CN112285623A - Magnetic resonance imaging water-fat image separation method and device and magnetic resonance imaging system - Google Patents
Magnetic resonance imaging water-fat image separation method and device and magnetic resonance imaging system Download PDFInfo
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Abstract
The embodiment of the invention discloses a method and a device for separating a water-fat image in magnetic resonance imaging and a magnetic resonance imaging system. The method comprises the following steps: acquiring echo data of two blades in a repetition time, wherein the echo data of each blade are at least two groups of echo data acquired by adopting the same readout gradient polarity and are respectively filled into at least two k spaces; wherein different sets of echo data have different echo times; taking a blade as a unit, obtaining at least two blade images of the blade according to data in at least two k spaces of each blade, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade; and synthesizing the water images of all the blades into a magnetic resonance water image of a complete k space, and synthesizing the fat images of the blades into a magnetic resonance fat image of the complete k space. The technical scheme in the embodiment of the invention can realize high-efficiency water-fat image separation.
Description
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a method and a device for separating a water-fat image in magnetic resonance imaging and a magnetic resonance imaging system.
Background
In a Magnetic Resonance Imaging (MRI) system, the resonance frequencies of hydrogen nuclei in fat and water inside a human body are different due to different molecular environments; when the hydrogen protons of fat and other tissues are excited by the rf pulse at the same time, their relaxation times are also different. The signals are acquired at different echo times, the adipose tissue and water exhibit different phases and signal strengths.
The Dixon method is a method for generating pure water proton images In magnetic resonance imaging, and its basic principle is to collect two echo signals of In Phase (In Phase) and Out Phase (Out Phase) of water and fat protons, respectively, and the two signals of different phases are operated to generate a pure water proton image and a pure fat proton image, respectively, so as to achieve the purpose of fat suppression. Various Dixon water-fat image separation methods exist at present, including: a single point Dixon method, a two point Dixon method, a three point Dixon method, and the like.
In Dixon-based Fast Spin Echo (Turbo Spin Echo, TSE, also called Fast Spin Echo, FSE) pulse sequences, a Radio Frequency (RF) pulse sequence includes a 90 ° excitation pulse and a 180 ° refocusing pulse (also called a complex phase pulse), and multiple echoes (Echo) can be acquired between two adjacent refocusing pulses (i.e., within one Echo interval). The echoes corresponding to the respective positions of the different refocusing pulses constitute a set of echoes, such as: the first echo appearing after the refocusing pulse 1, the first echo appearing after the refocusing pulse 2, … …, the first echo appearing after the refocusing pulse n may constitute one set of echoes, the second echo appearing after the refocusing pulse 1, the second echo appearing after the refocusing pulse 2, … …, the second echo appearing after the refocusing pulse n may constitute another set of echoes. Each set of echoes can independently reconstruct an image. Because the amplitude and phase information of different echoes are different, a pure water image and a pure fat image can be respectively reconstructed through further calculation.
Considering that some of the current water-fat image separation methods need phase correction, and some are sensitive to movement such as respiratory motion during imaging, which is easy to generate motion artifacts, those skilled in the art are also working to find other solutions.
Disclosure of Invention
In view of this, the embodiments of the present invention provide a method for separating a water-fat image in magnetic resonance imaging, and provide a device and a system for separating a water-fat image in magnetic resonance imaging for realizing efficient water-fat image separation.
The magnetic resonance imaging water-fat image separation method provided by the embodiment of the invention comprises the following steps: acquiring echo data of two blades in a repetition time, wherein the echo data of each blade are at least two groups of echo data acquired by adopting the same readout gradient polarity, and the at least two groups of echo data are respectively filled into at least two k spaces; the echo data of different groups have different echo times, and the number of echoes of each group of echo data is equal to the length of an echo chain in a repetition time; taking a blade as a unit, obtaining at least two blade images of the blade according to data in at least two k spaces of each blade, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade; and synthesizing the water images of all the blades into a magnetic resonance water image of a complete k space, and synthesizing the fat images of the blades into a magnetic resonance fat image of the complete k space.
In one embodiment, after obtaining the water image and the fat image of all the leaves, the method further comprises: performing Fourier transform on the water image of each blade to obtain water k-space data corresponding to the water image; rotating the water k space data by corresponding angles according to the corresponding angles of the blades in the complete k space to obtain the water k space data of the blades under a set angle; determining an oversampled k-space central area, and respectively extracting data corresponding to the central area from the water k-space data of each blade under the set angle to obtain the water k-space central area data of each blade; respectively carrying out Fourier transform on the water k space central region data of each blade to obtain an oversampling region water image of each blade; calculating the gray average value of the water images in the oversampling area of all the blades; and respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value, and interchanging the water image and the fat image of the corresponding blade when the calculated similarity is lower than a set threshold value.
In one embodiment, the acquiring echo data of two blades in one repetition time includes: acquiring echo data of two blades with low b values lower than a set threshold value in a repetition time; performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade comprises: carrying out Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a B0 field diagram corresponding to the uniformity of the reaction magnetic field of the blade; and calculating a water image and a fat image of the blade according to the B0 field diagram of the blade and at least two blade images of the blade.
In one embodiment, further comprising: storing the B0 field map; the acquiring echo data of two blades within one repetition time further comprises: acquiring echo data of two blades with high b values higher than the set threshold value in a repetition time; performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade comprises: and obtaining a water image and a fat image of the blade according to at least two blade images of the blade and a pre-stored B0 field map corresponding to the blade.
In one embodiment, said deriving at least two blade images of each blade from data in at least two k-spaces of said blade comprises: obtaining complete k-space data with low resolution by filling 0 in the data in each k-space of each blade according to the positions of other blades, and performing Fourier transform on the complete k-space data with low resolution to obtain a blade image corresponding to the k-space; the synthesizing of the water images of all the leaves into a magnetic resonance water image of a complete k-space and the synthesizing of the fat images of the leaves into a magnetic resonance fat image of a complete k-space comprises: respectively carrying out Fourier transform on the water image and the fat image of each blade to obtain water image data and fat image data of a low-resolution complete k space of the blade; extracting effective k space water image data corresponding to the blades from the low-resolution complete k space water image data, and extracting effective k space fat image data corresponding to the blades from the low-resolution complete k space fat image data; reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades; and respectively carrying out Fourier transform on the water image data and the fat image data of the high-resolution complete k space to obtain a magnetic resonance water image and fat image data of the complete k space.
The water-fat image separation device for magnetic resonance imaging provided by the embodiment of the invention comprises: the data acquisition unit is used for acquiring echo data of two blades in a repetition time, wherein the echo data of each blade are at least two groups of echo data acquired by adopting the same readout gradient polarity, and the at least two groups of echo data are respectively filled into at least two k spaces; the echo data of different groups have different echo times, and the number of echoes of each group of echo data is equal to the length of an echo chain in a repetition time; and a water-fat separation unit; the water-fat separation unit comprises: the first water-fat separation module is used for obtaining at least two blade images of each blade according to data in at least two k spaces of each blade by taking the blade as a unit, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of each blade; and the second water-fat separation module is used for synthesizing the water images of all the blades into a magnetic resonance water image of a complete k space and synthesizing the fat images of the blades into a magnetic resonance fat image of the complete k space.
In one embodiment, the water-fat separation unit further comprises: the water-fat image distinguishing and analyzing module is used for carrying out Fourier transform on the water image of each blade to obtain water k space data corresponding to the water image; rotating the water k space data by corresponding angles according to the corresponding angles of the blades in the complete k space to obtain the water k space data of the blades under a set angle; determining an oversampled k-space central area, and respectively extracting data corresponding to the central area from the water k-space data of each blade under the set angle to obtain the water k-space central area data of each blade; respectively carrying out Fourier transform on the water k space central region data of each blade to obtain an oversampling region water image of each blade; calculating the gray average value of the water images in the oversampling area of all the blades; and respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value, and interchanging the water image and the fat image of the corresponding blade when the calculated similarity is lower than a set threshold value.
In one embodiment, the data acquisition unit acquires echo data of two blades with a low b value lower than a set threshold value within one repetition time; the first water-fat separation module takes the blades as a unit, obtains at least two blade images with low B values of the blades according to data in at least two k spaces with low B values of each blade, and the second water-fat separation module performs Dixon water-fat separation calculation by utilizing the at least two blade images with low B values of each blade to obtain a B0 field diagram corresponding to the uniformity of the reaction magnetic field of the blades; and calculating a water image and a fat image of the blade according to the B0 field diagram of the blade and at least two blade images of the blade.
In one embodiment, the second water-fat separation module further stores the B0 field map; the data acquisition unit further acquires echo data of two blades with a high b value higher than the set threshold value within one repetition time; the first water-fat separation module takes the blades as a unit and obtains at least two blade images with high b values of the blades according to data in at least two k spaces with high b values of each blade; and the second water-fat separation module obtains a water image and a fat image of the blade according to at least two blade images with high B values of the blade and a pre-stored B0 field map corresponding to the blade.
In one embodiment, the first water-fat separation module obtains low-resolution complete k-space data by filling 0 in each k-space of each blade according to the positions of other blades by taking the blade as a unit, and performs fourier transform on the low-resolution complete k-space data to obtain a blade image corresponding to the k-space; carrying out Dixon water-fat separation calculation by utilizing at least two blade images of each blade to obtain a water image and a fat image of the blade; the second water-fat separation module respectively performs Fourier transform on the water image and the fat image of each blade to obtain water image data and fat image data of a low-resolution complete k space of the blade; extracting effective k space water image data corresponding to the blades from the low-resolution complete k space water image data, and extracting effective k space fat image data corresponding to the blades from the low-resolution complete k space fat image data; reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades; and respectively carrying out Fourier transform on the water image data and the fat image data of the high-resolution complete k space to obtain a magnetic resonance water image and fat image data of the complete k space.
The other kind of magnetic resonance imaging water fat image separation device that the embodiment of the invention proposes, including: at least one memory and at least one processor, wherein: the at least one memory is for storing a computer program; the at least one processor is configured to invoke a computer program stored in the at least one memory to perform the method for separating a water-fat image in magnetic resonance imaging according to any of the embodiments described above.
The magnetic resonance imaging system provided by the embodiment of the invention comprises the magnetic resonance imaging water-fat image separation device in any one of the specific implementation modes.
A computer-readable storage medium having a computer program stored thereon, the computer program being proposed in an embodiment of the present invention; characterized in that the computer program can be executed by a processor and implements the method for separating water-fat images in magnetic resonance imaging according to any one of the embodiments described above.
It can be seen from the above solution that, in the embodiment of the present invention, a Dixon method and a BLADE sequence are combined, two BLADEs are acquired within one TR, at least two echo data sets with different echo times are acquired by using a readout gradient of the same polarity for each BLADE, then, water-fat separation calculation is performed by using images of at least two echo data sets of the BLADE as a unit, and finally, the calculated water images of all the BLADEs and the calculated fat images of all the BLADEs are synthesized into a final water image and a final fat image, respectively. In this process, since at least two echo data sets for performing Dixon water-fat separation calculation for each blade are acquired at the same readout gradient polarity, phase correction is not necessary. In addition, because the water-fat separation calculation is performed in units of blades, and the echo data for the water-fat separation calculation of each blade is acquired in one echo chain acquisition, potential misregistration between different echoes is avoided. More importantly, the method can also be used for diffusion imaging because of strong robustness to motion. Moreover, the operation of acquiring two blades in one TR can accelerate the sampling efficiency.
In addition, by distinguishing the blade water image and the fat image obtained by the Dixon water-fat separation calculation by using redundant data of an oversampled k-space central region, the exchange of the water image and the fat image which can occur in the Dixon water-fat separation calculation can be identified and corrected.
In addition, the reliability of water-fat separation of the high B-value image can be improved by performing the water-fat separation calculation on the high B-value image as the intermediate result of the high B-value image by using the intermediate result of the uniformity of one reaction magnetic field, that is, the B0 field pattern, obtained when the water-fat separation calculation is performed on the low B-value image.
Drawings
The foregoing and other features and advantages of the invention will become more apparent to those skilled in the art to which the invention relates upon consideration of the following detailed description of a preferred embodiment of the invention with reference to the accompanying drawings, in which:
FIG. 1 is a schematic trajectory of k-space data acquired using a two-dimensional fast spin echo (TSE) BLADE artifact correction sequence (BLADE).
Fig. 2 is an exemplary flowchart of a method for separating a water-fat image in magnetic resonance imaging according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of a data acquisition sequence of a two-point Dixon method based on a fast spin echo (TSE) sequence in one example of the present invention.
Fig. 4A and 4B are graphs of k-space data acquired over a repetition time in one example of the invention.
Fig. 5 is an exemplary flowchart of a method for analyzing a water-fat image according to an embodiment of the present invention.
Fig. 6A and 6B are respectively an exemplary structural diagram of a magnetic resonance imaging water-fat image separating device in an embodiment of the present invention.
Fig. 7 is an exemplary structural diagram of a further magnetic resonance imaging water-fat image separating device in an embodiment of the present invention.
Fig. 8A shows 4 images of 40 water (first row) and fat (second row) images of the neck of a living body obtained by the magnetic resonance imaging water-fat image separation method in the embodiment of the present invention in one example.
Fig. 8B shows 4 diffusion weighted images of 20 images with B values of 50 (first row) and 800 (second row) obtained by the magnetic resonance imaging water-fat image separation method in the embodiment of the present invention in one example.
Wherein the reference numbers are as follows:
Detailed Description
MRI imaging includes images of various cross sections in a desired direction. k-space is the data space of each cross section, i.e. k-space data represents a set of raw data that can form an image. For example, after echo data acquisition in k-space using a two-dimensional or three-dimensional fast gradient echo sequence, the echo data is filled into a phase-encoded k-space. By then performing a fourier transform on the k-space data, a desired image can be obtained.
FIG. 1 is a schematic trajectory of k-space data acquired using a two-dimensional fast spin echo (TSE) BLADE artifact correction sequence (BLADE). As shown in fig. 1, one blade is taken during one repetition Time (TR), and the next blade is taken after rotating a certain angle (20 ° in fig. 1). In fig. 1, a case where the vane width is L and the echo chain length corresponding to the vane width L is 15 is taken as an example. Where each TR represents a time period from a 90 deg. excitation pulse to the next 90 deg. excitation pulse in the pulse sequence.
In the embodiment of the present invention, in order to overcome the problem that phase correction and motion artifact are required in magnetic resonance imaging, a Dixon method and a BLADE sequence are combined, two BLADEs are acquired in one TR, and at least two echo data sets with different echo times are acquired by using readout gradients of the same polarity for each BLADE. And finally, synthesizing the water images and the fat images of all the blades obtained by calculation into a final water image and a final fat image respectively. In this process, since at least two echo data sets for performing Dixon water-fat separation calculation for each blade are acquired at the same readout gradient polarity, phase correction is not necessary. In addition, because the water-fat separation is performed in units of blades, and the echo data for the water-fat separation calculation of each blade is acquired in one echo chain acquisition, the potential error registration between different echoes is avoided. More importantly, the method can also be used for diffusion imaging because of strong robustness to motion. Moreover, the operation of acquiring two blades in one TR can accelerate the sampling efficiency.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail by referring to the following examples.
Fig. 2 is an exemplary flowchart of a method for separating a water-fat image in magnetic resonance imaging according to an embodiment of the present invention, and as shown in fig. 2, the method may include the following steps:
step S22, acquiring echo data of two blades in a repetition time, wherein the echo data of each blade is at least two groups of echo data acquired by adopting the same readout gradient polarity, and the at least two groups of echo data are respectively filled into at least two k spaces; wherein, the echo data of different groups have different echo times, and the number of echoes of each group of echo data is equal to the length of an echo chain in a repetition time.
Fig. 3 shows a schematic diagram of a data acquisition sequence of a two-point Dixon method based on a fast spin Echo (TSE) sequence in an example of the present invention, and only one spin Echo period (ESP) is shown in fig. 3. The k-space data acquired over one repetition time is shown in fig. 4A and 4B. In fig. 4A and 4B, the case where the echo train length in one repetition time is 5 is taken as an example, that is, the width of each blade corresponds to 5 echo data lines, and based on the case shown in fig. 4A and 4B, 5 spin echo cycles as shown in fig. 3 are included in one repetition time, that is, 5 180-degree refocusing phase rf pulses are included between two 90-degree rf pulses. Wherein, RF, Gx, Gy and D correspond to the RF pulse, x axial gradient, y axial gradient and echo data, respectively. B1E1 corresponds to the first echo of blade 1, B1E2 corresponds to the second echo of blade 1, B2E1 corresponds to the first echo of blade 2, and B2E2 corresponds to the second echo of blade 2.
As shown in fig. 3, in one spin echo cycle, i.e. between two 180-degree rephasing radio frequency pulses RF _1, RF _2, at a first echo time (TE1) and a second echo time (TE2) from the echo center C, the magnetic resonance imaging apparatus applies two x-axial gradients for each of the first and second blades in the x-axis gradient direction, while applying two y-axial gradients for the second blade in the y-axis gradient direction, and the corresponding four echoes 1, 2, 3, 4 can be read out respectively based on the combined action of the x-axial gradients and the y-axial gradients. The four read echoes 1, 2, 3, 4 are filled into k-space, and after one repetition time is over, two k-space schematics as described in fig. 4A and 4B can be obtained, where fig. 4A shows a set of echo data of each of two blades acquired at a first echo time, and fig. 4B shows a set of echo data of each of two blades acquired at a second echo time.
In practical applications, more than two sets of echo data may be acquired for each blade in this step, for example, three sets of echo data, four sets of echo data, or more sets of echo data may be acquired. The specific value can be determined according to actual needs, and the method is not limited herein.
Step S24, taking the blade as a unit, obtaining at least two blade images of the blade according to the data in at least two k spaces of each blade, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade.
In this step, two sets of echo data of each blade are obtained in one echo train acquisition, so that the water-fat separation calculation is performed by taking the blade as a unit to avoid potential misregistration between different echoes. More importantly, the method can also be used in diffusion imaging because of strong robustness to motion.
One implementation is listed below:
based on data in k-space acquired at two different echo times of the same blade shown in fig. 4A and 4B, the data in each k-space can be filled with 0 at other blade positions to obtain low-resolution complete k-space data, and the low-resolution complete k-space data is subjected to fourier transform to obtain a blade image corresponding to the k-space, that is, a blade image of an a-blade can be obtained based on a group of echo data acquired at the first echo time of the a-blade shown in fig. 4A, and is denoted as a 1; another set of echo data acquired based on the second echo time of the a-blade shown in fig. 4B may result in another blade image of the a-blade, denoted as a 2. For the case where there are multiple coil units for data acquisition, for each of the leaf images a1 and a2, adaptive coil combination can be performed to obtain the final leaf image a1 and leaf image a2, respectively. Then, a two-point Dixon water-fat separation calculation can be performed by using the leaf image a1 and the leaf image a2, so that a water image and a fat image of the leaf are obtained. The leaf image a1 can be an approximate in-phase image, and the leaf image a2 can be an approximate anti-phase image. In practical applications, if three sets of echo data are acquired for each blade in step S22, the three-point Dixon water-fat separation calculation may be performed using the three obtained blade images in step S24. If four groups are obtained, four-point type Dixon water-fat separation calculation is carried out, and the rest is done in sequence. The specific situation can be determined according to actual needs, and the method is not limited herein.
And step S26, synthesizing the water images of all the blades into a magnetic resonance water image of a complete k space, and synthesizing the fat images of the blades into a magnetic resonance fat image of the complete k space.
In a specific implementation, in this step, for the water image and the fat image of each blade obtained in step S24, fourier transform may be performed on the water image and the fat image to obtain low-resolution water image data and fat image data of a complete k-space of the blade, respectively. Then, effective k-space water image data corresponding to the blades can be extracted from the water image data of the complete k-space with low resolution, and effective k-space fat image data corresponding to the blades can be extracted from the fat image data of the complete k-space with low resolution. Then, reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades; and then respectively carrying out Fourier transform on the water image data and the fat image data of the complete k space with high resolution to obtain the magnetic resonance water image and the fat image data of the complete k space.
In the embodiment of the present invention, it is considered that when the Dixon water-fat image separation calculation is performed in step S24, there is a possibility that the water image and the fat image are erroneously marked. For example, the water image is labeled as a fat image, and the fat image is labeled as a water image. Of course, the probability of this occurring is still relatively low. However, in order to ensure the correct division of the water and fat images and further acquire a higher quality water and fat image in step S26, the water and fat images of all the leaves may be resolved after the water and fat images of all the leaves are acquired in step S24. Consider that when using a BLADE sequence for k-space data acquisition, there is oversampling in the central region of k-space, i.e. the same content is repeatedly sampled. For the part of the oversampled content, the part of the oversampled content should have similar features in different blades, and taking a water image as an example, the part of the content should have similar gray values, so that a gray average value of the water image of all the blades in an oversampled central region can be calculated, then the gray value of each blade in the oversampled central region is compared with the gray average value, if the similarity reaches a preset threshold, it is considered that the water image and the fat image are not exchanged, and if the similarity is lower than the preset threshold, it is considered that the water image and the fat image are exchanged, and then the water image and the fat image are exchanged. For this reason, the method for separating water-fat images in magnetic resonance imaging in the embodiment of the present invention may further include the steps as shown in fig. 5:
and step S51, performing Fourier transform on the water image of each blade to obtain water k-space data corresponding to the water image.
And step S52, rotating the water k-space data by corresponding angles according to the corresponding angles of the blades in the complete k-space to obtain the water k-space data of the blades under the set angles.
And step S53, determining an oversampled k-space central area, and extracting data corresponding to the central area from the water k-space data of each blade under the set angle respectively to obtain the water k-space central area data of each blade.
And step S54, respectively carrying out Fourier transform on the water k space central area data of each blade to obtain an oversampled area water image of each blade.
And step S55, calculating the gray level average value of the water images in the oversampling area of all the blades.
And step S56, respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value to obtain a similarity value corresponding to each blade.
Step S57, determine whether the similarity value corresponding to the blade is lower than a set threshold? If so, step S58 is performed. Otherwise, the water and fat image markers of the leaf are kept unchanged.
Step S58, the water image and the fat image of the leaf are interchanged, i.e., the labels of the two images are interchanged.
In addition, when the water-fat separation calculation is carried out by taking the leaf as a unit, the method can be used in diffusion imaging due to the fact that the method is strong in robustness to movement. In diffusion imaging, echo data of different b values may be acquired in step S22, and it is considered that the reliability may be lower when performing water-fat separation calculation on a high b value image than that of performing water-fat separation calculation on a low b value image because the signal-to-noise ratio of the high b value image higher than a certain set threshold is low. Therefore, in the embodiment of the present invention, an intermediate result of the uniformity of the response magnetic field, i.e., the B0 field pattern, obtained when performing the water-fat separation calculation on the low B-value image is considered when performing the water-fat separation calculation on the high B-value image. That is, when the water-fat separation calculation is normally performed on at least two blade images with a low B value of a blade, a B0 field pattern corresponding to the uniformity of the reaction magnetic field of the blade is obtained in the process, and a water image and a fat image of the blade are calculated according to the B0 field pattern of the blade and the at least two blade images of the blade. In the embodiment of the present invention, in order to apply the B0 field map corresponding to the blade to the water-fat separation calculation of at least two blade images corresponding to the high B values of the blade, it is considered that the B0 field map corresponding to the blade is stored first, so that when the Dixon water-fat separation calculation is performed using at least two blade images corresponding to the high B values of each blade in the following, the water image and the fat image corresponding to the high B values of the blade are obtained from at least two blade images corresponding to the high B values of the blade and the pre-stored B0 field map corresponding to the blade.
The above describes the method for separating the magnetic resonance imaging water-fat image in the embodiment of the present invention in detail, and the following describes the device for separating the magnetic resonance imaging water-fat image in the embodiment of the present invention. The magnetic resonance imaging water-fat image separation device in the embodiment of the present invention can be used for implementing the magnetic resonance imaging water-fat image separation method in the embodiment of the present invention, and for the content which is not disclosed in detail in the embodiment of the present invention, reference may be made to the corresponding description in the embodiment of the method of the present invention, and details are not repeated here.
Fig. 6A and 6B are respectively an exemplary structural diagram of a magnetic resonance imaging water-fat image separating device in an embodiment of the present invention. As shown in fig. 6A, the apparatus may include a data acquisition unit 610 and a water-fat separation unit 620. Wherein, the water-fat separation unit 620 may specifically include: a first water-fat separation module 621 and a second water-fat separation module 622.
The data acquiring unit 610 is configured to acquire echo data of two blades in a repetition time, where the echo data of each blade is at least two sets of echo data acquired by using the same readout gradient polarity, and the at least two sets of echo data are respectively filled in at least two k spaces; wherein, the echo data of different groups have different echo times, and the number of echoes of each group of echo data is equal to the length of an echo chain in a repetition time.
The first water-fat separation module 621 is configured to obtain at least two blade images of each blade according to data in at least two k-spaces of each blade, and perform Dixon water-fat separation calculation using the at least two blade images of each blade to obtain a water image and a fat image of each blade, with the blade as a unit. In one embodiment, the first water-fat separation module 621 may use a blade as a unit, obtain low-resolution complete k-space data by filling 0 in each k-space of each blade according to the positions of other blades, and perform fourier transform on the low-resolution complete k-space data to obtain a blade image corresponding to the k-space; and performing Dixon water-fat separation calculation by using at least two blade images of each blade to obtain a water image and a fat image of the blade.
The second water-fat separation module 622 is configured to synthesize water images of all the blades into a magnetic resonance water image of a complete k-space, and synthesize a fat image of the blades into a magnetic resonance fat image of a complete k-space. In one embodiment, the second water-fat separation module 622 can perform fourier transform on the water image and the fat image of each leaf to obtain water image data and fat image data of a low-resolution complete k-space of the leaf; extracting effective k space water image data corresponding to the blades from the low-resolution complete k space water image data, and extracting effective k space fat image data corresponding to the blades from the low-resolution complete k space fat image data; reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades; and respectively carrying out Fourier transform on the water image data and the fat image data of the high-resolution complete k space to obtain a magnetic resonance water image and fat image data of the complete k space.
In one embodiment, the water-fat separation unit 620 may further include, as shown in fig. 6B: the water and fat image distinguishing and analyzing module 623 is used for performing Fourier transform on the water image of each blade to obtain water k space data corresponding to the water image; rotating the water k space data by corresponding angles according to the corresponding angles of the blades in the complete k space to obtain the water k space data of the blades under a set angle; determining an oversampled k-space central area, and respectively extracting data corresponding to the central area from the water k-space data of each blade under the set angle to obtain the water k-space central area data of each blade; respectively carrying out Fourier transform on the water k space central region data of each blade to obtain an oversampling region water image of each blade; calculating the gray average value of the water images in the oversampling area of all the blades; and respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value, and interchanging the water image and the fat image of the corresponding blade when the calculated similarity is lower than a set threshold value.
Based on the magnetic resonance imaging water-fat image separation apparatus shown in fig. 6A and 6B, in yet another embodiment, the data acquisition unit 610 may acquire echo data of two leaves with a low B value below a set threshold in each of certain repetition times; and acquiring echo data of two blades with high b value higher than the set threshold value in each of the other repetition times.
Correspondingly, the first water-fat separation module 621 may obtain at least two blade images with a low b value of each blade according to data in at least two k spaces with a low b value of the blade in units of blades; and obtaining at least two blade images of high b-value of each blade according to the data in at least two k-spaces of high b-value of the blade.
The second water-fat separation module 622 can perform Dixon water-fat separation calculation by using the at least two blade images with low B values of each blade to obtain a B0 field map corresponding to the uniformity of the reaction magnetic field of the blade; the B0 field image of the blade and the at least two blade images of the blade are used to calculate a low B-value water image and a low B-value fat image of the blade, and further, the second water-fat separation module 622 also stores the B0 field image, so as to obtain a high B-value water image and a high B-value fat image of the blade according to the at least two high B-value blade images of the blade and the pre-stored B0 field image corresponding to the blade.
Fig. 7 is an exemplary structural diagram of a further magnetic resonance imaging water-fat image separating device in an embodiment of the present invention. As shown in fig. 7, may include: at least one memory 71 and at least one processor 72. In addition, some other components may be included, such as a communications port, etc. These components communicate over a bus.
Wherein the at least one memory 71 is adapted to store a computer program. In one embodiment, the computer program may be understood to include the respective modules of the magnetic resonance imaging water-fat image separation apparatus shown in fig. 6. In addition, the at least one memory 71 may also store an operating system and the like. Operating systems include, but are not limited to: an Android operating system, a Symbian operating system, a Windows operating system, a Linux operating system, and the like.
The at least one processor 72 is configured to invoke a computer program stored in the at least one memory 71 to perform the method for separating a water-fat image for magnetic resonance imaging according to the embodiment of the present invention. The processor 72 may be a CPU, processing unit/module, ASIC, logic module, or programmable gate array, etc. Which can receive and transmit data through the communication port.
It should be noted that not all steps and modules in the above flows and structures are necessary, and some steps or modules may be omitted according to actual needs. The execution order of the steps is not fixed and can be adjusted as required. The division of each module is only for convenience of describing adopted functional division, and in actual implementation, one module may be divided into multiple modules, and the functions of multiple modules may also be implemented by the same module, and these modules may be located in the same device or in different devices.
It is understood that the hardware modules in the above embodiments may be implemented mechanically or electronically. For example, a hardware module may include a specially designed permanent circuit or logic device (e.g., a special purpose processor such as an FPGA or ASIC) for performing specific operations. A hardware module may also include programmable logic devices or circuits (e.g., including a general-purpose processor or other programmable processor) that are temporarily configured by software to perform certain operations. The implementation of the hardware module in a mechanical manner, or in a dedicated permanent circuit, or in a temporarily configured circuit (e.g., configured by software), may be determined based on cost and time considerations.
In addition, in an embodiment of the present invention, a computer-readable storage medium is further provided, on which a computer program is stored, where the computer program can be executed by a processor and is used to implement the method for separating a water-fat image in magnetic resonance imaging described in the embodiment of the present invention. Specifically, a system or an apparatus equipped with a storage medium on which a software program code that realizes the functions of any of the embodiments described above is stored may be provided, and a computer (or a CPU or MPU) of the system or the apparatus is caused to read out and execute the program code stored in the storage medium. Further, part or all of the actual operations may be performed by an operating system or the like operating on the computer by instructions based on the program code. The functions of any of the above-described embodiments may also be implemented by writing the program code read out from the storage medium to a memory provided in an expansion board inserted into the computer or to a memory provided in an expansion unit connected to the computer, and then causing a CPU or the like mounted on the expansion board or the expansion unit to perform part or all of the actual operations based on the instructions of the program code. Examples of the storage medium for supplying the program code include floppy disks, hard disks, magneto-optical disks, optical disks (e.g., CD-ROMs, CD-R, CD-RWs, DVD-ROMs, DVD-RAMs, DVD-RWs, DVD + RWs), magnetic tapes, nonvolatile memory cards, and ROMs. Alternatively, the program code may be downloaded from a server computer via a communications network.
Fig. 8A shows 4 images of 40 water (first row) and fat (second row) images of the neck of a living body obtained by the mr imaging water-fat image separation method according to the embodiment of the present invention, and the protocol parameters used include: FOV (field of view) 300X 300mm2The Matrix (sampling Matrix size) is 320 × 320, BW (sampling bandwidth) is 780 Hz/pixel, slice thickness is 3mm, slice number is 40, TE/TR (echo time/repetition time) is 97ms/4000ms, slice direction acceleration is 2 times, PE direction acceleration is also 2 times, and total sampling time is 3 minutes and 42 seconds.
As can be seen from fig. 8A, with the magnetic resonance imaging water-fat image separation method in the embodiment of the present invention, images with reduced motion artifacts and robust fat suppression can be obtained even in a complex region that is susceptible to motion and B0 non-uniform interference.
FIG. 8B shows an example of an embodiment of the present inventionThe magnetic resonance imaging water-fat image separation method obtains 4 pieces of 20 pieces of diffusion weighted imaging with the b value of 50 (first row) and the b value of 800 (second row), and the used protocol parameters comprise: FOV (field of view) 260X 260mm2The Matrix (sampling Matrix size) is 128 × 128, BW (sampling bandwidth) is 850 Hz/pixel, slice thickness is 5mm, slice number is 20, TE/TR (echo time/repetition time) is 43ms/4600ms, PE directional acceleration is also 2 times, and total sampling time is 6 minutes and 33 seconds.
As can be seen from fig. 8B, when the magnetic resonance imaging water-fat image separation method in the embodiment of the present invention is used for diffusion weighted imaging, a diffusion weighted image which is insensitive to motion and has a stable fat suppression effect can be obtained.
It can be seen from the above solution that, in the embodiment of the present invention, a Dixon method and a BLADE sequence are combined, two BLADEs are acquired within one TR, at least two echo data sets with different echo times are acquired by using a readout gradient of the same polarity for each BLADE, then, water-fat separation calculation is performed by using images of at least two echo data sets of the BLADE as a unit, and finally, the calculated water images of all the BLADEs and the calculated fat images of all the BLADEs are synthesized into a final water image and a final fat image, respectively. In this process, since at least two echo data sets for performing Dixon water-fat separation calculation for each blade are acquired at the same readout gradient polarity, phase correction is not necessary. In addition, because the water-fat separation calculation is performed in units of blades, and the echo data for the water-fat separation calculation of each blade is acquired in one echo chain acquisition, potential misregistration between different echoes is avoided. More importantly, the method can also be used for diffusion imaging because of strong robustness to motion. Moreover, the operation of acquiring two blades in one TR can accelerate the sampling efficiency.
In addition, by distinguishing the blade water image and the fat image obtained by the Dixon water-fat separation calculation by using redundant data of an oversampled k-space central region, the exchange of the water image and the fat image which can occur in the Dixon water-fat separation calculation can be identified and corrected.
In addition, the reliability of the water-fat separation of the high B-value image can be increased by performing the water-fat separation calculation on the high B-value image using the B0 field pattern, which is an intermediate result of the uniformity of the response magnetic field obtained when performing the water-fat separation calculation on the low B-value image, as an intermediate result of the high B-value image.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (13)
1. A water-fat image separation method for magnetic resonance imaging is characterized by comprising the following steps:
acquiring echo data of two blades in a repetition time, wherein the echo data of each blade are at least two groups of echo data acquired by adopting the same readout gradient polarity, and the at least two groups of echo data are respectively filled into at least two k spaces; wherein the echo data of different groups have different echo times, the number of echoes of each group of echo data is equal to the echo chain length in a repetition time (S22);
obtaining at least two blade images of each blade according to data in at least two k-spaces of each blade by taking the blade as a unit, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade (S24);
the water images of all the leaves are synthesized into a magnetic resonance water image of a complete k-space, and the fat images of the leaves are synthesized into a magnetic resonance fat image of a complete k-space (S26).
2. The method for separating water-fat images in magnetic resonance imaging according to claim 1, further comprising, after obtaining the water images and the fat images of all the leaves:
performing Fourier transform on the water image of each blade to obtain water k-space data corresponding to the water image (S51);
rotating the water k-space data by corresponding angles according to the corresponding angles of the blades in the complete k-space to obtain the water k-space data of the blades under a set angle (S52);
determining an oversampled k-space central region, and extracting data corresponding to the central region from the water k-space data of each blade under the set angle respectively to obtain water k-space central region data of each blade (S53);
fourier transform is carried out on the water k space central area data of each blade respectively to obtain an oversampled area water image of each blade (S54);
calculating a gray average of the water images of the oversampled regions of all the blades (S55);
and respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value, and when the calculated similarity is lower than a set threshold value, exchanging the water image and the fat image of the corresponding blade (S56-S58).
3. The method for separating water-fat image in magnetic resonance imaging according to claim 1 or 2,
the acquiring echo data of the two blades in one repetition time comprises: acquiring echo data of two blades with low b values lower than a set threshold value in a repetition time;
performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade comprises: carrying out Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a B0 field diagram corresponding to the uniformity of the reaction magnetic field of the blade; and calculating a water image and a fat image of the blade according to the B0 field diagram of the blade and at least two blade images of the blade.
4. The method for separating water-fat images in magnetic resonance imaging according to claim 3, further comprising: storing the B0 field map;
the acquiring echo data of two blades within one repetition time further comprises: acquiring echo data of two blades with high b values higher than the set threshold value in a repetition time;
performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of the blade comprises: and obtaining a water image and a fat image of the blade according to at least two blade images of the blade and a pre-stored B0 field map corresponding to the blade.
5. The method according to any one of claims 1 to 5, wherein the obtaining at least two blade images of each blade from data in at least two k-spaces of the blade comprises:
obtaining complete k-space data with low resolution by filling 0 in the data in each k-space of each blade according to the positions of other blades, and performing Fourier transform on the complete k-space data with low resolution to obtain a blade image corresponding to the k-space;
the synthesizing of the water images of all the leaves into a magnetic resonance water image of a complete k-space and the synthesizing of the fat images of the leaves into a magnetic resonance fat image of a complete k-space comprises:
respectively carrying out Fourier transform on the water image and the fat image of each blade to obtain water image data and fat image data of a low-resolution complete k space of the blade;
extracting effective k space water image data corresponding to the blades from the low-resolution complete k space water image data, and extracting effective k space fat image data corresponding to the blades from the low-resolution complete k space fat image data;
reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades;
and respectively carrying out Fourier transform on the water image data and the fat image data of the high-resolution complete k space to obtain a magnetic resonance water image and fat image data of the complete k space.
6. A kind of magnetic resonance imaging water fat image separation facility, characterized by that, comprising:
a data acquisition unit (610) which acquires echo data of two blades within a repetition time, wherein the echo data of each blade is at least two groups of echo data acquired by using the same readout gradient polarity, and the at least two groups of echo data are respectively filled into at least two k spaces; the echo data of different groups have different echo times, and the number of echoes of each group of echo data is equal to the length of an echo chain in a repetition time; and
a water-fat separation unit (620); the water-fat separation unit (620) includes:
the first water-fat separation module (621) is used for obtaining at least two blade images of each blade according to data in at least two k spaces of each blade by taking the blade as a unit, and performing Dixon water-fat separation calculation by using the at least two blade images of each blade to obtain a water image and a fat image of each blade; and
and the second water-fat separation module (622) is used for synthesizing the water images of all the blades into a magnetic resonance water image of a complete k space and synthesizing the fat images of the blades into a magnetic resonance fat image of the complete k space.
7. The magnetic resonance imaging water-fat image separation device according to claim 6, wherein the water-fat separation unit (620) further comprises:
the water and fat image distinguishing and analyzing module (623) is used for carrying out Fourier transform on the water image of each blade to obtain water k space data corresponding to the water image; rotating the water k space data by corresponding angles according to the corresponding angles of the blades in the complete k space to obtain the water k space data of the blades under a set angle; determining an oversampled k-space central area, and respectively extracting data corresponding to the central area from the water k-space data of each blade under the set angle to obtain the water k-space central area data of each blade; respectively carrying out Fourier transform on the water k space central region data of each blade to obtain an oversampling region water image of each blade; calculating the gray average value of the water images in the oversampling area of all the blades; and respectively carrying out similarity calculation on the gray value of the water image in the oversampling area of each blade and the gray average value, and interchanging the water image and the fat image of the corresponding blade when the calculated similarity is lower than a set threshold value.
8. The magnetic resonance imaging water-fat image separation device according to claim 6 or 7,
the data acquisition unit (610) acquires echo data of two blades with a low b value lower than a set threshold value within one repetition time;
the first water-fat separation module (621) obtains at least two blade images with low b values of the blades according to data in at least two k spaces with low b values of each blade by taking the blade as a unit;
the second water-fat separation module (622) performs Dixon water-fat separation calculation by using the at least two blade images with low B values of each blade to obtain a B0 field diagram corresponding to the uniformity of the reaction magnetic field of the blade; and calculating a water image and a fat image of the blade according to the B0 field diagram of the blade and at least two blade images of the blade.
9. The magnetic resonance imaging water-fat image separation device according to claim 8, wherein the second water-fat separation module (622) further stores the B0 field map;
the data acquisition unit (610) further acquires echo data of two blades of a high b value higher than the set threshold value within one repetition time;
the first water-fat separation module (621) obtains at least two blade images with high b values of the blades according to data in at least two k spaces with high b values of each blade by taking the blades as a unit;
and the second water-fat separation module (622) obtains a water image and a fat image of the blade according to at least two blade images with high B values of the blade and a pre-stored B0 field map corresponding to the blade.
10. The apparatus according to any one of claims 6 to 9, wherein the first water-fat separation module (621) is configured to obtain low-resolution complete k-space data for each k-space of each blade by filling 0 in other blade positions for the data in each k-space of each blade, and perform fourier transform on the low-resolution complete k-space data to obtain one blade image corresponding to the k-space; carrying out Dixon water-fat separation calculation by utilizing at least two blade images of each blade to obtain a water image and a fat image of the blade;
the second water-fat separation module (622) respectively performs Fourier transform on the water image and the fat image of each blade to obtain water image data and fat image data of a low-resolution complete k space of the blade; extracting effective k space water image data corresponding to the blades from the low-resolution complete k space water image data, and extracting effective k space fat image data corresponding to the blades from the low-resolution complete k space fat image data; reconstructing high-resolution water image data of a complete k space by using the effective k space water image data of all the blades, and reconstructing high-resolution fat image data of the complete k space by using the effective k space fat image data of all the blades; and respectively carrying out Fourier transform on the water image data and the fat image data of the high-resolution complete k space to obtain a magnetic resonance water image and fat image data of the complete k space.
11. A kind of magnetic resonance imaging water fat image separation facility, characterized by that, comprising: at least one memory (71) and at least one processor (72), wherein:
the at least one memory (71) is for storing a computer program;
the at least one processor (72) is configured to invoke a computer program stored in the at least one memory (71) to perform the magnetic resonance imaging water-fat image separation method according to any one of claims 1 to 5.
12. A magnetic resonance imaging system comprising a magnetic resonance imaging water-fat image separating device as claimed in claims 6 to 11.
13. A computer-readable storage medium having stored thereon a computer program; characterized in that the computer program is executable by a processor and implements the magnetic resonance imaging water-fat image separation method as claimed in any one of claims 1 to 5.
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| 李少蕊;翁得河;陈会城;: "基于反转恢复技术的水脂分离磁共振成像方法", 波谱学杂志, no. 04, pages 81 - 87 * |
| 田洪君;陈思平;汪天富;盛恒松;刁现芬;郑崇勋;: "一种有效的低场磁共振系统的水脂分离成像方法", 西安交通大学学报, no. 06, pages 58 - 62 * |
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