Echo calibration method for balanced steady-state free procession sequence

Abstract

The invention provides an echo calibration method for a balanced steady-state free procession sequence, which belongs to the technical field of magnetic resonance imaging and comprises the following steps of: setting a reverse gradient of a layer selection gradient as a system allowable maximum value; changing the forward gradient, and collecting the maximum value of the echo signal under each preset gradient; model fitting is substituted to determine a first offset factor; sequentially setting the first reverse gradient as a second preset gradient and a third preset gradient, and calculating a first slope of the phase difference of the Fourier transform result of the echo signal and a second slope corresponding to the maximum value of the central sampling point; calculating a gradient as a first reverse gradient based on the second preset gradient, the third preset gradient, the first slope and the second slope; according to the process, the method is used for reverse gradient adjustment, a second offset factor of the model is determined as a reverse gradient, and the calculation gradient is used as a second reverse gradient. The method has the beneficial effects of reducing artifacts and signal loss and improving the imaging quality by accurately adjusting the gradient and collecting and processing the echo signal.

Description

Technical Field

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

[0002] The balanced Steady-State Free Precession (bSSFP) sequence is a fast imaging technique commonly used 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) so that the transverse magnetization can be balanced in a steady state. Due to its high signal-to-noise ratio (SNR) and good contrast, the bSSFP sequence is widely used in the field of medical imaging diagnosis of key parts such as the heart, nerves, and abdomen.

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

[0004] In order to overcome the limitations of manual calibration methods, automatic calibration methods have emerged. Automatic calibration methods achieve automatic calculation and adjustment of gradient values ​​by introducing complex algorithms and models, thereby reducing human intervention and improving calibration efficiency and accuracy. However, automatic calibration methods rely on complex algorithms and models, which increases the difficulty of implementation and computational cost, and their calibration accuracy is also limited by algorithms and models. Summary of the invention

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

[0006] The technical problem solved by the present invention can be achieved by adopting the following technical solutions:

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

[0008] Step S1, when phase encoding is not working, the reverse gradient of the layer selection gradient is set to the maximum gradient value allowed by the system, and 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 are adjusted to the first preset gradient;

[0009] Step S2, changing the forward gradient of the layer selection gradient within a preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal;

[0010] Step S3, substituting each preset gradient obtained in step S2 and the corresponding maximum value of the echo signal into a signal strength-gradient model for fitting, so as to determine an offset factor of the signal strength-gradient model, recorded as a first offset factor, and using the first offset factor as the forward gradient;

[0011] Step S4, setting the forward gradient of the layer selection gradient to the first offset factor, and setting the gradient of the first 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 result; 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 first gradient, and taking the first gradient as the first reverse gradient;

[0012] Step S5, again in the phase encoding non-working state, setting the forward gradient of the slice selection gradient to the maximum gradient value allowed by the system, and adjusting the first reverse gradient and the second reverse gradient to the first gradient;

[0013] Step S6, changing the reverse gradient within the preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal;

[0014] Step S7, substituting each preset gradient obtained in step S6 and the corresponding maximum value of the echo signal into the signal strength-gradient model for fitting, so as to determine the offset factor of the signal strength-gradient model, recorded as the second offset factor, and using the second offset factor as the reverse gradient;

[0015] 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 result; 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, recorded as the second gradient, and using 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 the starting gradient of the preset gradient range;

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

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

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

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

[0023] Preferably, the phase difference is calculated as follows:

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

[0025] Among them, f n represents the Fourier transform result corresponding to the echo signal under the preset gradient of the nth sampling point; n+1 represents the Fourier transform result corresponding to the echo signal under the preset gradient of the n+1th sampling point; n Indicates the phase difference.

[0026] Preferably, the first slope of the phase difference is calculated as follows:

[0027]

[0028] Where M represents the maximum number of sampling points; n represents the sampling point number; P n represents the phase difference of the nth sampling point; k1 represents the first slope of the phase difference.

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

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

[0031] Based on the phase difference of the inverse Fourier transform results under the preset slopes, a second slope corresponding to the maximum value of the central sampling point 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] Among them, 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 central sampling point; and g represents the gradient.

[0035] The advantages or beneficial effects of the technical solution of the present invention are:

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

[0037] Figure 1 A timing diagram of a balanced steady-state free precession sequence in a preferred embodiment of the present invention;

[0038] Figure 2 FIG. 4 is a flow chart of a balanced steady-state free precession sequence method in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0041] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

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

[0043] G 11represents the positive gradient, that is, the area of ​​the layer selection gradient applied in the positive direction; G 12 Represents the reverse gradient, that is, the area of ​​the layer selection gradient applied in the reverse direction; G 21 represents the forward phase encoding gradient, that is, the area of ​​the phase encoding gradient applied in the forward direction, G 12 represents the reverse phase encoding gradient, that is, the area of ​​the reversely applied phase encoding gradient; G 31 represents the first reverse gradient on the left side of the sampled gradient, G 32 represents the second reverse gradient to the right of the sampling gradient, G 31 With G 32 The sum of the gradient areas is the area of ​​the frequency encoding gradient applied in the reverse direction, which is equal to the area of ​​the sampling gradient but in the opposite direction. This sampling gradient is the forward sampling gradient.

[0044] See also Figure 2 In a preferred embodiment of the present invention, based on the above problems existing in the prior art, a method for calibrating an echo of a balanced steady-state free precession sequence is provided, comprising:

[0045] Step S1, when phase encoding is not working, the reverse gradient of the layer selection gradient is set to the maximum gradient value allowed by the system, and 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 are adjusted to the first preset gradient;

[0046] Specifically, first, the forward phase encoding gradient G 21 and the reverse phase encoding gradient G 22 Set to zero to disable phase encoding and eliminate the influence of phase encoding. Then, set the reverse gradient G 12 Set to the maximum value allowed by the system to ensure the free induction decay (FID) signal dephasing, avoid signal overlap and interference, and thus improve the imaging quality. At the same time, the first reverse gradient of the frequency encoding gradient G 31 Adjust to the appropriate intensity, the second reverse gradient G 32 Can be combined with the first reverse gradient G 31 same.

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

[0048] Specifically, the first reverse gradient G 31 With the second reverse gradient G 32 Adjust to one-half the readout gradient area.

[0049] Furthermore, the readout gradient area can be determined according to the field of view in the readout direction and the number of sampling points. Readout gradient area = 1 / (FOV*n); where FOV is the field of view in the readout direction; n is the number of sampling points. The readout gradient area gradually decreases as the number of sampling points increases.

[0050] Step S2, changing the forward gradient of the layer selection gradient within a preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal;

[0051] As a preferred implementation, 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] Within the preset gradient range, the gradient value is divided into multiple preset gradient points, recorded as g1, g2, g3, ..., g n These preset gradient points are evenly or regularly distributed within the preset gradient range.

[0054] In step S2, the forward gradient G of the layer selection gradient is 11 Adjust to g1, g2, g3, ..., g n For each value in , the corresponding echo signal is collected under each preset gradient.

[0055] For each preset gradient, record the maximum value of the echo, which is denoted as s1, s2, s3, ..., s n The maximum values ​​of these echo signals reflect the peak values ​​of the magnetic resonance signal intensity received from the target tissue under this gradient.

[0056] Step S3, substituting each preset gradient obtained in step S2 and the corresponding maximum value of the echo signal into the signal strength-gradient model for fitting, so as to determine the offset factor of the signal strength-gradient model, recorded as the first offset factor, and taking the first offset factor as the forward gradient;

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

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

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

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

[0061] In step S3, the preset gradients g1, g2, g3, ..., g n and its corresponding echo maximum values ​​s1, s2, s3, ..., s n Substituting into the signal intensity-gradient model formula, the model parameters a, b and c can be fitted and obtained. Among them, the parameter c is the offset factor to be determined, which represents the gradient offset required to maximize the signal intensity or achieve specific optimization conditions.

[0062] In this embodiment, after the offset factor c is obtained, it is recorded as the first offset factor C. 11 , and use it as the forward gradient G of the layer selection gradient 11 The optimal value of .

[0063] Step S4, setting the forward gradient of the layer selection gradient to the first offset factor, and setting the gradient of the first 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 result; 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 first gradient, and taking the first gradient as the first reverse gradient;

[0064] Further, the second preset gradient and the third preset gradient are two adjacent 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., the above-mentioned g1;

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

[0067] Specifically, in step S4, first, the forward gradient G of the layer selection gradient is 11 Set to the first offset factor C determined in step S3 11 , so that the signal strength reaches the optimal layer selection gradient value.

[0068] Next, the first reverse gradient of the frequency-encoded gradient G 31 Set as g1, collect the echo signal s 11 , for the echo signal s 11 Perform Fourier transform, and the Fourier transform result is recorded as f1.

[0069] The first reverse gradient G 31 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.

[0070] As a preferred embodiment, the phase difference is calculated as follows:

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

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

[0073] Specifically, the Fourier transform results under the above two preset gradients 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}.

[0074] As a preferred embodiment, the calculation method of the first slope of the phase difference is:

[0075]

[0076] Where M represents the maximum number of sampling points; n represents the sampling point 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 to calculate the slope of the phase difference, which is recorded as the first slope k1.

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

[0079] Applying a slope within a preset range to the Fourier transform result corresponding to the echo signal under the second preset gradient, and then performing an inverse Fourier transform to obtain an inverse Fourier transform result under each preset slope;

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

[0081] Specifically, after applying a slope within a set range to the Fourier transform result f1, an inverse Fourier transform is performed, and then the inverse Fourier transform result is substituted into the above-mentioned phase difference calculation formula to calculate the phase difference; finally, the slope calculated using the above-mentioned phase difference slope calculation formula is the slope corresponding to the maximum value of the central sampling point, which is recorded as the second slope k2.

[0082] As a preferred implementation, 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] Among them, 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 central sampling point; and g represents the gradient.

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

[0086] Step S5, again in the phase encoding non-working state, setting the forward gradient of the slice selection gradient to the maximum gradient value allowed by the system, and adjusting 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 Set it to zero to disable phase encoding and eliminate the influence of phase encoding.

[0088] The forward gradient G of the layer selection gradient 11 Set to the maximum value allowed by the system to ensure the FID signal dephasing, avoid signal overlap and interference, and thus improve the imaging quality. At the same time, the first reverse gradient of the frequency encoding gradient G 31 Adjust to the first gradient C obtained in step S4 31 , the second reverse gradient G 32 Can be combined with the first reverse gradient G 31 same.

[0089] Step S6, changing the reverse gradient within a preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal;

[0090] Specifically, the preset gradient range is ±30% of half of the read gradient area, that is, 20% to 80% of the read gradient area. Within 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 For each value in , the corresponding echo signal is collected at each preset gradient. For each preset gradient, the maximum value of the echo is recorded, which is recorded as s1', s2', s3', ..., s n '.

[0092] Step S7, substituting 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 the offset factor of the signal strength-gradient model, recorded as the second offset factor, and using the second offset factor as the 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-selected 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] Further, 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-encoded gradient G 32 Set as g1, collect the echo signal s 11 ', for the echo signal s 11 'Perform Fourier transform, and the Fourier transform result is recorded 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 the slope of the 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 third slope k1 ′ is calculated in a similar manner to the first slope k1 , and the fourth slope k2 ′ is calculated in a similar manner to the second slope k2 , which will not be described in detail herein.

[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 is recorded as 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 present invention proposes a new bSSFP echo calibration method, which aims to improve the accuracy and efficiency of calibration. By bringing the gradient value and the echo signal into a specific fitting formula, the gradient value can be calibrated quickly and accurately, thereby optimizing the signal strength and image quality of the bSSFP sequence. The method has the advantages of simple implementation, low computational cost, and high calibration accuracy, and is suitable for various high-precision imaging applications.

[0108] The above are only preferred embodiments of the present invention, and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of this specification and illustrations should be included in the protection scope of the present invention.

Claims

1. A method for calibrating an echo of a balanced steady-state free precession sequence, characterized in that: include: Step S1, when phase encoding is not working, the reverse gradient of the layer selection gradient is set to the maximum gradient value allowed by the system, and 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 are adjusted to the first preset gradient; Step S2, changing the forward gradient of the slice selection gradient within a preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal; Step S3, substituting each preset gradient obtained in step S2 and the corresponding maximum value of the echo signal into a signal strength-gradient model for fitting, so as to determine an offset factor of the signal strength-gradient model, recorded as a first offset factor, and using the first offset factor as the forward gradient; Step S4, setting the forward gradient of the slice selection gradient to the first offset factor, and setting the gradient of the first reverse gradient to the second preset gradient and the third preset gradient in sequence, and obtaining the echo signals under the second preset gradient and the third preset gradient and the corresponding Fourier transform results; A first slope of the phase difference is calculated according to the Fourier transform result corresponding to the echo signal, and a second slope corresponding to the maximum value of the central sampling point is obtained; a gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, recorded as a first gradient, and the first gradient is used as the first reverse gradient; Step S5, again in the phase encoding non-working state, setting the forward gradient of the slice selection gradient to the maximum gradient value allowed by the system, and adjusting the first reverse gradient and the second reverse gradient to the first gradient; Step S6, changing the reverse gradient within the preset gradient range, collecting echo signals under each preset gradient, and recording the maximum value of each echo signal; Step S7, substituting each preset gradient obtained in step S6 and the corresponding maximum value of the echo signal into the signal strength-gradient model for fitting, so as to determine the offset factor of the signal strength-gradient model, recorded as the second offset factor, and using the second offset factor as the reverse gradient; 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 signals under the second preset gradient and the third preset gradient and the corresponding Fourier transform results; A first slope of the phase difference is calculated according to the Fourier transform result corresponding to the echo signal, and a second slope corresponding to the maximum value of the central sampling point is obtained; a gradient is calculated based on the second preset gradient, the third preset gradient, the first slope and the second slope, recorded as the second gradient, and the second gradient is used as the second reverse gradient.

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

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

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

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

6. The balanced steady-state free precession echo calibration method according to claim 1, characterized in that: The phase difference is calculated as follows: P n =angle(f n / f n+1 ) Among them, f n represents the Fourier transform result corresponding to the echo signal under the preset gradient of the nth sampling point; n+1 represents the Fourier transform result corresponding to the echo signal under the preset gradient of the n+1th sampling point; n Indicates the phase difference.

7. The balanced steady-state free precession echo calibration method according to claim 1, characterized in that: The calculation method of the first slope of the phase difference is: Where M represents the maximum number of sampling points; n represents the sampling point number; P n represents the phase difference of the nth sampling point; k1 represents the first slope of the phase difference.

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

9. The balanced steady-state free precession echo calibration method 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=g1+k1 / k2*(g2-g1) Among them, 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 central sampling point; and g represents the gradient.

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