WO2021248752A1 - 一种用于磁共振成像的梯度涡流补偿方法及系统 - Google Patents

一种用于磁共振成像的梯度涡流补偿方法及系统 Download PDF

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WO2021248752A1
WO2021248752A1 PCT/CN2020/119282 CN2020119282W WO2021248752A1 WO 2021248752 A1 WO2021248752 A1 WO 2021248752A1 CN 2020119282 W CN2020119282 W CN 2020119282W WO 2021248752 A1 WO2021248752 A1 WO 2021248752A1
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eddy current
gradient
time
echo
pulse sequence
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PCT/CN2020/119282
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French (fr)
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徐恺频
杨培强
张英力
周小龙
施群雁
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苏州纽迈分析仪器股份有限公司
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G01R33/5616—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using gradient refocusing, e.g. EPI

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  • This application relates to the field of nuclear magnetic resonance imaging technology, and in particular to a gradient eddy current compensation method and system for magnetic resonance imaging.
  • Gradient pulses also known as pulsed field gradients
  • the change in the actual output waveform of the gradient caused by the eddy current destroys the accuracy of the gradient spatial encoding, causing distortion and artifacts in the image, which seriously affects the quality of magnetic resonance imaging. Therefore, eliminating the influence of gradient eddy currents is very important for improving the quality of MRI images.
  • the current measurement and compensation methods of gradient eddy current rely on external equipment, have low accuracy, and have the disadvantages of complicated operation and long time-consuming.
  • the embodiments of the present application provide a gradient eddy current compensation method and system for magnetic resonance imaging, which solves the problem of the gradient eddy current measurement and compensation method in the prior art, which relies on external equipment, has low accuracy, and has complicated operations. , A time-consuming problem.
  • the embodiment of the application provides a gradient eddy current compensation method for magnetic resonance imaging, including: collecting echo signals of a preset sampling pulse sequence, the preset sampling pulse sequence is used to measure eddy current data; Data processing is performed on the echo signal to obtain eddy current component parameters by calculation; a pre-emphasis waveform is generated according to the eddy current component parameters; eddy current compensation is performed on the magnetic resonance imaging gradient pulse through the pre-emphasis waveform to obtain an expected target gradient waveform.
  • the step of collecting the echo signal of the preset sampling pulse sequence includes: Step S10: Place the test sample in the preset test area, complete the pre-scan, and obtain the correction parameters of the test area; Step S20: Pre-set the observation time, use the observation time, the calibration parameters of the test area, and the preset sampling pulse sequence parameters to run the preset sampling pulse sequence to obtain an echo signal.
  • the preset sampling pulse sequence parameters include: test gradient data, Phase gradient data and phase convergence gradient data, wherein the test gradient data includes: test gradient intensity, test gradient duration, and test gradient direction; step S30: change the test gradient direction, and replace the preset sampling pulse sequence parameters with After changing the preset sampling pulse sequence parameters, step S20 is repeated until the collection of echo signals in all gradient directions is completed.
  • the echo signal includes: a basic echo signal and a reference echo signal,
  • the step of using the observation time, the calibration parameters of the test area, and the preset sampling pulse sequence parameters to run the preset sampling pulse sequence to obtain the echo signal includes: Step 201: Preset the first observation Time; Step 202: use the first observation time, the calibration parameters of the test area and the first preset sampling pulse sequence parameters to run the preset sampling pulse sequence to obtain the basic echo signal; Step 203: sample the first preset The test gradient intensity and test gradient duration in the pulse sequence parameters are set to zero to obtain the second preset sampling pulse sequence parameter, and the preset sampling pulse sequence is run according to the first observation time to obtain the reference echo signal; step 204: The second observation time is preset as the first observation time, and step S202 is returned to until the collection of all echo signals of the preset sampling pulse sequence is completed.
  • the step of using a preset algorithm to perform data processing on the echo signal to obtain eddy current component parameters includes: performing a modulus operation on the echo signal to obtain a modulus echo signal; The main peak of the modulus echo signal is fitted and interpolated regression to the main peak of the modulus echo signal to obtain the phase gathering time of the echo signal; the time from the center of the sampling pulse to the sampling start time is added to the corresponding The focus time is used to obtain the echo time, and the focus time includes: the phase focus time corresponding to the basic echo signal and the reference phase focus time corresponding to the reference echo signal; using the echo time, the phase focus time and the time Calculate the eddy current composition parameters using the polyphasic gradient data.
  • the step of using the echo time, the phase convergence time and the phase convergence gradient data to calculate the eddy current component parameters includes: comparing the phase convergence time within the echo time with the reference phase convergence time The difference in time is determined as the delay time; the eddy current integral value is calculated by using the phasing gradient intensity in the phasing gradient data and the delay time; the echo time, eddy current integral value, and eddy current integral function model are used based on The preset constraint model calculates the eddy current component parameters, and the eddy current component parameters include: the amplitude of the exponential function and the decay time constant of the exponential function.
  • I eddy[m] G rph ( ⁇ rph[m] - ⁇ rph[ref] ),
  • I eddy[m] represents the integral value of the m- th group of eddy currents
  • G rph represents the intensity of the phase-convergence gradient
  • ⁇ rph[m] represents the phase-concentration time of the m- th group
  • ⁇ rph[ref] represents the reference phase-concentration time.
  • the step of using the echo time, the eddy current integral value, and the eddy current integral function model to calculate the eddy current component parameters includes:
  • the eddy current function model is calculated by the following formula:
  • G eddy (t) represents the eddy current function model
  • n represents the number of the exponential component
  • n 1, 2,...,N
  • a [n] represents the magnitude of the exponential function
  • T [n] represents the decay time constant of the exponential function
  • the vortex component parameters are calculated by the following formula:
  • I eddy[m] represents the m-th group of eddy current integral values
  • ⁇ obs[m] represents the m-th group of observation time
  • ⁇ echo[m] represents the m-th group of echo time
  • G eddy (t) represents the eddy current function model.
  • the embodiment of the present application also provides a gradient eddy current compensation system for magnetic resonance imaging, which includes: an acquisition module for acquiring echo signals of a preset sampling pulse sequence; Data processing of the echo signal is performed to calculate the eddy current component parameters; a waveform generation module is used to generate a pre-emphasis waveform according to the eddy current component parameters; a correction module is used to perform eddy current compensation on magnetic resonance imaging gradient pulses through the pre-emphasis waveform , Get the expected target gradient waveform.
  • An embodiment of the present application also provides a computer-readable storage medium that stores computer instructions to execute the gradient eddy current compensation method for magnetic resonance imaging provided by the embodiment of the present application.
  • An embodiment of the present application also provides an electronic device, including: a memory and a processor, the memory and the processor are communicatively connected to each other, the memory is stored with computer instructions, and the processor executes the The computer instructions are used to execute the gradient eddy current compensation method for magnetic resonance imaging provided by the embodiments of the present application.
  • the gradient eddy current compensation method and system for magnetic resonance imaging collects the echo signal of a preset sampling pulse sequence, uses a preset algorithm to perform data processing on the echo signal, and calculates the eddy current component parameters.
  • Magnetic resonance imaging gradient pulse is used for gradient eddy current compensation to obtain the expected target gradient waveform; no additional hardware equipment is required during the test and compensation process, and noise has little effect on determining the echo position, and the method of echo position analysis data is anti-noise Strong ability, and not easily affected by the main magnetic field and radio frequency field instability, high robustness; convenient operation, fast signal acquisition speed, reliable data processing results, and can efficiently and accurately realize gradient eddy current measurement and compensation .
  • the gradient eddy current compensation method and system for magnetic resonance imaging provided by this application do not have strict requirements on the shape and material of the test sample, only a single-component liquid sample with a long relaxation time; no need to accurately measure the position of the sample Once the sample is placed, there is no need to move its position again until the eddy current measurement in all gradient directions is completed; all the protons of the entire sample participate in the signal contribution, the signal amount is large, and the signal-to-noise ratio is high, which effectively solves the cumbersome operation of the traditional data acquisition process.
  • the time-consuming and long-term problems overcome the problem that the data analysis results of traditional methods are easily affected by uncertain factors such as noise.
  • FIG. 1 is a flowchart of a traditional gradient eddy current compensation method for magnetic resonance imaging provided in an embodiment of the application;
  • FIG. 2 is a schematic diagram of a signal phase method provided by an embodiment of the application.
  • FIG. 3 is a schematic diagram of a gradient echo phase method provided by an embodiment of the application.
  • FIG. 4 is a schematic diagram of a sampling pulse sequence provided by an embodiment of the application.
  • FIG. 5 is a flowchart of a specific example of running a preset sampling pulse sequence provided by an embodiment of the application.
  • FIG. 6 is a schematic diagram of the principle of the reference echo signal not affected by the eddy current and the basic echo signal affected by the eddy current provided by an embodiment of the application;
  • FIG. 7 is a schematic diagram of a modulus echo signal provided by an embodiment of the application.
  • FIG. 8 is a schematic diagram of main peak fitting and interpolation regression provided by an embodiment of the application.
  • Fig. 9 is a fitting effect diagram of the eddy current integral provided by an embodiment of the application.
  • FIG. 10 is a schematic diagram of the module composition of a gradient eddy current compensation system for magnetic resonance imaging provided by an embodiment of the application;
  • FIG. 11 is a composition diagram of a specific example of a computer device provided by an embodiment of the application.
  • Nuclear magnetic resonance can achieve non-destructive (non-invasive) detection, quantitative detection, and multi-modal detection. Based on these advantages, nuclear magnetic resonance technology and its applications have received extensive attention and will continue to develop for a long time in the future. And the signal acquisition of nuclear magnetic resonance is realized by pulse sequence, where pulse mainly refers to radio frequency pulse and gradient pulse.
  • the gradient pulse also known as pulsed field gradient, can make the magnetic field change linearly with space, so that the nuclei at different positions in the magnetic field are at different resonance frequencies. It is the key link of magnetic resonance imaging.
  • Gradient pulse The input waveform of is generally rectangular or trapezoidal, but the actual output cannot be satisfied.
  • the metal parts around the gradient coil will generate induced currents that hinder the change of the magnetic field, and form a vortex-shaped closed loop, that is, eddy current, or eddy current for short.
  • the change of the actual output waveform of the gradient caused by the eddy current destroys the accuracy of the gradient space encoding, and causes distortions and artifacts in the image.
  • the eddy current changes with time in the form of multi-exponential decay.
  • the gradient eddy current compensation method for magnetic resonance imaging specifically includes:
  • Step S1 Collect echo signals of a preset sampling pulse sequence, where the preset sampling pulse sequence is used to measure eddy current data.
  • the echo signal of a preset sampling pulse sequence is collected, where the preset sampling pulse sequence is used to measure eddy current data, and various parameters of the preset sampling pulse sequence are set, including a set of acquisition after setting the observation time ⁇ obs Echo signal, change ⁇ obs and collect again until all signals are collected.
  • ⁇ obs is taken at logarithmic intervals from 0.1 to 1000 milliseconds.
  • ⁇ obs is set to 1000 milliseconds, it can be considered that the eddy current of the test gradient has been completely attenuated, so This group of echo signals can be used as reference echo signals at the same time.
  • the current gradient eddy current measurement method is shown in Figure 2.
  • the signal phase method can calculate the phase change rate of the signal by turning off the gradient switch and waiting for a certain time ( ⁇ ) to collect the magnetic resonance signal.
  • the above-mentioned signal phase method has cumbersome operations ( The test process needs to change the position of the sample and accurately position the sample), the signal is small, and the accuracy is low; as shown in Figure 3, the gradient echo phase method turns off the gradient switch, waits for a certain time ( ⁇ ), and then uses the radio frequency pulse to excite Signal, apply a pair of gradient pulses with opposite polarity to collect the echo signal, and perform Fourier transform on the collected echo signal,
  • the echo peak appears earlier than expected, causing the true echo time to be smaller than the sequence set echo time, and the larger the vortex, the earlier the echo peak appears, and the smaller the true echo time. Therefore, the vortex measured by this method is less than the true value.
  • the larger the value the greater the deviation of this method;
  • the eddy current measured by this method is the geometric mean value of the eddy current in the entire echo time, not the eddy current size at the time of observation, and the true eddy current level is underestimated.
  • the gradient eddy current compensation method for magnetic resonance imaging does not require the support of additional hardware equipment, such as coils, circuits, digital signal receivers, etc.; it has strong noise immunity and is not susceptible to instability of the main magnetic field and radio frequency field The impact of sex and other issues.
  • the preset sampling pulse sequence in the implementation of this application is set according to the actual MRI system. In actual applications, the preset sampling pulse sequence can also be modified according to actual image requirements, and this application The embodiment only exemplifies the 8 sets of collected signals and the method for determining the reference echo signal, and the application is not limited thereto.
  • Step S2 Use a preset algorithm to perform data processing on the echo signal, and calculate the eddy current component parameters.
  • the echo signal includes: a basic echo signal and a reference echo signal
  • a preset algorithm is used to perform data processing on the echo signal.
  • the process of calculating the eddy current component parameters is as follows: first perform the modulus operation on the collected echo signal, and then perform fitting and interpolation regression on the main peak of the modulus echo signal obtained after the modulus operation to obtain a high-resolution Convergence time; after that, the echo time ⁇ echo is calculated through the convergence time and the integrated value I eddy of the test gradient eddy current in the echo time is calculated; taking the above 8 groups of echo signals as an example, the convergence time of the reference signal is equal to the 8th In the vortex integral function model of the phase gathering time of the group signal, all the other variables are determined except for the amplitude A and the decay time constant T of the exponential function to be solved.
  • the collected echo signals are 8 sets of data, and the methods for calculating the phase gathering time, echo time, and solving the eddy current integral function can be solved in multiple ways. This application Not limited to this.
  • Step S3 Generate a pre-emphasis waveform according to the eddy current component parameters.
  • the component parameters are input to the pre-emphasis module, and the pre-emphasis waveform is generated to realize eddy current compensation.
  • Step S4 Perform eddy current compensation on the magnetic resonance imaging gradient pulse through the pre-emphasis waveform to obtain the expected target gradient waveform.
  • pre-emphasis is a technology to solve the eddy current from the input terminal of the gradient pulse.
  • an additional “overcurrent” is input through the pre-emphasis unit of the gradient coil to form a distorted gradient waveform. If the distorted part is the same amplitude and opposite to the eddy current, the two can cancel each other out, and finally the expected gradient waveform is obtained.
  • setting the pre-emphasis parameters of the magnetic resonance equipment can generate the corresponding overcurrent components, flexibly realize the eddy current compensation, and finally obtain the expected target gradient waveform, avoiding the change of the actual output waveform of the gradient caused by the eddy current, and destroying the gradient space
  • the accuracy of encoding causes distortion and artifacts in the image, which seriously affects the quality of magnetic resonance imaging.
  • the gradient eddy current compensation method for magnetic resonance imaging collects echo signals of a preset sampling pulse sequence, uses a preset algorithm to perform data processing on the echo signals, and calculates the eddy current component parameters to obtain the gradient of magnetic resonance imaging.
  • the pulse is subjected to gradient eddy current compensation to obtain the expected target gradient waveform; no additional hardware equipment is required during the test and compensation process, and noise has little effect on determining the echo position.
  • the method of analyzing the data through the echo position has a strong anti-noise ability. It is not easily affected by the instability of the main magnetic field and radio frequency field, and has high robustness; it is easy to operate, fast in signal acquisition, reliable in data processing, and can efficiently and accurately realize gradient eddy current measurement and compensation.
  • step S1 may specifically include the following steps:
  • Step S10 Place the test sample in the preset test area, complete the pre-scan, and obtain the calibration parameters of the test area.
  • pure water is preferred as the test sample.
  • routine pre-scan operations including finding the center frequency, shimming, finding the RF pulse power and 90° pulse width are performed to complete the pre-scan and obtain the test area
  • the parameters are corrected to better eliminate noise interference for subsequent tests, thereby achieving the purpose of optimizing the preset sampling pulse sequence; among them, pure water is preferred as the test sample because there are no strict requirements on the shape and material of the test sample in the embodiments of this application.
  • Step S20 Pre-set the observation time, use the observation time, the calibration parameters of the test area and the preset sampling pulse sequence parameters to run the preset sampling pulse sequence to obtain the echo signal.
  • the preset sampling pulse sequence parameters include: test gradient data, scatter Phase gradient data and polyphasic gradient data, where the test gradient data includes: test gradient strength, test gradient duration, and test gradient direction.
  • the magnetic resonance signal is acquired by the sampling pulse sequence shown in FIG. 4.
  • the preset sampling pulse sequence parameters are first set, for example, the test gradient direction, such as x, y, or z; the sampling interval or Spectral width (the two are the reciprocal of each other); set the RF pulse width used for signal acquisition; set the test gradient strength G test , the duration ⁇ test can be set to a larger value, such as 20 milliseconds; set the defocused gradient strength G dph and dephasing time ⁇ dph ; set the phase gradient intensity G rph ; and the signal acquisition starts from the dephasing gradient pulse closed, that is, the phase phase gradient pulse is turned on, and continues until the complete echo signal can be observed.
  • the gradient pulse is always on during the signal acquisition period until it is turned off at the end of the signal acquisition.
  • the position of the maximum value is the time when the echo appears, from the beginning of the signal acquisition to the time when the echo appears. This period of time is the gathering time ⁇ rph .
  • Pre-set the observation time ⁇ obs that is, the time from when the test gradient pulse is turned off to the center of the RF pulse, and run the pulse sequence to collect the magnetic resonance signal to obtain the echo signal.
  • the embodiment of the present application only exemplifies the preset sampling pulse sequence parameters that are set, where the preset sampling pulse sequence parameters include: test gradient data, dephasing gradient data, and polyphasing gradient data, and the test gradient data includes: Test gradient intensity, test gradient duration and test gradient direction; dephasic gradient data includes: dephasic gradient intensity, dephasic gradient duration and dephasic gradient direction; polyphasic gradient data includes: polyphasic gradient intensity, polyphasic gradient duration Time and phase convergence gradient direction; the preset sampling pulse sequence parameters can also be adjusted and increased according to actual needs, and this application is not limited to this.
  • Step S30 Change the test gradient direction, replace the preset sampling pulse sequence parameters with the changed preset sampling pulse sequence parameters, and repeat step S20 until the collection of echo signals in all gradient directions is completed.
  • step S20 may specifically include the following steps:
  • Step S201 preset the first observation time.
  • Step S202 Using the first observation time, the calibration parameters of the test area, and the first preset sampling pulse sequence parameters, run the preset sampling pulse sequence to obtain a basic echo signal.
  • the first observation time, the calibration parameter of the test area, and the first preset sampling pulse sequence parameter are used to run the preset sampling pulse sequence to obtain the basic echo signal.
  • the basic echo signal in the embodiment of the present application is to set the correction parameters of the test area and the first preset sampling pulse sequence parameter within the first observation time, and then run the preset sampling pulse sequence to obtain the echo
  • the wave signal is used as the basic echo signal.
  • Step S203 Set the test gradient intensity and test gradient duration in the first preset sampling pulse sequence parameter to zero to obtain the second preset sampling pulse sequence parameter, and run the preset sampling pulse sequence according to the first observation time to obtain a reference Echo signal.
  • a set of signals that are not affected by the test gradient eddy current are collected as the reference echo signal, and the phase gathering time is ⁇ rph[ref] , as shown in Fig. 6, where the first preset sampling pulse
  • the test gradient intensity and test gradient duration in the sequence parameters are set to zero to obtain the second preset sampling pulse sequence parameter, and the preset sampling pulse sequence is run according to the first observation time to obtain the reference echo signal.
  • other methods may be used to obtain the reference echo signal, that is, the echo signal that is not affected by the eddy current, according to actual system functional requirements, and this application is not limited to this.
  • Step S204 preset the second observation time as the first observation time, and return to perform step S202 until the collection of all echo signals of the preset sampling pulse sequence is completed.
  • the first observation time is changed and the second observation time is used instead of the first observation time, that is , the size of ⁇ obs is changed and the above step S202 is repeated, and a total of M signal acquisitions are performed to obtain M groups of echo signals.
  • Let m represent the serial number of the collection, and ⁇ rph[m] represents the phase gathering time of the m-th signal collection.
  • the magnetic resonance signal collected by the method of the embodiment of the application has the echo appearing position affected by The influence of the eddy current generated by the test gradient pulse has shifted.
  • step S2 may specifically include the following steps:
  • Step S21 Perform a modulo operation on the echo signal to obtain a modulo echo signal.
  • the acquired echo signal of the preset sampling pulse sequence is subjected to the modulus operation to obtain the modulus echo signal, which mainly refers to the complex number of the echo signal of the acquired magnetic resonance signal
  • the signal is modulo.
  • Step S22 Obtain the main peak of the modulus echo signal, and perform fitting and interpolation regression on the main peak of the modulus echo signal to obtain the phase gathering time of the echo signal.
  • the main peak of the modulus echo signal is obtained, and the main peak of the modulus echo signal is fitted and interpolated regression to obtain the phase gathering time of the echo signal.
  • the main peak of the modulus echo signal is fitted and interpolated regression to obtain the phase gathering time of the echo signal.
  • the main echo peak and interpolating regression can get a more accurate phase gathering time, where a polynomial is used to fit the main echo peak, when the degree of the polynomial is not less than 3 When, the results obtained are more accurate and reliable.
  • interpolation regression can be performed based on the polynomial function model to obtain a high-precision phase gathering time.
  • phase gathering time can be obtained by fitting the main echo peaks and interpolation regression.
  • it can also be set according to actual experience and system requirements, and The function model used to fit the main echo peak is not unique. It can also be fitted with Lorentz linear function, Gauss linear function, extended exponential distribution function, etc., and this application is not limited to this. .
  • Step S23 The echo time is obtained by adding the time from the center of the sampling pulse to the sampling start time and the corresponding phase gathering time, where the phase gathering time includes: the phase gathering time corresponding to the basic echo signal and the reference phase gathering corresponding to the reference echo signal time.
  • Step S24 Calculate eddy current component parameters using echo time, phase convergence time and phase convergence gradient data.
  • step S24 may specifically include the following steps:
  • Step S241 Determine the difference between the phase gathering time and the reference phase gathering time within the echo time as the delay time.
  • the echo time ⁇ echo[m] of this group of signals is the time from the center of the sampled radio frequency pulse to the start of sampling plus its corresponding phase gathering time ⁇ rph[ m] , in this echo time, the sum of the integral values of all gradients and their vortices over time is zero.
  • the phase gathering time is ⁇ rph[ref] , and there is no eddy current generated by the test gradient pulse in the entire echo time. Since the dephasing gradient is in the same direction as the test gradient, and the phase gathering gradient is in the opposite direction, so for The echo time of the test signal is delayed due to the superposition of eddy currents. During this delay, the phase-focusing gradient pulse has played a role in making up for the eddy currents.
  • Step S242 Use the phase-concentration gradient intensity and the delay time in the phase-concentration gradient data to calculate the eddy current integral value.
  • the vortex integral value is calculated by the following formula by using the intensity and delay time of the phase phase gradient data in the phase phase gradient data:
  • I eddy[m] G rph ( ⁇ rph[m] - ⁇ rph[ref] ) (1)
  • I eddy[m] represents the integral value of the m- th group of eddy currents
  • G rph represents the intensity of the phase-convergence gradient
  • ⁇ rph[m] represents the phase-concentration time of the m- th group
  • ⁇ rph[ref] represents the reference phase-concentration time.
  • Step S243 Using the echo time, the eddy current integral value, and the eddy current integral function model, the eddy current component parameters are calculated based on the preset constraint model.
  • the eddy current component parameters include: the amplitude of the exponential function and the decay time constant of the exponential function.
  • the echo time and the eddy current integral value are used to calculate the eddy current function model by the following formula:
  • G eddy (t) represents the eddy current function model
  • n represents the number of the exponential component
  • n 1, 2,...,N
  • a [n] represents the magnitude of the exponential function
  • T [n] represents the decay time constant of the exponential function
  • the eddy current component parameters are calculated by the following formula:
  • I eddy[m] represents the m-th group of eddy current integral values
  • ⁇ obs[m] represents the m-th group of observation time
  • ⁇ echo[m] represents the m-th group of echo time
  • G eddy (t) represents the eddy current function model.
  • I eddy[m] represents the m-th group of eddy current integral values
  • ⁇ obs[m] represents the m-th group of observation time
  • ⁇ echo[m] represents the m-th group of echo time
  • G eddy (t) represents the eddy current function model
  • a [n] represents the amplitude of the exponential function
  • T [n] represents the decay time constant of the exponential function.
  • the goal of least squares fitting is to minimize the sum of squares of the distance between the observed value of the eddy current integral and the expected value of the eddy current integral function.
  • the minimization process needs to consider the constraint conditions for the eddy current component amplitude A and the decay time constant T: First, all A must be positive.
  • the embodiment of this application adopts the classic Levenberg-Marquardt nonlinear least squares algorithm combined with the logarithmic barrier function method to efficiently and robustly realize the constraint optimization of formula (4), and solve accurate and reliable eddy current components parameter.
  • the solution method considering the constraint conditions is not unique, it can also be other algorithms, such as gradient descent method, Newton method, trust region method, no matter what algorithm is used to solve formula (4) or its equivalent variants, All calculations can be performed, as long as the purpose of calculation and solution can be achieved, and this application is not limited to this.
  • Figure 7 is the first group of echo signals, that is, the main peak fitting and interpolation regression diagram when ⁇ obs takes 0.1 millisecond, and then the echo time ⁇ echo is calculated through the phase gathering time and the echo time test is calculated The integral value I eddy of the gradient eddy current.
  • the gradient eddy current compensation method for magnetic resonance imaging collects echo signals of a preset sampling pulse sequence, uses a preset algorithm to perform data processing on the echo signals, and calculates the eddy current component parameters to obtain the gradient of magnetic resonance imaging.
  • the pulse is subjected to gradient eddy current compensation to obtain the expected target gradient waveform; no additional hardware equipment is required during the test and compensation process, and noise has little effect on determining the echo position.
  • the method of analyzing the data through the echo position has a strong anti-noise ability. It is not easy to be affected by the instability of the main magnetic field and radio frequency field, and has high robustness; it is convenient to operate, fast in signal acquisition, and reliable in data processing results.
  • the embodiment of the present application also provides a gradient eddy current compensation system for magnetic resonance imaging, as shown in FIG. 10, including:
  • the acquisition module 1 is used to acquire the echo signal of the preset sampling pulse sequence. For details, please refer to the relevant description of step S1 in the foregoing method embodiment, which will not be repeated here.
  • the calculation module 2 is used to perform data processing on the echo signal using a preset algorithm, and calculate the eddy current component parameters. For details, please refer to the relevant description of step S2 in the above method embodiment, which will not be repeated here.
  • the waveform generation module 3 is used to generate the pre-emphasis waveform according to the eddy current component parameters. For details, please refer to the related description of step S3 in the above method embodiment, which will not be repeated here.
  • the correction module 4 is used to perform eddy current compensation on the magnetic resonance imaging gradient pulse through the pre-emphasis waveform to obtain the expected target gradient waveform.
  • step S4 in the above method embodiment, which will not be repeated here.
  • the gradient eddy current compensation system for magnetic resonance imaging collects echo signals of a preset sampling pulse sequence, uses a preset algorithm to perform data processing on the echo signals, and calculates Obtain the eddy current component parameters, and perform gradient eddy current compensation on the magnetic resonance imaging gradient pulse to obtain the expected target gradient waveform; no additional hardware equipment is required during the test and compensation process, and the noise has little effect on determining the echo position, and the echo position is passed
  • the method of analyzing data has strong anti-noise ability, and is not easily affected by the instability of the main magnetic field and radio frequency field, and has high robustness; and is convenient to operate, fast in signal acquisition, reliable in data processing results, and can be implemented efficiently and accurately Gradient eddy current measurement and compensation; there are no strict requirements on the shape and material of the test sample, only a single-component liquid sample with a long relaxation time; no need to accurately measure the position of the sample, once the sample is placed until the eddy
  • An embodiment of the application provides a computer device, as shown in FIG. 11, including: at least one processor 401, such as a CPU (Central Processing Unit, central processing unit), at least one communication interface 403, memory 404, and at least one communication bus 402 .
  • the communication bus 402 is used to implement connection and communication between these components.
  • the communication interface 403 may include a display screen (Display) and a keyboard (Keyboard), and the optional communication interface 403 may also include a standard wired interface and a wireless interface.
  • the memory 404 may be a high-speed RAM memory (Ramdom Access Memory, volatile random access memory), or a non-volatile memory (non-volatile memory), such as at least one disk memory.
  • the memory 404 may also be at least one storage device located far away from the aforementioned processor 401.
  • the processor 401 can execute a gradient eddy current compensation method for magnetic resonance imaging.
  • the memory 404 stores a set of program codes, and the processor 401 calls the program codes stored in the memory 404 to execute the aforementioned gradient eddy current compensation method for magnetic resonance imaging.
  • the communication bus 402 may be a peripheral component interconnection standard (peripheral component interconnect, PCI for short) bus or an extended industry standard architecture (EISA for short) bus, etc.
  • the communication bus 402 can be divided into an address bus, a data bus, a control bus, and so on. For ease of representation, only one line is used to represent in FIG. 11, but it does not mean that there is only one bus or one type of bus.
  • the memory 404 may include volatile memory (English: volatile memory), such as random access memory (English: random-access memory, abbreviation: RAM); the memory may also include non-volatile memory (English: non-volatile memory).
  • volatile memory such as random access memory (English: random-access memory, abbreviation: RAM); the memory may also include non-volatile memory (English: non-volatile memory).
  • memory such as flash memory (English: flash memory), hard disk (English: hard disk drive, abbreviation: HDD) or solid-state hard disk (English: solid-state drive, abbreviation: SSD); memory 404 may also include the above types The combination of memory.
  • the processor 401 may be a central processing unit (English: central processing unit, abbreviation: CPU), a network processor (English: network processor, abbreviation: NP), or a combination of CPU and NP.
  • CPU central processing unit
  • NP network processor
  • the processor 401 may further include a hardware chip.
  • the aforementioned hardware chip may be an application-specific integrated circuit (English: application-specific integrated circuit, abbreviation: ASIC), a programmable logic device (English: programmable logic device, abbreviation: PLD) or a combination thereof.
  • the above-mentioned PLD can be a complex programmable logic device (English: complex programmable logic device, abbreviation: CPLD), field programmable logic gate array (English: field-programmable gate array, abbreviation: FPGA), general array logic (English: generic array) logic, abbreviation: GAL) or any combination thereof.
  • the memory 404 is also used to store program instructions.
  • the processor 401 may call program instructions to implement the gradient eddy current compensation method for magnetic resonance imaging as in the present application.
  • An embodiment of the present application also provides a computer-readable storage medium, and computer-readable storage medium stores computer-executable instructions, and the computer-executable instructions can execute a gradient eddy current compensation method for magnetic resonance imaging.
  • the storage media can be magnetic disks, optical disks, read-only memory (Read-Only Memory, ROM), random access memory (RAM), flash memory (Flash Memory), hard disk (Hard Disk Drive) , Abbreviation: HDD) or solid-state drive (Solid-State Drive, SSD), etc.; the storage medium may also include a combination of the foregoing types of memories.

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Abstract

一种用于磁共振成像的梯度涡流补偿方法及系统，其中方法包括：采集预设采样脉冲序列的回波信号（S1），预设采样脉冲序列用于测量涡流数据；利用预设算法对回波信号进行数据处理，计算得到涡流成分参数（S2）；根据涡流成分参数生成预加重波形（S3）；通过预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形（S4）。本方法的测试与补偿的过程中无需额外硬件设备支持，通过回波位置分析数据的方法抗噪能力较强，稳健性高；操作便捷，信号采集速度快，能准确地实现梯度涡流的测量与补偿。

Description

一种用于磁共振成像的梯度涡流补偿方法及系统
相关申请的交叉引用
本申请要求在2020年06月10日提交中国专利局、申请号为202010524731.1、申请名称为“一种用于磁共振成像的梯度涡流补偿方法及系统”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本申请涉及核磁共振成像技术领域，具体涉及一种用于磁共振成像的梯度涡流补偿方法及系统。
背景技术
梯度脉冲，又称为脉冲场梯度(pulsed field gradient)，可使磁场随空间发生线性变化，进而使磁场中不同位置的原子核处在不同共振频率下，实现检测样品中不同位置原子核的空间编码，是磁共振成像的关键环节。然而涡流所引起的梯度实际输出波形的改变，破坏了梯度空间编码的准确性，导致图像产生畸变和伪影等失真现象，严重影响磁共振成像质量。因此，消除梯度涡流带来的影响对提升磁共振图像质量至关重要。目前梯度涡流的测量与补偿方法，依赖外部设备，精度较低，并且存在操作复杂，耗时长的缺点。
发明内容
有鉴于此，本申请实施例提供了一种用于磁共振成像的梯度涡流补偿方法及系统，解决现有技术中梯度涡流的测量与补偿方法，依赖外部设备，精度较低，并且存在操作复杂，耗时长的问题。
本申请实施例提供了一种用于磁共振成像的梯度涡流补偿方法，包括：采集预设采样脉冲序列的回波信号，所述预设采样脉冲序列用于测量涡流数据；利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数；根据所述涡流成分参数生成预加重波形；通过所述预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。
可选地，所述采集预设采样脉冲序列的回波信号的步骤，包括：步骤S10：将测试样品 放置于预设测试区域，完成预扫描，得到所述测试区域的校正参数；步骤S20：预设置观测时间，利用所述观测时间、测试区域的校正参数及预设采样脉冲序列参数，运行预设采样脉冲序列，得到回波信号，所述预设采样脉冲序列参数包括：测试梯度数据、散相梯度数据及聚相梯度数据，其中所述测试梯度数据包括：测试梯度强度、测试梯度持续时间及测试梯度方向；步骤S30：改变所述测试梯度方向，将预设采样脉冲序列参数替换为改变后的预设采样脉冲序列参数，重复步骤S20，直到完成所有梯度方向的回波信号的采集。
可选地，所述回波信号包括：基础回波信号和参考回波信号，
所述预设置观测时间，利用所述观测时间、测试区域的校正参数及预设采样脉冲序列参数，运行预设采样脉冲序列，得到回波信号的步骤，包括：步骤201：预设置第一观测时间；步骤202：利用第一观测时间、测试区域的校正参数及第一预设采样脉冲序列参数，运行预设采样脉冲序列，得到基础回波信号；步骤203：将所述第一预设采样脉冲序列参数中的测试梯度强度及测试梯度持续时间置为零，得到第二预设采样脉冲序列参数，根据第一观测时间运行预设采样脉冲序列，得到参考回波信号；步骤204：将第二观测时间预设置为第一观测时间，并返回执行步骤S202，直到完成预设采样脉冲序列的所有回波信号的采集。
可选地，所述利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数的步骤，包括：对所述回波信号进行取模值操作，得到模值回波信号；获取所述模值回波信号的主峰，对所述模值回波信号的主峰进行拟合和插值回归，得到回波信号的聚相时间；通过采样脉冲中心到采样开始时刻的时间加上对应的聚相时间得到回波时间，所述聚相时间包括：对应基础回波信号的聚相时间及对应参考回波信号的参考聚相时间；利用所述回波时间、所述聚相时间和所述聚相梯度数据计算涡流成分参数。
可选地，所述利用所述回波时间、所述聚相时间和所述聚相梯度数据计算涡流成分参数的步骤，包括：将回波时间内所述聚相时间与所述参考聚相时间之差确定为延迟时间；利用所述聚相梯度数据中的聚相梯度强度及所述延迟时间，计算得到涡流积分值；利用所述回波时间、涡流积分值、涡流积分函数模型，基于预设约束模型计算得到涡流成分参数，所述涡流成分参数包括：指数函数的幅度及指数函数的衰减时间常数。
可选地，通过以下公式计算涡流积分值：
I eddy[m]＝G rph(δ rph[m]-δ rph[ref])，
其中，I eddy[m]表示第m组涡流积分值，G rph表示聚相梯度强度，δ rph[m]表示第m组聚相时间，δ rph[ref]表示参考聚相时间。
可选地，所述利用所述回波时间、涡流积分值及涡流积分函数模型，计算得到涡流成分参数的步骤，包括：
利用所述回波时间及涡流积分值，通过以下公式计算涡流函数模型：
Figure PCTCN2020119282-appb-000001
其中，G eddy(t)表示涡流函数模型，n表示指数成分的序号，n＝1,2,…,N，A [n]表示指数函数的幅度，T [n]表示指数函数的衰减时间常数；
利用所述观测时间、所述回波时间、所述涡流函数模型及所述涡流积分值，通过以下公式计算涡流成分参数：
Figure PCTCN2020119282-appb-000002
其中，I eddy[m]表示第m组涡流积分值，τ obs[m]表示第m组观测时间，τ echo[m]表示第m组回波时间，G eddy(t)表示涡流函数模型。
本申请实施例还提供了一种用于磁共振成像的梯度涡流补偿系统，包括：采集模块，用于采集预设采样脉冲序列的回波信号；计算模块，用于利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数；波形生成模块，用于根据所述涡流成分参数生成预加重波形；校正模块，用于通过所述预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。
本申请实施例还提供了一种计算机可读存储介质，所述计算机可读存储介质存储计算 机指令，从而执行本申请实施例提供的用于磁共振成像的梯度涡流补偿方法。
本申请实施例还提供了一种电子设备，包括：存储器和处理器，所述存储器和所述处理器之间互相通信连接，所述存储器中存储有计算机指令，所述处理器通过执行所述计算机指令，用于所述计算机指令执行本申请实施例提供的用于磁共振成像的梯度涡流补偿方法。
本申请技术方案，具有如下优点：
1.本申请提供的用于磁共振成像的梯度涡流补偿方法及系统，通过采集预设采样脉冲序列的回波信号，利用预设算法对回波信号进行数据处理，计算得到涡流成分参数，对磁共振成像梯度脉冲进行梯度涡流补偿，得到预期的目标梯度波形；测试与补偿的过程中无需额外硬件设备支持，噪声对确定回波位置产生的影响小，通过回波位置分析数据的方法抗噪能力较强，且不易受主磁场及射频场不稳定性等问题的影响，稳健性高；并且操作便捷，信号采集速度快，数据处理结果可靠，能够高效、准确地实现梯度涡流的测量与补偿。
2.本申请提供的用于磁共振成像的梯度涡流补偿方法及系统，对测试样品的形状、材质没有严格要求，只需单组分、弛豫时间较长的液体样品；无需精确测量样品位置，样品一经放置，直至完成所有梯度方向的涡流测量前，无需再次移动其位置；整个样品所有质子均参与信号贡献，信号量大，信噪比高，有效解决了传统方法数据采集过程操作繁琐、耗时长等问题，克服了传统方法数据分析结果易受噪声等不确定性因素影响的问题。
附图说明
为了更清楚地说明本申请具体实施方式或现有技术中的技术方案，下面将对具体实施方式或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图是本申请的一些实施方式，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。
图1为本申请实施例中提供的传统用于磁共振成像的梯度涡流补偿方法的流程图；
图2为本申请实施例提供的信号相位法的示意图；
图3为本申请实施例提供的梯度回波相位法的示意图；
图4为本申请实施例提供的采样脉冲序列的示意图；
图5为本申请实施例提供的运行预设采样脉冲序列的一个具体示例的流程图；
图6为本申请实施例提供的未受涡流影响的参考回波信号与受涡流影响的基础回波信号原理示意图；
图7为本申请实施例提供的模值回波信号的示意图；
图8为本申请实施例提供的主峰拟合和插值回归的示意图；
图9为本申请实施例提供的涡流积分的拟合效果图；
图10为本申请实施例提供的用于磁共振成像的梯度涡流补偿系统的模块组成示意图；
图11为本申请实施例提供的计算机设备一个具体示例的组成图。
具体实施方式
下面将结合附图对本申请的技术方案进行清楚、完整地描述，显然，所描述的实施例是本申请一部分实施例，而不是全部的实施例。基于本申请中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本申请保护的范围。
此外，下面所描述的本申请不同实施方式中所涉及的技术特征只要彼此之间未构成冲突就可以相互结合。
核磁共振能够做到无损(非侵入)检测、定量检测，以及多模态检测，基于这些优势，核磁共振技术及其应用受到广泛关注，并将在今后很长一段时间内持续发展。并且核磁共振的信号采集通过脉冲序列来实现，这里的脉冲主要指射频脉冲和梯度脉冲。其中梯度脉冲，又称为脉冲场梯度(pulsed field gradient)，可使磁场随空间发生线性变化，进而使磁场中不同位置的原子核处在不同共振频率下，是磁共振成像的关键环节，梯度脉冲的输入波形一般是矩形或梯形，然而实际输出却无法满足。因为梯度磁场在经历瞬态变化过程时，梯度线圈周围的金属部件内部会产生阻碍磁场变化的感应电流，并形成涡旋形的闭合回路，即涡电流，简称涡流(eddy current)。由涡流所引起的梯度实际输出波形的改变，破坏了梯度空间编码的准确性，导致图像产生畸变和伪影等失真现象，涡流随时间的变化表现为多指数衰减的形式，通过设置磁共振设备的预加重参数，可产生相应的过电流成分，灵活地实现涡流补偿。
如图1所示，该用于磁共振成像的梯度涡流补偿方法具体包括：
步骤S1：采集预设采样脉冲序列的回波信号，其中预设采样脉冲序列用于测量涡流数 据。
本申请实施例中，采集预设采样脉冲序列的回波信号，其中预设采样脉冲序列用于测量涡流数据，设置预设采样脉冲序列的各项参数，包括设置观测时间τ obs后采集一组回波信号，改变τ obs再次采集，直到完成所有信号采集。例如共采集了8组不同τ obs条件下的回波信号，其中τ obs从0.1到1000毫秒对数等间隔取值，当τ obs取1000毫秒时，可认为测试梯度的涡流已完全衰减，故该组回波信号可同时作为参考回波信号使用。
目前梯度涡流的测量方法，如图2所示，信号相位法通过关闭梯度开关，等待一定时间(τ)采集磁共振信号，则能够计算出信号的相位变化率，上述信号相位法存在操作繁琐(测试过程需要改变样品位置并对样品精确定位)、信号量小、准确度低的问题；如图3所示，梯度回波相位法通过关闭梯度开关，等待一定时间(τ)后利用射频脉冲激发信号，再施加一对极性相反的梯度脉冲，采集回波信号，对采集到的回波信号进行傅里叶变换，
则涡流的大小可以计算得到，但是梯度回波相位法存在理论漏洞：(1)由于涡流的存在，
回波峰比预期提早出现，导致真实回波时间比序列设置回波时间小，且涡流越大回波峰出现越早，真实回波时间越小，故用该方法测得的涡流小于真值，涡流越大该方法的偏差越大；(2)该方法测得的涡流大小为整个回波时间里涡流的几何平均值，并非观测时刻处的涡流大小，真实涡流水平被低估。本申请实施例提供的用于磁共振成像的梯度涡流补偿方法无需额外硬件设备，如线圈、电路、数字信号接收机等的支持；抗噪能力较强，且不易受主磁场及射频场不稳定性等问题的影响。
需要说明的是，本申请实施中的预设采样脉冲序列是根据实际核磁共振成像系统进行设定的，在实际应用中，也可以根据实际图像需要对预设采样脉冲序列进行修改，并且本申请实施例仅举例说明采集的8组信号以及参考回波信号的确定方法，本申请并不以此为限。
步骤S2：利用预设算法对回波信号进行数据处理，计算得到涡流成分参数。
本申请实施例中，信号采集完毕后，对采集到的回波信号进行数据处理，其中回波信号包括：基础回波信号和参考回波信号，利用预设算法对回波信号进行数据处理，计算得到涡流成分参数的过程为：首先对采集到的回波信号进行取模值操作，然后对取模值操作后得到的模值回波信号的主峰进行拟合和插值回归，得到高分辨的聚相时间；之后通过聚相时间计算出回波时间τ echo并计算回波时间内测试梯度涡流的积分值I eddy；以上述8组回波信号为例，参考信号的聚相时间等于第8组信号的聚相时间，涡流积分函数模型中，除待求解的指数函数的幅度A和衰减时间常数T外，其余变量全部确定。
需要说明的是，本申请实施例仅仅举例说明，采集的回波信号为8组数据，并且计算聚相时间、回波时间以及求解涡流积分函数的方法都可以选择多种方式进行求解，本申请并不以此为限。
步骤S3：根据涡流成分参数生成预加重波形。
本申请实施例中，计算出涡流成分参数后，可准确拟合实际梯度涡流情况，将成分参数输入预加重模块，生成预加重波形实现涡流补偿。
步骤S4：通过预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。
本申请实施例中，通过预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形，其中，预加重(pre-emphasis)是从梯度脉冲输入端解决涡流的技术，是在输入梯度电流时，通过梯度线圈的预加重单元额外输入一个“过电流”，形成失真的梯度波形，失真部分若与涡流幅度相同、方向相反，则两者可相互抵消，最后得到预期的梯度波形。即设置磁共振设备的预加重参数，可产生相应的过电流成分，灵活地实现涡流补偿，最终得到预期的目标梯度波形，避免了由涡流所引起的梯度实际输出波形的改变，破坏了梯度空间编码的准确性，导致图像产生畸变和伪影等失真现象，严重影响磁共振成像质量的问题。
本申请提供的用于磁共振成像的梯度涡流补偿方法，通过采集预设采样脉冲序列的回波信号，利用预设算法对回波信号进行数据处理，计算得到涡流成分参数，对磁共振成像梯度脉冲进行梯度涡流补偿，得到预期的目标梯度波形；测试与补偿的过程中无需额外硬件设备支持，噪声对确定回波位置产生的影响小，通过回波位置分析数据的方法抗噪能力较强，且不易受主磁场及射频场不稳定性等问题的影响，稳健性高；并且操作便捷，信号采集速度快，数据处理结果可靠，能够高效、准确地实现梯度涡流的测量与补偿。
在一具体实施例中，执行步骤S1的过程可以具体包括如下步骤：
步骤S10：将测试样品放置于预设测试区域，完成预扫描，得到测试区域的校正参数。
本申请实施例中，优选纯水作为测试样品，放置样品后进行包括寻找中心频率、匀场、寻找射频脉冲功率与90°脉宽在内的常规预扫描操作，完成预扫描，得到测试区域的校正参数，为后续测试更好的排除噪声干扰，进而达到优化预设采样脉冲序列的目的；其中，优选纯水作为测试样品是因为本申请实施例中对测试样品的形状、材质没有严格要求，只需 单组分、弛豫时间较长的液体样品，因此选用生活中常见的纯水作为测试样品，并且整个样品所有质子均参与信号贡献，信号量大，信噪比高，无需精确测量样品位置，即预设测试区域，样品一经放置，直至完成所有梯度方向的涡流测量前，无需再次移动其位置。
需要说明的是，本申请实施例中，仅仅举例说明完成预扫描后得到的测试区域的校正参数，实际应用还可以包括其他参数，可根据实际精度需求进行计算，本申请并不以此为限。
步骤S20：预设置观测时间，利用观测时间、测试区域的校正参数及预设采样脉冲序列参数，运行预设采样脉冲序列，得到回波信号，预设采样脉冲序列参数包括：测试梯度数据、散相梯度数据及聚相梯度数据，其中测试梯度数据包括：测试梯度强度、测试梯度持续时间及测试梯度方向。
本申请实施例中，利用如图4所示的采样脉冲序列采集磁共振信号，采集前，首先设置预设采样脉冲序列参数，例如设置测试梯度方向，如x、y或z；设置采样间隔或谱宽(两者互为倒数)；设置用于信号采集的射频脉冲宽度；设置测试梯度强度G test，其持续时间δ test可设置为一较大值，例如20毫秒；设置散相梯度强度G dph及散相时间δ dph；设置聚相梯度强度G rph；并且信号采集从散相梯度脉冲关闭，即聚相梯度脉冲开启处开始，一直持续到保证能够观测到完整的回波信号，聚相梯度脉冲在信号采集期间始终开启，直到信号采集结束时关闭，观察采集到的磁共振信号的模值，其最大值所处位置即回波出现时刻，从信号采集开始时刻到回波出现时刻的这段时间即聚相时间δ rph。预设置观测时间τ obs，即从测试梯度脉冲关闭到射频脉冲中心的时间，运行脉冲序列采集磁共振信号，得到回波信号。
需要说明的是，本申请实施例仅仅举例说明设置的预设采样脉冲序列参数，其中预设采样脉冲序列参数包括：测试梯度数据、散相梯度数据及聚相梯度数据，并且测试梯度数据包括：测试梯度强度、测试梯度持续时间及测试梯度方向；散相梯度数据包括：散相梯度强度、散相梯度持续时间及散相梯度方向；聚相梯度数据包括：聚相梯度强度、聚相梯度持续时间及聚相梯度方向；还可以根据实际需要对预设采样脉冲序列参数进行调整与增加，本申请并不以此为限。
步骤S30：改变测试梯度方向，将预设采样脉冲序列参数替换为改变后的预设采样脉冲序列参数，重复步骤S20，直到完成所有梯度方向的回波信号的采集。
在一具体实施例中，如图5所示，执行步骤S20的过程可以具体包括如下步骤：
步骤S201：预设置第一观测时间。
步骤S202：利用第一观测时间、测试区域的校正参数及第一预设采样脉冲序列参数，运行预设采样脉冲序列，得到基础回波信号。
本申请实施例中，利用第一观测时间、测试区域的校正参数及第一预设采样脉冲序列参数，运行预设采样脉冲序列，得到基础回波信号。需要说明的是，本申请实施例中的基础回波信号就是在第一观测时间内，设置测试区域的校正参数及第一预设采样脉冲序列参数，然后运行预设采样脉冲序列，得到的回波信号作为基础回波信号。
步骤S203：将第一预设采样脉冲序列参数中的测试梯度强度及测试梯度持续时间置为零，得到第二预设采样脉冲序列参数，根据第一观测时间运行预设采样脉冲序列，得到参考回波信号。
本申请实施例中，采集一组未受测试梯度涡流影响的信号作为参考回波信号，其聚相时间为δ rph[ref]，如图6所示，其中可以通过将第一预设采样脉冲序列参数中的测试梯度强度及测试梯度持续时间置为零，得到第二预设采样脉冲序列参数，根据第一观测时间运行预设采样脉冲序列，得到参考回波信号，需要说明的是，本申请实施例中，还可以通过不改变测试梯度G test的强度和持续时间，设置观测时间τ obs为一较大值，如1000毫秒，然后采集信号，可认为此时涡流已完全衰减，在实际应用中，还可以根据实际系统功能要求，利用其他方法获取参考回波信号即未受涡流影响的回波信号，本申请并不以此为限。
步骤S204：将第二观测时间预设置为第一观测时间，并返回执行步骤S202，直到完成预设采样脉冲序列的所有回波信号的采集。
本申请实施例中，改变第一观测时间，用第二观测时间代替第一观测时间，即改变τ obs的大小并重复上述步骤S202操作，总共进行M次信号采集，得到M组回波信号，以m代表采集的序号，则δ rph[m]代表第m次信号采集的聚相时间，如图6所示，通过本申请实施例的方法采集到的磁共振信号，其回波出现位置受测试梯度脉冲产生的涡流影响发生了偏移。
在一具体实施例中，执行步骤S2的过程可以具体包括如下步骤：
步骤S21：对回波信号进行取模值操作，得到模值回波信号。
本申请实施例中，将采集到的预设采样脉冲序列的回波信号进行取模值操作，得到模值回波信号，其中主要是要将采集到的磁共振信号的回波信号中的复数信号进行取模。
步骤S22：获取模值回波信号的主峰，对模值回波信号的主峰进行拟合和插值回归，得到回波信号的聚相时间。
本申请实施例中，获取模值回波信号的主峰，对模值回波信号的主峰进行拟合和插值回归，得到回波信号的聚相时间。以任意梯度方向采集的第m组回波信号为例，对复数信号数据取模值，并通过回波出现时刻，即模值回波信号最大值所处的位置来确定聚相时间δ rph[m]，由于存在采样时间间隔、数据噪声等影响因素，对主回波峰进行拟合并插值回归可以得到更精确的聚相时间，其中采用多项式拟合主回波峰，当多项式的次数不小于3时，得到的结果比较准确、可靠，拟合得到多项式的各项系数后，可基于该多项式函数模型进行插值回归，得到高精度的聚相时间。
需要说明的是，本申请实施例中仅举例说明可以通过对主回波峰进行拟合并插值回归得到更精确的聚相时间，在实际应用中还可以根据实际经验及系统需求进行设定，并且对主回波峰进行拟合所采用的函数模型并不唯一，还可以采用如洛伦兹线型函数、高斯线型函数、广延指数分布函数等进行拟合，本申请并不以此为限。
步骤S23：通过采样脉冲中心到采样开始时刻的时间加上对应的聚相时间得到回波时间，其中聚相时间包括：对应基础回波信号的聚相时间及对应参考回波信号的参考聚相时间。
步骤S24：利用回波时间、聚相时间和聚相梯度数据计算涡流成分参数。
在一具体实施例中，执行步骤S24的过程可以具体包括如下步骤：
步骤S241：将回波时间内聚相时间与参考聚相时间之差确定为延迟时间。
本申请实施例中，以第m组回波信号为例，该组信号的回波时间τ echo[m]为采样射频脉冲中心到采样开始时刻的时间加上其对应的聚相时间δ rph[m],在这段回波时间内，所有梯度及其涡流随时间的积分值总和为零。对于参考信号，其聚相时间为δ rph[ref]，整个回波时间内不存在测试梯度脉冲产生的涡流，由于散相梯度与测试梯度方向相同，而聚相梯度与之方向相反，故对于测试信号，其回波时刻因涡流的叠加而产生延迟，延迟的这段时间内，聚相梯度脉冲起到了弥补涡流的作用。
步骤S242：利用聚相梯度数据中的聚相梯度强度及延迟时间，计算得到涡流积分值。
本申请实施例中，利用聚相梯度数据中的聚相梯度强度及延迟时间，通过以下公式计算涡流积分值：
I eddy[m]＝G rph(δ rph[m]-δ rph[ref])       (1)
其中，I eddy[m]表示第m组涡流积分值，G rph表示聚相梯度强度，δ rph[m]表示第m组聚相时间，δ rph[ref]表示参考聚相时间。
步骤S243：利用回波时间、涡流积分值、涡流积分函数模型，基于预设约束模型计算得到涡流成分参数，涡流成分参数包括：指数函数的幅度及指数函数的衰减时间常数。
本申请实施例中，利用回波时间及涡流积分值，通过以下公式计算涡流函数模型：
Figure PCTCN2020119282-appb-000003
其中，G eddy(t)表示涡流函数模型，n表示指数成分的序号，n＝1,2,…,N，A [n]表示指数函数的幅度，T [n]表示指数函数的衰减时间常数；
利用观测时间、回波时间、涡流函数模型及涡流积分值，通过以下公式计算涡流成分参数：
Figure PCTCN2020119282-appb-000004
其中，I eddy[m]表示第m组涡流积分值，τ obs[m]表示第m组观测时间，τ echo[m]表示第m组回波时间，G eddy(t)表示涡流函数模型。
在实际应用中，可以通过最小二乘拟合，因此涡流成分参数的计算可通过求解以下公式实现：
Figure PCTCN2020119282-appb-000005
其中，I eddy[m]表示第m组涡流积分值，τ obs[m]表示第m组观测时间，τ echo[m]表示第m组回波时间，G eddy(t)表示涡流函数模型，A [n]表示指数函数的幅度，T [n]表示指数函数的衰减时间常数。最小二乘拟合目标为最小化涡流积分观测值与涡流积分函数期望值距离的平方和，最小化过程需要考虑针对涡流成分幅度A和衰减时间常数T的约束条件：一是所有A必须为正值，且其总和不超过测试梯度强度G test；二是所有T必须落在[T min,T max]范围内，该范围应根据磁共振设备梯度预加重模块提供的衰减时间常数的可行范围进行调整。数据拟合过程中，I eddy[m]的具体数值由公式(1)给出，理论上，指数成分的数目N越大，拟合效果越好。然而在实际情况中，磁共振设备提供的预加重通道总数有限，N值若过大则无法实现所有指数成分的涡流补偿，故N的取值不应超过预加重通道总数。
本申请实施例采用经典的列文伯格-马夸尔特非线性最小二乘算法结合对数障碍函数法可高效、稳健地实现公式(4)的约束优化，求解出准确、可靠的涡流成分参数。需要说明的是，考虑约束条件的求解方法并不唯一，还可以是其它算法，如梯度下降法、牛顿法、信赖域法，无论采用何种算法求解公式(4)或其等效变型式，都可以进行计算，只要能达到计算求解的目的即可，本申请并不以此为限。
实际应用中，首先对采集到的复数信号进行取模值操作，得到的模值回波信号如图7所示，然后对每组回波信号主峰进行拟合和插值回归，得到高分辨的聚相时间δ rph，图8是第一组回波信号，即τ obs取0.1毫秒时的主峰拟合和插值回归示意图，之后通过聚相时间计算出回波时间τ echo并计算回波时间内测试梯度涡流的积分值I eddy。参考信号的聚相时间等于示例中的第八组信号的聚相时间(δ rph[ref]＝δ rph[8])。此时，涡流积分函数模型中，除待求解的指数成分幅度A和衰减时间常数T外，其余变量全部确定。涡流积分的拟合效果如图9所示，可见本申请提供的方法可准确拟合实际梯度涡流情况。
本申请提供的用于磁共振成像的梯度涡流补偿方法，通过采集预设采样脉冲序列的回波信号，利用预设算法对回波信号进行数据处理，计算得到涡流成分参数，对磁共振成像 梯度脉冲进行梯度涡流补偿，得到预期的目标梯度波形；测试与补偿的过程中无需额外硬件设备支持，噪声对确定回波位置产生的影响小，通过回波位置分析数据的方法抗噪能力较强，且不易受主磁场及射频场不稳定性等问题的影响，稳健性高；并且操作便捷，信号采集速度快，数据处理结果可靠，能够高效、准确地实现梯度涡流的测量与补偿；对测试样品的形状、材质没有严格要求，只需单组分、弛豫时间较长的液体样品；无需精确测量样品位置，样品一经放置，直至完成所有梯度方向的涡流测量前，无需再次移动其位置；整个样品所有质子均参与信号贡献，信号量大，信噪比高，有效解决了传统方法数据采集过程操作繁琐、耗时长等问题，克服了传统方法数据分析结果易受噪声等不确定性因素影响的问题。
本申请实施例还提供了一种用于磁共振成像的梯度涡流补偿系统，如图10所示，包括：
采集模块1，用于采集预设采样脉冲序列的回波信号。详细内容参见上述方法实施例中步骤S1的相关描述，在此不再赘述。
计算模块2，用于利用预设算法对回波信号进行数据处理，计算得到涡流成分参数。详细内容参见上述方法实施例中步骤S2的相关描述，在此不再赘述。
波形生成模块3，用于根据涡流成分参数生成预加重波形。详细内容参见上述方法实施例中步骤S3的相关描述，在此不再赘述。
校正模块4，用于通过预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。详细内容参见上述方法实施例中步骤S4的相关描述，在此不再赘述。
通过上述各个模块组成部分的协同合作，本申请提供的用于磁共振成像的梯度涡流补偿系统，通过采集预设采样脉冲序列的回波信号，利用预设算法对回波信号进行数据处理，计算得到涡流成分参数，对磁共振成像梯度脉冲进行梯度涡流补偿，得到预期的目标梯度波形；测试与补偿的过程中无需额外硬件设备支持，噪声对确定回波位置产生的影响小，通过回波位置分析数据的方法抗噪能力较强，且不易受主磁场及射频场不稳定性等问题的影响，稳健性高；并且操作便捷，信号采集速度快，数据处理结果可靠，能够高效、准确地实现梯度涡流的测量与补偿；对测试样品的形状、材质没有严格要求，只需单组分、弛豫时间较长的液体样品；无需精确测量样品位置，样品一经放置直至完成所有梯度方向的 涡流测量前，无需再次移动其位置；整个样品所有质子均参与信号贡献，信号量大，信噪比高，有效解决了传统方法数据采集过程操作繁琐、耗时长等问题，克服了数据分析结果易受噪声等不确定性因素影响的问题。
本申请实施例提供一种计算机设备，如图11所示，包括：至少一个处理器401，例如CPU(Central Processing Unit，中央处理器)，至少一个通信接口403，存储器404，至少一个通信总线402。其中，通信总线402用于实现这些组件之间的连接通信。其中，通信接口403可以包括显示屏(Display)、键盘(Keyboard)，可选通信接口403还可以包括标准的有线接口、无线接口。存储器404可以是高速RAM存储器(Ramdom Access Memory，易挥发性随机存取存储器)，也可以是非不稳定的存储器(non-volatile memory)，例如至少一个磁盘存储器。存储器404可选的还可以是至少一个位于远离前述处理器401的存储装置。其中处理器401可以执行用于磁共振成像的梯度涡流补偿方法。存储器404中存储一组程序代码，且处理器401调用存储器404中存储的程序代码，以用于执行上述的用于磁共振成像的梯度涡流补偿方法。
其中，通信总线402可以是外设部件互连标准(peripheral component interconnect，简称PCI)总线或扩展工业标准结构(extended industry standard architecture，简称EISA)总线等。通信总线402可以分为地址总线、数据总线、控制总线等。为便于表示，图11中仅用一条线表示，但并不表示仅有一根总线或一种类型的总线。
其中，存储器404可以包括易失性存储器(英文：volatile memory)，例如随机存取存储器(英文：random-access memory，缩写：RAM)；存储器也可以包括非易失性存储器(英文：non-volatile memory)，例如快闪存储器(英文：flash memory)，硬盘(英文：hard disk drive，缩写：HDD)或固态硬盘(英文：solid-state drive，缩写：SSD)；存储器404还可以包括上述种类的存储器的组合。
其中，处理器401可以是中央处理器(英文：central processing unit，缩写：CPU)，网络处理器(英文：network processor，缩写：NP)或者CPU和NP的组合。
其中，处理器401还可以进一步包括硬件芯片。上述硬件芯片可以是专用集成电路(英文：application-specific integrated circuit，缩写：ASIC)，可编程逻辑器件(英文：programmable logic device，缩写：PLD)或其组合。上述PLD可以是复杂可编程逻辑器件(英文：complex programmable logic device，缩写：CPLD)，现场可编程逻辑门阵列(英文：field-programmable  gate array，缩写：FPGA)，通用阵列逻辑(英文：generic array logic,缩写：GAL)或其任意组合。
可选地，存储器404还用于存储程序指令。处理器401可以调用程序指令，实现如本申请的用于磁共振成像的梯度涡流补偿方法。
本申请实施例还提供一种计算机可读存储介质，计算机可读存储介质上存储有计算机可执行指令，该计算机可执行指令可执行用于磁共振成像的梯度涡流补偿方法。其中，存储介质可为磁碟、光盘、只读存储记忆体(Read-Only Memory，ROM)、随机存储记忆体(Random Access Memory，RAM)、快闪存储器(Flash Memory)、硬盘(Hard Disk Drive，缩写：HDD)或固态硬盘(Solid-State Drive，SSD)等；存储介质还可以包括上述种类的存储器的组合。
显然，上述实施例仅仅是为清楚地说明所作的举例，而并非对实施方式的限定。对于所属领域的普通技术人员来说，在上述说明的基础上还可以做出其它不同形式的变化或变动。这里无需也无法对所有的实施方式予以穷举。而由此所引申出的显而易见的变化或变动仍处于本申请的保护范围之中。

Claims (10)

  1. 一种用于磁共振成像的梯度涡流补偿方法，其特征在于，包括：
    采集预设采样脉冲序列的回波信号，所述预设采样脉冲序列用于测量涡流数据；
    利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数；
    根据所述涡流成分参数生成预加重波形；
    通过所述预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。
  2. 根据权利要求1所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，所述采集预设采样脉冲序列的回波信号的步骤，包括：
    步骤S10：将测试样品放置于预设测试区域，完成预扫描，得到所述测试区域的校正参数；
    步骤S20：预设置观测时间，利用所述观测时间、测试区域的校正参数及预设采样脉冲序列参数，运行预设采样脉冲序列，得到回波信号，所述预设采样脉冲序列参数包括：测试梯度数据、散相梯度数据及聚相梯度数据，其中所述测试梯度数据包括：测试梯度强度、测试梯度持续时间及测试梯度方向；
    步骤S30：改变所述测试梯度方向，将预设采样脉冲序列参数替换为改变后的预设采样脉冲序列参数，重复步骤S20，直到完成所有梯度方向的回波信号的采集。
  3. 根据权利要求2所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，所述回波信号包括：基础回波信号和参考回波信号，
    所述预设置观测时间，利用所述观测时间、测试区域的校正参数及预设采样脉冲序列参数，运行预设采样脉冲序列，得到回波信号的步骤，包括：
    步骤201：预设置第一观测时间；
    步骤202：利用第一观测时间、测试区域的校正参数及第一预设采样脉冲序列参数，运行预设采样脉冲序列，得到基础回波信号；
    步骤203：将所述第一预设采样脉冲序列参数中的测试梯度强度及测试梯度持续时间置为零，得到第二预设采样脉冲序列参数，根据第一观测时间运行预设采样脉冲序列，得到参考回波信号；
    步骤204：将第二观测时间预设置为第一观测时间，并返回执行步骤S202，直到完 成预设采样脉冲序列的所有回波信号的采集。
  4. 根据权利要求3所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，所述利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数的步骤，包括：
    对所述回波信号进行取模值操作，得到模值回波信号；
    获取所述模值回波信号的主峰，对所述模值回波信号的主峰进行拟合和插值回归，得到回波信号的聚相时间；
    通过采样脉冲中心到采样开始时刻的时间加上对应的聚相时间得到回波时间，所述聚相时间包括：对应基础回波信号的聚相时间及对应参考回波信号的参考聚相时间；
    利用所述回波时间、所述聚相时间和所述聚相梯度数据计算涡流成分参数。
  5. 根据权利要求4所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，所述利用所述回波时间、所述聚相时间和所述聚相梯度数据计算涡流成分参数的步骤，包括：
    将回波时间内所述聚相时间与所述参考聚相时间之差确定为延迟时间；
    利用所述聚相梯度数据中的聚相梯度强度及所述延迟时间，计算得到涡流积分值；
    利用所述回波时间、涡流积分值、涡流积分函数模型，基于预设约束模型计算得到涡流成分参数，所述涡流成分参数包括：指数函数的幅度及指数函数的衰减时间常数。
  6. 根据权利要求5所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，通过以下公式计算涡流积分值：
    I eddy[m]＝G rph(δ rph[m]-δ rph[ref])，
    其中，I eddy[m]表示第m组涡流积分值，G rph表示聚相梯度强度，δ rph[m]表示第m组聚相时间，δ rph[ref]表示参考聚相时间。
  7. 根据权利要求5所述的用于磁共振成像的梯度涡流补偿方法，其特征在于，所述利用所述回波时间、涡流积分值及涡流积分函数模型，计算得到涡流成分参数的步骤，包括：
    利用所述回波时间及涡流积分值，通过以下公式计算涡流函数模型：
    Figure PCTCN2020119282-appb-100001
    其中，G eddy(t)表示涡流函数模型，n表示指数成分的序号，n＝1,2,…,N，A [n]表示指数函数的幅度，T [n]表示指数函数的衰减时间常数；
    利用所述观测时间、所述回波时间、所述涡流函数模型及所述涡流积分值，通过以下公式计算涡流成分参数：
    Figure PCTCN2020119282-appb-100002
    其中，I eddy[m]表示第m组涡流积分值，τ obs[m]表示第m组观测时间，τ echo[m]表示第m组回波时间，G eddy(t)表示涡流函数模型。
  8. 一种用于磁共振成像的梯度涡流补偿系统，其特征在于，包括：
    采集模块，用于采集预设采样脉冲序列的回波信号；
    计算模块，用于利用预设算法对所述回波信号进行数据处理，计算得到涡流成分参数；
    波形生成模块，用于根据所述涡流成分参数生成预加重波形；
    校正模块，用于通过所述预加重波形对磁共振成像梯度脉冲进行涡流补偿，得到预期的目标梯度波形。
  9. 一种计算机可读存储介质，其特征在于，所述计算机可读存储介质存储计算机指令，所述计算机指令被处理器执行时实现如权利要求1-7中任一项所述的用于磁共振成像的梯度涡流补偿方法。
  10. 一种电子设备，其特征在于，包括：
    存储器和处理器，所述存储器和所述处理器之间互相通信连接，所述存储器中存储有计算机指令，所述处理器通过执行所述计算机指令，从而执行如权利要求1-7中任一项所述的用于磁共振成像的梯度涡流补偿方法。
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CN117233677B (zh) * 2023-09-15 2024-06-04 安徽峻德医疗科技有限公司 非笛卡尔激发采样的磁共振梯度脉冲信号补偿方法及装置

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