Rapid eddy current field measurement and correction method and device

Abstract

The invention discloses a rapid eddy current field measurement and correction method, which comprises the following steps: preliminarily designing the size of a probe according to a probe signal-to-noise ratio formula, calculating a corresponding signal-to-noise ratio, and guiding the parameter design of a subsequent eddy current measurement sequence; the relaxation information corresponding to the T1 and T2 quantitative image test solution is used to ensure that the interval of each sampling window in the subsequent eddy current measurement sequence is greater than T1 and the length of each sampling window is less than T2; designing a positioning sequence, and positioning the probe; measuring the eddy current by using the eddy current field measurement sequence; and fitting the data through the composite model, reconstructing an eddy current field, and sending the eddy current field to a spectrometer system for compensation. The invention also discloses a rapid eddy current field measurement and correction device. The eddy current measurement efficiency and the eddy current measurement accuracy are greatly improved; and a stable and effective method is provided for rapid eddy current correction after portable magnetic resonance reaches different scenes.

Description

Technical Field

[0001] This invention relates to the field of low-field magnetic resonance imaging technology, and in particular to a method and apparatus for rapid eddy current field measurement and correction. Background Technology

[0002] Magnetic Resonance Imaging (MRI) is a non-contact, non-invasive, and radiation-free medical imaging technique that provides high-resolution soft tissue images. Through various sequence designs and parameter selections, it can provide multi-contrast structures, offering more accurate and comprehensive evidence for clinical disease diagnosis. With the development of MRI technology, portable low-field MRI devices and related technologies based on permanent magnets, which are smaller and less expensive, have gradually attracted more attention from researchers. Their low cost and ease of use have driven the popularization and development of MRI equipment in primary healthcare institutions and more flexible and diverse application scenarios. However, portable low-field MRI systems based on permanent magnets are more susceptible to eddy current artifacts, mainly because permanent magnet MRI systems contain a large amount of conductive material close to the imaging area. Furthermore, portable MRI applications are diverse and may require rapid calibration and scanning within a single scene. Therefore, exploring rapid and effective eddy current measurement and correction methods is of great significance for improving image quality and diagnostic accuracy.

[0003] Currently, eddy current measurement methods can be mainly divided into three categories: artifact mitigation imaging sequence design, image phase measurement-based methods, and methods based on external sensors.

[0004] Artifact mitigation sequences reduce accumulated extra eddy current phase or compensate by measuring the corresponding eddy current phase through optimized gradient design or the addition of phase navigation modules. However, this often comes at the cost of reduced temporal resolution and may introduce new types of artifacts. Image-based phase measurement is relatively easy to implement, often requiring only a uniform spherical water phantom to begin eddy current measurements. However, this method has high requirements for signal-to-noise ratio and field uniformity. In portable permanent magnet low-field MRI applications, it suffers from limitations such as long measurement times and reduced result validity. Methods based on external sensors often require additional spatial registration and high magnetic compatibility, resulting in long installation times and cumbersome operation.

[0005] In 2008, De Zanche et al. proposed a field probe technique based on Nuclear Magnetic Resonance (NMR). By using a miniature field probe array integrating a microcoil and a water phantom, combined with eddy current measurement sequences, rapid measurement of eddy current field distributions can be achieved. Compared to traditional eddy current measurement methods, this method offers advantages in time synchronization and rapid positioning, resulting in high eddy current measurement efficiency. Furthermore, the related technology can be extended to higher-order eddy current field measurement and real-time phase monitoring, demonstrating high application scalability. The application of this method in low-field conditions will help improve the efficiency of eddy current field measurement and provide a novel approach to addressing the slow low-field correction.

[0006] The existing technology has the following main problems: 1. Existing portable low-field MRI systems based on permanent magnets are susceptible to interference from large eddy current fields, resulting in artifacts that severely degrade image quality. Furthermore, advanced functional MRI sequences, such as diffusion-weighted imaging (DWI), cannot be implemented, reducing the system's clinical value. The most commonly used low-field eddy current measurement method based on image phase is often time-consuming, taking several hours, due to limitations in signal-to-noise ratio and the need for coded gradient-assisted localization. Since portable MRI systems lack temperature control systems, prolonged scanning can cause changes in magnet temperature, affecting the main magnetic field frequency and the eddy current field within the system. In addition, the extra eddy currents introduced by the coded gradients in the eddy current scanning sequence further reduce the accuracy of eddy current measurements, diminishing the effectiveness of eddy current correction.

[0007] 2. Current field probes are designed for low-temperature superconducting high-field magnetic resonance imaging systems and cannot work in low-field MRI.

[0008] 3. Current field probe measurement methods are not suitable for the non-uniformity and strong remanent magnetization of the main magnetic field (B0 field) in portable low-field MRI systems.

[0009] Therefore, those skilled in the art are dedicated to developing a rapid eddy current field measurement and correction method and apparatus to overcome the problems existing in the prior art. Summary of the Invention

[0010] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is the problem of long calibration time and great difficulty in calibration encountered in practical applications of low-field portable MRI systems.

[0011] To achieve the above objectives, the present invention provides a rapid eddy current field measurement and correction method, comprising the following steps: Step 1: Based on the probe signal-to-noise ratio formula, preliminarily design the probe size and calculate the corresponding signal-to-noise ratio to guide the parameter design of the subsequent eddy current measurement sequence; by using the relaxation information corresponding to the quantitative images of T1 and T2 to test the solution, ensure that the interval between each sampling window in the subsequent eddy current measurement sequence is greater than T1 and the sampling window length is less than T2. Step 2: Design a positioning sequence to locate the probe; Step 3: Measure the eddy current using the eddy current field measurement sequence; Step 4: Fit the data using a composite model to reconstruct the eddy current field, and then send it to the spectrometer system for compensation.

[0012] Furthermore, the probe diameter in step 1 covers 1 mm to 10 mm in portable low-field applications.

[0013] Furthermore, the probe in step 1 is spherical or cylindrical in shape, filled with test solution, and free of air bubbles.

[0014] Furthermore, step 2 specifically includes the following steps: After the X, Y, and Z gradients are successively raised to known non-zero gradient values ​​and maintained for 2 to 4 seconds, the probe signal is collected at the time point after the gradient stabilizes, and the position of the field probe is calculated using the formula (1) of phase and gradient. Among them, an additional gradient-free signal acquisition module is added before each gradient. The phase calculated by formula (1) is subtracted from the phase of the gradient-free signal acquisition module to obtain the accurate probe position information after removing the background phase containing non-uniform field information. (1) Where Φ represents the probe phase, γ Indicates the gyrometry ratio. G Represents non-zero gradient values. R Indicates the probe position. t This indicates the time during which the probe reception is activated and the gradient is applied. This represents the background field containing information about non-uniform fields.

[0015] Furthermore, in step 3, the eddy current measurement sequence consists of a large gradient that induces eddies followed by a series of probe signal acquisition modules composed of excitation pulses and a spectrometer receiving module. The time from the probe signal acquisition module to the large gradient that introduces the eddies changes sequentially.

[0016] Furthermore, the time from the probe signal acquisition module to the large gradient that causes the eddy current is varied by a linear time interval or by an exponential time interval, covering any time from 0.5 ms to 10 s.

[0017] Furthermore, the large gradient that induces the eddy current includes three combinations: a combination of one positive gradient and one zero gradient, a combination of two gradients with the same positive polarity but different amplitudes, and a combination of two gradients with the same negative polarity but different amplitudes.

[0018] Furthermore, step 4 specifically includes the following steps: Step 4.1: Using formula (2), calculate the corresponding parameters based on the basis of the spherical harmonic space function to obtain the probe's position information; (2) in, R These are the coordinates of each probe corresponding to the spherical harmonic space basis. R 'Represents the total position matrix of the probe. x , y , z , m These represent the coordinates of the probe on the x, y, and z axes, and the number of probes, respectively. Step 4.2: Use formulas (3) to (5) to obtain the relationship between the phase information Φ of each probe and the eddy current field. The phase Φ of each probe includes position and time information. Specifically, the field that generates the probe signal is decomposed into a zero-order eddy current field. B e With first-order and higher-order vortex fields R · K , K To characterize the first-order and higher-order spatial distribution coefficients of the eddy field, the phase information is differentiated. An exponential model is used to fit the short-time eddy part, and a linear model is used to fit the long-time eddy part (greater than 1 second) to improve computational efficiency and obtain the time variation model of the spatial distribution coefficients of the eddy field. (3) (4) (5) in, τ Indicates time, Φ + To incorporate phase information generated by the first gradient in the large gradient of the eddy current, Φ ref To incorporate phase information generated by the second gradient in the large gradient of the eddy current, Φ R This provides the phase information for the probe's placement position. Step 4.3: Calculate the eddy current field using formula (6) combined with the probe position R; (6) in, G e Represents a vortex field; Step 4.4: Using formula (7), fit the eddy current field into the form of an exponential sum, and input the obtained amplitude and time constant into the spectrometer; (7) y Represents the vortex field. τ i Represents the eddy current time constant. A i This represents the amplitude corresponding to each eddy current time constant.

[0019] This invention also provides a rapid eddy current field measurement and correction device, including a radio frequency power amplifier, a power divider, a control module or spectrometer containing an FPGA, a transceiver switch, a low-noise amplifier, and a field probe array composed of multiple probes. The radio frequency power amplifier is configured to amplify the transmitted pulse to the required power value, and the output pulse signal is connected to the power divider, which distributes the excitation pulse power equally to the transceiver switches corresponding to each probe. In addition to receiving the excitation pulse, the transceiver switch also receives a gating signal from the control module or spectrometer containing the FPGA, thereby switching the transmission and reception modes of the probe. The received signal of the probe is transmitted to the control module or spectrometer containing the FPGA via the transceiver switch and the low-noise amplifier, and converted into a digital signal. The size of the probe is optimized under the guidance of the signal-to-noise ratio and field measurement resolution formula (8), combined with the size of the measured eddy field; (8) in, SNR Indicates the signal-to-noise ratio. V sample Indicates the volume of the solution inside the probe. B 0 represents the main magnetic field. BW This indicates the sequence receiving bandwidth.

[0020] Furthermore, it also includes a probe positioning module and an eddy current generation module. The probe positioning module includes a B0 field reference module with no gradient and only a sampling window. The reference module outputs background phase information including the B0 field. The eddy current generation module includes gradient modules with the same polarity. The gradient modules with the same polarity include three combination methods: a combination of one positive gradient and one zero gradient, a combination of two gradients with the same positive polarity but different amplitudes, and a combination of two gradients with the same negative polarity but different amplitudes.

[0021] The beneficial effects of this invention are as follows: 1. This invention ultimately integrates a set of independent methods for rapid positioning, eddy current measurement, and correction of field probe arrays; it eliminates the reliance on repeated manual adjustment or confirmation of water model position in traditional eddy current measurement; it reduces the eddy current measurement time in low-field MRI from several hours to within five minutes, greatly improving the efficiency and accuracy of eddy current measurement; and it provides a stable and effective method for rapid eddy current correction of portable MRI in different scenarios.

[0022] 2. The present invention can maintain sufficient eddy current field measurement resolution while increasing the coil size.

[0023] 3. This invention significantly reduces the impact of B0 non-uniform field on probe positioning, providing accurate probe positioning information for low-field MRI; the gradient design with the same polarity makes eddy current measurement more accurate, especially for long-term eddy current terms.

[0024] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the hardware connection of a preferred embodiment of the present invention; Figure 2 This is a flowchart of a preferred embodiment of the eddy current field measurement of the present invention; Figure 3 yes Figure 2 A magnified view of the localization sequence; Figure 4 This is a schematic diagram of a preferred embodiment of the eddy current field test sequence of the present invention; Figure 5 This is a comparison of a preferred embodiment of the eddy current field test sequence of the present invention with a conventional method; Figure 6 This is a comparison of DWI-EPI imaging results of healthy subjects before and after eddy current field correction. Detailed Implementation

[0026] The preferred embodiments of the present invention are described below with reference to the accompanying drawings to make the technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0027] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings.

[0028] This invention addresses the problems of long calibration time and high calibration difficulty encountered in practical applications of low-field portable MRI systems. It utilizes the convenience of the spatial information inherent in the field probe and combines the characteristics of the low-field coil with the system characteristics to propose a complete set of rapid eddy current field measurement and calibration technology, including the field probe and rapid eddy current measurement sequence.

[0029] Compared to traditional image-phase-based eddy current measurement methods in low-field conditions, the field probe method offers a higher signal-to-noise ratio and inherent spatial location information, eliminating the need for additional coded gradients for positioning. This reduces measurement time from several hours to just 5 minutes, avoiding phase information interference introduced by additional gradients and system temperature drift caused by long scanning times, significantly improving the measurement efficiency and accuracy of eddy current fields in portable systems. Compared to traditional field probe methods, this method, based on mathematical calculations, improves the coil signal-to-noise ratio while meeting low-field resolution requirements. Furthermore, it optimizes the sequence positioning and eddy current generation modules, reducing the impact of height inhomogeneity and large remanent magnetization effects in portable low-field systems, effectively enhancing the accuracy of eddy current field measurements.

[0030] Currently, this invention has been applied to a self-built 110mT mobile scanning platform, demonstrating its ability to effectively achieve first-order eddy current field correction within 5 minutes. Compared to traditional image phase methods, this method not only increases measurement speed by a hundredfold but also exhibits better correction results in Fast Spin Echo (FSE) and Echo Planar Imaging - Diffusion Weighted Imaging (EPI-DWI) images, showcasing its higher accuracy in eddy current field measurement. In the future, this invention is expected to be more widely applied to portable low-field MRI systems, enabling rapid and convenient eddy current field measurement and correction, and improving the quality of low-field imaging.

[0031] This invention provides a method and apparatus for measuring and correcting eddy current fields based on a field probe, with the aim of significantly reducing the eddy current field correction process in portable magnetic resonance imaging (MRI) and ultimately improving the efficiency and accuracy of eddy current field correction in portable MRI.

[0032] This technology is based on field probe technology. It optimizes the probe to address the low signal-to-noise ratio issue caused by decreasing field strength. The positioning sequence eliminates the need for manual verification of probe placement, enabling rapid probe array positioning. The addition of a B0 reference module to the positioning sequence further enhances probe positioning accuracy. By designing the polarity of the eddy current trigger gradient in the eddy current field measurement sequence, the impact of remanent magnetization on eddy current field measurements is significantly reduced. Finally, a composite model is used to fit the data during reconstruction, significantly reducing eddy current field calculation time. The specific steps are as follows: 1. Based on the probe signal-to-noise ratio formula, the probe size is initially designed and the corresponding signal-to-noise ratio is calculated to guide the parameter design of subsequent eddy current measurement sequences. The relaxation information corresponding to the test solution is quantitatively measured using longitudinal relaxation time (T1) and transverse relaxation time (T2) to ensure that the interval between each sampling window in the subsequent eddy current measurement sequence is greater than T1 and the sampling window length is less than T2. 2. Design a positioning sequence to locate the probe; 3. Use eddy current field measurement sequences to measure eddy currents; 4. The data is fitted using a composite model to reconstruct the eddy current field, which is then sent to the spectrometer system for compensation.

[0033] Furthermore, the probe diameter in step 1 covers 1 mm to 10 mm in portable low-field applications. The probe can be spherical or cylindrical, filled with the test solution, and should be free of air bubbles. The signal-to-noise ratio (SNR) of each probe size is proportional to its solution volume. In actual measurements, the SNR of probes from 3 mm to 10 mm is sufficient for eddy current field measurements, with an average of 1 measurement.

[0034] Probes with a diameter of less than 3 mm have a low signal-to-noise ratio, requiring an increased averaging of subsequent eddy current field measurements to compensate for measurement accuracy. If the probe diameter is... D mm, using the volume of a cylinder or the volume of a sphere The signal-to-noise ratio difference is calculated, and the average value is increased accordingly based on the relationship that the signal-to-noise ratio is proportional to the square root of the mean. h This indicates the probe height. Numerical information about the solution is measured using quantitative images at T1 and T2. This information is then used in subsequent eddy current measurement sequences (such as...). Figure 4 As shown in the figure, the echo interval parameter must be greater than the solution T1 parameter, and the acquisition time of a single receiving module must be less than the T2 parameter, so as to ensure the quality of the probe signal.

[0035] Furthermore, the positioning sequence in step 2 is as follows: Figure 3 The diagram shows that the X, Y, and Z gradients rise sequentially at small rates to a known non-zero gradient value G and are maintained for 2 to 4 seconds. At the point where the gradients stabilize, the probe signal is acquired, and the position of the field probe is calculated using the formula for phase and gradient. This acquisition module is a positioning gradient module. Figure 2 The gradient term in the text is non-zero. In this step, an additional gradient-free signal acquisition module is added before each gradient, serving as a reference module. Figure 2 (The zero-gradient reference term in the model). By subtracting the phase from the reference module's phase from the phase measured by the positioning gradient module, the accurate probe position information, after removing the background phase containing inhomogeneous field information, is finally obtained. Figure 2 As shown in the upper right corner, the entire formula is: (1) Where Φ represents the probe phase, γ Indicates the gyrometry ratio. G Represents non-zero gradient values. R Indicates the probe position. t This indicates the time during which the probe reception is activated and the gradient is applied. This represents the background field containing information about non-uniform fields.

[0036] Furthermore, the eddy current field measurement sequence in step 3 should be performed after obtaining the positioning information from step 2. Based on the requirements of the spherical harmonic space function, the number of probes required for different orders of eddy current field measurements can be calculated. First-order measurements require at least 4 probes, and second-order measurements require 9 probes. N Steps required One. For example. Figure 2 (Bottom right) As shown in the eddy current measurement sequence module, the sequence is generated by a large gradient G that induces eddies. test This is followed by a probe signal acquisition module consisting of an excitation pulse and a spectrometer receiving module. The time from the probe signal acquisition module to the large gradient inducing the eddy current varies sequentially, covering any time range from 0.5 ms to 10 s, depending on the requirements. Both linear and exponential time interval variations can be used. test There are three combination methods: one with a positive gradient and one with a zero gradient, two gradients with the same positive polarity but different amplitudes, and two gradients with the same negative polarity but different amplitudes. These combination methods can avoid changes in gradient polarity during acquisition, thus preventing the introduction of additional residual magnetic fields.

[0037] Furthermore, the data processing in step 4 should be performed using the data obtained from the probe signal acquisition module in step 3. The processing procedure is as follows: (2) (3) (4) (5) (6) As shown in formula (2), the position information of the probe is used to calculate the corresponding parameters based on the basis of the spherical harmonic space function. ,in R These are the coordinates of each probe corresponding to the spherical harmonic space basis. R 'Represents the total position matrix of the probe. x , y , z , m These represent the coordinates of the probe on the x, y, and z axes, and the number of probes, respectively.

[0038] Next, equations (3) to (5) characterize the relationship between the phase information Φ of each probe and the eddy current field. The phase Φ of each probe includes position and time information, where, τ Indicates time, Φ + For G test The phase information generated by the first gradient, Φ ref For G test The phase information generated by the second gradient, Φ R This provides the phase information at the probe's placement position. The field generating the probe signal can be specifically decomposed into a zero-order eddy current field. B e With first-order and higher-order vortex fields R · K , K To characterize the first-order and higher-order spatial distribution coefficients of the eddy field. Based on the characteristics and hardware configuration of the portable low-field MRI system, measuring the first-order eddy field is generally sufficient. Finally, the phase information is differentiated, and an exponential model can be used to fit the short-time eddy part, while a linear model can be used to fit the long-time eddy part (greater than 1 second), thereby improving computational efficiency and obtaining the time-varying model of the spatial distribution coefficients of the eddy field, as shown in formula (6), where, Ge This represents the eddy current field. Finally, the model can be sent to the spectrometer system for gradient current compensation to reduce the influence of eddy current on the image, as shown in formula (7).

[0039] (7)

[0040] in, y Represents the vortex field. τ i Represents the eddy current time constant. A i This represents the amplitude corresponding to each eddy current time constant. This formula characterizes the modeling of each eddy current field in the time dimension. By decomposing it into several exponential terms, it can be simplified to eddy current time constants and corresponding amplitude information, which can then be stored and sent to the spectrometer system for subsequent eddy current correction.

[0041] Figure 1This diagram illustrates the hardware connection of the present invention, showing the probe connection diagram for measuring first-order eddy current fields. It includes a radio frequency (RF) power amplifier, a power divider, a control module or spectrometer containing a field-programmable gate array (FPGA), a switch for switching the probe's transmit / receive states, and a low-noise amplifier. The field probe array of this invention consists of probes 1-6. The RF power amplifier amplifies the transmitted pulse to the required power value, and the output pulse signal is connected to the power divider. The power divider distributes the excitation pulse power equally to the corresponding transmit / receive switch circuits of each probe. In addition to receiving the excitation pulse, the transmit / receive switch circuit also receives gating signals from the FPGA-containing control module or spectrometer, thereby switching the probe's transmit and receive modes. Finally, the received signal from the probe is transmitted to the spectrometer via the transmit / receive switch circuit and the low-noise amplifier, where it is converted into a digital signal.

[0042] Figure 2 This is a flowchart of the eddy current field measurement process with built-in positioning sequence proposed in this invention. A reference module Ref ( ) is added in the positioning stage to address the low-field inhomogeneity problem. Figure 3 The dashed box in the positioning sequence is used to remove background phase information. The gradient in the eddy current measurement stage shows a combination of a positive gradient and a zero gradient, and may also include two gradients of different amplitudes that are both positive or both negative.

[0043] Figure 4 The eddy field test sequence proposed in this invention includes a module that induces the eddy field, defined as G. test The module, along with a series of probe signal acquisition modules, has a pulse delay time that can start from as low as 0.5 ms, and the interval between each acquisition module (echo interval in the figure) can increase linearly or exponentially.

[0044] Figure 5 In the eddy current field test sequence for mitigating remanence effect proposed in this invention, G test A comparison of module features with traditional methods. The example given is a G with one positive and one zero. test Measurement results of the gradient module. The left side of (a) shows a sequence from a traditional probe-based eddy current measurement sequence in a high field, where its G... test It employs a gradient module with one positive and one negative gradient. The right side shows the G-axis module with one positive and one zero gradient used in this invention. test Gradient module. (b) shows these two types of G. test The eddy current measurement results corresponding to the module are derived from the residual eddy current field. G e With the gradient G that introduces eddies testThe percentage of the ratio between them is used for quantitative comparison; the smaller the value, the smaller the residual vortex field. Figure 5 (b) The nine small diagrams correspond to the nine terms of the first-order vortex field, marked with an asterisk ( G is a positive and a negative number. test The eddy current measured by the module, the box (□) represents the positive and zero G proposed in this invention. test The eddy current measured by the module, where G xx G xy G xz G yx G yy G yz G zx G zy G zz These represent the nine terms of the first-order gradient. The first letter of the subscript indicates the gradient direction of the applied eddy current field, and the second letter indicates the direction in which the eddy current field is linearly distributed. The final test results show that using the test sequence of this invention, the long-time eddy current term is closer to the theoretical result of the eddy current gradually decaying to 0, and is not affected by the remanent magnetization effect. At the same time, better measurement results are also achieved in the short-time portion of the eddy current cross term.

[0045] Figure 6 To compare the DWI-EPI imaging results of healthy subjects before and after eddy field correction, and also to compare them with the results of traditional image correction methods in low field, a pair of large-amplitude diffusion gradients are present in the DWI-EPI sequence. Their magnitude is generally measured by the b-value. In the figure, SS represents the diffusion gradient and layer-selection coding gradient being in the same direction, RO represents the diffusion gradient and readout coding gradient being in the same direction, PE represents the diffusion gradient and phase coding gradient being in the same direction, and No gradient (no gradient or zero gradient) represents no diffusion gradient. Because the diffusion gradient amplitude is large, it will induce a large eddy field, thus generating additional deformation information in the SS, RO, and PE diffusion images. The arrows in the figure indicate the deformation details caused by the eddy field in the image. As the b-value increases, the gradient increases; if the eddy field is not properly corrected, it will introduce an even larger eddy field, leading to intensified deformation. The final comparison shows that the method proposed in this invention (last row) achieves the most stable and best image quality with a shorter testing and correction time, and is almost unaffected by deformation caused by the eddy field.

[0046] This invention utilizes a field probe array-based measurement method to achieve rapid eddy current measurement and correction. By balancing signal-to-noise ratio and resolution, the probe size is optimized. This enables minute-level measurements of the spatial and temporal distribution of the eddy current field, allowing for eddy current field modeling and the acquisition of gradient system pre-correction parameters. This completes a comprehensive process for rapid measurement and correction of eddy current fields.

[0047] A field probe array combined with a self-designed eddy current measurement sequence was used to measure and calibrate first-order and zero-order eddy current fields. The field probe achieved a high fill factor by designing a small water phantom with a short T1 and a tightly encased micro-coil, compensating for the low signal-to-noise ratio of low-field MRI. The short T1 solution design shortened the probe's longitudinal relaxation time, reducing the signal acquisition time interval to less than 500 ms, further compressing the eddy current measurement time.

[0048] By placing multiple miniature probes at different locations in space, it is possible to model the spatial distribution of the eddy current field. Therefore, this eddy current measurement sequence does not require spatial encoding of the gradient; it only contains the gradient introduced by the eddy current, which can reduce the influence of additional phase on the eddy current measurement.

[0049] Guided by the formulas for signal-to-noise ratio and field measurement resolution, and taking into account the magnitude of the eddy current field being measured, the probe size is optimized to ensure the accuracy of the eddy current field measurement while meeting the signal-to-noise ratio requirements.

[0050] By analyzing the characteristics of low-field MRI, the signal-to-noise ratio of the probe is optimized at the hardware level, such as probe size. Simultaneously, sequence parameters can be quantitatively optimized for different probe sizes, demonstrating strong variability and compatibility.

[0051] This invention utilizes the characteristic that the signal-to-noise ratio is determined by the impedance of the low-field coil itself. It analyzes the relationship between MRI field strength and signal-to-noise ratio, and the volume of the solution inside the probe, V. sample The relationship between signal-to-noise ratio and the given formula is: (8) in, SNR Indicates the signal-to-noise ratio. V sample Indicates the volume of the solution inside the probe. BW This indicates the sequence receiving bandwidth.

[0052] The theoretical signal-to-noise ratio (SNR) is calculated using this formula. While ensuring a sufficient SNR, the probe size is reduced as much as possible to improve resolution. Simultaneously, the SNR differences between different probe sizes are calculated using the theoretical formula. Then, leveraging the relationship that the SNR is proportional to the square root of the mean number of scans, the average number of scans in the sequence is increased accordingly. Alternatively, a cylindrical probe combined with a small coil design can further improve the SNR.

[0053] To address the characteristics of low-field systems, a B0 reference module is added to the sequence, and the gradient module that generates eddies is optimized.

[0054] A B0 field reference module with no gradient and only a sampling window has been added to the probe positioning module. This module outputs background phase information including the B0 field. By removing the phase information from this module, the probe can be positioned more accurately.

[0055] The remanent magnetization effect in low field changes when the gradient polarity reverses. This method optimizes the traditional positive and negative eddy current gradient generation into a gradient with the same polarity, so that the remanent magnetization effect remains consistent throughout the measurement and can therefore be removed as a reference term.

[0056] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for rapid eddy current field measurement and correction, characterized in that, Includes the following steps: Step 1: Based on the probe signal-to-noise ratio formula, preliminarily design the probe size and calculate the corresponding signal-to-noise ratio to guide the parameter design of the subsequent eddy current measurement sequence; by using the relaxation information corresponding to the quantitative images of T1 and T2 to test the solution, ensure that the interval between each sampling window in the subsequent eddy current measurement sequence is greater than T1 and the sampling window length is less than T2. Step 2: Design a positioning sequence to locate the probe; Step 3: Measure the eddy current using the eddy current field measurement sequence; Step 4: Fit the data using a composite model to reconstruct the eddy current field, and then send it to the spectrometer system for compensation.

2. The rapid eddy current field measurement and correction method according to claim 1, characterized in that, The probe diameter in step 1 covers 1 mm to 10 mm in portable low-field applications.

3. The rapid eddy current field measurement and correction method according to claim 2, characterized in that, The probe in step 1 is spherical or cylindrical in shape, filled with test solution, and free of air bubbles.

4. The rapid eddy current field measurement and correction method according to claim 1, characterized in that, Step 2 specifically includes the following steps: After the X, Y, and Z gradients are successively raised to known non-zero gradient values ​​and maintained for 2 to 4 seconds, the probe signal is collected at the time point after the gradient stabilizes, and the position of the field probe is calculated using the formula (1) of phase and gradient. Among them, an additional gradient-free signal acquisition module is added before each gradient. The phase calculated by formula (1) is subtracted from the phase of the gradient-free signal acquisition module to obtain the accurate probe position information after removing the background phase containing non-uniform field information. （1） Where Φ represents the probe phase, γ Indicates the gyrometry ratio. G Represents non-zero gradient values. R Indicates the probe position. t This indicates the time during which the probe reception is activated and the gradient is applied. This represents the background field containing information about non-uniform fields.

5. The rapid eddy current field measurement and correction method according to claim 4, characterized in that, The eddy current measurement sequence in step 3 consists of a large gradient that induces eddy currents followed by a series of probe signal acquisition modules composed of excitation pulses and a spectrometer receiving module. The time from the probe signal acquisition module to the large gradient that introduces the eddy currents changes sequentially.

6. The rapid eddy current field measurement and correction method according to claim 5, characterized in that, The time from the probe signal acquisition module to the large gradient that causes the eddy current is varied by a linear time interval or by an exponential time interval, covering any time from 0.5 ms to 10 s.

7. The rapid eddy current field measurement and correction method according to claim 6, characterized in that, The large gradients that induce eddies include three combinations: a combination of one positive gradient and one zero gradient, a combination of two gradients of the same positive polarity but different amplitudes, and a combination of two gradients of the same negative polarity but different amplitudes.

8. The rapid eddy current field measurement and correction method according to claim 7, characterized in that, Step 4 specifically includes the following steps: Step 4.1: Using formula (2), calculate the corresponding parameters based on the basis of the spherical harmonic space function to obtain the probe's position information; （2） in, R These are the coordinates of each probe corresponding to the spherical harmonic space basis. R 'Represents the total probe position matrix, x , y , z , m These represent the coordinates of the probe on the x, y, and z axes, and the number of probes, respectively. Step 4.2: Use formulas (3) to (5) to obtain the relationship between the phase information Φ of each probe and the eddy current field. The phase Φ of each probe includes position and time information. Specifically, the field that generates the probe signal is decomposed into a zero-order eddy current field. B e With first-order and higher-order vortex fields R · K , K To characterize the first-order and higher-order spatial distribution coefficients of the eddy field, the phase information is differentiated. An exponential model is used to fit the short-time eddy part, and a linear model is used to fit the long-time eddy part (greater than 1 second) to improve computational efficiency and obtain the time variation model of the spatial distribution coefficients of the eddy field. （3） （4） （5） in, τ Indicates time, Φ + To incorporate phase information generated by the first gradient in the large gradient of the eddy current, Φ ref To incorporate phase information generated by the second gradient in the large gradient of the eddy current, Φ R This provides the phase information for the probe's placement position. Step 4.3: Calculate the eddy current field using formula (6) combined with the probe position R; （6） in, G e Represents a vortex field; Step 4.4: Using formula (7), fit the eddy current field into the form of an exponential sum, and input the obtained amplitude and time constant into the spectrometer; （7） y Represents the vortex field. τ i Represents the eddy current time constant. A i This represents the amplitude corresponding to each eddy current time constant.

9. A rapid eddy current field measurement and correction device, characterized in that, The system includes a radio frequency power amplifier, a power divider, a control module or spectrometer containing an FPGA, a transceiver switch, a low-noise amplifier, and a field probe array consisting of multiple probes. The radio frequency power amplifier is configured to amplify the transmitted pulse to the required power value, and the output pulse signal is connected to the power divider. The power divider distributes the excitation pulse power equally to the transceiver switches corresponding to each probe. In addition to receiving the excitation pulse, the transceiver switch also receives a gating signal from the control module or spectrometer containing the FPGA, thereby switching the transmit and receive modes of the probe. The received signal from the probe is transmitted to the control module or spectrometer containing the FPGA via the transceiver switch and the low-noise amplifier, and converted into a digital signal. The size of the probe is optimized under the guidance of the signal-to-noise ratio and field measurement resolution formula (8), combined with the size of the measured eddy field; （8） in, SNR Indicates the signal-to-noise ratio. V sample Indicates the volume of the solution inside the probe. B 0 represents the main magnetic field. BW This indicates the sequence receiving bandwidth.

10. The rapid eddy current field measurement and correction device according to claim 9, characterized in that, It also includes a probe positioning module and an eddy current generation module. The probe positioning module includes a B0 field reference module with no gradient and only a sampling window. The reference module outputs background phase information including the B0 field. The eddy current generation module includes gradient modules with the same polarity. The gradient modules with the same polarity include three combination methods: a combination of one positive gradient and one zero gradient, a combination of two gradients with the same positive polarity but different amplitudes, and a combination of two gradients with the same negative polarity but different amplitudes.

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