Magnetic field adjustment device and magnetic resonance imaging system

Abstract

The application relates to a magnetic field adjusting device and a magnetic resonance imaging system, the magnetic field adjusting device comprising: a ring array and a plurality of capacitor circuits, the ring array comprising a plurality of single rings, the plurality of single rings being arranged along an axis; one capacitor circuit is arranged on each single ring, and the capacitance values of the capacitor circuits are distributed in a gradient distribution increasing from the middle to both sides on the ring array. Such a distribution can change the equivalent permeability, compensate for the non-uniform magnetic field of the ring array itself, thereby realizing the uniformization of the magnetic field on the surface of the ring array. When the magnetic field adjusting device is applied to the magnetic resonance imaging system, the radio frequency magnetic field in the magnetic resonance imaging system can be uniformized, thereby improving the imaging quality of the magnetic resonance imaging system.

Description

Technical Field

[0001] This application relates to the field of magnetic resonance imaging technology, and in particular to a magnetic field adjustment device and a magnetic resonance imaging system. Background Technology

[0002] Magnetic Resonance Imaging (MRI) is a non-invasive diagnostic technique and a crucial foundational diagnostic technology in medicine, biology, and neuroscience. The signal strength transmitted by traditional MRI equipment primarily depends on the intensity of the static magnetic field B0. Using high or even ultra-high magnetic field systems can improve the signal-to-noise ratio, resolution, and shorten scan time. However, increasing the static magnetic field strength correspondingly increases equipment costs and increases heat generation in human tissue, thus posing safety risks. Therefore, achieving high image quality while using the lowest possible static magnetic field strength is a critical issue in MRI technology.

[0003] To address the aforementioned issues, researchers have proposed several solutions. The first is radio frequency coil optimization, which has significantly improved detector resolution and scanning speed in MRI. However, this approach requires redesigning the MRI system, leading to numerous inconveniences and high costs in practical applications. The second approach uses special contrast agents to enhance the local magnetic field, such as rare-earth magnetic atoms or magnetic nanoparticles. Since these contrast agents need to be administered orally or by injection into human tissues or organs, there are potential side effects and even life-threatening risks, making this not the most ideal solution. The third approach involves introducing a medium into the MRI system that enhances the intensity of the radio frequency magnetic field to reduce the signal-to-noise ratio. This method represents a new trend that can effectively improve MRI characteristics.

[0004] However, the third method mentioned above suffers from uneven magnetic field distribution. Summary of the Invention

[0005] Therefore, it is necessary to provide a magnetic field adjustment device and a magnetic resonance imaging system that can effectively improve the uniformity of the radio frequency magnetic field in a magnetic resonance system, in order to address the above-mentioned technical problems.

[0006] In a first aspect, this application provides a magnetic field adjustment device, which includes: a ring array and a plurality of capacitor circuits, wherein the ring array includes a plurality of single rings arranged along an axis;

[0007] Each of the single rings is provided with a corresponding capacitor circuit, and the capacitance value of each capacitor circuit in the ring array is distributed in a gradient that increases from the middle to both sides.

[0008] In one embodiment, the ring array constitutes a metasurface.

[0009] In one embodiment, the single ring is an open-circuit resonant ring.

[0010] In one embodiment, the capacitor circuit is disposed on the surface of the single ring.

[0011] In one embodiment, the capacitor circuit is disposed on the surface in a direction perpendicular to the plane of the single ring.

[0012] In one embodiment, the capacitor circuit includes at least two capacitors connected in parallel or in series.

[0013] In one embodiment, the gradient distribution includes any one of a linear gradient distribution, an exponential gradient distribution, a power gradient distribution, and a logarithmic gradient distribution.

[0014] In one embodiment, the capacitor circuit includes a first capacitor, a second capacitor, and a diode; the second capacitor is connected in series with the diode, and the first capacitor is connected in parallel with the series branch of the second capacitor and the diode.

[0015] The diode is turned off when the single ring senses a low electromotive force and turned on when it senses a high electromotive force.

[0016] Secondly, this application also provides a magnetic resonance imaging system, characterized in that the magnetic resonance imaging system includes: a transmitting coil, a magnetic field adjustment device as described in claim 1, a receiving coil, and an imaging device;

[0017] The transmitting coil is used to excite and generate a magnetic resonance signal;

[0018] The magnetic field adjustment device is used to enhance the intensity of the radio frequency field of the magnetic resonance signal and improve the uniformity of the radio frequency field.

[0019] The receiving coil is used to receive the magnetic resonance signal after passing through the magnetic field adjustment device and send it to the imaging device for imaging.

[0020] In one embodiment, the magnetic field adjustment device is disposed on the end surface of the receiving coil that receives the magnetic resonance signal.

[0021] This application provides a magnetic field adjustment device and a magnetic resonance imaging (MRI) system. The magnetic field on the surface of the ring array in the magnetic field adjustment device is distributed in a non-uniform manner, decreasing from the center outwards, which significantly reduces the image quality of MRI. Based on this, this embodiment uses capacitor circuits on the ring array to change the equivalent permeability of the ring array, thereby altering the magnetic field enhancement of the ring array. Furthermore, the capacitance values ​​of each capacitor circuit on the ring array exhibit a gradient distribution that increases from the center outwards, compensating for the non-uniform magnetic field of the ring array itself, thus homogenizing the magnetic field on the surface of the ring array. When this type of magnetic field adjustment device is applied to an MRI system, it can homogenize the radio frequency magnetic field in the MRI system, thereby improving the imaging quality of the MRI system. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the magnetic field adjustment device in Example 1;

[0023] Figure 2 This is a schematic diagram of the magnetic field adjustment device in Example 2;

[0024] Figure 3 This is a magnetic field distribution diagram during the receiving phase in one embodiment;

[0025] Figure 4 This is a magnetic field distribution diagram during the launch phase in one embodiment;

[0026] Figure 5 This is a capacitance distribution diagram for the transmission and reception phases in one embodiment;

[0027] Figure 6 This is a schematic diagram of the resonant performance during the transmission and reception phases in one embodiment;

[0028] Figure 7 This is a schematic diagram of the receiving coil and magnetic field adjustment device in Embodiment 3;

[0029] Figure 8 This is a graph showing the distribution of equivalent permeability in one embodiment;

[0030] Figure 9 This is a diagram illustrating the effect of magnetic field adjustment in one embodiment. Detailed Implementation

[0031] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0032] Before detailing the technical solutions of the embodiments of this disclosure, the technical background or evolution of the embodiments of this disclosure will be introduced first. Typically, the signal strength transmitted by an MRI device mainly depends on the strength of the static magnetic field B0. Using a high magnetic field or even an ultra-high magnetic field system can improve the signal-to-noise ratio, resolution, and shorten scan time. However, increasing the static magnetic field strength brings three problems: 1) Increased non-uniformity of the radio frequency (RF) magnetic field, increasing tuning difficulty; 2) Increased heat generation in human tissue, posing safety hazards, and patients are prone to adverse reactions such as dizziness and vomiting; 3) Significantly increased purchase costs, which is a burden for most small-scale hospitals. Therefore, how to obtain high image quality while using the smallest possible static magnetic field strength has become a crucial issue in MRI technology.

[0033] To address the aforementioned issues, researchers have proposed several solutions. The first is the radio frequency coil optimization method, which has significantly improved detector resolution and scanning speed in MRI. Studies have shown that parallel imaging can reduce scanning time, and multi-channel coils can achieve better image quality and a larger detection area. However, this approach is relatively mature, and optimizing the coil requires redesigning the MRI system, which presents many inconveniences for practical applications. The second approach uses special contrast agents to enhance the local magnetic field, such as rare-earth magnetic atoms or magnetic nanoparticles. Since contrast agents need to be administered orally or by injection into human tissues or organs, there are potential side effects and even life-threatening risks, making this not the most ideal solution. The third approach involves introducing plate- or columnar dielectric resonators with high dielectric constants into the MRI to increase the intensity of the radio frequency magnetic field and reduce the specific absorption rate, thereby improving imaging resolution and reducing the signal-to-noise ratio. This method represents a new trend that can effectively improve MRI characteristics. Based on this third approach, a novel metasurface material has emerged that can be used to improve or enhance MRI imaging quality and efficiency.

[0034] Currently proposed metasurfaces have identical properties across all their units, resulting in uniform magnetic field enhancement. However, due to the superposition effect of the magnetic fields within each unit, the metasurface exhibits a strong magnetic field in the central region and a weak magnetic field at the edges. Applying this structure to magnetic resonance imaging (MRI) leads to inhomogeneous MRI signals, thereby altering image contrast. Furthermore, the inhomogeneous magnetic field complicates the nonlinear control design of the metasurface, causing regions of magnetic field destructiveness during the radio frequency (RF) emission phase, which in turn leads to RF artifacts. Therefore, to utilize metasurfaces to enhance the RF magnetic field in MRI systems while maintaining image quality, it is essential to first address the issue of inhomogeneous magnetic fields within the metasurface.

[0035] Against this backdrop, through long-term model simulation development and the collection, demonstration, and verification of experimental data, the applicant discovered that the enhanced performance of metasurface structures is positively correlated with their equivalent magnetic permeability. Therefore, the magnetic field distribution of the metasurface can be re-optimized by adjusting the magnetic permeability of the metasurface to improve the uniformity of the magnetic field distribution. The applicant has devoted considerable creative effort to this approach and the technical solutions described in the following embodiments.

[0036] The technical solution of this application and how it solves the technical problems will be described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will be described below with reference to the accompanying drawings.

[0037] The serial numbers assigned to components in this document, such as "first" and "second," are used solely to distinguish the described objects and have no sequential or technical meaning. The terms "connection" and "linkage" used in this application, unless otherwise specified, include both direct and indirect connections (linkages). It should be understood that the terms "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are used solely for the convenience of describing this application and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0038] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0039] In Example 1, as Figure 1 As shown, a magnetic field adjustment device is provided, which is installed in the receiving coil of a magnetic resonance imaging system. Specifically, the magnetic field adjustment device includes a ring array 100 and a plurality of capacitor circuits 200, wherein the ring array 100 includes a plurality of single rings 110, which are arranged along an axis; a capacitor circuit 200 is correspondingly arranged on each single ring 110, and the capacitance value of each capacitor circuit 200 on the ring array 100 exhibits a gradient distribution that increases from the center to both sides.

[0040] The aforementioned ring array 100 is composed of multiple single rings 110. The specific number of single rings 110 can be determined according to actual application requirements. For example, a ring array 100 can contain 10, 20, or 30 single rings 110, etc. Furthermore, the multiple single rings 110 are arranged along an axis, specifically symmetrically arranged in a straight line with respect to the vertical axis (see [link to relevant documentation]). Figure 1 Each single ring 110 has the same size, and the specific size of a single ring can be determined according to actual needs. There is a preset distance between two adjacent single rings 110. This preset distance can be less than a preset distance threshold, and should be as small as possible to ensure the uniformity of the magnetic field enhancement on the surface of the ring array. The determination of the preset distance can be determined according to actual experiments and needs.

[0041] In practical applications, each individual ring 110 enhances the magnetic field, resulting in a superposition effect of the magnetic fields of the individual rings 110 on the ring array 100. This leads to a situation where the magnetic field is very strong in the middle region and very weak at the edges, thus the magnetic field on the surface of the ring array decreases from the middle to both sides. Optionally, the aforementioned ring array 100 constitutes a metasurface. Metasurfaces possess many special properties not found in natural materials. By utilizing the interaction between electromagnetic waves and the metallic or dielectric elements of the metasurface, as well as the coupling effect between these elements, the propagation path of electromagnetic waves and the distribution of electromagnetic field strength can be controlled. Optionally, the individual ring 110 is an open-ended resonant ring, which is one of the most important basic units for constructing a metasurface. Optionally, the configuration of the individual ring can be a closed conducting configuration such as circular, elliptical, or square, which is not limited in this embodiment. It should be noted that the magnetic field on the metasurface composed of multiple open-ended resonant rings exhibits a situation where the magnetic field is very strong in the middle region and very weak at the edges.

[0042] The capacitor circuit 200 described above may include only one capacitor, or it may include multiple capacitors, or it may include other components. As long as the capacitance values ​​of each capacitor circuit 200 on the ring array 100 exhibit a gradient distribution that increases from the center outwards, it meets the usage requirements of the magnetic field adjustment device in this embodiment. When the capacitor circuit 200 includes multiple capacitors, the multiple capacitors can be connected in parallel or in series. Optionally, the capacitor circuit 200 is disposed on the surface of the single ring along the vertical direction of the plane of the single ring, specifically including the inner surface and the outer surface. In practical applications, the capacitors can be soldered onto the surface of the single ring. It should be noted that a capacitor circuit is disposed at the same or different positions on each single ring 110, and the capacitance values ​​of the capacitor circuits 200 on different single rings 110 are different. Furthermore, the capacitance values ​​of each capacitor circuit 200 on the ring array 100 exhibit a gradient distribution that increases from the center outwards. Optionally, this gradient distribution includes any one of the following: linear gradient distribution, exponential gradient distribution, power gradient distribution, and logarithmic gradient distribution. Additionally, Figure 1 The examples given are multiple capacitors connected in parallel and capacitor circuits placed at the same location on multiple single rings. These are merely illustrative examples and do not constitute a limitation on the arrangement and location of the capacitors.

[0043] Correspondingly, Figure 1 The capacitor circuit 200 on each single ring 110 in the embodiment Figure 1 The inclusion of a capacitor in (a) is merely an example; the capacitance values ​​of each capacitor circuit 200 exhibit a gradient distribution that increases from the center outwards. For example, Figure 1 The capacitors C1, C2, C3, C4, and C5 are symmetrically distributed, and their capacitance values ​​increase linearly. The capacitance values ​​of each capacitor can be obtained using the following relationship (1):

[0044] C n =[1+(n-1)f α C1 n≥1 (1);

[0045] Where n is the number of the single ring, and the capacitor corresponding to the single ring numbered 1 is C1. C1 can be any value, which can be determined according to the actual application circuit requirements. The capacitors on the single rings arranged to both sides from the single ring numbered 1 are C2, C3, C4, and C5, and their corresponding capacitance values ​​can be determined according to the above relationship (1). α It is the gradient factor, which can be determined based on actual experiments or simulations.

[0046] Optional, if Figure 1 In the embodiment, each single ring 110 contains two capacitors in its capacitor circuit 200, and the two capacitors in the capacitor circuit 200 are connected in parallel (see [reference]). Figure 1 (b) Alternatively, they can be connected in series, where the equivalent capacitance of each capacitor circuit 200 exhibits a gradient distribution that increases from the middle to both sides, for example, Figure 1 The equivalent capacitances C1, C2, C3, C4, and C5 of each capacitor circuit 200 in the middle exhibit a linearly increasing distribution. Therefore, the capacitance values ​​of each capacitor can be obtained using the following relationship (2):

[0047] C n =[1+(n-1)f φ ](C p +C q ) n≥1 (2);

[0048] Where n is the number of the single ring, and the equivalent capacitance corresponding to the single ring numbered 1 is C1. C1 can be any value, which can be determined according to the actual application circuit requirements. The equivalent capacitances on the single rings arranged to both sides from the single ring numbered 1 are C2, C3, C4, and C5, and their corresponding equivalent capacitance values ​​can be determined according to the above relationship (1). φ It is the gradient factor, which can be determined based on actual experiments or simulations.

[0049] The magnetic field on the surface of the ring array in the magnetic field adjustment device provided in this embodiment exhibits a gradient distribution that decreases from the center to both sides, resulting in a non-uniform magnetic field that significantly degrades the image quality of magnetic resonance imaging (MRI). Therefore, this embodiment modifies the equivalent permeability of the ring array by incorporating capacitor circuits, thereby altering the magnetic field enhancement of the ring array. Furthermore, the capacitance values ​​of each capacitor circuit on the ring array exhibit a gradient distribution that increases from the center to both sides, compensating for the non-uniform magnetic field of the ring array itself. This achieves the homogenization of the magnetic field on the surface of the ring array. When this type of magnetic field adjustment device is applied to an MRI system, it can homogenize the radio frequency magnetic field in the MRI system, thereby improving the imaging quality of the MRI system.

[0050] Understandable, Figure 1 In this embodiment, the equivalent capacitance of the capacitor circuit 200 is symmetrically distributed and exhibits a gradient-symmetrical increase from the middle to both sides. Therefore, the gradient factors on both sides are the same, meaning the increasing slopes on both sides are the same. This design is intended for situations where the magnetic field on the surface of the ring array is generally strong in the middle region and symmetrically weak on both sides (see...). Figure 5 (Distribution curve of capacitance value in the middle). When the magnetic field on the surface of the actual ring array itself is strong in the middle region and asymmetrically weak on both sides, the equivalent capacitance value of the designed capacitor circuit 200 is also asymmetrically distributed. The equivalent capacitance values ​​on both sides can also be calculated by referring to the relationship (1) or (2), which will not be explained in detail here.

[0051] In Example 2, as Figure 2 As shown, a magnetic field adjustment device is provided. The capacitor circuit 200 in the magnetic field adjustment device includes a first capacitor 201, a second capacitor 202, and a diode 203; the second capacitor 202 and the diode 203 are connected in series, and the first capacitor 201 is connected in parallel or in series with the series branch of the second capacitor 202 and the diode 203. Figure 2 (The diagram only shows the parallel circuit); Diode 203 is turned off when single-ring 110 senses a low electromotive force and turned on when sensed a high electromotive force.

[0052] Exemplary Description Figure 2The magnetic field adjustment device described in this embodiment includes a ring array 100 and multiple capacitor circuits 200. Specifically, the ring array 100 can be a metasurface, consisting of 10 single rings 110. Each single ring 110 is an elliptical open-ended resonant ring with a major axis of 96 mm, a minor axis of 33 mm, and a width of 19 mm. The distance between two adjacent open-ended resonant rings is 1 mm, and the ring array period is 20 mm. The capacitance value on the ring array 100 increases linearly from the center to both sides, and is symmetrically distributed at both ends. For clarity, the ring array 100 is numbered 1-5 from the middle single ring 110 to the edge single rings 110. In this embodiment, a linear capacitance gradient is used, meaning the capacitance value on the ring array 100 increases linearly towards both sides. It should be noted that the dimensions of the above structure are reference values ​​provided by experimental research and are not intended to limit the scope of the device.

[0053] In this embodiment, the working state of the capacitor circuit 200 is as follows: During the receiving phase of the magnetic resonance imaging system, the diode 203 in the capacitor circuit 200 senses a low electromotive force. At this time, the diode 203 is turned off, the second capacitor 202 is turned off, and only the first capacitor 201 is connected to the circuit. At this time, the ring array 100 (metasurface) resonates at the Larmor frequency. During the transmitting phase of the magnetic resonance imaging system, the diode 203 in the capacitor circuit 200 senses a high electromotive force. At this time, the diode 203 is turned on, the second capacitor 202 is connected to the circuit and is connected in parallel with the first capacitor 201, which increases the equivalent capacitance of the ring array 100 (metasurface) and decreases the resonant frequency.

[0054] Assuming a linear capacitance gradient is used in this embodiment, since only the first capacitor 201 is connected to the circuit during the receiving stage, and the first capacitor 201 and the second capacitor 202 are connected in parallel during the transmitting stage, the ring array 100 (metasurface) has different gradient values ​​during the receiving and transmitting stages. Let the first capacitor be represented by C1 and the second capacitor by C2, then the capacitance values ​​of each single ring during the receiving and transmitting stages can be obtained using the following relationships (3) and (4):

[0055] C n =[1+(n-1)f r C i n≥1 (3);

[0056] C n =[1+(n-1)f t ](C i +C j n≥1 (4);

[0057] Among them, f r and f tThese are the linear gradient factors for the receive and transmit phases, respectively. n is the loop number, and C... i C is the equivalent capacitance corresponding to the capacitor circuit in the receiving stage. i +C j C is the equivalent capacitance corresponding to the capacitor circuit during the emission phase. i and C j It can be any value, and the specific value can be determined according to the actual application circuit requirements. The equivalent capacitance value corresponding to the capacitor circuit on each single ring can be determined according to the above relationships (3) and (4). r and f t These are the linear gradient factors for the receiving and transmitting phases, respectively, which can be determined based on actual experiments or simulations. The following explains f. r and f t The method for determining and the optimal value.

[0058] First, determine the gradient factor f for the receiving phase. r .based on Figure 2 The structure and specific dimensions of the magnetic field adjustment device described in the embodiment are shown in the foregoing example, and can be verified through simulation experiments (see [link]). Figure 3 The magnetic field distribution diagrams shown below (under different linear gradient factors) during the receiving stage can be used to determine f. r When f = 0.09, the ring array exhibits the best uniformity of the magnetic field, i.e., f r =0.09 is the optimal value, which allows the magnetic field adjustment device of the above configuration to have the best magnetic field uniformity in the receiving stage, and at this time it is in the critical gradient state; when f r When f < 0.09, the magnetic field distribution still exhibits a distribution characteristic of high in the middle and low at the edges, indicating a state of undergradient; when f r When the gradient factor f is greater than 0.09, two magnetic field intensity peaks appear in the magnetic field distribution, indicating poor magnetic field uniformity and a state of overgradient. Therefore, in the receiving phase of the magnetic resonance imaging system in this embodiment, the gradient factor f... r The range of values ​​for f can be r ≤0.09.

[0059] Next, determine the gradient factor f during the launch phase. t .based on Figure 2 The structure and specific dimensions of the magnetic field adjustment device described in the embodiment are shown in the foregoing example, and can be verified through simulation experiments (see [link]). Figure 4 The magnetic field distribution diagrams shown below (under different linear gradient factors) during the emission phase allow us to determine f. t When f = 0.12, the ring array exhibits the best uniformity of the magnetic field, i.e., f t =0.12 is the optimal value, which allows the magnetic field adjustment device of the above configuration to have the best magnetic field uniformity during the emission phase, and at this time it is in the critical gradient state; when f tWhen f < 0.12, the magnetic field distribution still exhibits a distribution characteristic of high in the middle and low at the edges, indicating a state of undergradient; when f t When the gradient factor f is greater than 0.12, two magnetic field intensity peaks appear in the magnetic field distribution, indicating poor magnetic field uniformity and a state of overgradient. Therefore, in the emission phase of the magnetic resonance imaging system in this embodiment, the gradient factor f... t The range of values ​​for f can be t ≤0.12. Based on the optimal values ​​of the two gradient factors mentioned above, the capacitance distribution of each single-loop (open-circuit resonant loop) 100 during the transmission and reception stages is as follows. Figure 5 As shown.

[0060] Figure 2 The embodiment proposes a field homogenization strategy for the magnetic permeability gradient by constructing a capacitance gradient. The capacitance value of each individual ring (open resonant ring) on ​​the ring array (metasurface) increases linearly from the middle ring to the edge rings, and there exists an optimal gradient, which greatly improves the uniformity of the magnetic field on the ring array surface. However, in practical applications, non-uniform magnetic fields can cause difficulties in the nonlinear control design of the ring array (metasurface). That is, if the magnetic field on the ring array (metasurface) is not uniform, nonlinear control of the ring array cannot be achieved, and effective nonlinear control design is a necessary condition for the clinical application of ring array (metasurface) MRI. This is because, typically, there are two radio frequency (RF) phases in MRI: the RF transmission phase and the RF reception phase, and the RF fields in these two phases have the same resonant frequency. If a ring array (metasurface) is used to enhance the intensity of the RF magnetic field in MRI, the ring array (metasurface) will significantly increase the RF transmission field while enhancing the RF reception field. After the radio frequency emission field is enhanced, the following two problems will occur: 1. The specific absorption rate (SAR) of the human body will increase significantly, SAR∝B1 2 Therefore, the addition of a ring array (metasurface) will cause a significant increase in heat generation in the human body, leading to safety issues. 2. For some sequences, such as the Fast Spin Echo (FSE) sequence, even after using a power standard, the obtained image still has significant artifacts and cannot obtain an ideal image.

[0061] In response to the above problems, Figure 2 In the embodiment, the capacitor circuit 200 on the ring array 100 can also realize nonlinear control of the resonant frequency of this type of component. That is, during the radio frequency transmission stage, each single ring 110 in the ring array (metasurface) 100 is in a resonant state, and during the radio frequency reception stage, each single ring 110 in the ring array 100 is in a detuned state (see...). Figure 6The diagram illustrates the resonant performance during the transmission and reception phases. Therefore, a capacitance gradient strategy (permeability gradient strategy) must be applied to both the resonant and non-resonant states of the ring array (metasurface) 100 to improve the magnetic field uniformity of the ring array (metasurface) 100 during the RF transmission and reception phases. Optionally, in this embodiment, the nonlinear control of the ring array (metasurface) 100 can also be achieved through nonlinear design methods such as parallel resonance and depletion-mode MOS transistors, the principle of which is the same as the nonlinear design principle of this embodiment.

[0062] Figure 2 The magnetic field adjustment device provided in this embodiment solves the problems of magnetic field inhomogeneity and interference with the radio frequency transmission field in current ring arrays composed of open resonant rings. This invention improves the magnetic field uniformity of the ring array (metasurface) during the radio frequency transmission and reception stages through permeability gradient design, thereby improving the uniformity of the magnetic resonance images obtained using the magnetic field adjustment device provided in this embodiment, and also paving the way for the nonlinear control design of the ring array (metasurface). Furthermore, this invention employs… Figure 2 The control method implemented by the capacitor circuit described in the embodiment allows the ring array (metasurface) to have different resonant frequencies during the RF transmission and reception phases. During reception, the ring array (metasurface) resonates at the Larmor frequency, significantly enhancing the signal field and thus improving the image signal-to-noise ratio. During transmission, the ring array (metasurface) resonant frequency decreases, moving away from the Larmor frequency, and does not enhance the RF transmission field, thereby overcoming the interference problem of the RF transmission field inherent in ring arrays composed of open resonant rings. Therefore, through permeability gradient design and capacitor circuit control, the increase in the field strength of the transmitted magnetic field in the magnetic resonance imaging system can be suppressed, thereby reducing image artifacts and achieving optimal clinical application of the ring array (metasurface) in MRI.

[0063] In Embodiment 3, a magnetic resonance imaging system is provided, the system comprising: a transmitting coil, Figure 2 The embodiment includes a magnetic field adjustment device, a receiving coil, and an imaging device; wherein the transmitting coil is used to excite and generate a magnetic resonance signal; the magnetic field adjustment device is used to enhance the intensity of the radio frequency field of the magnetic resonance signal and improve the uniformity of the radio frequency field; and the receiving coil is used to receive the magnetic resonance signal after passing through the magnetic field adjustment device and send it to the imaging device for imaging.

[0064] The transmitting coil, receiving coil, and imaging device in the above embodiments can be any existing MRI imaging system; specific details are not provided in this embodiment. The magnetic field adjustment device proposed in this invention can be disposed on the end surface of the receiving coil that receives the magnetic resonance signal. See [link to relevant documentation]. Figure 7As shown, the magnetic field adjustment device 400 is disposed on the upper surface of the receiving coil 300. This magnetic field adjustment device can homogenize the uniformity of the radio frequency magnetic field in the magnetic resonance imaging system based on the methods described in any of the above embodiments, thereby improving the imaging quality of the magnetic resonance imaging, such as increasing the signal-to-noise ratio of the image, thus increasing the image resolution, and shortening the image scanning time. The specific principles are explained above and will not be repeated here.

[0065] Finally, a comparative analysis is conducted between ring arrays (metasurfaces) with optimized permeability gradients and those without. In the ring array (metasurface) without permeability gradient design, the permeability of each individual ring is the same, while in the ring array (metasurface) with optimized permeability gradients, the permeability of each individual ring is different through permeability gradient design (see [link to analysis]). Figure 8 By employing a permeability gradient design, the magnetic field uniformity of the ring array (metasurface) is significantly improved during both RF transmission and reception. For details, please refer to [link to relevant documentation]. Figure 9 The diagram shows the magnetic field adjustment effect of the ring array (metasurface) during the RF transmission and reception phases. (See also...) Figure 9 The frequency response curves shown in (b) and (d) are as follows: Figure 9 The magnetic field distribution shown in (c) and (e) Figure 9 The MRI image shown in (f) and Figure 9 (g) The normalized signal intensity distribution curve corresponding to the MRI image described in section (g).

[0066] As can be seen from the MRI images of the uniform water film, significant artifacts and poor signal uniformity exist in the water film images without the magnetic permeability gradient design. After adopting the magnetic permeability gradient design, artifacts no longer appear in the water film images, and the signal uniformity is greatly improved. From the signal intensity distribution curve, without the magnetic permeability gradient design, the signal intensity distribution curve is a curve with high intensity values ​​in the middle and low intensity values ​​at both ends, indicating that the magnetic field of the ring array (metasurface) has a gradient distribution with strong values ​​in the middle region and weak values ​​at both ends, which is non-uniform. After adopting the magnetic permeability gradient design, the signal intensity distribution curve is a flat curve, indicating that the magnetic field of the ring array (metasurface) is uniform. In other words, the magnetic permeability gradient design achieves the adjustment of the magnetic field uniformity of the ring array (metasurface).

[0067] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0068] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A magnetic field adjustment device, characterized in that, The magnetic field adjustment device includes: a ring array and multiple capacitor circuits, wherein the ring array includes multiple single rings arranged along an axis; Each of the single rings is provided with a corresponding capacitor circuit, and on the ring array, the capacitance value of each capacitor circuit is distributed in a gradient that increases from the middle to both sides. The capacitor circuit is used to change the equivalent permeability of the ring array to compensate for the non-uniform magnetic field on the surface of the ring array and realize the homogenization of the magnetic field on the surface of the ring array. The capacitor circuit includes a first capacitor, a second capacitor, and a diode; the second capacitor is connected in series with the diode, and the first capacitor, the second capacitor, and the diode are connected in parallel or in series in a series branch; the diode is turned off when the single ring senses a low electromotive force and turned on when it senses a high electromotive force.

2. The magnetic field adjustment device according to claim 1, characterized in that, The ring array constitutes a metasurface.

3. The magnetic field adjustment device according to claim 2, characterized in that, The single ring is an open-ended resonant ring.

4. The magnetic field adjustment device according to any one of claims 1-3, characterized in that, The capacitor circuit is disposed on the surface of the single ring.

5. The magnetic field adjustment device according to claim 4, characterized in that, The capacitor circuit is disposed on the surface in a direction perpendicular to the plane of the single ring.

6. The magnetic field adjustment device according to claim 1, characterized in that, The capacitor circuit includes at least two capacitors, which are connected in parallel or in series.

7. The magnetic field adjustment device according to claim 1, characterized in that, The gradient distribution includes any one of the following: linear gradient distribution, exponential gradient distribution, power gradient distribution, and logarithmic gradient distribution.

8. A magnetic resonance imaging system, characterized in that, The magnetic resonance imaging system includes: a transmitting coil, a magnetic field adjustment device as described in claim 1, a receiving coil, and an imaging device; The transmitting coil is used to excite and generate a magnetic resonance signal; The magnetic field adjustment device is used to enhance the intensity of the radio frequency field of the magnetic resonance signal and improve the uniformity of the radio frequency field. The receiving coil is used to receive the magnetic resonance signal after passing through the magnetic field adjustment device and send it to the imaging device for imaging.

9. The magnetic resonance imaging system according to claim 8, characterized in that, The magnetic field adjustment device is disposed on the end surface of the receiving coil that receives the magnetic resonance signal.

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