Patent Publication Number: US-2022236355-A1

Title: Magnetic resonance water-fat image separation method and apparatus, imaging system and storage medium

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to Chinese Patent Application No. 202110113075.0, filed Jan. 27, 2021, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Field 
     The present disclosure relates to the technical field of magnetic resonance imaging, in particular to a magnetic resonance imaging water-fat image separation method and apparatus, a magnetic resonance imaging system and a computer readable storage medium. 
     Related Art 
     In a magnetic resonance imaging (MRI) system, due to the fact that hydrogen atom nuclei in water and hydrogen atom nuclei in fat inside the human body are in different molecular environments, their resonant frequencies are different; when hydrogen protons in fat and other tissues are excited by an RF pulse at the same time, their relaxation times are also different. When signals are acquired at different echo times, fat tissue and water exhibit different phases and signal intensities. 
     The Dixon method is a method used to produce a pure water proton image in MRI, the basic principle thereof being to separately acquire two types of echo signals, namely in-phase and out-of-phase echo signals, of water and fat protons; the two types of signals with different phases undergo arithmetic operations, each producing a pure water proton image and a pure fat proton image, thereby achieving the objective of fat suppression. At present, there are many types of Dixon water-fat image separation methods, including the single-point Dixon method, the two-point Dixon method and the three-point Dixon method, etc. 
     In a Dixon-based turbo spin echo (TSE, also called fast spin echo, FSE) pulse sequence, the radio frequency (RF) pulse sequence includes a 90° excitation pulse and a 180° refocusing pulse (also called a rephasing pulse), and multiple echoes can be acquired between two adjacent refocusing pulses, i.e. within one echo interval (also called an echo period). Echoes at corresponding positions corresponding to different refocusing pulses form a set of echoes, for example: a first echo appearing after refocusing pulse 1, a first echo appearing after refocusing pulse 2, . . . , a first echo appearing after refocusing pulse n can form one set of echoes, and a second echo appearing after refocusing pulse 1, a second echo appearing after refocusing pulse 2, . . . , a second echo appearing after refocusing pulse n can form another set of echoes. An image can be independently reconstructed from each set of echoes. Because different echoes carry different amplitude and phase information, a pure water image and a pure fat image can be separately reconstructed by further calculation. 
     In view of the fact that some of the existing water-fat image separation methods have a large number of dead zones and a low acquisition efficiency, while some have a low signal-to-noise ratio (SNR), those skilled in the art are still committed to searching for other solutions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the embodiments of the present disclosure and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the pertinent art to make and use the embodiments. 
         FIG. 1  is a flowchart of the MRI water-fat image separation method according to an exemplary embodiment of the disclosure. 
         FIG. 2A  is a schematic diagram of a data acquisition sequence of an existing two-point Dixon method based on a turbo spin echo (TSE) sequence according to an exemplary embodiment of the disclosure. 
         FIG. 2B  is a schematic diagram of a data acquisition sequence of a two-point Dixon method based on a turbo spin echo (TSE) sequence according to an exemplary embodiment of the disclosure. 
         FIG. 2C  is a schematic diagram of a data acquisition sequence of a three-point Dixon method based on a turbo spin echo (TSE) sequence according to an exemplary embodiment of the disclosure. 
         FIG. 3  is a flowchart of a process of using acquired images to perform image reconstruction with water-fat separation according to an exemplary embodiment of the disclosure. 
         FIG. 4  is a flowchart of a process of using acquired images to perform image reconstruction with water-fat separation according to an exemplary embodiment of the disclosure. 
         FIGS. 5A-5C  illustrate a MRI water-fat image separation apparatus according to exemplary embodiments of the disclosure. 
         FIG. 6  illustrates a MRI water-fat image separation apparatus according to an exemplary embodiment of the disclosure. 
         FIGS. 7A and 7B  illustrate an image quality comparison of in vitro phantom water images obtained using a conventional two-point Dixon water-fat separation algorithm and the MRI water-fat image separation method according to an exemplary embodiment of the disclosure, respectively. 
         FIG. 7C  illustrates ratio results for the two images in  FIGS. 7B and 7A , according to an exemplary embodiment of the disclosure. 
         FIGS. 8A and 8B  show an image quality comparison of hip joint water images obtained using the MRI water-fat image separation method and a combined-echo two-point Dixon water-fat separation algorithm with sampling performed twice, according to an exemplary embodiment of the disclosure. 
       The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings. Elements, features and components that are identical, functionally identical and have the same effect are—insofar as is not stated otherwise—respectively provided with the same reference character. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it will be apparent to those skilled in the art that the embodiments, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring embodiments of the disclosure. The connections shown in the figures between functional units or other elements can also be implemented as indirect connections, wherein a connection can be wireless or wired. Functional units can be implemented as hardware, software or a combination of hardware and software. 
     In view of the above, embodiments of the present disclosure propose an MRI water-fat image separation method, and further propose an MRI water-fat image separation apparatus, an MRI system and a computer readable storage medium, in order to increase the acquisition efficiency and SNR. 
     An MRI water-fat image separation method proposed in embodiments of the present disclosure comprises: within one echo period, acquiring a first echo set corresponding to Dixon water-fat separation calculation under a first readout gradient polarity, and acquiring a second echo set corresponding to Dixon water-fat separation calculation under a second readout gradient polarity, wherein the first readout gradient polarity and the second readout gradient polarity are opposite, and echoes in the second echo set are located at positions that are positionally symmetric to echoes in the first echo set with respect to the echo center of the echo period; obtaining a first echo image set based on first echo set data acquired in each echo period, and obtaining a second echo image set based on second echo set data acquired in each echo period; and using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In one embodiment, the first echo set and the second echo set each comprise: one nearly-in-phase echo meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is two-point Dixon water-fat separation calculation; alternatively, the first echo set and the second echo set each comprise: one nearly-in-phase echo meeting a set in-phase requirement and two nearly-out-of-phase echoes meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is three-point Dixon water-fat separation calculation. 
     In one embodiment, the step of using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image, comprises: separately subjecting each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to phase difference correction, then separately subjecting each pair of echo images resulting from phase difference correction to complex averaging, to obtain a corresponding echo averaged image set; and using the images in the echo averaged image set obtained to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In one embodiment, the step of separately subjecting each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to phase difference correction, comprises: subjecting either one of each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to complex conjugate transformation, to obtain a first processing image; subtracting, from the first processing image or the other echo image that has not undergone complex conjugate transformation, a low-pass phase difference between the first processing image and the echo image that has not undergone complex conjugate transformation, to obtain a second processing image, wherein the second processing image and the echo image which has not had the low-pass phase difference subtracted therefrom together form two echo images resulting from phase difference correction. 
     In one embodiment, the step of using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image, comprises: using the first echo image set to perform Dixon water-fat separation calculation, to obtain a first water image and a first fat image; using the second echo image set to perform Dixon water-fat separation calculation, to obtain a second water image and a second fat image; subjecting the first water image and the second water image to modulus averaging, to obtain a water image; and subjecting the first fat image and the second fat image to modulus averaging, to obtain a fat image. 
     An MRI water-fat image separation apparatus proposed in embodiments of the present disclosure comprises: a data acquisition module, for: within one echo period, acquiring a first echo set corresponding to Dixon water-fat separation calculation under a first readout gradient polarity, and acquiring a second echo set corresponding to Dixon water-fat separation calculation under a second readout gradient polarity, wherein the first readout gradient polarity and the second readout gradient polarity are opposite, and echoes in the second echo set are located at positions that are positionally symmetric to echoes in the first echo set with respect to the echo center of the echo period; an image extraction module, for obtaining a first echo image set based on first echo set data acquired in each echo period, and obtaining a second echo image set based on second echo set data acquired in each echo period; a water-fat separation module, for using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In one embodiment, the first echo set and the second echo set each comprise: one nearly-in-phase echo meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is two-point Dixon water-fat separation calculation; alternatively, the first echo set and the second echo set each comprise: two nearly-in-phase echoes meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is three-point Dixon water-fat separation calculation. 
     In one embodiment, the water-fat separation module comprises: a first image integration module, for separately subjecting each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to phase difference correction, then separately subjecting each pair of echo images resulting from phase difference correction to complex averaging, to obtain a corresponding echo averaged image set; and a first water-fat separation calculation module, for using the images in the echo averaged image set obtained to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In one embodiment, the first image integration module comprises: a complex conjugate transformation module, for subjecting either one of each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to complex conjugate transformation, to obtain a first processing image; a phase correction module, for subtracting, from the first processing image or the other echo image that has not undergone complex conjugate transformation, a low-pass phase difference between the first processing image and the echo image that has not undergone complex conjugate transformation, to obtain a second processing image; wherein the second processing image and the echo image which has not had the low-pass phase difference subtracted therefrom together form two echo images resulting from phase difference correction; and a complex averaging module, for separately subjecting each pair of echo images resulting from phase difference correction to complex averaging, to obtain a corresponding echo averaged image set. 
     In one embodiment, the water-fat separation module comprises: a second water-fat separation calculation module, for using the first echo image set to perform Dixon water-fat separation calculation, to obtain a first water image and a first fat image; and using the second echo image set to perform Dixon water-fat separation calculation, to obtain a second water image and a second fat image; and a second image integration module, for subjecting the first water image and the second water image to modulus averaging, to obtain a water image; and subjecting the first fat image and the second fat image to modulus averaging, to obtain a fat image. 
     An MRI water-fat image separation apparatus proposed in embodiments of the present disclosure comprises: at least one memory and at least one processor, wherein: the at least one memory is configured to store a computer program; the at least one processor is configured to call the computer program stored in the at least one memory, and perform the MRI water-fat image separation method as described in any one of the embodiments above. 
     An MRI system proposed in embodiments of the present disclosure comprises the MRI water-fat image separation apparatus as described in any one of the embodiments above. 
     A computer readable storage medium proposed in embodiments of the present disclosure has a computer program stored thereon, wherein the computer program is executable by a processor and realizes the MRI water-fat image separation method as described in any one of the embodiments above. 
     It can be seen from the solution above that because two echo sets corresponding to Dixon water-fat separation calculation are read in one echo period in embodiments of the present disclosure, nearly all of the time between two refocusing pulses is used for data acquisition, thus increasing the data acquisition efficiency and reducing dead zones. Moreover, each echo set is read out under the same readout gradient polarity, so there is no need to perform phase correction in each echo set. 
     In addition, the echo images corresponding to the two echo sets can undergo image merging to increase SNR followed by water-fat separation calculation, or undergo water-fat separation calculation followed by image merging to increase SNR; compared with a conventional TSE-Dixon water-fat separation calculation method, the number of echoes obtained in a very similar capture time is doubled, so the SNR is increased, and better water-fat imaging quality is achieved. Furthermore, by implementing the phase correction process before averaging in the merging process, it is possible to reduce redundancy and ensure consistency of local and global water-fat separation results. 
     In embodiments of the present disclosure, account is taken of the fact that at the present time, in order to increase the acquisition speed and reduce motion sensitivity, a fast-switching bipolar readout gradient can be used during data acquisition to acquire multiple echoes between each pair of refocusing pulses. In addition, account is taken of the fact that a recently developed Dixon reconstruction method allows sampling of nearly-out-of-phase (approximately out of phase, e.g. 120°-180°) and nearly-in-phase (approximately in phase, e.g. 0°-30°) echoes, rather than precisely out-of-phase and in-phase echoes, as was the case with the original Dixon method. Thus, in this embodiment, consideration is given to the following: acquiring two echo sets between each pair of refocusing pulses, each echo set comprising nearly out-of-phase and nearly-in-phase echoes for corresponding Dixon water-fat separation calculation, in order to increase the data acquisition speed. In addition, when using the acquired echo images to perform Dixon water-fat separation calculation, the images corresponding to different echo sets may be subjected to processing consisting of merging followed by water-fat separation calculation, or water-fat separation calculation followed by merging, thereby increasing the SNR of the water image and the fat image. 
     In order to clarify the object, technical solution and advantages of the present disclosure, the present disclosure is explained in further detail below by way of embodiments. 
       FIG. 1  is a demonstrative flow chart of the MRI water-fat image separation method in an embodiment of the present disclosure. As  FIG. 1  shows, the method may comprise the following steps: 
     Step  101 : within one echo period, acquiring a first echo set corresponding to Dixon water-fat separation calculation under a first readout gradient polarity, and acquiring a second echo set corresponding to Dixon water-fat separation calculation under a second readout gradient polarity, wherein the first readout gradient polarity and the second readout gradient polarity are opposite, and echoes in the first echo set are positionally one-to-one symmetric to echoes in the second echo set with respect to the echo center of the echo period, i.e. the echoes in the second echo set are located at positions that are positionally symmetric to the echoes in the first echo set with respect to the echo center of the echo period. 
       FIG. 2A  shows a schematic diagram of a data acquisition sequence of an existing two-point Dixon method based on a turbo spin echo (TSE) sequence.  FIG. 2B  shows a schematic diagram of a data acquisition sequence of a two-point Dixon method based on a turbo spin echo (TSE) sequence in one example of the present disclosure, and  FIG. 2C  is a schematic diagram of a data acquisition sequence of a three-point Dixon method based on a turbo spin echo (TSE) sequence in one example of the present disclosure. Only one spin echo period (echo spacing, ESP) is shown in  FIGS. 2A-2C . One repetition time includes multiple spin echo periods as shown in  FIGS. 2A-2C , i.e. multiple 180-degree rephasing RF pulses are included between two 90-degree RF pulses. RF, Gr and Ech correspond to the RF pulse, readout direction gradient and echo data, respectively. 
     As shown in  FIG. 2A , within one spin echo period of the echo sequence, i.e. between two 180-degree rephasing RF pulses RF_ 1  and RF_ 2 , the MRI equipment applies a readout gradient in the readout gradient direction, and reads an out-of-phase echo Ea and an in-phase echo Eb. 
     As shown in  FIG. 2B , within one spin echo period of the echo sequence, i.e. between two 180-degree rephasing RF pulses RF_ 1  and RF_ 2 , at first echo times (TE 1 ) and second echo times (TE 2 ) which are symmetrically distant from the echo center C, the MRI equipment applies two readout gradients in the readout gradient direction, reads four corresponding echoes E 1 , E 2 , E 3 , E 4  based on the effect of the readout gradients, and puts the four echoes E 1 , E 2 , E 3 , E 4  that are read into k-space. Echoes E 1  and E 3  are read out under the same readout gradient polarity and form an echo pair corresponding to a two-point Dixon water-fat separation algorithm; echoes E 2  and E 4  are read out under the same readout gradient polarity and form another echo pair corresponding to a two-point Dixon water-fat separation algorithm. These two echo pairs have the same dephasing factor, but opposite gradient polarities. Readout is repeated throughout the echo sequence. Each echo pair, E 1  and E 3 , E 2  and E 4 , is obtained with the same readout gradient polarity, and water-fat separation can be performed without performing phase correction. Echoes E 2  and E 3  are nearly-in-phase echoes which meet a set in-phase requirement and are positionally symmetric with respect to the echo center C of the echo period, with a complex conjugate relationship therebetween; echoes E 1  and E 4  are nearly-out-of-phase echoes which meet a set out-of-phase requirement and are positionally symmetric with respect to the echo center C of the echo period, with a complex conjugate relationship therebetween. 
     Of course, in other examples, the choice of TEs of echoes E 2 , E 3  and echoes E 1 , E 4  is flexible; as long as water-fat separation is stable, noise amplification is acceptable. For example, when using a 1.5 T system, echoes E 2 , E 3  and echoes E 1 , E 4  are nearly in phase and out of phase, respectively. When using a 3T system, they can set in another way, i.e. echoes E 2 , E 3  are nearly out of phase, while echoes E 1 , E 4  are nearly in phase. 
     As can be seen, for the two-point Dixon water-fat separation calculation method shown in  FIG. 2 , the first echo set and the second echo set are each an echo pair comprising one nearly-in-phase echo meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement. 
     As shown in  FIG. 2C , within one spin echo period of the echo sequence, i.e. between two 180-degree rephasing RF pulses RF_ 1  and RF_ 2 , at first echo times (TE 1 ), second echo times (TE 2 ) and third echo times (TE 3 ) which are symmetrically distant from the echo center C, the MRI equipment applies three readout gradients in the readout gradient direction, reads six corresponding echoes E 1 , E 2 , E 3 , E 4 , E 5 , E 6  based on the effect of the readout gradients, and puts the six echoes E 1 , E 2 , E 3 , E 4 , E 5 , E 6  that are read into k-space. Echoes E 1 , E 3  and E 5  are read out under the same readout gradient polarity and form an echo set corresponding to a three-point Dixon water-fat separation algorithm; echoes E 2 , E 4  and E 6  are read out under the same readout gradient polarity and form another echo set corresponding to a three-point Dixon water-fat separation algorithm. These two echo sets have the same dephasing factor, but opposite gradient polarities. Readout is repeated throughout the echo sequence. Each echo set, E 1 , E 3  and E 5 , E 2 , E 4  and E 6 , is obtained with the same readout gradient polarity, and water-fat separation can be performed without performing phase correction. Echoes E 3  and E 4  are nearly-in-phase echoes which meet a set in-phase requirement and are positionally symmetric with respect to the echo center C of the echo period, with a complex conjugate relationship therebetween; echoes E 1  and E 6  are nearly-out-of-phase echoes which meet a set out-of-phase requirement and are positionally symmetric with respect to the echo center C of the echo period, with a complex conjugate relationship therebetween; and echoes E 5  and E 2  are also nearly-out-of-phase echoes which meet a set out-of-phase requirement and are positionally symmetric with respect to the echo center C of the echo period, with a complex conjugate relationship therebetween. 
     As can be seen, for the three-point Dixon water-fat separation calculation method, the first echo set and the second echo set may each comprise: one nearly-in-phase echo meeting a set in-phase requirement and two nearly-out-of-phase echoes meeting a set out-of-phase requirement. Moreover, the nearly-in-phase echo in the first echo set and the nearly-in-phase echo in the second echo set are positionally symmetric with respect to the echo center of the echo period, and the two nearly-out-of-phase echoes in the first echo set and the two nearly-out-of-phase echoes in the second echo set are positionally symmetric with respect to the echo center of the echo period. 
     In addition, this embodiment is likewise suitable for other Dixon water-fat separation calculation methods. An item-by-item description is not repeated here. 
     Step  102 : obtaining a first echo image set based on first echo set data acquired in each echo period, and obtaining a second echo image set based on second echo set data acquired in each echo period. 
     In this step, a first echo image set is obtained based on the first echo set data acquired in each echo period of the echo sequence in k-space, and a second echo image set is obtained based on the second echo set data acquired in each echo period of the echo sequence in k-space. 
     Taking the data acquisition sequence shown in  FIG. 2B  as an example, an out-of-phase image corresponding to E 1  can be obtained for data of echo E 1  acquired in each echo period in k-space; an in-phase image corresponding to E 3  can be obtained for data of echo E 3  acquired in each echo period in k-space; and the out-of-phase image corresponding to E 1  and the in-phase image corresponding to E 3  together form the first echo image set. By the same principle, an in-phase image corresponding to E 2  can be obtained for data of echo E 2  acquired in each echo period in k-space; an out-of-phase image corresponding to E 4  can be obtained for data of echo E 4  acquired in each echo period in k-space; and the in-phase image corresponding to E 2  and the out-of-phase image corresponding to E 4  together form the second echo image set. 
     Step  103 : using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In this step, when particular implementation is performed, two implementation methods are possible: 
     First Implementation Method: Image Merging Followed by Water-Fat Separation Calculation 
     Each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set is separately subjected to phase difference correction, each pair of echo images that has undergone phase difference correction is then separately subjected to complex averaging, thereby obtaining a corresponding echo averaged image set by merging; then the images in the echo averaged image set obtained are used to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. Specifically, the process of subjecting the two echo images with symmetrical echo positions to phase difference correction may comprise: subjecting either one of the two echo images with symmetrical echo positions to complex conjugate transformation, to obtain a first processing image; subtracting, from the first processing image or the other echo image that has not undergone complex conjugate transformation, a low-pass phase difference between the first processing image and the echo image that has not undergone complex conjugate transformation, to obtain a second processing image; wherein the second processing image and the echo image which has not had the low-pass phase difference subtracted therefrom together form the two echo images resulting from phase difference correction. 
     Still taking the data acquisition sequence shown in  FIG. 2B  as an example,  FIG. 3  shows a schematic diagram of the process of using acquired images to perform image reconstruction with water-fat separation in one example of the present disclosure. As shown in  FIG. 3 , after obtaining the first echo image set formed of the out-of-phase image corresponding to E 1  and the in-phase image corresponding to E 3 , and the second echo image set formed of the in-phase image corresponding to E 2  and the out-of-phase image corresponding to E 4 , which were mentioned in step  102 , due to the fact that the out-of-phase image corresponding to E 1  and the out-of-phase image corresponding to E 4  are two images with symmetrical echo positions, and the in-phase image corresponding to E 3  and the in-phase image corresponding to E 2  are two images with symmetrical echo positions, the two images with symmetrical echo positions can be subjected to phase difference correction first; when particular processing is performed, the two images with positional symmetry have equal status, and either one may be taken as the standard for performing phase difference correction calculation. Only one implementation situation is shown in  FIG. 3 , and this may for example comprise the following steps: 
     Step  31 A: subjecting the in-phase image corresponding to E 2  to complex conjugate transformation, to obtain a first in-phase processing image; step  31 B: subjecting the out-of-phase image corresponding to E 4  to complex conjugate transformation, to obtain a first out-of-phase processing image. 
     In other embodiments, step  31 A may also comprise subjecting the in-phase image corresponding to E 3  to complex conjugate transformation, and by the same principle, step  31 B may also comprise subjecting the out-of-phase image corresponding to E 1  to complex conjugate transformation. 
     Step  32 A, subtracting, from the first in-phase processing image, a low-pass phase difference between the first in-phase processing image and the in-phase image corresponding to E 3 , to obtain a second in-phase processing image; step  32 B, subtracting, from the first out-of-phase processing image, a low-pass phase difference between the first out-of-phase processing image and the out-of-phase image corresponding to E 1 , to obtain a second out-of-phase processing image. 
     In other embodiments, step  32 A may also comprise subtracting, from the in-phase image corresponding to E 3 , the low-pass phase difference between the first in-phase processing image and the in-phase image corresponding to E 3 , and by the same principle, step  32 B may also comprise subtracting, from the out-of-phase image corresponding to E 1 , the low-pass phase difference between the first out-of-phase processing image and the out-of-phase image corresponding to E 1 . 
     Step  33 A: subjecting the second in-phase processing image and the in-phase image corresponding to E 3  to complex averaging, and integrating the two images as an echo averaged in-phase image; step  33 B, subjecting the second out-of-phase processing image and the out-of-phase image corresponding to E 1  to complex averaging, and integrating the two images as an echo averaged out-of-phase image. 
     Step  34 : using the echo averaged in-phase image and the echo averaged out-of-phase image to perform two-point Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     In addition, the three-point Dixon water-fat separation method is similar, so an item-by-item description is not repeated here. 
     Second Implementation Method: Water-Fat Separation Calculation Followed by Image Merging 
     The first echo image set is used to perform Dixon water-fat separation calculation, to obtain a first water image and a first fat image; the second echo image set is used to perform Dixon water-fat separation calculation, to obtain a second water image and a second fat image; the first water image and the second water image are subjected to modulus averaging, to obtain a water image; and the first fat image and the second fat image are subjected to modulus averaging, to obtain a fat image. 
     Still taking the data acquisition sequence shown in  FIG. 2B  as an example,  FIG. 4  shows a schematic diagram of the process of using acquired images to perform image reconstruction with water-fat separation in another example of the present disclosure. As  FIG. 4  shows, the process may comprise the following steps: 
     Step  41 A: using the out-of-phase image corresponding to E 1  and the in-phase image corresponding to E 3  to perform two-point Dixon water-fat separation calculation, to obtain a first water image W 1  and a first fat image F 1 ; step  41 B: using the in-phase image corresponding to E 2  and the out-of-phase image corresponding to E 4  to perform two-point Dixon water-fat separation calculation, to obtain a second water image W 2  and a second fat image F 2 . 
     Step  42 A: subjecting the first water image W 1  to complex conjugate transformation, to obtain a first processing water image W 1 *; step  42 B: subjecting the first fat image F 1  to complex conjugate transformation, to obtain a first processing fat image F 1 *. 
     In other embodiments, step  42 A may also comprise subjecting the second water image to complex conjugate transformation, and step  42 B may also comprise subjecting the second fat image to complex conjugate transformation. 
     Step  43 A: subtracting, from the first processing water image, a low-pass phase difference between the first processing water image and the second water image, to obtain a second processing water image; step  43 B: subtracting, from the first processing fat image, a low-pass phase difference between the first processing fat image and the second fat image, to obtain a second processing fat image. 
     Step  44 A: subjecting the second processing water image and the second water image to complex averaging, to obtain a water image by integration; step  44 B: subjecting the second processing fat image and the second fat image to complex averaging, to obtain a fat image by integration. 
     The MRI water-fat image separation method in embodiments of the present disclosure has been described in detail above; the MRI water-fat image separation apparatus in embodiments of the present disclosure is described in detail below. The MRI water-fat image separation apparatus in embodiments of the present disclosure can be used to implement the MRI water-fat image separation method in embodiments of the present disclosure. For particulars not disclosed in detail in the apparatus embodiments of the present disclosure, reference can be made to the corresponding description in the method embodiments of the present disclosure; an item-by-item description is not repeated here. 
       FIGS. 5A-5C  are demonstrative structural diagrams of the MRI water-fat image separation apparatus in embodiments of the present disclosure. As shown in  FIG. 5A , the apparatus may comprise: a data acquisition module  510 , an image extraction module  520  and a water-fat separation module  530 . 
     The data acquisition module  510  is used for: within one echo period, acquiring a first echo set corresponding to Dixon water-fat separation calculation under a first readout gradient polarity, and acquiring a second echo set corresponding to Dixon water-fat separation calculation under a second readout gradient polarity, wherein the first readout gradient polarity and the second readout gradient polarity are opposite, and echoes in the first echo set are positionally one-to-one symmetric to echoes in the second echo set with respect to the echo center of the echo period, i.e. the echoes in the second echo set are located at positions that are positionally symmetric to the echoes in the first echo set with respect to the echo center of the echo period. 
     The image extraction module  520  is used for obtaining a first echo image set based on first echo set data acquired in each echo period, and obtaining a second echo image set based on second echo set data acquired in each echo period. 
     The water-fat separation module  530  is used for using the first echo image set and the second echo image set to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     The first echo set and the second echo set each comprise: one nearly-in-phase echo meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is two-point Dixon water-fat separation calculation; alternatively, the first echo set and the second echo set each comprise: two nearly-in-phase echoes meeting a set in-phase requirement and one nearly-out-of-phase echo meeting a set out-of-phase requirement; and the Dixon water-fat separation calculation is three-point Dixon water-fat separation calculation. 
     In particular implementation, the water-fat separation module  530  may be implemented in various ways;  FIGS. 5B and 5C  each show one of these. 
     In one embodiment as shown in  FIG. 5B , the water-fat separation module  530  may comprise: a first image integration module  531  and a first water-fat separation calculation module  532 . The first image integration module  531  is used for separately subjecting each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to phase difference correction, then separately subjecting each pair of echo images that has undergone phase difference correction to complex averaging, to obtain a corresponding echo averaged image set. The first water-fat separation calculation module  532  is used for using the images in the echo averaged image set obtained to perform Dixon water-fat separation calculation, to obtain a water image and a fat image. 
     The first image integration module  531  may comprise: a complex conjugate transformation module  5311 , a phase correction module  5312  and a complex averaging module  5313 . 
     The complex conjugate transformation module  5311  is used for subjecting either one of each pair of echo images with symmetrical echo positions in the first echo image set and the second echo image set to complex conjugate transformation, to obtain a first processing image. 
     The phase correction module  5312  is used for subtracting, from the first processing image or the other echo image that has not undergone complex conjugate transformation, a low-pass phase difference between the first processing image and the echo image that has not undergone complex conjugate transformation, to obtain a second processing image; wherein the second processing image and the echo image which has not had the low-pass phase difference subtracted therefrom together form the two echo images resulting from phase difference correction. 
     The complex averaging module  5313  is used for separately subjecting each pair of echo images that has undergone phase difference correction to complex averaging, to obtain a corresponding echo averaged image set. 
     In another embodiment as shown in  FIG. 5B , the water-fat separation module  530  may comprise: a second water-fat separation calculation module  533  and a second image integration module  534 . 
     The second water-fat separation calculation module  533  is used for using the first echo image set to perform Dixon water-fat separation calculation, to obtain a first water image and a first fat image; and using the second echo image set to perform Dixon water-fat separation calculation, to obtain a second water image and a second fat image. 
     The second image integration module  534  is used for subjecting the first water image and the second water image to modulus averaging, to obtain a water image; and subjecting the first fat image and the second fat image to modulus averaging, to obtain a fat image. 
       FIG. 6  is a demonstrative structural diagram of another MRI water-fat image separation apparatus in embodiments of the present disclosure. As shown in  FIG. 6 , the apparatus may comprise: at least one memory  61  and at least one processor  62 . In addition, some other components may also be included, such as a communication port, etc. These components communicate via a bus  63 . 
     The at least one memory  61  is configured to store a computer program. In one embodiment, the computer program may be understood to comprise all of the modules of the MRI water-fat image separation apparatus shown in  FIG. 5 . Furthermore, the at least one memory  61  may also store an operating system, etc. Operating systems include but are not limited to: an Android operating system, a Symbian operating system, a Windows operating system and a Linux operating system, etc. 
     The at least one processor  62  is configured to call the computer program stored in the at least one memory  61 , to perform the MRI water-fat image separation method described in embodiments of the present disclosure. The processor  62  may be a CPU, a processing unit/module, an ASIC, a logic module or a programmable gate array, etc. It can receive and send data via the communication port. 
     It must be explained that not all of the steps and modules in the flows and structural diagrams above are necessary; certain steps or modules may be omitted according to actual requirements. The order in which steps are executed is not fixed, but may be adjusted as required. The partitioning of the modules is merely functional partitioning, employed for the purpose of facilitating description; during actual implementation, one module may be realized by multiple modules, and the functions of multiple modules may be realized by the same module; these modules may be located in the same device, or in different devices. 
     It can be understood that hardware modules in the embodiments above may be realized mechanically or electronically. For example, one hardware module may comprise a specially designed permanent circuit or logic device (such as a dedicated processor, such as an FPGA or ASIC) for completing a specific operation. The hardware module may also comprise a programmable logic device or circuit that is temporarily configured by software (e.g. comprising a general processor or another programmable processor) for executing a specific operation. The choice of whether to specifically use a mechanical method, or a dedicated permanent circuit, or a temporarily configured circuit (e.g. configured by software) to realize the hardware module can be decided according to considerations of cost and time. 
     In addition, in an embodiment of the present disclosure, a computer readable storage medium is further provided, having stored thereon a computer program that can be executed by a processor and realize the MRI water-fat image separation method described in embodiments of the present disclosure. Specifically, a system or apparatus equipped with a storage medium may be provided; software program code realizing the function of any one of the embodiments above is stored on the storage medium, and a computer (or CPU or MPU) of the system or apparatus is caused to read and execute the program code stored in the storage medium. Furthermore, it is also possible to cause an operating system etc. operating on a computer to complete a portion of, or all, actual operations by means of an instruction based on program code. It is also possible for program code read out from the storage medium to be written into a memory installed in an expansion board inserted in the computer, or written into a memory installed in an expansion unit connected to the computer, and thereafter instructions based on the program code cause a CPU etc. installed on the expansion board or expansion unit to execute a portion of and all actual operations, so as to realize the function of any one of the embodiments above. Embodiments of storage media used for providing program code include floppy disks, hard disks, magneto-optical disks, optical disks (such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-RW, DVD+RW), magnetic tapes, non-volatile memory cards and ROM. Optionally, program code may be downloaded from a server computer via a communication network. 
       FIGS. 7A and 7B  show image quality comparison figures of in vitro phantom water images obtained using a conventional two-point Dixon water-fat separation algorithm and the MRI water-fat image separation method in an embodiment of the present disclosure respectively, in one example, while  FIG. 7C  shows a schematic figure of ratio results for the two images in  FIGS. 7B and 7A . In this example, the Siemens Integrated Development Environment for Application (IDEA) is used to implement a sequence image reconstruction prototype system, and the in vitro phantom experiment is conducted on a Siemens instrument with a magnetic field strength of 0.55 T. As shown in  FIG. 7C , the image obtained by the MRI water-fat image separation method in an embodiment of the present disclosure has a 30%-40% higher SNR than the image obtained by the conventional two-point Dixon water-fat separation algorithm. As can be seen, for the same acquisition efficiency, the image quality is higher when the MRI water-fat image separation method in an embodiment of the present disclosure is used. 
       FIGS. 8A and 8B  show image quality comparison figures of hip joint water images obtained using a combined-echo two-point Dixon water-fat separation algorithm with sampling performed twice and the MRI water-fat image separation method in an embodiment of the present disclosure, in one example.  FIG. 8A  is the hip joint water image obtained using the combined-echo two-point Dixon water-fat separation algorithm with sampling performed twice;  FIG. 8B  is the hip joint water image obtained using the MRI water-fat image separation method in an embodiment of the present disclosure. The protocol parameters used in both cases include: FOV (field of view)=285×380×300 mm2, matrix size=180×320, sampling bandwidth=401 Hz/pixel, slice thickness=6 mm, number of slices=25, TE/TR for the two cases (echo time/repetition time)=22/778 and 17/650, sampling time TA corresponding to  FIG. 8A =2 minutes 18 seconds, sampling time TA corresponding to  FIG. 8B =1 minute 55 seconds. 
     As can be seen, the image quality is similar in  FIGS. 8B and 8A , but the sampling time corresponding to  FIG. 8B  is shorter than the sampling time corresponding to  FIG. 8A . As can be seen, for the same imaging quality, the sampling efficiency is higher when the MRI water-fat image separation method in an embodiment of the present disclosure is used. 
     It can be seen from the solution above that because two echo sets corresponding to Dixon water-fat separation calculation are read in one echo period in embodiments of the present disclosure, nearly all of the time between two refocusing pulses is used for data acquisition, thus increasing the data acquisition efficiency and reducing dead zones. Moreover, each echo set is read out under the same readout gradient polarity, so there is no need to perform phase correction in each echo set. 
     In addition, the echo images corresponding to the two echo sets can undergo image merging to increase SNR followed by water-fat separation calculation, or undergo water-fat separation calculation followed by image merging to increase SNR; compared with a conventional TSE-Dixon water-fat separation calculation method, the number of echoes obtained in a very similar capture time is doubled, so the SNR is increased, and better water-fat imaging quality is achieved. Furthermore, by implementing the phase correction process before averaging in the merging process, it is possible to reduce redundancy and ensure consistency of local and global water-fat separation results. 
     Additionally, because less echo spacing time is needed to realize the same dephasing factor, there is less T 2  blurring, so that a longer echo sequence can reduce the scanning time. Alternatively, with the same echo spacing time, the technical solution in embodiments of the present disclosure applies a lower readout gradient strength, resulting in less image distortion and less misregistration between water and fat images. 
     The embodiments above are merely preferred embodiments of the present disclosure, which are not intended to limit it. Any amendments, equivalent substitutions or improvements etc. made within the spirit and principles of the present disclosure shall be included in the scope of protection thereof. 
     To enable those skilled in the art to better understand the solution of the present disclosure, the technical solution in the embodiments of the present disclosure is described clearly and completely below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the embodiments described are only some, not all, of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art on the basis of the embodiments in the present disclosure without any creative effort should fall within the scope of protection of the present disclosure. 
     It should be noted that the terms “first”, “second”, etc. in the description, claims and abovementioned drawings of the present disclosure are used to distinguish between similar objects, but not necessarily used to describe a specific order or sequence. It should be understood that data used in this way can be interchanged as appropriate so that the embodiments of the present disclosure described here can be implemented in an order other than those shown or described here. In addition, the terms “comprise” and “have” and any variants thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or equipment comprising a series of steps or modules or units is not necessarily limited to those steps or modules or units which are clearly listed, but may comprise other steps or modules or units which are not clearly listed or are intrinsic to such processes, methods, products or equipment. 
     References in the specification to “one embodiment,” “an embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     The exemplary embodiments described herein are provided for illustrative purposes, and are not limiting. Other exemplary embodiments are possible, and modifications may be made to the exemplary embodiments. Therefore, the specification is not meant to limit the disclosure. Rather, the scope of the disclosure is defined only in accordance with the following claims and their equivalents. 
     REFERENCE LIST 
     
         
         
           
               101 - 103 , Operations 
               31 A- 34   
               41 A- 44 B 
               510  data acquisition module 
               520  image extraction module 
               530  water-fat separation module 
               531  first image integration module 
               5311  complex conjugate transformation module 
               5312  phase correction module 
               5313  complex averaging module 
               532  first water-fat separation calculation module 
               533  second water-fat separation calculation module 
               534  second image integration module 
               61  memory 
               62  processor 
               63  bus