A Structure for Eliminating Standing Waves in a Magnetic Resonance Radio Frequency Transmission Field and Its Design Method

Abstract

The present invention discloses a structure for eliminating standing waves in a magnetic resonance radio frequency emission field and a design method thereof, which relates to the technical field of magnetic resonance imaging. The structure for eliminating standing waves in the magnetic resonance radio frequency emission field includes a phase velocity matching body that is used to be placed in a waveguide of the magnetic resonance radio frequency emission field and filled between an imaging target body and a waveguide wall body. Wherein, the phase velocity matching body is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target body. Thus, through the foregoing structural design, after being applied in the waveguide of the magnetic resonance radio frequency emission field, the phase velocities of electromagnetic waves propagating in the surrounding area of the imaging target body and in the imaging target body can be kept consistent, and further, the problem of secondary standing waves caused by phase velocity mismatch can be eliminated, which is convenient for practical application and popularization.

Description

Technical Field

[0001] The present invention belongs to the field of magnetic resonance imaging technology, that is, to the application of electromagnetic field microwave technology in the field of medical imaging, and specifically relates to a magnetic resonance radio frequency transmission field standing wave elimination structure and a design method thereof for a radio frequency transmission system in a magnetic resonance imaging device. Background Art

[0002] Magnetic resonance imaging technology is an important means of clinical diagnosis in modern medicine. Improving the main magnetic field strength of the magnetic resonance imaging system can achieve advantages such as higher signal-to-noise ratio and contrast, but the operating frequency will also increase accordingly. The operating frequency is determined by the Larmor frequency (specifically, the frequency of the cyclotron motion of particles in the magnetic field, which describes how fast the particles rotate around the magnetic field lines), that is, the operating frequency Among them, γ represents the magnetic gyroscopic ratio, B0 represents the main magnetic field strength, and π represents pi. At high operating frequencies, the wavelength of the electromagnetic field will shorten and approach the size of the human body (when the main magnetic field strength is 7T, the operating frequency under magnetic resonance is 297MHz, and the wavelength in the human body is about 20cm). When the human body becomes electrically large (specifically refers to structures whose physical size is much smaller than a wavelength), the standing wave effect will cause the magnetic resonance RF excitation field to The amplitude of the RF field varies spatially, which leads to uneven RF field excitation in the human body. The amplitude of also determines the flip angle of the spin excitation and thus the contrast of the image, and non-uniform contrast can introduce significant errors in medical diagnostic applications, thus posing an engineering challenge that needs to be addressed.

[0003] At present, the mainstream engineering solution is to use a multi-channel RF transmission link to drive the phased array coil, such as Figure 1 As shown, by adjusting the amplitude and phase of each transmitting unit, the RF excitation field is realized. The field averaging function can obtain a uniform RF transmission field within a specific field of view. However, this solution cannot eliminate standing waves essentially, but it can regulate the distribution of standing wave effects in space through multi-channel control, such as Figure 2 As shown, the current distribution mode of the coil determines that the electromagnetic wave is transmitted from the periphery to the center, which will inevitably generate standing waves in the human body.

[0004] Another existing engineering solution is to use traveling wave transmission, such as Figure 3 As shown in the figure, the magnetic resonance magnet and the inner diameter of the gradient coil together form a waveguide for transmitting the radio frequency electromagnetic field, so as to excite the spin protons in the radiation area. In theory, this method will not form a standing wave because the electromagnetic wave propagates in a single direction, but due to the phase velocity v of the electromagnetic wave propagating in the air and in the human body, p ( Among them, μ represents the magnetic permeability of the propagation medium, and ε represents the permittivity of the propagation medium. Since they are inconsistent, when electromagnetic waves enter the human body, incident waves in multiple directions will be generated, thus forming secondary standing waves in the human body. Summary of the Invention

[0005] The object of the present invention is to provide a structure for eliminating standing waves in a magnetic resonance radio frequency emission field and its design method, so as to solve the problem that the existing engineering solution using the traveling wave emission method will still form secondary standing waves in the imaging target body.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In the first aspect, a structure for eliminating standing waves in a magnetic resonance radio frequency emission field is provided, including a phase velocity matching body placed in the waveguide of the magnetic resonance radio frequency emission field and filled between the imaging target body and the waveguide wall body. Among them, the phase velocity matching body is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target body.

[0008] Based on the above invention content, a new solution for eliminating standing waves applicable to the radio frequency traveling wave emission system in a magnetic resonance imaging device is provided, that is, including a phase velocity matching body placed in the waveguide of the magnetic resonance radio frequency emission field and filled between the imaging target body and the waveguide wall body. Among them, the phase velocity matching body is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target body. Thus, through the above structural design, after being applied in the waveguide of the magnetic resonance radio frequency emission field, the phase velocities of electromagnetic waves propagating in the surrounding area of the imaging target body and in the imaging target body can be kept consistent, and further the problem of secondary standing waves caused by phase velocity mismatch can be eliminated, which is convenient for practical application and popularization.

[0009] In a possible design, the waveguide of the magnetic resonance radio frequency emission field is surrounded by the inner wall of the magnet metal and the inner wall of the gradient coil metal in the magnetic resonance radio frequency emission system, where the waveguide wall body is specifically the inner wall of the gradient coil metal.

[0010] In a possible design, the conductivity of the dielectric material is less than or equal to 10 -8 S / m, and the tangent of the dielectric loss angle of the dielectric material is less than or equal to 0.5.

[0011] In a possible design, the relative magnetic permeability of the dielectric material is equal to 1.

[0012] In a possible design, the dielectric constant being close to the average dielectric constant of the imaging target body means that the dielectric constant belongs to the interval Among them, represents the average dielectric constant of the imaging target body.

[0013] In a possible design, the dielectric material is a natural material, an artificial electromagnetic material, or a metamaterial composed of periodic metal structure units.

[0014] In a possible design, the geometric shape of the phase velocity matching body adopts any geometric shape that can be reasonably placed in the waveguide gap, where the waveguide gap refers to the gap between the imaging target body and the waveguide wall body.

[0015] In a possible design, the phase velocity matching body is a solid, a liquid, or a gas.

[0016] In a possible design, when the phase velocity matching body is a liquid or a gas, a container for containing the phase velocity matching body is further included.

[0017] In a second aspect, a design method for a magnetic resonance radio frequency emission field standing wave elimination structure as described in the first aspect or any possible design in the first aspect is further provided, including the following steps S1 to S6:

[0018] S1. Model a magnetic resonance waveguide environment in electromagnetic simulation software, and then execute step S2, where the magnetic resonance waveguide environment refers to the waveguide environment of the magnetic resonance radio frequency emission field;

[0019] S2. Place the electromagnetic model of the imaging target body near the input-side waveguide port of the magnetic resonance waveguide environment, and simulate and measure to obtain the first propagation constant β1 of the magnetic resonance waveguide environment, and then execute step S3;

[0020] S3. Remove the electromagnetic model of the imaging target body from the magnetic resonance waveguide environment, and then place the initial electromagnetic model of the phase velocity matching body near the input-side waveguide port of the magnetic resonance waveguide environment and between the placement area of the electromagnetic model of the imaging target body and the waveguide sidewall of the magnetic resonance waveguide environment, and simulate and measure again to obtain the second propagation constant β2 of the magnetic resonance waveguide environment, and then execute step S4;

[0021] S4. Determine whether the second propagation constant β2 is equal to the first propagation constant β1. If so, execute step S6; otherwise, execute step S5;

[0022] S5. Optimize the current electromagnetic model of the phase velocity matching body by adjusting the geometric shape and / or dielectric constant of the phase velocity matching body to obtain a new electromagnetic model of the phase velocity matching body, and re-simulate and measure to obtain the new second propagation constant β2 of the magnetic resonance waveguide environment, and then return to execute step S4;

[0023] S6. Prepare a physical object of the phase velocity matching body according to the current electromagnetic model of the phase velocity matching body.

[0024] Advantages of the above solution:

[0025] (1) The present invention creatively provides a new solution for eliminating standing waves in a radio frequency traveling wave emission system applicable to a magnetic resonance imaging device, that is, it includes a phase velocity matching body placed in a waveguide of a magnetic resonance radio frequency emission field and filled between an imaging target body and a waveguide wall body. Among them, the phase velocity matching body is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target body. Through the above structural design, after being applied in the waveguide of the magnetic resonance radio frequency emission field, the phase velocities of electromagnetic waves propagating in the surrounding area of the imaging target body and in the imaging target body can be made consistent, thereby eliminating the secondary standing wave problem caused by phase velocity mismatch, which is convenient for practical application and popularization;

[0026] (2) Compared with the classical engineering solution based on multi-channel radio frequency emission technology and combined with a phased array volume emission coil, this embodiment has a simpler system architecture, low cost and high robustness, and when applied, the propagation direction of electromagnetic waves is single, and standing waves are not easily generated, and it is ensured that when the imaging target body is located in the radiation area of the antenna, the electromagnetic waves are easy to control. Description of the Drawings

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0028] Figure 1 It is a schematic diagram of driving a phased array coil by a multi-channel radio frequency emission link in the mainstream engineering solution provided by the prior art.

[0029] Figure 2 It is an example diagram of the application of a phased array coil in a waveguide of a magnetic resonance radio frequency emission field provided by the prior art.

[0030] Figure 3 It is an example diagram of a secondary standing wave problem formed during the application of another engineering solution provided by the prior art.

[0031] Figure 4 It is an example diagram of the application of the standing wave elimination structure in the magnetic resonance radio frequency emission field provided by the embodiment of the present application in a waveguide of the magnetic resonance radio frequency emission field.

[0032] Figure 5This is a comparative example diagram of electromagnetic simulation experiment results before and after introducing a standing wave elimination structure for the magnetic resonance radio frequency transmission field into the waveguide provided by the embodiment of the present application.

[0033] Figure 6 This is an example diagram of the placement of two electromagnetic models in the design method of the standing wave elimination structure for the magnetic resonance radio frequency transmission field provided by the embodiment of the present application.

[0034] In the above-mentioned drawings: 11 - Radio frequency transmission link; 12 - Phased array coil; 101 - Feeding coil; 102 - Inner wall of the magnet metal; 103 - Inner wall of the gradient coil metal; 200 - Imaging target body; 400 - Phase velocity matching body. Detailed implementation manners

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the present invention in combination with the drawings and the descriptions of the embodiments or the prior art. Obviously, the following descriptions of the drawing structures are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings. It should be noted here that the descriptions of these embodiment modes are used to help understand the present invention, but do not constitute a limitation to the present invention.

[0036] It should be understood that although terms such as first and second etc. may be used herein to describe various objects, these objects should not be limited by these terms. These terms are only used to distinguish one object from another. For example, the first object can be called the second object, and similarly, the second object can be called the first object, without departing from the scope of the exemplary embodiments of the present invention.

[0037] It should be understood that for the term "and / or" that may appear herein, it is only a description of the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, B exists alone, or A and B exist simultaneously, etc. three situations; again, for example, A, B and / or C can represent any one of A, B and C or any combination of them; for the term " / and" that may appear herein, it is a description of another association object relationship, indicating that two relationships can exist. For example, A / and B can represent: A exists alone or A and B exist simultaneously, etc. two situations; in addition, for the character " / " that may appear herein, generally it means that the front and rear associated objects are an "or" relationship.

[0038] Embodiment 1

[0039] As Figure 4As shown, the standing wave elimination structure for the magnetic resonance radio frequency emission field provided in this embodiment includes, but is not limited to, a phase velocity matching body 400 that is placed in the waveguide of the magnetic resonance radio frequency emission field and filled between the imaging target body 200 and the waveguide wall body. Among them, the phase velocity matching body 400 is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target body 200.

[0040] As Figure 4 shown, in the specific structure of the standing wave elimination structure for the magnetic resonance radio frequency emission field, the waveguide of the magnetic resonance radio frequency emission field is an existing structure for guiding electromagnetic waves in a specific direction, specifically surrounded by the magnet metal inner wall 102 and the gradient coil metal inner wall 103 in the existing magnetic resonance radio frequency emission system. Among them, the waveguide wall body is specifically the gradient coil metal inner wall 103. The imaging target body 200 is specifically but not limited to a human body to be subjected to magnetic resonance imaging. The phase velocity matching body 400 is used to fill the space between the imaging target body 200 and the waveguide wall body, and thus is used to reduce the phase velocity of electromagnetic wave propagation in the surrounding area of the imaging target body 200. (where β represents the propagation constant and ω represents the angular velocity of the electromagnetic wave; since the phase velocity is determined by the dielectric constant ε and the magnetic permeability μ of the propagation medium, filling the phase velocity matching body 400 with a dielectric constant equal to or close to the average dielectric constant of the imaging target body 200 between the imaging target body 200 and the waveguide wall body can reduce the phase velocity of electromagnetic wave propagation in the surrounding area of the imaging target body 200), so that the phase velocities of the electromagnetic wave propagating in the surrounding area of the imaging target body 200 and in the imaging target body 200 can be kept consistent, and thus the secondary standing wave problem caused by phase velocity mismatch can be eliminated. In addition, the magnetic resonance radio frequency emission system further includes as Figure 4 shown a feeding coil 101; the aforementioned example that the dielectric constant is close to the average dielectric constant of the imaging target body 200 means that the dielectric constant belongs to the interval Among them, represents the average dielectric constant of the imaging target body 200.

[0041] As Figure 5As shown, the left-side diagram shows the electromagnetic simulation experiment results before introducing the standing wave elimination structure of the magnetic resonance radio frequency emission field into the waveguide of the magnetic resonance radio frequency emission field. It can be seen that due to the phase velocity mismatch, after the traveling electromagnetic wave enters the interior of the imaging target body 200, the electromagnetic field amplitude alternates between high and low, indicating the existence of standing waves. The right-side diagram shows the electromagnetic simulation experiment results after introducing the standing wave elimination structure of the magnetic resonance radio frequency emission field into the waveguide of the magnetic resonance radio frequency emission field. It can be seen that the standing wave effect is effectively suppressed. In addition, since the boundary condition of the phase velocity matching body 400 can also ensure the continuity of the electromagnetic field components at the air-medium interface, even if the imaging target body-waveguide gap space is not completely filled, the phase velocity of the surrounding overall area can be uniformly reduced, so as to achieve the purpose of standing wave elimination / suppression to a certain extent.

[0042] Preferably, the conductivity of the dielectric material is less than or equal to 10 -8 S / m, and the tangent of the dielectric loss angle of the dielectric material is less than or equal to 0.5. Through the foregoing limitations, it is also possible to ensure a certain reduction in the propagation loss.

[0043] Preferably, the relative magnetic permeability of the dielectric material is equal to 1. Through the foregoing limitations, it is also possible to ensure a certain degree of avoiding interference with the main magnetic field of the magnetic resonance.

[0044] Specifically, the dielectric material can be a natural material, an artificial electromagnetic material, or a metamaterial composed of periodic metal structure units.

[0045] Specifically, the geometric shape of the phase velocity matching body 400 adopts any geometric shape that can be reasonably placed in the waveguide gap, where the waveguide gap refers to the gap between the imaging target body 200 and the waveguide wall body.

[0046] Specifically, the phase velocity matching body 400 is a solid, a liquid, or a gas. When the phase velocity matching body 400 is a liquid or a gas, it also includes a container for containing the phase velocity matching body 400.

[0047] In summary, adopting the standing wave elimination structure of the magnetic resonance radio frequency emission field provided by this embodiment has the following technical effects:

[0048] (1) This embodiment provides a new scheme for eliminating standing waves in a radio frequency traveling wave emission system applicable to magnetic resonance imaging equipment, that is, it includes a phase velocity matching body placed in a waveguide of a magnetic resonance radio frequency emission field and filled between an imaging target and a waveguide wall. Among them, the phase velocity matching body is made of a dielectric material with a dielectric constant equal to or close to the average dielectric constant of the imaging target. Through the above structural design, after being applied in the waveguide of the magnetic resonance radio frequency emission field, the phase velocities of electromagnetic waves propagating in the surrounding area of the imaging target and in the imaging target can be kept consistent, thereby eliminating the secondary standing wave problem caused by phase velocity mismatch, which is convenient for practical application and promotion;

[0049] (2) Compared with the classical engineering solution based on multi-channel radio frequency emission technology and combined with a phased array volume emission coil, this embodiment has a simpler system architecture, low cost, and high robustness. Moreover, when applied, the propagation direction of electromagnetic waves is single, standing waves are not easily generated, and it is ensured that when the imaging target is located in the radiation area of the antenna, the electromagnetic waves are easy to regulate.

[0050] Embodiment Two

[0051] Based on the technical solution of the foregoing Embodiment One, this embodiment also provides a design method for the standing wave elimination structure of the magnetic resonance radio frequency emission field as described in Embodiment One, specifically but not limited to including the following steps S1 to S6.

[0052] S1. Model a magnetic resonance waveguide environment in an electromagnetic simulation software, and then execute step S2, where the magnetic resonance waveguide environment refers to the waveguide environment of the magnetic resonance radio frequency emission field.

[0053] In step S1, the electromagnetic simulation software is existing software, and the specific modeling process is prior art.

[0054] S2. Place the electromagnetic model of the imaging target 200 near the input side waveguide port of the magnetic resonance waveguide environment, and simulate and measure to obtain the first propagation constant β1 of the magnetic resonance waveguide environment, and then execute step S3.

[0055] In step S2, the specific placement point of the electromagnetic model of the imaging target 200 is as Figure 6 shown in the upper content. The specific process of the simulation measurement is a conventional function of existing electromagnetic simulation software.

[0056] S3. Remove the electromagnetic model of the imaging target body 200 from the magnetic resonance waveguide environment, then place the initial electromagnetic model of the phase velocity matching body 400 near the input-side waveguide port of the magnetic resonance waveguide environment and between the placement area of the electromagnetic model of the imaging target body 200 and the waveguide sidewall of the magnetic resonance waveguide environment, and perform simulation measurement again to obtain the second propagation constant β2 of the magnetic resonance waveguide environment, and then execute step S4.

[0057] In step S3, the specific placement point of the initial electromagnetic model of the phase velocity matching body 400 is as Figure 6 shown in the following content.

[0058] S4. Judge whether the second propagation constant β2 is equal to the first propagation constant β1. If so, execute step S6; otherwise, execute step S5.

[0059] S5. Optimize the current electromagnetic model of the phase velocity matching body 400 by adjusting the geometry and / or dielectric constant of the phase velocity matching body 400 to obtain a new electromagnetic model of the phase velocity matching body 400, and re-perform simulation measurement to obtain a new second propagation constant β2 of the magnetic resonance waveguide environment, and then return to execute step S4.

[0060] In step S5, the specific adjustment method of the geometry and / or dielectric constant of the phase velocity matching body 400 can be a manual adjustment method or an automatic adjustment method based on a certain existing optimization algorithm (such as the grey wolf optimization algorithm).

[0061] S6. Prepare a physical object of the phase velocity matching body 400 according to the current electromagnetic model of the phase velocity matching body 400.

[0062] Thus, based on the phase velocity matching body design method described in the above steps S1 - S6, it can also be ensured that a qualified and usable magnetic resonance radio frequency emission field standing wave elimination structure can be obtained, and the design cost can be reduced.

[0063] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A design method for a magnetic resonance radio frequency transmission field standing wave elimination structure, characterized in that: The magnetic resonance radio frequency transmission field standing wave elimination structure comprises a phase velocity matching body (400) for being placed in a waveguide of the magnetic resonance radio frequency transmission field and filled between an imaging target body (200) and a waveguide wall body, wherein the phase velocity matching body (400) is made of a dielectric material having a dielectric constant equal to or close to an average dielectric constant of the imaging target body (200); The design method includes the following steps S1 to S6: S1. Modeling a magnetic resonance waveguide environment in electromagnetic simulation software, and then executing step S2, wherein the magnetic resonance waveguide environment refers to the waveguide environment of the magnetic resonance radio frequency transmission field; S2. placing the electromagnetic model of the imaging target (200) near the input side waveguide port of the magnetic resonance waveguide environment, and obtaining the first propagation constant β1 of the magnetic resonance waveguide environment by simulation measurement, and then executing step S3; S3. removing the electromagnetic model of the imaging target body (200) from the magnetic resonance waveguide environment, and then placing the initial electromagnetic model of the phase velocity matching body (400) near the input side waveguide port of the magnetic resonance waveguide environment and between the electromagnetic model placement area of ​​the imaging target body (200) and the waveguide side wall of the magnetic resonance waveguide environment, and again simulating and measuring to obtain the second propagation constant β2 of the magnetic resonance waveguide environment, and then executing step S4; S4. Determine whether the second propagation constant β2 is equal to the first propagation constant β1, if so, execute step S6, otherwise execute step S5; S5. By adjusting the geometric shape and / or dielectric constant of the phase velocity matching body (400), the current electromagnetic model of the phase velocity matching body (400) is optimized to obtain a new electromagnetic model of the phase velocity matching body (400), and re-simulation and measurement are performed to obtain a new second propagation constant β2 of the magnetic resonance waveguide environment, and then the process returns to step S4; S6. According to the current electromagnetic model of the phase velocity matching body (400), a physical object of the phase velocity matching body (400) is prepared.

2. The design method according to claim 1, characterized in that: The waveguide of the magnetic resonance radio frequency transmission field is surrounded by a metal inner wall (102) of a magnet and a metal inner wall (103) of a gradient coil in the magnetic resonance radio frequency transmission system, wherein the waveguide wall is specifically the metal inner wall (103) of the gradient coil.

3. The design method according to claim 1, characterized in that: The electrical conductivity of the dielectric material is less than or equal to 10 - 8 S / m, and the dielectric loss tangent of the dielectric material is less than or equal to 0.

5.

4. The design method according to claim 1, characterized in that: The relative magnetic permeability of the dielectric material is equal to 1.

5. The design method according to claim 1, characterized in that: The dielectric constant being close to the average dielectric constant of the imaging target (200) means that the dielectric constant belongs to the interval [ε×85%, ε×115%], wherein ε represents the average dielectric constant of the imaging target (200).

6. The design method according to claim 1, characterized in that: The dielectric material is a natural material, an artificial electromagnetic material or a metamaterial composed of periodic metal structural units.

7. The design method according to claim 1, characterized in that: The geometric shape of the phase velocity matching body (400) adopts any geometric shape that can be reasonably placed in a waveguide gap, wherein the waveguide gap refers to a gap between the imaging target body (200) and the waveguide wall body.

8. The design method according to claim 1, characterized in that: The phase velocity matching body (400) is solid, liquid or gas.

9. The design method according to claim 1, characterized in that: When the phase velocity matching body (400) is liquid or gas, a container for containing the phase velocity matching body (400) is also included.

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