A method for measuring remanence in a magnetic resonance imaging system using double echo

Abstract

The present application belongs to the technical field of magnetic resonance imaging, and relates to a method for measuring remanence in a magnetic resonance imaging system by using double echo. In the method, gradient echo and spin echo are measured continuously after a test gradient is applied, the difference in the effect of magnetic field inhomogeneity on the two echoes is used to measure the change in magnetic field homogeneity before and after a high-intensity gradient magnetic field is applied, and the change in echo peak value caused by the change is used to measure the strength of remanence. The method of the present application does not require additional measuring instruments, the measuring process is simple and easy to implement, and the spatial anisotropy of remanence can be measured. The measuring method simultaneously obtains gradient echo and spin echo in one scan, simplifies the test process, and can avoid the difference in the initial state of the two echoes caused by the magnetic hysteresis effect, thereby reducing the measurement error. The method can evaluate the influence of remanence on spatial positioning in imaging, and is particularly suitable for judging whether a permanent magnetic resonance imaging system meets the requirements of monitoring and navigation on spatial positioning in interventional therapy.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology and relates to a method for measuring residual magnetism in a magnetic resonance imaging system using dual-echo imaging. Background Technology

[0002] In clinical practice, magnetic resonance imaging (MRI) has become a very useful tool in medical diagnosis. Typically, in an MRI system, when the 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 B0, generating a macroscopic magnetization vector M0. This 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 system 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) systems 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 gradient and spin 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. Since this method compares the peak values ​​of the gradient and spin echoes, the same test gradient needs to be applied before both the gradient and spin echo sequences. However, if gradient and spin echo signals are obtained separately in two measurements, even with the same test gradient applied, the hysteresis effect will cause different initial states of the magnetic field distribution, leading to different changes in the magnetic field distribution caused by the same test gradient. Especially when the test gradient intensity is high, the measurement error of this method is relatively large if the initial states of the magnetic field distribution are different.

[0005] Based on the current state of the technology, the inventors of this application intend to provide a method for measuring residual magnetism in a magnetic resonance imaging system using dual echo. Summary of the Invention

[0006] The purpose of this invention is to propose a method for measuring residual magnetism in a magnetic resonance imaging system using dual echo, based on the current state of the technology and addressing existing problems.

[0007] In this invention, after applying a test gradient, the gradient echo and spin echo are continuously measured. The difference in the effect of magnetic field inhomogeneity on the gradient echo and spin echo is utilized to measure the change in magnetic field homogeneity before and after applying a high-intensity gradient magnetic field. The intensity of remanence is then measured by the change in the echo peak value caused by this change. This measurement method requires no additional measuring instruments, has a simple and easy-to-implement procedure, and can measure the spatial anisotropy of remanence. Furthermore, this method simultaneously obtains the gradient echo and spin echo in a single scan, simplifying the testing procedure and avoiding the difference in initial states of the two echoes due to hysteresis, thus reducing measurement errors. This method can assess the impact of remanence 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.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for measuring remanence in a magnetic resonance imaging system using dual-echo imaging, the method comprising the following specific steps:

[0010] Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction;

[0011] Step 2: Set the amplitude of the test gradient to 0 and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; apply a convergence pulse and apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the spin echo signal. Record the peak value A_ge_0 of the gradient echo signal and the peak value A_se_0 of the spin echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0.

[0012] Step 3: Set the amplitude of the test gradient to the maximum positive value allowed by the system, G_max_p, and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the gradient echo signal; apply a convergence pulse and apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal. Record the peak value A_ge_p of the gradient echo signal and the peak value A_se_p of the spin echo signal, and calculate the ratio Rp of A_ge_p to A_se_p.

[0013] Step 4: Set the magnitude of the test gradient to the system's allowed negative maximum value G_max_n, and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the gradient echo signal; apply a convergence pulse and apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal. Record the peak value A_ge_n of the gradient echo signal and the peak value A_se_n of the spin echo signal, and calculate the ratio Rn of A_ge_n to A_se_n.

[0014] Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0015] In a round of testing, the order of steps 2, 3, and 4 can be interchanged, and the testing direction and scanning direction of each step are the same.

[0016] After completing the test in one direction, change the test direction and repeat steps 1-5 above. Steps 1-5 should be completed in at least three mutually perpendicular test directions. After changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

[0017] In step 2 above, the areas of the three divergent gradients and the two convergent gradients are all equal. The area refers to the product of the gradient magnitude and the gradient duration. The first convergent gradient, the second divergent gradient, the third divergent gradient, and the second convergent gradient all have the same polarity, but their polarities are opposite to those of the first divergent gradient.

[0018] The divergent gradient and convergent gradient in steps 3 and 4 above have the same characteristics as the divergent gradient and convergent gradient in step 2 above.

[0019] Step 5 above can also be: calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denot the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denot it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0020] The method for measuring remanence in a magnetic resonance imaging system using dual-echo imaging according to the present invention may further include the following specific steps:

[0021] Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction;

[0022] Step 2: Set the amplitude of the test gradient to 0 and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence pulse and apply both convergence and dephasing gradients in the scanning direction, and acquire the spin echo signal; apply both convergence and dephasing gradients in the scanning direction, and acquire the gradient echo signal. Record the peak value A_se_0 of the spin echo signal and the peak value A_ge_0 of the gradient echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0.

[0023] Step 3: Set the amplitude of the test gradient to the maximum positive value allowed by the system, G_max_p, and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence pulse and apply both convergence and dephasing gradients in the scanning direction, and acquire the spin echo signal; apply both convergence and dephasing gradients in the scanning direction, and acquire the gradient echo signal. Record the peak value A_se_p of the spin echo signal and the peak value A_ge_p of the gradient echo signal, and calculate the ratio Rp of A_ge_p to A_se_p.

[0024] Step 4: Set the amplitude of the test gradient to the system's allowed negative maximum value G_max_n, and apply the test gradient in the test direction; apply an excitation pulse and a dephasing gradient in the scanning direction; apply a convergence pulse and apply both convergence and dephasing gradients in the scanning direction, and acquire the spin echo signal; apply both convergence and dephasing gradients in the scanning direction, and acquire the gradient echo signal. Record the peak value A_se_n of the spin echo signal and the peak value A_ge_n of the gradient echo signal, and calculate the ratio Rn of A_ge_n to A_se_n.

[0025] Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0026] In a round of testing, the order of steps 2, 3, and 4 can be interchanged, and the testing direction and scanning direction of each step are the same.

[0027] After completing the test in one direction, change the test direction and repeat steps 1-5 above. Steps 1-5 should be completed in at least three mutually perpendicular test directions. After changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

[0028] In step 2 above, the areas of the three diphasic gradients and the two converging gradients are all equal. The area refers to the product of the gradient magnitude and the gradient duration. The first diphasic gradient, the first converging gradient, and the second diphasic gradient have the same polarity, while their polarities are opposite to those of the second converging gradient and the third diphasic gradient.

[0029] The divergent gradient and convergent gradient in steps 3 and 4 above have the same characteristics as the divergent gradient and convergent gradient in step 2 above.

[0030] Step 5 above can also be: calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denot the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denot it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0031] Specifically,

[0032] In this invention, Figure 1This is a structural block diagram of the magnetic resonance imaging system described in this invention. In the magnetic resonance imaging system, the magnet 101 has a cavity for placing the sample. A gradient coil 102 is placed around the cavity to generate a gradient magnetic field 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.

[0033] 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.

[0034] 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 digital signal is transmitted to the computer 130 to reconstruct the image. The display / printer 126 is used to display / print the scanned image.

[0035] See Figure 2 In the diagram: Gt - test direction (test gradient); RF - radio frequency pulse (excitation pulse and inversion pulse); Gs - scan direction (dephasing gradient and convergence gradient); Echo - echo signal (gradient echo signal and spin echo signal); TE_GE - gradient echo time; TE_SE / 2 - half of the spin echo time.

[0036] A test gradient 201 is applied in the test direction. After a period of time, an excitation pulse 211 is applied, rotating the magnetization vector in the sample (from the Z direction) to the XY plane. The magnetization vector precesses around the Z-axis in the XY plane. A dephasing gradient 221, a convergence gradient 222, and a dephasing gradient 223 are applied in the scanning direction. The magnetization vector first dephases and then converges in the XY plane, and then dephases again, forming a gradient echo signal 231. After a period of time, a reversal pulse 212 is applied, reversing the phase of the magnetization vector in the XY plane. A convergence gradient 224 and a dephasing gradient 225 are applied in the scanning direction. The magnetization vector converges and then dephases in the XY plane, forming a spin echo signal 232.

[0037] When the amplitude of test gradient 201 is not zero, if the remanence of the imaging system is high, a residual gradient magnetic field will remain in the test direction after test gradient 201 ends. This residual gradient magnetic field causes different frequencies of the signal at different positions along the test direction; after a period of time, the phases of the signal at different positions along the test direction will also differ. Since the dephasing gradients 221, 223, and 225, as well as the converging gradients 222 and 224, are all applied in the scanning direction, which is perpendicular to the test direction, the phase difference between the signals at different positions along the test direction cannot be eliminated, resulting in insufficient convergence of the magnetization vector. Compared to when the amplitude of test gradient 201 is zero, when the amplitude of test gradient 201 is not zero, the peak value of the gradient echo signal 231 becomes smaller because the magnetization vector cannot converge sufficiently.

[0038] Whether it's the gradient in the test direction or the gradient in the scan direction, the phase difference in the signal (or magnetization vector) they cause accumulates starting from excitation pulse 211. This is because before applying excitation pulse 211, the magnetization vector does not contribute to either the gradient echo signal or the spin echo signal.

[0039] For the spin echo, the phase difference accumulates starting from the excitation pulse 211. After a time interval of TE_SE / 2, the accumulated phase difference is reversed by the inversion pulse 212, but the frequency of the signal at different positions along the test direction remains unchanged. After another time interval (TE_SE / 2), the accumulated phase difference exactly cancels out the reversed phase difference, thus allowing the magnetization vector to converge sufficiently. Compared to when the amplitude of the test gradient 201 is 0, when the amplitude of the test gradient 201 is not 0, the inversion pulse 212 eliminates the dephasing caused by the residual gradient magnetic field in the test direction, allowing the magnetization vector to converge sufficiently, thus the peak value of the spin echo signal 232 remains unchanged.

[0040] The time interval between the test gradient 201 and the excitation pulse 211 is to ensure that the scanning proceeds only after the transient process caused by the eddy current has ended. Therefore, the residual gradient magnetic field in the test direction is stable during the scanning process. The gradient echo signal 231 and the spin echo signal 232 are acquired after the same test gradient 201. Therefore, the residual gradient magnetic field in the test direction that affects the two echo signals is exactly the same.

[0041] Although the gradient echo time TE_GE differs from the spin echo time TE_SE, as long as TE_GE and TE_SE remain constant during a test run, the peak value of the spin echo signal 232 will not change, while the change in the peak value of the gradient echo signal 231 is only related to the change in the test gradient 201. Therefore, the ratio between the peak value of the gradient echo signal 231 and the peak value of the spin echo signal 232 is related to the degree of influence of remanence on magnetic field uniformity and can be used to measure the intensity of remanence in the imaging system.

[0042] See Figure 3 In the diagram: Gt - test direction (test gradient); RF - radio frequency pulse (excitation pulse and inversion pulse); Gs - scan direction (dephasing gradient and convergence gradient); Echo - echo signal (spin echo signal and gradient echo signal); TE_GE - gradient echo time; TE_SE / 2 - half of the spin echo time.

[0043] A test gradient 301 is applied in the test direction. After a period of time, an excitation pulse 311 is applied, rotating the magnetization vector in the sample (from the Z direction) to the XY plane. The magnetization vector precesses around the Z-axis in the XY plane. A dephasing gradient 321 is applied in the scanning direction, causing the magnetization vector to dephased in the XY plane. After a period of time, a reversal pulse 312 is applied, reversing the phase of the magnetization vector in the XY plane. A convergence gradient 322 and a dephasing gradient 323 are applied in the scanning direction, causing the magnetization vector to converge and then dephased in the XY plane, forming a spin echo signal 331. After a period of time, a convergence gradient 324 and a dephasing gradient 325 are applied in the scanning direction, causing the magnetization vector to converge and then dephased in the XY plane, forming a gradient echo signal 332.

[0044] When the amplitude of test gradient 301 is not zero, if the remanence of the imaging system is high, a residual gradient magnetic field will remain in the test direction after test gradient 301 ends. This residual gradient magnetic field causes different frequencies of the signal at different positions along the test direction; after a period of time, the phases of the signal at different positions along the test direction will also differ. Since the dephasing gradients 321, 323, and 325, as well as the converging gradients 322 and 324, are all applied in the scanning direction, which is perpendicular to the test direction, the phase difference between the signals at different positions along the test direction cannot be eliminated, resulting in insufficient convergence of the magnetization vector. Compared to when the amplitude of test gradient 301 is zero, when the amplitude of test gradient 301 is not zero, the peak value of the gradient echo signal 332 becomes smaller because the magnetization vector cannot converge sufficiently.

[0045] Whether it's the gradient in the test direction or the gradient in the scan direction, the phase difference in the signal (or magnetization vector) they cause accumulates starting from excitation pulse 311. This is because before applying excitation pulse 311, the magnetization vector does not contribute to either the gradient echo signal or the spin echo signal.

[0046] For the spin echo, the phase difference accumulates starting from the excitation pulse 311. After a time interval of TE_SE / 2, the accumulated phase difference is reversed by the inversion pulse 312, but the frequency of the signal at different positions along the test direction remains unchanged. After another time interval (TE_SE / 2), the accumulated phase difference exactly cancels out the reversed phase difference, thus allowing the magnetization vector to converge sufficiently. Compared to when the amplitude of the test gradient 301 is 0, when the amplitude of the test gradient 301 is not 0, the inversion pulse 312 eliminates the dephasing caused by the residual gradient magnetic field in the test direction, allowing the magnetization vector to converge sufficiently, thus keeping the peak value of the spin echo signal 331 unchanged.

[0047] The time interval between the test gradient 301 and the excitation pulse 311 is to ensure that the scanning proceeds only after the transient process caused by the eddy current has ended. Therefore, the residual gradient magnetic field in the test direction is stable during the scanning process. The spin echo signal 331 and the gradient echo signal 332 are acquired after the same test gradient 301. Therefore, the residual gradient magnetic field in the test direction that affects the two echo signals is exactly the same.

[0048] Although the gradient echo time TE_GE differs from the spin echo time TE_SE, as long as TE_GE and TE_SE remain constant during a test run, the peak value of the spin echo signal 331 will not change, while the change in the peak value of the gradient echo signal 332 is only related to the change in the test gradient 301. Therefore, the ratio between the peak value of the gradient echo signal 332 and the peak value of the spin echo signal 331 is related to the degree of influence of remanence on magnetic field uniformity and can be used to measure the intensity of remanence in the imaging system.

[0049] The beneficial effects of this invention are:

[0050] The measurement method proposed in this invention requires no additional measuring instruments, has a simple and easy-to-implement measurement procedure, and can measure the spatial anisotropy of remanent magnetization. Furthermore, the method simultaneously acquires gradient echo and spin echo in a single scan, simplifying the testing process and avoiding the initial differences in the two echoes caused by hysteresis, thus reducing measurement errors. This method can assess 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

[0051] Figure 1 This is a block diagram of the magnetic resonance imaging system described in this invention.

[0052] Figure 2 This is a schematic diagram of Embodiment 1 of the present invention.

[0053] Figure 3 This is a schematic diagram of Embodiment 2 of the present invention. Detailed Implementation

[0054] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0055] Example 1

[0056] See Figure 2 The method for measuring remanence in a magnetic resonance imaging system using dual-echo imaging provided by this invention includes the following specific steps:

[0057] Step 1: Select the test direction Gt and the scanning direction Gs, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction;

[0058] Step 2: Set the amplitude of test gradient 201 to 0, and apply test gradient 201 in the test direction; apply excitation pulse 211, and apply dephasing gradient 221 in the scanning direction; apply convergence gradient 222 and dephasing gradient 223 in the scanning direction, and acquire gradient echo signal 231; apply convergence pulse 212, and apply convergence gradient 224 and dephasing gradient 225 in the scanning direction, and acquire spin echo signal 232. Record the peak value A_ge_0 of gradient echo signal 231 and the peak value A_se_0 of spin echo signal 232, and calculate the ratio R0 of A_ge_0 to A_se_0.

[0059] Step 3: Set the amplitude of test gradient 201 to the maximum positive value G_max_p allowed by the system, and apply test gradient 201 in the test direction; apply excitation pulse 211, and apply dephasing gradient 221 in the scanning direction; apply convergence gradient 222 and dephasing gradient 223 in the scanning direction, and acquire gradient echo signal 231; apply convergence pulse 212, and apply convergence gradient 224 and dephasing gradient 225 in the scanning direction, and acquire spin echo signal 232. Record the peak value A_ge_p of gradient echo signal 231 and the peak value A_se_p of spin echo signal 232, and calculate the ratio Rp of A_ge_p to A_se_p.

[0060] Step 4: Set the amplitude of test gradient 201 to the system's allowed negative maximum value G_max_n, and apply test gradient 201 in the test direction; apply excitation pulse 211, and apply dephasing gradient 221 in the scanning direction; apply convergence gradient 222 and dephasing gradient 223 in the scanning direction, and acquire gradient echo signal 231; apply convergence pulse 212, and apply convergence gradient 224 and dephasing gradient 225 in the scanning direction, and acquire spin echo signal 232. Record the peak value A_ge_n of gradient echo signal 231 and the peak value A_se_n of spin echo signal 232, and calculate the ratio Rn of A_ge_n to A_se_n.

[0061] Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0062] In a round of testing, the order of steps 2, 3, and 4 can be interchanged, and the testing direction and scanning direction of each step are the same.

[0063] After completing the test in one direction, change the test direction and repeat steps 1-5 above. Steps 1-5 should be completed in at least three mutually perpendicular test directions. After changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

[0064] In step 2 above, the areas of the three dephasing gradients (221, 223, and 225) and the two converging gradients (222 and 224) are all equal. The area refers to the product of the gradient magnitude and the gradient duration. The first converging gradient 222, the second dephasing gradient 223, the second converging gradient 224, and the third dephasing gradient 225 have the same polarity, and their polarity is opposite to that of the first dephasing gradient 221.

[0065] The divergent gradient and convergent gradient in steps 3 and 4 above have the same characteristics as the divergent gradient and convergent gradient in step 2 above.

[0066] Step 5 above can also be: calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denot the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denot it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0067] Example 2

[0068] See Figure 3 The method for measuring remanence in a magnetic resonance imaging system using dual-echo imaging provided by this invention includes the following specific steps:

[0069] Step 1: Select the test direction Gt and the scanning direction Gs, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction;

[0070] Step 2: Set the amplitude of test gradient 301 to 0, and apply test gradient 301 in the test direction; apply excitation pulse 311, and apply dephasing gradient 321 in the scanning direction; apply convergence pulse 312, and apply convergence gradient 322 and dephasing gradient 323 in the scanning direction, and acquire spin echo signal 331; apply convergence gradient 324 and dephasing gradient 325 in the scanning direction, and acquire gradient echo signal 332. Record the peak value A_se_0 of spin echo signal 331 and the peak value A_ge_0 of gradient echo signal 332, and calculate the ratio R0 of A_ge_0 to A_se_0.

[0071] Step 3: Set the amplitude of test gradient 301 to the maximum positive value G_max_p allowed by the system, and apply test gradient 301 in the test direction; apply excitation pulse 311, and apply dephasing gradient 321 in the scanning direction; apply convergence pulse 312, and apply convergence gradient 322 and dephasing gradient 323 in the scanning direction, and acquire spin echo signal 331; apply convergence gradient 324 and dephasing gradient 325 in the scanning direction, and acquire gradient echo signal 332. Record the peak value A_se_p of spin echo signal 331 and the peak value A_ge_p of gradient echo signal 332, and calculate the ratio Rp of A_ge_p to A_se_p.

[0072] Step 4: Set the amplitude of test gradient 301 to the system's allowed negative maximum value G_max_n, and apply test gradient 301 in the test direction; apply excitation pulse 311, and apply dephasing gradient 321 in the scanning direction; apply convergence pulse 312, and apply convergence gradient 322 and dephasing gradient 323 in the scanning direction, acquiring spin echo signal 331; apply convergence gradient 324 and dephasing gradient 325 in the scanning direction, acquiring gradient echo signal 332. Record the peak value A_se_n of spin echo signal 331 and the peak value A_ge_n of gradient echo signal 332, and calculate the ratio Rn of A_ge_n to A_se_n.

[0073] Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

[0074] In a round of testing, the order of steps 2, 3, and 4 can be interchanged, and the testing direction and scanning direction of each step are the same.

[0075] After completing the test in one direction, change the test direction and repeat steps 1-5 above. Steps 1-5 should be completed in at least three mutually perpendicular test directions. After changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

[0076] In step 2 above, the areas of the three dephasing gradients (321, 323, and 325) and the two converging gradients (322 and 324) are all equal. The area refers to the product of the gradient magnitude and the gradient duration. Specifically, the first dephasing gradient 321, the first converging gradient 322, and the second dephasing gradient 323 have the same polarity, while their polarities are opposite to those of the second converging gradient 324 and the third dephasing gradient 325.

[0077] The divergent gradient and convergent gradient in steps 3 and 4 above have the same characteristics as the divergent gradient and convergent gradient in step 2 above.

[0078] Step 5 above can also be: calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denot the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denot it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system.

Claims

1. A method of measuring remanence in a magnetic resonance imaging system using double echo, characterized by, The method includes the following steps: Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction; Step 2: Set the magnitude of the test gradient to 0, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. A convergence pulse is applied, and convergence and dephasing gradients are applied in the scanning direction to acquire spin echo signals; Record the peak value A_ge_0 of the gradient echo signal and the peak value A_se_0 of the spin echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0; Step 3: Set the magnitude of the test gradient to the maximum positive value G_max_p allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. A convergence pulse is applied, and convergence and dephasing gradients are applied in the scanning direction to acquire spin echo signals; Record the peak value A_ge_p of the gradient echo signal and the peak value A_se_p of the spin echo signal, and calculate the ratio Rp of A_ge_p to A_se_p; Step 4: Set the magnitude of the test gradient to the maximum negative value G_max_n allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. A convergence pulse is applied, and convergence and dephasing gradients are applied in the scanning direction to acquire spin echo signals; Record the peak value A_ge_n of the gradient echo signal and the peak value A_se_n of the spin echo signal, and calculate the ratio Rn of A_ge_n to A_se_n; Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system. In one round of testing, the order of steps 2, 3 and 4 can be interchanged, the testing direction of each step is the same, and the scanning direction of each step is the same. After completing the test in one direction, change the test direction and repeat steps 1 to 5 above; steps 1 to 5 above should be completed in at least 3 mutually perpendicular test directions; after changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

2. The method according to claim 1, characterized in that, The areas of the three divergent gradients and the two convergent gradients in steps 2, 3, and 4 are all equal; the area refers to the product of the gradient magnitude and the gradient duration, wherein the first convergent gradient, the second divergent gradient, the second convergent gradient, and the third divergent gradient have the same polarity, and their polarities are opposite to those of the first divergent gradient.

3. A method for measuring remanence in a magnetic resonance imaging system using double echo, characterized by, The method includes the following specific steps: Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction; Step 2: Set the magnitude of the test gradient to 0, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. A convergence pulse is applied, and convergence and dephasing gradients are applied in the scanning direction to acquire spin echo signals; Record the peak value A_ge_0 of the gradient echo signal and the peak value A_se_0 of the spin echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0; Step 3: Set the magnitude of the test gradient to the maximum positive value G_max_p allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. Apply a convergence pulse, and apply convergence and dephasing gradients along the scanning direction to acquire spin. Echo signal; Record the peak value A_ge_p of the gradient echo signal and the peak value A_se_p of the spin echo signal, and calculate A_ge_p and... The ratio Rp of A_se_p; Step 4: Set the magnitude of the test gradient to the maximum negative value G_max_n allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; a convergence gradient and a dephasing gradient are applied in the scanning direction, and gradient echo signals are acquired. A convergence pulse is applied, and convergence and dephasing gradients are applied in the scanning direction to acquire spin echo signals; Record the peak value A_ge_n of the gradient echo signal and the peak value A_se_n of the spin echo signal, and calculate the ratio Rn of A_ge_n to A_se_n; Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denote the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denote it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system. In one round of testing, the order of steps 2, 3 and 4 can be interchanged, the testing direction of each step is the same, and the scanning direction of each step is the same. After completing the test in one direction, change the test direction and repeat steps 1 to 5 above; steps 1 to 5 above should be completed in at least 3 mutually perpendicular test directions; after changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

4. The method of claim 3, wherein, The areas of the three divergent gradients and the two convergent gradients in steps 2, 3, and 4 are all equal. The area refers to the product of the gradient magnitude and the gradient duration. The first convergent gradient, the second divergent gradient, the second convergent gradient, and the third divergent gradient have the same polarity, and their polarities are opposite to those of the first divergent gradient.

5. A method for measuring remanence in a magnetic resonance imaging system using double echo, characterized by, The method includes the following specific steps: Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction; Step 2: Set the magnitude of the test gradient to 0, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_0 of the spin echo signal and the peak value A_ge_0 of the gradient echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0. Step 3: Set the magnitude of the test gradient to the maximum positive value G_max_p allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_p of the spin echo signal and the peak value A_ge_p of the gradient echo signal, and calculate the ratio Rp of A_ge_p to A_se_p. Step 4: Set the magnitude of the test gradient to the maximum negative value G_max_n allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_n of the spin echo signal and the peak value A_ge_n of the gradient echo signal, and calculate the ratio Rn of A_ge_n to A_se_n. Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn. The maximum absolute value is denoted as R_max. R_max is used to measure the intensity of remanence in the magnetic resonance imaging system. In a round of testing, the order of steps 2, 3, and 4 can be interchanged, the testing direction of each step is the same, and the scanning of each step is... The directions are all the same; After completing the test in one direction, change the test direction and repeat steps 1 to 5 above; steps 1 to 5 above should be completed in at least 3 mutually perpendicular test directions; after changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

6. The method of claim 5, wherein, The areas of the three divergent gradients and the two convergent gradients in steps 2, 3, and 4 are all equal; the area refers to the product of the gradient magnitude and the gradient duration; wherein, the first convergent gradient, the second divergent gradient, the second convergent gradient, and the third divergent gradient have the same polarity, and their polarities are opposite to those of the first divergent gradient.

7. A method for measuring remanence in a magnetic resonance imaging system using double echo, characterized by, The method includes the following specific steps: Step 1: Select the test direction and scanning direction, with the scanning direction perpendicular to the test direction; place the uniform strip sample along the test direction; Step 2: Set the magnitude of the test gradient to 0, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_0 of the spin echo signal and the peak value A_ge_0 of the gradient echo signal, and calculate the ratio R0 of A_ge_0 to A_se_0. Step 3: Set the magnitude of the test gradient to the maximum positive value G_max_p allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_p of the spin echo signal and the peak value A_ge_p of the gradient echo signal, and calculate the ratio Rp of A_ge_p to A_se_p. Step 4: Set the magnitude of the test gradient to the maximum negative value G_max_n allowed by the system, and apply the test gradient in the test direction; An excitation pulse is applied, and a dephasing gradient is applied in the scanning direction; Apply a convergence pulse, apply a convergence gradient and a dephasing gradient in the scanning direction, and acquire the spin echo signal; apply a convergence gradient and a dephasing gradient in the scanning direction and acquire the gradient echo signal; record the peak value A_se_n of the spin echo signal and the peak value A_ge_n of the gradient echo signal, and calculate the ratio Rn of A_ge_n to A_se_n. Step 5: Calculate the absolute value of the difference between each pair of R0, Rp and Rn, and denote the maximum absolute value as R_max; take the maximum absolute value of G_max_p and G_max_n, and denote it as G_max; the ratio of R_max to G_max is used to measure the intensity of remanence in the magnetic resonance imaging system. In one round of testing, the order of steps 2, 3 and 4 can be interchanged, the testing direction of each step is the same, and the scanning direction of each step is the same. After completing the test in one direction, change the test direction and repeat steps 1 to 5 above; steps 1 to 5 above should be completed in at least 3 mutually perpendicular test directions; after changing the test direction, the scanning direction needs to be redefined to ensure that the scanning direction is perpendicular to the test direction.

8. The method of claim 7, wherein, The areas of the three divergent gradients and the two convergent gradients in steps 2, 3, and 4 are all equal; the area refers to the product of the gradient magnitude and the gradient duration; wherein, the first convergent gradient, the second divergent gradient, the second convergent gradient, and the third divergent gradient have the same polarity, and their polarities are opposite to those of the first divergent gradient.

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