CN104569882B - The method for correcting phase of magnetic resonance imaging system FSE and its application - Google Patents
The method for correcting phase of magnetic resonance imaging system FSE and its application Download PDFInfo
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Abstract
The invention discloses a kind of method for correcting phase of magnetic resonance imaging system FSE and its application.FSE (FSE) pulse train, so as to cause phase error, produces artifact because radio frequency returns the imperfect of poly- pulse and produces stimulated echo.In order to eliminate this artifact, the present invention is made spin echo be overlapped with stimulated echo position, is eliminated FSE zero and first order phase error using the method for prescan by the phase of gradient and the poly- pulse of adjustment radio frequency time, so as to eliminate the artifact as caused by phase error, to improve picture quality.
Description
Technical field
The invention belongs to magnetic resonance imaging arts, and in particular to a kind of phase of magnetic resonance imaging system FSE
Bearing calibration and its application.
Background technology
Mr imaging technique has become a kind of conventional technological means in modern medical diagnosis, using conventional MRI
Imaging sequence is scanned, and the single sweep operation time needs a few minutes.And be scanned using fast imaging sequences, during single sweep operation
Between can greatly shorten.Quick-speed spin echo pulse sequence turns into sequence the most frequently used in fast imaging sequences, in magnetic resonance imaging
It is used widely in system.Compared with the spin-echo sequence of standard, the scan efficiency of FSE sequences is greatly improved, and is about
The ETL (Echo Train Length) of standard spin echo sequence times.However, due to imperfect, the FSE sequences of radio-frequency pulse
Stimulated echo can be produced, so as to cause image to produce artifact, picture quality is reduced.In order to which this factor is reduced or eliminated to image
Influence, generally add prescan before formal image scanning.
United States Patent (USP) US5378985 and US6369568 provide a kind of FSE sequences prescan scheme.Zeroth order section error,
I.e. unrelated with a position amount of phase error.Single order phase error, i.e., related to a position single order amount of phase error.It is logical
The gradient for crossing closing phase-encoding direction is scanned, by choosing one strange one even two echoes in echo train, according to them
Zeroth order phase error come correct radio frequency return poly- pulse phase, corrected according to their single order phase error in " reading direction "
Dephasing gradient, then by closing " reading direction " gradient calibration phase encoding gradient, school is come finally by traversal echo-signal maximum
The gradient in positive layer choosing direction.
Above-mentioned prescan scheme has great role to improving FSE picture qualities, but the program is still present as next
A little problems：
1) program does not consider influence of the stimulated echo to picture quality, and stimulated echo is the master for causing image artifacts
Want one of reason.
2) program determines whether prescan continues using the method for threshold value, illustrates the scanning times of program needs and is
Repeatedly, this considerably increases sweep time.
The content of the invention
The present invention seeks to：A prescan need to be carried out by providing a kind of, you can complete the phase school to FSE pulse trains
Just, correction efficiency is greatly increased, the phase of the magnetic resonance imaging system FSE the time required to prescan is reduced
Bit correction method.
The technical scheme is that：A kind of method for correcting phase of magnetic resonance imaging system FSE, it is special
Levy and be, comprise the following steps：
(1) using FSE sequences respectively to every layer of progress prescan, and every layer is repeatedly excited (it is usually no more than 4
It is secondary)；
(2) gradient of phase-encoding direction is closed, the echo of collection K space center excites the N number of echo of collection every time；
(3) data that prescan goes out are handled, calculates the numerical value of correction gradient and return the initial phase of poly- pulse
Position；
(4) addition correction gradient and change back the initial phase of poly- pulse FSE sequences are corrected.
Specifically：
(1) prescan is carried out using FSE pulse trains to all layers, prescan echo train is N.By changing radio-frequency drive
Pulse and radio frequency return the phase of poly- pulse, and N number of echo is gathered respectively.
(2) data collected are handled, respectively obtains spin echo and stimulated echo data, by calculating first
The position of the peak-peak of individual spin echo and the position at sampling " window " center, calculate the value of compensation gradient G 11.
(3) by the position of the position and sampling " window " center of the peak-peak of stimulated echo, compensation gradient is calculated
G13 value.
(4) by the position and the position at sampling " window " center of second spin echo peak-peak, compensation is calculated
The value of gradient G 12.
(5) the first rank phase error of FSE pulse trains is eliminated by the compensation of three gradients.
(6) by the processing to data, the change value that radio frequency is returned needed for poly- impulse phase is calculated.So as to eliminate FSE sequences
The zeroth order phase error of row.
Whole trimming process only needs a sequence scanning, and all layers that need to be scanned are corrected, every layer school gradient
Offset and phase value are all different.
It is preferred that, the data progress processing gone out in the step (3) to prescan is comprised the following steps that：
(1) inverse Fourier transform is carried out to each channel data first；
(2) data processing is carried out to each passage respectively, then carries out multichannel synthesis；
(3) data after synthesis will be handled, by repeatedly exciting the data of acquisition, can respectively obtained from cycle
The data of ripple signal and stimulated echo signal；
(4) by the spin echo data obtained in (3), adjustment gradient G 11 is calculated with first spin echo data
Value；Calculate adjustment gradient G 13 by the data of stimulated echo, then again by second spin echo data and G11 and
G13 calculates the value of adjustment gradient G 12, so that the single order phase error of phase calibration；
(5) data to above-mentioned echo are handled, and spin echo data is entered by the single order phase error calculated
Line phase is compensated, and eliminates the influence that single order phase error is brought to phase of echo；
(6) by the spin echo data that correction is obtained in (5), then by linear fit, the first echo is calculated respectively
With the phase intercept of the second echo, the intercept for calculating two phase place is poor, the phase for adjusting back poly- pulse；
(7) according to the method described above, every layer is corrected, every layer is required for calculating adjustment gradient with returning the adjustment of poly- pulse
Phase.
It is an advantage of the invention that：
FSE (FSE) pulse train produces stimulated echo due to the imperfect of the poly- pulse of radio frequency time, so that
Cause phase error, produce artifact.In order to eliminate this artifact, the present invention passes through gradient and adjustment using the method for prescan
The phase that radio frequency returns poly- pulse makes spin echo be overlapped with stimulated echo position, eliminates FSE zero and first order phase error, from
And the artifact as caused by phase error is eliminated, to improve picture quality.
Brief description of the drawings
Below in conjunction with the accompanying drawings and embodiment the invention will be further described：
Fig. 1 phase of echos sequential develops schematic diagram.
Fig. 2 echo positions calculate schematic diagram.
Two spin echo intercept differentials of Fig. 3 are intended to.
Fig. 4 FSE phasing prescan schematic diagrames.
FSE normal scan sequence diagrams after Fig. 5 phasings.
Embodiment
Embodiment：Using normal picture scan FSE sequences, close phase-encoding direction gradient, to excitation pulse and return
The phase of poly- pulse is modulated.Key step is as follows：
1) prescan is carried out using FSE sequences, closes phase-encoding direction, every layer is scanned, and every layer repeatedly excites (one
As be no more than 4 times), the N number of echo of collection is excited every time.
2) by excitation pulse and return poly- pulse phase-modulation, repeatedly excited, in repeatedly exciting, excitation pulse with
The initial phase for returning poly- pulse is different.
3) data to collection are handled, and specific method can be by content of the invention explanation hereafter.
4) by phase-modulation above, the signal for N number of echo that prescan is gathered is spin echo or spin echo
With the mixed signal of stimulated echo.Pass through the data repeatedly excited, you can respectively obtain spin echo and stimulated echo.
5) by the spin echo data obtained in (4), adjustment gradient G 11 is calculated with first spin echo data
Value.Adjustment gradient G 13 is calculated by the data of spin echo, then passes through second spin echo data and G11 and G13 again
The value of adjustment gradient G 12 is calculated, so that the single order phase error of phase calibration.Specific computational methods are by invention hereinafter
Description of contents.
6) data to above-mentioned echo are handled, and spin echo data is carried out by the single order phase error calculated
Phase compensation, is that the correction of zeroth order phase error below is done to eliminate the influence that single order phase error is brought to phase of echo
Data prepare.
7) by the spin echo data that correction is obtained in (6), then by linear fit, calculate respectively the first echo and
The phase intercept of second echo, the intercept for calculating two phase place is poor, the phase for adjusting back poly- pulse, specific to calculate and place
The content of the invention explanation of the method for reason hereinafter.
8) according to the method described above, every layer is corrected, every layer is required for calculating adjustment gradient with returning the adjustment of poly- pulse
Phase.
By prescan, adjustment gradient G 11, G12, G13 and the adjustment phase place for returning poly- pulse are calculated, in normal picture
During scanning, direction addition adjustment gradient is being read, gradient locations are as shown in Figure 4.And the poly- impulse phase that each returns of sequence is carried out
Adjustment, adjusted value is calculated by prescan and obtained.
Sum it up, this paper pre-scanning process only needs single pass, every layer is excited, and is excited repeatedly, scanning K spaces
Then the data that scanning is obtained are handled by central row, calculate required adjusted value, and alignment type scanning sequence is adjusted
It is whole.Prescan sequence is also based on the sequence framework formally scanned, is swept in advance so new sequence need not be designed.Whole correction
Very short the time required to process, this greatlys save sweep time.To reach above-mentioned purpose, the present invention is adopted the following technical scheme that：
1) prescan is carried out using FSE sequences, respectively to every layer of progress prescan, every layer is repeatedly excited.
2) gradient of phase-encoding direction, the echo of collection K space center are closed.
3) in repeatedly exciting, the initial phase of excitation pulse and time poly- pulse is different.
4) after the completion of prescan, the data that scanning is obtained are handled, correction ladder is calculated by the data scanned
The numerical value of degree and the initial phase for returning poly- pulse.
5) in formal FSE sequence scannings, by adding correction gradient and changing back the initial phase of poly- pulse to FSE
Sequence is corrected.
The modulation by excitation pulse and the initial phase for returning poly- pulse of the invention main is come the collection of control signal, often
Layer is repeatedly excited, and the N number of echo data of acquisition is excited every time.
The data collected are handled, is to be synthesized the data of each passage first, comprises the following steps that：
1) inverse Fourier transform is carried out to each channel data first.
2) data processing is carried out to each passage respectively, then carries out multichannel synthesis.
Followed by the data after synthesis are handled, by repeatedly exciting the data of acquisition, spin can be respectively obtained
The data of echo-signal and stimulated echo signal.
In order to accurately calculate the value of adjustment gradient, the Grad of adjustment gradient is calculated herein by two methods, so
Afterwards by being compared to the Grad that two methods are calculated, the final Grad size for determining adjustment gradient.
1) obtained spin echo signal and stimulated echo signal data distribution is handled, calculates each echo data
Modulus value, the maximum of echo data, that is, echo center are found by traversal, the position X0 at each echo center is recorded.
As shown in Figure 1.Black line part is represented in the schematic diagram that Fig. 1 changes over time for the phase of echo that three radio-frequency pulses are produced, figure
The phase place change of spin echo, dotted portion represents the phase place change of stimulated echo, produce the magnetization vector of stimulated echo due to
The effect of second radio-frequency pulse can return to fore-and-aft plane, so the phase of stimulated echo is in second and the 3rd radio-frequency pulse
Between keep constant, as shown in figure 5, adjustment gradient G 11 be added in before first sample window, so G11 be used for correct first
The echo center of spin echo, and the echo center of stimulated echo is not influenceed by adjustment gradient G 11 and G12, only
Influenceed by adjustment gradient G 13, so G13 is used for the echo center for correcting stimulated echo.Second spin echo center
Position is by adjustment gradient G 11, the collective effect of tri- gradients of G12 and G13.
, can be by reducing sampling time interval, increase sampling in order to further improve the accuracy of echo center
Points, are kept for total sampling time constant, increase FOV, so as to further determine that echo center.Specifically, such as sampling bandwidth
For x, sampling number is y, and the error of fitting for obtaining central point is z, and correction gradient error tends to vary with the reduction of sampling bandwidth and increased
Greatly.In order to reduce correction error, with, generally using increase sampling bandwidth and sampling number, making control errors in pole in scanning process
In small scope, further reduce FSE artifact.
Obtain behind echo center, carry out quadratic fit, echo center is then calculated by the equation fitted.So
Echo center and the difference DELTA N of sampling window center are calculated afterwards, as shown in Figure 2.
2) data to spin echo and stimulated echo carry out one-dimensional inversefouriertransform, obtain each point in echo data
Phase value.
Δ N=Φ (Δ x) * N (2* π) formula (2)
R in above-mentioned formula is gyromagnetic ratio, and g is reads the reading Grad of gradient direction, and Δ x is that single volume elements is reading direction
Size, m is the points at interval, and N is sampling number, and Δ N is that sampling window center and the interval sampling at collection echo center are counted.
Formula (2) can be released by formula (1), it is only necessary to which (Δ x) just can be with by the value Φ of mutually dissipating for knowing in unit volume elements
Δ N is calculated, and (Δ x) can be tried to achieve Φ by formula (1), and (Δ x) can be by the following method by more accurate mutually scattered Φ
Obtain, try to achieve mutually dissipating for the volume elements in X position by formula (1) first, it is resulting then by the method for linear fit
Intercept be exactly each position volume elements mutually dissipate Φ (Δ x's) is averaged, so as to obtain echo the center displacement Δ N, and calculates in echo
Heart position X1.
By echo center X1 obtained above with before by seeking the echo center obtained by maximum
X0 is compared, if X1 differs larger with X0, by echo center of X0 calculate adjustment gradient, conversely, then using X1 for time
Ripple center calculates adjustment gradient.
Obtain behind each echo center, by formula, compensation gradient=(echo center-sample window center) * is adopted
Sample time interval * reads gradient/compensation gradient duration, the value of compensation gradient needed for calculating.
By the correction of the above method, echo the center displacement can be eliminated, in the intercept obtained in aforementioned manners, i.e. volume elements
The Φ that mutually dissipates (data gathered are corrected by Δ x), eliminate mutually dissipating of being brought of single order phase error.
After the mutually scattered elimination that single order phase error is brought, phase error is only left the zeroth order phase unrelated with position and missed
Difference, this needs eliminate zeroth order phase error by changing back the phase of poly- pulse.Specific trimming process is as follows：
1) using the good data of above-mentioned single order phase error correction, the correction of zeroth order phase error is carried out, two spins are taken
The correction data of echo, obtains the phase of echo each point, then carries out linear fit to the phase in respective point, tries to achieve two spins
The intercept of echo.
2) using the good data of above-mentioned single order phase error correction, the correction of zeroth order phase error is carried out, two spins are taken
The correction data of echo, obtains the phase of echo each point, is then weighted linear fit to the phase in respective point, tries to achieve two
The intercept of spin echo.
3) by the size of the error amount of the linear fit that compares above two method, select linear error of fitting less
Intercept.The intercept of tried to achieve two echoes is subtracted each other, the half of intercept difference is used for the phase for offseting back poly- pulse, that is, exists
Return on the basis of the poly- original phase of pulse plus the value.ΔΦ i.e. shown in Fig. 30Half.
Fig. 4 is FSE phasing prescan sequence diagrams, FSE of the prescan sequence basic framework based on normal scan
Sequence, closes the gradient of phase-encoding direction on the basis of original sequence, then to the phase of excitation pulse and time poly- pulse
It is modulated, every layer is repeatedly excited, the N number of echo of collection is excited every time.
Fig. 5 is the sequence diagram of normal scan after FSE phasings.Fig. 5 is shown in the FSE sequence bases of normal scan
On plinth, reading to add adjustment gradient on direction, it is shown that the position added by adjustment gradient.And the phase for returning poly- pulse is carried out
Adjustment.In Fig. 4 and Fig. 5：90 °-excitation pulse, 180 °-return poly- pulse, G11, G12, G13-adjustment gradient.
The present invention using prescan method, by gradient and adjustment radio frequency return poly- pulse phase make spin echo and by
Swash echo position to overlap, FSE zero and first order phase error is eliminated, so that the artifact as caused by phase error is eliminated, to carry
High image quality.
Certainly the above embodiments merely illustrate the technical concept and features of the present invention, and its object is to allow be familiar with technique
People can understand present disclosure and implement according to this, it is not intended to limit the scope of the present invention.It is all according to this hair
The modification that the Spirit Essence of bright main technical schemes is done, should all be included within the scope of the present invention.
Claims (4)
1. a kind of method for correcting phase of magnetic resonance imaging system FSE, it is characterised in that comprise the following steps：
(1) using FSE sequences respectively to every layer of progress prescan, and every layer is repeatedly excited；
(2) gradient of phase-encoding direction is closed, the echo of collection K space center excites the N number of echo of collection every time；
(3) data that prescan goes out are handled, calculates the numerical value of correction gradient and return the initial phase of poly- pulse；
(4) addition correction gradient and change back the initial phase of poly- pulse FSE sequences are corrected；
The data progress processing gone out in wherein described step (3) to prescan is comprised the following steps that：
(1) inverse Fourier transform is carried out to each channel data first；
(2) data processing is carried out to each passage respectively, then carries out multichannel synthesis；
(3) data after synthesis are handled, by repeatedly exciting the data of acquisition, spin echo signal can be respectively obtained
With the data of stimulated echo signal；
(4) by the spin echo data obtained in (3), the value of adjustment gradient G 11 is calculated with first spin echo data；
Adjustment gradient G 13 is calculated by the data of stimulated echo, is then counted again by second spin echo data and G11 and G13
The value of adjustment gradient G 12 is calculated, so that the single order phase error of phase calibration；
(5) data to above-mentioned echo are handled, and phase is carried out to spin echo data by the single order phase error calculated
Position compensation, eliminates the influence that single order phase error is brought to phase of echo；
(6) by the spin echo data that correction is obtained in (5), then by linear fit, the first echo and the are calculated respectively
The phase intercept of two echoes, the intercept for calculating two phase place is poor, the phase for adjusting back poly- pulse；
(7) according to the method described above, every layer is corrected, every layer is required for calculating adjustment gradient with returning the adjustment phase of poly- pulse
Position.
2. the method for correcting phase of magnetic resonance imaging system FSE according to claim 1, it is characterised in that
The initial phase of excitation pulse and time poly- pulse is different in the step (1).
3. the method for correcting phase of magnetic resonance imaging system FSE according to claim 1, it is characterised in that
What is repeatedly excited in the step (1) excites number of times to be no more than 4 times.
4. a kind of magnetic resonance system, it is characterised in that used magnetic resonance imaging system any in claim 1-3 quick
The method for correcting phase of spin echo is corrected.
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