CN108279393A - A kind of method of the full-automatic preemphasis correction vortex of magnetic resonance imaging - Google Patents
A kind of method of the full-automatic preemphasis correction vortex of magnetic resonance imaging Download PDFInfo
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Abstract
The present invention relates to a kind of methods of the full-automatic preemphasis correction vortex of magnetic resonance imaging, Levenberg Marquardt nonlinear regulations have been used to minimize fitting in eddy current signal dosing process, it is accurate and full-automatic, eddy current decay function in magnetic resonance imaging system fully automatically can accurately be judged, after eddy current decay form is correctly defined, the time constant of E indexes is fixed again in iteration preemphasis correction course, effectively nonlinear problem is converted to linear model to quantify, then correction is iterated to eddy current amplitude by SVD linear minimizations method, realize overall process full automation, manpower intervention is not needed.
Description
Technical field
It is vortexed the present invention relates to magnetic resonance imaging arts more particularly to a kind of full-automatic preemphasis correction of magnetic resonance imaging
Method.
Background technology
Magnetic resonance imaging has the characteristics that not damaged, soft tissue contrast is high, any direction tomography, is widely used at present
Medical clinic applications, and during magnetic resonance imaging, system will produce vortex, destroy the uniformity of main field and gradient fields,
To which artifact, especially fast spin echo (FS E), the imaging sequences such as Diffusion Imaging (DWI) can be generated in magnetic resonance image
It can be extremely sensitive to the vortex of magnetic resonance imaging system.Therefore it accurately and effectively quantifies and eliminates in magnetic resonance imaging system
Vortex is to influence the most key one of the factor of magnetic resonance image quality.
After the vortex of magnetic resonance imaging system occurs mainly with application imaging gradient, gradient current can be in gradient coil and magnetic
Induction generates at any time in the induced current of natural Exponents form decaying, to generate the vortex fields B0 of Decay Rate at any time in body
With gradient eddy field.The physical form of vortex is the attenuation function being composed of single or multiple E indexes, specifically by several E
The factors such as index constituent forms the design production by system performance, magnet and gradient coil, hardware device is installed determine, different
Magnetic resonance imaging system has different vorticity-stream function forms, even if in the same magnetic resonance imaging system, in three differences
The eddy current decay functional form in magnet direction (X, Y and Z) also have difference.
Correction at present is vortexed the most frequently used and effective two methods：It is to be compensated using self-shield coil and preemphasis respectively,
The former belongs to passive method, is inhibited by realizing the self-shileding effect to vortex field in gradient coil design and manufacture
Vortex in gradient coil, but the method depends on the coil design and manufacturing process of coil manufacturer, and can not be complete
The influence except vortex is totally disappeared, and often brings the vortex of negative polarity；And preemphasis compensation is then the method for belonging to active, is
By carrying out preemphasis amendment to the gradient waveform applied, application is identical as eddy current amplitude and damping time constant size, but
Opposite polarity gradient fields are cancelled out each other with system vortex, to eliminate vortes interference.Although this method can be corrected effectively
System vortex, but in the debugging of actual magnetic resonance imaging system, the number of the polarity of vortex, time parameter and damped expoential by
The equipment of system, hardware condition changed condition is widely different, and traditional method declines to vortex using nonlinear fitting mathematical tool
Cut signal can not guarantee accurate definition eddy current decay model.Therefore subjectivity of the actual quantitative vortex debugging dependent on engineer
Micro-judgment is often inaccurate, and it is by the ingredient of E indexes that can not provide suitable eddy current model or common practice
Number is configured to a fixed value (3 or 4), does so and often causes over-fitting, therefore traditional vortex pre-emphasis method
It can not be truly realized and automatically accurately debug correction system vortex.
Invention content
In view of this, the object of the present invention is to provide a kind of method of the full-automatic preemphasis correction vortex of magnetic resonance imaging,
To solve deficiency in the prior art.
In order to achieve the above object, the purpose of the present invention is be achieved through the following technical solutions：
A kind of method of the full-automatic preemphasis correction vortex of magnetic resonance imaging is provided, is included the following steps：
A) respectively in the gradient that X, Y or Z-direction independently apply a fixed amplitude position G and the duration is D, when by delay
Between after τ i, emit radio frequency, and acquire gtadient echo signal；
B it) using all signal phases are done with the swirl number size Ci that each delay time point τ i are quantitatively calculated, obtains
It is vortexed the curve decayed at any time；
C Levenberg-Marquardt algorithms) are utilized, more E exponential dampings are done to the eddy current decay curve Ci quantitatively obtained
It is excessive to prevent exponential term for the nonlinear fitting of model, and do weight regularization control to the range value of all E indexes
Over-fitting occurs, and after automatic rejection falls extra exponential term, all E exponential decay constants are fixed Kj；
D SVD linear fits) are done to vortex E Exponential Amplitudes, correspond to each time constant Kj according to what fitting quantitative obtained
E index eddy current amplitude Aj, and update the vortex pre-emphasis parameters of spectrometer, resurvey eddy current signal；
E it) iterates and carries out the above D processes, after swirl number is less than the threshold value T of a certain setting, programmed decision is most
Whole iteration convergence, vortex correction finish.
The method of the above-mentioned full-automatic preemphasis correction vortex of magnetic resonance imaging, wherein step B) it is middle using to all signals
Phase is the swirl number size Ci's that each delay time point τ i are quantitatively calculated, is expressed as formula [1]:
Ci=mi/2πγTE [1]
Wherein, mi indicates the slope (unit of the Fourier transformation back echo signal of ith acquisition：Radian/millimeter), γ tables
Show that physical constant gyromagnetic ratio (=0.267513 radian/millisecond/micro- tesla), TE indicate echo time (unit：Micro- tesla)；
Wherein, step C) used in more E exponential dampings function representation be formula [2]:
Wherein, n indicates the number of E index constituents, τiIndicate that gradient delay time, C (τ i) indicate that i-th of gradient postpones point
The eddy current being calculated is acquired, Aj and Kj indicate the amplitude and time constant of j-th of E exponential damping ingredient respectively；
Wherein, the regularization control done when being fitted to formula [2] can be expressed as formula [3]:
Wherein min { } indicates to minimize, and λ indicates the constant of regularization control item.
Compared with the prior art, the beneficial effects of the present invention are：
The present invention has used Levenberg-Marquardt nonlinear regulations minimum in eddy current signal dosing process
Change fitting, it is accurate and full-automatic, eddy current decay function in magnetic resonance imaging system fully automatically can accurately be sentenced
It is fixed, after eddy current decay form is correctly defined, the time constant of E indexes is consolidated again in iteration preemphasis correction course
It is fixed, effectively convert nonlinear problem to linear model and quantify, then by SVD linear minimizations method to eddy current amplitude into
Row iteration corrects, and realizes overall process full automation, does not need manpower intervention.
Description of the drawings
The attached drawing for constituting the part of the present invention is used to provide further understanding of the present invention, schematic reality of the invention
Example and its explanation are applied for explaining the present invention, is not constituted improper limitations of the present invention.In the accompanying drawings：
Attached drawing 1 is that the X-direction quantitatively obtained in the method for the present invention is vortexed the numerical value quantitation curves decayed at any time；
Attached drawing 2 is the effect contrast figure of the front and back system X-direction eddy axis of preemphasis correction in the method for the present invention, solid line institute
The system X-direction vortex quantitation curves of no preemphasis correction are shown as, dotted line show the system X-direction vortex of preemphasis correction
Quantitation curves；
Attached drawing 3 is the effect contrast figure that the front and back water mould FSE image authentications of preemphasis correction are vortexed in the method for the present invention：A is
Water mould FSE images without vortex preemphasis correction, b are the water mould FSE images for having vortex preemphasis correction.
Specific implementation mode
Following will be combined with the drawings in the embodiments of the present invention, and technical solution in the embodiment of the present invention carries out clear, complete
Site preparation describes, it is clear that described embodiments are only a part of the embodiments of the present invention, instead of all the embodiments.It is based on
Embodiment in the present invention, those of ordinary skill in the art are obtained every other without creative efforts
Embodiment shall fall within the protection scope of the present invention.
It should be noted that in the absence of conflict, the feature in embodiment and embodiment in the present invention can phase
Mutually combination.
The a direction (X, Y or Z) of the present invention in magnetic resonance imaging system applies a certain amount of gradient, passes through acquisition later
Gtadient echo signal measures swirl number, by measuring the swirl number at different delays time point, obtains eddy current decay curve, then
More E are done to eddy current decay signal value using the minimum algorithm of Levenberg-Marquardt (the literary Burger-Ma Kuaertefa of row)
The nonlinear fitting of exponential damping.And during minimum, weight regularization control is done to the range value of all E indexes,
The excessive over-fitting of exponential term is prevented to occur.Then after automatic rejection falls extra exponential term, all E exponential dampings are normal
Number is fixed, and is converted nonlinear model to linear model and is quantified, and SVD linear fits are done to vortex E Exponential Amplitudes, fixed according to fitting
The vortex pre-emphasis parameters of eddy current amplitude and time constant the update spectrometer measured, resurvey eddy current signal, iterate
Above procedure is carried out, all E exponential decay constants are fixed in iterative process, are only done vortex E Exponential Amplitudes and are done SVD linear fits,
After swirl number is less than the threshold value of a certain setting, programmed decision is final iteration convergence, and vortex correction finishes.Overall process is complete
Automation, does not need manpower intervention.A it is D's) independently to apply a fixed amplitude position G and duration in X, Y or Z-direction respectively
Gradient emits radio frequency, and acquire gtadient echo signal after delay time T i；
B it) using all signal phases are done with the swirl number size Ci that each delay time point τ i are quantitatively calculated, obtains
It is vortexed the curve decayed at any time；
C Levenberg-Marquardt algorithms) are utilized, more E exponential dampings are done to the eddy current decay curve Ci quantitatively obtained
It is excessive to prevent exponential term for the nonlinear fitting of model, and do weight regularization control to the range value of all E indexes
Over-fitting occurs, and after automatic rejection falls extra exponential term, all E exponential decay constants are fixed Kj；
D SVD linear fits) are done to vortex E Exponential Amplitudes, correspond to each time constant Kj according to what fitting quantitative obtained
E index eddy current amplitude Aj, and update the vortex pre-emphasis parameters of spectrometer, resurvey eddy current signal；
E it) iterates and carries out the above D processes, after swirl number is less than the threshold value T of a certain setting, programmed decision is most
Whole iteration convergence, vortex correction finish.
The method of the above-mentioned full-automatic preemphasis correction vortex of magnetic resonance imaging, wherein step B) it is middle using to all signals
Phase is the swirl number size Ci's that each delay time point τ i are quantitatively calculated, is expressed as formula [1]:
Ci=mi/2πγTE [1]
Wherein, mi indicates the slope (unit of the Fourier transformation back echo signal of ith acquisition：Radian/millimeter), γ tables
Show that physical constant gyromagnetic ratio (=0.267513 radian/millisecond/micro- tesla), TE indicate echo time (unit：Micro- tesla)；
Wherein, step C) used in more E exponential dampings function representation be formula [2]:
Wherein, n indicates the number of E index constituents, τiIndicate that gradient delay time, C (τ i) indicate that i-th of gradient postpones point
The eddy current being calculated is acquired, Aj and Kj indicate the amplitude and time constant of j-th of E exponential damping ingredient respectively；
Wherein, the regularization control done when being fitted to formula [2] can be expressed as formula [3]:
Wherein min { } indicates to minimize, and λ indicates the constant of regularization control item.
The specific implementation process of following instance step-by-step instructions the method for the present invention：Automatically vortex is adjusted used by the present embodiment
Examination technology is implemented in 1.5T superconducting magnetic resonance imaging machine devices.The present embodiment finally verify vortex calibration result magnetic resonance image be
The water mould image of fast spin echo FSE.
A, choose a magnet direction do vortex preemphasis correction, this example choose X-direction, apply a fixed amplitude position G and
Duration is the gradient of D, after delay time T i, emits radio frequency, and acquires the gtadient echo signal that the echo time is TE.
In the present embodiment, the micro- tesla/rice of gradient magnitude G=10000.0, duration D=20 millisecond, TE=6 milliseconds, delay time
τ i are arranged to 40, are respectively：1、2、3、4、5、6、8、10、15、20、25、30、40、50、60、70、80、90、100、110、
120、130、140、150、160、170、180、190、200、250、300、400、500、600、700、800、1000、1200、
1500 and 2000 milliseconds.
B, using formula [1] all signal phases are done with the swirl number size that each delay time point τ i are quantitatively calculated
Ci obtains being vortexed the curve decayed at any time, as shown in Fig. 1.In the present embodiment, the vortex of 40 obtained delay time points
Value size Ci is respectively：37.4620、35.5759、34.9731、34.7409、34.6033、34.7290、34.9395、
34.9363、34.0809、33.7751、32.8806 32.0224、30.3341、28.5487、26.9940、25.4155、
24.1821、22.7897 21.3279、19.7861、18.5141、17.5014、16.4106、15.2428、
14.438613.5914、12.7890、11.8998、11.1530、8.1253、5.8067、3.3796、1.9679、1.2117、
0.7434, the micro- tesla in 0.4086,0.2080,0.1391,0.0813 and 0.0127.
C, Levenberg-Marquardt (the literary Burger-Ma Kuaertefa of row) is done to vortex quantitative value using formula [2]
The nonlinear fitting of more E exponential decay models, and weight regularization is done to the range value of all E indexes using formula [3] and is controlled,
It prevents the excessive over-fitting of exponential term to occur, after automatic rejection falls extra exponential term, all E exponential decay constants is consolidated
It is fixed.Initial E exponential models have selected 4 exponential models in the present embodiment, if only doing nonlinear fitting with formula [2],
It is to fitting result：A1=-25.9644, A2=24.0478, A3=-5.68, A4=71.0602 (unit：Micro- tesla)；K1
=12.3483, K2=14.0353, K3=57.8750, K4=109.9339 (unit：Millisecond).Regularization fitting control is not done
If A values there is alternating polarity, over-fitting has occurred.And use formula [3] regularization be fitted then obtain result for：A1=
0.0281, A2=0.0047, A3=-0.0156, A4=35.9238 (unit：Micro- tesla)；K1=5.6944, K2=
18.9751, K3=53.6533, K4=176.3351 (unit：Millisecond).It can be found that the value of A1, A2 and A3 are respectively less than 0.1, just
Then change the generation that over-fitting is effectively prevented during minimizing, therefore eddy current decay model validation in the present embodiment
E index constituents for Single-Index Model, other small magnitudes are removed, and system time constant is fixed to K1=176.3351 millis
Second.
D, SVD linear fits are done to vortex E Exponential Amplitudes, corresponds to each time constant Kj according to what fitting quantitative obtained
E index eddy current amplitude Aj, and update the vortex pre-emphasis parameters of spectrometer, resurvey eddy current signal.Magnetic resonance in this implementation
Only there are one K1=176.3351 milliseconds of E index constituents for the X-direction swirl component of system, and A1=is retrieved after SVD fittings
37.0259 micro- tesla.
E, it iterates and carries out the above D processes, after swirl number is less than the threshold value T of a certain setting, programmed decision is most
Whole iteration convergence, vortex correction finish.Threshold value is set as 2.0 micro- teslas in the present embodiment.Convergence process has carried out 4 times repeatedly altogether
Generation.The swirl number size of 40 delay time points is respectively when final convergence：0.2293、-0.6179、-0.9193、-
0.7190、-0.5303、-0.3415、0.3137、0.5999、0.5898、1.0830、1.1886、1.2271、1.1131、
0.8150、0.6467、0.5886、0.4130、0.3089、0.1161、-0.3292、-0.5494、-0.4621、-0.4378、-
0.6785、-0.6720、-0.7268、-0.8124、-0.8818、-1.0654、-1.1561、-1.0061、-0.4635、-
0.0459, the micro- tesla in 0.1888,0.3444,0.3705,0.2728,0.1274,0.0856 and 0.1479.As shown in Fig. 2
For the effect contrast figure of the front and back system X-direction eddy axis of preemphasis correction.
F, repeat above step (A)-(E), in the same way to the Y-direction of magnetic resonance imaging system and Z-direction also into
Row vortex preemphasis correction.
G, the water mould image authentication vortex calibration result of fast spin echo FSE sequences is done.It is pre- for vortex as shown in Fig. 3
Aggravate the effect contrast figure of the front and back FSE water moulds of correction.
Specific embodiments of the present invention are described in detail above, but the present invention is not restricted to tool described above
Body embodiment, is intended only as example.To those skilled in the art, any equivalent modifications and replacement are also all in the present invention
Scope among.Therefore, made impartial conversion and modification without departing from the spirit and scope of the invention, should all cover
In the scope of the present invention.
Claims (2)
1. a kind of method of the full-automatic preemphasis correction vortex of magnetic resonance imaging, which is characterized in that include the following steps：
A) respectively in the gradient that X, Y or Z-direction independently apply a fixed amplitude position G and the duration is D, by delay time T i
Afterwards, emit radio frequency, and acquire gtadient echo signal；
B it) using all signal phases are done with the swirl number size Ci that each delay time point τ i are quantitatively calculated, is vortexed
The curve decayed at any time；
C Levenberg-Marquardt algorithms) are utilized, more E exponential decay models are done to the eddy current decay curve Ci quantitatively obtained
Nonlinear fitting, and weight regularization control is done to the range value of all E indexes, prevents excessive excessively quasi- of exponential term
It closes and occurs, after automatic rejection falls extra exponential term, all E exponential decay constants are fixed into Kj；
D SVD linear fits) are done to vortex E Exponential Amplitudes, the E corresponding to each time constant Kj obtained according to fitting quantitative
Index eddy current amplitude Aj, and the vortex pre-emphasis parameters of spectrometer are updated, resurvey eddy current signal；
E it) iterates and carries out the above D processes, after swirl number is less than the threshold value T of a certain setting, programmed decision is final changes
It withholds and holds back, vortex correction finishes.
2. the method for the full-automatic preemphasis correction vortex of magnetic resonance imaging as described in claim 1, which is characterized in that
Wherein, step B) in using all signal phases are done with the swirl number size that each delay time point τ i are quantitatively calculated
Ci's, it is expressed as formula [1]:
Ci=mi/2πγTE [1]
Wherein, mi indicates the slope (unit of the Fourier transformation back echo signal of ith acquisition：Radian/millimeter), γ expression things
Constant gyromagnetic ratio (=0.267513 radian/millisecond/micro- tesla) is managed, TE indicates echo time (unit：Micro- tesla)；
Wherein, step C) used in more E exponential dampings function representation be formula [2]:
Wherein, n indicates the number of E index constituents, τiIndicate that gradient delay time, C (τ i) indicate i-th of gradient delay point acquisition
The eddy current being calculated, Aj and Kj indicate the amplitude and time constant of j-th of E exponential damping ingredient respectively；
Wherein, the regularization control done when being fitted to formula [2] can be expressed as formula [3]:
Wherein min { } indicates to minimize, and λ indicates the constant of regularization control item.
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| CN109799472A (en) * | 2019-03-08 | 2019-05-24 | 沈阳工业大学 | A kind of magnetic resonance eddy current compensation method based on deep learning |
| CN112881959A (en) * | 2020-06-10 | 2021-06-01 | 苏州纽迈分析仪器股份有限公司 | Gradient eddy current compensation method and system for magnetic resonance imaging |
| CN113219391A (en) * | 2021-05-27 | 2021-08-06 | 成都鸣石峻致医疗科技有限公司 | Method and equipment for accelerating eddy current correction in magnetic resonance imaging system |
| CN113671434A (en) * | 2021-07-26 | 2021-11-19 | 深圳市联影高端医疗装备创新研究院 | Detection method and device of radio frequency board card, computer equipment and storage medium |
| CN114186584A (en) * | 2021-12-06 | 2022-03-15 | 无锡鸣石峻致医疗科技有限公司 | Method and equipment for denoising magnetic resonance signal based on extended exponential model |
| CN114252828A (en) * | 2021-12-21 | 2022-03-29 | 安徽福晴医疗科技有限公司 | Magnetic resonance imaging system and pre-emphasis eddy current correction method and device thereof |
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| CN114186584A (en) * | 2021-12-06 | 2022-03-15 | 无锡鸣石峻致医疗科技有限公司 | Method and equipment for denoising magnetic resonance signal based on extended exponential model |
| CN114252828A (en) * | 2021-12-21 | 2022-03-29 | 安徽福晴医疗科技有限公司 | Magnetic resonance imaging system and pre-emphasis eddy current correction method and device thereof |
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