A method for measuring residual magnetism based on echo phase shift difference of magnetic resonance

Abstract

The application discloses a remanence measuring method based on magnetic resonance echo phase shift difference. In the method, the test direction is the same as the scanning direction, the difference between the action of the magnetic field non-uniformity on the spin echo and the gradient echo is utilized, the change of the magnetic field uniformity caused by the test gradient is measured, and the remanence intensity is measured by using the difference between the phase changes of the two echoes caused by the change. The method overcomes the problem that the multi-path gradient is not completely balanced to affect the test gradient and further affect the remanence measurement, eliminates the influence of the flat echo signal peak shape and the peak value drop on the measurement accuracy and stability, and guarantees the effectiveness of the test in the non-coordinate axis direction. In addition, the method does not need additional measuring instruments, the measuring process is simple and easy to implement, and the spatial anisotropy of the remanence can be measured. The method can be used for evaluating the influence of the remanence on the imaging space positioning, and is particularly suitable for judging whether the permanent magnetic resonance imaging system meets the requirements of the space positioning for monitoring and navigation in the interventional treatment.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology and relates to a method for measuring remanence based on the phase shift difference of magnetic resonance echo. Background Technology

[0002] Magnetic resonance imaging (MRI) has become a highly valuable tool in medical diagnosis and the monitoring and navigation of interventional procedures. Typically, in an MRI scanner, when a sample (such as human tissue) is in equilibrium within a static magnetic field B0 (with B0 as the Z-axis of a Cartesian coordinate system), the atomic nuclei (nuclear spins) in the sample are polarized by B0, generating a macroscopic magnetization vector M0. This M0 is then rotated to the horizontal plane (XY plane) under the excitation of a radio frequency pulse and precesses around the Z-axis. A receiving coil placed around the sample induces the precession signal of the magnetization vector. The MRI signal acquired by the receiving coil is amplified and converted from analog to digital before being fed into a computer for image reconstruction. Generally, for imaging purposes, an MRI scanner also needs to generate three orthogonal gradient magnetic fields to perform three-dimensional spatial localization of the MRI signal.

[0003] Based on the method of generating the static magnetic field B0, magnetic resonance imaging (MRI) instruments can be divided into superconducting MRI systems and permanent magnet MRI systems. Compared to permanent magnet MRI systems, superconducting MRI systems typically have a higher static magnetic field strength, resulting in higher image resolution and signal-to-noise ratio, and faster scanning speed, but with lower spatial openness. Permanent magnet MRI systems, on the other hand, typically have higher spatial openness, making them suitable for monitoring and navigation in interventional procedures. During the imaging scan, the gradient magnetic field continuously switches. Because the magnets and their associated components in permanent magnet MRI systems have higher remanence, the magnetic field homogeneity of the scanned area is still affected after the gradient magnetic field switching is complete. Especially after applying a high-intensity gradient magnetic field, the residual gradient magnetic field caused by the remanence may affect the spatial localization of the image. Therefore, it is necessary to measure the remanence of the permanent magnet MRI system to assess whether it significantly affects spatial localization.

[0004] In magnetic resonance imaging (MRI) applications, gradient echo sequences and spin echo sequences belong to two different types of conventional scanning sequences. Gradient echo sequences are sensitive to magnetic field inhomogeneity, while spin echo sequences are not. Therefore, the difference in the effect of magnetic field inhomogeneity on the two types of echoes can be used to measure the change in magnetic field homogeneity before and after applying a high-intensity gradient magnetic field. The resulting change in echo peak value can then be used to measure the intensity of remanence. Testing schemes based on this principle, such as patent application number CN2021112087129 (Method for Measuring Remanence in an MRI System); patent application number CN2022103525218 (Method for Measuring Remanence in an MRI System Using Dual Echoes); and patent application number CN2022116329615 (Optimization Method for Remanence Measurement Process in an MRI System), typically require the echo signal scanning direction to be perpendicular to the test direction to ensure sharp echo signal peaks and stable peak points.

[0005] However, in practical applications, the structural block diagram of a magnetic resonance imaging system is as follows: Figure 1 As shown, magnetic resonance imaging (MRI) systems need to support tomographic scanning in any direction and multi-slice, multi-angle tomographic scanning. Therefore, the scanning direction may not be parallel to the X, Y, and Z coordinate axes. Similarly, the direction to be tested may also not be parallel to the X, Y, and Z coordinate axes. In this case, two or three gradients from the imaging system are needed to synthesize the test gradient. To ensure the validity of the test, the test gradient is usually the maximum gradient allowed by the system. Due to the limited accuracy of the gradient, the multiple gradients under the maximum output (or near maximum output) state cannot be completely balanced. Therefore, the direction of the synthesized test gradient will deviate from the direction to be tested. This deviation will cause the actual test direction to be not completely perpendicular to the scanning direction, resulting in a flattened echo signal peak and a decrease in peak value, changing from a "sharp peak" to a "plateau," significantly affecting the confirmation of echo peak points and peak value measurement, and thus affecting the accuracy and stability of remanence measurement.

[0006] Chinese Patent No. ZL011431881 discloses a magnetic resonance imaging method, a method for measuring remanence, and a magnetic resonance imaging device. To suppress the influence of remanence caused by the previous MR imaging pulse sequence on an MR image, a demagnetizing gradient pulse sequence RS1-RS4 is applied before the previous MR imaging pulse sequence to eliminate the remanence caused by the previous MR imaging pulse sequence and reduce the amount of remanence. Chinese Patent No. ZL2021111902875 discloses a method for detecting the focus of a gradient magnetic field in a magnetic resonance imaging system. This method utilizes the characteristic that the magnetic field strength at the center of the gradient magnetic field remains unchanged when positive and negative gradients are applied. Combined with the relationship between magnetic field strength and proton resonance frequency, the offset of the gradient magnetic field center in each direction is measured to determine whether there is a common center among the three gradient magnetic fields, i.e., whether there is a gradient magnetic field focus, and to determine the location of the gradient magnetic field focus. This method does not require additional detection instruments, the detection process is simple and easy to implement, and it can measure the offset of the gradient magnetic field center in each direction. It can not only provide a reference for correcting gradient coil assembly errors but also provide useful information for spatial coordinate correction during image reconstruction. This method is particularly suitable for magnetic resonance imaging that uses a non-grid point scanning mode to acquire data. It also failed to solve the problem that incomplete balance of multiple gradients affects the test gradient and thus the measurement of residual magnetism. Summary of the Invention

[0007] To address the existing problems, the purpose of this invention is to propose a remanence measurement method based on magnetic resonance echo phase shift difference. This method overcomes the problem that incomplete balance of multiple gradients affects the test gradient and thus the remanence measurement. It also eliminates the impact of flat echo signal peaks and peak value drops on measurement accuracy and stability, thereby ensuring the effectiveness of testing in non-coordinate axis directions.

[0008] Another object of the present invention is to provide an application of the remanence measurement method based on magnetic resonance echo phase shift difference.

[0009] The objective of this invention is achieved through the following scheme: a method for measuring remanence based on the phase shift difference of magnetic resonance echoes. In this method, the test direction is the same as the scanning direction. The method utilizes the difference in the effect of magnetic field inhomogeneity on spin echoes and gradient echoes to measure the change in magnetic field homogeneity caused by the test gradient. The intensity of remanence is measured using the difference in phase change between the two types of echoes caused by this change (i.e., echo phase shift difference). The method includes the following steps: (1) Obtain the phase of the scanning signal under the negative and positive combined test gradient: First, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tnp_n. Then, set the magnitude of the test gradient to 0 and hold it for a time Tnp_0. Next, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tnp_p. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. (2) Obtain the phase of the scanning signal under the positive and negative combined test gradient: First, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tpn_p. Then, set the magnitude of the test gradient to 0 and hold it for a time Tpn_0. Next, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tpn_n. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. (3) Measuring remanence: The difference in phase shift between the two scanning signals is calculated to measure the intensity of residual magnetism in the magnetic resonance imaging system.

[0010] The order of steps (1) and (2) can be interchanged.

[0011] The present invention also proposes the method for evaluating the effect of residual magnetism on imaging spatial positioning, which is particularly suitable for determining whether a permanent magnet magnetic resonance imaging system meets the spatial positioning requirements for monitoring and navigation in interventional therapy.

[0012] Furthermore, the present invention provides a method for measuring remanence based on magnetic resonance echo phase shift difference, comprising the following specific steps: (1) Selecting a direction: Select the test direction and place the uniform strip sample along the test direction; (2) Obtain the phase of the scanning signal under the negative and positive combined test gradient: First, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tnp_n. Then, set the magnitude of the test gradient to 0 and hold it for a time Tnp_0. Next, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tnp_p. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. (3) Obtain the phase of the scanning signal under the positive and negative combined test gradient: First, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tpn_p. Then, set the magnitude of the test gradient to 0 and hold it for a time Tpn_0. Next, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tpn_n. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. (4) Measuring remanence: The difference in phase shift between the two scanning signals is calculated to measure the intensity of residual magnetism in the magnetic resonance imaging system.

[0013] In a preferred embodiment, the hold times Tnp_n, Tnp_0 and Tnp_p in step (2) are equal and are 100 times the minimum gradient pulse width allowed by the system.

[0014] Step (2) includes: applying a test gradient in the test direction; performing a one-dimensional spin echo scan in the same direction as the test direction; and recording the phase P_se_np of all sampling points of the spin echo signal. P_se_np is a vector in which each element records the phase of a sampling point.

[0015] Step (2) includes: applying the test gradient in the test direction; performing a one-dimensional gradient echo scan in the same direction as the test direction; and recording the phase P_ge_np of all sampling points of the gradient echo signal. P_ge_np is a vector, where each element records the phase of a sampling point.

[0016] In a preferred embodiment, the hold times Tpn_p, Tpn_0 and Tpn_n in step (3) are equal and are 100 times the minimum gradient pulse width allowed by the system.

[0017] Step (3) includes: applying a test gradient in the test direction; performing a one-dimensional spin echo scan in the same direction as the test direction; and recording the phase P_se_pn of all sampling points of the spin echo signal. P_se_pn is a vector in which one element records the phase of a sampling point.

[0018] Step (3) includes: applying the test gradient in the test direction; performing a one-dimensional gradient echo scan in the same direction as the test direction; and recording the phase P_ge_pn of all sampling points of the gradient echo signal. P_ge_pn is a vector, where each element records the phase of a sampling point.

[0019] In a preferred embodiment, step (4) includes: calculating the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn using a linear regression model; calculating the difference between P_ge and P_se, denoted as P_ge_se; P_ge_se is used to measure the intensity of remanent magnetization in the magnetic resonance imaging system.

[0020] In a preferred embodiment, step (4) includes: calculating the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn using a linear regression model; calculating the difference between P_np and P_pn, denoted as P_np_pn; and using P_np_pn to measure the intensity of remanence in the magnetic resonance imaging system.

[0021] In a preferred embodiment, step (4) includes: calculating the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn using a linear regression model; calculating the difference between P_ge and P_se, denoted as P_ge_se; taking the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; and using the ratio of P_ge_se to G_max to measure the intensity of remanence in the magnetic resonance imaging system.

[0022] In a preferred embodiment, step (4) includes: calculating the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn using a linear regression model; calculating the difference between P_np and P_pn, denoted as P_np_pn; taking the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; and using the ratio of P_np_pn to G_max to measure the intensity of remanence in the magnetic resonance imaging system.

[0023] Steps (2) and (3), as well as the scanning records of one-dimensional spin echo and one-dimensional gradient echo, can be interchanged in order.

[0024] In a preferred embodiment, after testing in one direction is completed, the testing direction is changed, and steps 1 to 5 are repeated. Steps (1) to (4) should be completed in at least 3 mutually perpendicular directions.

[0025] The method proposed in this invention can be used to evaluate the impact of residual magnetism on imaging spatial positioning.

[0026] Furthermore, it is particularly suitable for determining whether a permanent magnet magnetic resonance imaging system meets the spatial positioning requirements for monitoring and navigation during interventional therapy.

[0027] The advantages of this invention are: it overcomes the problem of incomplete balance of multi-path gradients affecting the test gradient and thus the remanent magnetization measurement; it eliminates the influence of flat echo signal peaks and peak value drops on measurement accuracy and stability; and it ensures the effectiveness of testing in non-coordinate axis directions. Furthermore, this method does not require additional measuring instruments, the measurement process is simple and easy to implement, and it can measure the spatial anisotropy of remanent magnetization. The method proposed in this invention can be used to evaluate the impact of remanent magnetization on imaging spatial positioning, and is particularly suitable for determining whether a permanent magnet magnetic resonance imaging system meets the spatial positioning requirements for monitoring and navigation in interventional therapy. Attached Figure Description

[0028] Figure 1 This is a block diagram of a magnetic resonance imaging system. Figure 2 is a schematic diagram of the negative-positive or positive-negative combination test gradient and one-dimensional gradient echo scanning according to the present invention, including: Figure 2a, a schematic diagram of the negative-positive combination test gradient and one-dimensional gradient echo scanning and Figure 2b, a schematic diagram of the positive-negative combination test gradient and one-dimensional gradient echo scanning. Figure 3 is a schematic diagram of the negative-positive or positive-negative combination test gradient and one-dimensional spin echo scanning according to the present invention, including: Figure 3a, a schematic diagram of the negative-positive combination test gradient and one-dimensional spin echo scanning, and Figure 3b, a schematic diagram of the positive-negative combination test gradient and one-dimensional spin echo scanning. Explanation of the labels in the diagram: Figure 1 middle: 101—Magnet; 102—Gradient coil; 112—Gradient current amplifier; 122—Gradient waveform generator; 103 – RF transmitting coil; 113 – RF power amplifier; 123 – Transmitter; 104 – RF receiving coil; 114 – Preamplifier; 124 – Receiver; 125—Pulse sequence storage circuit; 126 – Monitor / Printer; 130 – Computer; In Figures 2a and 2b: RF – Radio frequency excitation channel; Gt / Gs – Test gradient channel and scan gradient channel; Echo – Echo signal channel; TE – Echo time; 202—Excitation pulse; 203—Dephasing gradient; 204—Converging gradient; 205—Gradient echo signal; G_np — Test gradient using a combination of positive and negative values; G_pn — Gradient testing using positive and negative combinations; G_max_n — The maximum negative gradient allowed by the system; G_max_p — The maximum positive gradient allowed by the system; Tnp_n — The duration of the negative gradient in the negative-positive combination test gradient; Tnp_0 — The time interval between positive and negative gradients in the negative-positive combination test gradient; Tnp_p — The duration of the positive gradient in the negative-positive combination test gradient; Tpn_p — The duration of the positive gradient in the positive-negative combination test gradient; Tpn_0 — The time interval between the positive and negative gradients in the positive and negative combination test gradient; Tpn_n — The duration of the negative gradient in the positive-negative combination test gradient; In Figures 3a and 3b: (RF, Gt / Gs, Echo, TE in the figures are synonyms with those in Figure 2) 302 – Excitation pulse; 303 – Dephasing gradient; 304 – Converging gradient; 305 – Spin echo signal; 306 – Inversion pulse; G_np — Test gradient using a combination of positive and negative values; G_pn — Gradient testing using positive and negative combinations; G_max_p — The maximum positive gradient allowed by the system; G_max_n — The maximum negative gradient allowed by the system; Tnp_n — The duration of the negative gradient in the negative-positive combination test gradient; Tnp_0 — The time interval between positive and negative gradients in the negative-positive combination test gradient; Tnp_p — The duration of the positive gradient in the negative-positive combination test gradient; Tpn_p — The duration of the positive gradient in the positive-negative combination test gradient; Tpn_0 — The time interval between the positive and negative gradients in the positive and negative combination test gradient; Tpn_n — The duration of the negative gradient in the positive-negative combination test gradient. Detailed Implementation

[0029] The present invention will now be described in detail with reference to specific embodiments.

[0030] Figure 1This is a structural block diagram of the MRI system described in this invention. In the MRI system, the magnet 101 has a cavity for placing the sample. A gradient coil 102 is placed around the cavity to generate gradient magnetic fields in the slice selection direction, phase encoding direction, and readout direction for spatial positioning of the sample. An RF transmitting coil 103 and an RF receiving coil 104 are placed around the cavity. The transmitting coil transmits RF pulses to excite the magnetization vector of the sample, and the receiving coil receives the precession signal of the magnetization vector. The gradient coil 102 is connected to a gradient current amplifier 112, and the transmitting coil 103 and the receiving coil 104 are connected to an RF power amplifier 113 and a preamplifier 114, respectively.

[0031] Based on instructions from computer 130, pulse sequence storage circuit 125 controls gradient waveform generator 122 and transmitter 123 according to the pulse sequence stored therein. Gradient waveform generator 122 outputs gradient pulse signals with predetermined timing and waveform. This signal is amplified by gradient current amplifier 112 and then generates a gradient magnetic field in the magnet cavity through gradient coil 102. Transmitter 123 outputs radio frequency pulse signals with predetermined timing and envelope. This signal is amplified by radio frequency power amplifier 113 and then excites nuclear spins in the sample through radio frequency transmission coil 103.

[0032] The radio frequency receiving coil 104 detects the magnetization vector precession signal, which is amplified by the preamplifier 114 and then input to the receiver 124. Under the control of the pulse sequence storage circuit 125, the receiver 124 performs detection and digital-to-analog conversion on the amplified signal to obtain a digital signal, which is then transmitted to the computer 130 to reconstruct the image. The display / printer 126 is used to display / print the scanned image.

[0033] In Figure 2, after either the negative-positive combined test gradient G_np (Figure 2a) or the positive-negative combined test gradient G_pn (Figure 2b), the magnetization vector in the sample is rotated from the Z direction to the XY plane under the action of excitation pulse 202. The magnetization vector precesses around the Z axis in the XY plane and undergoes dephasing under the action of dephasing gradient 203, and then converges under the action of converging gradient 204, forming a gradient echo signal 205. The time interval between the peak of excitation pulse 202 and the peak of gradient echo signal 205 is the echo time TE.

[0034] In Figure 3, after the negative-positive combined test gradient G_np (Figure 3a) or the positive-negative combined test gradient G_pn (Figure 3b), the magnetization vector in the sample is rotated from the Z direction to the XY plane under the action of excitation pulse 302. The magnetization vector precesses around the Z axis in the XY plane and undergoes dephasing under the action of dephasing gradient 303, then undergoes phase reversal under the action of inversion pulse 306, and finally converges under the action of convergence gradient 304, forming a spin echo signal 305. The time interval between the peak of excitation pulse 302 and the peak of spin echo signal 305 is the echo time TE. Inversion pulse 306 is located between excitation pulse 302 and spin echo signal 305. Example 1 A method for measuring remanence based on magnetic resonance echo phase shift difference, wherein the magnetic resonance imaging system under test has the following characteristics: Figure 1 The structure shown is illustrated in Figures 2a, 2b, 3a, and 3b. Follow these specific steps: Step 1: Select the test direction and place the uniform strip sample along the test direction; Step 2.1: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 3a, with hold times Tnp_n, Tnp_0, and Tnp_p equal to 100 times the minimum gradient pulse width allowed by the system. Apply the test gradient in the test direction. Perform a one-dimensional spin echo scan in the same direction as the test direction. Record the phase P_se_np of all sampling points of the spin echo signal 305. P_se_np is a vector, where each element records the phase of a sampling point.

[0035] Step 2.2: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 2a, with hold times Tnp_n, Tnp_0, and Tnp_p equal to 100 times the minimum allowable gradient pulse width of the system. Apply the test gradient in the test direction. Perform a one-dimensional gradient echo scan in the same direction as the test direction. Record the phase P_ge_np of all sampling points of the gradient echo signal 205. P_ge_np is a vector, where each element records the phase of a sampling point.

[0036] Step 3.1: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 3b, with holding times Tpn_p, Tpn_0, and Tpn_n equal to 100 times the minimum gradient pulse width allowed by the system. Apply the test gradient in the test direction. Perform a one-dimensional spin echo scan in the same direction as the test direction. Record the phase P_se_pn of all sampling points of the spin echo signal 305. P_se_pn is a vector, where each element records the phase of a sampling point.

[0037] Step 3.2: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 2b, with holding times Tpn_p, Tpn_0, and Tpn_n equal to 100 times the minimum gradient pulse width allowed by the system. Apply the test gradient in the test direction. Perform a one-dimensional gradient echo scan in the same direction as the test direction. Record the phase P_ge_pn of all sampling points of the gradient echo signal 205. P_ge_pn is a vector, where each element records the phase of a sampling point.

[0038] Step 4: Calculate the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn using a linear regression model; calculate the difference between P_ge and P_se, denoted as P_ge_se; P_ge_se is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0039] The linear regression model in step 4 is Y = kX + b. The model coefficients k and b can be solved using the linear fitting method. Y and X in the model are taken as P_ge_np and P_ge_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_ge. Similarly, Y and X in the model are taken as P_se_np and P_se_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_se.

[0040] The difference between P_ge and P_se in step 4 is denoted as vector P_ge_se. The first-order phase shift difference in vector P_ge_se is used to measure the remanent magnetization component parallel to the test direction, and the zero-order phase shift difference is used to measure the remanent magnetization component perpendicular to the test direction.

[0041] The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0042] After completing the test in one direction, change the test direction and repeat steps 1-4 above. Steps 1-4 should be completed in at least three mutually perpendicular directions.

[0043] Example 2 A method for measuring remanence based on magnetic resonance echo phase shift difference, wherein the magnetic resonance imaging system under test has the following characteristics: Figure 1 The structure shown is illustrated in Figures 2a, 2b, 3a, and 3b. Follow these specific steps: Step 1: Select the test direction and place the uniform strip sample along the test direction; Step 2.1: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 3a, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_np of all sampling points of the spin echo signal 305. P_se_np is a vector, where each element records the phase of a sampling point.

[0044] Step 2.2: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 2a, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_np of all sampling points of the gradient echo signal 205. P_ge_np is a vector, where each element records the phase of a sampling point.

[0045] Step 3.1: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 3b, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_pn of all sampling points of the spin echo signal 305. P_se_pn is a vector, where each element records the phase of a sampling point.

[0046] Step 3.2: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 2b, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_pn of all sampling points of the gradient echo signal 205. P_ge_pn is a vector, where each element records the phase of a sampling point.

[0047] Step 4: Calculate the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn using a linear regression model; calculate the difference between P_np and P_pn, denoted as P_np_pn; P_np_pn is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0048] The linear regression model in step 4 is Y = kX + b. The model coefficients k and b can be solved using the linear fitting method. Y and X in the model are taken as P_ge_np and P_se_np, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_np. Similarly, Y and X in the model are taken as P_ge_pn and P_se_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_pn.

[0049] The difference between P_np and P_pn in step 4 is denoted as vector P_np_pn. The first-order phase shift difference in vector P_np_pn is used to measure the remanent magnetization component parallel to the test direction, and the zero-order phase shift difference is used to measure the remanent magnetization component perpendicular to the test direction.

[0050] The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0051] After completing the test in one direction, change the test direction and repeat steps 1-4 above. Steps 1-4 should be completed in at least three mutually perpendicular directions.

[0052] Example 3 A method for measuring remanence based on magnetic resonance echo phase shift difference, wherein the magnetic resonance imaging system under test has the following characteristics: Figure 1 The structure shown is illustrated in Figures 2a, 2b, 3a, and 3b. Follow these specific steps: Step 1: Select the test direction and place the uniform strip sample along the test direction; Step 2.1: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 3a, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_np of all sampling points of the spin echo signal 305. P_se_np is a vector, where each element records the phase of a sampling point.

[0053] Step 2.2: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 2a, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_np of all sampling points of the gradient echo signal 205. P_ge_np is a vector, where each element records the phase of a sampling point.

[0054] Step 3.1: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 3b, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_pn of all sampling points of the spin echo signal 305. P_se_pn is a vector, where each element records the phase of a sampling point.

[0055] Step 3.2: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 2b, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_pn of all sampling points of the gradient echo signal 205. P_ge_pn is a vector, where each element records the phase of a sampling point.

[0056] Step 4: Calculate the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn using a linear regression model; calculate the difference between P_ge and P_se, denoted as P_ge_se; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of P_ge_se to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0057] The linear regression model in step 4 is Y = kX + b. The model coefficients k and b can be solved using the linear fitting method. Y and X in the model are taken as P_ge_np and P_ge_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_ge. Similarly, Y and X in the model are taken as P_se_np and P_se_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_se.

[0058] The difference between P_ge and P_se in step 4 is denoted as vector P_ge_se. The ratio of the first-order phase shift difference to G_max in vector P_ge_se is used to measure the remanent magnetization component parallel to the test direction, and the ratio of the zero-order phase shift difference to G_max is used to measure the remanent magnetization component perpendicular to the test direction.

[0059] The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0060] After completing the test in one direction, change the test direction and repeat steps 1-4 above. Steps 1-4 should be completed in at least three mutually perpendicular directions.

[0061] Example 4 A method for measuring remanence based on magnetic resonance echo phase shift difference, wherein the magnetic resonance imaging system under test has the following characteristics: Figure 1 The structure shown is illustrated in Figures 2a, 2b, 3a, and 3b. Follow these specific steps: Step 1: Select the test direction and place the uniform strip sample along the test direction; Step 2.1: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 3a, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_np of all sampling points of the spin echo signal 305. P_se_np is a vector, where each element records the phase of a sampling point.

[0062] Step 2.2: Set the test gradient to the negative-positive combination test gradient G_np shown in Figure 2a, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_np of all sampling points of the gradient echo signal 205. P_ge_np is a vector, where each element records the phase of a sampling point.

[0063] Step 3.1: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 3b, and apply the test gradient in the test direction; perform a one-dimensional spin echo scan, with the scan direction being the same as the test direction; record the phase P_se_pn of all sampling points of the spin echo signal 305. P_se_pn is a vector, where each element records the phase of a sampling point.

[0064] Step 3.2: Set the test gradient to the positive and negative combination test gradient G_pn shown in Figure 2b, and apply the test gradient in the test direction; perform a one-dimensional gradient echo scan, with the scan direction being the same as the test direction; record the phase P_ge_pn of all sampling points of the gradient echo signal 205. P_ge_pn is a vector, where each element records the phase of a sampling point.

[0065] Step 4: Calculate the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn using a linear regression model; calculate the difference between P_np and P_pn, denoted as P_np_pn; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of P_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0066] The linear regression model in step 4 is Y = kX + b. The model coefficients k and b can be solved using the linear fitting method. Y and X in the model are taken as P_ge_np and P_se_np, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_np. Similarly, Y and X in the model are taken as P_ge_pn and P_se_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, denoted as vector P_pn.

[0067] The difference between P_np and P_pn in step 4 is denoted as vector P_np_pn. The ratio of the first-order phase shift difference to G_max in vector P_np_pn is used to measure the remanent magnetization component parallel to the test direction, and the ratio of the zero-order phase shift difference to G_max is used to measure the remanent magnetization component perpendicular to the test direction.

[0068] The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0069] After completing the test in one direction, change the test direction and repeat steps 1-4 above. Steps 1-4 should be completed in at least three mutually perpendicular directions.

Claims

1. A method for measuring remanence based on magnetic resonance echo phase shift difference, characterized in that, The test direction is the same as the scanning direction. Utilizing the difference in the effect of magnetic field inhomogeneity on spin echoes and gradient echoes, the change in magnetic field homogeneity induced by the test gradient is measured. The difference in phase shift between the two echoes caused by this change, i.e., the echo phase shift difference, is used to measure the intensity of remanence in the magnetic resonance imaging system. This includes the following steps: (1) Selecting a direction: Select the test direction and place the uniform strip sample along the test direction; (2) Obtain the phase of the scanning signal under the negative and positive combined test gradient: First, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tnp_n. Then, set the magnitude of the test gradient to 0 and hold it for a time Tnp_0. Next, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tnp_p. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. (3) Obtain the phase of the scanning signal under the positive and negative combined test gradient: First, set the magnitude of the test gradient to the maximum positive gradient G_max_p allowed by the system and hold it for a time Tpn_p. Then, set the magnitude of the test gradient to 0 and hold it for a time Tpn_0. Next, set the magnitude of the test gradient to the maximum negative gradient G_max_n allowed by the system and hold it for a time Tpn_n. Finally, set the magnitude of the test gradient to 0 and perform a scan, recording the phase of the scan signal. Alternatively, the order of steps (2) and (3) can be reversed. (4) Measuring remanence: The phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn are calculated using a linear regression model; the difference between P_ge and P_se is calculated and denoted as P_ge_se; P_ge_se is used to measure the intensity of remanence in a magnetic resonance imaging system; or, The phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn are calculated using a linear regression model; the difference between P_np and P_pn is calculated and denoted as P_np_pn; P_np_pn is used to measure the intensity of remanence in a magnetic resonance imaging system; or, The phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn are calculated using a linear regression model; the difference between P_ge and P_se is calculated and denoted as P_ge_se; the sum of the absolute values ​​of G_max_p and G_max_n is denoted as G_max; the ratio of P_ge_se to G_max is used to measure the intensity of remanence in a magnetic resonance imaging system; or, The phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn are calculated using a linear regression model; the difference between P_np and P_pn is calculated and denoted as P_np_pn; the sum of the absolute values ​​of G_max_p and G_max_n is taken and denoted as G_max; the ratio of P_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system. Steps (2) to (4) should be completed in at least three mutually perpendicular directions. In step (2), the test gradient is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction being the same as the test direction; the phase P_se_np of all sampling points of the spin echo signal is recorded; P_se_np is a vector, where each element records the phase of a sampling point; and / or, In step (2), the test gradient is applied in the test direction; a one-dimensional gradient echo scan is performed in the same direction as the test direction; the phase P_ge_np of all sampling points of the gradient echo signal is recorded; P_ge_np is a vector in which one element records the phase of a sampling point. In step (3), the test gradient is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction being the same as the test direction; the phase P_se_pn of all sampling points of the spin echo signal is recorded; P_se_pn is a vector, where each element records the phase of a sampling point; and / or, In step (3), the test gradient is applied in the test direction; a one-dimensional gradient echo scan is performed in the same direction as the test direction; the phase P_ge_pn of all sampling points of the gradient echo signal is recorded; P_ge_pn is a vector in which one element records the phase of a sampling point.

2. The remanence measurement method based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, The following steps are followed: In step (2), the holding times Tnp_n, Tnp_0 and Tnp_p are equal and are 100 times the minimum gradient pulse width allowed by the system.

3. The remanence measurement method based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, In step (3), the holding times Tpn_p, Tpn_0 and Tpn_n are equal and are 100 times the minimum gradient pulse width allowed by the system.

4. The remanence measurement method based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, In step (4), the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn are calculated using a linear regression model; The difference between P_ge and P_se is calculated and denoted as P_ge_se, which is used to measure the intensity of remanence in a magnetic resonance imaging system; where, The linear regression model is Y = kX + b. The model coefficients k and b are solved using the linear fitting method. Y and X in the model are taken as P_ge_np and P_ge_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_ge. Y and X in the model are taken as P_se_np and P_se_pn, respectively. The solved k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_se. The difference between P_ge and P_se is denoted as vector P_ge_se; The first-order phase shift difference in the vector P_ge_se is used to measure the remanent magnetization component parallel to the test direction, and the zero-order phase shift difference is used to measure the remanent magnetization component perpendicular to the test direction.

5. The method for measuring remanence based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, In step (4), the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn are calculated using a linear regression model; the difference between P_np and P_pn is calculated and denoted as P_np_pn; P_np_pn is used to measure the intensity of remanence in the magnetic resonance imaging system. The linear regression model is Y = kX + b, and the model coefficients k and b are obtained by linear fitting. Y and X in the model are P_ge_np and P_se_np, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_np. Y and X in the model are P_ge_pn and P_se_pn, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_pn. The difference between P_np and P_pn is denoted as vector P_np_pn; the first-order phase shift difference in vector P_np_pn is used to measure the remanent magnetization component parallel to the test direction, and the zero-order phase shift difference is used to measure the remanent magnetization component perpendicular to the test direction.

6. The method for measuring remanence based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, Step 4: Calculate the phase shift P_ge between P_ge_np and P_ge_pn, and the phase shift P_se between P_se_np and P_se_pn using a linear regression model; calculate the difference between P_ge and P_se, denoted as P_ge_se; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of P_ge_se to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system. The linear regression model is Y = kX + b, and the model coefficients k and b are obtained by linear fitting. Y and X in the model are taken as P_ge_np and P_ge_pn, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_ge. Y and X in the model are taken as P_se_np and P_se_pn, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_se. The difference between P_ge and P_se is denoted as vector P_ge_se. The ratio of the first-order phase shift difference in vector P_ge_se to G_max is used to measure the remanent magnetization component parallel to the test direction, and the ratio of the zero-order phase shift difference to G_max is used to measure the remanent magnetization component perpendicular to the test direction.

7. The method for measuring remanence based on magnetic resonance echo phase shift difference according to claim 1, characterized in that, Step 4: Calculate the phase shift P_np between P_ge_np and P_se_np, and the phase shift P_pn between P_ge_pn and P_se_pn using a linear regression model; calculate the difference between P_np and P_pn, denoted as P_np_pn; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of P_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system. The linear regression model is Y = kX + b, and the model coefficients k and b are obtained by linear fitting. Y and X in the model are P_ge_np and P_se_np, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_np. Y and X in the model are P_ge_pn and P_se_pn, respectively, and the obtained k and b are the first-order phase shift and the zero-order phase shift, respectively, denoted as vector P_pn. The difference between P_np and P_pn is denoted as vector P_np_pn. The ratio of the first-order phase shift difference in vector P_np_pn to G_max is used to measure the remanent magnetization component parallel to the test direction, and the ratio of the zero-order phase shift difference to G_max is used to measure the remanent magnetization component perpendicular to the test direction.

8. An application of the remanence measurement method based on magnetic resonance echo phase shift difference according to any one of claims 1 to 7 in evaluating the influence of remanence on imaging spatial positioning.

9. The application according to claim 8, characterized in that: Used to determine whether a permanent magnet magnetic resonance imaging system meets the spatial positioning requirements for monitoring and navigation in interventional therapy.

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