also applicable to the preparation method of the cylindrical metasurface device used in the nuclear magnetic resonance imaging system of this embodiment. I won't repeat it here.

According to the preparation method of the cylindrical metasurface device for the nuclear magnetic resonance imaging system proposed by the embodiment of the present invention, according to the Biot-Savart theorem, the central region of the metasurface has good magnetic field uniformity and can be uniformly improved The signal-to-noise ratio in this area; and the structure is cylindrical, and the arms and legs of the human body are also cylindrical. This design is more in line with the human body shape. On the one hand, it can save space and on the other hand, it can also make superstructures. The surface is closer to the inspected part, and a better enhancement effect is obtained; in addition, the resonance frequency of the cylindrical metasurface device is tunable, and can be applied to MRI inspection of test objects with different load effects.

In the description of this specification, descriptions with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean specific features described in conjunction with the embodiment or example , Structure, materials or features are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics can be combined in any one or N embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine the different embodiments or examples and the features of the different embodiments or examples described in this specification without contradicting each other.

In addition, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined with "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, "N number" means at least two, such as two, three, etc., unless specifically defined otherwise.

Any process or method description in the flowchart or described in other ways herein can be understood as a module, segment or part of code that includes one or N executable instructions for implementing custom logic functions or steps of the process , And the scope of the preferred embodiments of the present invention includes additional implementations, which may not be in the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order according to the functions involved. It is understood by those skilled in the art to which the embodiments of the present invention belong.

The logic and/or steps represented in the flowchart or described in other ways herein, for example, can be considered as a sequenced list of executable instructions for implementing logic functions, and can be embodied in any computer-readable medium, For use by instruction execution systems, devices, or equipment (such as computer-based systems, systems including processors, or other systems that can fetch and execute instructions from instruction execution systems, devices, or equipment), or combine these instruction execution systems, devices Or equipment. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transmit a program for use by an instruction execution system, device, or device or in combination with these instruction execution systems, devices, or devices. More specific examples (non-exhaustive list) of computer readable media include the following: electrical connections (electronic devices) with one or N wires, portable computer disk cases (magnetic devices), random access memory (RAM), Read only memory (ROM), erasable and editable read only memory (EPROM or flash memory), fiber optic devices, and portable compact disk read only memory (CDROM). In addition, the computer-readable medium can even be paper or other suitable media on which the program can be printed, because it can be done, for example, by optically scanning the paper or other media, and then editing, interpreting, or other suitable media if necessary. The program is processed in a way to obtain the program electronically and then stored in the computer memory.

It should be understood that each part of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above embodiments, the N steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if it is implemented by hardware as in another embodiment, it can be implemented by any one or a combination of the following technologies known in the art: Discrete logic gate circuits with logic functions for data signals Logic circuit, application specific integrated circuit with suitable combinational logic gate circuit, programmable gate array (PGA), field programmable gate array (FPGA), etc.

A person of ordinary skill in the art can understand that all or part of the steps carried in the method of the foregoing embodiments can be implemented by a program instructing relevant hardware to complete. The program can be stored in a computer-readable storage medium. When executed, it includes one of the steps of the method embodiment or a combination thereof.

In addition, the functional units in the various embodiments of the present invention may be integrated into one processing module, or each unit may exist alone physically, or two or more units may be integrated into one module. The above-mentioned integrated modules can be implemented in the form of hardware or software function modules. If the integrated module is implemented in the form of a software function module and sold or used as an independent product, it can also be stored in a computer readable storage medium.

The above-mentioned storage medium may be a read-only memory, a magnetic disk or an optical disk, etc. Although the embodiments of the present invention have been shown and described above, it can be understood that the above-mentioned embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can comment on the above-mentioned embodiments within the scope of the present invention. The embodiment undergoes changes, modifications, substitutions, and modifications.

Claims

A cylindrical metasurface device used in a nuclear magnetic resonance imaging system, which is characterized in that it comprises:

A printed circuit board, the printed circuit board comprising a dielectric board and a first electrode and a second electrode respectively located on the front and back of the dielectric board, and the orthographic projection of the second electrode on the dielectric board is located on the first The electrodes are on both ends of the orthographic projection of the dielectric plate to form a parallel plate capacitor;

A variable capacitor, the variable capacitor is connected in parallel with the parallel plate capacitor;

A first annular guide piece and a second annular guide piece, the first annular guide piece and the second annular guide piece are arranged at both ends of the device, and the first annular guide piece and the second annular guide piece are respectively Connected to the second electrodes at both ends, so that the structural capacitance formed by the first electrode and the second electrode is connected in series;

The cylindrical support is used to make the printed circuit board be arranged in a stable and regular circular array and generate an imageable area.
The device according to claim 1, wherein the first electrode and the second electrode are both made of conductive non-magnetic materials.
The device according to claim 2, wherein the conductive non-magnetic material comprises one or more of copper, gold and silver.
The device according to claim 1, wherein the dielectric plate has a preset thickness and dielectric constant to serve as the dielectric of the structural capacitor, so that the metasurface has a target resonant frequency.
The device according to claim 4, wherein a variable capacitance is connected in parallel with the parallel plate capacitor formed by the first electrode and the second electrode to adjust the equivalent capacitance of the device and reduce the resonant frequency Adjust to the target resonance frequency.
The device according to claim 1, wherein after the first annular guide piece and the second annular guide piece are connected to the second electrode, the ends of the guide piece are connected, so that the entire device is isotropic .
The device according to claim 1, wherein the initial resonance frequency of the metasurface device is obtained from the operating frequency of the nuclear magnetic resonance imaging system.
7. The device according to claim 7, wherein the dielectric constant and thickness of the dielectric plate and the length of the second electrode are determined according to the initial resonance frequency and the diameter of the cylinder of the metasurface device.
A method for preparing a cylindrical metasurface device for an MRI system according to any one of claims 1-8, which is characterized in that it comprises the following steps:

Determining the initial resonance frequency of the metasurface device according to the working frequency of the nuclear magnetic resonance imaging system;

According to the initial resonance frequency of the metasurface device and the designed diameter of the cylinder, the method of numerical simulation is used to determine the permittivity and thickness of the dielectric plate and the length of the second electrode;

Fabricating the printed circuit board and the cylindrical support according to the dielectric constant and thickness of the dielectric board and the length of the second electrode;

Arranging the printed circuit boards regularly around the cylindrical support, and using two conductive sheets to connect the second electrodes at both ends respectively, and the conductive sheets themselves are connected end to end; and

A welding method is used to connect variable capacitors in parallel at both ends of a pair of first electrodes and second electrodes to prepare cylindrical superstructure surface devices.
The method according to claim 9, wherein the initial resonance frequency is 3-5 MHz higher than the working frequency of the nuclear magnetic resonance imaging system.