CN105232046B - A kind of single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo - Google Patents
A kind of single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo Download PDFInfo
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Abstract
A kind of single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo, it is related to the method for magnetic resonance imaging.The echo-signal of two different times of developing is produced by adding two excitation pulses for having an equal deflection angle in single sweep operation, although the evolution time of two echo-signals is different, cause the T2 weightings of two echo-signals different, the two echo-signals come from same imaging slice, can pass through the priori between two echo-signals：Both structures are similar, the openness of joint edge separates the two echo-signals, using sparse transformation coordinate corresponding separation algorithm to separate the two echo-signals；T2 finally is carried out to two isolated signals quantitative T2 images are calculated.The quantitative T2 imagings of single sweep operation are obtained, by the time that quantitative T2 is imaged by second level even minute level, are reduced to ms levels, and obtained T2 picture qualities can be suitable with the picture quality that the single sweep operation EPI sequences of routine obtain.
Description
Technical field
The present invention relates to the method for magnetic resonance imaging, is total to more particularly, to a kind of quantitative magnetic of single sweep based on overlapping echo
Shake T2 imaging methods.
Background technology
Magnetic resonance parameters imaging (T2 imagings, T2*Imaging and diffusion imaging) belong to because it can provide abundant tissue signature
The quantitative information of property and had a wide range of applications in clinical diagnosis ([1] B.Zhao, F.Lam, and Z.P.Liang,
“Model-based MR Parameter mapping with sparsity constraints:parameter
estimation and performance bounds,”IEEE Trans.Med.Imag.,vol.33,no.9,pp.1832-
1844,2014), such as:Whether iron content is excessive etc. in the diagnosis of myocardial infarction, measurement liver.Especially, T2 relaxation times
Quantitative analysis causes increasing concern in the clinical medicine magnetic resonance imaging such as psychiatry and neuropathy section.So
And magnetic resonance parameters are imaged on and a series of image for obtaining contrast weights is generally required in its imaging process, thus general its obtains
The time for evidence of fetching is very long.Although put forward to overcome above mentioned problem now with many different imaging methods, such as：Down-sampling
Spin echo magnetic resonance imaging (Spin-Echo MRI), gradient spin echo magnetic resonance imaging (Gradient Spin Echo
MRI), parallel imaging based on compressed sensing (Parallel Imaging with CS) etc..Meanwhile some weights based on model
Construction method and based on Bloch emulation method for reconstructing be suggested in succession, the speed of imaging is further speeded up with this.It is but more
The secondary magnetic resonance parameters imaging method excited still needs the time for expending the several seconds in the acquisition stage, so to not reproducible god
The task of hardly possible realization has been reformed into through activity progress real-time parameter imaging.Therefore, the plane of more echoes of single sweep
The imaging method of echo-wave imaging (echo-planar imaging, EPI) be suggested ([2] S.Posse, S.Wiese,
D.Gembris,K.Mathiak,C.Kessler,M.L.Grosse-Ruyken,B.Elghawaghi,T.Richards,
S.R.Dager,and V.G.Kiselev,“Enhancement of BOLD-Contrast Sensitivity by
Single-Shot Multi-Echo Functional MR Imaging,”Magn.Reson.Med.,vol.42,pp.87–
97,1999), the method passes through multiple echoes by a series of acquisition of contrast weight images obtained in single pass
In.But limitation be present in this method, be on the one hand this method need extend echo train, necessarily cause increase obtain when
Between decay with signal；On the other hand the realization for being this method is to extend the repetition time (TR) compared with conventional EPI methods
For cost, this may need to the spatial resolution for sacrificing gained echo；And above all this method is current only
T2* quantitative imagings can be used for, be still not used to T2 quantitative imagings.In addition, in spite of different quick T2 imaging methods quilt in succession
It is proposed, including gradient spin echo sequence, but these methods are all to carry out T2 imagings with multiple excitation sequence, so not only
Effect is barely satisfactory, and imaging efficiency is also in urgent need to be improved.
The content of the invention
It is an object of the invention to provide a kind of single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo.
The present invention comprises the following steps：
(1) on magnetic resonance imager operating desk, the operation software in magnetic resonance imager is opened, first to imaging object
Area-of-interest positioning is carried out, is then tuned, shimming, capability correction and frequency correction；
(2) OLED imaging sequences compiled in advance are imported；According to specific experimental conditions, each of pulse train is set
Parameter；
The structure of the OLED imaging sequences is followed successively by：Flip angle is that α piece selects pulse, (TE2-TE1)/2, flip angle α
Piece select pulse, TE1/ 2,180 ° of reunion pulses, sampled echo chains；
Two low-angle excitation pulses are combined into two echo shift gradient Gs1And G2So that two echoes are in K spaces
The heart produce skew, 180 ° of reunion pulses and two low-angle excitation pulses all with layer choosing gradient GssIt is combined carry out layer
Choosing；Apply echo delay (TE before and after second low-angle excitation pulse respectively2-TE1)/2 and TE1/ 2,180 ° of reunion pulses
It is front and rear to have x, the destruction gradient effect in tri- directions of y, z；
The sampled echo chain is by being respectively acting on x, the gradient chain composition in y directions；The gradient chain in x directions is by a series of
Positive negative gradient is formed, and the area of each gradient is first echo shift gradient G1Three times；The gradient chain in y directions is by one
Equal-sized " blips " gradient of series is formed, and the gross area of " blips " gradient is equal to the four of displacement gradient area
Times；
Before the sampled echo chain, x and y directions are applied with reunion gradient G respectivelyrorAnd Gar, the GrorArea
It is the half of first gradient area in x directions, direction is opposite with first, x directions gradient direction；The GarArea be all
The half of the gross area of " blips " gradient, direction are opposite with " blips " gradient direction；
(3) the OLED imaging sequences that step (2) is set are performed, carry out data sampling；After the completion of data sampling
To the K space data of two echo-signals；
(4) K space data obtained to step (3) is analyzed and to echo-signal magnetization vector M+Evolution carry out
Theory deduction, in second echo shift gradient G2Afterwards, following formula can be obtained：
In formulaIt is spin density distribution, α is the flip angle of excitation pulse, is found through experiments that, when α=45 °,
The intensity of two echo-signals is all of a relatively high, δ TE=(TE2-TE1)/2,Wherein δ1,
δ2The duration of first and second echo shift gradient corresponding to respectively, γ is magnetic rotaion comparison；From above formula, actually
There are three echo-signals modulated by out of phase, wherein Section 1 is as caused by second excitation pulse, and then two are
As caused by first excitation pulse；But it is to be separated go out the signal that is obtained with respect to single sweep of these three signals for be very multiple
Miscellaneous, by two after analysis, it is known that the echo center of latter two is different, and last signal intensity is relative
It is smaller for Section 2, therefore last echo can be ignored by simply handling；
(5) echo-signal obtained to step (4) is separated with following separation algorithms, theoretical according to Fourier transformation,
Two echo-signals are different in the linear phase of image area, although in addition, two echo-signals are due to developing the time not
With causing, T2 weightings are different, but they are to come from same image layer；Therefore, the similar priori of both picture structures is utilized
Information can carry out joint reconstruction to two echo-signals, and algorithm for reconstructing is as follows：
Wherein, x1, x2It is that the image come is reconstructed from first and second echo-signal respectively；It is scale factor, x10, x20It is the initial pictures of first and second echo-signal respectively；λ1, λ2With
λ3It is the adjustable constraint weight of lagrange's method of multipliers respectively；▽ is gradient operator；Section 1 is fidelity item, Section 2 and the 3rd
Item is the sparsity constraints to the first width and the second width image, and last is two images profile similarity constraint；This two width
Image has following relation：
Wherein,It is the linear phase displacement of the first width and the second width image respectively；x0It is primary signal,
It is to carry out inverse Fourier transform by the primary signal comprising first and second echo-signal to obtain, is asked by iterative algorithm
Solve above-mentioned formula and can be obtained by image caused by first after separation and second echo-signal；
(6) image that step (5) is separated carries out T2 imagings and calculated；For the T2 imaging methods of single sweep, only
Needing the different echo time image of two width, T2 value is directly tried to achieve by T2 relaxation equations with regard to feasible：
WhereinIt is correction factor, Δ TE=TE2-TE1；S1And S2It is first echo-signal and respectively
The image intensity of two echo-signals, full variation (Total Variation) extrapolation is added to strengthen the resolution ratio of image, and
And one threshold value of setting, when obtained numerical value is less than threshold value, it can be considered as noise and be ignored, equally as the T2 calculated
It is also irrational when being worth excessive, can be also omitted；The Gao Pin with preferable resolution ratio has been calculated finally by T2 imagings
The T2 images of matter.
The present invention provides one kind in the case of single pass, obtains overlapping echo-signal, then utilizes separation algorithm
Overlapping signal is separated, finally carry out T2 calculating, just can obtain and conventional single sweep EPI sequences required for acquisition when
Between and a kind of suitable new imaging method of resolution ratio.
The present invention is not played together by adding two excitation pulses for having equal deflection angle in single sweep operation to produce two
Change the echo-signal of time, although the evolution time of two echo-signals is different, cause the T2 weightings of two echo-signals different,
But the two echo-signals come from same imaging slice, therefore the priori between two echo-signals can be passed through：
Both structures are similar, the openness of joint edge separates the two echo-signals, so as to be coordinated using sparse transformation corresponding
Separation algorithm separates to the two echo-signals.T2 finally is carried out to two isolated signals quantitative T2 is calculated
Image.Obtain the quantitative T2 imagings of single sweep operation first using the method, the time that quantitative T2 is imaged even is divided by second level
Clock level, ms levels are reduced to, and the picture quality that obtained T2 picture qualities can obtain with the single sweep operation EPI sequences of routine
Quite.
Brief description of the drawings
Fig. 1 is OLED imaging sequences structure chart in the present invention.
Fig. 2 illustrates the comparative result figure of the model experiment of OLED imaging sequences.Wherein：
(a) image for including two echo-signals that OLED rebuilding series go out before being separation；
(b) it is first echo-signal image after being separated from (a)；
(c) it is second echo-signal image after being separated from (a)；
(d) it is the signal pattern for scanning single echo spin-echo sequences (SE sequences) and reconstructing more；
(e) it is the signal pattern that goes out of single sweep spin EPI rebuilding series；
(g) it is that the T2 images come are reconstructed from (a)；
(h) it is respectively along the signal strength values and T2 values of respective dashed part of horizontal section in (d) and (e).
Fig. 3 is that the T2 images come are reconstructed from Fig. 2 (d).
Embodiment
Below in conjunction with the drawings and the specific embodiments, the present invention will be further described.
Each step in specific implementation process of the present invention is as follows：
(1) on magnetic resonance imager operating desk, corresponding operation software in imager is opened, imaging object is entered first
Row area-of-interest positions, and is then tuned, shimming, power and frequency correction；
(2) OLED imaging sequences compiled in advance are imported；According to specific experimental conditions, each of pulse train is set
Parameter；
The structure of the OLED imaging sequences is followed successively by：Flip angle is that α piece selects pulse, (TE2-TE1)/2, flip angle α
Piece select pulse, TE1/ 2,180 ° of reunion pulses, sampled echo chains；
Two low-angle excitation pulses are combined into two echo shift gradient Gs1And G2, so that two echoes are in K spaces
Center produce skew, 180 ° of reunion pulses and two low-angle excitation pulses all with layer choosing gradient GssIt is combined progress
Layer choosing；Apply echo delay (TE before and after second low-angle excitation pulse respectively2-TE1)/2 and TE1/ 2,180 ° of reunion arteries and veins
There are x, the destruction gradient effect in tri- directions of y, z before and after punching；
The sampled echo chain is by being respectively acting on x, the gradient chain composition in y directions；The gradient chain in x directions is by a series of
Positive negative gradient is formed, and the area of each gradient is the displacement gradient G1Three times；The gradient chain in y directions is by a series of big
Small equal " blips " gradient is formed, and the gross area of " blips " gradient is equal to four times of the displacement gradient area；
Before the sampled echo chain, x and y directions are applied with reunion gradient G respectivelyrorAnd Gar, the GrorArea
It is the half of first gradient area in x directions, direction is opposite with first, x directions gradient direction；The GarArea be all
The half of the gross area of " blips " gradient, direction are opposite with " blips " gradient direction；
(3) the OLED imaging sequences that step (2) is set are performed, carry out data sampling；After the completion of data sampling
To the K space data of two echo-signals.
(4) K space data obtained to step (3) is analyzed and to echo-signal magnetization vector M+Evolution carry out
Theory deduction, gradient G is shifted at second2Afterwards, following formula can be obtained：
In formulaIt is spin density distribution, α is the flip angle of excitation pulse, is found through experiments that, when α=45 ° two
The intensity of individual echo-signal is all of a relatively high, δ TE=(TE2-TE1)/2,Wherein δ1, δ2
The duration of first and second echo shift gradient corresponding to respectively, γ is magnetic rotaion comparison.Understand actually have by above formula
Three by out of phase modulate echo-signals, wherein Section 1 be as caused by second excitation pulse, then two be by
Caused by first excitation pulse；But it is to be separated go out the signal that is obtained with respect to single sweep of these three signals for be extremely complex
, by two after analysis, it is known that the echo center of latter two is different, and last signal intensity is with respect to
It is smaller for binomial, therefore last echo can be ignored by simply handling.
(5) echo-signal obtained to step (4) is separated with following separation algorithms, theoretical according to Fourier transformation,
Two echo-signals are different in the linear phase of image area, although in addition, two echo-signals are due to developing the time not
With causing, T2 weightings are different, but they are to come from same image layer.Therefore, the similar priori of both picture structures is utilized
Information can carry out joint reconstruction to two echo-signals.Algorithm for reconstructing is as follows：
Wherein x1, x2It is that the image come is reconstructed from first and second echo-signal respectively；It is scale factor, x10, x20It is the initial pictures of first and second echo-signal respectively；λ1, λ2With
λ3It is the adjustable constraint weight of lagrange's method of multipliers respectively；▽ is gradient operator.Section 1 is fidelity item, Section 2 and the 3rd
Item is the sparsity constraints to the first width and the second width image, and last is two images profile similarity constraint.This two width
Image has following relation：
WhereinIt is the linear phase displacement of the first width and the second width image respectively；x0It is primary signal,
It is to carry out inverse Fourier transform by the primary signal comprising first and second echo-signal to obtain.Asked by iterative algorithm
Solve above-mentioned formula and can be obtained by image caused by first after separation and second echo-signal.
(6) image that step (5) is separated carries out T2 imagings and calculated.For the T2 imaging methods of single sweep, only
Needing the different echo time image of two width, T2 value is directly tried to achieve by T2 relaxation equations with regard to feasible：
WhereinIt is correction factor, Δ TE=TE2-TE1；S1And S2It is first echo-signal and respectively
The image intensity of two echo-signals.Here we add full variation (Total Variation) extrapolation to strengthen image
Resolution ratio, and we set a threshold value, can be considered as noise when obtained numerical value is less than threshold value and be ignored,
It is also irrational equally when the T2 values calculated are excessive, can be also omitted.Being calculated finally by T2 imagings has
The T2 images of the high-quality of preferable resolution ratio.
Specific embodiment given below：
Water model experiment is carried out with the single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo, for verifying this
The feasibility of invention.Experiment before first by seven ampoules concentration be 0.5% agar gel with contain various concentrations
The aqueous solution mixing of manganous chloride (MnCl2,0.01~0.16mM), for producing a series of T2 values close to tissue, and
The ratio for making T1/T2 is about 10 in 3T imagers, and the T1 under 3T magnetic fields is suitable with T2 ratio with tissue for this.Experiment is surveyed
T1 values its time ranges be about 350~1500ms.In addition, to scan single echo spin-echo sequence (SE sequences) production more
For raw T2 images as reference, it is 2mm that it, which is imaged thickness,.We import compiled as shown in Figure 1 based on overlapping time first
The single sweep separating plane imaging sequence of ripple, setting test parameters, the test parameters setting of the present embodiment are as follows：45 ° excite arteries and veins
The firing time of punching is 3ms, x directions sampling number NxFor 128, y directions sampling number NyFor 64, sampling bandwidth sw is
91.4kHz.The visual field FOV in x directionsxFor 20cm, the visual field FOV in y directionsyFor 20cm.Above test parameters is set
After good, the sampling time for directly running whole sequence is about 160ms.After sampling terminates, adopting for overlapping echo-signal is just obtained
Sample data.Then we using the separation algorithm in step (5) to two overlapping echo-signals with regard to being separated, point here
From algorithm, regularization parameter is respectively set to λ by us1=1.4, λ2=1, λ3=1.Result after separation as shown in Figures 2 and 3,
SE sequences are delayed (from 8.8~120ms) using 8 different echoes, and its sampling matrix size is 128 × 128, TR=
3.5s, total sweep time are about 1h.It can see from figure a and figure e, there is obvious distortion, this is due to ambient field inequality
Caused by even.It is worth noting that figure d and arrow indicating section in figure e, the signal intensity in each water pipe should be equal originally
Even, and the signal intensity of indicating section is uneven in figure.This be probably as caused by many factors comprehensive function,
Wherein uneven ambient field and the sensitive sex differernce of detection coil are cause spin-echo sequence image uneven main
Reason, and the pile up effect of signal is also that single sweep EPI sequences the main reason for this phenomenon occur in non-uniform field.From figure
2h is it can be found that although EPI signal intensity is more uneven, but its T2 value is but than signal intensity evenly.Therefore experiment institute
The sequence pair non-uniform field and coil sensitivity diversity ratio spin echo EPI sequence robustness of use are stronger.It is possible thereby to prove,
OLED imaging methods, overlapping echo-signal can be obtained in the case where once exciting, be carried out using corresponding separation algorithm
Separation, reduce and obtain the time, improve the spatial resolution of image.
Table 1
Each symbol description is referring to table 1.
Claims (1)
1. a kind of single sweep Quantitative MRI Measurement T2 imaging methods based on overlapping echo, it is characterised in that comprise the following steps：
(1) on magnetic resonance imager operating desk, the operation software in magnetic resonance imager is opened, imaging object is carried out first
Area-of-interest positions, and is then tuned, shimming, capability correction and frequency correction；
(2) OLED imaging sequences compiled in advance are imported；According to specific experimental conditions, each ginseng of pulse train is set
Number；
The structure of the OLED imaging sequences is followed successively by：Flip angle is that α piece selects pulse, (TE2-TE1)/2, the piece that flip angle is α
Select pulse, TE1/ 2,180 ° of reunion pulses, sampled echo chains；
Two low-angle excitation pulses are combined into two echo shift gradient Gs1And G2So that two echoes produce at the center in K spaces
Raw skew, 180 ° of reunion pulses and two low-angle excitation pulses all with layer choosing gradient GssIt is combined carry out layer choosing；The
Apply echo delay (TE before and after two low-angle excitation pulses respectively2-TE1)/2 and TE1/ 2, before and after 180 ° of reunion pulses
There are x, the destruction gradient effect in tri- directions of y, z；
The sampled echo chain is by being respectively acting on x, the gradient chain composition in y directions；The gradient chain in x directions is by a series of positive and negative
Gradient is formed, and the area of each gradient is first echo shift gradient G1Three times；The gradient chain in y directions is by a series of
Equal-sized " blips " gradient is formed, and the gross area of " blips " gradient is equal to four times of displacement gradient area；
Before the sampled echo chain, x and y directions are applied with reunion gradient G respectivelyrorAnd Gar, the GrorArea be x side
To the half of first gradient area, direction is opposite with first, x directions gradient direction；The GarArea be all described
The half of the gross area of " blips " gradient, direction are opposite with " blips " gradient direction；
(3) the OLED imaging sequences that step (2) is set are performed, carry out data sampling；Two are obtained after the completion of data sampling
The K space data of individual echo-signal；
(4) K space data obtained to step (3) is analyzed and to echo-signal magnetization vector M+Evolution carry out it is theoretical
Derive, in second echo shift gradient G2Afterwards, following formula can be obtained：
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In formulaIt is spin density distribution, α is the flip angle of excitation pulse, is found through experiments that, when α=45 °, two are returned
The intensity of ripple signal is all of a relatively high, δ TE=(TE2-TE1)/2,Wherein δ1, δ2Respectively
The duration of corresponding first and second echo shift gradient, γ is magnetic rotaion comparison；From above formula, actually there are three
The echo-signal modulated by out of phase, wherein Section 1 is as caused by second excitation pulse, and then two are by first
Caused by individual excitation pulse；But it is to be separated go out the signal that is obtained with respect to single sweep of these three signals for be it is extremely complex,
By two after analysis, it is known that the echo center of latter two is different, and last signal intensity is with respect to second
It is smaller for, therefore last echo can be ignored by simply handling；
(5) echo-signal obtained to step (4) is separated with following separation algorithms, theoretical according to Fourier transformation, two
Echo-signal is different in the linear phase of image area, although in addition, two echo-signals are led due to the time difference that develops
Cause T2 weightings different, but they are to come from same image layer；Therefore, the similar prior information of both picture structures is utilized
Joint reconstruction can be carried out to two echo-signals, algorithm for reconstructing is as follows：
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Wherein, x1, x2It is that the image come is reconstructed from first and second echo-signal respectively；It is
Scale factor, x10, x20It is the initial pictures of first and second echo-signal respectively；λ1, λ2And λ3It is Lagrange respectively
The adjustable constraint weight of Multiplier Method；It is gradient operator；Section 1 is fidelity item, and Section 2 and Section 3 are to the first width and
The sparsity constraints of two width images, last is two images profile similarity constraint；This two images has following relation：
Wherein,It is the linear phase displacement of the first width and the second width image respectively；x0Primary signal, be by
Primary signal comprising first and second echo-signal carries out what inverse Fourier transform obtained, is solved by iterative algorithm
State formula and can be obtained by image caused by first after separation and second echo-signal；
(6) image that step (5) is separated carries out T2 imagings and calculated；For the T2 imaging methods of single sweep, it is only necessary to
The different echo time image of two width with regard to feasible, directly by T2 relaxation equations tried to achieve by T2 value：
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WhereinIt is correction factor, Δ TE=TE2-TE1；S1And S2It is first echo-signal and second respectively
The image intensity of echo-signal, full variation (Total Variation) extrapolation is added to strengthen the resolution ratio of image, Er Qieshe
A fixed threshold value, when obtained numerical value is less than threshold value, can be considered as noise and be ignored, equally when the T2 values calculated
When big, omit；The T2 images of the high-quality with preferable resolution ratio have been calculated finally by T2 imagings.
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| CN107045115A (en) * | 2017-05-04 | 2017-08-15 | 厦门大学 | Single sweep Quantitative MRI Measurement T2* imaging methods based on double echo |
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| CN108294753B (en) * | 2018-01-03 | 2021-03-05 | 上海东软医疗科技有限公司 | Method and device for acquiring magnetic resonance quantitative information map |
| CN108663644B (en) * | 2018-03-14 | 2020-02-14 | 厦门大学 | Single-scan quantitative magnetic resonance T based on residual error network reconstruction2Imaging method |
| CN109100669B (en) * | 2018-07-12 | 2020-01-14 | 厦门大学 | Single-scan synchronous magnetic resonance diffusion and T based on overlapped echoes2Image forming method |
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