CN113917379A - High-signal-to-noise-ratio magnetic resonance imaging k-space trajectory measurement method - Google Patents

High-signal-to-noise-ratio magnetic resonance imaging k-space trajectory measurement method Download PDF

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CN113917379A
CN113917379A CN202111169939.7A CN202111169939A CN113917379A CN 113917379 A CN113917379 A CN 113917379A CN 202111169939 A CN202111169939 A CN 202111169939A CN 113917379 A CN113917379 A CN 113917379A
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CN113917379B (en
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刘朝阳
杨春升
陈俊飞
张志�
陈黎
李文帅
冯继文
陈方
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Hubei Optics Valley Laboratory
Institute of Precision Measurement Science and Technology Innovation of CAS
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Abstract

The invention discloses a magnetic resonance imaging k-space trajectory measurement method with high signal-to-noise ratio, which comprises the following steps: planning a k space filling mode of an imaging scanning sequence; setting parameters of radio frequency pulses of a radio frequency channel and parameters of gradient pulses of a gradient channel, and setting the occurrence sequence of the radio frequency pulses, the gradient pulses and sampling events of each experiment; mapping the gradient channels to physical gradient channels in the x direction, the y direction and the z direction respectively, sequentially carrying out experiments respectively, and calculating k space trajectories corresponding to the three physical gradient channels; a k-space trajectory of the imaging scan sequence is estimated. The phase information acquired by the method comes from a data source with high signal-to-noise ratio, is more stable and reliable, is suitable for high-resolution imaging, has no requirement on delta s, and has less parameter setting constraint.

Description

High-signal-to-noise-ratio magnetic resonance imaging k-space trajectory measurement method
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a magnetic resonance imaging k-space trajectory measurement method with high signal-to-noise ratio.
Background
Magnetic Resonance Imaging (MRI) is a non-radiative, non-invasive imaging modality with high soft tissue contrast and numerous imaging parameters, and has found widespread use in preclinical and clinical studies. MRI raw data is a complex data set acquired in a 'k space' according to a designed track or coordinate, and accurate measurement of k space tracks is a premise for realizing high fidelity and artifact-free image reconstruction. Various factors in the MRI system, such as time delays associated with magnetic fields, eddy currents, and electronics, can cause deviations between the actual k-space trajectory and the theoretical trajectory set by the MRI pulse sequence during the experiment. Accordingly, MRI scanners alleviate this problem by various methods, such as gradient active shielding and gradient pre-emphasis to minimize eddy current effects during gradient switching. However, residual eddy currents still exist and remain a major problem in cartesian fast imaging (EPI, Rare, etc.), non-cartesian imaging (radial UTE, helical, Rosette, etc. trajectories), phase contrast imaging (Flow, MR vessel imaging) techniques.
All measurements of k-space sampling trajectories are impractical and time consuming, a more efficient method is to apply test gradient pulses to the three physical gradient channels separately, measure the time-varying course of the test gradient pulses, and estimate the sampling trajectories in the experiment using a linear combination of the three, thereby improving the image reconstruction quality.
Currently, one popular approach is to calibrate the test gradient waveforms using self-encoding gradients, and this arbitrary k-space trajectory calibration technique requires additional self-encoding gradients and long acquisition times. Another common method is to calibrate the gradient pulse shape with a small volume water model (point sample) off the center of the gradient, characterizing the k-space trajectory of the test gradient with the phase change between consecutive data points of the FID. This method requires the use ofThe additional test water model, the manual change water model position, it is tedious and relatively time-consuming to operate. The method is further developed into a track measurement method based on slice selection, namely, under the coordination of slice selection gradient and radio frequency pulse, a thin-layer voxel deviating from the center of the gradient magnetic field in an imaging object is excited to replace a small sample, and the improved method is easy to realize and quick and is widely applied. But the method is also based on the phase of the FID signal in nature, and the measurement accuracy is easily influenced by T2 *The influence of low signal-to-noise ratio caused by attenuation and gradient dispersion, especially in the periphery of k-space, when the signal-to-noise ratio is low, the phase information is extremely inaccurate, and the k-space trajectory represented by the phase is inaccurate. A more serious problem is that the amplitude of the FID signal acquired by this method is modulated by the fourier transform of the excited layer, and signals with amplitudes close to or equal to zero appear when the FID signal is acquired for a long time. For example, in MRI, usually a designated slice is excited by using SINC radio frequency pulses in conjunction with slice selection gradients, the excited slice is rectangular along the test gradient direction, and the amplitude of the FID signal S (k (t)) obtained by the test trace procedure is modulated by the SINC function, which is expressed by the following formula:
Figure BDA0003292550160000021
where k (t) is the trace tested, Δ s is the thickness of the layer excited for the trace tested, and d is the distance of the layer from the center of the gradient magnetic field. It is readily understood that S (k (t)) will approach zero when k (t) x Δ S is close to an integer, and zero when equal to an integer, and that S (k (t)) will actually be greater than k (t) x Δ S
Figure BDA0003292550160000022
In this case, even if the sum is accumulated for multiple times, the signal-to-noise ratio cannot be effectively improved, so that it cannot be guaranteed that the acquired phase information is accurate, and especially in the case where Δ S is set to be large, the modulation of the SINC function is easy to zero, and the amplitude of S (k (t)) is difficult to obtain the zero crossing pointA reliable k-space trajectory.
Maximum value k of resolution Δ x and k (t) of MRI imagesmax(t) the relationship is as follows:
Figure BDA0003292550160000023
when high fraction imaging, Δ x is small, and when Δ x ≦ Δ s, the maximum value k of k (t)maxThe product of (t) and Δ S is more than 1 or even 2, and the acquired FID signal S (k (t)) crosses zero for many times according to the formula of the FID signal S (k (t)) obtained in the process of testing the track, so that the track measurement method based on the slice is not suitable for the application scene of high-fraction imaging.
Disclosure of Invention
The invention aims to solve the problems in the prior art, and provides a magnetic resonance imaging k-space trajectory measurement method with high signal-to-noise ratio.
The above object of the present invention is achieved by the following technical solutions:
a magnetic resonance imaging k-space trajectory measurement method with high signal-to-noise ratio comprises the following steps:
step 1, planning a k space filling mode of an imaging scanning sequence;
step 2, setting parameters of radio frequency pulses of a radio frequency channel F1 and parameters of gradient pulses of a gradient channel Gs, and setting the occurrence sequence of the radio frequency pulses, the gradient pulses and sampling events of each experiment;
step 3, mapping the gradient channel Gs to an x-direction physical gradient channel Gx, sequentially implementing each experiment, and calculating a k-space track k corresponding to the x-direction physical gradient channel Gxx(t)；
Mapping the gradient channel Gs to a y-direction physical gradient channel Gy, sequentially implementing each experiment, and calculating a k-space track k corresponding to the y-direction physical gradient channel Gyy(t)；
Connecting gradient channel Gs toMapping the channel to a z-direction physical gradient channel Gz, sequentially carrying out each experiment, and calculating a k-space track k corresponding to the z-direction physical gradient channel Gzz(t)；
Step 4, according to the k space trajectory kx(t), k space trajectory ky(t), k space trajectory kz(t) and a k-space filling mode of the imaging scan sequence, estimating a k-space trajectory k (t) of the imaging scan sequence.
Step 1 as described above comprises the steps of: setting an inspection area FOV, a layer thickness delta s required to be excited by a test track, a layer surface off-center distance d, a sampling spectrum width SW and the number of sampling points NP; and combining the test gradient pulse and the imaging scanning sequence to determine the shape of the test gradient pulse and calculate the intensity value Grevel of the test gradient pulse.
The radio frequency pulse parameters of the radio frequency channel F1 in step 2 include the shape of the excitation pulse and the excitation pulse width P1, and the frequency domain width BW of the excitation pulse is calculated according to the attributes of the excitation pulse;
the gradient pulse comprises a layer selection gradient pulse, a layer selection echo gradient pulse and a test gradient pulse in sequence, and the parameters of the gradient pulse comprise: time tg of a slice selection gradient pulse platform, slice selection gradient pulse intensity Gslevel, slice selection echo gradient pulse intensity G0 and test gradient pulse duration tm.
The slice selection gradient pulse plateau time tg as described above has a value equal to the excitation pulse width P1,
the value of the test gradient pulse duration tm being not less than
Figure BDA0003292550160000031
Gradient pulse intensity of selected layer
Figure BDA0003292550160000032
Gamma is the magnetic spin ratio of the imaging nuclear species,
and calculating the intensity G0 of the selective layer echo gradient pulse according to the fact that the area of the selective layer echo gradient pulse is equal to half of the area of the selective layer gradient pulse.
Setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the nth experiment in step 2 as described above includes the following steps:
step 2.3.1, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the first half experiment of the nth experiment: the center of the selected layer gradient pulse of the gradient channel Gs is aligned with the center of the excitation pulse of the radio frequency channel F1, the selected layer echo gradient pulse is applied next to the selected layer gradient pulse, then the test gradient pulse is applied, the strength of the test gradient pulse is set as the strength value Grlevel of the test gradient pulse, gamma is the magnetic rotation ratio of the imaging nuclide, the starting time of the sampling window corresponding to the sampling event on the radio frequency channel is aligned with the starting time of the test gradient pulse of the gradient channel Gs, the FID signal is collected as the FID signal FID1(n) while the test gradient pulse of the gradient channel Gs is applied,
step 2.3.2, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the second half experiment of the nth experiment: the center of the selected layer gradient pulse of the gradient channel Gs is aligned with the center of the excitation pulse of the radio frequency channel F1, the selected layer echo gradient pulse is applied next to the selected layer gradient pulse, then the test gradient pulse is applied, the strength of the test gradient pulse is set to be zero, the starting time of the sampling window corresponding to the sampling event on the radio frequency channel is aligned with the starting time of the test gradient pulse of the gradient channel Gs, and the FID signal is acquired as the FID signal FID2(n) while the test gradient pulse of the gradient channel Gs is applied.
And adjusting the gradient pulse intensities of the gradient channel Gs of different times of experiments to be different by taking the calculated gradient pulse intensity G0 of the selective layer echo as reference, wherein the gradient pulse intensities of the selective layer echo of the first half experiment and the gradient pulse intensity of the selective layer echo of the second half experiment of the same time of experiments are the same.
The total number of times M of the above-mentioned experiments is more than 1 and less than or equal to 5, 1 < n < M, after the mapping of the gradient channel Gs channel is completed and the experiments are sequentially performed, the two FID signals obtained from the n-th experiment are marked as FID1(n) and FID2(n),
subtracting the phase of the FID signal FID2(n) from the phase of the FID signal FID1(n), and recording the obtained data as the nth differential phase (n);
extracting each low signal-to-noise ratio distribution interval of the FID signal FID1(1) according to the low signal-to-noise ratio threshold value, searching a FID signal FID1(i) with the highest data signal-to-noise ratio corresponding to the same low signal-to-noise ratio distribution interval as an FID signal to be replaced for each low signal-to-noise ratio distribution interval of the FID signal FID1(1), wherein i is more than or equal to 2 and less than or equal to M, replacing data corresponding to each low signal-to-noise ratio distribution interval in the first differential phase diffphase (1) with data corresponding to the same low signal-to-noise ratio distribution interval in the differential phase corresponding to the searched FID signal to be replaced, performing phase unwrapping on the replaced first differential phase (1), and marking as an unwrapped differential phase diffphase after unwrapping,
k-space trajectory ks(t) the calculation formula is as follows:
Figure BDA0003292550160000041
ks(t)∈{kx(t)，ky(t)，kz(t)}。
compared with the prior art, the invention has the following beneficial effects:
(1) the data source signal-to-noise ratio used for calculating the k track is high, the obtained phase information is more accurate, the result is more stable and reliable, and the method can be used for a high-resolution imaging scene;
(2) when the method is applied to high-resolution MRI, the condition that the signal-to-noise ratio of partial data points is low and even zero and the accurate phase cannot be obtained due to the fact that original FID data is modulated by excitation layer fourier transform when a gradient track is tested can be effectively avoided;
(3) the invention has no requirement on the layer thickness delta s selected by the test track, can be properly increased and is beneficial to obtaining a data source with high signal-to-noise ratio.
(4) Each gradient channel only needs to adjust the intensity of the selective layer echo gradient pulse once or for many times, is simple and easy to operate, can be used as a pre-scanning sequence, is loaded in an MRI system, measures and estimates the real k-space track of the imaging scanning sequence, and compensates the imperfection of a hardware system.
Drawings
Fig. 1 is a pulse sequence diagram of a radio frequency channel F1 and a slice direction gradient channel Gs of a three-dimensional radial acquisition UTE pulse sequence;
fig. 2 is a pulse sequence diagram of a radio frequency channel F1, a readout gradient channel Gr, a plane direction gradient channel Gs, and a phase encoding gradient channel Gp of a three-dimensional radial acquisition UTE trajectory measurement pulse sequence;
FIG. 3 is a graph of FID data obtained in example 1;
FIG. 4 is a k-space trajectory k corresponding to the x-direction physical gradient channel Gx obtained in example 1x(t)；
Fig. 5 is an image obtained after image reconstruction in example 1.
Detailed Description
The present invention will be described in further detail with reference to examples for the purpose of facilitating understanding and practice of the invention by those of ordinary skill in the art, and it is to be understood that the present invention has been described in the illustrative embodiments and is not to be construed as limited thereto.
Example 1:
as shown in fig. 2, in this embodiment, the imaging scanning sequence adopts a three-dimensional radial acquisition UTE pulse sequence, the imaging scanning sequence includes a radio frequency pulse and a sampling event of a radio frequency channel F1, gradient pulses of a readout gradient channel Gr, a slice direction gradient channel Gs, and a phase encoding gradient channel Gp are matched with the imaging scanning sequence, and the pulse sequence of the magnetic resonance imaging k-space trajectory measurement method of the present invention is shown in fig. 1. The pulse sequences shown in fig. 1 and 2 are loaded on a magnetic resonance imaging system console for UTE imaging.
A magnetic resonance imaging k-space trajectory measurement method with high signal-to-noise ratio comprises the following steps:
step 1, planning a k-space filling mode of an imaging scanning sequence, wherein the imaging scanning sequence is a three-dimensional radial acquisition UTE pulse sequence in the embodiment, the imaging scanning sequence is not limited to the three-dimensional radial acquisition UTE pulse sequence, an inspection area FOV (field of view), a layer thickness delta s required to be excited by a test track, a layer surface eccentric distance d, a sampling spectrum width SW and a sampling point number NP are set to be combined into the imaging scanning sequence, the shape of a test gradient pulse is determined, and a test gradient pulse strength value Grevel is calculated; in this example, the gradient pulse strength was testedThe value Grlevel is
Figure BDA0003292550160000061
Gamma is the magnetic spin ratio of the imaging nuclear species.
And 2, setting parameters of radio frequency pulses of a radio frequency channel F1 and parameters of gradient pulses of a gradient channel Gs, and setting the occurrence sequence of the radio frequency pulses, the gradient pulses and sampling events of each experiment.
Step 2.1, setting parameters of a radio frequency pulse of a radio frequency channel F1, wherein the parameters of the radio frequency pulse specifically comprise: the shape and the width of the excitation pulse P1 of the excitation pulse, and the frequency domain width BW of the excitation pulse is calculated according to the attribute parameters of the radio frequency pulse and the excitation pulse width P1; in this embodiment, the excitation pulse is set to an SINC excitation pulse;
step 2.2, setting parameters of gradient pulses of the gradient channel Gs, wherein the gradient pulses sequentially comprise a layer selection gradient pulse, a layer selection echo gradient pulse and a test gradient pulse, and the parameters of the gradient pulses specifically comprise: time tg of a layer selection gradient pulse platform, the intensity Gslevel of a layer selection gradient pulse, the intensity G0 of a layer selection echo gradient pulse and duration tm of a test gradient pulse;
wherein the value of the slice selection gradient pulse plateau time tg is equal to the excitation pulse width P1, and the value of the test gradient pulse duration tm is not less than
Figure BDA0003292550160000062
According to the formula:
Figure BDA0003292550160000063
calculating the intensity Gslevel of the gradient pulse of the selected layer; calculating the gradient intensity G0 of the selective layer echo according to the fact that the area of the selective layer echo gradient pulse is equal to half of the area of the selective layer gradient pulse;
and 2.3, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the nth experiment.
Step 2.3.1, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the first half experiment of the nth experiment: the center of a layer-selecting gradient pulse of a gradient channel Gs is aligned with the center of an excitation pulse of a radio frequency channel F1, a layer-selecting echo gradient pulse is applied next to the layer-selecting gradient pulse, then a testing gradient pulse is applied, the strength of the testing gradient pulse is set as a testing gradient pulse strength value Grlevel, the starting time of a sampling window corresponding to a sampling event on the radio frequency channel is aligned with the starting time of the testing gradient pulse of the gradient channel Gs, and a FID signal is acquired as a FID signal FID1(n) while the testing gradient pulse of the gradient channel Gs is applied.
Step 2.3.2, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the second half experiment of the nth experiment: the center of the selected layer gradient pulse of the gradient channel Gs is aligned with the center of the excitation pulse of the radio frequency channel F1, the selected layer echo gradient pulse is applied next to the selected layer gradient pulse, then the test gradient pulse is applied, the strength of the test gradient pulse is set to be zero, the starting time of the sampling window corresponding to the sampling event on the radio frequency channel is aligned with the starting time of the test gradient pulse of the gradient channel Gs, and the FID signal is acquired as the FID signal FID2(n) while the test gradient pulse of the gradient channel Gs is applied.
And adjusting the gradient pulse intensities of the gradient channel Gs of different times of experiments to be different by taking the calculated gradient pulse intensity G0 of the selective layer echo as reference, wherein the gradient pulse intensities of the selective layer echo of the first half experiment and the gradient pulse intensity of the selective layer echo of the second half experiment of the same time of experiments are the same.
Step 3, mapping the gradient channel Gs to an x-direction physical gradient channel Gx, sequentially implementing each experiment, and calculating a k-space track k corresponding to the x-direction physical gradient channel Gxx(t)；
Mapping the gradient channel Gs to a y-direction physical gradient channel Gy, sequentially implementing each experiment, and calculating a k-space track k corresponding to the y-direction physical gradient channel Gyy(t)；
Mapping the gradient channel Gs to a z-direction physical gradient channel Gz, sequentially carrying out each experiment, and calculating a k-space track k corresponding to the z-direction physical gradient channel Gzz(t)；
After the gradient channel Gs is mapped to physical gradient channels in different axial directions, a plurality of tests are carried out (the total times M of the tests are more than or equal to 1 and less than or equal to 5, and n is more than or equal to 1 and less than or equal to M), in the first half of the test of each test, an FID signal is collected as an FID signal FID1(n) while a test gradient pulse of the gradient channel Gs is applied; in the second half of the experiment, the FID signal was acquired as the FID signal FID2(n) while the test gradient pulse of the gradient channel Gs was applied;
subtracting the phase of the FID signal FID2(n) from the phase of the FID signal FID1(n), and recording the obtained data as the nth differential phase (n);
extracting each low signal-to-noise ratio distribution interval of the FID signal FID1(1) according to the low signal-to-noise ratio threshold, searching a FID signal FID1(i) with the highest data signal-to-noise ratio corresponding to the same low signal-to-noise ratio distribution interval as an FID signal to be replaced for each low signal-to-noise ratio distribution interval of the FID signal FID1(1), wherein i is more than or equal to 2 and less than or equal to M, replacing data corresponding to each low signal-to-noise ratio distribution interval in the first differential phase diffphase (1) with data corresponding to the same low signal-to-noise ratio distribution interval in the differential phase corresponding to the searched FID signal to be replaced, performing phase unwrapping on the replaced first differential phase (1), and marking the unwrapped first differential phase as an unwrapped differential phase.
In this embodiment, because the experiment is performed in combination with the three-dimensional radial acquisition UTE pulse sequence (the present invention is not limited to be applied to only the three-dimensional radial acquisition UTE pulse sequence), the present embodiment can measure an accurate k trajectory in the coordinate axis direction by performing two experiments, and therefore, in this embodiment, the total number of times M of the experiment in each physical gradient direction is 2. In the first experiment, the intensity of the selective layer echo gradient pulse of the gradient channel Gs is adjusted and is set as G1; in the second experiment, the intensity of the selective layer echo gradient pulse of the gradient channel Gs is adjusted to be G2, and G2 is not equal to G1.
The first experiment of this example obtained the FID signal FID1(1) and the FID signal FID2 (1);
the second experiment of this example obtained the FID signal FID1(2) and the FID signal FID2 (2);
subtracting the phase of the FID signal FID2(1) from the phase of the FID signal FID1(1), and recording the obtained data as a first differential phase (1);
subtracting the phase of the FID signal FID2(2) from the phase of the FID signal FID1(2), and recording the obtained data as a second differential phase (2);
extracting a low snr distribution interval [ t1 t2] of the FID signal FID1(1) according to the low snr threshold, in this embodiment, there is only one low snr distribution interval, but in an actual situation, there may be a plurality of low snr distribution intervals, searching the FID signal FID1(i) with the highest data snr corresponding to the same low snr distribution interval as the FID signal to be replaced, in this embodiment, the FID signal to be replaced is the FID signal FID1(2), replacing the data corresponding to the low snr distribution interval [ t1 t2] in the first differential phase (1) with the data corresponding to the searched to-be-replaced FID signal [ t1 t2] in the low snr distribution interval [ t1 t2] (in this embodiment, the data corresponding to the low snr distribution interval [ t 8296 t2] in the first differential phase (2)), after unwrapping, the differential phase is noted as unwrapped.
k-space trajectory ks(t) the calculation formula is as follows:
Figure BDA0003292550160000081
in this example, G1 ═ G0 × (1 to 10%), and G2 ═ G0 × (1+ 10%). G0 is the slice selection echo gradient pulse intensity, k, of the gradient channel Gs obtained by calculation in step 2.2s(t)∈{kx(t)，ky(t)，kz(t)}。
Implementing three-dimensional radial acquisition of UTE sequences to obtain k-space data kdata;
step 4, estimating a k-space trajectory, specifically as follows:
according to the k space track k corresponding to the x-direction physical gradient channel GxxK space trajectory k corresponding to (t) and y-direction physical gradient channel Gyy(t) k-space trajectory k corresponding to z-direction physical gradient channel Gzz(t) estimating the k-space trajectory k of the imaging scan sequence in a k-space filling manner of the imaging scan sequenceute(t)；
Reconstructing the image as follows: according tok-space data kdata and k-space trajectory kuteAnd (t) reconstructing an image by adopting a gridding method. In this embodiment, the central slice image in the three-dimensional volume of the water model is obtained as shown in fig. 5.
It should be noted that the specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims (7)

1. A magnetic resonance imaging k space trajectory measurement method with high signal-to-noise ratio is characterized by comprising the following steps:
step 1, planning a k space filling mode of an imaging scanning sequence;
step 2, setting parameters of radio frequency pulses of a radio frequency channel F1 and parameters of gradient pulses of a gradient channel Gs, and setting the occurrence sequence of the radio frequency pulses, the gradient pulses and sampling events of each experiment;
step 3, mapping the gradient channel Gs to an x-direction physical gradient channel Gx, sequentially implementing each experiment, and calculating a k-space track k corresponding to the x-direction physical gradient channel Gxx(t)；
Mapping the gradient channel Gs to a y-direction physical gradient channel Gy, sequentially implementing each experiment, and calculating a k-space track k corresponding to the y-direction physical gradient channel Gyy(t)；
Mapping the gradient channel Gs to a z-direction physical gradient channel Gz, sequentially carrying out each experiment, and calculating a k-space track k corresponding to the z-direction physical gradient channel Gzz(t)；
Step 4, according to the k space trajectory kx(t), k space trajectory ky(t), k space trajectory kz(t) and a k-space filling mode of the imaging scan sequence, estimating a k-space trajectory k (t) of the imaging scan sequence.
2. A method as claimed in claim 1, wherein the step 1 comprises the following steps: setting an inspection area FOV, a layer thickness delta s required to be excited by a test track, a layer surface off-center distance d, a sampling spectrum width SW and the number of sampling points NP; and combining the test gradient pulse and the imaging scanning sequence to determine the shape of the test gradient pulse and calculate the intensity value Grevel of the test gradient pulse.
3. The method as claimed in claim 2, wherein the parameters of the rf pulse of the rf channel F1 in step 2 include the shape of the excitation pulse and the excitation pulse width P1, and the frequency domain width BW of the excitation pulse is calculated according to the properties of the excitation pulse;
the gradient pulse comprises a layer selection gradient pulse, a layer selection echo gradient pulse and a test gradient pulse in sequence, and the parameters of the gradient pulse comprise: time tg of a slice selection gradient pulse platform, slice selection gradient pulse intensity Gslevel, slice selection echo gradient pulse intensity G0 and test gradient pulse duration tm.
4. A method as claimed in claim 3, wherein the value of the slice selection gradient pulse plateau time tg is equal to the excitation pulse width P1,
the value of the test gradient pulse duration tm being not less than
Figure FDA0003292550150000011
Gradient pulse intensity of selected layer
Figure FDA0003292550150000012
Gamma is the magnetic spin ratio of the imaging nuclear species,
and calculating the intensity G0 of the selective layer echo gradient pulse according to the fact that the area of the selective layer echo gradient pulse is equal to half of the area of the selective layer gradient pulse.
5. The method as claimed in claim 4, wherein the step 2 of setting the occurrence sequence of the RF pulse, the gradient pulse and the sampling event of the n-th experiment comprises the following steps:
step 2.3.1, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the first half experiment of the nth experiment: the center of the selected layer gradient pulse of the gradient channel Gs is aligned with the center of the excitation pulse of the radio frequency channel F1, the selected layer echo gradient pulse is applied next to the selected layer gradient pulse, then the test gradient pulse is applied, the strength of the test gradient pulse is set as the strength value Grlevel of the test gradient pulse, gamma is the magnetic rotation ratio of the imaging nuclide, the starting time of the sampling window corresponding to the sampling event on the radio frequency channel is aligned with the starting time of the test gradient pulse of the gradient channel Gs, the FID signal is collected as the FID signal FID1(n) while the test gradient pulse of the gradient channel Gs is applied,
step 2.3.2, setting the occurrence sequence of the radio frequency pulse, the gradient pulse and the sampling event of the second half experiment of the nth experiment: the center of the selected layer gradient pulse of the gradient channel Gs is aligned with the center of the excitation pulse of the radio frequency channel F1, the selected layer echo gradient pulse is applied next to the selected layer gradient pulse, then the test gradient pulse is applied, the strength of the test gradient pulse is set to be zero, the starting time of the sampling window corresponding to the sampling event on the radio frequency channel is aligned with the starting time of the test gradient pulse of the gradient channel Gs, and the FID signal is acquired as the FID signal FID2(n) while the test gradient pulse of the gradient channel Gs is applied.
6. The method as claimed in claim 5, wherein the gradient channels Gs of different experiments have different slice-selective echo gradient pulse intensities, and the slice-selective echo gradient pulse intensities of the first half experiment and the second half experiment of the same experiment are the same.
7. The method as claimed in claim 6, wherein the total number of the experiments M is greater than 1 and less than or equal to 5, 1 < n < M, the gradient channel Gs channel mapping is completed and the experiments are performed sequentially, and the two FID signals obtained from the n-th experiment are marked as FID1(n) and FID2(n),
subtracting the phase of the FID signal FID2(n) from the phase of the FID signal FID1(n), and recording the obtained data as the nth differential phase (n);
extracting each low signal-to-noise ratio distribution interval of the FID signal FID1(1) according to the low signal-to-noise ratio threshold value, searching a FID signal FID1(i) with the highest data signal-to-noise ratio corresponding to the same low signal-to-noise ratio distribution interval as an FID signal to be replaced for each low signal-to-noise ratio distribution interval of the FID signal FID1(1), wherein i is more than or equal to 2 and less than or equal to M, replacing data corresponding to each low signal-to-noise ratio distribution interval in the first differential phase diffphase (1) with data corresponding to the same low signal-to-noise ratio distribution interval in the differential phase corresponding to the searched FID signal to be replaced, performing phase unwrapping on the replaced first differential phase (1), and marking as an unwrapped differential phase diffphase after unwrapping,
k-space trajectory ks(t) the calculation formula is as follows:
Figure FDA0003292550150000031
ks(t)∈{kx(t)，ky(t)，kz(t)}。
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