CN100580476C - Correcting method for quick-speed spin echo pulse sequence and application thereof - Google Patents
Correcting method for quick-speed spin echo pulse sequence and application thereof Download PDFInfo
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Abstract
The invention provides a method for correcting fast spin echo sequence, aiming at solving the problem that the prior FSE images have the factors of an imaging system such as artifacts, magnetic field stability, eddy current, etc. which have impact on the quality of images. The method can be used to correct the parameter of the fast spin echo (FSE) sequence of a magnetic resonance diagnostic, in particular to correct the parameter of the fast spin echo (FSE) sequence of a magnetic resonance diagnostic which can switch frequence of a transmitter and a receiver. The method for correcting fast spin echo sequence of the invention eliminates the impact of the factors of the imaging system such as artifacts, magnetic field stability, eddy current, etc. on the quality of the images; and as the parameter which is obtained by the correcting method and used in the pre-scanning is the optimal parameter, thereby solving the problem of artifacts of the prior FSE images.
Description
Technical field
The present invention relates to a kind of bearing calibration and application thereof of quick-speed spin echo pulse sequence, belong to the mr imaging technique field.
Background technology
Magnetic resonance imaging (MRI) technology has become in the medical diagnosis very useful means.Adopt conventional magnetic resonance imaging sequence to scan, the single sweep operation time can reach a few minutes.Not only can bring discomfort to patient so long sweep time, and because breathing, heartbeat, gastrointestinal peristalsis and pseudo-shadow that some autokinetic movement caused are degenerated picture quality greatly.Adopt the fast imaging sequence single sweep operation time can be foreshortened to several seconds, thereby reduce the motion artifacts and the discomfort of patient in scanning process of image.Fast spin echo (FSE) sequence is a kind of in the fast imaging sequence, is applied in most MRI system.Compare with the standard spin-echo sequence, shorten the sweep time of FSE sequence widely.Yet FSE sequence formation method is had relatively high expectations to magnetic field homogeneity, gradient amplitude distortion, radio-frequency pulse shape and the eddy current state of system.In order to eliminate or to reduce of the harmful effect of these factors, need before formal scanning, carry out prescan usually to image.
U.S. Pat 5378985 and US6369568 disclose a kind of FSE sequence prescan scheme and improvement project thereof.This scheme makes the correction of pulse train partial parameters be independent of other parameter at selecting layer, phase encoding and reading direction (corresponding to XYZ direction in the rectangular coordinate system) and respectively imaging sequence is made amendment; And choose in the echo train two echoed signals as a reference, according to 0 grade of phase differential and the 1 grade of phase differential between them, relevant parameters is proofreaied and correct.
Although above-mentioned prescan scheme has great role to improving FSE picture quality, yet still there is following problem in this scheme:
1) the prescan sequence is different with the imaging sequence that actual scanning adopts in this scheme, this has not only increased the difficulty of sequences Design, and because the amplitude of eddy current state and pulsed gradient and to apply mode relevant, so the portability of optimized parameter between different sequences (prescan sequence and imaging sequence) is difficult to be guaranteed.
2) this scheme is not considered stimulated echo by in the process of prescan correcting imaging sequential parameter, and stimulated echo is one of reason that produces image artifacts.
3) in this scheme implementation process, only chosen two echoes in the echo train and verified the prescan effect, this correction for partial parameters is feasible.Because the correction of other a part of parameter need be taken all factors into consideration whole echoes in the echo train, so the correction result that this scheme obtains is not an optimum solution.
Summary of the invention
The present invention is directed to the deficiencies in the prior art, a kind of bearing calibration of quick-speed spin echo pulse sequence and the application in mr imaging technique thereof are provided, exist imaging system factors such as pseudo-shadow, magnetic field stability and eddy current state image quality to be existed the problem of influence to solve existing FSE image.
For achieving the above object, the present invention has taked following technical scheme:
The bearing calibration of quick-speed spin echo pulse sequence provided by the invention comprises following sequential steps:
A, adopt FSE pulse train to scan, the range value of the diffusing phase gradient pulse on " reading direction " according to the position correction of first echoed signal peak dot in the echo train, the range value trimming process of described diffusing phase gradient pulse comprises following sequential steps:
1) selects one deck, adopt FSE pulse train to scan;
2) calculate the position of first echoed signal peak dot that this sampling obtains, if the peak dot of first echoed signal is positioned at the center of sampling " window ", the range value trimming process end of the phase gradient pulse of then loosing, otherwise execution in step 3;
3) if the current range value of the phase gradient pulse of loosing is GA4, then new range value GA4 '=GA4 * TD/ (Nt * 2), wherein TD is a sampling number, Nt is the ordinal number of the pairing sampled point of current peak dot, the numerical value of GA4 ' is composed to GA4, carried out the scanning of FSE pulse train with the range value of the diffusing phase gradient pulse after upgrading;
4) circulation execution in step 2 and step 3 are positioned at the center of sampling " window " until the peak dot of echoed signal, and the range value trimming process of the phase gradient pulse of then loosing finishes;
The range value of the diffusing phase gradient pulse that B, employing have been proofreaied and correct carries out the scanning of FSE pulse train, read the range value of gradient pulse according to the position correction of second echoed signal peak dot in the echo train, the described range value trimming process of reading gradient pulse comprises following sequential steps:
1) the range value GA4 with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, as if Ac＜Ar, and execution in step 5, otherwise execution in step 3;
3) be GA6 if read the current range value of gradient pulse, then new range value GA6 '=GA6+ δ, δ wherein gets 1% of GA6 initial value, or system allows the minimum step of GA6 stepping, the numerical value of GA6 ' is composed to GA6, carried out the scanning of FSE pulse train once more with the GA6 value after upgrading;
4) circulation execution in step 2 and step 3, until Ac＜Ar, execution in step 5;
5) carry out the scanning of FSE pulse train with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct and the range value GA6 that reads gradient pulse after preliminary the correction;
6) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, if Ac＜Ar, the range value trimming process end of then reading gradient pulse, otherwise execution in step 7;
7) if reading the current range value of gradient pulse is GA6, then new range value GA6 '=GA6-δ (δ is the same) composes the numerical value of GA6 ' to GA6, carries out the scanning of FSE pulse train once more with the GA6 value after upgrading;
8) circulation execution in step 6 and step 7, until Ac＜Ar, the range value trimming process of then reading gradient pulse finishes;
The range value of the diffusing phase gradient pulse that C, employing have been proofreaied and correct and the range value of having proofreaied and correct of reading gradient pulse carry out the scanning of FSE pulse train, according to whole phase place of the phase correction of echoed signal peak dots counter-rotating radio-frequency pulse in the echo train, the phase correction process of described counter-rotating radio-frequency pulse comprises following sequential steps:
1) range value GA4 and the range value GA6 that reads gradient pulse after the correction with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0, σ wherein
0Be predefined reference value, then execution in step 5, otherwise execution in step 3;
3) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ, δ wherein gets 5% of FQ2 initial value, and the numerical value of FQ2 ' is composed to FQ2, carries out the scanning of FSE pulse train once more with the FQ2 value after upgrading;
4) circulation execution in step 2 and step 3 are until σ＜σ
0, execution in step 5;
5) range value GA6 that reads gradient pulse with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct, after proofreading and correct and the FQ2 value after preliminary the correction are carried out FSE pulse train and are scanned;
6) calculate the standard deviation sigma of whole echo peak dot phase places of this sampling gained, if σ＜σ
0, σ wherein
0Be predefined reference value, the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7;
7) if the frequency offset of current choosing layer is FQ3, then new frequency offset FQ3 '=FQ3+ δ, δ wherein gets 5% of FQ3 initial value, and the numerical value of FQ3 ' is composed to FQ3, carries out the scanning of FSE pulse train once more with FQ 3 values after upgrading;
8) circulation execution in step 6 and step 7 are until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes;
Or the reverse phase correction process of radio-frequency pulse of step in the following order:
1) range value GA4 and the range value GA6 that reads gradient pulse after the correction with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) calculate the standard deviation sigma of echo (not comprising first echo) peak dot phase place of this sampling gained, if σ＜σ
0, σ wherein
0Be predefined reference value, then execution in step 5, otherwise execution in step 3;
3) if the frequency offset of current choosing layer is FQ3, then new frequency offset FQ3 '=FQ3+ δ, δ wherein gets 5% of FQ3 initial value, and the numerical value of FQ3 ' is composed to FQ3, carries out the scanning of FSE pulse train once more with the FQ3 value after upgrading;
4) circulation execution in step 2 and step 3 are until σ＜σ
0, execution in step 5;
5) range value GA6 that reads gradient pulse with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct, after proofreading and correct and the FQ3 value after preliminary the correction are carried out FSE pulse train and are scanned;
6) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0, σ wherein
0Be predefined reference value, the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7;
7) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ, δ wherein gets 5% of FQ2 initial value, and the numerical value of FQ2 ' is composed to FQ2, carries out the scanning of FSE pulse train once more with the FQ2 value after upgrading;
8) circulation execution in step 6 and step 7 are until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes;
The phase place of D, the range value that adopts the diffusing phase gradient pulse of having proofreaied and correct, the range value of reading gradient pulse and counter-rotating radio-frequency pulse is carried out the scanning of FSE pulse train, heavily assemble the range value of gradient pulse according to the phase place in the position correction of first echoed signal peak dot in the echo train " a choosing layer direction ", the range value trimming process that the phase place on described " a choosing layer direction " is heavily assembled gradient pulse comprises following sequential steps:
1) carries out the scanning of FSE pulse train with range value GA4, the range value GA6 that reads gradient pulse, FQ2 value and the FQ3 value of the diffusing phase gradient pulse after proofreading and correct;
2) the range value Ac with first echoed signal of this sampling gained compares with the preceding once range value Ar of first echoed signal of sampling, and as if Ac＜Ar, then execution in step 5, otherwise execution in step 3;
3) be GA1 if the phase place on " choosing layer direction " is heavily assembled the current range value of gradient pulse, then new range value GA1 '=GA1+ δ, δ wherein gets 1% of GA1 initial value, or system allows the minimum step of GA1 stepping, the numerical value of GA1 ' is composed to GA1, carried out the scanning of FSE pulse train once more with the GA1 value after upgrading;
4) circulation execution in step 2 and step 3, until Ac＜Ar, execution in step 5;
5) carry out the scanning of FSE pulse train with the GA1 value after range value GA4, the range value GA6 that reads gradient pulse, FQ2 value, FQ3 value and preliminary correction of the diffusing phase gradient pulse after proofreading and correct;
6) the range value Ac of first echoed signal of this sampling gained and the range value Ar of preceding first echoed signal of once sampling are compared, if Ac＜Ar, then the phase place on " choosing layer direction " is heavily assembled the range value trimming process end of gradient pulse, otherwise execution in step 7;
7) if it is GA1 that the phase place on " choosing layer direction " is heavily assembled the current range value of gradient pulse, then new range value GA1 '=GA1-δ (δ is the same) composes the numerical value of GA1 ' to GA1, carries out the scanning of FSE pulse train once more with the GA1 value after upgrading;
8) circulation execution in step 6 and step 7, until Ac＜Ar, then the phase place on " choosing layer direction " is heavily assembled the range value trimming process end of gradient pulse.
The bearing calibration of described quick-speed spin echo pulse sequence can be applicable to the parameter correction of fast spin echo (FSE) sequence of magnetic resonance imaging system, especially can be applicable to the transmitter and receiver parameter correction of the fast spin echo of the magnetic resonance imaging system of switching frequency (FSE) sequence synchronously.
The influence that imaging system factors such as magnetic field stability, eddy current state, stimulated echo produce image quality has been eliminated in the bearing calibration of quick-speed spin echo pulse sequence of the present invention, the parameter of using described bearing calibration to obtain during prescan is optimized parameter, has therefore solved the problem that there is pseudo-shadow in existing FSE image.
Description of drawings
Fig. 1 is the synoptic diagram of the embodiment of the invention 1 described spin-echo sequence;
Fig. 2 is the synoptic diagram of the embodiment of the invention 2 described spin-echo sequences.
Among the figure: the RF-radio-frequency pulse; 90 °
x-excitation pulse; 180 °
y-inversion pulse; The Gs-slice selective gradient; The Gp-phase encoding gradient; Gr-reads gradient; The Echo-echo; The Tx-transmitter; The Rx-receiver.
Embodiment
Below in conjunction with drawings and Examples the present invention is done further detailed, complete explanation:
Embodiment 1
The bearing calibration of quick-speed spin echo pulse sequence provided by the invention comprises following sequential steps:
A, adopt FSE pulse train to scan, the range value of the diffusing phase gradient pulse on " reading direction " according to the position correction of first echoed signal peak dot in the echo train;
The range value of the diffusing phase gradient pulse that B, employing have been proofreaied and correct carries out the scanning of FSE pulse train, reads the range value of gradient pulse according to the position correction of second echoed signal peak dot in the echo train;
C, adopt the range value of the diffusing phase gradient pulse proofreaied and correct and the range value of reading gradient pulse proofreaied and correct carries out the scanning of FSE pulse train, according to whole phase place of the phase correction of echoed signal peak dots counter-rotating radio-frequency pulse in the echo train;
The phase place of D, the range value that adopts the diffusing phase gradient pulse of having proofreaied and correct, the range value of reading gradient pulse and counter-rotating radio-frequency pulse is carried out the scanning of FSE pulse train, heavily assembles the range value of gradient pulse according to the phase place in the position correction " choosing layer direction " of first echoed signal peak dot in the echo train.
Select one deck to proofread and correct (frequency offset corresponding to this aspect among FQL1, FQL2 and the FQL3 is respectively FQ1, FQ2 and FQ3).
The range value trimming process of the diffusing phase gradient pulse of present embodiment comprises following sequential steps:
1) carry out FSE pulse train shown in Figure 1, wherein: on phase-encoding direction, the range value of gradient pulse 231～236 is 0; On choosing layer direction, gradient pulse 221 and 222 range value (GA0 and GA2) with formally scan identical; The range value of gradient pulse 223 is GA1 (initial value=GA0 of GA1 * T0/T1, T0 and T1 are respectively the time of GA0 and GA1 effect); Reading on the direction, the range value of gradient pulse 242 (GA5) is identical with formal scanning; Gradient pulse 241 and 243 range value are GA4 and GA6 (T4, T5 and T6 are respectively the time of GA4, GA5 and GA6 effect for initial value=GA5 of GA4 * T5/T4, initial value GA5 * T5/T6 of GA6); Initial value=FQ1 of FQ2 * t1/t2, initial value=FQ1 of FQ3 * t1 '/t2 (t1 is the time of the pairing FQ1 effect of excitation pulse, and t1 ' is the time of the pairing FQ1 effect of inversion pulse, and t2 is the time of FQ2 effect, and t3 is the time of FQ3 effect).
2) calculate the position of first echoed signal peak dot that this sampling obtains, if the peak dot of first echoed signal 201 is positioned at the center of sampling " window ", the range value trimming process end of the phase gradient pulse of then loosing, otherwise execution in step 3.
3) if the phase gradient pulse 241 current range values that loose are GA4, then new range value GA4 '=GA4 * TD/ (Nt * 2), wherein TD is a sampling number, Nt is the ordinal number of the pairing sampled point of current peak dot, the numerical value of GA4 ' is composed to GA4, carry out FSE pulse train shown in Figure 1 once more, and adopt the GA4 that upgraded.
4) circulation execution in step 2 and step 3 are positioned at the center of sampling " window " until the peak dot of echoed signal 201, and the range value trimming process of the phase gradient pulse of then loosing finishes.
The range value trimming process of reading gradient pulse of present embodiment comprises following sequential steps:
1) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt the GA4 that proofreaies and correct.
2) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, as if Ac＜Ar, and execution in step 5, otherwise execution in step 3.
3) be GA6 if read gradient pulse 243 current range values, (δ can get 1% of GA6 initial value to then new range value GA6 '=GA6+ δ, or system allows the minimum step of GA6 stepping), the numerical value of GA6 ' is composed to GA6, carry out FSE pulse train shown in Figure 1 once more, and adopt the GA6 that upgraded.
4) circulation execution in step 2 and step 3, until Ac＜Ar, execution in step 5.
5) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read the GA6 after gradient pulse 243 range values adopt preliminary the correction.
6) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, if Ac＜Ar, the range value trimming process end of then reading gradient pulse, otherwise execution in step 7.
7) if reading gradient pulse 243 current range values is GA6, then new range value GA6 '=GA6-δ (δ is the same) composes the numerical value of GA6 ' to GA6, carries out FSE pulse train shown in Figure 1 once more, and adopts the GA6 that upgraded.
8) circulation execution in step 6 and step 7, until Ac＜Ar, the range value trimming process of then reading gradient pulse finishes.
The phase correction process of the counter-rotating radio-frequency pulse of present embodiment comprises following sequential steps:
1) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read gradient pulse 243 range values adopts GA6 after proofreading and correct.
2) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0(σ
0Be predefined reference value), then execution in step 5, otherwise execution in step 3.
3) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ (δ can get FQ2 initial value 5%) composes the numerical value of FQ2 ' to FQ2, carries out FSE pulse train shown in Figure 1 once more, and adopts the FQ2 that upgraded.
4) circulation execution in step 2 and step 3 are until σ＜σ
0, execution in step 5.
5) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read the FQ2 value after frequency offset that gradient pulse 243 range values adopt GA6 after proofreading and correct and current choosing layer adopts preliminary the correction.
6) calculate the standard deviation sigma of whole echo peak dot phase places of this sampling gained, if σ＜σ
0(σ
0Be predefined reference value), the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7.
7) if the frequency offset of current choosing layer is FQ 3, then new frequency offset FQ3 '=FQ3+ δ (δ can get FQ3 initial value 5%) composes the numerical value of FQ3 ' to FQ3, carries out FSE pulse train shown in Figure 1 once more, and adopts the FQ3 that upgraded.
8) circulation execution in step 6 and step 7 are until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes.
The range value trimming process that phase place on described " choosing layer direction " is heavily assembled gradient pulse comprises following sequential steps:
1) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read frequency offset that gradient pulse 243 range values adopt GA6 after proofreading and correct, choosing layer adopts FQ2 value and FQ3 value after proofreading and correct.
2) the range value Ac with first echoed signal of this sampling gained compares with the preceding once range value Ar of first echoed signal of sampling, and as if Ac＜Ar, then execution in step 5, otherwise execution in step 3.
3) be GA1 if the phase place on " choosing layer direction " is heavily assembled gradient pulse 223 current range values, (δ can get 1% of GA1 initial value to then new range value GA1 '=GA1+ δ, or system allows the minimum step of GA1 stepping), the numerical value of GA1 ' is composed to GA1, adopt the GA1 that upgraded, carry out FSE pulse train shown in Figure 1 once more.
4) circulation execution in step 2 and step 3, until Ac＜Ar, execution in step 5.
5) carry out FSE pulse train shown in Figure 1, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read frequency offset that gradient pulse 243 range values adopt GA6 after proofreading and correct, choosing layer adopts FQ2 value after proofreading and correct and the phase place on FQ3 value and " the choosing layer direction " heavily to assemble GA1 value after gradient pulse 223 range values adopt tentatively correction.
6) the range value Ac of first echoed signal of this sampling gained and the range value Ar of preceding first echoed signal of once sampling are compared, if Ac＜Ar, then the phase place on " choosing layer direction " is heavily assembled the range value trimming process end of gradient pulse, otherwise execution in step 7.
7) if it is GA1 that the phase place on " choosing layer direction " is heavily assembled gradient pulse 223 current range values, then new range value GA1 '=GA1-δ (δ is the same) composes the numerical value of GA1 ' to GA1, adopts the GA1 that upgraded, and carries out FSE pulse train shown in Figure 1 once more.
8) circulation execution in step 6 and step 7, until Ac＜Ar, then the phase place on " choosing layer direction " is heavily assembled the range value trimming process end of gradient pulse.
The diffusing phase gradient pulse 241 that obtains through above-mentioned correction and read " phase shift " that gradient pulse 243 can proof reading gradient pulse 242 causes, and eliminated the influence that stimulated echo produces.The FQ2 that obtains through above-mentioned correction can proofread and correct counter-rotating radio-frequency pulse 212 with respect to 212 phase place, make their 90 ° of phase phasic differences, the FQ3 that obtains can proofread and correct the phase place between the adjacent counter-rotating radio-frequency pulse 212, the phase differential that makes them is 0, thereby avoids the error accumulation that causes because of the inversion pulse out of true.Phase place in " choosing layer direction " that obtains through above-mentioned correction is heavily assembled the phase place that gradient pulse 223 can proofread and correct on " a choosing layer direction " and is heavily assembled " phase shift " that gradient pulse 221 causes.
All need through above-mentioned correction with the corresponding structure at all levels of FQL1.Structure at all levels is all proofreaied and correct and is finished, the prescan EOP (end of program).Be used for " phase shift " due to the correction gradient pulse 223,241,243 and be used to proofread and correct FQ2, the FQ3 of counter-rotating radio-frequency pulse phase place, all relevant with factors such as system magnetic field stability and eddy current states, need the parameter that basis scans at every turn be determined.Before formal scanning, need determine above-mentioned each correction parameter by prescan.
Embodiment 2
The difference of present embodiment and embodiment 1 only is, the step operation in the following order of the phase correction process of described counter-rotating radio-frequency pulse:
1) carry out FSE pulse train shown in Figure 2, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read gradient pulse 243 range values adopts GA6 after proofreading and correct, and other sequential parameter is identical with embodiment 1.
2) calculate the standard deviation sigma of echo (not comprising first echo) peak dot phase place of this sampling gained, if σ＜σ
0(σ
0Be predefined reference value), then execution in step 5, otherwise execution in step 3.
3) if the frequency offset of current choosing layer is FQ3, then new frequency offset FQ3 '=FQ3+ δ (δ can get FQ3 initial value 5%) composes the numerical value of FQ3 ' to FQ3, carries out FSE pulse train shown in Figure 2 once more, and adopts the FQ3 that upgraded.
4) circulation execution in step 2 and step 3 are until σ＜σ
0, execution in step 5.
5) carry out FSE pulse train shown in Figure 2, phase gradient pulse 241 range values that wherein loose adopt GA4 after proofreading and correct, read the FQ3 value after frequency offset that gradient pulse 243 range values adopt GA6 after proofreading and correct and current choosing layer adopts preliminary the correction.
6) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0(σ
0Be predefined reference value), the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7.
7) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ (δ can get FQ2 initial value 5%) composes the numerical value of FQ2 ' to FQ2, carries out FSE pulse train shown in Figure 2 once more, and adopts the FQ2 value of upgrading.
8) circulation execution in step 6 and step 7 are until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes.
Claims (3)
1. the bearing calibration of a quick-speed spin echo pulse sequence is characterized in that, described bearing calibration comprises following sequential steps:
A, adopt FSE pulse train to scan, the range value of the diffusing phase gradient pulse on " reading direction " according to the position correction of first echoed signal peak dot in the echo train, the range value trimming process of described diffusing phase gradient pulse comprises following sequential steps:
1) selects one deck, adopt FSE pulse train to scan;
2) calculate the position of first echoed signal peak dot that this sampling obtains, if the peak dot of first echoed signal is positioned at the center of sampling " window ", the range value trimming process end of the phase gradient pulse of then loosing, otherwise execution in step 3);
3) if the current range value of the phase gradient pulse of loosing is GA4, then new range value GA4 '=GA4 * TD/ (Nt * 2), wherein TD is a sampling number, Nt is the ordinal number of the pairing sampled point of current peak dot, the numerical value of GA4 ' is composed to GA4, carried out the scanning of FSE pulse train with the range value of the diffusing phase gradient pulse after upgrading;
Circulation execution in step 2) and step 3) 4), be positioned at the center of sampling " window " until the peak dot of echoed signal, the range value trimming process of the phase gradient pulse of then loosing finishes;
The range value of the diffusing phase gradient pulse that B, employing have been proofreaied and correct carries out the scanning of FSE pulse train, read the range value of gradient pulse according to the position correction of second echoed signal peak dot in the echo train, the described range value trimming process of reading gradient pulse comprises following sequential steps:
1) the range value GA4 with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, as if Ac＜Ar, and execution in step 5), otherwise execution in step 3);
3) be GA6 if read the current range value of gradient pulse, then new range value GA6 '=GA6+ δ, δ wherein gets the minimum step of the permission GA6 of 1% or the system stepping of GA6 initial value, and the numerical value of GA6 ' is composed to GA6, carries out the scanning of FSE pulse train once more with the GA6 value after upgrading;
Circulation execution in step 2) and step 3) 4), until Ac＜Ar, execution in step 5);
5) carry out the scanning of FSE pulse train with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct and the range value GA6 that reads gradient pulse after preliminary the correction;
6) the range value Ac with second echoed signal of this sampling gained compares with the preceding once range value Ar of second echoed signal of sampling, if Ac＜Ar, the range value trimming process end of then reading gradient pulse, otherwise execution in step 7);
7) if the current range value of reading gradient pulse is GA6, then new range value GA6 '=GA6-δ, δ wherein is identical with δ in the step 3), and the numerical value of GA6 ' is composed to GA6, carries out FSE pulse train once more with the GA6 value after the renewal and scans;
8) circulation execution in step 6) and step 7), until Ac＜Ar, the range value trimming process of then reading gradient pulse finishes;
The range value of the diffusing phase gradient pulse that C, employing have been proofreaied and correct and the range value of having proofreaied and correct of reading gradient pulse carry out the scanning of FSE pulse train, according to whole phase place of the phase correction of echoed signal peak dots counter-rotating radio-frequency pulse in the echo train, the phase correction process of described counter-rotating radio-frequency pulse comprises following sequential steps:
1) range value GA4 and the range value GA6 that reads gradient pulse after the correction with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0, σ wherein
0Be predefined reference value, then execution in step 5), otherwise execution in step 3); Or
3) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ, δ wherein gets 5% of FQ2 initial value, and the numerical value of FQ2 ' is composed to FQ2, carries out the scanning of FSE pulse train once more with the FQ2 value after upgrading;
4) circulation execution in step 2) and step 3), until σ＜σ
0, execution in step 5);
5) range value GA6 that reads gradient pulse with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct, after proofreading and correct and the FQ2 value after preliminary the correction are carried out FSE pulse train and are scanned;
6) calculate the standard deviation sigma of whole echo peak dot phase places of this sampling gained, if σ＜σ
0, σ wherein
0Be predefined reference value, the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7);
7) if the frequency offset of current choosing layer is FQ3, then new frequency offset FQ3 '=FQ3+ δ, δ wherein gets 5% of FQ3 initial value, and the numerical value of FQ3 ' is composed to FQ3, carries out the scanning of FSE pulse train once more with the FQ3 value after upgrading;
8) circulation execution in step 6) and step 7), until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes;
Or the reverse phase correction process of radio-frequency pulse of step in the following order:
1) range value GA4 and the range value GA6 that reads gradient pulse after the correction with the diffusing phase gradient pulse after proofreading and correct carries out the scanning of FSE pulse train;
2) calculate the standard deviation sigma of echo peak dot phase place of this sampling gained, described echo does not comprise first echo; If σ＜σ
0, σ wherein
0Be predefined reference value, then execution in step 5), otherwise execution in step 3);
3) if the frequency offset of current choosing layer is FQ3, then new frequency offset FQ3 '=FQ3+ δ, δ wherein gets 5% of FQ3 initial value, and the numerical value of FQ3 ' is composed to FQ3, carries out the scanning of FSE pulse train once more with the FQ3 value after upgrading;
4) circulation execution in step 2) and step 3), until σ＜σ
0, execution in step 5);
5) range value GA6 that reads gradient pulse with the range value GA4 of the diffusing phase gradient pulse after proofreading and correct, after proofreading and correct and the FQ3 value after preliminary the correction are carried out FSE pulse train and are scanned;
6) calculate the first echo peak dot of this sampling gained and the phase differential σ of the second echo peak dot, if σ＜σ
0, σ wherein
0Be predefined reference value, the phase correction process of the radio-frequency pulse that then reverses finishes, otherwise execution in step 7);
7) if the frequency offset of current choosing layer is FQ2, then new frequency offset FQ2 '=FQ2+ δ, δ wherein gets 5% of FQ2 initial value, and the numerical value of FQ2 ' is composed to FQ2, carries out the scanning of FSE pulse train once more with the FQ2 value after upgrading;
8) circulation execution in step 6) and step 7), until σ＜σ
0, the phase correction process of the radio-frequency pulse that then reverses finishes;
The phase place of D, the range value that adopts the diffusing phase gradient pulse of having proofreaied and correct, the range value of reading gradient pulse and counter-rotating radio-frequency pulse is carried out the scanning of FSE pulse train, heavily assemble the range value of gradient pulse according to the phase place in the position correction of first echoed signal peak dot in the echo train " a choosing layer direction ", the range value trimming process that the phase place on described " a choosing layer direction " is heavily assembled gradient pulse comprises following sequential steps:
1) carries out the scanning of FSE pulse train with range value GA4, the range value GA6 that reads gradient pulse, FQ2 value and the FQ3 value of the diffusing phase gradient pulse after proofreading and correct;
2) the range value Ac with first echoed signal of this sampling gained compares with the preceding once range value Ar of first echoed signal of sampling, and as if Ac＜Ar, then execution in step 5), otherwise execution in step 3);
3) be GA1 if the phase place on " choosing layer direction " is heavily assembled the current range value of gradient pulse, then new range value GA1 '=GA1+ δ, δ wherein gets the minimum step of the permission GA1 of 1% or the system stepping of GA1 initial value, the numerical value of GA1 ' is composed to GA1, carried out the scanning of FSE pulse train once more with the GA1 value after upgrading;
Circulation execution in step 2) and step 3) 4), until Ac＜Ar, execution in step 5);
5) carry out the scanning of FSE pulse train with the GA1 value after range value GA4, the range value GA6 that reads gradient pulse, FQ2 value, FQ3 value and preliminary correction of the diffusing phase gradient pulse after proofreading and correct;
6) the range value Ac of first echoed signal of this sampling gained and the range value Ar of preceding first echoed signal of once sampling are compared, if Ac＜Ar, then the phase place on " choosing layer direction " is heavily assembled the range value trimming process end of gradient pulse, otherwise execution in step 7);
7) be GA1 if the phase place on " choosing layer direction " is heavily assembled the current range value of gradient pulse, then new range value GA1 '=GA1-δ, δ wherein is identical with δ in the step 3), and the numerical value of GA1 ' is composed to GA1, carries out FSE pulse train once more with the GA1 value after upgrading and scans;
8) circulation execution in step 6) and step 7), until Ac＜Ar, then the range value trimming process of heavily assembling gradient pulse of the phase place on " a choosing layer direction " finishes.
2. the application of the bearing calibration of the described quick-speed spin echo pulse sequence of claim 1 is characterized in that, described bearing calibration is used for the parameter correction of fast spin echo (FSE) sequence of magnetic resonance imaging system.
3. the application of the bearing calibration of quick-speed spin echo pulse sequence according to claim 2, it is characterized in that described bearing calibration is used for the transmitter and receiver parameter correction of the fast spin echo of the magnetic resonance imaging system of switching frequency (FSE) sequence synchronously.
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| CN102096054B (en) * | 2010-12-22 | 2013-03-13 | 华东师范大学 | Gradient system time delay correction method for fast spin echo pulse sequence |
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