Birdcage coil for extremely high field magnetic resonance imaging and uniformity evaluation structure thereof

Abstract

The invention discloses a birdcage coil for extremely high field magnetic resonance imaging and a uniformity evaluation structure thereof, and relates to the field of magnetic resonance imaging, the coil comprises two circular end rings, a plurality of parallel support legs and a PCB; the two circular end rings are oppositely arranged, and the plurality of parallel supporting legs are connected between the first circular end ring and the second circular end ring, so that a cylindrical birdcage structure is formed; the PCB is arranged along the birdcage structure; a strip-shaped inner side clad copper is arranged on the inner side face of the PCB and located at the position of each parallel supporting leg in the extending direction of the supporting legs; the outer side surface of the PCB and the two ends of each piece of inner side clad copper are provided with a piece of outer side clad copper with a predetermined shape, every two pieces of outer side clad copper are not connected, and the outer side clad copper and the inner side clad copper have an overlapped part to form a distributed capacitor; and a lumped capacitor is arranged between every two adjacent pieces of outer-side clad copper and close to the edge of the end ring. According to the invention, the signal-to-noise ratio of the birdcage coil is improved.

Description

Technical Field

[0001] The present application relates to the technical field of magnetic resonance imaging, and in particular to a birdcage coil for extremely high field magnetic resonance imaging and a uniformity evaluation structure thereof. Background Art

[0002] Extremely high-field magnetic resonance imaging (MRI) systems are widely used in the field of high-resolution imaging of small animals due to their inherent high signal-to-noise ratio (SNR) advantage. In ultra-high-field MRI, the radio frequency coil is a key component that determines imaging quality. It mainly includes various types such as surface coils, phased array coils, and volume coils. Among them, the birdcage coil, as a classic volume coil design, is widely adopted due to its good field uniformity and moderate imaging depth. However, as the magnetic field strength increases, conventional birdcage coil designs face many 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 value) of the birdcage coil. To address this problem, researchers have proposed alternatives such as fully distributed capacitor designs and slot coils, but these solutions also have obvious drawbacks: fully distributed capacitor designs are generally only applicable to low-pass birdcage configurations, which have a worse signal-to-noise ratio than high-pass and bandpass designs at extremely high fields. Summary of the Invention

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

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

[0005] In a first aspect, the present application provides a birdcage coil for very 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 between the first circular end ring and the second circular end ring at equal intervals, 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 cladding is provided along the extending direction of the legs;

[0009] On the outer side of the PCB board, and at both ends of each of the inner copper clads, an outer copper clad of a predetermined shape is provided, and each two outer copper clads are not connected to each other, and there is an overlapping portion between the outer copper clads and the inner copper clads to form a distributed capacitor;

[0010] A lumped capacitor is provided between each two adjacent outer copper covers and near the edge of the end ring.

[0011] Optionally, when the birdcage structure is placed vertically, the outer copper cladding at the upper portion is in a T-shape, and the outer copper cladding at the lower portion is in an inverted T-shape.

[0012] Optionally, matching capacitors are provided at the orthogonal ports.

[0013] Optionally, tuning capacitors are installed at opposite positions.

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

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

[0016] Optionally, the uniformity evaluation structure further includes: a radio frequency shielding layer installed around the birdcage coil structure.

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

[0018] The present 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. Distributed capacitance on the coil legs is achieved using a double-sided copper-clad PCB, while the end rings utilize traditional lumped capacitors. This novel structure, combining distributed capacitance on the legs with lumped capacitance on the end rings, creates a hybrid distributed capacitance birdcage coil. This effectively improves the coil's signal-to-noise ratio (SNR) and Q factor, while reducing SAR (Specular Absorption Spectrum), thereby better meeting the requirements of extremely high-field magnetic resonance imaging technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 A schematic diagram of a birdcage coil for extremely high field magnetic resonance imaging provided by one embodiment of the present application;

[0021] Figure 2A schematic diagram of a birdcage coil for extremely high field magnetic resonance imaging provided by one embodiment of the present 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 one embodiment of the present application;

[0023] Figure 4 A schematic diagram of the results of water phantom imaging at 14.1 T using a distributed capacitance birdcage coil is provided in another embodiment of the present application.

[0024] In the figure, the lumped capacitor is installed at position 1-24; the copper cover on the outside of the PCB is 25; the copper cover on the inside of the PCB is 26; the PCB board is 27; the orthogonal port is 28; and the overlapping part is 29. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0026] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0027] In an exemplary embodiment, see Figure 1-Figure 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, a plurality of 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 between the first circular end ring and the second circular end ring at equal intervals, 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 clad 26 is provided along the extending direction of the legs;

[0031] On the outer side of the PCB board, and at both ends of each inner copper clad 26, there is provided an outer copper clad 25 of a predetermined shape, and each two outer copper clads 25 are not connected to each other. There is an overlapping portion 29 between the outer copper clad 25 and the inner copper clad 26, forming a distributed capacitor;

[0032] A lumped capacitor is set between each two adjacent outer copper clads 25 and near the edge of the end ring. Figure 2 At the position indicated by 1-24.

[0033] Among them, extremely high magnetic fields are magnetic fields with a magnetic field intensity greater than 10.5 Tesla.

[0034] Among them, the first circular end ring, the second circular end ring, and multiple parallel legs are made of metal. After forming a columnar structure, they are used to provide support for the entire birdcage coil.

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

[0036] The lumped capacitors on the end rings together with the distributed capacitors form a bandpass configuration.

[0037] In addition, the birdcage coil is used for imaging small animals, so the diameter and length of the hybrid distributed capacitance birdcage coil can be determined according to the size of the small animal. The leg length is set within the range suitable for small animal imaging.

[0038] That is, considering the needs of small animal imaging, the coil diameter and length should match the size of the small animal to obtain the best signal-to-noise ratio. In this embodiment, the birdcage coil has a diameter of 30 mm, a total length of 40 mm, a leg width of 3 mm, a length of 34 mm, and an end ring width of 3 mm.

[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 as needed, for example, 8, 12, 16, etc. If the number of legs is too small, the field distribution will be uneven; if the number of legs is too large, the coil structure will be more complex and the manufacturing difficulty will increase. In this embodiment, 12 legs are selected to form the birdcage coil to achieve optimal field uniformity.

[0040] In addition, experiments have shown that it is particularly suitable for small animal imaging at field strengths such as 14.1T.

[0041] The present 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. Distributed capacitance on the coil legs is achieved using a double-sided copper-clad PCB, while the end rings utilize traditional lumped capacitors. This novel structure, combining distributed capacitance on the legs with lumped capacitance on the end rings, creates a hybrid distributed capacitance birdcage coil. This effectively improves the coil's signal-to-noise ratio (SNR) and reduces SAR, thereby better meeting the requirements of extremely high-field magnetic resonance imaging technology.

[0042] In addition, with existing technologies, birdcage coils require a large number of lumped capacitors, which not only introduce magnetic susceptibility artifacts but also significantly reduce the quality factor (Q value) of the birdcage coil. Moreover, in extremely high field environments, the nonlinear behavior exhibited by the lumped elements will further degrade the performance of the RF coil. In addition, for small-sized RF coils, the coil losses themselves account for a large proportion of the total noise, and the widespread use of lumped elements will significantly reduce the final image quality. In addition, for the slot-type coils in existing technologies, their design is limited by the radial size, which limits the effective imaging area and may reduce the RF field uniformity. Therefore, it is of great significance to develop a new RF coil design that can simultaneously optimize field uniformity and signal-to-noise ratio and has good tuning flexibility and performance in extremely high field environments.

[0043] Optionally, in another exemplary embodiment of the present application, when the birdcage structure is placed vertically, the outer copper cladding 25 at the upper portion is in a T-shape, and the outer copper cladding 25 at the lower portion is in an inverted T-shape.

[0044] Among them, vertical placement is as follows Figure 1 Convenient placement shown.

[0045] Among them, T-shaped copper cladding can construct distributed capacitors with copper cladding on both the inside and outside, so there must be overlapping area inside and outside, and the rectangle with the inner T-shape on the outside is undoubtedly the most suitable.

[0046] Optionally, in another exemplary embodiment of the present application, a matching capacitor is provided at the orthogonal port.

[0047] The orthogonal port is the upper port formed between each two adjacent T-shaped copper clads. Further, the upper port is the port formed by the two horizontal copper clads in the two adjacent T-shaped copper clads. Similarly, the lower port is the port formed by the two vertical copper clads in the two adjacent T-shaped copper clads.

[0048] Among them, the matching capacitor is used to achieve load-related matching.

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

[0050] Here, matching means that the birdcage coil can achieve good impedance matching under load.

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

[0052] Optionally, in another exemplary embodiment of the present application, tuning capacitors are installed at relative positions.

[0053] Among them, the relative position is: the position relative to the port, all on the same end ring. For example, position 28 is the port, then a capacitor 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 capacitance birdcage coil.

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

[0056] The choice of PCB material has a significant impact on capacitance. The higher the relative dielectric constant of the material, the greater the capacitance that can be achieved with the same overlap area. In this embodiment, a material with a relative dielectric constant of 10 is selected as the dielectric layer to achieve the desired capacitance with a smaller overlap area.

[0057] In addition, the thickness of the PCB can be set according to the requirements. For example, if the thickness is 0.2mm, Taconic CER-10 (ε r =10) as the dielectric layer material, achieving a capacitance of 0.44 pF per square millimeter of overlapping area.

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

[0059] The resonance frequency of hydrogen nuclei is calculated based on the magnitude of the main magnetic field, and this frequency is the operating frequency of the birdcage coil. In this embodiment, the operating frequency of the birdcage coil is determined to be 600 MHz based on the field strength characteristics of the extremely high field magnetic resonance imaging system used.

[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 dielectric constant, ε r is the relative dielectric constant, A is the overlap area, and d is the dielectric layer thickness. Therefore, once the required capacitance is determined, this formula can be used to calculate the overlap area, which in turn can guide the fabrication of the birdcage coil, ensuring it resonates at the specified operating frequency.

[0063] Among them, 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 factor, and better signal-to-noise ratio. Furthermore, the electric field distribution generated by the distributed capacitance birdcage coil is more optimized, with lower average and maximum electric field values, and a corresponding reduction in RF power requirements, thereby effectively reducing SAR values and improving RF safety. Figure 4 This is the result of water phantom imaging at 14.1 T using the distributed capacitance birdcage coil of this embodiment, indicating that highly uniform imaging can be achieved at extremely high fields.

[0065] This invention provides a birdcage coil with a hybrid capacitor structure that combines the advantages of distributed and lumped capacitors. Specifically, this solution uses a distributed capacitor design on the legs while retaining traditional lumped capacitors on the end rings. This allows the coil to have the performance advantages of a bandpass configuration while also having lower capacitance losses. Its technical advantages include:

[0066] (1) A hybrid birdcage coil with a capacitor structure is provided, combining the advantages of distributed and lumped capacitors. Specifically, this solution uses a distributed capacitor design on the legs and retains traditional lumped capacitors on the end rings, giving the coil the performance advantages of a bandpass configuration while also having lower capacitive losses.

[0067] (2) A radio frequency coil structure optimized for extremely high-field small animal imaging is provided. Compared with the traditional bandpass birdcage coil, the hybrid distributed capacitance birdcage coil has higher receiving sensitivity and higher Q value, significantly improving the imaging signal-to-noise ratio.

[0068] (3) 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 demand accordingly, effectively reduces the SAR value, and improves RF safety.

[0069] (4) 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] The present application provides a birdcage coil uniformity evaluation structure, comprising: any one of the birdcage structures described above, and a cylindrical phantom suitable for the birdcage coil structure, wherein the interior of the phantom 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 is calculated and evaluated based on the imaging results.

[0072] The dielectric parameters of the phantom are close 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 electromagnetic simulation software is used to calculate the magnetic field and electric field distribution generated in the target area.

[0074] Further, based on The calculation method of field uniformity is used to calculate and evaluate the magnetic field uniformity in the target area.

[0075] Furthermore, the average electric field and the maximum electric field inside the phantom are calculated to evaluate the SAR value distribution generated by the coil.

[0076] Use electromagnetic simulation software to calculate the B1 generated by the coil + Field distribution. Electromagnetic simulation software may produce some calculation errors when meshing the model, resulting in abnormal values in the simulation results and affecting the correct evaluation of the uniformity of the RF field distribution. The method for calculating the RF field uniformity in this solution is as follows:

[0077]

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

[0079] In addition, in order to eliminate outliers in the simulation, a statistical method can be used to filter the data and remove 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. This allows for direct calculation of the maximum and average electric field generated by the coil within the phantom, which can be used to estimate the SAR value. In this example, the maximum electric field generated by the coil within the water phantom was 670.11 V / m, and the average electric field was 370.53 V / m.

[0081] Optionally, in another exemplary embodiment of the present application, the uniformity evaluation structure further comprises: installing a radio frequency shielding layer around the outside of the birdcage coil structure.

[0082] Through this RF shielding layer, external electromagnetic interference can be minimized and the accuracy of the calculation and evaluation of the birdcage coil uniformity can be improved.

[0083] Therefore, the RF shielding layer is a circle of copper foil added to the outside of the birdcage coil structure to shield electromagnetic signals from the external environment.

[0084] The common-mode current in the RF shield of the coaxial cable is suppressed by a coaxial balun, thereby improving the accuracy of the calculation and evaluation of the birdcage coil uniformity.

[0085] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, 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 description of the above examples is only intended to help understand the method and core concept of this application. At the same time, for those skilled in the art, based on the concept of this application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A birdcage coil for extremely high field magnetic resonance imaging, characterized in that include: a first circular end ring, a second circular end ring, a plurality of parallel legs, and a PCB; 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 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 of the parallel legs, a long strip of inner copper cladding is provided along the extending direction of the legs; On the outer side of the PCB board, and at both ends of each of the inner copper clads, an outer copper clad of a predetermined shape is provided, and each two outer copper clads are not connected to each other, and there is an overlapping portion between the outer copper clads and the inner copper clads to form a distributed capacitor; A lumped capacitor is provided between each two adjacent outer copper covers and near the edge of the end ring.

2. The birdcage coil for extremely high field magnetic resonance imaging according to claim 1, characterized in that: When the birdcage structure is placed vertically, the outer copper cladding at the upper portion is in a T-shape, and the outer copper cladding at the lower portion is in an inverted T-shape.

3. The birdcage coil for extremely high field magnetic resonance imaging according to claim 1, characterized in that: Set matching capacitors at the quadrature ports.

4. The birdcage coil for extremely high field magnetic resonance imaging according to claim 1, characterized in that: Install the tuning capacitors at opposite positions.

5. The birdcage coil for extremely high field magnetic resonance imaging according to claim 1, characterized in that: The PCB is a flexible PCB with a relative dielectric constant of 10.

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

7. The birdcage coil uniformity evaluation structure according to claim 6, characterized in that: The uniformity evaluation structure further includes a radio frequency shielding layer installed around the birdcage coil structure.

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