CN105249967B - A kind of positive contrast MR imaging method and device - Google Patents

A kind of positive contrast MR imaging method and device Download PDF

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CN105249967B
CN105249967B CN201510796227.6A CN201510796227A CN105249967B CN 105249967 B CN105249967 B CN 105249967B CN 201510796227 A CN201510796227 A CN 201510796227A CN 105249967 B CN105249967 B CN 105249967B
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magnetic field
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CN105249967A (en
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谢国喜
史彩云
陈敏
苏适
纪秀全
刘新
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Shenzhen Institute of Advanced Technology of CAS
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Abstract

The present invention relates to a kind of positive contrast MR imaging method and device, wherein, method includes：Data acquisition twice is carried out to the same aspect of same target, obtains the TSE data with the offset of echo readout gradient and the TSE data without the offset of echo readout gradient respectively；In the same aspect of same target, the phase of the TSE data with the offset of echo readout gradient subtracts the phase of the TSE data without the offset of echo readout gradient, obtains the phase difference that local magnetic field is influenced and produced；The local magnetic field that metal device produces surrounding tissue in target area is obtained using the phase difference；The Convolution target local magnetic field expression formula of polarized nucleus and magnetization rate matrix, Fourier transform is carried out to target local magnetic field expression formula, utilize the local magnetic field, regularization constraint reconstruction is carried out to the target local magnetic field expression formula after Fourier transform, the magnetization rate matrix of target is obtained, realizes positive contrast magnetic resonance imaging.

Description

A kind of positive contrast MR imaging method and device
Technical field
The present invention relates to mr imaging technique field, more particularly to a kind of positive contrast MR imaging method and device.
Background technology
Magnetic resonance imaging (Magnetic Resonance Imaging, MRI) it is radiationless, good soft tissue contrast and appoint Meaning planar imaging causes it to become one of presently the most popular clinical diagnosis instrument.MRI is to utilize some Hydrogen Protons in human body Occur to resonate and produce signal to be imaged under the action of main field (i.e. external magnetic field) and excitation pulse, and it is most of Metal be easily magnetized in magnetic field, local magnetic field can be produced, so that interference can be produced to main field, this interference be imaged In be known as metal magnetic susceptibility artifact.Since the magnetic resonance compatible device of human body implantation is generally metal material, device and its week Enclose neighborhood and show black hole phenomenon (negative contrast) in the picture.This black hole is difficult area with tissue void and compared with low signal-to-noise ratio region Point, so that precise positioning and the assessment of device can not be carried out.
At present, University of Regensburg doctor Lenhart of Jiang's generation skilful doctor life of China Beijing Union Medical College and Germany etc. People carries out stent magnetic resonance noninvasive test using CE-MRA technologies, and research finds that CE-MRA technologies can be used for assessing NiTi conjunction It is whether unobstructed in golden stent cavity, but due to there are magnetic susceptibility artifact, can not be accurately classified to the stenosis of tube chamber.But One common ground of these researchs is the negative contrast images that can only obtain stent.In recent years, in order to be accurately positioned and assess these Metal implant, researchers propose a series of just (bright) the contrast imaging technique of magnetic resonance, these technologies are broadly divided into two classes. The first kind is that positive contrast imaging is produced by changing pulse train, such as Seppenwoolde JH in 2003 et al. are proposed The GRASP technologies that white marker and Venkatesh Mani in 2006 et al. are proposed, these technologies are simply to layer direction Gradient is changed, and is easier to implement, but only can produce positive contrast to the SPIO (0.8 concentration) of low concentration, and is only existed Layer carries out gradient compensation on direction, so what is just contrasted preparatory also needs to investigate.In 2007, Stuber M et al. were proposed in succession IRON (Inversion-Recovery With ON-Resonant Water Suppression) technology, although can obtain Obtain and just contrast image effect well, but the technical operation is complex, fails to be used widely in clinic.Second class It is the method by post processing, the data obtained to GRE sequence scannings post-process, and are produced just using distinctive algorithm for reconstructing Contrast imaging, as Liu T et al. propose quantitative susceptibility imaging (Quantitative Susceptibility Mapping, QSM), but existing QSM methods largely apply to superparamagnetic iron oxide, and organization internal paramagnet is determined Amount assessment, and it is not suitable for positioning the metal implanting device of this superelevation magnetism.In 2008, Dahnke, H et al. proposed one kind Susceptibility gradient imaging technique (susceptibility gradient mapping, SGM), is the local magnetic to three directions Field gradient is imaged, but the region that the technology is just contrasting imaging is greater than the actual position region of actual implanted device. 2014, Dong Ying etc. proposed a kind of positive contrast imaging SE (spin echo) sequence, different from classical SE sequences, its 180 ° of pulses move T not at TE centers to 90 ° of pulsesshiftUnit, therefore phase change acts on caused by magnetic susceptibility 2TshiftUnit, but the SE imaging sequences sequence scanning times are long.
Since traditional QSM is basic using gtadient echo (GRE) as data acquisition, removal background is taken phase information A local field figure is obtained after the processing of field, magnetic susceptibility image is reconstructed in conjunction with distinctive algorithm for reconstructing, to organization internal paramagnetic Property material carry out qualitative assessment and what is just contrasted show metallic position.But for the larger intervention device of magnetic susceptibility Speech, due to its high magnetic susceptibility so that the quick dephasing position of implanted metal particle periphery tissue, because it is difficult to using its neighborhood information Go to rebuild magnetic susceptibility value, therefore can not accurately position implanted device.
The content of the invention
To solve problem of the prior art, the present invention proposes a kind of positive contrast MR imaging method and device, based on one Improved fast spin echo (the Turbo spin echo of kind：TSE) sequence acquisition data, and utilize improved quantitative magnetic susceptibility (Quantitative Susceptibility Mapping, the QSM) technology of imaging, realizes that high-resolution just contrasts magnetic resonance imaging, The accurate size and location for showing implanted device.
To achieve the above object, the present invention provides a kind of positive contrast MR imaging method, including：
Data acquisition twice is carried out to the same aspect of same target, is obtained respectively with the offset of echo readout gradient TSE data and the TSE data without the offset of echo readout gradient；
In the same aspect of same target, the phase of the TSE data with the offset of echo readout gradient subtracts institute The phase of the TSE data of no echo readout gradient offset is stated, obtains the phase difference that local magnetic field is influenced and produced；
The local magnetic field that metal device produces surrounding tissue in target area is obtained using the phase difference；
Polarized nucleus with magnetization rate matrix Convolution target local magnetic field expression formula, to target local magnetic field expression formula into Row Fourier transform, using the local magnetic field, regularization is carried out about to the target local magnetic field expression formula after Fourier transform Shu Chongjian, obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
Preferably, the expression formula of the local magnetic field is：
Δ B=Δ θ/γ B0Tshift
Wherein, Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent the main field of magnetic resonance imaging；Tshift Represent echo readout gradient shift time；Δ B represents the local magnetic field that metal device produces surrounding tissue in target area, That is local field figure.
Preferably, the target local magnetic field expression formula is：
Wherein, Δ B (r) represents target local magnetic field, is the hydrogen that metal device is adjusted the distance at the r of its position in target area The size for the local magnetic field that proton produces；χ represents the magnetization rate matrix of target, and d (r) is the polarized nucleus of imageable target.
Preferably, the target local magnetic field expression formula after the Fourier transform is：
Wherein, D represents polarization nuclear matrix, kxIt is expressed as As the coordinate value in target x directions in three-dimensional imaging space, kyRepresent the coordinate in imageable target y directions in three-dimensional imaging space Value, kzRepresent the coordinate value in imageable target z directions in three-dimensional imaging space.
Preferably, the target local magnetic field expression formula after the Fourier transform uses regularization constraint, its expression is：
Wherein, Δ B=Δs θ/γ B0Tshift；λ represents regularization parameter；W is weighting matrix, and M is mask matrix；G is represented The gradient operator in three directions in three dimensions.
Accordingly, to achieve the above object, present invention also offers a kind of positive contrast MR imaging apparatus, including：
Data acquisition unit, for carrying out data acquisition twice to the same aspect of same target, obtain has back respectively The TSE data and the TSE data without the offset of echo readout gradient of ripple readout gradient offset；
Phase difference acquiring unit, it is described to have what echo readout gradient deviated for the same aspect in same target The phase of TSE data subtracts the phase of the TSE data without the offset of echo readout gradient, and obtaining local magnetic field influences and produce Raw phase difference；
Local magnetic field determination unit, produces surrounding tissue for obtaining metal device in target area using the phase difference Raw local magnetic field；
Imaging unit, for the Convolution target local magnetic field expression formula of polarized nucleus and magnetization rate matrix, to target office Portion's magnetic field expression formula carries out Fourier transform, and using the local magnetic field, the target local magnetic field after Fourier transform is expressed Formula carries out regularization constraint reconstruction, obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
Preferably, the expression formula for the local magnetic field that the local magnetic field determination unit obtains is：
Δ B=Δ θ/γ B0Tshift
Wherein, Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent the main field of magnetic resonance imaging；Tshift Represent echo readout gradient shift time；Δ B represents the local magnetic field that metal device produces surrounding tissue in target area, That is local field figure.
Preferably, the target local magnetic field expression formula of the imaging unit acquisition is：
Wherein, Δ B (r) represents target local magnetic field, is the group that metal device is adjusted the distance at the r of its position in target area Knit the size of the local magnetic field of generation；χ represents the magnetization rate matrix of target, and d (r) is the polarized nucleus of imageable target.
Preferably, the imaging unit is to the expression formula after target local magnetic field expression formula Fourier transform：
Wherein, D represents polarization nuclear matrix, kxIt is expressed as picture The coordinate value in target x directions in three-dimensional imaging space, kyRepresent the coordinate in imageable target y directions in three-dimensional imaging space Value, kzRepresent the coordinate value in imageable target z directions in three-dimensional imaging space.
Preferably, the target local magnetic field expression formula of the imaging unit regularization constraint reconstruction is：
Wherein, Δ B=Δs θ/γ B0Tshift；λ represents regularization parameter；W is weighting matrix, and M is mask matrix；G is represented The gradient operator in three directions in three dimensions.
Above-mentioned technical proposal has the advantages that：
1st, it is different from existing QSM, present invention can apply to the larger intervention of magnetic susceptibility/implanted metal device imaging, fit It is good with property.
2nd, TSE sequences of the invention are improved echo readout gradient in traditional TSE sequence basis so that it is suitable The data acquisition of Ying Yuzheng contrasts, only need to gather two difference TshiftImage come produce one only phase caused by magnetic susceptibility Difference, obtains a field figure to solve the magnetic susceptibility of metallic implants by the phase.
3rd, TSE imaging sequences speed is fast, than the SE imaging sequences of the propositions such as Dong Ying, is not losing signal-to-noise ratio, image Picking rate improves 2 to 3 times under the premise of the higher grade of quality, with reference to the post processing of image algorithm for reconstructing of robust, improves device and determines Position and the accuracy assessed.
Brief description of the drawings
In order to illustrate more clearly about the embodiment of the present invention or technical scheme of the prior art, below will be to embodiment or existing There is attached drawing needed in technology description to be briefly described, it should be apparent that, drawings in the following description are only this Some embodiments of invention, for those of ordinary skill in the art, without creative efforts, can be with Other attached drawings are obtained according to these attached drawings.
Fig. 1 is a kind of positive contrast MR imaging method flow chart proposed by the present invention；
Fig. 2 is tradition TSE sequence acquisition schematic diagrames；
Fig. 3 is tradition TSE sequence k-space blank maps；
Fig. 4 is that tradition TSE sequences are schemed with the TSE alignments that the technical program is related to；
Fig. 5 is a kind of positive contrast MR imaging apparatus block diagram proposed by the present invention；
Fig. 6 is one of sequence acquisition amplitude figure of the present embodiment；
Fig. 7 is one of positive contrast image of the present embodiment；
Fig. 8 is the two of the sequence acquisition amplitude figure of the present embodiment；
Fig. 9 is the two of the positive contrast image of the present embodiment.
Embodiment
Below in conjunction with the attached drawing in the embodiment of the present invention, the technical solution in the embodiment of the present invention is carried out clear, complete Site preparation describes, it is clear that described embodiment is only part of the embodiment of the present invention, instead of all the embodiments.It is based on Embodiment in the present invention, those of ordinary skill in the art are obtained every other without making creative work Embodiment, belongs to the scope of protection of the invention.
The operation principle of the technical program：Since traditional QSM is basis using gtadient echo (GRE) as data acquisition, Phase information is taken and obtains a local field figure after removing ambient field processing, magnetization is reconstructed in conjunction with distinctive algorithm for reconstructing Rate image, carries out organization internal paramagnet qualitative assessment and what is just contrasted shows metallic position.But for magnetic For the larger intervention device of rate, due to its high magnetic susceptibility so that the quick dephasing position of implanted metal particle periphery tissue, Because it is difficult to be gone to rebuild magnetic susceptibility value with its neighborhood information, therefore it can not accurately position implanted device.In view of the above-mentioned problems, this Technical solution is unfolded based on QSM, proposes a kind of positive contrast MR imaging method based on fast spin echo, phase Than in GRE sequences, there is higher signal-to-noise ratio and less image deformation.Dexterously will on the basis of traditional TSE sequences The position of echo readout gradient, can be in very short T into line displacementshiftTime gathered data, carries out necessarily phase information Pretreatment, including unwrapping is carried out to phase information.Using two images at different moments of TSE sequence acquisitions of proposition, pass through Difference is done to phase can avoid doing ambient field processing, in conjunction with distinctive QSM algorithm for reconstructing, reconstruct magnetic susceptibility image.Obtain The positive contrast image for the implanted metal device that must stablize, ultimately forms the positive contrast magnetic resonance imaging with clinical value Technology, a kind of noninvasive technical guarantee of safety is provided for the accurate positionin and assessment of implanted device.
Based on above-mentioned operation principle, the present invention proposes a kind of positive contrast MR imaging method, as shown in Figure 1.Including：
Step 101)：Data acquisition twice is carried out to the same aspect of same target, obtaining respectively, there is echo to read ladder Spend the TSE data of offset and the TSE data without the offset of echo readout gradient；
It is compared to traditional QSM differences, data acquisition is used as without using gtadient echo (GRE), but with TSE (Fast spin echo) is used as sequence basis.TSE grows up on the basis of SE sequences.As shown in Fig. 2, it is tradition TSE sequence acquisition schematic diagrames.As shown in figure 3, it is tradition TSE sequence k-space blank maps.Compared with spin-echo sequence, its Using multiple (more than 2) 180 ° times poly- more spin echoes of pulses generation after 90 ° of radio-frequency pulse excitations, and each echo is carried out Phase code, realizes the collection for once exciting a plurality of phase line, therefore has the advantages that acquisition speed is fast, signal-to-noise ratio is high. But 180 ° of the sequence return poly- pulse and cause all phase signals that lost caused by main field inhomogeneities gather, it is impossible to use In just contrast magnetic resonance imaging.
Step 102)：In the same aspect of same target, the phase of the TSE data with the offset of echo readout gradient Position subtracts the phase of the TSE data without the offset of echo readout gradient, obtains the phase difference that local magnetic field is influenced and produced；
Echo readout gradient is deviated very short T by we in traditional TSE sequence basisshiftWhen (0.2ms~0.7ms) Between, as shown in figure 4, scheming for the TSE alignments that traditional TSE sequences and the technical program are related to.So that effective echo time is very It is short (for Tshift), the phase place change caused by organizing the difference of magnetic susceptibility itself is obtained while signal serious loss is avoided, Realize the data acquisition of the positive contrast magnetic resonance imaging of implanted device.Made to eliminate magnetic resonance system magnetic field bump itself Into phse conversion, we, which also gather one group of echo readout gradient, does not have the TSE sequence datas of any offset, passes through this two groups of numbers According to the phase-contrast for subtracting each other to obtain by organizing itself susceptibility difference (local magnetic field influence) and producing.This change just embodies In phase diagram.Therefore, using the phase information of image, and the amplitude of the TSE data of no echo readout gradient offset is combined Figure, using QSM as algorithm basis, can solve the magnetic susceptibility figure of tissue, realize the face using tissue magnetization rate intensity as contrast Compare magnetic resonance imaging.
Step 103)：The local magnetic that metal device produces surrounding tissue in target area is obtained using the phase difference ；
Wherein, the expression formula of local magnetic field is：
Δ B=Δ θ/γ B0Tshift
Wherein, Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent the main field of magnetic resonance imaging；Tshift Represent echo readout gradient shift time；Δ B represents the local magnetic field that metal device produces surrounding tissue in target area, That is local field figure.
Step 104)：The Convolution target local magnetic field expression formula of polarized nucleus and magnetization rate matrix, to target part magnetic Expression formula carries out Fourier transform, using the local magnetic field, to the target local magnetic field expression formula after Fourier transform into Row regularization constraint is rebuild, and obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
It can be write as its magnetic susceptibility characteristics and the convolution of polarized nucleus for the local magnetic field of Arbitrary Target：
Wherein, Δ B (r) represents target local magnetic field, is the hydrogen that metal device is adjusted the distance at the r of its position in target area The size for the local magnetic field that proton produces；χ represents the magnetization rate matrix of target, and d (r) is the polarized nucleus of imageable target.
Since in above formula, convolution is inconvenient to calculate, therefore, above-mentioned Convolution Formula is changed into multiplying in Fourier by us Accumulate to solve.Expression formula after the target local magnetic field expression formula Fourier transform is：
Wherein, D represents polarization nuclear matrix, kxIt is expressed as As the coordinate value in target x directions in three-dimensional imaging space, kyRepresent the coordinate in imageable target y directions in three-dimensional imaging space Value, kzRepresent the coordinate value in imageable target z directions in three-dimensional imaging space.
Above-mentioned product formula can be reduced to Δ B=D χ.If going out χ from formula Δ B=D χ direct solutions, can be related to asking Dematrix it is reversible, but matrix D existsWhen be discrete.Therefore direct solution χ is an ill-conditioning problem, can be made Obtain noise extension.A regularization mode constraint solving is must be introduced into, to obtain an accurate approximate solution.In view of implantation The magnetic susceptibility of metal device is much larger than the characteristics of tissue, increases by two constraint matrixes and uses L1 norm constraints to improve figure As reconstruction quality, its regularization constraint expression formula is：
Wherein, Δ B=Δs θ/γ B0Tshift；λ represents regularization parameter；W is weighting matrix, which is by not having Acquisition is normalized in the amplitude for having the TSE data of echo readout gradient offset.M is mask matrix；G is represented in three dimensions The gradient operator in three directions.Different from QSM, M can precisely be consistent with the position of material.This convex programming is similar to compression and passes Introduced just in sense method (Compressed Sensing, CS) in order to reconstruct target image in lack sampling K space data Then change constraint.By CS theories it is recognised that when lack sampling data are approximate sparse in transform domain, non-linear reconstruction method can be utilized Image is reconstructed from the K space data for owing to adopt at random.
The technical program is related to a kind of follow-on TSE and just contrasts acquisition sequence, does difference after to phase unwrapping, removes Ambient field must show up figure；On this basis, new improvement has been done to original QSM imaging algorithms, using follow-on QSM algorithms Realize positive contrast magnetic resonance imaging, improve positioning accurate accuracy.
Accordingly, based on above-mentioned operation principle, the present invention also proposes a kind of positive contrast MR imaging apparatus, such as Fig. 5 institutes Show.Including：
Data acquisition unit 501, for carrying out data acquisition twice to the same aspect of same target, is had respectively The TSE data and the TSE data without the offset of echo readout gradient of echo readout gradient offset；
Phase difference acquiring unit 502, it is described that there is the offset of echo readout gradient for the same aspect in same target TSE data phase subtract it is described without echo readout gradient offset TSE data phase, obtain local magnetic field influence and The phase difference of generation；
Local magnetic field determination unit 503, for metal device in utilization phase difference acquisition target area to surrounding group Knit the local magnetic field of generation；
Imaging unit 504, for the Convolution target local magnetic field expression formula of polarized nucleus and magnetization rate matrix, to target Local magnetic field expression formula carries out Fourier transform, using the local magnetic field, to the target local magnetic field table after Fourier transform Regularization constraint reconstruction is carried out up to formula, obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
As shown in fig. 6, one of sequence acquisition amplitude figure for the present embodiment.The puncture needle of a diameter of 2mm is placed in 1mg/ In the bottle of the copper-bath of ml, titanium pin generates very big artifact, which is shown as black hole in figure 6.By this technology The processing of scheme, obtains final positive contrast as a result, as shown in Figure 7.In figure, the black hole of artifact becomes a bright spot.
As shown in figure 8, two of sequence acquisition amplitude figure for the present embodiment.In pork, titanium is sequentially inserted into from top to bottom Pin, toothpick, sticking plaster and another piece of titanium pin.It can be seen in fig. 8 that the high titanium pin of magnetic susceptibility can just be contrasted and shown well Come, the low toothpick of magnetic susceptibility, sticking plaster, which are then displayed without, to be come.By the processing of the technical program, final positive contrast knot is obtained Fruit.As shown in Figure 9.From the point of view of reconstructed results, the technical program effectively inhibits metal artifacts really, realizes high time-space resolution On the premise of rate, clearly positive contrast images are obtained, are conducive to device positioning and assessment.
Above-described embodiment, has carried out the purpose of the present invention, technical solution and beneficial effect further Describe in detail, it should be understood that the foregoing is merely the embodiment of the present invention, be not intended to limit the present invention Protection domain, within the spirit and principles of the invention, any modification, equivalent substitution, improvement and etc. done, should all include Within protection scope of the present invention.

Claims (10)

  1. A kind of 1. positive contrast MR imaging method, it is characterised in that including：
    Data acquisition twice is carried out to the same aspect of same target, obtains the TSE numbers with the offset of echo readout gradient respectively According to the TSE data deviated with no echo readout gradient；
    In the same aspect of same target, the phases of the TSE data with the offset of echo readout gradient, which subtracts, described not to be had There is the phase of the TSE data of echo readout gradient offset, obtain the phase difference that local magnetic field is influenced and produced；
    The local magnetic field that metal device produces surrounding tissue in target area is obtained using the phase difference；
    Using polarized nucleus with magnetization rate matrix Convolution target local magnetic field expression formula, to target local magnetic field expression formula into Row Fourier transform, using the local magnetic field, regularization is carried out about to the target local magnetic field expression formula after Fourier transform Shu Chongjian, obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
  2. 2. the method as described in claim 1, it is characterised in that the expression formula of the local magnetic field is：
    Δ B=Δ θ/γ B0Tshift
    Wherein, Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent the main field of magnetic resonance imaging；TshiftRepresent Echo readout gradient shift time；Δ B represents the local magnetic field that metal device produces surrounding tissue in target area, Ye Jiju Portion figure.
  3. 3. the method as described in claim 1, it is characterised in that the target local magnetic field expression formula is：
    <mrow> <mi>&amp;Delta;</mi> <mi>B</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>&amp;CircleTimes;</mo> <mi>&amp;chi;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> </mrow>
    Wherein, Δ B (r) represents target local magnetic field, is the Hydrogen Proton that metal device is adjusted the distance at the r of its position in target area The size of the local magnetic field of generation；χ (r) represents the magnetization rate matrix that metal device is adjusted the distance at the r of its position in target area, d (r) it is that metal device is adjusted the distance the polarized nucleus of the imageable target at the r of its position in target area.
  4. 4. method as claimed in claim 3, it is characterised in that the target local magnetic field expression formula after the Fourier transform For：
    <mrow> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;Delta;</mi> <mi>B</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>d</mi> <mo>)</mo> </mrow> <mo>&amp;times;</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>)</mo> </mrow> <mo>=</mo> <mrow> <mo>(</mo> <mfrac> <mn>1</mn> <mn>3</mn> </mfrac> <mo>-</mo> <mfrac> <msubsup> <mi>k</mi> <mi>z</mi> <mn>2</mn> </msubsup> <msup> <mi>k</mi> <mn>2</mn> </msup> </mfrac> <mo>)</mo> </mrow> <mo>&amp;times;</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>)</mo> </mrow> </mrow>
    Wherein, D represents polarization nuclear matrix,kxIt is expressed as mesh It is marked on the coordinate value in x directions in three-dimensional imaging space, kyRepresent the coordinate value in imageable target y directions in three-dimensional imaging space, kz Represent the coordinate value in imageable target z directions in three-dimensional imaging space.
  5. 5. method as claimed in claim 4, it is characterised in that the target local magnetic field expression formula after the Fourier transform is adopted With regularization constraint, its expression is：
    <mrow> <mi>arg</mi> <mi>min</mi> <mi> </mi> <mi>f</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>,</mo> <mi>&amp;lambda;</mi> <mo>)</mo> </mrow> <mo>=</mo> <mo>|</mo> <mo>|</mo> <mi>W</mi> <mrow> <mo>(</mo> <mi>D</mi> <mi>&amp;chi;</mi> <mo>-</mo> <mi>&amp;Delta;</mi> <mi>B</mi> <mo>)</mo> </mrow> <mo>|</mo> <msubsup> <mo>|</mo> <mn>2</mn> <mn>2</mn> </msubsup> <mo>+</mo> <mi>&amp;lambda;</mi> <mo>|</mo> <mo>|</mo> <mi>M</mi> <mi>G</mi> <mi>&amp;chi;</mi> <mo>|</mo> <msub> <mo>|</mo> <mn>1</mn> </msub> </mrow>
    Wherein, Δ B=Δs θ/γ B0Tshift；Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent magnetic resonance imaging Main field；TshiftRepresent echo readout gradient shift time；Δ B represents that metal device produces surrounding tissue in target area Local magnetic field, namely local field figure；λ represents regularization parameter；W is weighting matrix, and M is mask matrix；G is represented in three-dimensional space Between in three directions gradient operator.
  6. A kind of 6. positive contrast MR imaging apparatus, it is characterised in that including：
    Data acquisition unit, for carrying out data acquisition twice to the same aspect of same target, obtaining respectively, there is echo to read Go out the TSE data of gradient offset and the TSE data without the offset of echo readout gradient；
    Phase difference acquiring unit, for the same aspect in same target, the TSE numbers with the offset of echo readout gradient According to phase subtract it is described without echo readout gradient offset TSE data phase, obtain local magnetic field influence and produce Phase difference；
    Local magnetic field determination unit, for obtaining what metal device in target area produced surrounding tissue using the phase difference Local magnetic field；
    Imaging unit, for the Convolution target local magnetic field expression formula using polarized nucleus and magnetization rate matrix, to target office Portion's magnetic field expression formula carries out Fourier transform, and using the local magnetic field, the target local magnetic field after Fourier transform is expressed Formula carries out regularization constraint reconstruction, obtains the magnetization rate matrix of target, realizes positive contrast magnetic resonance imaging.
  7. 7. device as claimed in claim 6, it is characterised in that the table for the local magnetic field that the local magnetic field determination unit obtains It is up to formula：
    Δ B=Δ θ/γ B0Tshift
    Wherein, Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent the main field of magnetic resonance imaging；TshiftRepresent Echo readout gradient shift time；Δ B represents the local magnetic field that metal device produces surrounding tissue in target area, Ye Jiju Portion figure.
  8. 8. device as claimed in claim 6, it is characterised in that the target local magnetic field expression formula that the imaging unit obtains For：
    <mrow> <mi>&amp;Delta;</mi> <mi>B</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>&amp;CircleTimes;</mo> <mi>&amp;chi;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> </mrow>
    Wherein, Δ B (r) represent target local magnetic field, be target area in metal device adjust the distance its position r place tissue produce The size of raw local magnetic field；χ (r) represents the magnetization rate matrix that metal device is adjusted the distance at the r of its position in target area, d (r) It is that metal device is adjusted the distance the polarized nucleus of imageable target at the r of its position in target area.
  9. 9. device as claimed in claim 8, it is characterised in that the imaging unit is to target local magnetic field expression formula Fourier Expression formula after conversion is：
    <mrow> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;Delta;</mi> <mi>B</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>d</mi> <mo>)</mo> </mrow> <mo>&amp;times;</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>)</mo> </mrow> <mo>=</mo> <mrow> <mo>(</mo> <mfrac> <mn>1</mn> <mn>3</mn> </mfrac> <mo>-</mo> <mfrac> <msubsup> <mi>k</mi> <mi>z</mi> <mn>2</mn> </msubsup> <msup> <mi>k</mi> <mn>2</mn> </msup> </mfrac> <mo>)</mo> </mrow> <mo>&amp;times;</mo> <mi>F</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>)</mo> </mrow> </mrow>
    Wherein, D represents polarization nuclear matrix,kxRepresent imageable target The coordinate value in x directions, k in three-dimensional imaging spaceyRepresent the coordinate value in imageable target y directions in three-dimensional imaging space, kzTable Show the coordinate value in imageable target z directions in three-dimensional imaging space.
  10. 10. device as claimed in claim 9, it is characterised in that the target that the imaging unit regularization constraint is rebuild is local Magnetic field expression formula is：
    <mrow> <mi>arg</mi> <mi>min</mi> <mi> </mi> <mi>f</mi> <mrow> <mo>(</mo> <mi>&amp;chi;</mi> <mo>,</mo> <mi>&amp;lambda;</mi> <mo>)</mo> </mrow> <mo>=</mo> <mo>|</mo> <mo>|</mo> <mi>W</mi> <mrow> <mo>(</mo> <mi>D</mi> <mi>&amp;chi;</mi> <mo>-</mo> <mi>&amp;Delta;</mi> <mi>B</mi> <mo>)</mo> </mrow> <mo>|</mo> <msubsup> <mo>|</mo> <mn>2</mn> <mn>2</mn> </msubsup> <mo>+</mo> <mi>&amp;lambda;</mi> <mo>|</mo> <mo>|</mo> <mi>M</mi> <mi>G</mi> <mi>&amp;chi;</mi> <mo>|</mo> <msub> <mo>|</mo> <mn>1</mn> </msub> </mrow>
    Wherein, Δ B=Δs θ/γ B0Tshift；Δ θ represents phase difference；γ represents gyromagnetic ratio, is constant；B0Represent magnetic resonance imaging Main field；TshiftRepresent echo readout gradient shift time；Δ B represents that metal device produces surrounding tissue in target area Local magnetic field, namely local field figure；λ represents regularization parameter；W is weighting matrix, and M is mask matrix；G is represented in three-dimensional space Between in three directions gradient operator.
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