Integrated circularly polarized radio frequency transmitting-receiving coil for ultralow field magnetic resonance imaging

Abstract

The invention provides an ultra-low field magnetic resonance imaging integrated circular polarization radio frequency transmit-receive coil, and relates to the ultra-low field magnetic resonance imaging technology, the ultra-low field magnetic resonance imaging integrated circular polarization radio frequency transmit-receive coil comprises a medium substrate, split copper sheet ring structures and split straight copper sheet structures, the medium substrate is a cylindrical structure, the medium substrate is uniformly provided with multiple groups of split copper sheet ring structures, and the split straight copper sheet structures are arranged on the medium substrate. The split straight copper sheet structures are vertically and uniformly arranged on the split copper sheet ring structures, and feed ports are formed in the split copper sheet ring structures and connected with coaxial cables through SMA connectors. According to the invention, the transmitting and receiving of the circularly polarized magnetic field are realized through orthogonal feed and the adjustable element, the circularly polarized magnetic field can obviously improve the received signal strength and realize the effective improvement of the signal-to-noise ratio, the parameterized design is adopted, the flexible adaptation of multiple scenes is supported, and the antenna can be applied to different field strengths by adjusting the parameters of the adjustable capacitor, the adjustable resistor and the like. And the application scene is widened.

Description

Technical Field

[0001] This invention relates to the field of ultra-low field magnetic resonance imaging technology, and in particular to an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging. Background Technology

[0002] Magnetic resonance imaging (MRI) is a commonly used medical imaging technique. Its basic principle is that a transmitting coil emits a sequence of pulses, generating a resonant electromagnetic field within the human body. This causes the atomic nuclei within the body to spontaneously emit signals, which are detected and received by a receiving coil. The MRI machine acquires Fourier spatial data of the object being examined and reconstructs the internal structure of a specific region of the body based on the acquired data, thus obtaining an image of the internal structure. Compared to computed tomography (CT), MRI has advantages such as high tissue resolution, multi-sequence imaging, three-dimensional imaging, and the absence of ionizing radiation. This technology is widely used in clinical medicine, biomedical research, and materials science.

[0003] Traditional 1.5T and higher field strength MRI systems have become standard equipment in hospitals due to their high resolution and signal-to-noise ratio (SNR) advantages. However, these high field systems, which rely on liquid helium-cooled superconducting magnet technology, not only weigh several tons or even tens of tons, but also cost tens of millions of RMB. They also require strict electromagnetic shielding facilities and dedicated machine rooms, making them difficult to use in scenarios such as mobile healthcare, bedside monitoring, or disease screening in remote areas.

[0004] With the shift of medical resources to primary care and the development of precision medicine, ultra-low field MRI (ULF MRI, field strength ≤0.1T), characterized by its small size and low cost, has gradually attracted research interest. ULF MRI utilizes permanent magnets or low-field electromagnets to achieve a lightweight design, eliminating the need for liquid helium cooling and complex shielding, significantly reducing equipment costs and lowering the barrier to entry. For example, a 54.6mT ULF MRI system can be fitted into an ambulance or mobile testing vehicle, providing immediate imaging services to emergency and remote areas. However, ULF MRI faces the challenge of a significantly lower main magnetic field strength, resulting in a signal-to-noise ratio (SNR) far lower than high-field equipment. SNR, a core indicator of MRI image quality, directly affects the ability to detect lesions and diagnostic accuracy. Furthermore, factors such as magnetic field temperature drift, human body coupling noise, and the lack of reliable electromagnetic shielding make the design of permanent magnet ULF MRI radio frequency coils even more challenging.

[0005] The radio frequency transceiver coil is a core component of the MRI system, and its performance directly determines the imaging quality. It has two key functions: in transmit mode, it converts the pulse signal generated by the radio frequency generator into a radio frequency magnetic field to excite the spin resonance of the atomic nuclei of the imaged object; in receive mode, it captures the magnetic resonance signal generated by the atomic nucleus resonance and converts it into an electrical signal to be transmitted to the processing system.

[0006] The performance of the radio frequency (RF) transceiver coil is directly related to key indicators of MRI images such as signal-to-noise ratio (SNR), resolution, and uniformity. A high SNR clearly displays details of human tissues and lesions, helping doctors make accurate diagnoses. High resolution facilitates the identification of minute tissue structures and lesions, improving the early diagnosis rate of diseases. Good uniformity reduces image distortion and artifacts, ensuring diagnostic accuracy. Improper coil design or insufficient performance can cause problems such as signal attenuation, noise interference, and image distortion, and in severe cases, even lead to misdiagnosis or missed diagnosis. Therefore, designing high-performance RF transceiver coils is extremely important for improving the performance of low-field magnetic resonance imaging (MRI) machines and expanding their clinical application value.

[0007] In recent years, ultra-low field magnetic resonance imaging (ULF) technology has attracted widespread attention due to its low cost, portability, and safety. Significant progress has been made abroad in the design of ULF MRI radio frequency coils. In 2015, Matthew Rosen's team at Harvard Medical School developed a 6.5 mT ULF MRI system, proposing a very simple, non-cryoprehytic method to achieve high-performance human MRI under ultra-low magnetic fields. In 2020, O'Reilly's team at Leiden University Medical Center researched a ULF MRI system using Halbach magnets, with an imaging aperture diameter of 27 cm. A discretized Halbach permanent magnet array achieved a main magnetic field strength of 50 mT, and custom gradient coils, radio frequency coils, gradient amplifiers, and radio frequency amplifiers were integrated and tested in models and in vivo. In February 2020, Hyperfine launched the first commercially available portable 64 mT ULF MRI system, which received FDA approval. The system is approximately 1.5 m high and weighs approximately 650 kg. This device, combined with noise cancellation technology, can operate in an unshielded open environment, achieving impressive image SNR and can be used for bedside monitoring of patients infected with the novel coronavirus (COVID-19).

[0008] In recent years, my country has made significant progress in the research of ULF MRI radio frequency transceiver coils, with major universities and research institutions actively carrying out related work and producing independent innovative results. In 2022, researchers at Chongqing University designed and optimized a head radio frequency receiving coil for a 50.4 mT ULF MRI system. By optimizing the orthogonal radio frequency coil and using phantom imaging, they increased the signal-to-noise ratio of the orthogonal radio frequency coil by approximately 40% compared to the single-channel radio frequency coil. In 2023, the Chongqing University team developed a flexible orthogonal coil suitable for 50.4 mT ultra-low field knee joint magnetic resonance imaging, achieving for the first time high-quality images of different degrees of knee flexion on a ULF MRI system. In 2024, researchers at Chongqing University designed and optimized a helmet-mounted dual-channel radio frequency receiving coil for brain imaging environments with a magnetic field strength of 54 mT (2.32 MHz).

[0009] In general, existing ULF MRI coils are mostly single-function transmit or receive coils for local imaging (such as the head and knee joint), lacking an integrated design for whole-body imaging. Furthermore, most coils are linearly polarized, and even those designed as circularly polarized rely on two orthogonal linearly polarized coils in different directions, rather than an integrated design. In MRI, circularly polarized coils offer higher signal-to-noise ratio, higher RF energy efficiency, higher RF field uniformity, and better tissue coupling efficiency compared to linearly polarized coils. They also support multi-channel parallel imaging and reduce specific absorption rate (SAR), making them a superior choice for ultra-low field MRI. In addition, existing coils often employ Helmholtz or saddle-shaped structures, resulting in significant magnetic field fluctuations in the peripheral regions during large-aperture whole-body imaging, with a 45cm radius ROI uniformity of only 60%-65%, leading to image distortion. Finally, traditional coils are mostly single-function transmit or receive coils, lacking integrated transmit / receive switching circuitry, and cannot meet the requirements of transmit / receive switching for MRI pulse sequences, resulting in relatively limited functionality.

[0010] In summary, current research findings still have many limitations. There is a significant lack of research in the existing scientific field and related markets regarding integrated circularly polarized radio frequency transceiver coils suitable for ultra-low field magnetic resonance imaging (MRI). Therefore, designing such a coil is essential. It would not only improve the quality of ultra-low field MRI images, aiding in early disease screening, optimizing equipment performance, and promoting the widespread adoption of low-cost portable MRI devices, but also provide direction for theoretical innovation in magnetic resonance technology and advance the life sciences. Therefore, it is necessary to design an integrated circularly polarized radio frequency transceiver coil suitable for ultra-low field MRI, covering whole-body organs, with stable structure, easy tuning, good magnetic field uniformity, integrated transmission and reception, and capable of achieving a stable circularly polarized magnetic field. Summary of the Invention

[0011] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging.

[0012] To achieve the above objectives, the present invention provides the following solution: This invention provides an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging, comprising: a dielectric substrate, a split copper ring structure, and a split straight copper sheet structure. The dielectric substrate is a cylindrical structure, and multiple sets of the split copper ring structures are uniformly arranged on the dielectric substrate. Multiple sets of the split straight copper sheet structures are vertically and uniformly arranged on the multiple sets of split copper ring structures. A feed port is provided on the split copper ring structure, and the feed port is connected to a coaxial cable through an SMA connector.

[0013] Preferably, the split copper ring structure includes split copper rings, adjustable capacitors, and adjustable resistors. Multiple split copper rings are uniformly arranged on the dielectric substrate, and each split copper ring is provided with two sets of adjustable capacitors and adjustable resistors.

[0014] Preferably, a plurality of slots are uniformly arranged on the medium substrate, and a plurality of split copper rings are correspondingly arranged in the slots and welded to them.

[0015] Preferably, each of the split copper rings is provided with a pair of radially symmetrical notches, and each notch is provided with a set of adjustable capacitors and adjustable resistors.

[0016] Preferably, two power supply ports with a 90° phase difference are provided on the split copper ring located in the middle position, and the power supply ports are connected to a coaxial cable through an SMA connector.

[0017] Preferably, the split copper strip structure includes split copper strips and adjustable capacitors. Multiple split copper strips are arranged axially on the dielectric substrate, and the split copper strips are welded to the split copper strip ring. Multiple adjustable capacitors are arranged at the center of the split copper strips.

[0018] Preferably, the dielectric substrate is provided with a plurality of axial through holes corresponding to the split copper strips, the split copper strips are disposed inside the axial through holes, and the first and last ends of each split copper strip are welded and fixed to the first and last split copper strip rings from top to bottom, respectively, and the middle part is welded and fixed to the corresponding position of each layer of split copper strip rings.

[0019] Preferably, an axial gap is provided at the center of the split straight copper sheet, and two adjustable capacitors are provided inside the axial gap.

[0020] Preferably, the material of the dielectric substrate is polyimide.

[0021] Preferably, the integrated circularly polarized RF transceiver coil further includes a transmit / receive switching circuit and an impedance matching circuit. The transmit / receive switching circuit is used to achieve rapid switching between transmit and receive modes, and the impedance matching circuit is used to tune the RF coil using a parallel capacitor structure and to perform frequency selection matching using a multi-stage Π-type network cascade structure.

[0022] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: This invention provides an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging, comprising a dielectric substrate, split copper ring structures, and split straight copper sheet structures. The dielectric substrate is cylindrical, and multiple sets of the split copper ring structures are uniformly arranged on the dielectric substrate. Multiple sets of split straight copper sheet structures are vertically and uniformly arranged on the multiple sets of split copper ring structures. A feed port is provided on the split copper ring structure, and the feed port is connected to a coaxial cable via an SMA connector. This invention has the following beneficial effects: 1. High Magnetic Field Uniformity and Large Aperture Adaptability: Simulation results of the RF coil show that the ROI uniformity is 82.82% for a 40cm radius, 91.20% for a 30cm radius, and 96.15% for a 20cm radius, which is superior to existing Helmholtz or saddle-shaped coils. In practical magnetic resonance imaging applications, it can cover the target area symmetrically over a large area with good uniformity. This enables targeted magnetic resonance imaging of specific structures or organs such as the breast, brain, and knee. Compared with most existing single-function transmitting or receiving coils for local imaging, the large aperture structure of this invention is more suitable for the actual needs of the medical imaging field. Technical points: The axial uniform arrangement of multi-layer split copper sheet rings and the radially spaced split straight copper sheet structure form a distributed LC resonant network, and the magnetic field superposition is optimized through Ampere's circuital law; the parameter tuning of the adjustable capacitor and adjustable resistor compensates for the magnetic field fluctuations caused by structural asymmetry. 2. High signal-to-noise ratio and signal stability: This invention significantly improves the signal-to-noise ratio of ultra-low field magnetic resonance imaging and ensures the stability of signal transmission through structural design and parameter optimization. The technical point is that the circularly polarized magnetic field is synthesized by orthogonal fields (generated by the current in the split copper sheet loop) and fields (generated by the current in the split straight copper sheet loop). The signal superposition effect improves the SNR by about twice. 3. High biocompatibility and low specific absorption rate (SAR): This invention demonstrates excellent biocompatibility. Through structural optimization and parameter design, the amount of radio frequency energy deposited in human tissue is far below international safety standards. Simulation results for an adult male phantom show that the average maximum SAR value for 1g of tissue is only 0.0611W / kg, and the average SAR value for 10g of tissue is 0.0011W / kg, far below the limits of 1.6W / kg for 1g of tissue and 2W / kg for 10g of tissue specified in the IEEE C95.1 standard. Technical advantages include: optimized energy distribution of low-frequency resonance (2.12MHz) and circularly polarized magnetic field, reducing radio frequency energy deposition in tissue; and adjustable resistors lowering the circuit Q value, avoiding energy concentration caused by excessively high resonant peak values.

[0023] 4. Full-Function Integration and Scalability: Regarding full-function integration, this invention achieves seamless connection between RF signal transmission and reception by integrating a transmit / receive switching circuit and an impedance matching circuit, overcoming the limitations of separate transmit and receive coils in existing technologies. In terms of scalability, the parametric design of this invention supports flexible adaptation to multiple scenarios. Based on a 60cm large aperture, by adjusting the number of layers, axial length, and aperture size of the split copper ring, it can be adapted for head, knee, and whole-body imaging. Simultaneously, this invention can also be used under ultra-low field strengths. Taking 50mT as an example, different field strengths can be applied by adjusting parameters such as adjustable capacitors and resistors, broadening the application scenarios. Key technical points include: a dual-PIN diode bridge switching circuit and orthogonal feed port design to achieve automatic switching between transmit / receive modes; and the parametric adjustability of the number of layers, length, and aperture of the split copper ring, supporting expansion of different imaging ranges. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The following are schematic diagrams of the components of an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging provided in this embodiment of the invention: (a) is a schematic diagram of a split copper ring structure; (b) is a schematic diagram of a dielectric substrate structure; and (c) is a schematic diagram of a split straight copper sheet structure. Figure 2 (a) is a schematic diagram of the geometric structure of the coil; (b) is a schematic diagram of the coil along the y-axis; (c) is a schematic diagram of the coil along the z-axis; (d) is a schematic diagram of the coil along the x-axis. Figure 3 This is a schematic diagram of the port location (coil along the z-axis); Figure 4 A schematic diagram of a Hugo male phantom placed inside a coil (with a hidden dielectric layer). Figure 5 A schematic diagram showing the magnetic field amplitude collected at different frequencies by a probe at the geometric center of the coil; (a) shows the magnetic field amplitude in the x-direction; (b) shows the magnetic field amplitude in the y-direction; (c) shows the magnetic field amplitude in both the x-direction and y-direction. Figure 6 Here are schematic diagrams of the electric field simulation results for the copper sheet coil; (a) is the H-field diagram of the coil along the z-axis; (b) is the H-field diagram of the coil along the x-axis; (c) is the H-field diagram of the coil along the y-axis; (d) is the field diagram of the coil along the z-axis; (d) is the field diagram of the coil along the z-axis; (d) is the field diagram of the coil along the z-axis. Figure 7 The simulation results show the electric field strength of a Hugo human model placed inside a copper coil; (a) is the H-field diagram along the z-axis; (b) is the H-field diagram along the x-axis; (c) is the H-field diagram along the y-axis; and (d) is the H-field diagram along the z-axis. Field diagram; (d) shows the coil along the z-axis. Field diagram; (d) shows the coil along the z-axis. Field diagram.

[0026] Reference numerals in the attached diagram: 1. Split copper ring; 2. Adjustable resistor; 3. Adjustable capacitor; 4. Dielectric substrate; 5. Split straight copper strip. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The purpose of this invention is to provide an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging. Through orthogonal feeding and adjustable components, it achieves the transmission and reception of a circularly polarized magnetic field. This circularly polarized magnetic field can significantly improve the received signal strength, effectively enhancing the signal-to-noise ratio. In terms of scalability, the parametric design of this invention supports flexible adaptation to multiple scenarios. Based on a 60cm large aperture, by adjusting the number of layers, axial length, and aperture size of the split copper ring, it can be adapted for head, knee, and whole-body imaging. Furthermore, this invention can also be used under different ultra-low field strengths. Taking 50mT as an example, it can be applied to different field strengths by adjusting parameters such as adjustable capacitors and adjustable resistors, thus broadening the application scenarios.

[0029] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] like Figure 1 As shown, the present invention provides an integrated circularly polarized radio frequency transceiver coil for ultra-low field magnetic resonance imaging, comprising: a dielectric substrate 4, a split copper ring structure and a split straight copper sheet structure. The dielectric substrate 4 is a cylindrical structure. Multiple sets of the split copper ring structures are uniformly arranged on the dielectric substrate 4. Multiple sets of the split straight copper sheet structures are vertically and uniformly arranged on the multiple sets of split copper ring structures. A feed port is provided on the split copper ring structure. The feed port is connected to a coaxial cable through an SMA connector.

[0031] The split copper ring structure includes a split copper ring 1, an adjustable capacitor 3, and an adjustable resistor 2. Multiple slots are uniformly arranged on the dielectric substrate 4. Multiple split copper rings 1 are correspondingly arranged in the slots and welded to them. Each split copper ring 1 is provided with a pair of radially symmetrical notches. Each notch is provided with a set of adjustable capacitors 3 and adjustable resistors 2.

[0032] Two power supply ports with a 90° phase difference are provided on the split copper ring 1 located in the middle position. The power supply ports are connected to a coaxial cable through an SMA connector.

[0033] The split copper sheet structure includes split copper sheets 5 and adjustable capacitors 3. The dielectric substrate 4 is provided with multiple axial through holes corresponding to the split copper sheets 5. The split copper sheets 5 are disposed inside the axial through holes. The first and last ends of each split copper sheet 5 are welded and fixed to the first split copper sheet ring 1 and the last split copper sheet ring 1 from top to bottom, respectively. The middle part is welded and fixed to the corresponding position of each layer of split copper sheet ring 1. An axial gap is provided at the center of the split copper sheet 5. Two adjustable capacitors 3 are disposed inside the axial gap.

[0034] The material of the dielectric substrate 4 is polyimide.

[0035] The integrated circularly polarized RF transceiver coil also includes a transmit / receive switching circuit and an impedance matching circuit. The transmit / receive switching circuit is used to achieve rapid switching between transmit and receive modes. The impedance matching circuit is used to tune the RF coil using a parallel capacitor structure and to perform frequency selection matching using a multi-stage Π-type network cascade structure.

[0036] This invention uses a 60cm large aperture as an example to illustrate this: It should be noted that the outer diameter of the dielectric substrate 4 is designed to match the inner diameter of the split copper strip ring 1, and its length is designed to match the size of the split straight copper strip 5. The structure is described below: 1. Split copper ring structure (1) Split copper ring In this embodiment, it is actually a radially symmetrical gap copper ring, with each ring containing two radial gaps (180° apart). An adjustable capacitor 3 (such as a ceramic trimmer capacitor) and an adjustable resistor 2 (thin-film resistor) are welded within the gaps. The ring is 5mm wide and 0.5mm thick (meeting a skin depth of 1.44μm for 2.12MHz, reducing resistance loss). It has nine layers (adjustable from 9 to 29 layers; the resonant frequency decreases by approximately 0.02MHz for every two layers added). When the diameter is adapted to a 60cm aperture, the inner diameter of a single-layer copper ring is 60cm and the outer diameter is 60.5cm (adjustable according to the aperture expansion ratio). Its function is to generate an axial magnetic field B1y through a co-phase alternating current; increasing the number of layers increases the magnetic field strength. (2) Adjustable capacitor A ceramic trimmer capacitor (SMD0805 package) can be used, installed in the radial gap of the split copper ring 1, and fixed by surface mounting or soldering. It can be adjusted within the range of 1000pF-5000pF. It is used to compensate for parasitic capacitance introduced by the number of layers, length changes of the split copper ring 1 and human body load through capacitance adjustment, so as to ensure the frequency stability of the LC resonant network. For example, when the number of coil layers increases, the distributed inductance increases and the resonant frequency decreases. The capacitor value needs to be reduced to maintain the 2.12MHz resonant point. (3) Adjustable resistor A thin-film adjustable resistor 2 (0603 or 1206 package) is used, employing a glass glaze film or metal film as the resistive element, with an insulating coating on the surface. It is soldered into the gap of the split copper ring 1 and connected in parallel with the adjustable capacitor 3. The value is adjusted between 0-2Ω. It is used to change the circuit quality factor (Q value) by adjusting the resistance value, balance the difference in resonant peak values ​​between the orthogonal magnetic fields B1x and B1y. In addition, the resistor can also optimize the impedance matching between the coil and the RF source (from 30Ω to 50Ω), reduce the standing wave ratio to below 1.2, and reduce energy reflection loss.

[0037] 2. Split straight copper sheet structure (1) Split straight copper sheet Its structure consists of a straight copper sheet axially penetrating the split copper sheet ring 1, with an axial gap in the middle, and an adjustable capacitor 3 (such as a tantalum electrolytic capacitor) built in. The length of the capacitor is 75cm (consistent with the axial length of the coil), the width is 5mm, and the thickness is 0.5mm. There is one gap for each straight copper sheet, with a built-in capacitance of 3180pF (tuned to match the 1530pF of the split copper sheet ring 1). This is used to form an orthogonal loop with the split copper sheet ring 1 to generate a transverse magnetic field and synthesize a circularly polarized field. (2) Adjustable capacitor Tantalum electrolytic capacitors (axial lead type) are used and placed in the axial gap of the split copper strip 5. They are fixed by surface mounting or soldering and can be adjusted within the range of 15000pF-22000pF. They are used to compensate for parasitic capacitance introduced by changes in the number of layers and length of the split copper strip ring 1 and human body load through capacitance adjustment, so as to ensure the frequency stability of the LC resonant network. For example, when the number of coil layers increases, the distributed inductance increases and the resonant frequency decreases. The capacitor value needs to be reduced to maintain the 2.12MHz resonant point.

[0038] 3. Dielectric substrate Polyimide (dielectric constant ε=3.5, loss factor tanσ=0.0027 at 1.0MHz) is used to form the structure by 3D printing. Its outer diameter matches the inner diameter of the split copper ring 1 (e.g., the outer diameter is 60cm when the aperture is 60cm); its length is consistent with the split straight copper sheet 5 (75cm), and its thickness is 2mm. It is used to support the copper sheet structure, reduce parasitic capacitance interference, and optimize the resonance efficiency.

[0039] like Figure 2 As shown, the overall geometric structure is described. From an axial perspective, the 21-layer split copper sheet ring 1 is evenly arranged axially with a layer spacing of 3.5cm, forming a 75cm long cylindrical structure. From a radial perspective, split straight copper sheets 5 penetrate the copper sheet ring at 90° intervals, forming an orthogonal feeding network with the feed port, as shown... Figure 3 As shown, the intermediate layer split copper ring 1 has two feed ports with a 90° phase difference, which are connected to a coaxial cable (characteristic impedance 50Ω) through an SMA connector.

[0040] The transmit / receive switching circuit and impedance matching circuit are described below: 1. Transceiver switching circuit (1) Structure: A dual-PIN diode bridge switching circuit (such as MA4AGRF17 type PIN diode) is used in conjunction with an RC delay network (10kΩ resistor + 0.1μF capacitor) to achieve fast switching between transmit and receive modes. The circuit is based on a bridge structure, and the conduction state of the diode is controlled by positive and negative bias to ensure electrical isolation between the transmit and receive channels; (2) Working principle: Transmit mode: The control voltage forward biases the PIN diode to conduct, short-circuiting the receiving channel to ground to prevent radio frequency signal interference to the receiver; Receive mode: The control voltage reverse biases the diode to cut off, blocking the high-power signal from the transmitting end from entering the receiving link, while connecting the coil to a low-noise amplifier (LNA). Switching timing: The switching time is ≤1μs, which meets the timing requirements of the transmit pulse (approximately 10μs) and the receiving window (ms level) in the MRI pulse sequence, avoiding signal distortion; (3) Compatibility design: The circuit integrates an ESD protection diode (such as P6SMB15CA) to prevent transient impact of RF pulses on the PIN diode; a 100μF electrolytic capacitor is connected in parallel at the power supply end for filtering to reduce ripple interference.

[0041] 2. Impedance matching circuit (1) Structure: The RF coil is tuned using a parallel capacitor structure and the frequency selection matching is performed using a multi-stage Π-type network cascade structure; (2) Working principle: A capacitor is connected in parallel across the RF coil to tune it to the desired Larmor frequency. A multi-stage Π-type network is cascaded between the parallel capacitor and the Rec terminal to match the high impedance of the RF coil with the low impedance of the receiving circuit, thereby achieving impedance transformation and maximizing power transmission; (3) Design advantages: Tuning the parallel capacitor of the RF coil can effectively reduce coil losses and improve the flexibility of impedance matching and the stability of the resonant frequency compared to series capacitor. The cascaded structure of the multi-stage Π-type network allows for fine adjustment of capacitor and inductor values ​​to adapt to different frequency ranges. When combined with diodes, it can significantly improve the isolation between the RF coil, signal input terminal and output terminal. When combined with the RF coil tuning capacitor, it can effectively reduce system noise and improve the signal-to-noise ratio.

[0042] For ease of understanding, the connection relationships of the various components are described in this invention: 1. Split copper ring and dielectric substrate The surface of the dielectric substrate 4 is provided with a slot. Each layer of split copper sheet ring 1 is positioned through the slot on the surface of the polyimide dielectric substrate 4 and fixed by welding with high temperature resistant epoxy resin. The interlayer maintains a uniform spacing of 3.5cm and is fixed by through-hole polyimide bolts to avoid inductive interference caused by metal contact. 2. Split straight copper sheet and split copper sheet ring The dielectric substrate 4 is provided with an axial through hole corresponding to the split straight copper sheet 5. Two split straight copper sheets 5 pass through the axial through hole of the dielectric substrate 4 at 180° intervals. The two ends are electrically connected to the edge copper sheets of the first and last layer split copper sheet rings 1 by laser welding. The middle part is welded and fixed to the corresponding position of each layer split copper sheet ring 1 to ensure the continuity of the axial current path. 3. Dielectric substrate The surface has pre-set copper strip ring grooves and straight copper strip through holes, with a groove depth of 0.5mm, to ensure that the copper strips are tightly attached to the substrate and reduce parasitic capacitance fluctuations.

[0043] This invention describes the circuit connection relationships of the various components: Electrical connections are achieved through welding and shielded wiring to ensure signal transmission efficiency and anti-interference capabilities. 1. Adjustable element and copper sheet structure The adjustable capacitor 3 and adjustable resistor 2 in the gap of the split copper ring 1 are soldered to the end of the copper sheet using surface mount technology, with a lead length ≤3mm to reduce parasitic inductance; the adjustable capacitor 3 in the gap of the split straight copper sheet 5 is soldered with axial leads, and the contact area between the leads and the copper sheet is ≥0.5mm² to reduce contact resistance. 2. Transceiver switching circuit connection The anode of the PIN diode is soldered to the feed point of the split copper ring 1 through a 0.1mm copper foil, and the cathode is connected to the receiver input terminal through a DC blocking capacitor; the control signal line is connected to the external control module through a shielded twisted pair cable, and the shielding layer is soldered to the metal grounding layer of the dielectric substrate 4 to suppress electromagnetic interference. 3. Impedance matching network connection The inductor (15μH) of the L-type matching circuit is connected in series with the feed terminal of the split copper ring 1. One end of the capacitor is soldered to the copper sheet, and the other end is grounded. The resistor is connected across the output terminal of the matching network and forms a closed loop with the copper sheet and the switching circuit through PCB traces. 4. Power supply port connection Two orthogonal feed points (90° out of phase) of the intermediate layer split copper ring 1 are welded with SMA connectors. The inner pins of the connectors are directly connected to the copper rings, and the outer ground layer is welded to the metal shielding layer of the dielectric substrate 4. The SMA connectors are connected to the RF source / receiver through a 50Ω coaxial cable, and the cable shielding layer is reliably connected to the connector grounding layer.

[0044] Finally, the working principle of this invention is introduced: When the coil proposed in this invention is working, it mainly generates a circularly polarized radio frequency excitation magnetic field B1, which is synthesized from a linearly polarized radio frequency excitation magnetic field B1y and a linearly polarized radio frequency excitation magnetic field B1x. When the coil resonates, a radio frequency alternating current of the same phase flows through the split copper ring 1. The current direction in all the split copper rings 1 is simultaneously clockwise or counterclockwise, and reverses periodically with the radio frequency signal. According to Ampere's circuital law, the current in each layer of split copper rings 1 will generate a ring magnetic field around it, the direction of which is perpendicular to the split copper rings 1 (i.e., along the coil axis). Since the split copper rings 1 are uniformly distributed on the circumference, the magnetic fields generated by each ring are superimposed in the central region of the coil to form a magnetic field rotating in the vertical direction (vertical, parallel to the coil axis), namely the B1y field.

[0045] Furthermore, on both sides of the split copper ring structure, a split straight copper strip 5 penetrates and connects to the multiple layers of split copper rings. When the coil resonates, these two copper pillars and each layer of split copper ring 1 together form a longitudinal loop. Similar to the magnetic field generation mechanism perpendicular to the split copper ring 1 and pointing vertically upward (y direction), the loop current formed between the split straight copper strip structure and the split copper ring structure will generate a ring magnetic field around it, the direction of which is parallel to the split copper ring 1 (x direction), i.e., the B1x field.

[0046] This invention simulates this embodiment, employing a highly realistic human body model in the electromagnetic simulation, such as... Figure 4 As shown, the core object is the Hugo adult male model built into CST Studio Suite; Figure 5The relationship between the magnetic field strength and frequency in the x and y directions of the copper sheet coil is shown. According to the results, at a frequency of 2.12MHz, the magnetic fields in the x and y directions of the coil reach resonance simultaneously, and the amplitudes of the magnetic fields in the two directions are almost exactly the same. To more intuitively illustrate the electric field distribution inside the coil, Figure 6 The H-field diagram of the copper sheet coil and the post-processed field diagram are shown to more accurately simulate the actual MRI environment. Figure 7 The H-field diagram obtained after placing a Hugo human model inside a copper coil is shown. Field diagram.

[0047] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0048] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, 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 the present invention.

Claims

1. An integrated circular polarized radio frequency transmit-receive coil for ultra-low field magnetic resonance imaging, characterized in that, The application relates to an integrated circularly polarized radio frequency (RF) coil, which comprises a medium substrate, split copper ring structures and split straight copper structures. The split copper ring structure comprises a split copper ring, an adjustable capacitor and an adjustable resistor.

2. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 1, wherein, The medium substrate is uniformly provided with a plurality of clamping grooves, and a plurality of split copper rings are correspondingly arranged in the clamping grooves and welded.

3. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 2, wherein, Each split copper ring is provided with a pair of radially symmetrical notches, and each notch is provided with a set of adjustable capacitors and adjustable resistors.

4. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 3, wherein, The split straight copper structure comprises a split straight copper and an adjustable capacitor.

5. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 4, wherein, The medium substrate is provided with a plurality of axial through holes corresponding to the split straight copper, and the split straight copper is arranged in the axial through hole.

6. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 5, wherein, The medium substrate is made of polyimide.

7. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 6, wherein, The integrated circularly polarized RF coil further comprises a transceiving switching circuit and an impedance matching circuit.

8. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 7, wherein, The transceiving switching circuit is used for realizing quick switching of a transmitting mode and a receiving mode.

9. The integrated circular polarized RF receive coil for ultra-low field MRI of claim 1, wherein, The impedance matching circuit is used for tuning the RF coil by adopting a capacitor parallel structure and frequency selection matching by adopting a multi-stage Pi network cascade structure.

10. The integrated circular polarized RF receive and transmit coil for ultra-low field MRI of claim 1, wherein, ​

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