Method for suppressing transverse electromagnetic interference in magnetic resonance imager cavity without electromagnetic shielding

Abstract

The invention relates to a method for suppressing transverse electromagnetic interference in a cavity of a magnetic resonance imager without electromagnetic shielding, and belongs to the technical field of nuclear magnetic resonance. The method is based on an active noise reduction hardware system, an electromagnetic wave sensor arranged around an imager collects space electromagnetic interference signals in real time, and after the phase and amplitude of the interference signals are adjusted through a noise signal processing circuit, noise counteracting coils placed on the two sides of a magnet cavity generate a noise counteracting coil which is opposite to the phase of the interference signals in an imaging area, and the opposite-phase offset electromagnetic waves with the same spatial distribution rule form an electromagnetic field which is mutually offset with the original interference signal, and finally dynamic suppression of transverse electromagnetic interference in the imaging area is realized. The problem of transverse electromagnetic interference suppression in an electromagnetic-shielding-free magnetic resonance imaging system is solved, and the signal-to-noise ratio of the electromagnetic-shielding-free magnetic resonance imaging instrument is effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear magnetic resonance and relates to a method for suppressing transverse electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding. Background Art

[0002] The electromagnetically shielded MRI system, with its open structure and low hardware cost, has shown significant application potential in mobile healthcare, bedside diagnosis, and medical care in remote areas. However, the system has serious technical shortcomings in its anti-interference ability in complex electromagnetic environments:

[0003] 1) Electromagnetic interference sensitivity: In the absence of electromagnetic shielding, MRI systems are extremely susceptible to wide-band interference (0.1-100MHz frequency band) generated by power line harmonics (50 / 60Hz and its multiples) and medical electronic equipment (such as monitors and electrosurgical units) that are widely present in the environment, resulting in a decrease in the image signal-to-noise ratio.

[0004] 2) Noise anisotropy challenge: Experiments show that, if Figure 2 As shown in the figure, in the orthogonal RF coil system, the electromagnetic interference noise shows significant anisotropy in spatial distribution: the electromagnetic interference intensity in the horizontal direction (transverse direction) can be more than twice that in the vertical direction (longitudinal direction). This anisotropy of electromagnetic noise makes it difficult for traditional algorithm-based active noise reduction technologies (such as adaptive filtering, spectrum suppression, etc.) to effectively eliminate lateral interference. Especially in a dynamically changing electromagnetic environment, the real-time and stability of the algorithm face great challenges.

[0005] To avoid lateral noise interference, the current mainstream solution uses a spiral tube single-channel RF coil to reduce noise sensitivity through a longitudinal layout. Although this solution can reduce lateral noise sensitivity, it seriously sacrifices the system's multi-channel parallel acquisition capabilities, resulting in limited imaging speed (usually taking tens of minutes), low signal-to-noise ratio (SNR), and inability to achieve high-resolution imaging.

[0006] Therefore, there is an urgent need for an innovative method that integrates the active electromagnetic wave cancellation mechanism and hardware topology optimization to build a targeted suppression strategy from the source of interference. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a method for suppressing lateral electromagnetic interference in the cavity of a magnetic resonance imaging device without electromagnetic shielding, which is suitable for suppressing lateral noise of a saddle-shaped radio frequency receiving coil in a magnetic resonance imaging device without electromagnetic shielding, solves the problem of suppressing lateral electromagnetic interference in a magnetic resonance imaging system without electromagnetic shielding, and effectively improves the signal-to-noise ratio of the magnetic resonance imaging device without electromagnetic shielding.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] A method for suppressing lateral electromagnetic interference in the cavity of an MRI without electromagnetic shielding is based on an active noise reduction hardware system. Electromagnetic wave sensors (such as noise detection coils) arranged around the imager collect spatial electromagnetic interference signals in real time. After the phase and amplitude of the interference signals are adjusted by a noise signal processing circuit, anti-phase cancellation electromagnetic waves with opposite phases and the same spatial distribution pattern as the interference signals are generated in the imaging area by noise cancellation coils placed on both sides of the magnet cavity. This forms an electromagnetic field that cancels out the original interference signal, ultimately achieving dynamic suppression of lateral electromagnetic interference in the imaging area.

[0010] The basic principle of this method is to collect the spatial electromagnetic noise around the magnetic resonance device, invert the noise through the noise signal processing circuit, drive the noise cancellation coil to form an anti-phase magnetic field that matches the interference field in the imaging area, and suppress lateral electromagnetic interference from space in a destructive way by wave interference.

[0011] In the U-shaped magnet cavity of the magnetic resonance imaging system, the intensity of electromagnetic interference forms a gradient distribution from strong to weak from the opening to the inside. Therefore, the noise cancellation coil needs to generate a gradient field that matches the gradient of the interference field and has the opposite direction.

[0012] Furthermore, the stream function method is used to optimize the design of the noise cancellation coil structure to ensure that the generated magnetic field can accurately match the gradient distribution of the interference noise.

[0013] Preferably, the noise cancellation coil comprises at least two symmetrically arranged noise cancellation coil groups, which are respectively located on the left and right sides of the imaging area inside the magnet.

[0014] Preferably, each noise cancellation coil group is composed of a pair of sub-coils with the same geometric structure; the sub-coils are electrically connected in series; each sub-coil is composed of a plurality of concentric or non-concentric conductive coil units arranged axially along the cavity; the coil units adopt at least one of the geometric configurations of circular, rectangular or elliptical, or the winding path of the noise cancellation coil can also adopt an irregular curve, and the winding density is distributed in a decreasing manner from the cavity opening to the depth of the cavity.

[0015] Preferably, the noise cancellation coil is realized in any of the following forms: a planar or curved coil structure directly printed on the inner surface of the iron yoke on both sides of the cavity or the outer surface of the RF excitation coil, or a deformable coil structure encapsulated on a flexible PCB substrate, or a three-dimensional winding structure conformally arranged with the RF excitation coil.

[0016] Preferably, the noise cancellation coil assembly is fixed at any of the following locations: fixed to the inner wall surface of the iron yoke on both sides, or attached to the radio frequency excitation coil (i.e. Figure 1The outer surface of the MRI transmitting coil in the apparatus, or fixed at any position between the RF excitation coil and the iron yokes on both sides.

[0017] Preferably, the noise signal processing circuit includes: a matching circuit, a low-noise preamplifier, an operational amplifier circuit and a phase modulation circuit;

[0018] The matching circuit is used to achieve impedance matching with the electromagnetic wave sensor on the input side and the noise cancellation coil on the output side;

[0019] The low-noise preamplifier is used to amplify the sensed weak electromagnetic signal while maintaining minimal noise gain to ensure signal quality and clarity;

[0020] The op-amp circuit consists of two operational amplifier stages: a non-inverting op-amp circuit and an inverting op-amp circuit, which perform initial non-inverting amplification and phase inversion, respectively. The feedback network in the op-amp circuit uses an adjustable resistor design to match the interference signal amplitude in different environments, ensuring the system's adaptability to various application scenarios.

[0021] The phase modulation circuit performs phase compensation on the anti-phase signal through precise phase adjustment, thereby ensuring that the compensated signal cancels out the original interference signal and maximizing the noise suppression effect.

[0022] The beneficial effects of the present invention are:

[0023] 1) Dynamic interference suppression

[0024] Active noise reduction technology is used to collect electromagnetic interference in real time and generate anti-phase offset electromagnetic waves, which achieves dynamic and real-time suppression of lateral electromagnetic interference in the imaging area and solves the anti-interference problem of magnetic resonance systems without electromagnetic shielding.

[0025] 2) Spatial gradient matching design

[0026] Based on the gradient distribution characteristics of electromagnetic interference in the magnet cavity, the stream function method is used to inversely optimize the noise cancellation coil structure to ensure that the generated anti-phase magnetic field accurately matches the gradient distribution of the interference field, significantly improving the cancellation efficiency.

[0027] 3) Flexible structure and low complexity implementation

[0028] The noise cancellation coil supports multi-form integration (such as planar printing, flexible PCB or conformal winding) and can be flexibly arranged on the inner wall of the iron yoke on both sides of the magnet cavity, the outside of the RF coil, and other locations. By connecting sub-coils in series and designing a decreasing winding density, gradient field control can be achieved without increasing the complexity of the system.

[0029] 4) Anti-interference signal processing optimization

[0030] The analog circuit solution of low-noise preamplifier and adjustable feedback network is adopted, combined with phase modulation technology, taking into account low noise, high-speed response and environmental adaptability, ensuring the real-time inversion processing accuracy of interference signals.

[0031] 5) Compatibility and cost advantages

[0032] The need for traditional electromagnetic shielding layers reduces equipment weight and manufacturing costs. At the same time, the conformal design makes the noise cancellation coil compatible with existing RF excitation coils, making it easy to integrate into ultra-low field magnetic resonance systems.

[0033] 6) Universality of experimental verification

[0034] Through one-dimensional magnetic resonance signal denoising and water phantom imaging experiments, it was demonstrated that this method significantly suppresses background noise while retaining effective signals, and is suitable for practical application scenarios of ultra-low-field MRI systems.

[0035] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0037] Figure 1 Schematic diagram of the transverse electromagnetic interference suppression system designed for the present invention;

[0038] Figure 2 Schematic diagram of electromagnetic interference induced by the orthogonal coil (a), and a comparison diagram of the transverse and longitudinal electromagnetic interference noise induced by the orthogonal coil (b);

[0039] Figure 3 A two-dimensional planar structural diagram of the sub-coils of the noise cancellation coil assembly designed for this embodiment;

[0040] Figure 4 It is the noise reduction effect of one-dimensional magnetic resonance signal;

[0041] Figure 5 are the imaging results, where (a) is the water phantom imaging before noise reduction (SNR=2.6); (b) is the water phantom imaging after noise reduction (SNR=9.6). DETAILED DESCRIPTION

[0042] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0043] Among them, the accompanying drawings are only for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the dimensions of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the accompanying drawings.

[0044] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "back", etc. indicating directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0045] See also Figures 1 to 5 This embodiment provides a method for suppressing lateral electromagnetic interference in ultra-low-field magnetic resonance imaging using electromagnetic wave cancellation. Based on an active noise reduction hardware system, electromagnetic wave sensors (such as noise detection coils) arranged around the imager collect spatial electromagnetic interference in real time. After the phase and amplitude of the interference signal are adjusted by a noise signal processing circuit, cancellation coils placed on both sides of the magnet cavity generate anti-phase cancellation electromagnetic waves within the imaging area that have opposite phases and the same spatial distribution as the interference electromagnetic wave, forming an electromagnetic field that cancels out the original interference wave, ultimately achieving dynamic suppression of lateral electromagnetic interference within the imaging area. This invention solves the problem of suppressing lateral electromagnetic interference in magnetic resonance imaging systems without electromagnetic shielding, effectively improving the signal-to-noise ratio of magnetic resonance imaging devices without electromagnetic shielding.

[0046] The reverse design of the noise cancellation coil structure takes into account the influence of the iron yoke material. Considering the noise gradient distribution characteristics observed in the experiment, it is necessary to design a cancellation coil that can output a gradient field distribution. This is a typical electromagnetic field inverse problem. The stream function method is used to optimize the design of the noise cancellation coil to ensure that the generated magnetic field can accurately match the gradient distribution of the interference noise, thereby achieving efficient electromagnetic interference suppression. The core steps are:

[0047] (1) The current density in the wiring area is expanded exponentially using the idea of stream function. In the ultra-low field MRI system, the magnet structure adopts a dual-plane magnetic pole design, and the noise cancellation coils are placed on the left and right sides of the cavity. It is confined to a finite plane region of y = ±a, and its radius ρ is in the range of ρ0 < ρ < ρ m .

[0048] Continuous current density Distributed within the coil surface, it can be written as two components: tangential and radial and e ρ Represents the polar coordinates in the plane The unit vectors in the directions of and ρ. For a biplanar coil, the stream function is set to

[0049]

[0050] Stream function expansion basis s q (ρ) can take any value, such as the simplest trigonometric function s q (ρ)=sin[qc(ρ-ρ0)], so the two current density components can be written as:

[0051]

[0052] Where c = π / (ρ m -ρ0), q is an integer, ρ0 is the minimum radius, ρ m is the maximum radius, U q is the current Fourier expansion coefficient, Q is the number of the expansion series. From formula (2), we can see that when ρ≤ρ0 and ρ≥ρ m When in the region, J ρ = 0, ensuring that the current is confined to a limited area. It must be emphasized that when Q → ∞, the current density within the line surface can be accurately expressed and is completely determined by the current expansion coefficients of each order.

[0053] (2) Establish an optimization model that integrates mirror magnetic current and noise cancellation coil current. Based on the mirror model, the influence of ferromagnetic materials is integrated into the target magnetic field calculation in the form of current. As shown in formula (3), combined with the magnetic dipole method, the magnetic induction excitation coil wiring area is equivalent to Q small current loops, and the small current loop is equivalent to L layers of mirror current at the ferromagnetic material position (when the magnetic permeability or plate thickness is large enough, L is 1). The magnetic field of the small current loop and its mirror current loop is superimposed in the target area to form a total magnetic field. The left side of formula (3) is the total magnetic field of the target area, and the two terms on the right are the magnetic fields generated by the cancellation coil and the mirror current respectively.

[0054]

[0055] Among them, c(r,r q ), S q (r q ), c(r,r qi ), S i (r qi ) represent the position parameters and stream function values of the coil area and the mirror area, respectively. Here we only focus on the y-axis component of the magnetic field, that is, the transverse magnetic field distribution. The functional relationship between the target magnetic field and the field source is established through the above formula:

[0056]

[0057] Among them, D q The elements are functions of the field points P(x,y,z), B y is the magnetic field at point P(x,y,z), U q is the expansion coefficient of the current density series in equation (2). It can be written in matrix form:

[0058]

[0059] If at least Q target field points are known, by solving Equation (5) q , the current density can be determined, and finally the structure of the noise cancellation coil can be obtained through the stream function.

[0060] The noise signal processing circuit is a key part of the active noise reduction system, which directly determines the real-time suppression capability of external electromagnetic interference. The analog circuit is used to implement the inversion function to meet the requirements of low noise and high-speed response, and the stability and anti-interference capability of the system are improved by optimizing signal processing and power management. Figure 1As shown, the noise signal processing circuit includes a matching circuit, a low-noise preamplifier, an op amp circuit (including a non-inverting op amp circuit and an inverting op amp circuit), and a phase modulation circuit. The noise detection coil is used to sense external electromagnetic interference and achieve impedance matching through a precisely designed matching circuit, reducing signal attenuation and load effects, ensuring efficient and stable signal transmission. The low-noise preamplifier is used to amplify the sensed weak electromagnetic signal while minimizing noise gain to ensure signal quality and clarity. The op amp circuit consists of two operational amplifier stages (i.e., a non-inverting op amp circuit and an inverting op amp circuit), which are used for initial non-inverting amplification and phase inversion, respectively. The feedback network in the op amp circuit uses an adjustable resistor design to match the interference signal amplitude in different environments, ensuring the system's good adaptability in different application scenarios. The phase modulation circuit compensates the inverted signal through precise phase adjustment, ensuring that the compensated signal cancels out the original interference signal and maximizing the noise suppression effect.

[0061] In order to verify the performance of the method of the present invention, a water phantom imaging experiment was carried out using the above method. Figure 4 The figure shows the noise reduction effect of the one-dimensional magnetic resonance signal. The gray triangle arrow curve represents the one-dimensional magnetic resonance signal before noise reduction, and the black circle curve represents the one-dimensional magnetic resonance signal after noise reduction. The peak in the middle represents the magnetic resonance signal, and the surrounding area is the noise signal. It can be found that the noise interference can be effectively suppressed without changing the magnetic resonance signal.

[0062] This method was used to conduct magnetic resonance imaging experiments, and the experimental results are as follows: Figure 5 As shown in Figure 1, (a) is the imaging result before noise reduction, and (b) is the effect after noise reduction. The signal-to-noise ratio of the image is improved by about 3.7 times. In summary, the feasibility of the method proposed in this patent has been verified through experiments.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for suppressing lateral electromagnetic interference in a magnetic resonance imaging device cavity without electromagnetic shielding, characterized in that: Based on the active noise reduction hardware system, electromagnetic wave sensors arranged around the imager collect spatial electromagnetic interference signals in real time. After the phase and amplitude of the interference signal are adjusted by the noise signal processing circuit, anti-phase cancellation electromagnetic waves with the opposite phase and the same spatial distribution pattern as the interference signal are generated in the imaging area through noise cancellation coils placed on both sides of the magnet cavity, forming an electromagnetic field that cancels out the original interference signal, ultimately achieving dynamic suppression of lateral electromagnetic interference in the imaging area.

2. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging device cavity without electromagnetic shielding according to claim 1, characterized in that: The noise cancellation coil includes at least two symmetrically arranged noise cancellation coil groups, which are respectively located on the left and right sides of the imaging area inside the magnet.

3. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging device cavity without electromagnetic shielding according to claim 2, characterized in that: Each noise cancellation coil group is composed of a pair of sub-coils with the same geometric structure; the sub-coils are electrically connected in series; each sub-coil is composed of multiple concentric or non-concentric conductive coil units arranged along the axial direction of the cavity; the coil units adopt at least one of the geometric configurations of circle, rectangle or ellipse, or the winding path of the noise cancellation coil adopts an irregular curve, and the winding density is distributed in a decreasing manner from the opening of the cavity to the depth of the cavity.

4. The method for suppressing transverse electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding according to any one of claims 1 to 3, characterized in that: The noise cancellation coil is realized in any of the following forms: a flat or curved coil structure directly printed on the inner surface of the iron yoke on both sides of the cavity or the outer surface of the RF excitation coil, or a deformable coil structure encapsulated on a flexible PCB substrate, or a three-dimensional winding structure conformally arranged with the RF excitation coil.

5. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding according to claim 2 or 3, characterized in that: The noise cancellation coil group is fixed at any of the following positions: fixed on the inner wall surface of the iron yokes on both sides, or attached to the outer surface of the radio frequency excitation coil, or fixed at any position between the radio frequency excitation coil and the iron yokes on both sides.

6. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding according to claim 1, characterized in that: The stream function method is used to optimize the noise cancellation coil structure to ensure that the generated magnetic field can accurately match the gradient distribution of the interference noise.

7. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding according to claim 1, characterized in that: The noise signal processing circuit includes: a matching circuit, a low-noise preamplifier, an operational amplifier circuit and a phase modulation circuit; The matching circuit is used to achieve impedance matching with the electromagnetic wave sensor on the input side and the noise cancellation coil on the output side; The low-noise preamplifier is used to amplify the sensed weak electromagnetic signal while maintaining a minimized noise gain; The operational amplifier circuit is composed of two stages of operational amplifiers, namely a non-inverting operational amplifier circuit and an inverting operational amplifier circuit, which are used for preliminary non-inverting amplification and phase inversion respectively; The phase modulation circuit performs phase compensation on the anti-phase signal through precise phase adjustment, thereby ensuring that the compensated signal cancels out the original interference signal and maximizing the noise suppression effect.

8. The method for suppressing lateral electromagnetic interference in a magnetic resonance imaging apparatus cavity without electromagnetic shielding according to claim 7, characterized in that: The feedback network in the operational amplifier circuit is designed with adjustable resistance to match the interference signal amplitude under different environments.

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