Method and system for magnetic resonance radio frequency gain calibration based on signal phase

Abstract

The application discloses a kind of based on signal phase's magnetic resonance radio frequency gain calibration method, comprising the following steps: S1: design a fermi pulse, calculate corresponding radio frequency amplitude;S2: design pre-scan sequence;S3: the actual maximum radio frequency amplitude of fermi pulse under fixed radio frequency gain is measured to the data acquisition;S4: according to the actual maximum radio frequency amplitude measured and the maximum integer value of fermi pulse waveform file, the radio frequency conversion coefficient of radio frequency system at this time is calculated;S5: predict the flip angle of any radio frequency pulse under a certain radio frequency gain preset value;S6: according to multiple radio frequency gain preset values and corresponding flip angle, the radio frequency gain value required for target flip angle is predicted.The application is also disclosed a kind of based on signal phase's magnetic resonance radio frequency gain calibration system.The radio frequency conversion coefficient of radio frequency system is indirectly measured by more stable signal phase information, and will not be influenced by background magnetic field inhomogeneity, tissue signal attenuation and other environments.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance radio frequency technology, and in particular to a magnetic resonance radio frequency gain calibration method and system based on signal phase. Background Technology

[0002] Before the formal MRI scan, radio frequency gain calibration is required through a pre-scan to ensure that the magnetization vector under the action of the radio frequency pulse reaches the required flip angle, and ultimately achieve the same imaging quality for different subjects and different imaging sites.

[0003] Currently, the main method for RF gain calibration is based on signal amplitude. This type of method usually first corrects the flip angle by adjusting the signal amplitude (for example, adjusting the RF gain so that the peak values ​​of the two echo signals are equal, at which point the flip angle corresponding to the RF gain is 90 degrees), and then measures intermediate values ​​related to the RF system, such as the RF conversion coefficient, and then uses these to estimate the RF gain required for any RF pulse to reach a certain flip angle.

[0004] The key to this type of amplitude-based method lies in the accuracy of amplitude measurement. However, in practical applications, the amplitude of a signal is easily affected by the inhomogeneity of the background magnetic field, signal attenuation characteristics, and moving objects (such as abdominal breathing). Furthermore, it requires a certain number of iterative scanning steps and cannot complete the scan in one go before data post-processing. All of these factors may lead to unstable calibration results.

[0005] Therefore, there is an urgent need to provide a novel magnetic resonance radio frequency gain calibration method and system based on signal phase to solve the above problems. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a magnetic resonance radio frequency gain calibration method and system based on signal phase, which can indirectly measure the radio frequency conversion coefficient of the radio frequency system through more stable signal phase information, and is not affected by environmental factors such as background magnetic field inhomogeneity and tissue signal attenuation.

[0007] To solve the above-mentioned technical problems, one technical solution adopted by the present invention is to provide a magnetic resonance radio frequency gain calibration method based on signal phase, comprising the following steps:

[0008] S1: Design a Fermi pulse to apply an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculate the corresponding radio frequency amplitude;

[0009] S2: Pre-scanning involves acquiring data in a single step and designing the pre-scanning sequence;

[0010] S3: Measure the actual maximum radio frequency amplitude of the Fermi pulse under a fixed radio frequency gain from the collected data;

[0011] S4: Calculate the RF conversion coefficient of the RF system at this time based on the actual maximum RF amplitude measured in step S3 and the maximum integer value of the Fermi pulse waveform file;

[0012] S5: Predicts the flip angle of any RF pulse at a preset RF gain value;

[0013] S6: Based on multiple preset RF gain values ​​and corresponding flip angles, predict the RF gain value required for the target flip angle.

[0014] In a preferred embodiment of the present invention, in step S1, the waveform RF of the normalized Fermi pulse is... uni (t) is shown in equation (1) below.

[0015]

[0016] In a, t represents the time variable, t=0 is the center position of the pulse, t0 is a parameter used to measure the width of the Fermi pulse, and a is a parameter used to measure the width of the Fermi pulse transition band.

[0017] First, the ratio t0 / a is preset, and then t0 and a are determined by the Fermi pulse duration. The Fermi pulse duration T satisfies the following relationship:

[0018] T=2t0+13.81a#[2]

[0019] Finally, the Fermi pulse waveform is calculated using equation (1).

[0020] In a preferred embodiment of the present invention, the specific steps for designing the pre-scan sequence in step S2 include:

[0021] S201: In accordance with the scanning time sequence, an excitation pulse with layer selection characteristics and a gradient magnetic field are applied simultaneously as a layer selection gradient to excite the magnetic resonance signal of a certain layer.

[0022] S202: Apply the Fermi pulse preset in step S1 and set it to partial resonant excitation, with the partial resonant frequency amplitude being ω. RF ;

[0023] S203: After applying the Fermi pulse, data acquisition of the free decay signal is performed. The polarity of the off-resonance frequency of the Fermi pulse is modified and scanned twice. The data obtained are ksp. + With KSP - .

[0024] In a preferred embodiment of the present invention, step S3 specifically includes:

[0025] S301: Ksp of the data collected twice. +With KSP - Perform a one-dimensional inverse Fourier transform to the image domain, and take the first complex data point, which corresponds to the center of the image domain, for subsequent calculations:

[0026]

[0027] Here, sort1(·) represents the operation of taking the first point of the data, and iFT(·) represents the inverse Fourier transform;

[0028] S302: The actual maximum RF amplitude B1 of the Fermi pulse under the set RF gain is calculated by the following formula (4):

[0029]

[0030] The calculated radio frequency amplitude B1 is in microtesla (μT), angle(·) represents the operation of taking the amplitude angle of the complex data, and the Fermi pulse constant K is... RF It is the value calculated based on the pulse parameters according to the following formula (5):

[0031]

[0032] Where γ = 42.57, Δt is the time interval between each data point of the RF waveform, and ω RF It is the frequency offset of the Fermi pulse, RF uni (i) is the value of the i-th point after Fermi pulse normalization.

[0033] In a preferred embodiment of the present invention, step S4 specifically includes:

[0034] After measuring the actual maximum RF amplitude B1 of the Fermi pulse under a fixed RF gain A, the RF conversion coefficient k of the RF system at this time is calculated according to the maximum integer value N_max of the Fermi pulse waveform file by formula (6).

[0035] k = A - 20log 10 N_max+20log 10 B1#[6]

[0036] In a preferred embodiment of the present invention, step S5 specifically includes:

[0037] S501: Reads the integer array N of the RF pulse waveform file to be predicted and the user-input flip angle θ. x ;

[0038] S502: Read an RF gain preset value A, and convert the integer array N into a physical quantity B1 with the unit μT:

[0039]

[0040] Where k is the radio frequency conversion coefficient;

[0041] S503: The evolution curve of the magnetization vector under arbitrary radio frequency pulse waveforms is obtained by using the Cayley-Klein parameter method;

[0042] S504: Calculate the flip angle evolution curve based on the magnetization vector evolution curve and perform unwinding;

[0043] S505: Calculate the flip angle under the preset RF gain based on the flip angle evolution curve after unwinding.

[0044] In a preferred embodiment of the present invention, step S6 specifically includes:

[0045] S601: Change multiple RF gain preset values ​​A j Repeat step S5 to iteratively calculate the flip angle θ corresponding to these preset values. j ;

[0046] S602: According to the following formula (10) based on the preset A j and the calculated θ j Predict the target flip angle θ input by the user. x The corresponding RF gains are averaged to obtain the RF gain value A required for the target flip angle. x ,

[0047]

[0048] Where mean j (·) indicates the operation of averaging over all index j.

[0049] To solve the above-mentioned technical problems, another technical solution adopted by the present invention is: to provide a magnetic resonance radio frequency gain calibration system based on signal phase, comprising:

[0050] The pre-scan measurement module is used to design the waveform of the Fermi pulse, modify the pre-scan sequence, and measure the RF conversion coefficient determined by the hardware system.

[0051] The RF gain calculation module is used to read the RF pulse file to be calibrated and the user-defined target flip angle, and calculate the required RF gain value.

[0052] In a preferred embodiment of the present invention, the pre-scan measurement module includes:

[0053] The Fermi pulse design unit is used to design a Fermi pulse, which applies an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculates the corresponding radio frequency amplitude.

[0054] The pre-scan sequence design unit is used to design pre-scan sequences for one-time data acquisition.

[0055] The maximum RF amplitude measurement unit is used to measure the actual maximum RF amplitude of the Fermi pulse under a fixed RF gain from the collected data.

[0056] The radio frequency conversion coefficient calculation unit is used to calculate the radio frequency conversion coefficient of the radio frequency system at this time based on the actual maximum radio frequency amplitude measured by the maximum radio frequency amplitude measurement unit and the maximum integer value of the Fermi pulse waveform file.

[0057] In a preferred embodiment of the present invention, the radio frequency gain calculation module includes a magnetization vector calculation unit, a flip angle calculation unit, and an iterative update calculation unit;

[0058] The magnetization vector calculation unit is used to calculate the magnetization vector evolution curve generated by the input radio frequency pulse;

[0059] The flip angle calculation unit is used to calculate the flip angle based on the magnetization vector evolution curve obtained by the magnetization vector calculation module;

[0060] The iterative update calculation unit is used to iteratively update and calculate based on multiple preset RF gain values ​​and corresponding flip angles to predict the RF gain value required for the target flip angle.

[0061] The beneficial effects of this invention are:

[0062] (1) This invention utilizes the signal phase information under Fermi pulse partial resonance excitation to measure the radio frequency conversion coefficient. It is not affected by the background magnetic field inhomogeneity, tissue signal attenuation and other environmental factors. It can achieve a measurement accuracy of 0.01dB and the predicted radio frequency amplitude range can cover 0-30uT, which fully covers the design and application range of magnetic resonance radio frequency coils.

[0063] (2) The present invention can complete the acquisition of all data within 100ms without the need for iterative scanning process, thus effectively avoiding the measurement error of the radio frequency conversion coefficient introduced by motion and iterative scanning, and ensuring the accuracy of the calibration effect. Attached Figure Description

[0064] Figure 1 This is a three-dimensional structural schematic diagram of a preferred embodiment of the magnetic resonance radio frequency gain calibration method based on signal phase of the present invention;

[0065] Figure 2 This is a schematic diagram of the pre-scan sequence based on Fermi pulses;

[0066] Figure 3 This is a block diagram of the magnetic resonance radio frequency gain calibration system based on signal phase. Detailed Implementation

[0067] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0068] Please see Figure 1 The embodiments of the present invention include:

[0069] A magnetic resonance radio frequency gain calibration method based on signal phase includes the following steps:

[0070] S1: Design a Fermi pulse to apply an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculate the corresponding radio frequency amplitude;

[0071] normalized Fermi pulse waveform RF uni (t) is shown below, where t represents the time variable (t = 0 is the center position of the pulse), t0 and a are two characteristic parameters that control the profile characteristics of the Fermi pulse layer selection. t0 is a parameter used to measure the width of the Fermi pulse, and a is a parameter used to measure the width of the Fermi pulse transition band. When a approaches 0, the Fermi pulse is close to a constant rectangular pulse.

[0072]

[0073] To improve the layer selection characteristics of the Fermi pulse and avoid the excitation of resonant signals, the Fermi pulse duration T, derived using a 60dB attenuation definition, satisfies the following relationship:

[0074] T=2t0+13.81a#[2]

[0075] First, the ratio of t0 / a is preset, then t0 and a are determined by the duration T of the Fermi pulse, and finally the Fermi pulse waveform is calculated by equation (1).

[0076] In a specific embodiment of this invention, the ratio of t0 / a is 16, and T is 8ms. After designing the normalized Fermi pulse, the waveform curve is discretized into integers between [0, 2^17] and saved as the radio frequency pulse file required by the magnetic resonance spectrometer.

[0077] S2: Pre-scanning involves acquiring data in a single step and designing the pre-scanning sequence;

[0078] The pre-scan sequence used in this invention is based on a sequence structure of free decaying signals, such as... Figure 2 As shown, the specific steps include:

[0079] S201: In accordance with the scanning time sequence, an excitation pulse with layer selection characteristics and a gradient magnetic field are applied simultaneously as a layer selection gradient to excite the magnetic resonance signal of a certain layer.

[0080] S202: Apply the Fermi pulse preset in step S1 and set it to partial resonant excitation, with the partial resonant frequency amplitude being ω. RF ;

[0081] S203: After applying the Fermi pulse, data acquisition of the free decay signal is performed. The polarity of the off-resonance frequency of the Fermi pulse is modified and scanned twice. The data obtained are ksp. + With KSP - .

[0082] In a specific embodiment of the present invention, the scanning parameters used are: echo time = 8ms, sequence repetition time = 50ms, number of data points acquired per session = 128, and the off-resonance frequency ±ω of the Fermi pulse. rf =±4kHz, and the RF gain of both the excitation pulse and the Fermi pulse is fixed at 15dB.

[0083] Introducing Fermi pulses to measure the RF conversion coefficient can effectively improve the accuracy and stability of the measurement results, thereby enhancing the final RF gain calibration effect. Because Fermi pulses are used, the required information can be directly extracted from two scans, eliminating the need for multiple iterations during data acquisition and allowing for a one-time acquisition process. This also accelerates the entire pre-scan calibration process.

[0084] S3: Measure the actual maximum RF amplitude of the Fermi pulse under a fixed RF gain from the collected data; specific steps include:

[0085] S301: Ksp of the data collected twice. + With KSP - Perform a one-dimensional inverse Fourier transform to the image domain, and take the first complex data point, which corresponds to the center of the image domain, for subsequent calculations:

[0086] I ± =sort1(IPT(ksp) ± ))#[3]

[0087] Here, sort1(·) represents the operation of taking the first point of the data, and iFT(·) represents the inverse Fourier transform;

[0088] S302: The actual maximum RF amplitude B1 of the Fermi pulse under the set RF gain is calculated by the following formula (4):

[0089]

[0090] The calculated radio frequency amplitude B1 is in microtesla (μT), angle(·) represents the operation of taking the amplitude angle of the complex data, and the Fermi pulse constant K is... RF It is the value calculated based on the pulse parameters according to the following formula (5):

[0091]

[0092] Where γ = 42.57, Δt is the time interval (in milliseconds) between each data point of the RF waveform, and ω RF It is the frequency offset of the Fermi pulse (in kHz), RF uni (i) is the value of the i-th point after Fermi pulse normalization.

[0093] In a specific embodiment of this invention, the Fermi pulse parameters used are: the ratio of t0 / a is 16, T is 8 ms, and the calculated constant is K. RF =74.49.

[0094] S4: Calculate the RF conversion coefficient of the RF system at this time based on the actual maximum RF amplitude measured in step S3 and the maximum integer value of the Fermi pulse waveform file;

[0095] After measuring the actual maximum RF amplitude B1 (in μT) of the Fermi pulse under a fixed RF gain A, the RF conversion coefficient k of the RF system is calculated using the following formula based on the maximum integer value N_max of the Fermi pulse waveform file:

[0096] k = A - 20log 10 N_max+20log 10 B1#[6]

[0097] In a specific embodiment of the present invention, the fixed RF gain A is 15dB, and the maximum integer value N_max of the Fermi pulse waveform file is 2. 17 When B1 = 7.3 μT, the RF conversion coefficient k of the RF system is calculated to be -70 dB.

[0098] S5: Predict the flip angle of any RF pulse at a preset RF gain value; specific steps include:

[0099] S501: Reads the integer array N of the RF pulse waveform file to be predicted and the user-input flip angle θ. x ;

[0100] S502: Read an RF gain preset value A, and convert the integer array N into a physical quantity B1 with the unit μT:

[0101]

[0102] Where k is the radio frequency conversion coefficient;

[0103] S503: The evolution curve of the magnetization vector under arbitrary radio frequency pulse waveforms is obtained by using the Cayley-Klein parameter method;

[0104] The magnetization vector evolution curve was calculated using the Cayley-Klein parameter method. First, the Cayley-Klein parameters at the resonant frequency corresponding to the B1 value at the j-th time point were calculated: (α j ,β j Then, by solving the Bloch equation, the magnetization vector (M) that varies with time t during the radio frequency application process is obtained. x (t), M y (t), M z The evolution curve of (t)).

[0105] S504: Calculate the flip angle evolution curve based on the magnetization vector evolution curve and perform unwinding;

[0106] The flip angle at each moment is calculated using the following formula, and the final moment t is taken. end The corresponding value is the final flip angle under the action of this radio frequency pulse.

[0107] θ(t)=unwrap(arccos(M z (t end )))#[8]

[0108] Here, `unwrap(·)` represents the phase unwrapping operation, and `arccos(·)` represents the inverse cosine operation. The specific implementation of the phase unwrapping operation is as follows:

[0109] 1. Starting from the first time point, iterate through each time point and determine whether "entanglement" occurs. The criterion is: the first derivative of the flip angle evolution curve passes through a zero point, and at that time point t... j The two magnetization vector components (M) x (t j M y (t j Any one of them undergoes a sign change;

[0110] 2. If the problem is determined to be "entanglement", then perform the untangling operation as shown in the following formula and return to step 1, where round(·) represents the rounding operation:

[0111]

[0112] Where θ represents the flip angle before unwinding, θ unwrap Indicates the flip angle after unwinding;

[0113] 3. If it is not determined to be an "entanglement phenomenon", then go directly back to step 1.

[0114] S505: Calculate the flip angle under the preset RF gain based on the flip angle evolution curve after unwinding.

[0115] S6: The RF gain value required to predict the target flip angle. Specific steps include:

[0116] S601: Change multiple RF gain preset values ​​A j Repeat step S5 to iteratively calculate the flip angle θ corresponding to these preset values. j That is, to obtain a set (A) j θ j The value of ) is updated iteratively to ensure that the algorithm of this invention can be more stable and effectively applied to various complex situations.

[0117] S602: According to the following formula (10) based on the preset A j and the calculated θ j Predict the target flip angle θ input by the user. x The corresponding RF gains are averaged to obtain the RF gain value A required for the target flip angle. x :

[0118]

[0119] Where mean j (·) indicates the operation of averaging over all index j.

[0120] It should be noted that in this step, only one arbitrary RF gain preset value A is used. j The required RF gain for the target flip angle can also be predicted. Multiple preset RF gain values ​​are used here to increase the stability of the calculation results. The principle for selecting the preset values ​​is to cover a wide range as much as possible, while avoiding excessive deviation from the predicted RF gain value. Empirically, the RF pulse gain range used in actual magnetic resonance scanning is generally between [0, 60 dB], therefore the preset values ​​should also be selected within this range. In a specific embodiment of this invention, the preset RF gain values ​​used are [15, 20, 25, 30, 35] dB.

[0121] See Figure 3 This invention also provides a magnetic resonance radio frequency gain calibration system based on signal phase, including a pre-scan measurement module and a radio frequency gain calculation module.

[0122] The pre-scan measurement module is used to design the waveform of the Fermi pulse, modify the pre-scan sequence, and measure the RF conversion coefficient determined by the hardware system.

[0123] The RF gain calculation module is used to read the RF pulse file to be calibrated and the user-defined target flip angle, and calculate the required RF gain value.

[0124] Specifically, the pre-scan measurement module includes:

[0125] The Fermi pulse design unit is used to design a Fermi pulse, which applies an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculates the corresponding radio frequency amplitude.

[0126] The pre-scan sequence design unit is used to design pre-scan sequences for one-time data acquisition.

[0127] The maximum RF amplitude measurement unit is used to measure the actual maximum RF amplitude of the Fermi pulse under a fixed RF gain from the collected data.

[0128] The radio frequency conversion coefficient calculation unit is used to calculate the radio frequency conversion coefficient of the radio frequency system at this time based on the actual maximum radio frequency amplitude measured by the maximum radio frequency amplitude measurement unit and the maximum integer value of the Fermi pulse waveform file.

[0129] Specifically, the RF gain calculation module includes a magnetization vector calculation unit, a flip angle calculation unit, and an iterative update calculation unit;

[0130] The magnetization vector calculation unit is used to calculate the magnetization vector evolution curve generated by the input radio frequency pulse;

[0131] The flip angle calculation unit is used to calculate the flip angle based on the magnetization vector evolution curve obtained by the magnetization vector calculation module;

[0132] The iterative update calculation unit is used to iteratively update and calculate based on multiple preset RF gain values ​​and corresponding flip angles to predict the RF gain value required for the target flip angle.

[0133] The system described in this invention first designs the waveform of the Fermi pulse, modifies the pre-scan sequence, measures the RF conversion coefficient determined by the hardware system, converts the arbitrary dimensionless RF pulse waveform file into the units of actual physical quantities, solves the magnetization vector evolution curve under the arbitrary RF pulse waveform using Cayley-Klein parameters, and finally calculates the RF gain required for the target flip angle.

[0134] To address the shortcomings of existing signal amplitude-based methods that are susceptible to hardware environment influences, this invention indirectly measures the RF conversion coefficient of an RF system using more stable signal phase information. The method adds a cumulative phase difference to the signal by applying an additional Fermi pulse, which is only related to the actual RF intensity of the Fermi pulse, thus ensuring the accuracy of the RF conversion coefficient measurement.

[0135] To address the shortcomings of existing signal amplitude-based methods, which require iterative scanning and are susceptible to the influence of moving parts, this invention optimizes the pre-scan sequence to complete all data acquisition within 100ms, and subsequent data processing can be completed within 0.5s, ensuring a fast and stable calibration effect.

[0136] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A magnetic resonance radio frequency gain calibration method based on signal phase, characterized in that, Includes the following steps: S1: Design a Fermi pulse to apply an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculate the corresponding radio frequency amplitude; S2: Pre-scanning involves acquiring data in a single step and designing the pre-scanning sequence; S3: Measure the actual maximum radio frequency amplitude of the Fermi pulse under a fixed radio frequency gain from the collected data; S4: Calculate the RF conversion coefficient of the RF system at this time based on the actual maximum RF amplitude measured in step S3 and the maximum integer value of the Fermi pulse waveform file; S5: Predicts the flip angle of any RF pulse at a preset RF gain value; S6: Based on multiple preset RF gain values ​​and corresponding flip angles, predict the RF gain value required for the target flip angle.

2. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, In step S1, the waveform RF of the normalized Fermi pulse is... uni (t) is shown in equation (1) below. Where t represents the time variable, t=0 is the center position of the pulse, t0 is a parameter used to measure the width of the Fermi pulse, and a is a parameter used to measure the width of the Fermi pulse transition band. First, the ratio t0 / a is preset, and then t0 and a are determined by the Fermi pulse duration. The Fermi pulse duration T satisfies the following relationship: T=2t0+13.81a#[2] Finally, the Fermi pulse waveform is calculated using equation (1).

3. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, In step S2, the specific steps for designing the pre-scan sequence include: S201: In accordance with the scanning time sequence of the sequence, an excitation pulse with layer selection characteristics and a gradient magnetic field are applied simultaneously as a layer selection gradient to excite the magnetic resonance signal of a certain layer. S202: Apply the Fermi pulse preset in step S1 and set it to partial resonant excitation, with the partial resonant frequency amplitude being ω. RF ; S203: After applying the Fermi pulse, data acquisition of the free decay signal is performed. The polarity of the off-resonance frequency of the Fermi pulse is modified and scanned twice. The data obtained are ksp respectively. + With KSP - .

4. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, The specific steps of step S3 include: S301: Ksp of the data collected twice. + With KSP - Perform a one-dimensional inverse Fourier transform to the image domain, and take the first complex data point, which corresponds to the center of the image domain, for subsequent calculations: I ± =sort1(iFT(ksp ± ))#[3] Here, sort1(·) represents the operation of taking the first point of the data, and iFT(·) represents the inverse Fourier transform; S302: The actual maximum RF amplitude B1 of the Fermi pulse under the set RF gain is calculated by the following formula (4): The calculated RF amplitude B1 is in microtesla (μT), angle(·) represents the operation of taking the amplitude angle of the complex data, and the Fermi pulse constant K is... RF It is the value calculated based on the pulse parameters according to the following formula (5): Where γ = 42.57, Δt is the time interval between each data point of the RF waveform, and ω RF It is the frequency offset of the Fermi pulse, RF uni (i) is the value of the i-th point after Fermi pulse normalization.

5. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, The specific steps of step S4 include: After measuring the actual maximum RF amplitude B1 of the Fermi pulse under a fixed RF gain A, the RF conversion coefficient k of the RF system at this time is calculated according to the maximum integer value N_max of the Fermi pulse waveform file by formula (6). k=A-20log 10 N_max+20log 10 B1#[6]。 6. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, The specific steps of step S5 include: S501: Reads the integer array N of the RF pulse waveform file to be predicted and the user-input flip angle θ. x ; S502: Read an RF gain preset value A, and convert the integer array N into a physical quantity B1 with the unit μT: Where k is the radio frequency conversion coefficient; S503: The evolution curve of the magnetization vector under arbitrary radio frequency pulse waveforms is obtained by using the Cayley-Klein parameter method; S504: Calculate the flip angle evolution curve based on the magnetization vector evolution curve and perform unwinding; S505: Calculate the flip angle under the preset RF gain based on the flip angle evolution curve after unwinding.

7. The magnetic resonance radio frequency gain calibration method based on signal phase according to claim 1, characterized in that, The specific steps of step S6 include: S601: Change multiple RF gain preset values ​​A j Repeat step S5 to iteratively calculate the flip angle θ corresponding to these preset values. j ; S602: According to the following formula (10) based on the preset A j and the calculated θ j Predict the target flip angle θ input by the user. x The corresponding RF gains are averaged to obtain the RF gain value A required for the target flip angle. x : Where mean j (·) indicates the operation of averaging over all index j.

8. A magnetic resonance radio frequency gain calibration system based on signal phase, characterized in that, include: The pre-scan measurement module is used to design the waveform of the Fermi pulse, modify the pre-scan sequence, and measure the RF conversion coefficient determined by the hardware system. The RF gain calculation module is used to read the RF pulse file to be calibrated and the user-defined target flip angle, and calculate the required RF gain value.

9. The magnetic resonance radio frequency gain calibration system based on signal phase according to claim 8, characterized in that, The pre-scan measurement module includes: The Fermi pulse design unit is used to design a Fermi pulse, which applies an additional phase to the magnetic resonance signal through off-resonance modulation, and then calculates the corresponding radio frequency amplitude. The pre-scan sequence design unit is used to design pre-scan sequences for one-time data acquisition. The maximum RF amplitude measurement unit is used to measure the actual maximum RF amplitude of the Fermi pulse under a fixed RF gain from the collected data. The radio frequency conversion coefficient calculation unit is used to calculate the radio frequency conversion coefficient of the radio frequency system at this time based on the actual maximum radio frequency amplitude measured by the maximum radio frequency amplitude measurement unit and the maximum integer value of the Fermi pulse waveform file.

10. The magnetic resonance radio frequency gain calibration system based on signal phase according to claim 8, characterized in that, The radio frequency gain calculation module includes a magnetization vector calculation unit, a flip angle calculation unit, and an iterative update calculation unit; The magnetization vector calculation unit is used to calculate the magnetization vector evolution curve generated by the input radio frequency pulse; The flip angle calculation unit is used to calculate the flip angle based on the magnetization vector evolution curve obtained by the magnetization vector calculation module; The iterative update calculation unit is used to iteratively update and calculate based on multiple preset RF gain values ​​and corresponding flip angles to predict the RF gain value required for the target flip angle.

Images