Flexible metasurface enhancement device and method for low-field magnetic resonance imaging

Abstract

The invention discloses a flexible metasurface enhancement device and method for low-field magnetic resonance imaging. The open type spiral metasurface antenna is composed of open type spiral metasurface units arranged periodically, and each unit comprises a flexible dielectric substrate and an open type spiral metal pattern rotating 90 degrees in a double-face space. Through the depth sub-wavelength design, the unit side length is 0.03 times smaller than the working wavelength, the ultra-thin flexible structure is adopted, and the total thickness is smaller than or equal to 0.15 mm, preferably 0.136 mm. Double-sided patterns are synchronously manufactured by adopting a printed circuit board process, so that the manufacturing cost is remarkably reduced. During application, the device is attached to a scanning part, a magnetic field is enhanced through inductive coupling with a radio frequency coil, the image signal-to-noise ratio is increased by 97.4%-133.2% in a 0.2 T MRI system, the field uniformity is improved, meanwhile, the local value is smaller than or equal to 0.2 W / kg, and the contradiction between the insufficient low-field MRI signal-to-noise ratio and the clinical adaptability of the device is solved.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology, specifically relating to a flexible metasurface enhancement device and method for low-field magnetic resonance imaging. The device improves the signal-to-noise ratio and magnetic field uniformity of low-field magnetic resonance imaging through a deep subwavelength structure and a double-sided orthogonal spiral design. Background Technology

[0002] Magnetic resonance imaging (MRI), an indispensable non-invasive diagnostic tool in modern medicine, provides complementary tissue information for clinicians through its multi-contrast imaging capabilities (such as T1-weighted, T2-weighted, and proton density-weighted imaging). However, while low-field MRI systems (≤0.5T) offer advantages such as cost-effectiveness and portability, they are limited by their inherently low signal-to-noise ratio (SNR) and limited spatial resolution, resulting in insufficient detection of minute lesions. More seriously, weak polarization effects force longer scan times, easily causing patient motion artifacts and directly affecting diagnostic accuracy. Traditional SNR improvement solutions face fundamental contradictions: increasing the static magnetic field strength can improve SNR, but significantly increases hardware costs and introduces biosafety issues; multi-channel surface coils, while supporting parallel imaging and accelerated scanning, greatly increase system complexity; and solutions such as capacitively loaded surface resonators or water-based resonators are difficult to meet clinical needs due to complex manufacturing processes, bulky size, and poor anatomical adaptability. Existing technologies such as CN108226834B, while employing metal spirals and capacitors to construct resonant units, suffer from rigid substrates (≥0.6mm) that cannot conform to the curvature of the human body. Furthermore, they rely on external surface-mount capacitors (56-560pF) for frequency tuning, leading to the need for precise soldering during manufacturing, poor mass production consistency, and the risk of capacitor detuning. More seriously, their unit size (…) (Not exceeding wavelength limitations, it is still prone to causing problems in low-field MRI) Field inhomogeneity and unverified clinical SAR safety.

[0003] The emergence of metamaterials and their two-dimensional forms (metasurfaces) has provided a new path to overcome the aforementioned bottlenecks. These subwavelength artificial structures can theoretically achieve a leap in SNR by precisely controlling the distribution of the radio frequency field. Existing research has verified their effectiveness in high-field MRI (≥1.5T): for example, a 10 mm thick metal wire array increases SNR by 2.7 times at 1.5T, a 0.8 mm thick Hilbert curve structure achieves a fourfold magnetic field enhancement in deep tissue at 7T, and a 0.025 mm copper-based metasurface can optimize field uniformity in brain regions. However, when these achievements are transferred to low-field MRI, a fatal flaw is exposed: traditional rigid metasurfaces require a unit size of over 300 mm in the 8.5 MHz band (…). The current low-field compatible solutions also have obvious shortcomings: the planar spiral resonant coil only improves the SNR by 2.2 times at 54.6mT, which is a limited enhancement effect; although the 40 mm thick capacitive load open ring lens shows potential in the 0.3T system, it cannot conformally fit the human body surface due to its rigid structure.

[0004] Therefore, the field of low-field MRI urgently needs an innovative solution to simultaneously address three major contradictions: how to achieve this under depth subwavelength size constraints (e.g., cell side length) This technology aims to achieve enhanced magnetic fields while maintaining ultra-thin flexibility to adapt to the anatomical curves of the human body, and ensures clinical scalability in manufacturing processes. This technological gap severely hinders the improvement of the diagnostic value of low-field MRI in emergency and bedside settings. Summary of the Invention

[0005] To overcome the core deficiency of insufficient signal-to-noise ratio in low-field MRI, this invention provides a flexible metasurface enhancement device and method for low-field magnetic resonance imaging, specifically a signal-to-noise ratio and field homogeneity enhancement device for low-field magnetic resonance imaging systems based on an open-ring spiral metasurface (ORSM). This invention designs an ultrathin, flexible, deep subwavelength bi-faced orthogonal open-ring spiral metasurface structure. By precisely controlling the radio frequency (RF) field distribution, it significantly enhances the magnetic field strength and homogeneity of the target region, thereby effectively improving the image quality of low-field MRI (especially 0.2T). This device is easy to manufacture, conformally fits the curved surface of the human body, and can be seamlessly integrated into existing low-field MRI hardware.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A flexible metasurface enhancement device for low-field magnetic resonance imaging includes: a periodically arranged array of open spiral metasurface units, each unit comprising a flexible dielectric substrate, a top layer open spiral metal pattern located on a first surface of the dielectric substrate, and a bottom layer open spiral metal pattern located on a second surface of the substrate and orthogonally arranged with the top layer pattern.

[0008] The unit is a deep subwavelength structure with a side length of 104±5mm and a total thickness of 0.136±0.01mm.

[0009] A flexible metasurface enhancement method for low-field magnetic resonance imaging based on the above-mentioned device, characterized in that it includes:

[0010] The flexible metasurface enhancement device is conformally attached to the surface of a phantom with a curvature radius ≥100mm and placed inside the magnet cavity of an MRI scanner. The near-field magnetic field distribution is reconstructed through inductive coupling with the radio frequency coil.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] First, because the ORSM structure proposed in this invention has a unique double-layer open spiral design and deep subwavelength characteristics, it can efficiently modulate and enhance the local radio frequency magnetic field, thereby significantly improving the signal-to-noise ratio (SNR) and image uniformity of low-field MRI (especially 0.2T system) without increasing the static magnetic field strength.

[0013] Secondly, due to the use of a flexible polyimide substrate and an ultra-thin design (only 0.136 mm), the ORSM device can conformally fit tightly to the curved surfaces of the human body (such as the knee joint and abdomen) with a curvature radius ≥100 mm, and the resonant frequency shift is minimal when bending (<0.05 MHz), completely solving the clinical fit barrier of rigid solutions such as CN108226834B (substrate ≥0.6 mm).

[0014] Third, due to the synergistic effect of the optimized helical topology and the double-layer field localization mechanism, the present invention can achieve deep magnetic field enhancement and penetration, which is crucial for improving the imaging clarity of deep tissues in low-field MRI.

[0015] Fourth, due to the use of standard PCB manufacturing process, the device of this invention has the advantages of being mature, reliable and cost-controllable, which facilitates large-scale production and seamless integration into existing low-field MRI hardware systems without requiring major modifications to the core scanner equipment, thus having significant practical and promotional value.

[0016] Fifth, through rigorous electromagnetic simulation verification, the SAR value generated by the device of this invention under typical working conditions is far lower than the international safety standard, which fully ensures the safety of patients when undergoing MRI examinations.

[0017] Sixth, unlike complex structures that require capacitor tuning, the passive double-sided orthogonal design of this invention is formed in one step using standard PCB processes, avoiding the risk of soldering mismatch. The top and bottom open spiral metal patterns are electromagnetically coupled through a flexible dielectric substrate, without the need for metallized vias or external lumped capacitor components. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an open spiral metasurface (ORSM) structure provided in an embodiment of the present invention. In the figure: 101 is the top spiral metal pattern, 102 is the flexible dielectric substrate, and 103 is the bottom spiral metal pattern.

[0019] Figure 2 This is a schematic diagram of the equivalent circuit model of the ORSM unit provided in an embodiment of the present invention;

[0020] Figure 3A schematic diagram of ORSM integrated application provided in an embodiment of the present invention: 301 is a radio frequency coil, 302 is a phantom, and 303 is an enhancement device;

[0021] Figure 4 The MRI image effect comparison diagram provided for the embodiments of the present invention: the top five images are cross-sectional images of the phantom without ORSM, and the bottom five images are images after ORSM is applied. The green box marks the SNR analysis area. Detailed Implementation

[0022] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0023] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0024] This invention provides a flexible metasurface enhancement device for low-field magnetic resonance imaging (MRI). Its core lies in its unique ultrathin flexible structure design and its integrated application in low-field MRI systems, aiming to significantly improve image signal-to-noise ratio (SNR) and uniformity. Specific implementation is as follows:

[0025] The basic unit of the enhancement device presents a three-layer structure: the top layer is a spiral metal pattern 101 made of a highly conductive material (preferably copper, with high conductivity). The open spiral metal pattern is formed, with a preferred thickness of 0.018 mm. The intermediate layer, namely the dielectric substrate 102, is a flexible dielectric material substrate (preferably polyimide, with a relative permittivity of 10 ... The loss angle tangent tan δ = 0.0025, and the thickness H is preferably 0.1 mm. Bottom spiral metal pattern 103: An open spiral metal pattern made of the same material as the top layer, with the same shape as the top layer pattern, but precisely rotated 90° in space.

[0026] The physical dimensions of a single basic unit are designed for deep subwavelength: length 104 mm ( ), width 104mm ( ), total thickness 0.136 mm ( ),in The free space wavelength (≈ 3.551 m) corresponding to the 0.2T MRI operating frequency (8.5 MHz).

[0027] The key geometric parameters of the helical pattern include: the outer edge length of the helix C = 100 mm, the width of the helical metal wire W = 1 mm, the spacing between adjacent helical lines D = 1.5 mm, and a specific internal spacing S = 2 mm. These parameters are optimized using electromagnetic simulations (such as HFSS software) to ensure resonance at the target frequency (8.5 MHz). Full-wave electromagnetic simulation was used to establish... Figure 2 The equivalent circuit model shown contains four sub-modules: Part I (top spiral) is equivalent to a series capacitor. With inductance Part II (inter-spiral coupling) is equivalent to a series inductor. With capacitor Part III (flexible dielectric substrate) is equivalent to a series capacitor. with parallel inductor Part IV (bottom spiral) is equivalent to Part I. Circuit. This model fully describes the electromagnetic coupling mechanism of the two-sided orthogonal spiral: the top / bottom spirals dominate the fundamental resonance ( ), inter-spiral coupling ( Enhanced magnetic field orthogonal components, substrate transmission line effect ( By regulating the energy transfer phase, these three factors work together to achieve strong magnetic field enhancement at the deep subwavelength level. Actual performance is subject to MRI test results. Figure 1 The top view, bottom view, and side view of the ORSM unit are shown.

[0028] The enhancement device is manufactured using a mature standard printed circuit board (PCB) process: a top spiral metal pattern 101 and a bottom spiral metal pattern 103 rotated 90° are formed by simultaneous photolithography and etching on a double-sided copper-clad polyimide substrate. There are no metallization vias or external capacitor soldering steps throughout the process. Finally, after cleaning and drying, a flexible structure with a total thickness of 0.136 mm is obtained.

[0029] When integrating applications (refer to) Figure 3 The enhancement device 303 is tightly attached to the surface of the phantom 302 or the human body to be scanned, and placed inside the magnet cavity of the 0.2T MRI scanner. It redistributes the near-field magnetic field through inductive coupling with the radio frequency coil 301, significantly enhancing the target area. Field uniformity. Its flexible design can adapt to the curved surfaces of the human body with a curvature radius ≥100 mm, and the resonant frequency shift is <0.05 MHz.

[0030] Performance verification experiments were conducted based on standard cylindrical water models (including...) The image was scanned using a 3.65 g / L NaCl solution on a 0.2 T MRI system with a T1-weighted spin echo sequence (TR=500 ms, TE=17 ms, FA=90°, matrix 256×256, FOV=300×300 mm²). Figure 4 The image shown is a comparison of MRI image effects provided by an embodiment of the present invention. The top five images are cross-sectional images of the phantom without ORSM, and the bottom five images are images after ORSM is applied. The green boxes indicate the SNR analysis area. Experimental results show that after applying the ORSM device, the average signal-to-noise ratio (SNR) of the region of interest (ROI) is significantly improved. Where S is the signal mean, The improvement in noise standard deviation reached 97.4%-133.2%, with the SNR value of typical imaging site Slice #1 significantly increasing from 68.83 to 135.84; the SNR dispersion coefficient of the five standard imaging positions decreased from 18.9% to 3.8%, and the field uniformity improvement rate reached 79.9%; the local SAR10g peak value verified by full-wave electromagnetic simulation (according to IEC / IEEE 62704-1 standard) was 0.1805 W / kg, which is only 5.64% of the head safety limit (3.2 W / kg) of IEC 60601-2-33, with a safety margin as high as 94.36%.

[0031] Summary of implementation results:

[0032] As can be seen from the above specific embodiments, the Open Helical Metasurface (ORSM) device provided by this invention, with its innovative ultrathin (0.136 mm) flexible, deep subwavelength structure and optimized double-helix design, successfully achieves effective enhancement and homogenization of the magnetic field in the target region in a 0.2T low-field MRI system. Experiments confirm that it can significantly improve image SNR (up to >97%) and greatly improve image uniformity (CV <4%), while meeting electromagnetic safety requirements (SAR << limit). The device has a mature manufacturing process (standard PCB) and is easy to integrate and apply, providing an effective and practical technical solution to solve the core performance bottleneck of low-field MRI.

[0033] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A flexible metasurface enhancement device for low-field magnetic resonance imaging, characterized in that, include: A periodically arranged open spiral metasurface unit array, each unit comprising a flexible dielectric substrate, a top open spiral metal pattern located on the first surface of the dielectric substrate, and a bottom open spiral metal pattern located on the second surface of the substrate and orthogonally arranged with the top pattern. The unit is a deep subwavelength structure with a side length of 104±5mm and a total thickness of 0.136±0.01mm.

2. The apparatus according to claim 1, characterized in that, The top and bottom open spiral metal patterns are spirals with the following geometric parameters: outer edge length C = 100 ± 5 mm; spiral width W = 1 ± 0.2 mm; spacing between adjacent spirals D = 1.5 ± 0.3 mm; and internal specific spacing S = 2 ± 0.4 mm.

3. The apparatus according to claim 1, characterized in that, When the unit is fitted onto a curved surface with a curvature radius ≥ 100 mm, the resonant frequency shift is ≤ 0.05 MHz.

4. The apparatus according to claim 1, characterized in that, The device is manufactured using standard printed circuit board technology, specifically by photolithography and etching simultaneously forming a top and bottom spiral pattern on a double-sided copper-clad polyimide substrate, without soldering capacitor components.

5. The apparatus according to claim 1, characterized in that, The device generates localized noise at an operating frequency of 8.5MHz. Value ≤ 0.18 W / kg.

6. The apparatus according to claim 1, characterized in that, Polyimide is used, with a relative permittivity of The loss tangent tan δ = 0.0025, and the thickness is preferably 0.1 mm.

7. The apparatus according to claim 1, characterized in that, The top and bottom open spiral metal patterns are continuous metal films made of highly conductive materials, with a preferred thickness of 0.018 mm.

8. The apparatus according to claim 7, characterized in that, Copper is preferred as a high conductivity material, with high conductivity. .

9. The apparatus according to claim 1, characterized in that, Orthogonal arrangement is an arrangement rotated 90°±5°.

10. A flexible metasurface enhancement method for low-field magnetic resonance imaging based on the device described in claims 1-9, characterized in that, include: The flexible metasurface enhancement device according to any one of claims 1-9 is conformally attached to the surface of a phantom with a curvature radius ≥100mm, placed inside the magnet cavity of an MRI scanner, and reconstructed with the near-field magnetic field distribution through inductive coupling with a radio frequency coil.

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