Echo calibration method for balanced steady-state free precession sequences

Abstract

The application provides a balanced steady-state free precession sequence echo calibration method, belonging to the technical field of magnetic resonance imaging, and comprising the following steps: setting the reverse gradient of the layer-selected gradient as the maximum value allowed by the system; changing the forward gradient, collecting the maximum value of the echo signal under each preset gradient; substituting into a model to determine a first offset factor; setting the first reverse gradient as the second and third preset gradients in sequence, calculating the first slope of the phase difference of the Fourier transform result of the echo signal and the second slope corresponding to the maximum value of the center sampling point; calculating the gradient based on the second preset gradient, the third preset gradient, the first slope and the second slope as the first reverse gradient; according to the above process, the reverse gradient is adjusted, the model second offset factor is determined as the reverse gradient, and the gradient is calculated as the second reverse gradient. Beneficial effects: by accurately adjusting the gradient and collecting and processing the echo signal, the artifacts and signal loss are reduced, and the imaging quality is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnetic resonance imaging, and in particular to an echo calibration method of a balanced steady-state free precession sequence. BACKGROUND

[0002] Balanced steady-state free precession (bSSFP) sequence is a commonly used fast imaging technique in the field of magnetic resonance imaging (MRI). The bSSFP sequence generates high-quality images by applying a series of radio frequency (RF) pulses and gradient pulses within each repetition time (TR), allowing transverse magnetization to remain balanced in a steady state. Due to its high signal-to-noise ratio (SNR) and good contrast, the bSSFP sequence is widely used in medical imaging diagnosis fields of key parts such as heart, nerve and abdomen.

[0003] Currently, the echo calibration method of the bSSFP sequence mainly relies on manual adjustment of gradient values, that is, in actual operation, by repeatedly observing the image quality and signal intensity changes, the gradient value parameters are gradually fine-tuned to achieve the best imaging effect. This manual calibration method is time-consuming, low in calibration efficiency, and affects the imaging speed; and requires the operator to have deep professional knowledge and rich practical experience, and the calibration result is easily affected by human factors, resulting in low calibration stability and accuracy.

[0004] In order to overcome the limitations of manual calibration method, automatic calibration method emerges as the times require. The automatic calibration method realizes automatic calculation and adjustment of gradient values by introducing complex algorithms and models, thereby reducing human intervention and improving calibration efficiency and accuracy. However, the automatic calibration method relies on complex algorithms and models, increasing the implementation difficulty and calculation cost, and its calibration accuracy is also limited by the algorithms and models. SUMMARY

[0005] In order to solve the above technical problems, the present application provides an echo calibration method of a balanced steady-state free precession sequence.

[0006] The technical problems solved by the present application can be realized by the following technical solutions:

[0007] An echo calibration method of a balanced steady-state free precession sequence, comprising:

[0008] Step S1, in a phase encoding non-working state, set the reverse gradient of the layer selection gradient to the maximum gradient value allowed by the system, and adjust the first reverse gradient on the left side of the sampling gradient of the frequency encoding gradient and the second reverse gradient on the right side to the first preset gradient;

[0009] Step S2, change the positive gradient of the layer selection gradient in a preset gradient range, and collect echo signals under each preset gradient to record the maximum value of each echo signal;

[0010] Step S3, substitute each preset gradient and the corresponding echo signal maximum value obtained in the step S2 into a signal intensity-gradient model for fitting to determine the offset factor of the signal intensity-gradient model, denoted as a first offset factor, and take the first offset factor as the positive gradient;

[0011] Step S4, set the positive gradient of the layer selection gradient as the first offset factor, and set the gradient of the first reverse gradient as a second preset gradient and a third preset gradient in turn, obtain the echo signals under the second preset gradient and the third preset gradient and the corresponding Fourier transform results; calculate the first slope of the phase difference according to the Fourier transform results corresponding to the echo signals, and obtain the second slope corresponding to the maximum value of the center sampling point; calculate the gradient based on the second preset gradient, the third preset gradient, the first slope and the second slope, denoted as a first gradient, and take the first gradient as the first reverse gradient.

[0012] Step S5, again in the phase encoding non-working state, set the positive gradient of the layer selection gradient as the maximum gradient allowed by the system, and adjust the first reverse gradient and the second reverse gradient to the first gradient;

[0013] Step S6, change the reverse gradient in the preset gradient range, and collect echo signals under each preset gradient to record the maximum value of each echo signal;

[0014] Step S7, substitute each preset gradient and the corresponding echo signal maximum value obtained in the step S6 into the signal intensity-gradient model for fitting to determine the offset factor of the signal intensity-gradient model, denoted as a second offset factor, and take the second offset factor as the reverse gradient;

[0015] Step S8, set the reverse gradient of the layer selection gradient as the second offset factor, and set the gradient of the second reverse gradient as a second preset gradient and a third preset gradient in turn, obtain the echo signals under the second preset gradient and the third preset gradient and the corresponding Fourier transform results; calculate the first slope of the phase difference according to the Fourier transform results corresponding to the echo signals, and obtain the second slope corresponding to the maximum value of the center sampling point; calculate the gradient based on the second preset gradient, the third preset gradient, the first slope and the second slope, denoted as a second gradient, and take the second gradient as the second reverse gradient.

[0016] Preferably, the first preset gradient is half of the readout gradient area.

[0017] Preferably, the preset gradient range is 20% to 80% of the readout gradient area.

[0018] Preferably, the second preset gradient is a starting gradient of the preset gradient range.

[0019] The third preset gradient is a next step gradient of the starting gradient.

[0020] Preferably, the signal strength-gradient model is:

[0021] s = a * cos(b * (g + c))

[0022] Wherein, a represents an amplitude factor; b represents a frequency factor; c represents an offset factor; g represents a gradient; s represents a maximum value of the echo signal.

[0023] Preferably, the method for calculating the phase difference is:

[0024] P n = angle(f n / f n+1 )

[0025] Wherein, f n represents a Fourier transform result corresponding to an echo signal under a preset gradient of an nth sampling point; f n+1 represents a Fourier transform result corresponding to an echo signal under a preset gradient of an (n+1)th sampling point; P n represents a phase difference.

[0026] Preferably, the method for calculating the first slope of the phase difference is:

[0027]

[0028] Wherein, M represents a maximum sampling point number; n represents a sampling point serial number; P n represents a phase difference of an nth sampling point; k1 represents a first slope of the phase difference.

[0029] Preferably, the obtaining of the second slope corresponding to the center sampling point maximum value comprises:

[0030] Applying a preset range of slope to the Fourier transform result corresponding to the echo signal under the second preset gradient, and then performing inverse Fourier transform to obtain inverse Fourier transform results under each preset slope;

[0031] Based on the phase difference of the inverse Fourier transform results under each preset slope, the second slope corresponding to the center sampling point maximum value is obtained.

[0032] Preferably, the gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, and the calculation formula is:

[0033] g=g1+k1 / k2*(g2-g1)

[0034] Wherein, g1 represents the second preset gradient; g2 represents the third preset gradient; k1 represents the first slope of the phase difference; k2 represents the second slope corresponding to the maximum value of the center sampling point; and g represents the gradient.

[0035] The advantages or beneficial effects of the technical scheme of the present application are as follows:

[0036] The bSSFP echo calibration method provided by the present application reduces image artifacts and signal loss phenomenon by accurately adjusting the acquisition and processing of gradient and echo signal, significantly improves the imaging quality of the bSSFP sequence, and further improves the accuracy and reliability of medical imaging diagnosis, which has important application value. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 The timing diagram of the balanced steady-state free precession sequence in the preferred embodiment of the present application is shown.

[0038] Figure 2 The flowchart of the balanced steady-state free precession sequence method in the preferred embodiment of the present application is shown. DETAILED DESCRIPTION

[0039] The technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0040] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0041] The present application will be further described below with reference to the drawings and specific embodiments, but not as a limitation of the present application.

[0042] Figure 1 The timing diagram of the balanced steady-state free precession sequence is shown. Wherein, RF represents frequency excitation; sPE represents layer selection gradient; PE represents phase encoding gradient; and RO represents frequency encoding gradient (or readout gradient).

[0043] G 11represents a forward gradient, i.e. the area of a forward applied slice selection gradient; G 12 represents a reverse gradient, i.e. the area of a reverse applied slice selection gradient; G 21 represents a forward phase encoding gradient, i.e. the area of a forward applied phase encoding gradient, G 12 represents a reverse phase encoding gradient, i.e. the area of a reverse applied phase encoding gradient; G 31 represents a first reverse gradient on the left side of the sampling gradient, G 32 represents a second reverse gradient on the right side of the sampling gradient, G 31 the sum of the gradient areas of G 32 is the area of a reverse applied frequency encoding gradient, which is equal to the area of the sampling gradient but in the opposite direction. The sampling gradient is a forward sampling gradient.

[0044] Referring to Figure 2 , in order to solve the above problems existing in the prior art, the present application provides a method for echo calibration of a balanced steady state free precession sequence, comprising:

[0045] In step S1, a reverse gradient of a slice selection gradient is set to a maximum gradient value allowed by the system in a phase encoding non-working state, and a first reverse gradient on the left side of a sampling gradient of a frequency encoding gradient and a second reverse gradient on the right side thereof are adjusted to a first preset gradient.

[0046] Specifically, first, a forward phase encoding gradient G 21 and a reverse phase encoding gradient G 22 are set to zero, so that the phase encoding is in a non-working state, and the influence of the phase encoding is eliminated. Then, a reverse gradient G 12 of the slice selection gradient is set to a maximum value allowed by the system, so as to ensure that a free induction decay (FID) signal is scattered, and signal overlapping and interference are avoided, thereby improving the imaging quality. Meanwhile, a first reverse gradient G 31 of the frequency encoding gradient is adjusted to a suitable strength, and a second reverse gradient G 32 may be the same as the first reverse gradient G 31 .

[0047] As a preferred embodiment, the first preset gradient is half of the area of a readout gradient.

[0048] Specifically, the first reverse gradient G 31 and the second reverse gradient G 32 are adjusted to one half of the area of a readout gradient.

[0049] Further, the readout gradient area can be determined according to the field of view in the readout direction and the number of sampling points. The readout gradient area = 1 / (FOV*n); wherein, FOV is the field of view in the readout direction; n is the sampling point sequence number. The readout gradient area gradually decreases with the increase of the sampling point sequence number.

[0050] In step S2, the positive gradient of the slice selection gradient is changed in a preset gradient range, and echo signals under each preset gradient are collected, and the maximum values of the echo signals are recorded.

[0051] As a preferred embodiment, the preset gradient range is 20% to 80% of the readout gradient area.

[0052] Specifically, the preset gradient range is ±30% of half of the readout gradient area.

[0053] In the preset gradient range, the gradient values are divided into a plurality of preset gradient points, denoted as g1, g2, g3, …, g n These preset gradient points are uniformly or regularly distributed in the preset gradient range.

[0054] In step S2, the positive gradient G 11 of the slice selection gradient is sequentially adjusted to each value in g1, g2, g3, …, g n , and the corresponding echo signals are collected under each preset gradient.

[0055] For each preset gradient, the maximum value of the echo signal is recorded, denoted as s1, s2, s3, …, s n . These maximum values of the echo signals reflect the peak value of the magnetic resonance signal strength received from the target tissue under the gradient.

[0056] In step S3, each preset gradient obtained in step S2 and the corresponding maximum value of the echo signal are substituted into the signal strength-gradient model for fitting to determine the offset factor of the signal strength-gradient model, denoted as the first offset factor, and the first offset factor is taken as the positive gradient.

[0057] As a preferred embodiment, the signal strength-gradient model is:

[0058] s = a * cos(b * (g + c))

[0059] Wherein, a represents the amplitude factor; b represents the frequency factor; c represents the offset factor; g represents the gradient; s represents the maximum value of the echo signal.

[0060] Specifically, the signal strength-gradient model is established to describe the relationship between the signal strength and the gradient, so as to reflect the influence of the gradient change on the signal strength.

[0061] In step S3, preset gradients g1, g2, g3, …, g n and their corresponding echo maximum values s1, s2, s3, …, s n are substituted into the signal intensity-gradient model formula to fit the model parameters a, b and c. The parameter c is the offset factor to be determined, which represents the gradient offset required to achieve the maximum signal intensity or a specific optimization condition.

[0062] In this embodiment, after obtaining the offset factor c, it is denoted as the first offset factor C 11 , which is used as the optimal value of the positive gradient G 11 of the slice selection gradient.

[0063] In step S4, the positive gradient of the slice selection gradient is set to the first offset factor, and the gradient of the first reverse gradient is set to the second preset gradient and the third preset gradient in turn, to obtain the echo signals and the corresponding Fourier transform results under the second preset gradient and the third preset gradient; the first slope of the phase difference is calculated according to the Fourier transform results corresponding to the echo signals, and the second slope corresponding to the maximum value of the center sampling point is obtained; the gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, denoted as the first gradient, and the first gradient is used as the first reverse gradient.

[0064] Further, the second preset gradient and the third preset gradient are adjacent two preset gradients in the preset gradient range.

[0065] As a preferred embodiment, the second preset gradient is the starting gradient of the preset gradient range, i.e. g1 mentioned above.

[0066] The third preset gradient is the next step gradient of the starting gradient, i.e. g2 mentioned above.

[0067] Specifically, in step S4, first, the positive gradient G 11 of the slice selection gradient is set to the first offset factor C 11 determined in step S3, so that the signal intensity reaches the optimal slice selection gradient value.

[0068] Next, the first reverse gradient G 31 of the frequency encoding gradient is set to g1, and the echo signal s 11 is acquired, and the Fourier transform result of the echo signal s 11 is denoted as f1.

[0069] The first reverse gradient G 31 is set to g2, and the echo signal s 21 is acquired, and the Fourier transform result of the echo signal s 12 is denoted as f2.

[0070] As a preferred implementation, the method for calculating the phase difference is:

[0071] P n = angle(f n / f n+1 )

[0072] Wherein, f n represents the Fourier transform result corresponding to the preset gradient echo signal of the nth sampling point; f n+1 represents the Fourier transform result corresponding to the preset gradient echo signal of the n+1th sampling point; P n represents the phase difference.

[0073] Specifically, the above two Fourier transform results under the preset gradient are substituted into the above phase difference calculation formula P1=angle(f1 / f2) to evaluate the phase change of the signal under different reverse gradients. In this way, the phase difference P={P1, P2,..., P n} of each sampling point is calculated.

[0074] As a preferred implementation, the method for calculating the first slope of the phase difference is:

[0075]

[0076] Wherein, M represents the maximum sampling point number; n represents the sampling point serial number; P n represents the phase difference of the nth sampling point; k1 represents the first slope of the phase difference.

[0077] Specifically, the phase difference of each sampling point calculated above is substituted into the above phase difference slope calculation formula, and the slope of the phase difference can be calculated, which is recorded as the first slope k1.

[0078] As a preferred implementation, the method for obtaining the second slope corresponding to the maximum value of the center sampling point includes:

[0079] After applying a preset range of slope to the Fourier transform result corresponding to the echo signal under the second preset gradient, inverse Fourier transform is performed to obtain the inverse Fourier transform result under each preset slope;

[0080] Based on the phase difference of the inverse Fourier transform result under each preset slope, the second slope corresponding to the maximum value of the center sampling point is obtained.

[0081] Specifically, the slope of a set range is applied to the Fourier transform result f1, and then the inverse Fourier transform is performed, and the inverse Fourier transform result is substituted into the above phase difference calculation formula to calculate the phase difference; finally, the slope calculated by the above phase difference slope calculation formula is the slope corresponding to the maximum value of the center sampling point, which is denoted as the second slope k2.

[0082] As a preferred embodiment, the gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, and the calculation formula is:

[0083] g=g1+k1 / k2*(g2-g1)

[0084] Wherein, g1 represents the second preset gradient; g2 represents the third preset gradient; k1 represents the first slope of the phase difference; k2 represents the second slope corresponding to the maximum value of the center sampling point; g represents the gradient.

[0085] Specifically, g calculated by the above formula is the value of the first reverse gradient G 31 , denoted as the first gradient C 31 .

[0086] Step S5, again in the phase encoding non-working state, set the forward gradient of the layer selection gradient to the maximum value of the gradient allowed by the system, and adjust the first reverse gradient and the second reverse gradient to the first gradient;

[0087] Specifically, the forward phase encoding gradient G 21 and the reverse phase encoding gradient G 22 are set to zero, so that the phase encoding is in a non-working state, and the influence of the phase encoding is eliminated.

[0088] The forward gradient G 11 of the layer selection gradient is set to the maximum value allowed by the system to ensure that the FID signal is scattered, avoid signal overlap and interference, and thus improve the imaging quality. At the same time, the first reverse gradient G 31 of the frequency encoding gradient is adjusted to the first gradient C 31 obtained in step S4, and the second reverse gradient G 32 may be the same as the first reverse gradient G 31 .

[0089] Step S6, change the reverse gradient in the preset gradient range, and collect the echo signal under each preset gradient, and record the maximum value of each echo signal;

[0090] Specifically, the preset gradient range is ±30% of the readout gradient area, i.e. 20% to 80% of the readout gradient area. In the preset gradient range, the gradient value is divided into a plurality of preset gradient points, denoted as g1, g2, g 3 , …, gn .

[0091] In step S6, the reverse gradient G of the layer selection gradient is 12 Adjust to g1, g2, g3, ..., g n Each value in , and collect the corresponding echo signal under each preset gradient. For each preset gradient, record the maximum value of the echo, which is recorded as s1', s2', s3', ..., s n '.

[0092] Step S7: Substitute each preset gradient obtained in step S6 and the corresponding maximum value of the echo signal into the signal strength-gradient model for fitting to determine an offset factor of the signal strength-gradient model, recorded as a second offset factor, and use the second offset factor as a reverse gradient;

[0093] Specifically, the preset gradients g1, g2, g3, ..., g n and its corresponding echo maximum values ​​s1', s2', s3', ..., s n ', substitute the following signal intensity-gradient model formula:

[0094] s=a*cos(b*(g+c))

[0095] The model parameters a, b and c are obtained by fitting.

[0096] In this embodiment, the parameter c obtained at this time is recorded as the second offset factor C 12 , and use it as the reverse gradient G of the layer selection gradient 12 The optimal value of .

[0097] Step S8, setting the reverse gradient of the layer selection gradient to the second offset factor, and setting the gradient of the second reverse gradient to the second preset gradient and the third preset gradient in sequence, obtaining the echo signal under the second preset gradient and the third preset gradient and the corresponding Fourier transform results; calculating the first slope of the phase difference according to the Fourier transform result corresponding to the echo signal, and obtaining the second slope corresponding to the maximum value of the central sampling point; calculating the gradient based on the second preset gradient, the third preset gradient, the first slope and the second slope, recording it as the second gradient, and using the second gradient as the second reverse gradient.

[0098] Furthermore, the second preset gradient and the third preset gradient are two adjacent preset gradients in the preset gradient range. Preferably, the second preset gradient is the starting gradient of the preset gradient range, i.e., g1; and the third preset gradient is the next step gradient of the starting gradient, i.e., g2.

[0099] Specifically, in step S8, the reverse gradient G of the layer selection gradient is 12Set to the second offset factor C determined in step S7 12 , so that the signal strength reaches the optimal layer selection gradient value.

[0100] Next, the second reverse gradient of the frequency encoding gradient G 32 Set as g1, collect the echo signal s 11 ', for the echo signal s 11 'Perform Fourier transform, and record the Fourier transform result as f1'.

[0101] The second reverse gradient G 32 Set as g2, collect the echo signal s 21 ', for the echo signal s 12 'Perform Fourier transform, and the Fourier transform result is recorded as f2'.

[0102] Then, the Fourier transform results f1' and f2' under the above two preset gradients are substituted into the phase difference calculation formula P1'=angle(f1' / f2') to evaluate the phase change of the signal under different reverse gradients. In this way, the phase difference P'={P1', P2', ..., P n '}.

[0103] Then, the slope of the phase difference is calculated and recorded as the third slope k1'. After applying a slope within a set range to f1', an inverse Fourier transform is performed to obtain the slope corresponding to the maximum value of the central sampling point, which is recorded as the fourth slope k2'.

[0104] The calculation method of the third slope k1 ′ is similar to that of the first slope k1 , and the calculation method of the fourth slope k2 ′ is similar to that of the second slope k2 , which will not be described in detail here.

[0105] Finally, the second preset gradient g1, the third preset gradient g2, the third slope k1' and the fourth slope k2' are substituted into the formula g'=g1+k1' / k2'*(g2-g1) to calculate g', which is the second reverse gradient G 32 The value of the second gradient C 32 .

[0106] Through the above steps, we can obtain the forward gradient G 11 , reverse gradient G 12 , the first reverse gradient G 31 , the second reverse gradient G 32 The corresponding value C 11 、C 12 、C 31 、C 32 , thus completing the echo calibration.

[0107] Specifically, in view of the limitations of the prior art, the application proposes a new bSSFP echo calibration method to improve the accuracy and efficiency of calibration. By introducing the gradient value and echo signal into a specific fitting formula, the gradient value can be quickly and accurately calibrated, thereby optimizing the signal strength and image quality of the bSSFP sequence. This method has the advantages of simple implementation, low computational cost, high calibration accuracy, and is suitable for various high-precision imaging applications.

[0108] The above merely describes the preferred embodiments of the present application, and is not intended to limit the implementation and protection scope of the present application. Those skilled in the art should realize that any equivalent replacement and obvious changes made according to the content of the present application and drawings should be included in the protection scope of the present application.

Claims

1. A method of echo calibration for a balanced steady-state free precession sequence, the method comprising: Comprise: Step S1, in the phase encoding non-working state, set the reverse gradient of the layer selection gradient to the maximum gradient allowed by the system, and adjust the first reverse gradient on the left of the sampling gradient of the frequency encoding gradient and the second reverse gradient on the right to the first preset gradient; Step S2, change the forward gradient of the layer selection gradient in the preset gradient range, and collect the echo signal under each preset gradient, and record the maximum value of each echo signal; Step S3, put each preset gradient and the corresponding echo signal maximum value obtained in the step S2 into the signal intensity-gradient model for fitting to determine the offset factor of the signal intensity-gradient model, denoted as the first offset factor, and take the first offset factor as the forward gradient; Step S4, set the forward gradient of the layer selection gradient to the first offset factor, and set the gradient of the first reverse gradient to the second preset gradient and the third preset gradient in turn, and obtain the echo signal under the second preset gradient and the third preset gradient and the corresponding Fourier transform result; According to the Fourier transform result corresponding to the echo signal, the first slope of the phase difference is calculated, and the second slope corresponding to the maximum value of the center sampling point is obtained; based on the second preset gradient, the third preset gradient, the first slope and the second slope, a gradient is calculated, denoted as the first gradient, and the first gradient is taken as the first reverse gradient; Step S5, again in the phase encoding non-working state, set the forward gradient of the layer selection gradient to the maximum gradient allowed by the system, and adjust the first reverse gradient and the second reverse gradient to the first gradient; Step S6, change the reverse gradient in the preset gradient range, and collect the echo signal under each preset gradient, and record the maximum value of each echo signal; Step S7, put each preset gradient and the corresponding echo signal maximum value obtained in the step S6 into the signal intensity-gradient model for fitting to determine the offset factor of the signal intensity-gradient model, denoted as the second offset factor, and take the second offset factor as the reverse gradient; Step S8, set the reverse gradient of the layer selection gradient to the second offset factor, and set the gradient of the second reverse gradient to the second preset gradient and the third preset gradient in turn, and obtain the echo signal under the second preset gradient and the third preset gradient and the corresponding Fourier transform result; According to the Fourier transform result corresponding to the echo signal, the first slope of the phase difference is calculated, and the second slope corresponding to the maximum value of the center sampling point is obtained; based on the second preset gradient, the third preset gradient, the first slope and the second slope, a gradient is calculated, denoted as the second gradient, and the second gradient is taken as the second reverse gradient.

2. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The first preset gradient is half of the readout gradient area.

3. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The preset gradient range is 20% ~ 80% of the readout gradient area.

4. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The second preset gradient is the starting gradient of the preset gradient range; The third preset gradient is the next step gradient of the starting gradient.

5. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The signal intensity-gradient model is: s = a * cos ( b* ( g + c )) wherein, a denotes an amplitude factor; b denotes a frequency factor; c denotes an offset factor; g denotes a gradient; s denotes a maximum of the echo signal.

6. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The method for calculating the phase difference is: P n = angle ( f n / f n+1 ) in, f n Indicates the n Fourier transform results corresponding to the echo signal under the preset gradient of sampling points; f n+1 Indicates the n+ The Fourier transform result corresponding to the echo signal under the preset gradient of 1 sampling point; P n Indicates the phase difference.

7. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The method for calculating the first slope of the phase difference is: ; wherein, M represents the maximum number of sampling points; n represents the sampling point number; P n represents the phase difference of the n th sampling point; k 1 represents the first slope of the phase difference.

8. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The second slope corresponding to the maximum value of the center sampling point is obtained by: Applying a preset range of slope to the Fourier transform result corresponding to the echo signal under the second preset gradient, and then performing inverse Fourier transform to obtain the inverse Fourier transform result under each preset slope; Based on the phase difference of the inverse Fourier transform result under each preset slope, the second slope corresponding to the maximum value of the center sampling point is obtained.

9. The echo calibration method of balanced steady-state free precession sequences according to claim 1, characterized in that, The gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, and the calculation formula is: g = g 1 +k 1 / k 2 * ( g 2 -g 1) wherein, g 1 represents the second preset gradient; g 2 represents the third preset gradient; k 1 represents the first slope of the phase difference; k 2 represents the second slope corresponding to the maximum value of the center sampling point; g represents the gradient.

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