A method for optimizing a remanence measurement process in a magnetic resonance imaging system

Abstract

The application discloses a kind of optimization methods of residual magnetism measurement process in magnetic resonance imaging system, adopt positive and negative combination gradient as test gradient, the difference of the action of spin echo and gradient echo is combined with magnetic field inhomogeneity, the change of magnetic field uniformity caused by test gradient is measured, then the change of echo peak caused by the change is used to measure the strength of residual magnetism.The method avoids the reference test under the condition that test gradient is zero, overcomes the adverse effects of magnetic hysteresis on reference signal, ensures that the magnetic field distribution state when collecting echo signal in measurement process is determined and stable, eliminates the influence of test sequence on test result, so as to improve the accuracy and stability of residual magnetism measurement result.The method of the application is simple and easy to operate, can measure the spatial anisotropy of residual magnetism, can be used to evaluate the influence of residual magnetism on imaging space positioning, and is especially suitable for judging whether permanent magnetic resonance imaging system meets the requirements of space positioning for monitoring and navigation in interventional therapy.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology and relates to an optimization method for the remanence measurement process in a magnetic resonance imaging system. Background Technology

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

[0003] Magnetic resonance imaging (MRI) instruments can be divided into two types: superconducting MRI systems and permanent magnet MRI systems. Compared to each other, superconducting MRI systems typically have a higher static magnetic field strength, resulting in higher image resolution and signal-to-noise ratio, as well as faster scanning speeds, but with lower spatial openness. Permanent magnet MRI systems, on the other hand, generally have higher spatial openness, making them suitable for monitoring and navigation during 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 due to 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.

[0005] However, testing schemes based on this principle typically require benchmark testing. That is, a benchmark scan is performed under conditions where the test gradient is zero, and the resulting gradient echo signal and spin echo signal serve as the reference signal. However, because hysteresis affects the magnetic field distribution, and this effect persists until the next non-zero test gradient appears, the reference signal is still affected by previously applied non-zero gradients (including but not limited to the test gradient), even though the benchmark test uses a zero test gradient. In particular, applying a strong gradient before the benchmark test can significantly impact the reference signal, thereby affecting the accuracy and stability of the remanence measurement results. Summary of the Invention

[0006] To address the existing problems, the purpose of this invention is to propose an optimized method for remanence measurement in a magnetic resonance imaging system. This method aims to avoid benchmark testing under conditions where the test gradient is zero, thereby improving the accuracy and stability of remanence measurement results.

[0007] Another object of the present invention is to provide an application of an optimized method for measuring residual magnetism in the magnetic resonance imaging system.

[0008] The objective of this invention is achieved through the following scheme: an optimized method for measuring remanence in a magnetic resonance imaging system, which uses a combination of positive and negative gradients as the test gradient, combines the difference in the effect of magnetic field inhomogeneity on spin echo and gradient echo, measures the change in magnetic field homogeneity caused by the test gradient, and then uses the change in echo peak value caused by this change to measure the intensity of remanence, including the following steps:

[0009] (1) Obtain the peak value of the scanning signal under the negative and positive combined test gradient:

[0010] First, set the magnitude of the test gradient to the negative maximum value 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 positive maximum value 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 peak value of the scan signal.

[0011] (2) Obtain the peak value of the scanning signal under the positive and negative combined test gradient:

[0012] First, set the magnitude of the test gradient to the maximum positive value 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 value 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 peak value of the scan signal.

[0013] (3) Measuring remanence:

[0014] The difference between the ratios of the peak values ​​of the two scanning signals is calculated to measure the intensity of residual magnetism in the magnetic resonance imaging system.

[0015] The present invention also proposes the aforementioned optimization method for evaluating the impact 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.

[0016] Furthermore, the present invention provides an optimization method for the remanence measurement process in a magnetic resonance imaging system, comprising the following specific steps:

[0017] (1) Selecting a direction:

[0018] Select the test direction and place the uniform strip sample along the test direction;

[0019] (2) Obtain the peak value of the scanning signal under the negative and positive combined test gradient:

[0020] First, set the magnitude of the test gradient to the negative maximum value 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 positive maximum value 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 peak value of the scan signal.

[0021] (3) Obtain the peak value of the scanning signal under the positive and negative combined test gradient:

[0022] First, set the magnitude of the test gradient to the maximum positive value 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 value 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, perform a scan, and record the peak value of the scan signal.

[0023] (4) Measuring remanence:

[0024] The difference between the ratios of the peak values ​​of the two scanning signals is calculated to measure the intensity of residual magnetism in the magnetic resonance imaging system.

[0025] 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.

[0026] Step (2) includes: applying a test gradient in the test direction; performing a one-dimensional spin echo scan with the scan direction perpendicular to the test direction; and recording the peak value A_se_np of the spin echo signal.

[0027] Step (2) includes: applying the test gradient in the test direction; performing a one-dimensional gradient echo scan with the scan direction perpendicular to the test direction; and recording the peak value A_ge_np of the gradient echo signal.

[0028] 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.

[0029] Step (3) includes: applying a test gradient in the test direction; performing a one-dimensional spin echo scan with the scan direction perpendicular to the test direction; and recording the peak value A_se_pn of the spin echo signal.

[0030] In a preferred embodiment, step (3) includes: applying a test gradient in the test direction; performing a one-dimensional gradient echo scan with the scan direction perpendicular to the test direction; and recording the peak value A_ge_pn of the gradient echo signal.

[0031] In a preferred embodiment, the scanning directions of steps (2) and (3) are the same and perpendicular to the test direction.

[0032] In a preferred embodiment, step (4) includes: calculating the ratio R_ge of A_ge_np to A_ge_pn and the ratio R_se of A_se_np to A_se_pn; calculating the difference between R_ge and R_se, denoted as R_ge_se; R_ge_se is used to measure the intensity of remanent magnetization in the magnetic resonance imaging system.

[0033] In a preferred embodiment, step (4) includes: calculating the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculating the difference between R_np and R_pn, denoted as R_np_pn; R_np_pn is used to measure the intensity of remanent magnetization in the magnetic resonance imaging system.

[0034] In a preferred embodiment, step (4) includes: calculating the ratio R_ge of A_ge_np to A_ge_pn, and the ratio R_se of A_se_np to A_se_pn; calculating the difference between R_ge and R_se, denoted as R_ge_se; taking the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of R_ge_se to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0035] In a preferred embodiment, step (4) includes: calculating the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculating the difference between R_np and R_pn, denoted as R_np_pn; taking the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of R_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0036] 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.

[0037] The method proposed in this invention can be used to evaluate the impact of residual magnetism on imaging spatial positioning.

[0038] 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.

[0039] The advantages of this invention are: it avoids benchmark testing under conditions of zero test gradient, overcomes the adverse effects of hysteresis on the reference signal, ensures that the magnetic field distribution is definite and stable when acquiring echo signals during measurement, and eliminates the influence of test sequence on test results, thereby improving the accuracy and stability of remanence measurement results. Furthermore, the method of this invention does not require additional measuring instruments, the measurement process is simple and easy to implement, and it can measure the spatial anisotropy of remanence. Attached Figure Description

[0040] Figure 1 Block diagram of a magnetic resonance imaging system;

[0041] Figure 2 is a schematic diagram of the negative-positive or positive-negative combination test gradient and one-dimensional gradient echo scanning of the present invention, including: Figure 2a Schematic diagram of negative and positive combination test gradient and one-dimensional gradient echo scanning Figure 2b Schematic diagram of positive and negative combination test gradient and one-dimensional gradient echo scan;

[0042] Figure 3 is a schematic diagram of the negative-positive or positive-negative combination test gradient and one-dimensional spin echo scanning described in this invention, including: Figure 3a Schematic diagram of negative and positive combination test gradient and one-dimensional spin echo scan Figure 3b Schematic diagram of positive and negative combination test gradient and one-dimensional spin echo scan;

[0043] Explanation of the labels in the diagram:

[0044] Figure 1 middle:

[0045] 101—Magnet;

[0046] 102 – Gradient coil; 112 – Gradient current amplifier; 122 – Gradient waveform generator;

[0047] 103 – RF transmitting coil; 113 – RF power amplifier; 123 – Transmitter;

[0048] 104 – RF receiving coil; 114 – Preamplifier; 124 – Receiver;

[0049] 125—Pulse sequence storage circuit;

[0050] 126 – Monitor / Printer; 130 – Computer;

[0051] In Figures 2a and 2b:

[0052] Gt—Test gradient channel; RF—Radio frequency excitation channel; Gs—Scan gradient channel; Echo—Echo signal channel:

[0053] 201a – Gradient test of negative-positive combination; 201b – Gradient test of positive-negative combination;

[0054] 202—Excitation pulse; 203—Dephasing gradient; 204—Converging gradient; 205—Gradient echo signal;

[0055] Tnp_n - Negative-positive combination test gradient duration of negative gradient;

[0056] The interval between positive and negative gradients in the Tnp_0-negative-positive combination test gradient;

[0057] Tnp_p - Tests the duration of the positive gradient in a negative-positive combination gradient test;

[0058] Tpn_n - The duration of the positive gradient in the positive-negative combination test gradient;

[0059] Tpn_0 - The interval between positive and negative gradients in the positive and negative combination test gradient;

[0060] Tpn_p - Duration of the negative gradient in the positive-negative combination test gradient;

[0061] G_max_n - The maximum negative gradient allowed by the system;

[0062] G_max_p - The maximum positive gradient allowed by the system;

[0063] G_np - Test gradient of negative and positive combinations;

[0064] G_pn - Positive and negative combination test gradient;

[0065] In Figures 3a and 3b: (Gt, RF, Gs, and Echo in the figures are synonyms of those in Figure 2)

[0066] 301a - Negative-positive combination test gradient; 301b - Positive-negative combination test gradient;

[0067] 302—Excitation pulse; 303—Dephasing gradient; 304—Converging gradient; 305—Spin echo signal;

[0068] 306—Reverse Pulse;

[0069] Tnp_n - Negative-positive combination test gradient duration of negative gradient;

[0070] The interval between positive and negative gradients in the Tnp_0-negative-positive combination test gradient;

[0071] Tnp_p - Tests the duration of the positive gradient in a negative-positive combination gradient test;

[0072] Tpn_p - The duration of the positive gradient in the positive-negative combination test gradient;

[0073] Tpn_0 - The interval between positive and negative gradients in the positive and negative combination test gradient;

[0074] Tpn_n - The duration of the negative gradient in the positive-negative combination test gradient;

[0075] G_max_p - The maximum positive gradient allowed by the system;

[0076] G_max_n - The maximum negative gradient allowed by the system. Detailed Implementation

[0077] The present invention will now be described in detail with reference to specific embodiments.

[0078] The magnetic resonance imaging systems used in the following embodiments are shown in [reference needed]. Figure 1 The structural block diagram shown includes a magnet 101, an imaging system controlled by a computer 130, and an image output via a display / printer 126; it includes: 1) a receiving module, where information received by the RF receiving coil 104 is passed through a preamplifier 114, received by the receiver 124, and transmitted to the computer 130. The computer 130 processes the information, stores it in a pulse sequence storage circuit 125, and compares it with the information from the receiver 124; 2) a transmitting module, where the pulse sequence storage circuit 125 transmits a signal to the transmitter 123, which is then transmitted to the RF transmitting coil 103 via an RF power amplifier 113; 3) a gradient module, where the pulse sequence storage circuit 125 sends a signal to the gradient waveform generator 122, which is amplified by a gradient current amplifier 112 and transmitted to the gradient coil 102.

[0079] Figure 1The diagram below shows the structural block diagram of the MRI system described in this invention. In the MRI system, a cavity for placing a sample is provided on a magnet 101. 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, thereby spatially locating 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.

[0080] 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 the nuclear spins in the sample through radio frequency transmitting coil 103.

[0081] 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. The obtained digital signal is transmitted to the computer 130 to reconstruct the image, and the monitor / printer 126 is used to display / print the scanned image.

[0082] In Figure 2, after testing gradient G_np or G_pn, 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 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.

[0083] In Figure 3, after testing gradient G_np or G_pn, 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.

[0084] Example 1

[0085] An optimization method for the remanence measurement process in a magnetic resonance imaging system, using testing equipment with features such as Figure 1 The block diagram of the magnetic resonance imaging system shown is provided in the reference document. Figure 2a , Figure 2b , Figure 3a and Figure 3b Follow these specific steps:

[0086] Step 1: Select the test direction and place the uniform strip sample along the test direction;

[0087] Step 2.1: Set the test gradient to Figure 3a The negative and positive combined test gradient 301a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 301a is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_se_np of the spin echo signal 305 is recorded.

[0088] Step 2.2: Set the test gradient to Figure 2a The negative and positive combined test gradient 201a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 201a is applied in the test direction; a one-dimensional gradient echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_ge_np of the gradient echo signal 205 is recorded.

[0089] Step 3.1: Set the test gradient to Figure 3bThe positive and negative combined test gradient 301b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 301b is applied in the test direction. A one-dimensional spin echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_se_pn of the spin echo signal 305 is recorded.

[0090] Step 3.2: Set the test gradient to Figure 2b The positive and negative combined test gradient 201b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 201b is applied in the test direction. A one-dimensional gradient echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_ge_pn of the gradient echo signal 205 is recorded.

[0091] Step 4: Calculate the ratio R_ge of A_ge_np to A_ge_pn, and the ratio R_se of A_se_np to A_se_pn; calculate the difference between R_ge and R_se, denoted as R_ge_se; R_ge_se is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0092] In one round of testing, the scanning directions in steps 2.1, 2.2, 3.1, and 3.2 are all the same and perpendicular to the testing direction. The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0093] After completing the test in one direction, change the test direction and repeat steps 1 to 4 above. Steps 1 to 4 should be completed in at least three mutually perpendicular directions.

[0094] Example 2

[0095] An optimization method for the remanence measurement process in a magnetic resonance imaging system, see reference. Figure 2a , Figure 2b , Figure 3a and Figure 3b The testing equipment is the same as in Example 1, and the following steps are followed:

[0096] Step 1: Select the test direction and place the uniform strip sample along the test direction;

[0097] Step 2.1: Set the test gradient to Figure 3aThe negative and positive combined test gradient 301a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 301a is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_se_np of the spin echo signal 305 is recorded.

[0098] Step 2.2: Set the test gradient to Figure 2a The negative and positive combined test gradient 201a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 201a is applied in the test direction; a one-dimensional gradient echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_ge_np of the gradient echo signal 205 is recorded.

[0099] Step 3.1: Set the test gradient to Figure 3b The positive and negative combined test gradient 301b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 301b is applied in the test direction. A one-dimensional spin echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_se_pn of the spin echo signal 305 is recorded.

[0100] Step 3.2: Set the test gradient to Figure 2b The positive and negative combined test gradient 201b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 201b is applied in the test direction. A one-dimensional gradient echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_ge_pn of the gradient echo signal 205 is recorded.

[0101] Step 4: Calculate the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculate the difference between R_np and R_pn, denoted as R_np_pn; R_np_pn is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0102] In one round of testing, the scanning directions in steps 2.1, 2.2, 3.1, and 3.2 are all the same and perpendicular to the testing direction. The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0103] 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.

[0104] Example 3

[0105] An optimization method for the remanence measurement process in a magnetic resonance imaging system, see reference. Figure 2a , Figure 2b , Figure 3a and Figure 3b The testing equipment is the same as in Example 1, and the following steps are followed:

[0106] See Figure 2a , Figure 2b , Figure 3a and Figure 3b The optimization method for remanence measurement in a magnetic resonance imaging system provided by this invention includes the following specific steps:

[0107] Step 1: Select the test direction and place the uniform strip sample along the test direction;

[0108] Step 2.1: Set the test gradient to Figure 3a The negative and positive combined test gradient 301a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 301a is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_se_np of the spin echo signal 305 is recorded.

[0109] Step 2.2: Set the test gradient to Figure 2a The negative and positive combined test gradient 201a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 201a is applied in the test direction; a one-dimensional gradient echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_ge_np of the gradient echo signal 205 is recorded.

[0110] Step 3.1: Set the test gradient to Figure 3b The positive and negative combined test gradient 301b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 301b is applied in the test direction. A one-dimensional spin echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_se_pn of the spin echo signal 305 is recorded.

[0111] Step 3.2: Set the test gradient to Figure 2bThe positive and negative combined test gradient 201b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 201b is applied in the test direction. A one-dimensional gradient echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_ge_pn of the gradient echo signal 205 is recorded.

[0112] Step 4: Calculate the ratio R_ge of A_ge_np to A_ge_pn, and the ratio R_se of A_se_np to A_se_pn; calculate the difference between R_ge and R_se, denoted as R_ge_se; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of R_ge_se to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0113] In one round of testing, the scanning directions in steps 2.1, 2.2, 3.1, and 3.2 are all the same and perpendicular to the testing direction. The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0114] 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.

[0115] Example 4

[0116] An optimization method for the remanence measurement process in a magnetic resonance imaging system, see reference. Figure 2a , Figure 2b , Figure 3a and Figure 3b The testing equipment is the same as in Example 1, and the following steps are followed:

[0117] Step 1: Select the test direction and place the uniform strip sample along the test direction;

[0118] Step 2.1: Set the test gradient to Figure 3a The negative and positive combined test gradient 301a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 301a is applied in the test direction; a one-dimensional spin echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_se_np of the spin echo signal 305 is recorded.

[0119] Step 2.2: Set the test gradient to Figure 2aThe negative and positive combined test gradient 201a is shown. In the combined test gradient, the magnitude of the negative gradient is taken as the maximum negative value G_max_n allowed by the system, and the magnitude of the positive gradient is taken as the maximum positive value G_max_p allowed by the system. The test gradient 201a is applied in the test direction; a one-dimensional gradient echo scan is performed, with the scan direction perpendicular to the test direction; the peak value A_ge_np of the gradient echo signal 205 is recorded.

[0120] Step 3.1: Set the test gradient to Figure 3b The positive and negative combined test gradient 301b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 301b is applied in the test direction. A one-dimensional spin echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_se_pn of the spin echo signal 305 is recorded.

[0121] Step 3.2: Set the test gradient to Figure 2b The positive and negative combined test gradient 201b is shown. The magnitude of the positive gradient in the combined test gradient is the maximum positive value G_max_p allowed by the system, and the magnitude of the negative gradient is the maximum negative value G_max_n allowed by the system. The test gradient 201b is applied in the test direction. A one-dimensional gradient echo scan is performed with the scan direction perpendicular to the test direction. The peak value A_ge_pn of the gradient echo signal 205 is recorded.

[0122] Step 4: Calculate the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculate the difference between R_np and R_pn, denoted as R_np_pn; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of R_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0123] In one round of testing, the scanning directions in steps 2.1, 2.2, 3.1, and 3.2 are all the same and perpendicular to the testing direction. The order of steps 2.1, 2.2, 3.1, and 3.2 can be interchanged.

[0124] 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 of optimizing a remanence measurement procedure in a magnetic resonance imaging system, characterized by: Using a combination of positive and negative gradients as the test gradient, and considering the difference in the effect of magnetic field inhomogeneity on spin echo and gradient echo, the change in magnetic field homogeneity induced by the test gradient is measured. The intensity of remanence is then measured using the change in echo peak value resulting from this change. The process includes the following steps: Step 1: Select the test direction and place the uniform strip sample along the test direction; Step 2.1: Set the negative-positive combination test gradient: First, set the magnitude of the test gradient to the system's allowed negative maximum value G_max_n and hold time Tnp_n, then set the magnitude of the test gradient to 0 and hold time Tnp_0. Next, set the magnitude of the test gradient to the system's allowed positive maximum value G_max_p and hold time Tnp_p, then set the magnitude of the test gradient to 0. Apply the set test gradient to the test direction and perform a one-dimensional spin echo scan, with the scan direction perpendicular to the test direction. Record the peak value A_se_np of the spin echo signal (305); Step 2.2: Set the negative-positive combination test gradient: First, set the magnitude of the test gradient to the system's allowed negative maximum value G_max_n and hold it for Tnp_n, then set the magnitude of the test gradient to 0 and hold it for Tnp_0. Next, set the magnitude of the test gradient to the system's allowed positive maximum value G_max_p and hold it for Tnp_p, then set the magnitude of the test gradient to 0. Apply the set test gradient to the test direction and perform a one-dimensional gradient echo scan, with the scan direction perpendicular to the test direction. Record the peak value A_ge_np of the gradient echo signal (205); Step 3.1: Set the positive and negative combination test gradient: First, set the magnitude of the test gradient to the system's allowed positive maximum value G_max_p and hold time Tpn_p; then set the magnitude of the test gradient to 0 and hold time Tpn_0; next, set the magnitude of the test gradient to the system's allowed negative maximum value G_max_n and hold time Tpn_n; set the magnitude of the test gradient to 0; apply the set test gradient to the test direction and perform a one-dimensional spin echo scan, with the scan direction perpendicular to the test direction; Record the peak value A_se_pn of the spin echo signal (305); Step 3.2: Set the positive and negative combination test gradient: First, set the magnitude of the test gradient to the system's allowed positive maximum value G_max_p and hold it for Tpn_p. Then, set the magnitude of the test gradient to 0 and hold it for Tpn_0. Next, set the magnitude of the test gradient to the system's allowed negative maximum value G_max_n and hold it for Tpn_n. Finally, set the magnitude of the test gradient to 0. Apply the set test gradient to the test direction and perform a one-dimensional gradient echo scan, with the scan direction perpendicular to the test direction. Record the peak value A_ge_pn of the gradient echo signal (205); Step 4: Calculate the ratio R_ge of A_ge_np to A_ge_pn, and the ratio R_se of A_se_np to A_se_pn; calculate the difference between R_ge and R_se, denoted as R_ge_se; R_ge_se is used to measure the intensity of remanence in the magnetic resonance imaging system, completing one round of testing; After completing the test in one direction, steps 1 to 4 above should be completed in at least three mutually perpendicular directions.

2. The method of optimizing a remanence measurement process in a magnetic resonance imaging system of claim 1, wherein, In step (2), the holding times Tnp_n, Tnp_0 and Tnp_p are equal, which is 100 times the minimum gradient pulse width allowed by the system.

3. The method of optimizing a remanence measurement process in a magnetic resonance imaging system of claim 1, wherein, In step (3), the holding times Tpn_p, Tpn_0 and Tpn_n are equal, which is 100 times the minimum gradient pulse width allowed by the system.

4. The method of optimizing a remanence measurement process in a magnetic resonance imaging system of claim 1, wherein, Follow these steps: In step 4, calculate the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculate the difference between R_np and R_pn, denoted as R_np_pn; R_np_pn is used to measure the intensity of remanent magnetization in a magnetic resonance imaging system.

5. The method of optimizing a process of measuring a magnetic remanence in a magnetic resonance imaging system according to claim 1, characterized in that, In step 4, the ratio of A_ge_np to A_ge_pn, R_ge, and the ratio of A_se_np to A_se_pn, R_se, are calculated; the difference between R_ge and R_se is calculated and denoted as R_ge_se; the sum of the absolute values ​​of G_max_p and G_max_n is taken and denoted as G_max; the ratio of R_ge_se to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

6. The optimization method for the remanence measurement process in the magnetic resonance imaging system according to claim 1, characterized in that, In step 4, calculate the ratio R_np of A_ge_np to A_se_np and the ratio R_pn of A_ge_pn to A_se_pn; calculate the difference between R_np and R_pn, denoted as R_np_pn; take the sum of the absolute values ​​of G_max_p and G_max_n, denoted as G_max; the ratio of R_np_pn to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

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