CN103064047B - Correcting method and correcting system for elastic rebuilding of magnetic resonance elastography (MRE) - Google Patents

Abstract

The invention relates to a correcting method and correcting system for elastic rebuilding of magnetic resonance elastography (MRE). The method comprises the following steps: acquiring a shear wave generated by an imaging object; comparing the size of the imaging object with the wavelength of the shear wave, and when the size of the imaging object and the wavelength of the shear wave are of an identical order of magnitude, the shear wave is introduced into a higher order term to correct an equation of motion and then obtain an objective function through calculation, wherein the objective function is shown in the description, gamma is the correction factor, mu is the lame constant, rho is the medium density, omega is the angular frequency of an incentive, and U is the displacement vector of a medium. According to the correcting method and correcting system for the elastic rebuilding of the MRE, through acquisition of the shear wave generated by the imaging object, comparison of the wavelength of the shear wave with the size of the imaging object and then adoption of an algorithm of the objective function, the elastic rebuilding can be relatively accurately achieved.

Description

The elasticity of magnetic resonance elastography is rebuild modification method and system

Technical field

The present invention relates to magnetic resonance elastography (Magnetic Resonance Elastography, MRE) technology, the elasticity that particularly relates to magnetic resonance elastography is rebuild modification method and system.

Background technology

Elasticity reconstruction algorithm is a most important link in magnetic resonance elastography (MRE) process, has determined spatial resolution and the elasticity measurement accuracy of MRE image.

At present, the research of MRE elasticity reconstruction algorithm mainly launches around solving Helmholtz algorithm.Suppose that shearing wave propagates in isotropic Infinite medium, and ignore the impact of compressional wave, Particles Moving meets Helmholtz equation wherein U is particle displacement, and ρ is Media density, and ω is the angular frequency of excitation, μ modulus of shearing.

Also have some other methods simultaneously, local frequency estimation method (local frequency estimation for example, LFE), method (the Directly Inversion that directly inverts, DI), matched filter method (Matched Filter, MF), phase gradient method (Phase Gradient, PG), finite element iterative method (FEM) etc., yet all there is a problem in above-mentioned MRE elasticity reconstruction algorithm: in algorithm, all suppose that shearing wave propagates in Infinite medium.But in fact, owing to being subject to edge effect, there is reflection and refraction in shearing wave, what MRE captured is the result that has been mingled with a plurality of ripple stacks such as reflection wave and reflection wave, this and assumed condition are runed counter to, especially when the Spatial Dimension of imaging object approaches or is less than shearing wave wavelength, the elasticity number that LFE, DI and PG method calculate can be significantly less than actual value, has error.

Summary of the invention

Based on this, be necessary for the problems referred to above, provide the elasticity of the magnetic resonance elastography that a kind of accuracy is higher to rebuild modification method.

In addition, also by the necessary elasticity of the magnetic resonance elastography that a kind of accuracy is higher that provides, rebuild update the system.

The elasticity of magnetic resonance elastography is rebuild a modification method, comprises the steps: to obtain the shearing wave that imaging object produces; The wavelength of described imaging object and described shearing wave compares, when the wavelength of described imaging object and described shearing wave is during at an order of magnitude, described shearing wave is introduced to higher order term and the equation of motion is revised, calculate objective function, described objective function is:

wherein γ is correction factor, and μ is Lame's constant, and ρ is Media density, and ω is the angular frequency of excitation, and U is dielectric displacement vector.

In an embodiment, also comprise therein: when described imaging object is greater than the wavelength of three times of described shearing waves, described shearing wave includes ejected wave and reflection wave:

Described incident wave equation is: $\stackrel{\→}{r}\left(t\right)={\stackrel{\→}{r}}_0+{\stackrel{\→}{\mathrm{\ξ}}}_0\exp \left(j(\stackrel{\→}{k}\·\stackrel{\→}{r}-\mathrm{\ωt})\right),$

Described reflection wave equation is: ${\stackrel{\→}{r}}^{\′}\left(t\right)={{\stackrel{\→}{r}}_0}^{\′}+{{\stackrel{\→}{\mathrm{\ξ}}}_0}^{\′}\exp \left(j({\stackrel{\→}{k}}^{\′}\·{\stackrel{\→}{r}}^{\′}-\mathrm{\ωt})\right),$

Wherein be respectively the incident wave of r and k direction, with incident wave take border is the reflection wave of Mirror Symmetry relation as axis, be respectively the particle equilibrium position of incident wave and reflection wave, be respectively incident wave and reflection wave amplitude, ω is the angular frequency of excitation, and t is the time;

By wave filter, carry out filtering, the filtering of described wave filter is defined as:

Q(ω)＝R(ω)D _k(ω)， $R\left(\mathrm{\ω}\right)=\frac{1}{\sqrt{1+{(\mathrm{\ω}/{\mathrm{\ω}}_c)}^{2n}}},$

Wherein Q (ω) is anisotropic filter, the frequency response function that R (ω) is wave filter, D _k(ω) be the directional response function of wave filter, ω is angular frequency, ω _cfor filter cutoff frequency, n is filter order, for filter direction, for self-defining direction vector.

In an embodiment, the direction of described wave filter is described reflection wave line of propagation therein, described wave filter filtering be described reflection wave.

In an embodiment, the filtering definition of described wave filter is carried out to parameter selection therein, described parameter is R (ω) and D _k(ω), described parameter selects to adopt linear search method to be: investigate the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is to adjust parameter, and fixing another parameter is preset parameter; The curve that drawing image quality changes with described adjustment parameter; Select with the described adjustment parameter of goldstandard error minimum as most suitable parameter; Described goldstandard is to set in advance the accurate reference value of obtaining.

In an embodiment, described parameter selects to adopt linear search method also to comprise: described adjustment setting parameter, be new preset parameter, described preset parameter is set as to new adjustment parameter therein; Return to described drawing image quality with the step of the curve of described adjustment parameter variation.

In addition, be also necessary to provide a kind of elasticity of magnetic resonance elastography to rebuild update the system, comprise: shearing wave acquisition module, the shearing wave producing for obtaining imaging object; Comparison module, for comparing the wavelength of described imaging object and described shearing wave; First selects module, when the wavelength of described imaging object and described shearing wave is during at an order of magnitude, described shearing wave introducing higher order term and the equation of motion is revised, calculates objective function.

In an embodiment, also comprise therein: second selects module, and when described imaging object is greater than the wavelength of three times of described shearing waves, described shearing wave includes ejected wave and reflection wave; Wave filter, for carrying out filtering to described shearing wave.

In an embodiment, the direction of described wave filter is described reflection wave line of propagation therein, described wave filter filtering be described reflection wave.

In an embodiment, described wave filter comprises therein: parameter selection unit, and for investigating the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is adjustment parameter, fixing another parameter is preset parameter; Image Rendering unit, the curve changing with described adjustment parameter for drawing image quality; Comparing unit, for selecting with the described adjustment parameter of goldstandard error minimum as most suitable parameter; Described goldstandard is to set in advance the accurate reference value of obtaining.

In an embodiment, described wave filter also comprises therein: exchange unit is new preset parameter described adjustment setting parameter, and described preset parameter is set as to new adjustment parameter; Return to unit, return to described Image Rendering unit.

Adopt the elasticity of the application's magnetic resonance elastography to rebuild modification method and system, the shearing wave producing by obtaining imaging object, the size of the wavelength of this shearing wave and imaging object is compared, then adopt objective function algorithm, can carry out more accurately elasticity reconstruction.

Accompanying drawing explanation

Fig. 1 is the process flow diagram that the elasticity of a magnetic resonance elastography in embodiment is rebuild modification method;

Fig. 2 is the process flow diagram that the elasticity of the magnetic resonance elastography in another embodiment is rebuild modification method;

Fig. 3 is the process flow diagram that a parameter in embodiment selects to adopt linear search method;

Fig. 4 is the process flow diagram that the parameter in another embodiment selects to adopt linear search method;

Fig. 5 is the logic diagram that the elasticity of a magnetic resonance elastography in embodiment is rebuild update the system;

Fig. 6 is the logic diagram that the elasticity of the magnetic resonance elastography in another embodiment is rebuild update the system;

Fig. 7 is the logic diagram of a wave filter in embodiment;

Fig. 8 is the logic diagram of the wave filter in another embodiment.

Embodiment

In reconstruction algorithm, imaging object receives that extraneous excitation produces shearing wave, yet can produce reflection wave etc. on the border of imaging object, and what collect is the stack result such as shearing wave, transmitted wave.Usually, there are three kinds of situations on the border of imaging object, respectively: rigid boundary, free boundary and elastic restraint border.In rigid boundary condition: reflection does not have the variation of waveform, and the shearing wave of incident (compressional wave) only produces the shearing wave (compressional wave) of a reflection; In free boundary condition: reflection wave is relevant to the Poisson ratio of incident angle; At elastic restraint boundary condition: reflection wave is relevant with incident angle, also depends on the rigidity on border and the wavelength of incident wave.

As shown in Figure 1, in one embodiment, in the elasticity of the application's magnetic resonance elastography, rebuild in modification method, comprise the steps:

Step S10: obtain the shearing wave that imaging object produces.In the process of elastogram, by exciting bank, imaging object or body surface are produced to shearing wave, this shearing wave frequency, wavelength can be adjusted by exciting bank.Now, the imaging object of excited target produces shearing wave, and the shearing wave frequency that this imaging object produces, wavelength etc. are different and different according to the hardness of imaging object etc., and the information of this shearing wave can reflect the elastic information of imaging object.For imaging object, can be the organs such as liver, heart.

Step S20: the wavelength of imaging object and shearing wave compares, when the wavelength of imaging object and shearing wave is during at an order of magnitude, introduces shearing wave higher order term and the equation of motion is revised, and calculates objective function.Particularly, for imaging object, the size of imaging object is suitable with the wavelength of shearing wave, and the border for imaging object can be considered as not producing reflection wave, can adopt this objective function to calculate.

Objective function is: $\min {\left|\right|\mathrm{\μ}{\▿}^{2}U+\mathrm{\γ\μ}{\mathrm{\Δ}}^{2}U+\mathrm{\ρ}{\mathrm{\ω}}^{2}U\left|\right|}_2^2$

Wherein γ is correction factor, and μ is Lame's constant, and ρ is Media density, and ω is the angular frequency of excitation, and U is dielectric displacement vector.

The shearing wave producing by obtaining imaging object, compares the size of the wavelength of this shearing wave and imaging object, adopts the objective function algorithm of this programme, can carry out more accurately elasticity reconstruction.

In one embodiment, consult accompanying drawing 2, the elasticity of magnetic resonance elastography is rebuild modification method and is also comprised: step S30: when imaging object is greater than the wavelength of three times of shearing waves, when the size of imaging object is much larger than the wavelength of shearing wave in other words, the shearing wave that imaging object produces mainly includes ejected wave and reflection wave, now:

Incident wave equation is: $\stackrel{\→}{r}\left(t\right)={\stackrel{\→}{r}}_0+{\stackrel{\→}{\mathrm{\ξ}}}_0\exp \left(j(\stackrel{\→}{k}\·\stackrel{\→}{r}-\mathrm{\ωt})\right),$

Reflection wave equation is: ${\stackrel{\→}{r}}^{\′}\left(t\right)={{\stackrel{\→}{r}}_0}^{\′}+{{\stackrel{\→}{\mathrm{\ξ}}}_0}^{\′}\exp \left(j({\stackrel{\→}{k}}^{\′}\·{\stackrel{\→}{r}}^{\′}-\mathrm{\ωt})\right),$

Wherein be respectively the incident wave of r and k direction, with incident wave take border is the reflection wave of Mirror Symmetry relation as axis, be respectively the particle equilibrium position of incident wave and reflection wave, be respectively incident wave and reflection wave amplitude, ω is the angular frequency of excitation, and t is the time.

Step S40: carry out filtering by wave filter, the filtering of wave filter is defined as:

Q(ω)＝R(ω)D _k(ω)， $R\left(\mathrm{\ω}\right)=\frac{1}{\sqrt{1+{(\mathrm{\ω}/{\mathrm{\ω}}_c)}^{2n}}},$

Wherein Q (ω) is anisotropic filter, the frequency response function that R (ω) is wave filter, D _k(ω) be the directional response function of wave filter, ω is angular frequency, ω _cfor filter cutoff frequency, n is filter order, for filter direction, for self-defining direction vector.

For wave filter, this wave filter is anisotropic filter.Because the directivity of reflection wave is very strong, and the direction of wave filter is reflection wave line of propagation, therefore by anisotropic filter filtering reflection wave, can further reduce reconstruction error.

By reference to the accompanying drawings 3, further, for the filtering definition of wave filter, carry out parameter selection, this parameter is R (ω) and D _k(ω), parameter is selected to adopt linear search method, is specially:

S41: investigate the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is adjustment parameter, and fixing another parameter is preset parameter.Particularly, can select parameters R (ω) or D _k(ω) parameter is adjusted at one of them seat, then fixing another one.

S42: drawing image quality is with the curve of adjusting parameter variation.Particularly, constantly linear change is adjusted parameter, and obtains the curve of adjusting parameter variation.

S43: select with the adjustment parameter of goldstandard error minimum as most suitable parameter; Goldstandard is to set in advance the accurate reference value of obtaining.Particularly, preset reference value, this reference value is real value, is goldstandard.The comparison of curve map and goldstandard by picture quality, error minimum be best adjustment parameter value.

In addition, by reference to the accompanying drawings 4, parameter selects to adopt linear search method also to comprise:

S44: be new preset parameter adjusting setting parameter, preset parameter is set as to new adjustment parameter.Particularly, exchanging former adjustment parameter and preset parameter is new adjustment parameter and preset parameter, and wherein new preset parameter is determined optimal adjusting parameters in step S43, then with the new adjustment parameter of this new preset parameter adjustment.

S45: return to drawing image quality with the step of adjusting the curve of parameter variation, return to step S42, adopt above-mentioned identical method to determine new adjustment parameter.

The elasticity of the magnetic resonance elastography based on above-mentioned is rebuild modification method, and by reference to the accompanying drawings 5, in one embodiment, provide the elasticity of magnetic resonance elastography to rebuild modification method system, comprising:

Shearing wave acquisition module 10, the shearing wave producing for obtaining imaging object.Concrete, the shearing wave that the imaging object of excited target produces, this shearing wave is sheared ripple acquisition module 10 and gets.

Comparison module 20, for comparing the wavelength of imaging object and shearing wave.

First selects module 30, when the wavelength of imaging object and shearing wave is during at an order of magnitude, shearing wave introducing higher order term and the equation of motion is revised, calculates objective function.Particularly, for imaging object, the size of imaging object is suitable with the wavelength of shearing wave, and the border for imaging object can be considered as not producing reflection wave, can adopt this objective function to calculate.

Objective function is: $\min {\left|\right|\mathrm{\μ}{\▿}^{2}U+\mathrm{\γ\μ}{\mathrm{\Δ}}^{2}U+\mathrm{\ρ}{\mathrm{\ω}}^{2}U\left|\right|}_2^2$

Wherein γ is correction factor, and μ is Lame's constant, and ρ is Media density, and ω is the angular frequency of excitation, and U is dielectric displacement vector.

The shearing wave producing by obtaining imaging object, compares the size of the wavelength of this shearing wave and imaging object, adopts the objective function algorithm of this programme, can carry out more accurately elasticity reconstruction.

By reference to the accompanying drawings 6, in one embodiment, the elasticity of magnetic resonance elastography is rebuild modification method system, also comprises:

Second selects module 40, and when imaging object is greater than the wavelength of three times of shearing waves, shearing wave includes ejected wave and reflection wave.Particularly, for the size of imaging object, during much larger than the wavelength of shearing wave, the shearing wave that imaging object produces mainly includes ejected wave and reflection wave, now:

Incident wave equation is: $\stackrel{\→}{r}\left(t\right)={\stackrel{\→}{r}}_0+{\stackrel{\→}{\mathrm{\ξ}}}_0\exp \left(j(\stackrel{\→}{k}\·\stackrel{\→}{r}-\mathrm{\ωt})\right),$

Reflection wave equation is: ${\stackrel{\→}{r}}^{\′}\left(t\right)={{\stackrel{\→}{r}}_0}^{\′}+{{\stackrel{\→}{\mathrm{\ξ}}}_0}^{\′}\exp \left(j({\stackrel{\→}{k}}^{\′}\·{\stackrel{\→}{r}}^{\′}-\mathrm{\ωt})\right),$

Wherein be respectively the incident wave of r and k direction, with incident wave take border is the reflection wave of Mirror Symmetry relation as axis, be respectively the particle equilibrium position of incident wave and reflection wave, be respectively incident wave and reflection wave amplitude, ω is the angular frequency of excitation, and t is the time.

Wave filter 50, for carrying out filtering to shearing wave.The filtering of wave filter 50 is defined as:

Q(ω)＝R(ω)D _k(ω)， $R\left(\mathrm{\ω}\right)=\frac{1}{\sqrt{1+{(\mathrm{\ω}/{\mathrm{\ω}}_c)}^{2n}}},$

Wherein Q (ω) is anisotropic filter, the frequency response function that R (ω) is wave filter, D _k(ω) be the directional response function of wave filter, ω is angular frequency, ω _cfor filter cutoff frequency, n is filter order, for filter direction, for self-defining direction vector.

For wave filter, this wave filter is anisotropic filter.Because the directivity of reflection wave is very strong, and the direction of wave filter is reflection wave line of propagation, therefore by anisotropic filter filtering reflection wave, can further reduce reconstruction error.

By reference to the accompanying drawings 7, further, for the filtering definition of wave filter 50, carry out parameter selection, this parameter is R (ω) and D _k(ω), wave filter 50 comprises:

Parameter selection unit 51, for investigating the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is adjustment parameter, fixing another parameter is preset parameter.Particularly, can select parameters R (ω) or D _k(ω) parameter is adjusted at one of them seat, then fixing another one.

Image Rendering unit 52, the curve changing with adjustment parameter for drawing image quality.Particularly, constantly linear change is adjusted parameter, and obtains the curve of adjusting parameter variation.

Comparing unit 53, for selecting with the adjustment parameter of goldstandard error minimum as most suitable parameter; Goldstandard is to set in advance the accurate reference value of obtaining.Particularly, preset reference value, this reference value is real value, is goldstandard.The comparison of curve map and goldstandard by picture quality, error minimum be best adjustment parameter value.

In addition, by reference to the accompanying drawings 8, wave filter 50 also comprises:

Exchange unit 54, is new preset parameter adjusting setting parameter, and preset parameter is set as to new adjustment parameter.Particularly, exchanging former adjustment parameter and preset parameter is new adjustment parameter and preset parameter, and wherein new preset parameter is determined optimal adjusting parameters in comparing unit 53, then with the new adjustment parameter of this new preset parameter adjustment.

Return to unit 55, return to Image Rendering unit 52.

The above embodiment has only expressed several embodiment of the present invention, and it describes comparatively concrete and detailed, but can not therefore be interpreted as the restriction to the scope of the claims of the present invention.It should be pointed out that for the person of ordinary skill of the art, without departing from the inventive concept of the premise, can also make some distortion and improvement, these all belong to protection scope of the present invention.Therefore, the protection domain of patent of the present invention should be as the criterion with claims.

Claims (6)

1. the elasticity of magnetic resonance elastography is rebuild a modification method, comprises the steps:

Obtain the shearing wave that imaging object produces;

The wavelength of described imaging object and described shearing wave compares, when the wavelength of described imaging object and described shearing wave is during at an order of magnitude, described shearing wave is introduced to higher order term and the equation of motion is revised, calculate objective function, described objective function is:

wherein γ is correction factor, and μ is Lame's constant, and ρ is Media density, and ω is the angular frequency of excitation, and U is dielectric displacement vector;

Described method also comprises: when described imaging object is greater than the wavelength of three times of described shearing waves, described shearing wave includes ejected wave and reflection wave:

Described incident wave equation is: $\stackrel{\→}{r}\left(t\right)={\stackrel{\→}{r}}_0+{\stackrel{\→}{\mathrm{\ξ}}}_0\exp \left(j(\stackrel{\→}{k}\·\stackrel{\→}{r}-\mathrm{\ωt})\right),$

Described reflection wave equation is: ${\stackrel{\→}{r}}^{\′}\left(t\right)={{\stackrel{\→}{r}}_0}^{\′}+{{\stackrel{\→}{\mathrm{\ξ}}}_0}^{\′}\exp \left(j({\stackrel{\→}{k}}^{\′}\·{\stackrel{\→}{r}}^{\′}-\mathrm{\ωt})\right),$

Wherein be respectively the incident wave of r and k direction, with incident wave take border is the reflection wave of Mirror Symmetry relation as axis, be respectively the particle equilibrium position of incident wave and reflection wave, be respectively incident wave and reflection wave amplitude, ω is the angular frequency of excitation, and t is the time;

By wave filter, carry out filtering, the filtering of described wave filter is defined as:

Q(ω)＝R(ω)D _k(ω)， $R\left(\mathrm{\ω}\right)=\frac{1}{\sqrt{1+{(\mathrm{\ω}/{\mathrm{\ω}}_c)}^{2n}}},$

Wherein Q (ω) is anisotropic filter, the frequency response function that R (ω) is wave filter, D _k(ω) be the directional response function of wave filter, ω is angular frequency, ω _cfor filter cutoff frequency, n is filter order, for filter direction, for self-defining direction vector;

The direction of described wave filter is described reflection wave line of propagation, described wave filter filtering be described reflection wave.

2. the elasticity of magnetic resonance elastography according to claim 1 is rebuild modification method, it is characterized in that, the filtering definition of described wave filter is carried out to parameter selection, and described parameter is R (ω) and D _k(ω), described parameter selects to adopt linear search method to be:

Investigate the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is adjustment parameter, and fixing another parameter is preset parameter;

The curve that drawing image quality changes with described adjustment parameter;

Select with the described adjustment parameter of goldstandard error minimum as most suitable parameter; Described goldstandard is to set in advance the accurate reference value of obtaining.

3. the elasticity of magnetic resonance elastography according to claim 2 is rebuild modification method, it is characterized in that, described parameter selects to adopt linear search method also to comprise:

Described adjustment setting parameter, be new preset parameter, described preset parameter is set as to new adjustment parameter;

Return to described drawing image quality with the step of the curve of described adjustment parameter variation.

4. the elasticity of magnetic resonance elastography is rebuild a update the system, it is characterized in that, comprising:

Shearing wave acquisition module, the shearing wave producing for obtaining imaging object;

Comparison module, for comparing the wavelength of described imaging object and described shearing wave;

First selects module, when the wavelength of described imaging object and described shearing wave is during at an order of magnitude, described shearing wave introducing higher order term and the equation of motion is revised, calculates objective function;

Described system also comprises: second selects module, and when described imaging object is greater than the wavelength of three times of described shearing waves, described shearing wave includes ejected wave and reflection wave;

Wave filter, for carrying out filtering to described shearing wave;

The direction of described wave filter is described reflection wave line of propagation, described wave filter filtering be described reflection wave.

5. the elasticity of magnetic resonance elastography according to claim 4 is rebuild update the system, it is characterized in that, described wave filter comprises:

Parameter selection unit, for investigating the wherein impact of a parameter on reconstruction quality, selecting a wherein parameter is adjustment parameter, fixing another parameter is preset parameter;

Image Rendering unit, the curve changing with described adjustment parameter for drawing image quality;

Comparing unit, for selecting with the described adjustment parameter of goldstandard error minimum as most suitable parameter; Described goldstandard is to set in advance the accurate reference value of obtaining.

6. the elasticity of magnetic resonance elastography according to claim 5 is rebuild update the system, it is characterized in that, described wave filter also comprises:

Exchange unit, is new preset parameter described adjustment setting parameter, and described preset parameter is set as to new adjustment parameter;

Return to unit, return to described Image Rendering unit.