CN110604570A

CN110604570A - Time-division coded hydrogen and sodium synchronous magnetic resonance imaging method

Info

Publication number: CN110604570A
Application number: CN201910863274.6A
Authority: CN
Inventors: 刘朝阳; 杨春升; 陈俊飞; 周波; 陈黎; 张震; 冯继文; 陈方; 鲍庆嘉
Original assignee: Wuhan Institute of Physics and Mathematics of CAS
Current assignee: Wuhan Institute of Physics and Mathematics of CAS
Priority date: 2019-09-12
Filing date: 2019-09-12
Publication date: 2019-12-24
Anticipated expiration: 2039-09-12
Also published as: CN110604570B

Abstract

The invention discloses a hydrogen and sodium synchronous magnetic resonance imaging method based on time-sharing coding.A sodium is excited twice before and after hydrogen back-focusing pulse, the same phase coding and frequency coding are carried out twice in a time-sharing manner, the overall effect of the applied coding gradient has no influence on the hydrogen, a sodium image accumulated twice is obtained, and the signal-to-noise ratio of the sodium image is improved; the strength of the first damage gradient of the hydrogen nuclei is adjusted, the hydrogen phase encoding gradient and the hydrogen reading preparation gradient are applied at the same time when the first damage gradient of the hydrogen nuclei is applied, no additional pulse event is added, and the weighting effect of the hydrogen nuclei T2 which is the same as that of the conventional spin echo imaging method can be obtained. Meanwhile, the echo time of the sodium nucleus is effectively shortened, the uniform convergence of the dispersed phase signals in the gradient direction and the readout gradient direction of the two nuclides is ensured, and the thickness consistency of the layers of hydrogen and sodium is realized; the image resolution of the two reconstructed nuclides is consistent, and the pixel positions correspond to one another.

Description

Time-division coded hydrogen and sodium synchronous magnetic resonance imaging method

Technical Field

The invention relates to the technical field of magnetic resonance imaging, in particular to a hydrogen and sodium synchronous magnetic resonance imaging method based on time-sharing coding.

Background

The image brightness and contrast of Magnetic Resonance Imaging (MRI) are related to the density, relaxation parameters (T1, T2), diffusion coefficient and the like of the tested nuclei in the sample, and the difference of the density, relaxation, diffusion and other parameters of the tested nuclei in different parts of the human body causes the difference of MRI signal intensity, and various structures and tissue components are separated by utilizing the difference.

Abundant water content in human body, high nuclear magnetic sensitivity of hydrogen nuclei, hydrogen: (¹H) The nuclear species becomes the first nuclear species for human body imaging and is used for analyzing the structure, the function and the like of tissues or organs. Sodium ion (a)²³Na⁺) Is one of the ubiquitous electrolytes in organisms and widely participates in physiological and pathological activities of organisms. Sodium is the second most powerful element with nuclear magnetic resonance activity in living organisms, and has become a nuclide widely used for studying organism metabolism and other physiological activities. The hydrogen and sodium synchronous magnetic resonance imaging technology is more and more emphasized.

The gyromagnetic ratio of hydrogen is about 3.8 times that of sodium, and the concentration of hydrogen from water in human tissues is about 1000-2000 times that of sodium, so that the signal of sodium is very low relative to hydrogen, the signal-to-noise ratio of hydrogen is 3000-20000 times higher than that of sodium, and in order to obtain a sodium image which can be used by a clinician, sodium needs to be repeatedly scanned and accumulated.

In the existing hydrogen and sodium synchronous magnetic resonance imaging technology, after a hydrogen nucleus and a sodium nucleus are excited by radio frequency, a spatial coding gradient is applied, and the hydrogen nucleus and the sodium nucleus are simultaneously spatially coded, however, the difference of the magnetic rotation ratio of the two nuclides is large, and for the hydrogen nucleus and the sodium nucleus, the layer thickness and the size of a layer area corresponding to an acquired signal are different, that is, the layer thickness, the image size and the resolution of a hydrogen image and a sodium image are different, which is not beneficial to the pixel correspondence between the hydrogen image and the sodium image.

In addition, the hydrogen nuclei in biological tissues have longer T1 and T2, such as grey brain matter, T1 is about 1.4s and T2 is about 110ms at a field strength of 3.0T; whereas sodium has a T1 of about 25 to 45ms, T2 has two components, distributed in both the 1-3ms and 20-30ms ranges. A long T2 of hydrogen may be used to obtain differently weighted contrast images using a sequence of long echo times, such as the widely used spin echo; however, the short sodium T2 requires shortening the echo time as much as possible for imaging, and cannot use a spin echo sequence but only gradient echoes, ultrashort gradient echoes, or the like. In the spin echo sequence, in order to suppress non-ideal factors such as the inaccuracy of the 180-degree radio frequency echo pulse, it is necessary to apply a destructive gradient before and after the spin echo sequence, and the intensity value is generally several times higher than the intensity of the slice selection gradient.

Disclosure of Invention

Aiming at the difference of the relaxation properties of hydrogen and sodium and the problems in the prior art, the invention provides a hydrogen and sodium synchronous magnetic resonance imaging method with time division encoding, which integrates a spin echo and gradient echo imaging method, wherein the frequency encoding and phase encoding of the hydrogen and sodium are applied in a time division manner, and for the hydrogen, a T2 weighted image of the hydrogen is obtained by using the spin echo imaging method in one scanning period; for sodium, a density weighted image accumulated twice is obtained by utilizing gradient echoes, so that the signal-to-noise ratio is effectively improved; meanwhile, the layer thickness Th and the size FOV of the inspection area corresponding to the hydrogen and sodium images are kept consistent, and the pixels of the two nuclide images correspond to one another.

The above object of the present invention is achieved by the following technical solutions:

a time-division coded hydrogen and sodium synchronous magnetic resonance imaging method comprises the following steps:

step 1, setting an inspection area FOV, a layer thickness Th and a sampling matrix [ M N ];

wherein M is the number of sampling points, N is the number of encoding steps in the phase encoding direction, and the inspection area FOV comprises a readout gradient direction inspection area FOVr and a phase encoding direction inspection area FOVp;

step 2, setting a hydrogen radio frequency channel, wherein a hydrogen refocusing pulse in the hydrogen radio frequency channel is applied after a hydrogen excitation pulse;

step 3, a sodium radio frequency channel is set, and a second sodium excitation pulse in the sodium radio frequency channel is applied after the first sodium excitation pulse;

step 4, a layer selection gradient channel GS is arranged, a first layer selection gradient and a first layer selection rephasing gradient are successively and closely applied to the layer selection gradient channel GS, a first hydrogen nucleus damage gradient, a hydrogen nucleus layer selection gradient and a second hydrogen nucleus damage gradient are applied to the layer selection gradient channel GS after the first layer selection rephasing gradient, the center of the hydrogen nucleus layer selection gradient is aligned with the center of the hydrogen rephasing pulse, and finally a sodium nucleus layer selection rephasing gradient is closely applied to the hydrogen nucleus second damage gradient;

the first selection layer gradient is used as a selection layer gradient shared by the hydrogen core and the sodium core, the second damage gradient of the hydrogen core is used as a selection layer gradient for exciting the sodium core for the second time, and the strength of the first damage gradient of the hydrogen core is adjusted, so that all dispersed phases of the hydrogen core and the sodium core are reunited in the layer direction;

step 5, setting a phase encoding gradient channel GP, wherein a sodium first-time phase encoding gradient is applied to the phase encoding gradient channel GP, a hydrogen phase encoding gradient is applied after the first sodium magnetic resonance signal echo1 is collected, and a sodium second-time phase encoding gradient is applied after the sodium second excitation pulse acts on the hydrogen phase encoding gradient;

step 6, setting a readout gradient channel GR, and applying a sodium first readout preparation gradient, a sodium first readout gradient, a hydrogen readout preparation gradient, a sodium second readout gradient and a hydrogen readout gradient in sequence in the readout gradient channel GR;

the center of the first slice segregation gradient, the center of the sodium first phase encoding gradient and the center of the sodium first readout preparation gradient are aligned; aligning the first hydrogen nucleus damage gradient, the hydrogen nucleus phase encoding gradient and the hydrogen reading preparation gradient center; aligning the center of the sodium nuclear selection layer echo gradient, the center of the sodium secondary phase encoding gradient and the center of the sodium second readout preparation gradient;

step 7, setting a sodium receiving channel R2, wherein in the sodium receiving channel R2, a first sodium magnetic resonance signal echo1 is acquired during the application of a first sodium readout gradient, and a second sodium magnetic resonance signal echo2 is acquired during the application of a second sodium readout gradient;

step 8, setting a hydrogen receiving channel R1, wherein a hydrogen magnetic resonance signal echo3 is acquired during the hydrogen reading gradient is applied in the hydrogen receiving channel R1;

step 9, accumulating the first sodium magnetic resonance signal echo1 and the second sodium magnetic resonance signal echo2, and filling the accumulated signals into a sodium K space for image reconstruction to obtain a sodium image; the hydrogen magnetic resonance signal echo3 is filled in the hydrogen K space, and is turned over end to end along the phase encoding gradient direction of the K space matrix to be reconstructed, so that the hydrogen image is obtained.

The hydrogen excitation pulse, the sodium first excitation pulse, and the first slice selection gradient pulse as described above remain right aligned; the sodium second excitation pulse is right aligned with the hydrogen nuclei second destruction gradient pulse.

The sodium first time phase encoding gradient and the sodium second time phase encoding gradient as described above are equal in area.

As described above, the sodium first read preparation gradient and the sodium second read preparation gradient are equal in area, and the sodium first read gradient and the sodium second read gradient are equal in area.

Compared with the prior art, the invention has the following beneficial effects:

1. according to the method, sodium is excited twice before and after hydrogen back-focusing pulse, the same phase coding and frequency coding are carried out twice in a time-sharing manner, the overall effect of the applied coding gradient is not influenced on hydrogen nuclei, and a sodium image accumulated twice is obtained.

2. The invention applies hydrogen phase encoding gradient and hydrogen readout preparation gradient at the same time of applying the first damage gradient of hydrogen nuclei, has compact time sequence design, and obtains the same T2 weighting effect without adding extra pulse event compared with the conventional spin echo image for hydrogen.

3. The invention effectively shortens the echo time of the sodium nucleus by adjusting the parameters of the first sodium excitation pulse and the second sodium excitation pulse and the strength and the relative position of the first hydrogen nucleus damage gradient, ensures that the two nuclides read out the phase signals in the gradient direction and realize the consistent layer thickness of the hydrogen and the sodium at the same time;

4. the invention reconstructs that the image resolution of two nuclides is consistent, and the pixel positions are in one-to-one correspondence.

5. In one scanning period, the invention obtains a hydrogen T2 weighted image, and simultaneously collects and accumulates signals of the sodium nucleus twice in short echo time, thereby improving the signal-to-noise ratio of the sodium nucleus.

Drawings

Fig. 1 is a timing diagram of a magnetic resonance imaging pulse sequence of one scan (TR) according to an embodiment of the present invention, wherein the horizontal axis represents the time axis.

Detailed Description

The present invention will be described in further detail with reference to examples for the purpose of facilitating understanding and practice of the invention by those of ordinary skill in the art, and it is to be understood that the present invention has been described in the illustrative embodiments and is not to be construed as limited thereto.

Loading a pulse sequence as shown in figure 1 on a magnetic resonance imaging system console supporting two or more simultaneous transmit-receive channels; one of the synchronous transceiving channels comprises a hydrogen radio frequency channel and a hydrogen receiving channel, and the other synchronous transceiving channel comprises a sodium radio frequency channel and a sodium receiving channel.

step 1, setting an inspection area FOV (field of View), a layer thickness Th and a sampling matrix [ M N ];

step 2, setting a hydrogen radio frequency channel, comprising: setting hydrogen excitation pulse and hydrogen excitation pulse width P₁Hydrogen refocusing pulse, hydrogen refocusing pulse width and hydrogen channel sampling spectrum width SW, the hydrogen refocusing pulse in the hydrogen radio frequency channel is applied after the hydrogen excitation pulse,

in this embodiment, the hydrogen excitation pulse is set as an SINC excitation pulse, and the hydrogen refocusing pulse is set as an SINC refocusing pulse.

According to the property parameter and the width P of the SINC excitation pulse₁Calculating the frequency domain width BW of the hydrogen excitation pulse₁Keeping the frequency domain width BW of the hydrogen refocusing pulse and the frequency domain width BW of the hydrogen excitation pulse₁Consistently, the frequency domain width BW of the hydrogen excitation pulse is determined according to the attribute parameters of the SINC echo pulse₁Calculating hydrogen back polymerization pulse width P₂。

Step 3, setting a sodium radio frequency channel, comprising: setting a first excitation pulse of sodium and a first excitation pulse width of sodium₃Sodium second excitation pulse, sodium second excitation pulse width P₄Sodium channel sampling spectral width SW₂A second excitation pulse of sodium in the sodium radio frequency channel is applied after the first excitation pulse of sodium;

in the parameters of the sodium radio frequency channel, the first excitation pulse of sodium and the second excitation pulse of sodium are selected as Gauss pulses,

according to gamma₁×Gslevel×Th＝BW₁Calculating the gradient intensity Gslevel, gamma of the first selected layer₁Is the magnetic rotation ratio of the hydrogen,

according to gamma₂×Gslevel×Th＝BW₃Calculating the frequency domain width BW of the first excitation pulse₃，γ₂Is the magnetic rotation ratio of sodium,

calculating the frequency domain width BW of the first sodium excitation pulse according to the attribute parameters of the Gauss pulse₃Corresponding sodium first excitation pulse width P₃Calculating the frequency domain width of the sodium second excitation pulse equal to the frequency domain width BW of the hydrogen excitation pulse₁Corresponding sodium second trigger pulsePunch width P₄Sodium channel sampling spectral width SW₂Consistent with the hydrogen channel sampling spectral width SW.

Step 4, setting a layer selection gradient channel GS, comprising: setting the width of the gradient of the first selected layer, the strength and the width of the repolymerization gradient of the first selected layer, the strength and the width of the first damage gradient of the hydrogen nuclei, the strength and the width of the gradient of the selected layer of the hydrogen nuclei, the strength and the width of the second damage gradient of the hydrogen nuclei and the strength and the width of the repolymerization gradient of the selected layer of the sodium nuclei.

Wherein, the first selective layer gradient is used as the selective layer gradient shared by the hydrogen nucleus and the sodium nucleus, and the second damage gradient of the hydrogen nucleus is used as the selective layer gradient for exciting the sodium nucleus for the second time. And calculating or fine-adjusting the strength of the first damage gradient of the hydrogen nuclei to ensure that all the dispersed phases of the hydrogen nuclei and the sodium nuclei are converged back in the layer direction.

Step 4.1, setting the width of the first layer selection gradient:

the width of the first selective layer gradient is set as the hydrogen excitation pulse width P₁And sodium first excitation pulse width P₃The larger of these.

Step 4.2, setting the intensity and width of the first layer-selection echo gradient:

and setting the width of the first selective layer echo gradient as Tref.

Calculating the intensity of the first slice echo gradient according to the area S1 of the first slice echo gradient and the width Tref of the first slice echo gradient, wherein S1 is 0.5 × Gslevel × P₃At this time, the layer direction of the sodium nucleus after the first excitation is scattered and gathered back,

step 4.3, setting the gradient strength and width of the hydrogen nuclear selection layer:

the gradient strength of the hydrogen nuclear selection layer is consistent with the gradient strength Gslevel of the first selection layer, and the gradient width of the hydrogen nuclear selection layer is equal to the pulse width P of the hydrogen back-focusing pulse₂；

And 4.4, setting the strength and width of a second hydrogen nucleus damage gradient:

the width of the second destruction gradient is set to be the sodium second excitation pulse width P₄；

Intensity of the second spoil gradient Gcrush₂Is arranged as

Step 4.5, setting the intensity and width of the sodium nuclear selective layer echo gradient

The width of the sodium nuclear sorting layer echo gradient is consistent with the width Tref of the first sorting layer echo gradient;

calculating the strength of the sodium nuclear selection layer segregation gradient according to the area S2 of the sodium nuclear selection layer segregation gradient and the width (Tref) of the sodium nuclear selection layer segregation gradient, wherein S2 is 0.5 XGcrush₂×P₄To ensure the diffusion of the sodium nucleus in the layer direction after the second excitation to be converged.

Step 4.6, setting the strength and width of the first damage gradient of the hydrogen nuclei:

the width of the first damage gradient of the hydrogen nuclei is set as the width P of the second excitation pulse of the sodium nuclei₄，

According to the area S3 of the first damage gradient of the hydrogen nuclei and the width P of the first damage gradient of the hydrogen nuclei₄Calculating the first damage gradient strength Gburst of hydrogen nuclei₁Wherein S3 ═ S2- (0.5 XGslevel × p₁-S1)。

The strength Gburst of the first damage gradient of the hydrogen nuclei can be finely adjusted in the practical experimental process₁Optimizing the maximum hydrogen echo signal;

at this time, all the scattered phases in the direction of the hydrogen nuclear layer are converged while eliminating the free decay signal FID which may be introduced by the non-ideality of the RF refocusing pulse.

In time sequence, a first layer selection gradient and a first layer selection rephasing gradient are sequentially and closely applied in a layer selection gradient channel GS, a first damage gradient of a hydrogen nucleus, a hydrogen nucleus layer selection gradient and a second damage gradient of the hydrogen nucleus are applied after the first layer selection rephasing gradient, the center of the hydrogen nucleus layer selection gradient is aligned with the center of a hydrogen rephasing pulse, and finally a sodium nucleus layer selection rephasing gradient is closely applied after the second damage gradient of the hydrogen nucleus.

Preferably, the hydrogen excitation pulse, the sodium first excitation pulse and the first slice selection gradient pulse are kept in right alignment in time sequence; the center of the second sodium excitation pulse is aligned with the right of the second hydrogen nucleus destroying gradient pulse, so that the echo time of sodium twice sampling is short.

And step 5, setting a phase encoding gradient channel GP, wherein the setting of the sodium primary phase encoding gradient strength array and width, the hydrogen phase encoding gradient strength array and width and the sodium secondary phase encoding gradient strength array and width is included.

The width of the sodium primary phase encoding gradient and the width of the sodium secondary phase encoding gradient are both consistent with the width Tref of the first segregation gradient;

hydrogen phase encoding gradient width and first spoiled gradient width P₄The consistency is achieved;

according to the functional forms and widths of the sodium primary phase encoding gradient, the hydrogen phase encoding gradient and the sodium secondary phase encoding gradient, the respective magnetic rotation ratios, the phase encoding direction examination region FOVp and the sampling theorem, a hydrogen phase encoding gradient strength array Gp1, a sodium primary phase encoding gradient strength array Gp2 and a sodium secondary phase encoding gradient strength array Gp3 are respectively calculated.

The hydrogen phase encoding gradient strength array Gp1, the sodium first time phase encoding gradient strength array Gp2 and the sodium second time phase encoding gradient strength array Gp3 respectively have N phase encoding gradient strength values.

In the phase encoding gradient channel GP, a first sodium phase encoding gradient is applied first, after the acquisition of the first sodium magnetic resonance signal echo1 is completed, a hydrogen phase encoding gradient is applied next, and after the action of a second sodium excitation pulse, a second sodium phase encoding gradient is applied.

The sodium primary phase encoding gradient and the sodium secondary phase encoding gradient are distributed before and after the hydrogen back-focusing pulse, and the areas of the sodium primary phase encoding gradient and the sodium secondary phase encoding gradient are equal, so that the total result has no influence on hydrogen nuclei, and the FOVp of the hydrogen nuclei and the FOVp of the sodium nuclei phase encoding direction examination area are kept consistent.

Step 6, setting a readout gradient channel GR, including: the strength and width of the sodium first readout gradient, the strength and width of the hydrogen readout preparation gradient, the strength and width of the sodium second readout gradient, and the strength and width of the hydrogen readout gradient are set.

The width of the first sodium readout preparation gradient and the width of the second sodium readout preparation gradient are the same as the width of the first selective layer echo gradient and are set as Tref;

the hydrogen readout preparation gradient width is consistent with the first damage gradient width of the hydrogen nuclei and is set as P₄；

A sodium first readout preparation gradient, a sodium first readout gradient, a hydrogen readout preparation gradient, a sodium second readout gradient, and a hydrogen readout gradient are applied sequentially.

The sodium first readout preparation gradient, the hydrogen readout preparation gradient, and the sodium second readout preparation gradient can all be fine tuned to obtain the maximum echo signal.

In time sequence, the center of the first selective layer echo gradient, the center of the first sodium phase encoding gradient and the center of the first sodium readout preparation gradient are aligned; aligning the first hydrogen nucleus damage gradient, the hydrogen nucleus phase encoding gradient and the hydrogen reading preparation gradient center; the center of the sodium nuclear selection layer echo gradient, the center of the sodium second phase encoding gradient and the center of the sodium second readout preparation gradient are aligned.

The first sodium readout preparation gradient and the first sodium readout gradient are distributed before the hydrogen refocusing pulse, the second sodium readout preparation gradient and the second sodium readout gradient are distributed after the hydrogen refocusing pulse, the first sodium readout preparation gradient and the second sodium readout gradient are equal in area, and the first sodium readout gradient and the second sodium readout gradient are equal in area, so that the overall result has no influence on hydrogen nuclei, the phase dispersion refocuses in the hydrogen nuclei readout direction, and the hydrogen nuclei and the sodium nuclei readout gradient direction inspection region FOVr are kept consistent.

Step 7, setting a sodium receiving channel R2, and setting the sampling spectral width to SW₂In the sodium reception channel R2, a first sodium magnetic resonance signal echo1 is acquired during application of a sodium first readout gradient, and a second sodium magnetic resonance signal echo2 is acquired during application of a sodium second readout gradient.

Step 8, a hydrogen receiving channel R1 is provided, and in the hydrogen receiving channel R1, a hydrogen magnetic resonance signal echo3 is acquired during the application of the hydrogen readout gradient.

Step 9, image reconstruction, namely filling the accumulated first sodium magnetic resonance signal echo1 and the second sodium magnetic resonance signal echo2 into a sodium K space for image reconstruction to obtain a sodium image; the hydrogen magnetic resonance signal echo3 is filled in the hydrogen K space, and is turned over end to end along the phase encoding gradient direction of the K space matrix to be reconstructed, so that the hydrogen image is obtained.

At the moment, the hydrogen image and the sodium image have consistent layer thickness and same resolution, and the pixel positions correspond to each other one by one, so that accurate registration is realized.

It should be noted that the specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims

1. A time-division coded hydrogen and sodium synchronous magnetic resonance imaging method is characterized by comprising the following steps:

step 1, setting an inspection areaFOVThickness of the layerThAnd a sampling matrix [ M N]；

Where M is the number of sampling points, N is the number of encoding steps in the phase encoding direction, the examination regionFOVIncluding readout gradient direction examination regionFOVrAnd phase encoding direction examination regionFOVp；

2. The time-division coded hydrogen and sodium synchronous magnetic resonance imaging method according to claim 1, wherein the hydrogen excitation pulse, the sodium first excitation pulse and the first slice selection gradient pulse are kept in right alignment; the sodium second excitation pulse is right aligned with the hydrogen nuclei second destruction gradient pulse.

3. The time-division coded hydrogen and sodium synchronous magnetic resonance imaging method according to claim 1, wherein the sodium first-time phase encoding gradient and the sodium second-time phase encoding gradient are equal in area.

4. The time-division coded hydrogen and sodium synchronous magnetic resonance imaging method according to claim 1, wherein the sodium first read preparation gradient and the sodium second read preparation gradient are equal in area, and the sodium first read preparation gradient and the sodium second read preparation gradient are equal in area.