Magnetic resonance imaging magnetic field focusing intensifier based on metasurface

Abstract

The invention relates to a metasurface-based magnetic resonance imaging magnetic field focusing intensifier, which comprises a unit array arranged on the same plane, and is characterized in that each unit comprises a substrate and a spiral metal wire fixed on the substrate; the head end and the tail end of the spiral metal wire are connected through a variable capacitor. The side length and the number of the units are determined through the size of the imaging target area; a unit located in a magnetic resonance imaging target area resonates at the working frequency of magnetic resonance imaging through tuning of a variable capacitor; other units which are not in a magnetic resonance imaging target area are tuned through a variable capacitor, and the resonance frequency is lower than the magnetic resonance imaging working frequency; when magnetic resonance imaging is carried out, the magnetic field intensity in an imaging target area is enhanced by Fano resonance, the magnetic field intensity in an area outside an imaging target is weakened, and magnetic field energy is gathered in the imaging target area, so that the signal-to-noise ratio of magnetic resonance imaging is improved.

Description

Technical Field

[0001] The invention relates to the field of magnetic resonance imaging, and in particular to a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface. Background Art

[0002] Magnetic resonance imaging (MRI) is a non-invasive diagnostic technique commonly used in clinical practice. Its principle is to destroy the precession balance of atomic nuclei in an external enhanced static magnetic field by transmitting radio frequency pulses at Larmor frequency through a radio frequency transmitting coil. After the radio frequency pulse stops, the radio frequency receiving coil receives the magnetic resonance signal in the process of restoring the precession balance of atomic nuclei based on Faraday's law of electromagnetic induction, and reconstructs the image through a computer.

[0003] Metasurface is a type of artificial composite sub-wavelength periodic structure material, which usually has special electromagnetic properties such as negative dielectric constant or negative magnetic permeability that natural materials do not have. By designing the unit size and geometric shape, the metasurface can control the propagation path of electromagnetic waves and the redistribution of field intensity. As the two-dimensional counterpart of metamaterials, metasurfaces reduce one dimension compared to metamaterials. Therefore, they are smaller in size, lighter in weight, and more advantageous in processing and integration.

[0004] Fano resonance is an asymmetric linear resonance phenomenon. Due to its significant field enhancement effect, it has been widely used in sensing, switching and wireless energy transmission in recent years.

[0005] In magnetic resonance imaging, improving the signal-to-noise ratio (SNR) can achieve higher image resolution or shorter signal acquisition time. Since the signal-to-noise ratio is proportional to the static magnetic flux density, high-field (≥7T) magnetic resonance has the potential to achieve high signal-to-noise ratio imaging. However, due to the close proximity of the radio frequency wavelength to the geometric dimensions of the human body, the standing wave effect produced by high-field magnetic resonance causes light and dark artifacts to appear on the image, resulting in a serious loss of image contrast. 1.5T magnetic resonance imaging has been widely used in clinical diagnosis due to its advantages such as low cost and high safety. However, due to its low field strength and shallow penetration depth, the imaging clarity is low and the imaging time is long, which seriously affects the imaging quality and the comfort of the patient being tested.

[0006] In order to improve the signal-to-noise ratio of magnetic resonance imaging without increasing the strength of the static magnetic field, many types of metasurfaces have been used to enhance magnetic resonance imaging. However, traditional metasurfaces have limited effect on improving the signal-to-noise ratio, and cannot concentrate the collected radio frequency magnetic field within a small field of view (FOV) to further improve the signal-to-noise ratio in a small FOV. Based on this, how to provide a metasurface that can control the focusing of the magnetic field to significantly enhance the signal-to-noise ratio in a small FOV has become an urgent problem to be solved. Summary of the invention

[0007] The present invention overcomes the above-mentioned deficiencies of the prior art and provides a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface, which forms Fano resonance by controlling the different resonant frequencies of each unit in the magnetic field focusing enhancer based on a metasurface. By utilizing the characteristics of constructive interference and destructive interference of the field strength of Fano resonance, the magnetic field strength in the imaging target area is enhanced, and the magnetic field strength outside the imaging target area is weakened, so as to achieve the purpose of focusing the magnetic field energy in the imaging target area, thereby improving the signal-to-noise ratio of magnetic resonance imaging. The technical solution is as follows:

[0008] A magnetic resonance imaging magnetic field focusing enhancer based on a metasurface comprises a unit array arranged on the same plane, each unit comprising a substrate and a spiral metal wire fixed on the substrate; the two ends of the spiral metal wire are connected by a variable capacitor; the side length and number of the units are determined by the size of the imaging target area; the units in the magnetic resonance imaging target area resonate at the working frequency of the magnetic resonance imaging through the tuning of the variable capacitor; the remaining units not in the magnetic resonance imaging target area resonate at a frequency lower than the working frequency of the magnetic resonance imaging through the tuning of the variable capacitor; so that when performing magnetic resonance imaging, the magnetic field intensity in the imaging target area is enhanced by Fano resonance, the magnetic field intensity in the area outside the imaging target is weakened, and the magnetic field energy is concentrated in the imaging target area, thereby improving the signal-to-noise ratio of the magnetic resonance imaging.

[0009] Furthermore, in the electromagnetic wave finite element simulation software, the remaining geometric parameters including the line width, gap, number of turns and thickness of the spiral metal wire are optimized according to the side length and number of the required units, so that the magnetic field enhancement multiple is maximized.

[0010] Furthermore, the resonant frequencies of the units located in and not located in the target area of ​​magnetic resonance imaging are determined respectively, and appropriate capacitance values ​​are calculated in electromagnetic wave finite element simulation software according to the determined unit geometric parameters, so that each unit can resonate at its required target frequency.

[0011] Furthermore, it includes 3×3 units with equal spacing, and the side length of the unit is 80 mm; the position of one central unit is aligned with the position of the magnetic resonance imaging target area.

[0012] Furthermore, the spiral metal wire is circular, has an inner diameter of 25 mm, a wire width of 2 mm, a turn spacing of 1 mm, and 3 turns.

[0013] Furthermore, for a 1.5T MRI system, the unit within the imaging target area resonates at the operating frequency of 63.8 MHz of the 1.5T MRI system and is loaded with a capacitance of 5.31 pF; the unit outside the imaging target area is loaded with a capacitance of 6.08 pF and resonates at 60.8 MHz.

[0014] Furthermore, the spiral metal wire is in a circular or square shape.

[0015] The beneficial effects of the present invention are as follows: the magnetic field focusing enhancer based on the metasurface of the present invention is composed of a unit array on the same plane. Each unit adjusts the resonant frequency through a separate variable capacitor. Among all the units, the unit corresponding to the target area of ​​magnetic resonance imaging resonates at the operating frequency of the magnetic resonance imaging system through the tuning of the variable capacitor; the remaining units outside the imaging target area are tuned by the variable capacitor, and their resonant frequencies are lower than the operating frequency of the magnetic resonance imaging system. According to the principle of Fano resonance, when two resonant modes of different frequencies are coupled, the electromagnetic field of the high-frequency mode will interfere constructively, and the electromagnetic field of the low-frequency mode will interfere destructively. Since the resonant frequency of the unit corresponding to the imaging target area is higher than the resonant frequency of the remaining units, the magnetic field strength in the imaging target area is enhanced by the constructive interference of Fano resonance, while the magnetic field strength outside the imaging target area is weakened due to the destructive interference of Fano resonance, and finally the magnetic field energy is concentrated in the imaging target area, thereby improving the signal-to-noise ratio of magnetic resonance imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A top view of a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface provided in an embodiment of the present invention;

[0017] Figure 2 The relationship between the magnetic field enhancement and frequency at the center of the imaging target when the magnetic field focusing enhancer based on the metasurface provided in the embodiment of the present invention operates at 63.8MHz;

[0018] Figure 3 A schematic diagram of a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface applied to a magnetic resonance imaging system model;

[0019] Figure 4 The B1 field distribution diagram along the ZOY section of the model when the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided in the embodiment of the present invention is applied to a 1.5T magnetic resonance imaging system;

[0020] Figure 5 The B1 field distribution diagram along the ZOY section of the model when the conventional metasurface magnetic field enhancer is applied to a 1.5T magnetic resonance imaging system;

[0021] Figure 6 A comparison of the signal-to-noise ratio gain of the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided in an embodiment of the present invention and the traditional metasurface magnetic field enhancer in 1.5T magnetic resonance imaging with the penetration depth.

[0022] In the figure:

[0023] 1. Dielectric substrate

[0024] 2. Spiral metal wire

[0025] 3. Variable capacitor

[0026] 4. RF Coil

[0027] 5. Planar metamaterial resonators

[0028] 6. Water model DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0030] The present invention provides a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface, comprising a unit array arranged at equal intervals on the same plane, each unit comprising a spiral metal wire fixed on a substrate, and the ends of the metal wire are connected by welding a variable capacitor.

[0031] The array units are arranged in an n×n arrangement.

[0032] In the unit array, one or more units corresponding to the magnetic resonance imaging target area resonate at the working frequency of the magnetic resonance imaging system through the tuning of the variable capacitor. The remaining units outside the imaging target area resonate at a frequency lower than the working frequency of the magnetic resonance imaging system through the tuning of the variable capacitor. According to the principle of Fano resonance, when two resonance modes with different resonance frequencies interact with each other, the electromagnetic field of the frequency corresponding to the high-frequency resonance mode will undergo constructive interference, and the electromagnetic field of the frequency corresponding to the low-frequency resonance mode will undergo destructive interference. Since the resonance frequency of the corresponding unit in the imaging target area is higher than the resonance frequency of the remaining units outside the imaging target area, the magnetic field strength in the imaging target area is enhanced by the constructive interference of the Fano resonance, and the magnetic field strength outside the imaging target area is weakened due to the destructive interference of the Fano resonance, so that the magnetic field energy is finally concentrated in the imaging target area.

[0033] The spiral metal wire is in a circular or square shape.

[0034] The metasurface-based magnetic resonance imaging magnetic field focusing enhancer can be used in magnetic resonance imaging systems with different field strengths such as 0.7T, 1.5T, 3T and 7T by tuning the resonance frequency through a variable capacitor.

[0035] Figure 1A top view of a magnetic resonance imaging magnetic field focusing enhancer based on a metasurface provided in an embodiment of the present invention. The unit structure includes a dielectric substrate 1, a spiral metal wire 2 and a variable capacitor 3.

[0036] The magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided in this embodiment is applied to the focused imaging of the internal organs of the human body in a 1.5T magnetic resonance imaging system. In this embodiment, a water model with similar electromagnetic properties is used instead of the internal organs as a test object. The specific construction process is as follows:

[0037] Step 1:

[0038] The side length and number of the unit are determined according to the size of the imaging target area. If the unit size is too small, the area of ​​magnetic field enhancement is not enough to cover the imaging target area; if the unit size is too large, the magnetic field energy is not fully concentrated in the imaging target area, and the magnetic field enhancement effect cannot reach the best. In the embodiment, the side length of the unit is designed to be 80 mm, and the entire magnetic field focusing enhancer is designed with 3×3 units.

[0039] Step 2:

[0040] Through computer calculation, the rest of the geometric parameters including the line width, gap, number of turns and thickness of the spiral metal wire are optimized according to the determined unit side length in the electromagnetic wave finite element simulation software to achieve the best magnetic field enhancement performance. Through computer calculation, for a circular spiral metal wire, the inner diameter is 25mm, the line width is 2mm, the turn spacing is 1mm, and the number of turns is 3 turns, the performance is the best.

[0041] Step 3:

[0042] The spiral metal wire of each unit is connected at both ends by welding a variable capacitor. Through computer calculation, the appropriate capacitance value is calculated according to the determined geometric parameters of the magnetic field focusing enhancer in the electromagnetic wave finite element simulation software, so that the unit resonates at the target frequency. Taking the 1.5T magnetic resonance imaging system as an example, through simulation calculation, the capacitance value required for the unit corresponding to the imaging target area to resonate at the operating frequency of 63.8MHz of the 1.5T magnetic resonance imaging system is 5.31pF. Through further simulation calculation, when the unit outside the imaging target area resonates at 60.8MHz by loading a 6.08pF capacitor, the Fano resonance generated by the interaction of the two resonance modes with different frequencies has the best magnetic field enhancement effect. Figure 1In the figure, the central unit of the magnetic field focusing enhancer is loaded with a 5.31pF capacitor and resonates at 63.8MHz, and the surrounding 8 units are loaded with a 6.08pF capacitor and resonate at 60.8MHz. At this time, the resonance of the central unit will produce constructive interference under the action of Fano resonance, and the resonance of the surrounding 8 units will produce destructive interference under the action of Fano resonance. Therefore, in the application, the position of the central unit should be aligned with the target imaging area.

[0043] Figure 2 The relationship between the magnetic field enhancement and frequency at the center of the imaging target when the magnetic field focusing enhancer based on the metasurface provided in the embodiment of the present invention works at 63.8MHz. Figure 2 As shown, when the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided in the embodiment resonates, constructive interference of Fano resonance occurs at a frequency of 63.8MHz, and destructive interference of Fano resonance occurs at a frequency of 60.8MHz. The magnetic field intensity excited by resonance of one unit at a frequency of 63.8MHz is higher than the magnetic field intensity excited by resonance of eight units at a frequency of 60.8MHz. There are fewer units, but the magnetic field intensity excited by resonance is higher, which can prove the feasibility and effectiveness of Fano resonance to enhance the magnetic field intensity.

[0044] Figure 3 The figure is a schematic diagram of a metasurface-based magnetic resonance imaging magnetic field focusing enhancer applied to a magnetic resonance imaging system model, which includes a radio frequency coil 4, a metamaterial resonator 5 and a water phantom 6. The radio frequency coil is a 16-legged birdcage coil widely used in commercial magnetic resonance imaging systems. The water phantom is an imaging target, which is cylindrical in shape with a radius of 50 mm and a height of 100 mm. The relative dielectric constant of the constituent material of the water phantom is 65 and the electrical conductivity is 0.4 S / m, which are close to the parameters of human tissue. The water phantom is placed in the center of the radio frequency coil. According to the design of step three above, Figure 1 In the metasurface-based magnetic resonance imaging magnetic field focusing enhancer shown, the position of the central unit is aligned with the position of the water model. When the imaging system is working, the radio frequency coil emits electromagnetic waves with a frequency of 63.8MHz, and the magnetic field strength can be detected in the imaging target area corresponding to the water model. When a magnetic field focusing enhancer is placed and its resonant frequency matches the operating frequency of the imaging system, the electromagnetic waves emitted by the radio frequency coil will also cause the resonance of the magnetic field focusing enhancer, resulting in further enhancement of the magnetic field strength in space. The higher the magnetic field strength, the higher the signal-to-noise ratio in magnetic resonance imaging.

[0045] Figure 4 The B1 field distribution diagram along the ZOY section of the model in a 1.5T magnetic resonance imaging system using the metasurface-based magnetic resonance imaging magnetic field focusing enhancer provided in an embodiment of the present invention, and the results are normalized according to the 1W input power of the RF coil. Figure 4As shown, the peak value of the B1 field excited in the water phantom by the 1.5T magnetic resonance imaging system using the metasurface-based magnetic resonance imaging magnetic field focusing enhancer provided by the embodiment of the present invention reaches 7.47μT.

[0046] Different from the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided by the present invention, all units in the existing traditional metasurface magnetic field enhancer have the same electromagnetic characteristics, that is, the same resonant frequency, so there is no Fano resonance between the units.

[0047] Figure 5 This is the B1 field distribution diagram along the ZOY section of the model when the traditional metasurface magnetic field enhancer is applied in a 1.5T magnetic resonance imaging system. The results are normalized according to the 1W input power of the RF coil. Figure 5 As shown, the B1 field peak excited by the 1.5T magnetic resonance imaging system using a traditional metasurface magnetic field enhancer in the water phantom is 4.41μT.

[0048] Combination Figure 4 and Figure 5 It can be seen that in the MRI system using the traditional metasurface magnetic field enhancer, the electromagnetic waves emitted by the RF coil are dispersed in a space as wide as the metasurface magnetic field enhancer after being acted upon by the traditional metasurface magnetic field enhancer. In the MRI system using the metasurface-based MRI magnetic field focusing enhancer, the electromagnetic waves emitted by the RF coil are concentrated in a smaller space approximately as wide as the central unit after being acted upon by the metasurface-based MRI magnetic field focusing enhancer, and the magnetic field intensity is higher.

[0049] Figure 6 A comparison is made between the magnetic resonance imaging magnetic field focusing enhancer based on a metasurface provided in an embodiment of the present invention and the traditional metasurface magnetic field enhancer in terms of signal-to-noise ratio gain as a function of penetration depth in 1.5T magnetic resonance imaging.

[0050] In the magnetic resonance system, the signal-to-noise ratio coefficient is calculated as follows:

[0051]

[0052] Where B1 is the RF field strength excited by the system, P coil is the power loss of the RF coil, P meta is the loss power of the metamaterial resonator, P phantom is the power loss of the object being measured.

[0053] The calculation formula of signal-to-noise ratio gain is:

[0054]

[0055] Wherein, SNR1 represents the signal-to-noise ratio coefficient of the magnetic resonance system when the enhancer is loaded, and SNR2 represents the signal-to-noise ratio coefficient of the magnetic resonance system when the enhancer is not loaded.

[0056] like Figure 6 As shown, the peak signal-to-noise ratio gain of the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface provided by the embodiment of the present invention and the traditional metasurface magnetic field enhancer in the water model are 26.23 and 14.88 respectively, which is an improvement of 76.48%. In addition, at different positions in the entire water model, the improvement of the signal-to-noise ratio of the magnetic resonance imaging magnetic field focusing enhancer based on the metasurface is better than the performance of the traditional metasurface magnetic field enhancer.

[0057] Based on the above results, it can be concluded that the magnetic resonance imaging magnetic field focusing enhancer based on metasurface uses the field strength enhancement principle of Fano resonance and is highly feasible and effective in improving the signal-to-noise ratio in magnetic resonance focusing imaging.

[0058] Matters not covered by the present invention are known technologies.

[0059] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A magnetic resonance imaging magnetic field focusing enhancer based on a metasurface, comprising a unit array arranged on the same plane, each unit comprising a substrate and a spiral metal wire fixed on the substrate; the two ends of the spiral metal wire are connected by a variable capacitor; the side length and number of the unit are determined by the size of the imaging target area; the unit in the magnetic resonance imaging target area resonates at the working frequency of the magnetic resonance imaging through the tuning of the variable capacitor; the remaining units not in the magnetic resonance imaging target area resonate at a frequency lower than the working frequency of the magnetic resonance imaging through the tuning of the variable capacitor; so that when performing magnetic resonance imaging, the magnetic field strength in the imaging target area is enhanced by Fano resonance, the magnetic field strength in the area outside the imaging target is weakened, and the magnetic field energy is concentrated in the imaging target area, thereby improving the signal-to-noise ratio of the magnetic resonance imaging.

2. The magnetic resonance imaging magnetic field focusing enhancer according to claim 1, characterized in that: In the electromagnetic wave finite element simulation software, the remaining geometric parameters including the width, gap, number of turns and thickness of the spiral metal wire are optimized according to the side length and number of the required units to maximize the magnetic field enhancement multiple.

3. The magnetic resonance imaging magnetic field focusing enhancer according to claim 2, characterized in that: The resonant frequencies of the units located in and not located in the target area of ​​magnetic resonance imaging are determined respectively, and appropriate capacitance values ​​are calculated in electromagnetic wave finite element simulation software according to the determined unit geometric parameters so that each unit can resonate at its required target frequency.

4. The magnetic resonance imaging magnetic field focusing enhancer according to claim 3, characterized in that: It contains 3×3 units with equal spacing, and the side length of the unit is 80 mm; the position of the central unit is aligned with the position of the magnetic resonance imaging target area.

5. The magnetic resonance imaging magnetic field focusing enhancer according to claim 4, characterized in that: The spiral metal wire is circular, with an inner diameter of 25 mm, a wire width of 2 mm, a turn spacing of 1 mm, and 3 turns.

6. The magnetic resonance imaging magnetic field focusing enhancer according to claim 5, characterized in that: For a 1.5T MRI system, the unit within the imaging target area resonates at the operating frequency of the 1.5T MRI system, 63.8MHz, with a loading capacitance of 5.31pF; the unit outside the imaging target area resonates at 60.8MHz with a loading capacitance of 6.08pF.

7. The magnetic resonance imaging magnetic field focusing enhancer according to claim 1, characterized in that: The shape of the spiral metal wire is circular or square.

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