Birdcage coil for ultra-high field magnetic resonance imaging and its homogeneity evaluation structure

Abstract

This application discloses a birdcage coil for extremely high field magnetic resonance imaging and its uniformity evaluation structure, relating to the field of magnetic resonance imaging. The coil includes: two circular end rings, multiple parallel legs, and a PCB board. The two circular end rings are arranged opposite each other, and the multiple parallel legs are connected between the first and second circular end rings, thus forming a cylindrical birdcage structure. The PCB board is arranged along the birdcage structure. On the inner side of the PCB board, at the position of each parallel leg, a long strip of inner copper plating is provided along the extension direction of the leg. On the outer side of the PCB board, at both ends of each inner copper plating, a predetermined shape of outer copper plating is provided. Each pair of outer copper platings is not connected, and there is an overlap between the outer and inner copper platings, forming a distributed capacitance. A lumped capacitor is provided between each pair of adjacent outer copper platings, near the edge of the end ring. This application improves the signal-to-noise ratio of the birdcage coil.

Description

Technical Field

[0001] This application relates to the field of magnetic resonance imaging technology, and in particular to a birdcage coil for extremely high field magnetic resonance imaging and its uniformity evaluation structure. Background Technology

[0002] Extremely high field magnetic resonance imaging (UHMR) systems are widely used in high-resolution imaging of small animals due to their inherent high signal-to-noise ratio (SNR). In UHMR, the radio frequency (RF) coil is a key component determining image quality, and it includes various types such as surface coils, phased array coils, and volumetric coils. Among these, the birdcage coil, a classic volumetric coil design, is widely used due to its good field uniformity and suitable imaging depth. However, with increasing magnetic field strength, conventional birdcage coil designs face several challenges. For example, traditional birdcage coils require a large number of lumped capacitors for frequency tuning and impedance matching, but these capacitors significantly reduce the quality factor (Q) of the birdcage coil. To address this issue, researchers have proposed alternatives such as fully distributed capacitance designs and slotted coils, but these solutions also have significant drawbacks: fully distributed capacitance designs are typically only suitable for low-pass birdcage configurations, and this configuration has a lower SNR than high-pass and band-pass designs at extremely high fields. Summary of the Invention

[0003] The purpose of this application is to provide a hybrid distributed capacitance birdcage coil for extremely high field magnetic resonance imaging to solve the problem of low signal-to-noise ratio in existing coils described in the background art.

[0004] To achieve the above objectives, this application provides the following solution:

[0005] In one aspect, this application provides a birdcage coil for extremely high field magnetic resonance imaging, comprising: a first circular end ring, a second circular end ring, a plurality of parallel legs, and a PCB board;

[0006] The first circular end ring and the second circular end ring are arranged opposite to each other, and the plurality of parallel legs are connected at equal intervals between the first circular end ring and the second circular end ring, thereby forming a cylindrical birdcage structure.

[0007] The PCB board is arranged along the birdcage structure;

[0008] On the inner side of the PCB board, and at the position of each of the parallel legs, a long strip of inner copper plating is provided along the extension direction of the legs.

[0009] On the outer side of the PCB board, and at both ends of each inner copper layer, there is a predetermined shape of outer copper layer. Each pair of outer copper layers is not connected, and there is an overlap between the outer copper layer and the inner copper layer, forming a distributed capacitance.

[0010] A lumped capacitor is placed between each pair of adjacent outer copper layers, near the edge of the end ring.

[0011] Optionally, when the birdcage structure is placed vertically, the outer copper plating at the top is T-shaped, and the outer copper plating at the bottom is inverted T-shaped.

[0012] Optionally, a matching capacitor can be provided at the quadrature port.

[0013] Optionally, a tuning capacitor may be installed in a relative position.

[0014] Optionally, the PCB is a flexible PCB with a relative permittivity of 10.

[0015] Secondly, this application provides a birdcage coil uniformity evaluation structure, comprising: any of the birdcage coil structures described in the first aspect above, and a cylindrical mold suitable for the birdcage coil structure, wherein the interior of the mold is filled with a solution having specific dielectric parameters.

[0016] Optionally, the uniformity evaluation structure further includes an RF shielding layer mounted around the birdcage coil structure.

[0017] According to the specific embodiments provided in this application, the following technical effects are disclosed:

[0018] This invention also provides a birdcage coil for extremely high-field magnetic resonance imaging, comprising multiple legs and two end rings connected to the legs, forming a cylindrical birdcage structure. The distributed capacitance on the coil legs is implemented using a double-sided copper-coated PCB board, while the end rings employ traditional lumped capacitors. This novel structure, combining the distributed capacitance on the legs and the lumped capacitors on the end rings, forms a hybrid distributed capacitance birdcage coil, which can effectively improve the coil's signal-to-noise ratio and Q value while reducing the SAR value, thus better meeting the needs of extremely high-field magnetic resonance imaging technology. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A schematic diagram of a birdcage coil for extremely high field magnetic resonance imaging provided in an embodiment of this application;

[0021] Figure 2A schematic diagram of a birdcage coil for extremely high field magnetic resonance imaging provided in an embodiment of this application;

[0022] Figure 3 A comparison diagram of the inner side, outer side, and inner side combined with outer side of a birdcage coil provided in an embodiment of this application;

[0023] Figure 4 This is a schematic diagram showing the results of underwater model imaging at 14.1T using a distributed capacitance birdcage coil, as provided in another embodiment of this application.

[0024] In the diagram, the lumped capacitor is installed at positions 1-24; copper plating on the outer side of the PCB is 25; copper plating on the inner side of the PCB is 26; PCB board is 27; orthogonal port is 28; overlapping part is 29. Detailed Implementation

[0025] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0026] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] In one exemplary embodiment, see Figures 1-3 As shown, a birdcage coil for extremely high field magnetic resonance imaging is provided, comprising: a first circular end ring, a second circular end ring, multiple parallel legs, and a PCB board;

[0028] The first circular end ring and the second circular end ring are arranged opposite to each other, and the plurality of parallel legs are connected at equal intervals between the first circular end ring and the second circular end ring, thereby forming a cylindrical birdcage structure.

[0029] The PCB board is arranged along the birdcage structure;

[0030] On the inner side of the PCB board, and at the position of each of the parallel legs, a long strip of inner copper cladding 26 is provided along the extension direction of the legs.

[0031] On the outer side of the PCB board, and at both ends of each inner copper cladding 26, there is a predetermined outer copper cladding 25. Each pair of outer copper cladding 25 is not connected. The outer copper cladding 25 and the inner copper cladding 26 have an overlap portion 29, forming a distributed capacitor.

[0032] A lumped capacitor is placed between every two adjacent outer copper cladding 25, and near the edge of the end ring, and mounted on... Figure 2 The location indicated by 1-24.

[0033] Among them, the extremely high magnetic field is a magnetic field with a magnetic field strength greater than 10.5 Tesla.

[0034] The first circular end ring, the second circular end ring, and multiple parallel legs are made of metal and form a columnar structure to provide support for the entire birdcage coil.

[0035] The PCB board is laid along the side wall of the birdcage and wraps the first circular end ring, the second circular end ring, and multiple parallel support legs. The PCB board is used to support the copper plating.

[0036] In this configuration, the lumped capacitors on the end ring, together with the distributed capacitors, form a bandpass configuration.

[0037] Furthermore, since the birdcage coil is used for imaging small animals, the diameter and length of the hybrid distributed capacitance birdcage coil can be determined based on the size of the small animal. The leg length is set within a range suitable for imaging small animals.

[0038] In other words, considering the needs of imaging small animals, the coil diameter and length should be matched with the size of the small animal to obtain the best signal-to-noise ratio. In this embodiment, the birdcage coil diameter is 30mm, the total length is 40mm, the leg width is 3mm, the length is 34mm, and the end ring width is 3mm.

[0039] Furthermore, the birdcage coil provided in this application can be considered a distributed capacitance birdcage coil. The number of legs in a distributed capacitance birdcage coil has a significant impact on field uniformity. The number of legs can be set according to requirements, for example, 8, 12, 16, etc. If the number of legs is too small, it will lead to uneven field distribution; if the number of legs is too large, it will increase the complexity of the coil structure and the difficulty of manufacturing. In this embodiment, 12 legs are selected to form the birdcage coil to obtain the best field uniformity.

[0040] In addition, experiments have shown that it is particularly suitable for imaging small animals under field strengths of 14.1T.

[0041] This invention also provides a birdcage coil for extremely high-field magnetic resonance imaging, comprising multiple legs and two end rings connected to the legs, forming a cylindrical birdcage structure. The distributed capacitance on the coil legs is implemented using a double-sided copper-coated PCB board, while the end rings employ traditional lumped capacitors. This novel structure, combining the distributed capacitance on the legs and the lumped capacitors on the end rings, forms a hybrid distributed capacitance birdcage coil, which can effectively improve the coil's signal-to-noise ratio and reduce the SAR value, thereby better meeting the needs of extremely high-field magnetic resonance imaging technology.

[0042] Furthermore, existing birdcage coils require a large amount of lumped capacitance, which not only introduces magnetic susceptibility artifacts but also significantly reduces the quality factor (Q value) of the birdcage coil. Moreover, in extremely high-field environments, the nonlinear behavior exhibited by lumped elements further degrades the performance of the RF coil. Additionally, for small-sized RF coils, the coil's own losses account for a large proportion of the total noise, and the extensive use of lumped elements significantly reduces the final image quality. Furthermore, the design of slotted coils in existing technologies is limited by their radial dimensions, restricting the effective imaging area and potentially reducing RF field uniformity. Therefore, developing a novel RF coil design that can simultaneously optimize field uniformity and signal-to-noise ratio while exhibiting good tuning flexibility and performance in extremely high-field environments is of great significance.

[0043] Optionally, in another exemplary embodiment of this application, when the birdcage structure is placed vertically, the outer copper plating 25 located at the top is T-shaped, and the outer copper plating 25 located at the bottom is inverted T-shaped.

[0044] Among them, vertical placement is as follows Figure 1 As shown, it is easy to place.

[0045] Among them, T-shaped copper plating can construct distributed capacitance on both the inner and outer sides, so there must be overlapping area between the inner and outer sides. The outer T-shape and the inner rectangular shape are undoubtedly the most suitable.

[0046] Alternatively, in another exemplary embodiment of this application, a matching capacitor is provided at the quadrature port.

[0047] The orthogonal port is the upper port formed between each pair of adjacent T-shaped copper pours. Further, the upper port is a port formed by two horizontal copper pours in two adjacent T-shaped copper pours; similarly, the lower port is a port formed by two vertical copper pours in two adjacent T-shaped copper pours.

[0048] The matching capacitor is used to achieve load-related matching.

[0049] The load refers to the water mold, which is a cylindrical mold suitable for the birdcage coil structure. The interior of the mold is filled with a solution with specific dielectric parameters and serves as the load for the distributed capacitance birdcage coil.

[0050] Here, matching refers to the ability of a birdcage coil to achieve good impedance matching under load.

[0051] In this embodiment, at an operating frequency of 600MHz, the two orthogonal ports are matched to 50Ω, and the isolation between the ports is better than -20dB.

[0052] Alternatively, in another exemplary embodiment of this application, a tuning capacitor is installed at a relative position.

[0053] The relative position refers to the position opposite to the port, which is on the same end ring. For example, if port 28 is the port, then capacitors can be added at positions 12 and 9 respectively to adjust the resonant frequency.

[0054] The tuning capacitor is used to adjust the resonant frequency of the distributed capacitor birdcage coil.

[0055] Optionally, in another exemplary embodiment of this application, the PCB is a flexible PCB with a relative permittivity of 10.

[0056] The choice of PCB material has a significant impact on capacitance. The higher the relative permittivity of the material, the greater the capacitance that can be obtained with the same overlapping area. In this embodiment, a material with a relative permittivity of 10 is selected as the dielectric layer to obtain the required capacitance with a smaller overlapping area.

[0057] Additionally, the PCB thickness can be set according to requirements; for example, a thickness of 0.2mm can be achieved using Taconic CER-10 (ε) PCBs. r =10) as the dielectric layer material to achieve a capacitance of 0.44pF per square millimeter of overlap area.

[0058] In addition, the operating frequency of the birdcage coil can be determined by the following method: determining the main magnetic field strength of the magnetic resonance imaging system; and then, based on the main magnetic field strength and combined with Larmor's theorem, calculating the resonance frequency of the hydrogen nucleus, or the operating frequency of the hybrid distributed capacitance birdcage coil.

[0059] The resonance frequency of the hydrogen nucleus is calculated based on the magnitude of the main magnetic field; this frequency is the operating frequency of the birdcage coil. In this embodiment, considering the field strength characteristics of the applied ultra-high field magnetic resonance imaging system, the operating frequency of the birdcage coil is determined to be 600 MHz.

[0060] In addition, the size of the overlapping area can be calculated using the following formula:

[0061] C=ε0ε r A / d

[0062] Where C is the required capacitance value, ε0 is the vacuum permittivity, and ε r Let A be the relative permittivity, d be the overlap area, and d be the dielectric layer thickness. Therefore, once the required capacitance value is determined, this formula can be used to calculate the size of the overlap area, which can then guide the fabrication of the birdcage coil, thereby enabling the birdcage coil to resonate at a specified operating frequency.

[0063] The selection of capacitance value is based on frequency. Generally speaking, the frequency is determined first, the capacitance value is determined based on experience, and then the overlapping area is determined based on the capacitance value.

[0064] Furthermore, compared to traditional bandpass birdcage coils, the distributed capacitance birdcage coil in this embodiment exhibits higher receiver sensitivity, higher Q value, and better signal-to-noise ratio. Simultaneously, the electric field distribution generated by the distributed capacitance birdcage coil is more optimized, with both the average and maximum electric field values ​​reduced, resulting in a corresponding decrease in RF power requirements. This effectively reduces SAR values ​​and improves RF security. Figure 4 The results are obtained by using the distributed capacitance birdcage coil of this embodiment to perform water model imaging at 14.1T, which shows that it can achieve high uniformity imaging under extremely high fields.

[0065] This invention provides a birdcage coil with a hybrid capacitor structure, combining the advantages of distributed capacitance and lumped capacitance. Specifically, this solution employs a distributed capacitance design on the legs while retaining traditional lumped capacitance on the end rings, enabling the coil to possess both the performance advantages of a bandpass configuration and lower capacitance losses. Its technical advantages include:

[0066] (i) A birdcage coil with a hybrid capacitor structure is provided, combining the advantages of distributed capacitance and lumped capacitor. Specifically, this solution adopts a distributed capacitance design on the legs and retains a traditional lumped capacitor on the end ring, so that the coil has both the performance advantages of bandpass configuration and lower capacitance loss.

[0067] (ii) A radio frequency coil structure optimized for imaging small animals in extremely high fields is provided. Compared with the traditional bandpass birdcage coil, the hybrid distributed capacitance birdcage coil has higher receiving sensitivity and higher Q value, which significantly improves the imaging signal-to-noise ratio.

[0068] (III) The coil design of the present invention reduces the electric field concentration, optimizes the electric field distribution, effectively reduces the average electric field value and the maximum electric field value, and at the same time reduces the RF power requirement accordingly, effectively reducing the SAR value and improving radio frequency security.

[0069] (iv) Compared with the fully distributed capacitor design, the hybrid design provided by the present invention has better tuning flexibility; compared with the slot coil design, the present invention maintains a larger effective imaging area while maintaining excellent field uniformity.

[0070] This application provides a birdcage coil uniformity evaluation structure, comprising: any of the birdcage structures described above, and a cylindrical mold suitable for the birdcage coil structure, wherein the interior of the mold is filled with a solution having specific dielectric parameters.

[0071] The size of the phantom is similar to that of the birdcage coil, so that the birdcage coil can image the phantom, and then the uniformity of the birdcage coil can be calculated and evaluated based on the imaging results.

[0072] The dielectric parameters of the phantom are similar to those of living tissue (such as small animals) to simulate the actual imaging environment.

[0073] In addition, in this application, the phantom is placed at the center of the hybrid distributed capacitor birdcage coil, and the magnetic field and electric field distribution generated in the target area are calculated using electromagnetic simulation software.

[0074] Furthermore, based on A method for calculating field uniformity is used to evaluate the magnetic field uniformity of a target region.

[0075] Furthermore, the average and maximum electric fields inside the phantom are calculated, and the distribution of SAR values ​​generated by the coils is evaluated.

[0076] Calculate B1 generated by the coil using electromagnetic simulation software. + Field distribution. Electromagnetic simulation software may introduce computational errors during model mesh generation, leading to outliers in the simulation results and affecting the accurate assessment of the uniformity of the radio frequency field distribution. The method for calculating radio frequency field uniformity in this scheme is as follows:

[0077]

[0078] Where Hom represents uniformity, represent The maximum value of the modulus, represent The minimum value of the modulus.

[0079] In addition, to eliminate outliers in the simulation, statistical methods can be used to filter the data, removing extreme values ​​before calculating the field uniformity. In this embodiment, the coil uniformity is 92%.

[0080] Alternatively, electromagnetic simulation software can be used to calculate the E-field distribution generated by the coil. The maximum and average values ​​of the electric field generated by the coil inside the model can be directly calculated to evaluate the SAR value. In this embodiment, the maximum electric field generated by the coil inside the water model is 670.11 V / m, and the average electric field is 370.53 V / m.

[0081] Optionally, in another exemplary embodiment of this application, the uniformity evaluation structure further includes an externally mounted radio frequency shielding layer surrounding the birdcage coil structure.

[0082] This radio frequency shielding layer minimizes external electromagnetic interference and improves the accuracy of calculation and evaluation of birdcage coil uniformity.

[0083] Therefore, the radio frequency shielding layer is for birdcage coil structures. It is a copper foil layer added to the outside of the birdcage coil structure to shield electromagnetic signals from the external environment.

[0084] By using a coaxial balun to suppress common-mode current in the RF shielding layer of the coaxial cable, the accuracy of the calculation and evaluation of birdcage coil uniformity can be improved.

[0085] 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.

[0086] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A birdcage coil for very high field magnetic resonance imaging, characterized in that, Comprise: a first circular end ring, a second circular end ring, a plurality of parallel legs and a PCB board; the first circular end ring and the second circular end ring are oppositely arranged, and the plurality of parallel legs are connected between the first circular end ring and the second circular end ring at equal intervals, thereby forming a cylindrical birdcage structure; the PCB board is arranged along the birdcage structure; on the inner side of the PCB board and at the position of each parallel leg, a long strip-shaped inner side copper coating is arranged along the extension direction of the leg; on the outer side of the PCB board and at the two ends of each inner side copper coating, a predetermined shape of outer side copper coating is arranged, and each two outer side copper coatings are not connected with each other, the outer side copper coating and the inner side copper coating have an overlapping part, thereby forming a distributed capacitor; between each adjacent two outer side copper coatings and close to the edge of the end ring, a lumped capacitor is arranged, and the lumped capacitor and the distributed capacitor form a bandpass configuration structure.

2. The birdcage coil for very high field magnetic resonance imaging of claim 1, wherein, When the birdcage structure is vertically placed, the shape of the outer side copper coating at the upper part is T-shaped, and the shape of the outer side copper coating at the lower part is inverted T-shaped.

3. The birdcage coil for very high field magnetic resonance imaging of claim 1, wherein, A matching capacitor is arranged at the orthogonal port.

4. The birdcage coil for very high field magnetic resonance imaging of claim 1, wherein, A tuning capacitor is installed at the opposite position.

5. The birdcage coil for very high field magnetic resonance imaging of claim 1, wherein, The PCB is a flexible PCB, and the relative dielectric constant is 10.

6. A birdcage coil homogeneity evaluation structure comprising: The birdcage coil according to any one of claims 1-5, and a cylindrical mold suitable for the birdcage coil structure, and the mold is filled with a solution with specific dielectric parameters.

7. The birdcage coil homogeneity evaluation structure of claim 6, wherein, The uniformity evaluation structure further comprises a radio frequency shielding layer installed around the birdcage coil structure.

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