Low frequency magnetic field interference active cancellation device for ultra-low field magnetic resonance imaging

Abstract

A low-frequency magnetic field interference active cancellation device for ultra-low field magnetic resonance imaging (MRI) is characterized by comprising: a magnetic field sensing subsystem, including: a three-axis fluxgate magnetometer for acquiring changes in the external environmental magnetic field; a miniature reverse solenoid for generating a static bias magnetic field opposite to the direction of the leakage magnetic field of the main magnetic field of the ultra-low field MRI device, thereby canceling the net magnetic field at the probe to near zero and enabling the three-axis fluxgate magnetometer to operate in the linear region; a signal processing and control subsystem electrically connected to the magnetic field sensing subsystem, including a digital signal processor (DSP) that executes a PID control algorithm to calculate a compensation control quantity based on the negative feedback error signal; and a magnetic field execution subsystem electrically connected to the signal processing and control subsystem, including a ceiling coil and a floor coil, for generating a compensation magnetic field uniformly distributed along the direction of the main magnetic field within the imaging area of ​​the ultra-low field MRI device.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic environment compatibility and shielding technology for magnetic resonance imaging (MRI) equipment, specifically to an active magnetic field shielding system for suppressing quasi-DC and low-frequency magnetic field interference generated by urban rail transit systems such as subways and trams.

[0002] In particular, this invention proposes a real-time active magnetic field cancellation scheme based on a large Helmholtz coil architecture on the ceiling and floor for ultra-low field (magnetic field strength less than 0.1 T) permanent magnet MRI devices with a magnetic field direction perpendicular to the vertical (up and down) direction. Background Technology

[0003] Magnetic resonance imaging (MRI) relies on the nuclear magnetic resonance phenomenon of atomic nuclei (typically hydrogen protons) in a strong static magnetic field (B0). The proton precession frequency (larmor frequency) ω0 is strictly proportional to the strength of the main magnetic field B0, i.e., ω0 = γB0 (where γ is the gyromagnetic ratio). Any uncontrolled fluctuation in the external environmental magnetic field ΔB will be directly superimposed on the main magnetic field B0, causing an unexpected shift in the proton precession frequency. For vertical field MRI systems, the direction of the main magnetic field (Z-axis) is usually perpendicular to the ground, making these devices particularly sensitive to interference from vertical magnetic fields in the environment.

[0004] Modern subways and trams typically use direct current (DC) traction power. Current flows to the locomotive via the overhead contact line or third rail and returns via the running rail. Due to the spatial distance between the power supply arm and the return rail, and the potential for return current leakage through the ground, a large current loop can be formed. This interference is known as "quasi-DC" interference, with its main energy concentrated in the extremely low frequency band of 0 Hz to 10 Hz, manifesting as slow, step-like or ramp-like fluctuations that change with the train's position. For MRI equipment located near subway lines, this interfering magnetic field can penetrate building floors and ceilings, creating a significant vertical component that severely impacts image quality.

[0005] Traditional passive shielding shunts magnetic flux by laying high-permeability materials (such as silicon steel sheets), but for low-frequency strong magnetic field interference, extremely thick shielding layers are required, resulting in high costs and difficult construction. While some existing active shielding systems (such as the ETS-Lindgren MACS system) can cancel interference with a reverse magnetic field, they face the following key problems when applied to ultra-low field vertical field permanent magnets: sensor saturation, where the background static magnetic field far exceeds the range of conventional high-sensitivity fluxgate sensors (typically ±100 μT), causing the sensor to malfunction in the high-sensitivity pseudo-region, limiting signal acquisition or even causing saturation failure; and coil mismatch, where general active shielding coil designs are not optimized for the specific geometry of open vertical field MRI, making it difficult to generate a uniform compensating magnetic field within the imaging field of view (DSV), resulting in limited compensation effects. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides an active cancellation device for low-frequency magnetic field interference in ultra-low field magnetic resonance imaging. The device comprises: a magnetic field sensing subsystem, including: a three-axis fluxgate magnetometer for acquiring changes in the external environmental magnetic field, with a noise floor not exceeding 10 ohms. The sampling bandwidth covers DC to 1kHz; a miniature anti-reverse solenoid, integrated at the probe of the triaxial fluxgate magnetometer, driven by a constant current source, is used to generate a static bias magnetic field opposite to the direction of the leakage magnetic field of the main magnetic field of the ultra-low field magnetic resonance imaging device, canceling the net magnetic field at the probe to near zero, so that the triaxial fluxgate magnetometer operates in the linear region; a signal processing and control subsystem, electrically connected to the magnetic field sensing subsystem, includes a digital signal processor, which executes a PID control algorithm to calculate the compensation control quantity based on the negative feedback error signal; and a magnetic field execution subsystem, electrically connected to the signal processing and control subsystem, includes a ceiling coil and a floor coil, the ceiling coil being installed at the top of the ultra-low field magnetic resonance imaging device room, and the floor coil being installed at the bottom of the room, the ceiling coil and the floor coil forming a Helmholtz structure, used to generate a compensation magnetic field uniformly distributed along the direction of the main magnetic field within the imaging area of ​​the ultra-low field magnetic resonance imaging device.

[0007] In one embodiment, the compensating magnetic field is uniformly distributed within the approximately 280mm uniform spherical field of view of a 0.08T ultra-low field magnetic resonance imaging device.

[0008] In one embodiment, the ceiling coil and the floor coil are wound with multi-strand Litz wire.

[0009] In one embodiment, the signal processing and control subsystem further includes a negative feedback loop, which comprises a low-pass filter and a notch filter; the low-pass filter has a passband range of DC to 500Hz and is used to filter out high-frequency noise; the notch filter is used to filter out power frequency interference.

[0010] In one embodiment, the signal processing and control subsystem performs a transfer function calibration process before operation, including: outputting a test signal to the magnetic field execution subsystem, calculating the transfer function between the coil in the magnetic field execution subsystem and the sensor in the magnetic field sensing subsystem, and automatically adjusting the parameters of the PID control algorithm according to the transfer function.

[0011] In one embodiment, the magnetic field actuation subsystem further includes a power amplifier, which is a bipolar four-quadrant power amplifier operating in constant current source mode.

[0012] In one embodiment, the constant current source mode of the power amplifier is used to eliminate phase hysteresis caused by coil inductance.

[0013] In one embodiment, the miniature reverse solenoid performs a static zeroing operation during the system initialization phase to cancel the net magnetic field at the probe to near zero.

[0014] In one embodiment, the direction of the compensation magnetic field is opposite to that of the external interference magnetic field, and its axis coincides with the direction of the main magnetic field of the ultra-low field magnetic resonance imaging device.

[0015] This invention also provides an active cancellation method for low-frequency magnetic field interference in ultra-low field magnetic resonance imaging, applied to the active cancellation device. The method comprises: static zeroing: driving a miniature reverse solenoid integrated into the probe of a fluxgate magnetometer to generate a static bias magnetic field opposite in direction to the leakage magnetic field of the main magnetic field of the ultra-low field magnetic resonance imaging device, thus canceling the net magnetic field at the probe to near zero, completing static zeroing; transfer function calibration: outputting a test signal to the magnetic field execution subsystem, calculating the transfer function between the coil in the magnetic field execution subsystem and the sensor in the magnetic field sensing subsystem, and adjusting the parameters of the PID control algorithm according to the transfer function; real-time cancellation: detecting the change in the external environmental magnetic field ΔB_ext in real time through the magnetic field sensing subsystem, and the signal processing and control subsystem calculating the required compensation current I_comp according to the change in the external environmental magnetic field using the PID control algorithm, driving the coil in the magnetic field execution subsystem with a power amplifier to generate a canceling magnetic field -ΔB_comp, making the change in the net magnetic field around the ultra-low field magnetic resonance imaging device approach zero.

[0016] In one embodiment, during the real-time cancellation step, the signal processing and control subsystem filters the acquired signal through a negative feedback loop. The negative feedback loop includes a low-pass filter for filtering out high-frequency noise and a notch filter for filtering out power frequency interference.

[0017] In one embodiment, the compensating magnetic field is generated by a pair of Helmholtz coils installed on the ceiling and floor of the computer room. The direction of the compensating magnetic field generated by the coils is opposite to the direction of the external interference magnetic field, and the axis coincides with the direction of the main magnetic field of the ultra-low field magnetic resonance imaging device.

[0018] In one embodiment, the power amplifier operates in constant current source mode to eliminate phase hysteresis caused by coil inductance.

[0019] In one embodiment, during the real-time cancellation step, the triaxial fluxgate magnetometer acquires the change signal of the external environmental magnetic field in real time, with a sampling bandwidth covering DC to 1kHz.

[0020] In one embodiment, during the real-time cancellation step, the dynamic response time of the compensation magnetic field meets the sequence timing requirements of ultra-low field magnetic resonance imaging.

[0021] In one embodiment, during the real-time cancellation step, when the frequency of the external interference magnetic field is in the range of 0Hz to 10Hz, the signal processing and control subsystem responds to the interference signal through a low-pass filter.

[0022] In one embodiment, during the real-time cancellation step, when the frequency of the external interference magnetic field is at the power frequency of 50Hz or 60Hz, the signal processing and control subsystem suppresses the interference signal through a notch filter.

[0023] In one embodiment, within a 280mm uniform spherical field of view, the attenuation factor of the compensating magnetic field against the external interfering magnetic field is not less than 30dB.

[0024] This invention addresses ultra-low field (magnetic field strength less than 0.1 T) permanent magnet MRI equipment with a vertical (up and down) magnetic field direction. It proposes a real-time active magnetic field cancellation scheme based on a large Helmholtz coil architecture on the ceiling and floor, which generates an opposing magnetic field to counteract external magnetic field interference. This scheme offers the following advantages: The sensor saturation problem was solved: by integrating a miniature reverse solenoid (Bucking Coil) at the probe and applying a static bias magnetic field, the sensor's operating point was "zeroed" to its linear range, breaking through the sensor saturation bottleneck under a strong static magnetic field environment of 0.08T and realizing reliable acquisition of high-sensitivity magnetic field signals.

[0025] Wide-range uniform compensation: The ceiling-floor Helmholtz pair generates a compensation magnetic field uniformly distributed along the Z-axis within the imaging area. Simulation results show that within a 280mm uniform spherical field of view, the interference magnetic field attenuation factor can reach more than 30dB, which is significantly better than the general solution.

[0026] Effective suppression across the entire frequency band: The low-pass filter (DC-500Hz) combined with the dedicated feedback control strategy of the power frequency notch filter can accurately identify and suppress quasi-DC (0-10Hz) and low-frequency interference generated by the subway, while avoiding false responses to signals in other frequency bands, and effectively suppressing magnetic field interference in the DC to 1kHz frequency band.

[0027] Good spatial compatibility: The ceiling / floor coil design does not occupy the floor space of the machine room, maintaining the convenience of doctors' operation and patient comfort in open MRI, and is suitable for mobile or open ultra-low field MRI systems.

[0028] Large dynamic range and fast response speed: The active cancellation system, combined with the constant current source working mode of the bipolar four-quadrant power amplifier, can handle large magnetic field jumps, eliminate phase lag caused by coil inductance, and ensure fast response to rapidly changing magnetic fields. Attached Figure Description

[0029] Other features, objects, and advantages of this disclosure will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a system architecture block diagram of the active cancellation device of the present invention; Figure 2 This is a schematic diagram of the Helmholtz coil layout; Figure 3 For control flow and timing diagrams; Figure 4 Imaging effect without the technical solution of this invention under external magnetic field interference environment; Figure 5 Imaging effect after applying the technical solution of this invention under external magnetic field interference environment. Detailed Implementation

[0030] The present disclosure will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present disclosure, but do not limit the present disclosure in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present disclosure. These all fall within the protection scope of the present disclosure.

[0031] Example 1: Overall Structure of the Active Cancellation Device Reference Figure 1The overall architecture of the active cancellation device in this embodiment is as follows.

[0032] The system comprises three core subsystems: a magnetic field sensing subsystem, a signal processing and control subsystem, and a magnetic field execution subsystem.

[0033] The magnetic field sensing subsystem uses a triaxial fluxgate magnetometer as its core sensor. This sensor features low noise characteristics, with a noise floor not exceeding 10 pT / √Hz and a bandwidth covering DC to 1kHz, enabling it to detect weak quasi-DC magnetic field interference signals. To address the sensor saturation problem in a strong static magnetic field environment of 0.08T, this invention innovatively integrates a miniature reverse solenoid (Bucking Coil) at the probe. This Bucking Coil is driven by a constant current source, generating a static bias magnetic field of equal magnitude and opposite direction to the leakage magnetic field of the MRI main magnetic field, thus canceling the net magnetic field at the probe to near zero, allowing it to operate in the linear region. The probe is mounted on a non-magnetic support approximately 1.5 meters from the center of the magnet, positioned between the magnet and the interference source.

[0034] The signal processing and control subsystem is built on a high-performance DSP chip. This subsystem receives the magnetic field signal output from the magnetic field sensing subsystem and calculates the compensation control quantity through the following processing flow: First, the signal passes through a low-pass filter (passband range DC-500Hz) to filter out high-frequency noise; then, it passes through a power frequency notch filter (center frequency 50Hz or 60Hz) to filter out power grid interference and its harmonics; next, the DSP executes a PID control algorithm to calculate the required compensation current value based on the negative feedback error signal; finally, the digital control quantity is converted into an analog drive signal and output to the power amplifier.

[0035] The magnetic field actuation subsystem consists of a pair of large square coils mounted on the ceiling and floor of the MRI room. The two coils form a Helmholtz structure to generate a compensating magnetic field that is uniformly distributed along the Z-axis (direction of the main magnetic field) within the imaging area.

[0036] Reference Figure 2 The specific design parameters of the coil are as follows: the coil is square in shape, with a side length of 3 to 5 meters; the spacing between the two coils is designed to be about 3 meters (close to the height of a typical machine room), which meets the Helmholtz condition; the coil is wound with multi-strand Litz wire to reduce the skin effect and reduce high-frequency impedance; the direction of the compensation magnetic field generated by the coil is opposite to the direction of the external interference magnetic field, and the axis coincides with the MRI main magnetic field B0.

[0037] The power amplifier is a bipolar four-quadrant power amplifier operating in constant current source mode. This mode eliminates phase hysteresis caused by coil inductance, ensuring a fast response to rapidly changing magnetic fields. The power amplifier receives analog drive signals from the DSP and outputs a compensation current to the Helmholtz coil pair.

[0038] Example 2: Bucking Coil Desaturation Principle The following details the working principle of Bucking Coil desaturation.

[0039] In a 0.08T vertical field MRI environment, the range of a conventional high-sensitivity fluxgate sensor is typically ±100 μT, which is much smaller than the background magnetic field of 0.08T (80,000 μT). Even if the probe is installed in a region with a weak main magnetic field (such as outside the 5 Gauss line), the background field strength may still reach hundreds of μT, far exceeding the sensor's range, causing the sensor output to saturate and preventing it from working properly.

[0040] The solution proposed in this invention is to integrate a miniature reverse solenoid (Bucking Coil) at the probe. This Bucking Coil has the following characteristics: Structural features: The Bucking Coil is a miniature solenoid wound around the probe housing, with the number of turns designed according to the required bias magnetic field strength; Drive method: Driven by a constant current source, which provides a stable drive current to ensure that the bias magnetic field strength generated by the Bucking Coil is constant; Magnetic field direction: The bias magnetic field generated by the Bucking Coil is opposite to the leakage magnetic field direction of the MRI main magnetic field; Working principle: When the constant current source drives the Bucking Coil to work, the Bucking Coil generates a reverse bias magnetic field B_buck. This bias magnetic field is superimposed on the leakage magnetic field B_leak of the main magnetic field, so that the net magnetic field at the probe B_net = B_leak - B_buck ≈ 0, thereby "zeroing" the sensor's operating point to its linear range.

[0041] During system initialization, a static zeroing operation is performed: first, the actual background magnetic field strength at the probe is measured; then, a constant current source is driven to generate a reverse magnetic field of corresponding magnitude using the Bucking Coil, precisely canceling the background static magnetic field. After static zeroing is completed, the sensor can operate normally and is used to acquire the change signal ΔB_ext of the external environmental magnetic field.

[0042] Example 3: Helmholtz Coil Design and Layout Reference Figure 2The design and spatial layout of the Helmholtz coil are explained in detail below.

[0043] Helmholtz condition: For a pair of parallel circular or square coils, when the distance between the two coils is equal to half the radius of curvature or side length of the coils, an approximately uniform magnetic field can be generated in the region near the midpoint of the line connecting the centers of the two coils. This invention employs a ceiling-floor layout, with the distance between the two coils designed to be approximately 3 meters (close to the height of a typical computer room), satisfying the Helmholtz condition with the side length of the coils (half of 3-5 meters, i.e., 1.5-2.5 meters).

[0044] Coil size: The coil is designed to be square with a side length of 3-5 meters. The reasons for choosing this size range are as follows: (1) The coverage area needs to be larger than the typical imaging field of ultra-low field MRI; (2) A coil that is too large will increase power consumption and cost, while a coil that is too small will make it difficult to guarantee the uniform area range; (3) Combined with the typical machine room height of 3 meters, this size range can generate a uniform compensation magnetic field in the central area.

[0045] Uniform coverage: Simulation and field measurements show that the Helmholtz coil pair provides uniform magnetic field distribution within a uniform spherical field of view (DSV) of approximately 280 mm on a 0.08 T magnet, meeting clinical imaging requirements. Within the 280 mm FOV, the attenuation factor of the interfering magnetic field can reach over 30 dB.

[0046] Winding process: The coil is wound with multi-strand Litz wire. Litz wire is made of multiple strands of fine insulated copper wire, which can effectively reduce the increase in AC resistance caused by the skin effect, reduce high-frequency impedance, and improve the coil's performance over a wide bandwidth.

[0047] Spatial arrangement: The ceiling coil is installed at the top of the MRI room (below the ceiling), and the floor coil is installed at the bottom of the room (above the ground or embedded in the floor), with both coils parallel to each other and parallel to the ground. This vertical layout is particularly suitable for compensating for quasi-DC interference magnetic fields from subways / tramways that have a vertical component.

[0048] Example 4: Signal Processing and Control Flow Reference Figure 3 The following details the system's workflow and control strategies.

[0049] The control system of the present invention includes three working stages: system initialization, transfer function calibration, and real-time cancellation.

[0050] Phase 1: System Initialization. Upon system startup, a static zero-adjustment operation is first performed: the BuckingCoil is driven by a constant current source to generate a static bias magnetic field, which cancels out the background static magnetic field at the probe, ensuring the three-axis fluxgate magnetometer operates in the linear region. After static zero-adjustment is complete, the sensor can normally acquire changes in the external environmental magnetic field.

[0051] Second stage: Transfer function calibration. To ensure the accuracy of the control system, transfer function calibration is performed before formal operation: (1) The DSP controller outputs a preset test signal (usually a sine wave or step signal) to the magnetic field execution subsystem; (2) The test signal is amplified by the power amplifier and drives the Helmholtz coil to generate a test magnetic field; (3) The sensor of the magnetic field sensing subsystem synchronously collects the response signal of the test magnetic field; (4) The DSP calculates the transfer function of the coil-sensor (i.e., the gain and phase relationship between the compensated magnetic field and the driving current); (5) Based on the transfer function calculation results, the proportional, integral, and derivative parameters of the PID control algorithm are automatically adjusted to make the closed-loop control system achieve optimal performance.

[0052] Phase 3: Real-time cancellation. After calibration, the system enters the real-time cancellation mode: (1) The three-axis fluxgate magnetometer collects the change signal ΔB_ext of the external environment magnetic field in real time; (2) The signal is filtered by a low-pass filter (DC-500Hz) to remove high-frequency noise; (3) The signal is filtered by a power frequency notch filter (50Hz / 60Hz) to remove power grid interference; (4) The DSP executes the PID control algorithm and calculates the required compensation current I_comp based on the negative feedback error signal e(t): I_comp = K_p·e(t) + K_i·∫e(t)dt + K_d·de(t) / dt, where e(t) = ΔB_ext (the target value is set to zero); (5) The compensation current I_comp is output to the power amplifier after digital-to-analog conversion (DAC); (6) The power amplifier drives the Helmholtz coil in constant current source mode to generate the cancellation magnetic field -ΔB_comp; (7) The cancellation magnetic field is opposite to the direction of the external interference magnetic field and is superimposed in the imaging area to make the net magnetic field change approach zero.

[0053] Filter Design: The low-pass filter has a cutoff frequency of 500Hz to cover the quasi-DC component (0-10Hz) of subway interference and possible low-frequency harmonic components, while filtering out instrument noise and high-frequency environmental interference. The power frequency notch filter deeply suppresses the power grid frequency and its harmonics at 50Hz (domestic) or 60Hz (some overseas regions), preventing power frequency interference from entering the control loop.

[0054] Constant current source mode: The power amplifier operates in constant current source mode, and the output current strictly tracks the calculation instructions of the DSP, unaffected by changes in coil inductance. This mode can eliminate phase hysteresis caused by inductive loads in the coil, improving the system's response speed to rapidly changing magnetic fields.

[0055] Example 5: System Performance and Application Scenarios The active cancellation system of this invention can effectively solve the problem of low-frequency magnetic field interference faced by ultra-low field MRI. Its performance indicators are as follows: Frequency coverage: Effectively suppresses magnetic field interference in the DC to 1kHz frequency band, covering major interference sources such as DC impact during subway start-up and power frequency harmonics; Compensation effectiveness: Within a 280mm FOV, the interference magnetic field attenuation factor can reach over 30dB, significantly improving image quality; Dynamic range: The active cancellation system, combined with Bucking Coil desaturation technology, can handle large magnetic field jumps (such as mT-level transients generated when entering / exiting a subway station), and has a large dynamic range; Space compatibility: The ceiling / floor Helmholtz coil layout does not occupy floor space in the machine room, maintaining the ease of operation for doctors and the comfort for patients in open MRI.

[0056] This invention is particularly suitable for the following application scenarios: Deployment of MRI equipment in urban hospitals: Hospitals located near subway lines can achieve stable operation of 0.08T ultra-low field MRI equipment without the need for expensive shielding projects; Mobile / vehicle-mounted MRI system: 0.08T ultra-low field MRI equipment used in mobile stroke units or field hospitals to maintain image quality in complex electromagnetic environments; Emergency room / ICU bedside MRI: Achieving rapid, low-cost bedside magnetic resonance imaging without the need for a heavily shielded magnetic room.

[0057] Explanation of the technical effects of this invention: The technical problem this invention aims to solve is how to reduce the impact of external magnetic field interference on image quality during ultra-low field magnetic resonance imaging (ULMRI) scanning under external magnetic field interference. Parts of the Shanghai Sixth People's Hospital Xuhui Branch are located near the tunnel of Shanghai Metro Line 9. In an unshielded environment, the imaging quality of ULMRI is severely affected by electromagnetic interference from the metro. To verify the technical effectiveness of this invention, the low-frequency magnetic field interference active cancellation device provided by this invention was installed and tested in an ordinary room (unshielded) near the metro tunnel.

[0058] As shown in Figure 4, with the active magnetic field cancellation system of this invention turned off, the system acquired an image of a square water phantom under strong environmental interference. The baseline scan was configured with a head coil and a T1-weighted three-dimensional fast field echo sequence (T1-FFE3D), and the scanning field of view reached 270 mm × 270 mm. For low-field permanent magnet MRI equipment such as 0.08T with the main magnetic field direction perpendicular to the direction of the magnetic field, the vertical component formed by the "quasi-DC" interference generated by subways, etc., will be directly superimposed on the main magnetic field. This intrusion of external spatial magnetic fields seriously disrupts the temporal stability of the static magnetic field during sequence execution. The continuous fluctuation of the main magnetic field magnitude will cause significant Larmor frequency drift in the system, which in turn will lead to severe intravoxel spin astigmatism and spatial readout positioning errors, which are visually manifested as large-area signal attenuation and loss inside the water phantom, boundary geometric distortion, and a significant decrease in the overall signal-to-noise ratio.

[0059] As shown in Figure 5, after activating the active cancellation system provided by this invention, a highly uniform compensating magnetic field opposite to the direction of the interference vector is generated within a uniform spherical field of view of approximately 280 mm. This physical-level active cancellation effect cancels out external low-frequency magnetic field interference in real time, restoring the time stability of the main magnetic field. Comparing with Figure 4, it can be seen that after suppressing the fluctuations of the main magnetic field at the physical level, the signal intensity inside the water model is completely restored to a uniform distribution. Geometric distortion and uneven brightness artifacts caused by frequency drift are completely eliminated, and the signal-to-noise ratio is greatly improved.

[0060] The specific embodiments of this disclosure have been described above. It should be understood that this disclosure is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the substantive content of this disclosure. The above-described preferred features can be used in any combination without conflict.

Claims

1. A device for actively canceling low-frequency magnetic field interference in ultra-low field magnetic resonance imaging, characterized in that, include: The magnetic field sensing subsystem includes: a three-axis fluxgate magnetometer, used to acquire the change signal of the external environmental magnetic field, with a noise floor not exceeding 10 pT / √Hz and a sampling bandwidth covering DC to 1kHz; and a miniature anti-solenoid, integrated at the probe of the three-axis fluxgate magnetometer, driven by a constant current source, used to generate a static bias magnetic field opposite to the direction of the main magnetic field leakage magnetic field of the ultra-low field magnetic resonance imaging equipment, canceling the net magnetic field at the probe to near zero, so that the three-axis fluxgate magnetometer operates in the linear region. A signal processing and control subsystem, electrically connected to the magnetic field sensing subsystem, includes a digital signal processor that executes a PID control algorithm and calculates a compensation control quantity based on a negative feedback error signal; and The magnetic field execution subsystem, electrically connected to the signal processing and control subsystem, includes a ceiling coil and a floor coil. The ceiling coil is installed at the top of the ultra-low field magnetic resonance imaging equipment room, and the floor coil is installed at the bottom of the room. The ceiling coil and the floor coil form a Helmholtz structure for generating a compensation magnetic field uniformly distributed along the main magnetic field direction within the imaging area of ​​the ultra-low field magnetic resonance imaging equipment.

2. The active cancellation device according to claim 1, characterized in that, Within the approximately 280mm uniform spherical field of view of the 0.08T ultra-low field magnetic resonance imaging device, the compensating magnetic field is uniformly distributed.

3. The active cancellation device according to claim 1, characterized in that, The ceiling coil and the floor coil are wound with multi-strand Litz wire.

4. The active cancellation device according to claim 1, characterized in that, The signal processing and control subsystem further includes a negative feedback loop, which comprises a low-pass filter and a notch filter. The passband of the low-pass filter is from DC to 500Hz and is used to filter out high-frequency noise. The notch filter is used to filter out power frequency interference.

5. The active cancellation device according to claim 1, characterized in that, Before operation, the signal processing and control subsystem performs a transfer function calibration process, including: outputting a test signal to the magnetic field execution subsystem, calculating the transfer function between the coil in the magnetic field execution subsystem and the sensor in the magnetic field sensing subsystem, and automatically adjusting the parameters of the PID control algorithm based on the transfer function.

6. The active cancellation device according to claim 1, characterized in that, The magnetic field execution subsystem also includes a power amplifier, which is a bipolar four-quadrant power amplifier operating in constant current source mode.

7. The active cancellation device according to claim 6, characterized in that, The constant current source mode of the power amplifier is used to eliminate phase hysteresis caused by coil inductance.

8. The active cancellation device according to claim 1, characterized in that, The miniature reverse solenoid performs a static zeroing operation during the system initialization phase, canceling the net magnetic field at the probe to near zero.

9. The active cancellation device according to claim 1, characterized in that, The direction of the compensation magnetic field is opposite to that of the external interference magnetic field, and its axis coincides with the direction of the main magnetic field of the ultra-low field magnetic resonance imaging device.

10. A method for actively canceling low-frequency magnetic field interference in ultra-low field magnetic resonance imaging, applied to the active cancellation device according to any one of claims 1 to 9, characterized in that, include: Static zeroing: Drive the miniature reverse solenoid integrated in the fluxgate magnetometer probe to generate a static bias magnetic field opposite to the direction of the main magnetic field leakage magnetic field of the ultra-low field magnetic resonance imaging equipment, cancel the net magnetic field at the probe to near zero, and complete the static zeroing. Transfer function calibration: Output a test signal to the magnetic field actuation subsystem, calculate the transfer function between the coil in the magnetic field actuation subsystem and the sensor in the magnetic field sensing subsystem, and adjust the parameters of the PID control algorithm according to the transfer function; Real-time cancellation: The magnetic field sensing subsystem detects the change in the external environmental magnetic field ΔB_ext in real time. The signal processing and control subsystem calculates the required compensation current I_comp based on the change in the external environmental magnetic field using a PID control algorithm. The power amplifier drives the coil in the magnetic field execution subsystem to generate a cancellation magnetic field -ΔB_comp, so that the net magnetic field change around the ultra-low field magnetic resonance imaging device approaches zero.

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