CN107167752B - Ultra-fast magnetic resonance water-fat separation imaging method - Google Patents

Ultra-fast magnetic resonance water-fat separation imaging method Download PDF

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CN107167752B
CN107167752B CN201710536325.5A CN201710536325A CN107167752B CN 107167752 B CN107167752 B CN 107167752B CN 201710536325 A CN201710536325 A CN 201710536325A CN 107167752 B CN107167752 B CN 107167752B
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罗会俊
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Nanjing Tuobao Medical Technology Co., Ltd.
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Abstract

The invention discloses an ultra-fast magnetic resonance water-fat separation imaging method. The magnetic resonance imaging system comprises a two-dimensional/three-dimensional imaging sequence and a data processing algorithm, is composed of a magnet for generating a uniform magnetic field, a gradient system for generating spatial coding gradient, a radio frequency system for transmitting radio frequency pulse and receiving magnetic resonance signal and clinical operation software, and is characterized in that the imaging sequence and the data processing algorithm have the functions of clinical scanning acceleration or image signal to noise ratio improvement and can eliminate the field non-uniform effect, and can generate an in-phase image, an inverse phase image, a water image, a fat field image and a distribution diagram delta B0And is precise
Figure DDA0001340687500000011
And (5) distribution diagram.

Description

Ultra-fast magnetic resonance water-fat separation imaging method
Technical Field
The invention relates to the technical field of nuclear magnetic imaging, in particular to an ultra-fast magnetic resonance water-fat separation imaging method.
Background
Dixon water-fat separation imaging technology has gradually replaced early fat-pressing imaging technology to become a high-grade magnetic resonance image diagnosis means with more clinical applications, and has unique value for clear development of focus and surrounding tissues under fat inhibition condition and accurate diagnosis of fat diseases. Compared with a two-point Dixon technology, the three-point Dixon technology is beneficial to accurately attributing to pixels at a water-fat boundary and eliminating a channeling effect, and the two technologies can actually accurately calculate the initial phi of an echo on the basis of correct phase unwrapping0And phase error phi to eliminate interference from hardware imperfections and field inhomogeneity effects. The Dixon water-fat separation imaging technique uses the chemical shift difference of water and fat to perform phase encoding to realize independent imaging of different spectrum components. For the three-point Dixon imaging technique, the phase encoding includes three phases, which are commonly used clinically as (0, pi, 2 pi), (-pi, 0, pi), and even (theta)0+θ,θ0+2θ,θ0+3 θ); although the three phases can be selected arbitrarily in the magnetic resonance imaging theory, it is clinically necessary to facilitate the effective separation of the water and fat signals through certain phase differences, and at the same time, to maintain a high signal-to-noise ratio, and to ensure that the algorithm efficiency of the image data analysis is high enough to meet the clinical real-time application requirements. Three-point Dixon imaging techniqueThe implementation of (1) mainly includes two modes: one is to obtain three phase-coded images (including two in-phase images and one anti-phase image) by three times of scanning, and the other is to obtain three phase-coded images by one time of scanning by a single excitation of three echoes, wherein the scanning efficiency of the latter is improved by three times compared with that of the former. The three-point Dixon imaging technology adopting the single-shot mode can be divided into two types, namely gradient echo based imaging and spin echo based imaging, the former has higher clinical efficiency, which is equivalent to the efficiency of SPGR-based T1 weighted imaging, but the scanning speed of the three-point Dixon imaging technology especially for infants and restless patients still has difficulty in fully meeting the clinical diagnosis requirement under the abdominal imaging condition. Moreover, the currently used three-point Dixon imaging technology of a single excitation mode performs frequency encoding by using bipolar gradients, which requires that the strength and linearity of the gradients in all directions have isotropic characteristics and have no interference of eddy current fields and the like, but an actual gradient system generates different zero-order, linear and high-order eddy current fields and maxwell fields in different directions, so that the phase error correction of echo signals required by water-fat separation is sometimes quite complex and depends on the performance parameters of the gradient system.
Disclosure of Invention
The invention aims to provide a three-point Dixon water-fat separation technology which can improve the scanning efficiency or the signal-to-noise ratio and avoid gradient polarity inversion.
The invention provides an ultra-fast magnetic resonance water-fat separation imaging method, which comprises a two-dimensional/three-dimensional imaging sequence and a magnetic resonance imaging system required by executing a data processing algorithm, wherein the system consists of a magnet for generating a uniform magnetic field, a gradient system for generating a spatial coding gradient, a radio frequency system for transmitting radio frequency pulses and receiving magnetic resonance signals and clinical operation software, and is characterized in that the imaging sequence and the data processing algorithm have the functions of clinical scanning acceleration or image signal-to-noise ratio improvement and can eliminate the field non-uniform effect, and can generate an in-phase image, an inverse image, a water image, a fat image and a field distribution image delta B0And precise T2 *And (5) distribution diagram.
The imaging sequence adopts a sectional type excitation mode, N groups of echoes are collected after each sinc or SLR pulse excitation, and each group of three echoes is subjected to phase coding based on chemical shift difference of chemical components; the frequency encoding gradient is composed of a preparatory gradient and a series of trapezoidal gradients which are equally spaced and have the same polarity, the preparatory gradient and the series of trapezoidal gradients are divided into N groups, each group of three trapezoidal gradients are applied to a triangular gradient which has opposite polarity and equal integral area between adjacent trapezoidal gradients, and each group of three echo signals are always acquired during the application period of the trapezoidal gradients with the same polarity.
Preferably, in the fast scan scheme, the N sets of echoes generated by each excitation are separately phase encoded, and the phase encoding gradient is formed by a preliminary phase encoding gradient Gp0And a series of triangular phase encoding gradients G of opposite polarity△pComposition of each phase encoding gradient G△pCorresponds to the middle position of the immediately adjacent gradient of trapezoids of the same polarity, Gp0And performing phase coding circulation, wherein each group of three echoes shares each step of phase coding.
Preferably, the upper and lower portions of k-space are each divided into N regions, NPEFor half the number of all phase encoding steps Dim2, each region is filled with NPEN k-space lines, set G△p＝±GPN, where GPRepresenting the maximum amplitude of the phase encoding gradient, GP0Is added with j.G△pJ is 0 to N-1, and the start position of the k-space line of the j-th region in the phase cycle is determined to be + -G in the center-to-outer periphery filling modep/NPE·i+G△pJ, i ═ 1 to NPEDetermining the position of the current k-space line, wherein the position is +/-G in the outer circumferential center filling modep/
Figure GDA0002599679950000038
/NPE·i+G△pJ, i ═ 1 to NPEN, j ═ N-1 to 0, and the position of the current k-space line is determined, where the positive sign corresponds to the upper k-space half and the negative sign corresponds to the lower k-space half.
Preferably, N sets of echoes are provided, each set of three echoes is phase-coded based on chemical shift differences of chemical components, and the phases of the three echoes can be set to phi0，φ0+△φ，φ0+2Δ φ, wherein φ0Preferably, Δ Φ is preferably pi, and after the three echoes are acquired, three k-space complex matrices are respectively constructed, which respectively correspond to two preferred, and the data processing algorithm respectively calculates the following when Δ τ is 1/. DELTA.f/2 and N is 1 in the fast scanning scheme:
Figure GDA0002599679950000031
Figure GDA0002599679950000032
wherein κ ═ Re (S)2)/|S2|，
Figure GDA0002599679950000033
S1Denotes the first in-phase diagram, S2Denotes the inverse phase diagram, S3Showing a second in-phase diagram.
Preferably, the data processing algorithm is applied when Δ τ is 1/. DELTA.f/2 and N is applied in the fast scan scheme>1, if the Nth group of echoes fills the k-space centerline, i.e., T2 *Weighted case, then S1、S2And S3Can be described by the following formula:
Figure GDA0002599679950000034
Figure GDA0002599679950000035
Figure GDA0002599679950000036
will S1、S2And S3Are respectively multiplied by ei·(2N-2)φ、ei·(2N-1)φAnd ei·(2N)φThereby eliminating the effect of field inhomogeneity, then passing through0＝arctan[Im(S1)/Re(S1)]Calculating an initial phase phi0Then, the S is1、S2And S3Are respectively multiplied by
Figure GDA0002599679950000037
Thus eliminating the initial phase, and then calculating according to the water image and the fat image.
Preferably, the data processing algorithm performs phase encoding and k-space filling in a conventional manner in the snr enhancement scanning scheme, each scanning slice obtains 2N in-phase maps and N anti-phase maps, and N water images and N fat images generated based on the in-phase maps and the anti-phase maps are accumulated respectively to achieve snr enhancement, and the data processing manner is as follows:
Figure GDA0002599679950000041
Figure GDA0002599679950000042
Figure GDA0002599679950000043
in the above formula
Figure GDA0002599679950000044
Figure GDA0002599679950000045
And
Figure GDA0002599679950000046
respectively representing an in-phase diagram complex matrix corresponding to a first in-phase echo, an inverse diagram complex matrix corresponding to a first inverse echo and an in-phase diagram complex matrix corresponding to a second in-phase echo of the mth group of three-point Dixon echoes, wherein m is a natural number between 1 and N.
Preferably, the phase correction method will be
Figure GDA0002599679950000047
And
Figure GDA0002599679950000048
are respectively multiplied by ei·(2m-2)φ、ei·(2m-1)φAnd ei·(2m)φWherein
Figure GDA0002599679950000049
Phase unwrapping by conventional polynomial fitting or region growing methods, and
Figure GDA00025996799500000410
calculating an initial phase phi0Then, then
Figure GDA00025996799500000411
Figure GDA00025996799500000412
And
Figure GDA00025996799500000413
are respectively multiplied by
Figure GDA00025996799500000414
Preferably, after the in-phase map and the inverse map are subjected to phase correction, the water-fat separation image with enhanced signal-to-noise ratio is obtained according to the following formula:
Figure GDA00025996799500000415
Figure GDA00025996799500000416
where κ may be calculated as κ ═ Re (S) after phase correction based on the inverse phase map with the highest signal-to-noise ratio2)/|S2I, here S2Selected as the first inverse phase diagram
Figure GDA00025996799500000417
And generating accurate two-dimensional or three-dimensional values by single exponential fitting using the amplitude of each pixel or voxel in the 2N in-phase maps as a function of the echo time under the condition of larger N
Figure GDA00025996799500000418
And (5) distribution diagram.
Preferably, two-dimensional or three-dimensional
Figure GDA00025996799500000419
Distribution diagram, from the first in-phase diagram complex matrix
Figure GDA00025996799500000420
And a second in-phase diagram complex matrix
Figure GDA0002599679950000051
Respectively extracting the amplitudes of the pixels (k, l)
Figure GDA0002599679950000052
Or
Figure GDA0002599679950000053
Where m denotes the group number of Dixon echoes, and will
Figure GDA0002599679950000054
Or
Figure GDA0002599679950000055
The echo times t ═ (3m-2 · Δ τ or 3m · Δ τ of the sequence shown in fig. 5 in turn form a sequence of numbers; next, the sequence is fitted to as a function of t ═ 3 m-2. DELTA.τ or 3 m. DELTA.τ
Figure GDA0002599679950000056
To obtain
Figure GDA0002599679950000057
A value; then, all the pixels in the in-phase map are traversed to obtain their corresponding pixels in the manner described above
Figure GDA0002599679950000058
Value and plot
Figure GDA0002599679950000059
A distribution diagram; similarly, when m sets of three-point Dixon echoes are acquired after applying a refocusing pulse in the scanning scheme, the echo time is t ═ 3 m-3. delta. tau or (3 m-1. delta. tau), and we can obtain the image domain complex matrix in the same way
Figure GDA00025996799500000510
And
Figure GDA00025996799500000511
and calculate
Figure GDA00025996799500000512
And (5) distribution diagram.
Has the advantages that: the water-fat separation technology can improve the scanning efficiency by N times, simultaneously obtain an in-phase diagram, an inverse diagram, a water image and a fat image with enhanced signal-to-noise ratio and no field non-uniform effect, and also can obtain a field distribution diagram delta B0And is precise
Figure GDA00025996799500000513
The distribution map can also be specially used for enhancing the signal-to-noise of the water-fat separation image or carrying out thin-layer scanning, thereby meeting various clinical requirements.
Drawings
FIG. 1 shows a two-dimensional water-fat separation sequence (version I) according to the present invention.
The two-dimensional water-fat separation sequence is formed by a gradient echo-based single-shot three-point Dixon sequence, and comprises N sequence repeating units. Gp0Is a preliminary phase encoding gradient, G△pIs a triangular phase encoding gradient, GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks.
FIG. 2 shows a two-dimensional water-fat separation sequence (version II) according to the present invention.
Wherein, the two-dimensional water-fat separation sequenceThe initial part of the sequence is composed of a spin echo based single shot three point Dixon sequence, comprising N sequence repeats. Gp0Is a preliminary phase encoding gradient, G△pIs a triangular phase encoding gradient, GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks.
FIG. 3 shows a three-dimensional water-fat separation sequence (version I) according to the present invention.
Wherein, the three-dimensional water-fat separation sequence, the initial part of the sequence is composed of a single-shot three-point Dixon sequence based on gradient echo, and the sequence comprises N sequence repeating units, Gp0Is a preliminary phase encoding gradient, G△pIs a triangular phase encoding gradient, GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks. The layer-selecting direction phase encoding gradient includes phase refocusing gradient, and the radio frequency excitation pulse can also adopt small-angle soft pulse.
FIG. 4 shows a three-dimensional water-fat separation sequence (version II) according to the present invention.
Wherein, the three-dimensional water-fat separation sequence, the initial part of the sequence is formed by a spin echo-based single-shot three-point Dixon sequence, and the sequence comprises N sequence repeating units, Gp0Is a preliminary phase encoding gradient, G△pIs a triangular phase encoding gradient, GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks. The slice-select direction phase encoding gradient includes a phase refocusing gradient.
FIG. 5 shows a two-dimensional water-fat separation sequence (version I) according to the present invention
The two-dimensional water-fat separation sequence is formed by a gradient echo-based single-shot three-point Dixon sequence, and comprises N sequence repeating units. GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks.
FIG. 6 shows a two-dimensional water-fat separation sequence (version II) according to the present invention.
Wherein, the two-dimensional water-fat separation sequence, the initial part of the sequence is a single-shot three-point Dixon sequence based on spin echoComprising N sequence repeat units. GspoilerIs the phase-loss gradient, TE is the echo time, and Δ τ is the time interval between the echo peaks.
Fig. 7 is a flow chart of the present invention.
Wherein, the work flow chart. SS represents the slice selection gradient direction, RO represents the frequency encoding gradient direction, PE represents the phase encoding gradient direction, A represents the echo amplitude attenuation ratio, and N is the number of sequence repeat units or the number of Dixon echo groups. In the two-dimensional imaging mode, Dim3 is set to 1.
Detailed Description
In order to make the technical problems solved, technical solutions adopted and technical effects achieved by the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some but not all of the relevant aspects of the present invention are shown in the drawings.
The invention relates to an ultra-fast magnetic resonance water-fat separation imaging method, which comprises an imaging sequence and a magnetic resonance imaging system required by executing a data processing algorithm, wherein the system consists of a magnet for generating a uniform magnetic field, a gradient system for generating three-dimensional space encoding gradient, a radio frequency system for transmitting radio frequency pulse and receiving magnetic resonance signal and clinical operation software, and is characterized in that the imaging sequence (see figures 1 to 6) adopts single sinc (or SLR) pulse excitation and N groups of echo acquisition modes, each group of three echoes carries out phase encoding based on chemical shift difference of chemical components (such as water and fat), and the phases of the three echoes are respectively set to phi0，φ0+△φ，φ0+ 2. delta. phi, where phi0Preferably 0, and preferably pi, and is equipped with three scanning schemes and data processing modules, the workflow of which is shown in fig. 7. For these imaging sequences, slice-select gradients and slice-select pulses are used for imaging slab or slice selection, which are designed in the same way as for conventional gradient echo sequences, with the frequency-encoding gradients consisting of a preliminary gradient and a series of equally spaced trapezoidal gradients between adjacent trapezoidal gradientsApplying a triangular gradient with opposite polarity and equal integration area, the phase encoding gradient is composed of a preliminary phase encoding gradient Gp0And a series of triangular phase encoding gradients G of opposite polarity△pComposition of each phase encoding gradient G△pCorresponds to the middle position of the adjacent homopolar trapezoidal gradient, and the echo signal is always acquired during the application of the homopolar trapezoidal gradient.
For the scanning scheme (one), the sequences shown in fig. 1 and fig. 2 both use the fast scanning mode to perform segmented phase encoding, and the phase encoding mode and the k-space filling mode are as follows:
dividing the upper and lower parts of k space into N regions, NPEFor half of all phase encoding steps (Dim2), each region is filled with NPEThe number of data points of each k-space line is 3-Dim 1, wherein Dim1 is the number of frequency codes. Number of phase encoding cycles NPE/N, phase cycle time Gp0Is gradually increased or decreased from zero to the maximum value of the phase encoding gradient + -G P1/N, step by + -Gp/NPEAnd N is>Setting G at 1 hour△p＝±GP/N，GP0Is added with j.G△p(j is 0 to N-1) determining the start position of the k-space line of the jth region in the phase cycle, and ± G in the center-to-outer periphery filling modep/NPE·i+G△pJ (i ═ 1 to N)PE/N, j ═ 0 to N-1) determines the position of the current k-space line, ± G in the outer circumference center-fill modep/
Figure GDA0002599679950000073
/NPE·i+G△pJ (i ═ 1 to N)PEN, j ═ N-1 to 0) determines the position of the current k-space line, where the positive sign corresponds to the upper k-space half and the negative sign corresponds to the lower k-space half. And acquiring a group of Dixon echoes by every three-point Dixon sequence unit, wherein the first group of echoes fills the central region, the second group of echoes fills the adjacent region, and the like until the k space is filled. Then, the first in-phase echo data, the reverse phase echo data and the second in-phase echo data are extracted from the k-space file according to the single echo data point number (Dim1)Respectively constructing three k-space data matrixes for echo data, respectively performing one-dimensional inverse Fourier transform along a frequency coding direction, and respectively performing one-dimensional inverse Fourier transform along a phase coding direction to obtain three two-dimensional complex arrays of an image domain, wherein the size of each array is Dim 1.2NPE。
The first phase diagram S is formed when the time interval between the echo peaks is set to Δ τ equal to 1/. DELTA.f/2 and N equal to 11Phase inversion scheme S2And a second in-phase diagram S3The data analysis of (a) is based on the following formula:
Figure GDA0002599679950000071
Figure GDA0002599679950000072
Figure GDA0002599679950000081
here, the first and second liquid crystal display panels are,
Figure GDA0002599679950000082
wherein
Figure GDA0002599679950000083
Is the apparent transverse relaxation time constant. In the case where N is 1, a can be determined based on the intensity of each pixel of the previous two in-phase maps, i.e.,
Figure GDA0002599679950000084
calculation by four-quadrant arctangent function
Figure GDA0002599679950000085
Phase unwrapping by conventional polynomial fitting or region growing methods and based on Delta B0Calculating field distribution diagram delta B according to actual echo time (phi/(2 pi gamma delta tau))0. Then the S is mixed1、S2And S3Are respectively multiplied by
Figure GDA00025996799500000815
Eliminating initial phase and multiplying by 1, e respectivelyi·φAnd ei·2φThereby eliminating the field inhomogeneity effect. In addition, according to S after eliminating phase error2Calculation of κ ═ Re (S)2)/|S2And determining the attribution of the water or fat pixels at the boundary.
Then, the water image and the fat image are separated and cumulatively averaged, respectively, according to the following formula:
Figure GDA0002599679950000086
Figure GDA0002599679950000087
when Δ τ is 1/. DELTA.f/2 and N >1, the in-phase and anti-phase maps can still be analyzed based on equations (1) to (3) and the water and fat images with enhanced signal-to-noise ratio are obtained based on equations (4) and (5) as long as the first set of echoes fills the k-space centerline (i.e., T1 weighted case).
When Δ τ is 1/. DELTA.f/2 and N>1, if the Nth group of echoes fills the k-space centerline (i.e., T)2 *Weighted case), then S1、S2And S3Can be described by the following formula:
Figure GDA0002599679950000088
Figure GDA0002599679950000089
Figure GDA00025996799500000810
can be obtained from the above formula
Figure GDA00025996799500000811
And according to
Figure GDA00025996799500000812
Can calculate
Figure GDA00025996799500000813
And (5) distribution diagram. Calculation by four-quadrant arctangent function
Figure GDA00025996799500000814
And performing phase unwrapping by conventional polynomial fitting or region growing method, and based on Delta B0Calculating according to time echo time to obtain field distribution diagram delta B0. Will S1、S2And S3Are respectively multiplied by ei·(2N-2)φ、ei·(2N-1)φAnd ei·(2N)φThereby eliminating the effect of field inhomogeneity, then passing through0＝arctan[Im(S1)/Re(S1)]Calculating an initial phase phi0Then S1、S2And S3Are respectively multiplied by
Figure GDA0002599679950000091
Thereby eliminating the initial phase. In addition, according to S after eliminating phase error2Calculation of κ ═ Re (S)2)/|S2And determining the attribution of the water or fat pixels at the boundary. Finally, a water image and a fat image are obtained according to the formulas (4) and (5).
For the scanning scheme (ii), the sequences shown in fig. 1 and fig. 2 each add an additional phase encoding gradient in the slice selection direction and acquire signals in the same manner, as shown in fig. 3 and fig. 4, the matched data processing module adds a one-dimensional discrete fourier transform in the slice selection direction during image reconstruction, and then performs phase correction in the same manner, and finally obtains a thin-layer water-fat separation image.
For scan scheme (three), the sequence shown in fig. 1 and 2 is performed using phase encoding and k-space filling for conventional T1 weighted imaging, specifically for image snr enhancement scans, as shown in fig. 5 andas shown in fig. 6. The N sequence repeating units acquire 2N in-phase echoes and N reverse phase echoes in total, and a matched data processing module carries out image reconstruction through discrete inverse Fourier transform and then carries out phase correction and water-fat separation in the same way, which is described in detail below. And, taking a larger value at N (e.g. N)>4) And Δ τ is 1/. DELTA.f/2, the accuracy can be obtained by
Figure GDA0002599679950000092
Distribution diagram:
for each set of Dixon echoes acquired in the sequence shown in fig. 5, the complex matrix is derived from the first in-phase diagram
Figure GDA0002599679950000093
And a second in-phase diagram complex matrix
Figure GDA0002599679950000094
Respectively extracting the amplitudes of the pixels (i, j)
Figure GDA0002599679950000095
Or
Figure GDA0002599679950000096
Where m denotes the group number of the Dixon echoes, m takes a value between 1 and N and will
Figure GDA0002599679950000097
Or
Figure GDA0002599679950000098
The echo times t ═ (3m-2 · Δ τ or 3m · Δ τ of the sequence shown in fig. 5 in turn form a sequence of numbers; next, the sequence is fitted to as a function of t ═ 3 m-2. DELTA.τ or 3 m. DELTA.τ
Figure GDA0002599679950000099
To obtain
Figure GDA00025996799500000910
A value; then, all the pixels in the in-phase map are traversed to obtain their corresponding pixels in the manner described above
Figure GDA00025996799500000911
Value and plot
Figure GDA00025996799500000912
And (5) distribution diagram. For each set of Dixon echoes acquired in the sequence shown in fig. 6, the echo time is either (3m-3) · Δ τ or (3m-1) · Δ τ, which we can obtain in the same way
Figure GDA00025996799500000913
And (5) distribution diagram.
Example 1
The sequence of figure 1 (or figure 2) is loaded on a 1.5T magnetic resonance imaging system and a parameter table is set up, with Dim1 ═ 256, Dim2 ═ 192, NPE96, N2, Δ τ 2.3ms, and TR 160 ms. A sequence is performed and T1-weighted two sets of echoes, each set consisting of three Dixon echoes, are acquired. The upper part and the lower part of the k space are divided into two regions according to the number of echo groups, the number j of the regions is from 1 to 2, and each region is filled with NPEand/N is 48 k-space lines.
For the top half of k-space filling, let GPThe phase encoding is divided into N for 1/2 of the maximum value of the phase encoding gradientPE48 steps, cycle number i from 1 to 48, first set of echoes filling the central region, GP0Sequentially taking values of (i-1) G in the phase coding cycleP96; the jth group of echoes fills the jth peripheral region with a phase encoding gradient of (i-1) · GP/96+(j-1)·G△pWherein G is△p＝Gp/2. For k-space lower half fill, let GPIs 1/2 of the negative maximum of the phase encoding gradient and the sign is negative, the phase encoding gradient can be processed in a similar manner. In the case of partial Fourier reconstruction, N is setPEDim2 · X% ═ 115, where X% ═ 55%. Then, extracting first in-phase echo data, first reverse phase echo data, second in-phase echo data and second reverse phase echo data from the k-space file according to the number (256) of single echo data points, respectively constructing three two-dimensional complex arrays with the size of 256 multiplied by 105, and performing one-dimensional discrete inverse Fourier transform along the frequency coding directionAnd the phase correction is carried out on the reverse phase data according to the mode, then the one-dimensional discrete Fourier transform is carried out to the space frequency domain for partial Fourier reconstruction, or the one-dimensional discrete inverse Fourier transform is carried out after the zero filling is carried out along the phase coding direction, thereby obtaining the in-phase diagram for eliminating the phase error
Figure GDA0002599679950000101
And
Figure GDA0002599679950000102
and a phase inversion diagram
Figure GDA0002599679950000103
And calculating factor k therefrom, and finally obtaining water images and fat images according to the formula (5) and the formula (6). The scanning time used was calculated to be 160ms × 192 × 55%/2 ═ 8.4(s).
Example 2
The sequence of figure 5 (or figure 6) is loaded on a 0.35T magnetic resonance imaging system and a parameter table is set, where Dim1 is 256, Dim2 is 192, Δ τ is 9.8ms, and TR is 160ms, and based on T2 *The degree of weighting sets N, for example, where N is 4. In the phase encoding cycle, 4 groups of echoes share one phase encoding gradient, as shown in fig. 5 (or fig. 6), and k space is filled in a conventional manner to obtain 4 groups of three-point Dixon echoes. Then, data processing is performed as follows:
Figure GDA0002599679950000104
Figure GDA0002599679950000105
Figure GDA0002599679950000111
in the above formula
Figure GDA0002599679950000112
And
Figure GDA0002599679950000113
respectively showing an in-phase diagram complex matrix corresponding to a first in-phase echo, an inverse diagram complex matrix corresponding to a first inverse echo and an in-phase diagram complex matrix corresponding to a second in-phase echo of the mth group of three-point Dixon echoes. A can be determined based on the intensity of each pixel of the previous two in-phase maps, i.e.,
Figure GDA0002599679950000114
in addition, calculate
Figure GDA0002599679950000115
And phase unwrapping is performed by a commonly used polynomial fitting or region growing method. Will be provided with
Figure GDA0002599679950000116
And
Figure GDA0002599679950000117
are respectively multiplied by ei·(2m-2)φ、ei·(2m-1)φAnd ei·(2m)φThen pass through
Figure GDA0002599679950000118
Calculating an initial phase phi0Then, then
Figure GDA0002599679950000119
And
Figure GDA00025996799500001110
are respectively multiplied by
Figure GDA00025996799500001111
In addition, since the assignment of pixels to water-fat boundaries does not depend on the choice of m, κ may be calculated as the phase correction after the above-described phase correction based on the inverse map with the highest signal-to-noise ratio
Figure GDA00025996799500001112
Then, a water-fat separation image with enhanced signal-to-noise ratio is obtained according to the following formula:
Figure GDA00025996799500001113
Figure GDA00025996799500001114
finally, selecting the same phase diagram
Figure GDA00025996799500001115
The amplitude of the first pixel in the same phase diagram is one by one
Figure GDA00025996799500001116
Or
Figure GDA00025996799500001117
According to the echo time delta tau, 3 delta tau, 4 delta tau, 6 delta tau, 7 delta tau, 9 delta tau,
10 Δ τ,12 Δ τ., (3N-2) · Δ τ,3N · Δ τ are arranged in a series and fitted to the following equation:
Figure GDA00025996799500001118
thereby obtaining a first pixel
Figure GDA00025996799500001119
The value is obtained. Likewise, for all other pixels
Figure GDA00025996799500001120
Or
Figure GDA00025996799500001121
Processed in a similar manner and fitted to the following equation:
Figure GDA00025996799500001122
thereby obtaining each pixel
Figure GDA00025996799500001123
Value and plot
Figure GDA00025996799500001124
And (5) distribution diagram. When using volume scanning or other field strength conditions, three-dimensional can be obtained in a similar manner
Figure GDA00025996799500001125
And (5) distribution diagram.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: modifications of the technical solutions described in the embodiments or equivalent replacements of some or all technical features may be made without departing from the scope of the technical solutions of the embodiments of the present invention.

Claims (5)

1. An ultra-fast magnetic resonance water-fat separation imaging method comprises a two-dimensional/three-dimensional imaging sequence and a magnetic resonance imaging system necessary for executing a data processing algorithm, wherein the system consists of a magnet for generating a uniform magnetic field, a gradient system for generating a spatial coding gradient, a radio frequency system for transmitting radio frequency pulses and receiving magnetic resonance signals and clinical operation software, and is characterized in that the imaging sequence and the data processing algorithm have the functions of clinical scanning acceleration or image signal-to-noise ratio improvement and can eliminate the field non-uniform effect, and can generate an in-phase image, an anti-phase image, a water image, a fat image and a field distribution diagram delta B0And is precise
Figure FDA0002599679940000011
A distribution diagram;
the imaging sequence adopts a sectional type excitation mode, N groups of echoes are collected after each sinc or SLR pulse excitation, and each group of three echoes is subjected to phase coding based on chemical shift difference of chemical components; the frequency coding gradient consists of a preparatory gradient and a series of trapezoidal gradients which are equally spaced and have the same polarity, the preparatory gradient and the series of trapezoidal gradients are divided into N groups, each group of three trapezoidal gradients are applied with a triangular gradient which has opposite polarity and equal integral area between adjacent trapezoidal gradients, and each group of three echo signals are always acquired during the application period of the trapezoidal gradients with the same polarity;
the data processing algorithm calculates the water and fat images when Δ τ is 1/. DELTA.f/2 and N is 1 in the fast scan scheme as follows:
Figure FDA0002599679940000012
Figure FDA0002599679940000013
wherein κ ═ Re (S)2)/|S2|，
Figure FDA0002599679940000014
S1Denotes the first in-phase diagram, S2Denotes the inverse phase diagram, S3Represents a second in-phase diagram;
the data processing algorithm, in a fast scan scheme, when Δ τ is 1/. DELTA.f/2 and N>1, if the Nth group of echoes fills the k-space centerline, i.e., T2 *Weighted case, then S1、S2And S3Can be described by the following formula:
Figure FDA0002599679940000015
Figure FDA0002599679940000016
Figure FDA0002599679940000017
will S1、S2And S3Are respectively multiplied by ei·(2N-2)φ、ei·(2N-1)φAnd ei·(2N)φWherein
Figure FDA0002599679940000021
Thereby eliminating the effect of field inhomogeneity, then passing phi0＝arctan[Im(S1)/Re(S1)]Calculating an initial phase phi0Then, the S is1、S2And S3Are respectively multiplied by
Figure FDA0002599679940000022
Thus eliminating the initial phase, and then calculating according to the water image and the fat image to obtain a water image and a fat image;
the data processing algorithm adopts a conventional mode to carry out phase coding and k space filling in a signal-to-noise ratio enhancement scanning scheme, each scanning layer surface obtains 2N in-phase images and N anti-phase images, N water images and N fat images generated based on the in-phase images and the anti-phase images are respectively accumulated to realize signal-to-noise ratio enhancement, and the data processing mode is as follows:
Figure FDA0002599679940000023
Figure FDA0002599679940000024
Figure FDA0002599679940000025
in the above formula
Figure FDA0002599679940000026
Figure FDA0002599679940000027
And
Figure FDA0002599679940000028
respectively representing an in-phase diagram complex matrix corresponding to a first in-phase echo, an inverse diagram complex matrix corresponding to a first inverse echo and an in-phase diagram complex matrix corresponding to a second in-phase echo of an m-th group of three-point Dixon echoes, wherein m represents the group number of the Dixon echoes, and the value of m is between 1 and N;
the phase correction method is to
Figure FDA0002599679940000029
And
Figure FDA00025996799400000210
are respectively multiplied by ei·(2m-2)φ、ei·(2m-1)φAnd ei·(2m)φWherein
Figure FDA00025996799400000211
Phase unwrapping by conventional polynomial fitting or region growing methods, and
Figure FDA00025996799400000212
calculating an initial phase phi0Then, then
Figure FDA00025996799400000213
And
Figure FDA00025996799400000214
are respectively multiplied by
Figure FDA00025996799400000215
And after the in-phase diagram and the inverse diagram are subjected to phase correction, obtaining a water-fat separation image with enhanced signal-to-noise ratio according to the following formula:
Figure FDA00025996799400000216
Figure FDA0002599679940000031
where κ may be calculated as κ ═ Re (S) after phase correction based on the inverse phase map with the highest signal-to-noise ratio2)/|S2I, here S2Selected as the first inverse phase diagram
Figure FDA0002599679940000032
And generating accurate two-dimensional or three-dimensional values by single exponential fitting using the amplitude of each pixel or voxel in the 2N in-phase maps as a function of the echo time under the condition of larger N
Figure FDA0002599679940000033
And (5) distribution diagram.
2. The method of claim 1, wherein each of the N echoes generated by each excitation is phase-encoded separately in a fast scan scheme, and the phase-encoding gradient is defined by a preliminary phase-encoding gradient Gp0And a series of triangular phase encoding gradients G of opposite polarity△pComposition of each phase encoding gradient G△pCorresponds to the middle position of the immediately adjacent gradient of trapezoids of the same polarity, Gp0And performing phase coding circulation, wherein each group of three echoes shares each step of phase coding.
3. The ultra-fast MRI water-fat separation imaging method as claimed in claim 2, wherein the upper and lower portions of k-space are divided into N regions, NPEFor half the number of all phase encoding steps Dim2, each region is filled with NPEN k-space lines, set G△p＝±GPN, where GPRepresenting the maximum amplitude of the phase encoding gradient, GP0Is added with j.G△pJ is 0 to N-1, when determining the phase cycleStarting position of k-space line of jth region in center-to-periphery filling mode + -Gp/NPE·i+G△pJ, i ═ 1 to NPEDetermining the position of the current k-space line, under the mode of outer circumferential center filling
Figure FDA0002599679940000036
To NPEN, j ═ N-1 to 0, and the position of the current k-space line is determined, where the positive sign corresponds to the upper k-space half and the negative sign corresponds to the lower k-space half.
4. The method of claim 1, wherein N sets of echoes are used, each set of three echoes is phase-coded based on chemical shift differences of chemical components, and the phases of the three echoes are set to phi 0 and phi respectively0+△φ，φ0+ 2. delta. phi, where phi0The three echoes are acquired to form three k-space complex matrixes which respectively correspond to two in-phase images and an anti-phase image.
5. The ultrafast magnetic resonance water-fat separation imaging method as claimed in claim 1, wherein the two-dimensional or three-dimensional
Figure FDA0002599679940000034
Distribution diagram, from the first in-phase diagram complex matrix
Figure FDA0002599679940000035
And a second in-phase diagram complex matrix
Figure FDA0002599679940000041
Respectively extracting the amplitudes of the pixels (k, l)
Figure FDA0002599679940000042
Or
Figure FDA0002599679940000043
Where m denotes the group number of Dixon echoes, and will
Figure FDA0002599679940000044
Or
Figure FDA0002599679940000045
According to the signal-to-noise ratio enhancement scanning scheme, the echo time t ═ 3 m-2. delta. tau or 3 m. delta. tau form a sequence in sequence; next, the sequence is fitted to as a function of t ═ 3 m-2. DELTA.τ or 3 m. DELTA.τ
Figure FDA0002599679940000046
To obtain
Figure FDA0002599679940000047
A value; then, all the pixels in the in-phase map are traversed to obtain their corresponding pixels in the manner described above
Figure FDA0002599679940000048
Value and plot
Figure FDA0002599679940000049
A distribution diagram; similarly, when m sets of three-point Dixon echoes are acquired after applying a refocusing pulse in the scanning scheme, the echo time is t ═ 3 m-3. delta. tau or (3 m-1. delta. tau), and we can obtain the image domain complex matrix in the same way
Figure FDA00025996799400000410
And
Figure FDA00025996799400000411
and calculate
Figure FDA00025996799400000412
And (5) distribution diagram.
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