CN104504656B - The quick Scattering correction method in pyramidal CT image domain - Google Patents
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Abstract
The invention discloses a kind of quick Scattering correction method in pyramidal CT image domain, including：Using filter back-projection algorithm according to actual measurement backprojection reconstruction three-dimensional butterfly wave filter mask and initial three-dimensional CT images；Described initial three-dimensional CT images include m layers of two-dimensional CT image, and described three-dimensional butterfly wave filter mask includes m layers of butterfly wave filter mask；A modifying factor is set for every layer of two-dimensional CT image, and builds the correction function based on modifying factor, three-dimensional butterfly wave filter mask and initial three-dimensional CT images, for being modified to initial three-dimensional CT images；Majorized function is built for modifying factor, described majorized function includes optimization object function and optimization constraint function；The optimal value that described majorized function obtains each modifying factor is solved, and the optimal value of the modifying factor is substituted into majorized function and obtain revised three-dimensional CT image.The correction result high precision that the quick scatter artefacts modification method of Cone-Beam CT of the invention is obtained, and erection rate is fast.
Description
Technical field
The present invention relates to engineering in medicine technical field, and in particular to a kind of quick Scattering correction method in pyramidal CT image domain.
Background technology
Current Scattering correction can be realized in projection domain or image area.Projection domain Scattering correction is conventional method, according to
The characteristics of additivity that scattered signal belongs to pollutes and presentation space low-frequency is distributed, by the side such as measurement, analog simulation or analytical Calculation
Method estimates scatter distributions, then is subtracted from the resultant signal for measuring, and obtains the projection after Scattering correction.Revised projection can be with
Using the CT algorithm for reconstructing of standard, such as filtered back projection reconstructs three-dimensional body.Mainly include following method：Reduce detection
Collimater method (i.e. backscattering wiregrating method), solution that device opens solid angle (i.e. increase air gap method) to light source, suppresses scattered photon
Analysis method, Monte Carlo simulation, scattered signal Measurement Algorithm and source signal modulator approach etc..
First two method is not often used alone, because their methods can only to a certain extent reduce scattering pollution shadow
Ring, it is necessary to increase sufferer dose of radiation to compensate projection signal snr loss.Analytic method receives much attention in clinical practice, because
For it realizes succinct and calculates efficient；But estimated accuracy is scattered when running into complex object can be deteriorated, and be primarily due to scattering nucleus quilt
It is considered linear or translation invariant.Monte-carlo Simulation Method by statistical method estimate scatter distributions, its precision it is very high but
Amount of calculation is very big, generally to share to realize the Scattering correction algorithm of highly effective with analytic method.Scatter measurement method by
Small size beam stop is placed between light source and object come source signal of decaying, signal conduct in detector shadow region is then measured
Scatter distributions are sampled.Based on scattered signal space low-frequency distribution character, whole audience scatter distributions are sampled into row interpolation by above-mentioned
Obtain.This method realizes accurate scatter distributions, but cost is to cause source signal to lose, thus generally needs in the projection of every width
Two groups of data of middle collection, one group has stopper and another group does not have.Source modulator approach according to scattering and source signal to modulator not
Them are separated with response characteristic, but the method heavy dependence system calibrating precision is, it is necessary to carry out algorithm according to clinical setting excellent
Change design.
Image area Scattering correction need not access Raw projection data, directly to the image containing scatter artefacts after standard reconstruction
Processed.The strategy is generally divided into two classes, and a class is alignd in same two groups of CT images of patient by anamorphose registration technique
Anatomical structure, reuse plan CT gray values substitution Cone-Beam CT number does the application such as Rapid Dose Calculation.The obvious of this kind of " replacement " algorithm lacks
It is to be completely dependent on deformable registration precision to fall into, and accurate deformable registration is difficult to accomplish, proves under many circumstances or even in theory
It is infeasible.
Up-to-date information so directly related with patient in Cone-Beam CT projection is not used, and final image information is counted completely
Draw CT to dominate, it is impossible to play the treatment monitoring characteristic of Cone-Beam CT.Another kind of method is based on the space to shading artifact in Cone-Beam CT
Frequecy characteristic is observed, it is assumed that the artifact is mainly low frequency distribution signal in image area, and image area amendment is produced by planning CT
Mapping graph, digital picture correction is carried out to Cone-Beam CT gray value.This kind of method can effectively keep sufferer information in Cone-Beam CT, but
Needs realize mapping graph by existing plan CT images, it is impossible to as general-purpose algorithm.
The content of the invention
In view of the shortcomings of the prior art, the present invention proposes a kind of quick Scattering correction method in pyramidal CT image domain.
A kind of quick Scattering correction method in pyramidal CT image domain, including：
(1) using filter back-projection algorithm according to actual measurement backprojection reconstruction three-dimensional butterfly wave filter mask and initial three-dimensional CT figures
Picture；
Described initial three-dimensional CT images include m layers of two-dimensional CT image, and described three-dimensional butterfly wave filter mask includes m layers
Two-dimentional butterfly wave filter mask；
(2) a modifying factor is set for every layer of two-dimensional CT image, and is built based on described modifying factor, three-dimensional butterfly
The correction function of mode filter mask and initial three-dimensional CT images, for being modified to described initial three-dimensional CT images；
(3) majorized function is built for modifying factor, described majorized function includes that optimization object function and optimization are constrained
Function；
(4) optimal value that described majorized function obtains each modifying factor is solved, and by the optimal of the modifying factor
Value substitutes into correction function and obtains revised three-dimensional CT image.
The size of m depends on the size of CT scanner and sweep object, usual m=200 in the present invention.
Actual measurement projection includes the data for projection of the data for projection of sweep object and butterfly wave filter, root in the step (1)
Initial three-dimensional CT images are reconstructed using filter back-projection algorithm according to the actual measurement projection of sweep object；According to the reality of butterfly wave filter
Survey projection and three-dimensional butterfly wave filter mask is reconstructed using filter back-projection algorithm.
Wherein, the projection number that measurement is obtained when the data for projection of butterfly wave filter is CT scanner zero load (no-raster object)
According to.
Three-dimensional butterfly wave filter masking operations are obtained simply by above method, and is substantially increased compared with prior art
Speed is built, the time loss of whole makeover process is advantageously reduced.
The reason for so selecting correction function is MxThe shape of itself gives the shape information of artifact, but MxItself
Monochrome information and artifact brightness also have a certain distance, this luminance difference is away from can be by MxMultiply modifying factor to remove,
I.e. modifying factor multiplies MxJust can simultaneously obtain the shape information and monochrome information of artifact.
Preferably, the correction function that the step (2) builds is as follows：
L=L0+[α1,α2... ..., αm]·[M1, M2... ..., Mm]；
L is revised three-dimensional CT image, L0It is initial three-dimensional CT images, αxIt is xth layer two dimension in initial three-dimensional CT images
CT images correspondence modifying factor, MxIt is the two-dimentional butterfly wave filter mask of xth layer in three-dimensional butterfly wave filter mask, wherein, x=1,
2 ... ..., m.
Revised three-dimensional CT image L and initial three-dimensional CT images L in the present invention0Actually include m layers of two-dimensional ct figure
Picture.
Further preferably, the step (3) builds majorized function by the following method：
(3-1) is made with the maximum of the histogram functions of the three-dimensional CT image L after being modified using the correction function
It is optimization object function；
(3-2) is using the difference of the CT numbers of adjacent two layers two-dimensional CT image less than predetermined threshold value as optimization constraint function.
Obligating condition relative to the interlayer for rule of thumb obtaining modifying factor in the prior art has certain master
The interference of sexual factor is seen, the principle of modifying factor should be able to meet rip cutting figure in the present invention will not produce the jump of monochrome information
Become, row constraint is entered come the deviation to interlayer modifying factor.
The maximum for calculating histogram functions has less computation complexity, is conducive to improving erection rate, in addition, because
For pixel number is certain in three-dimensional CT image, thus the maximum of histogram functions is bigger, illustrates that histogram is got over " thin
It is long ", i.e., more pixels are on similar gray value, i.e., flat, and corresponding revised result is also more accurate.
Further preferably, described predetermined threshold value is 30~50HU.The size of predetermined threshold value is directly connected to final amendment
The precision of result and the when consumption of amendment.
The step (4) majorized function described as follows：
(4-1) determines the initial value of each modifying factor, specific as follows：
Using the number of unknown number (modifying factor) in the lax optimization object function of lax optimization, to relax be 1：Make each
The corresponding modifying factor of layer two-dimensional CT image is identical, solution is optimized to the majorized function and obtains the first of each modifying factor
Initial value；
(4-2) is based on the initial value of the modifying factor, and solution is optimized to described majorized function, obtains final product each and repaiies
The optimal value of positive divisor.
Based on the initial value of the modifying factor, solution is optimized to described majorized function, obtain final product each modifying factor
The optimal value of son.
Just unknown number number relaxes as after 1, due to every layer of modifying factor have been set to it is identical, thus according to
When modifying factor is modified, the difference for obtaining the CT numbers of adjacent two layers two-dimensional CT image in revised three-dimensional CT image is basic
It is zero, and then causes optimization constraint function to fail.
If X-ray tube activity in itself can be obtained in actual applications, can be obtained by hardware means and corrected
The initial value of the factor.
To improve solution efficiency, preferably, being carried out to described majorized function by interior point method in the step (4-2)
Optimization Solution.
Can be that every 3~6 layers of two-dimensional CT image sets a phase to further speed up solving speed, in the step (2)
Same modifying factor, after solution obtains the optimal value of each modifying factor, further carries out B- spline interpolations to the result for obtaining
To keep the uniformity of interlayer).
The quick Scattering correction method in pyramidal CT image domain of the invention, it is also pre- including being carried out to three-dimensional butterfly wave filter mask
Treatment obtains pretreated three-dimensional butterfly wave filter mask；
The peripheral bright ring artifact and ripple artifact of the three-dimensional butterfly wave filter mask are eliminated during pretreatment successively；
Corresponding three-dimensional butterfly wave filter mask is pretreated three-dimensional butterfly wave filter in the corresponding correction function
Mask.
Peripheral bright ring artifact is removed using Hough transform method during pretreatment：
(S1-1) using the circle in Hough transformation identification butterfly wave filter mask, and identification butterfly is determined according to recognition result
The peripheral bright ring artifact region of wave filter mask；
(S1-2) gray value of all pixels point in peripheral bright ring artifact region is set to zero and then eliminates butterfly wave filter
Peripheral bright ring artifact region in mask, obtains peripheral bright ring artifact butterfly wave filter mask.
When carrying out Hough transformation in the present invention, only it is more than 200 pictures with the distance of central point in traversal butterfly wave filter mask
The region of element.
Peripheral bright ring artifact region in the present invention in butterfly wave filter mask pixel gray value be more than 0.1mm-1(i.e.
CT numbers are more than 5000HU).
Ripple artifact is removed using mean filter method during pretreatment：
Mean filter is carried out to the central area for removing peripheral bright ring artifact butterfly wave filter mask, butterfly filtering is eliminated
The ripple artifact of the central area of device mask.
It to eliminate the center of butterfly wave filter mask is the center of circle that central area of the invention is, radius is n pixel
Region.For size is 512 × 512 butterfly wave filter masks, n=50~200.
The vicinity points number used when carrying out mean filter in the present invention is 9.
Compared with prior art, the correction result precision that the quick Scattering correction method in pyramidal CT image domain of the invention is obtained
Height, and erection rate is fast, for the initial three-dimensional CT images that size is 512 × 512, the correction time for being spent is 2min.
Brief description of the drawings
Fig. 1 is the flow chart of the quick scatter artefacts modification method in pyramidal CT image domain of the present embodiment；
Fig. 2 (a) is the two-dimensional CT image in initial three-dimensional CT images；
Fig. 2 (b) is the two-dimentional butterfly wave filter mask in three-dimensional butterfly wave filter mask；
Fig. 3 (a) is the two-dimentional butterfly wave filter mask in pretreated three-dimensional butterfly wave filter mask；
Fig. 3 (b) is the two-dimensional CT image in revised three-dimensional CT image.
Specific embodiment
Below in conjunction with the drawings and specific embodiments, the present invention will be described in detail.
Fig. 1 is the flow chart of the quick scatter artefacts modification method in pyramidal CT image domain of the present embodiment, is comprised the following steps：
(1) covered according to actual measurement backprojection reconstruction initial three-dimensional CT images and three-dimensional butterfly wave filter using filter back-projection algorithm
Film：Initial three-dimensional CT images include m layers of two-dimensional CT image, and described three-dimensional butterfly wave filter mask includes m layers of two-dimentional butterfly filter
Ripple device mask；
M=200 in the present embodiment, shown in the two-dimensional CT image such as Fig. 2 (a) in the initial three-dimensional CT images for obtaining, it is seen that
Initial three-dimensional CT images have the larger dark circle of scatter artefacts pollution.
Shown in two-dimentional butterfly wave filter mask such as Fig. 2 (b) in the three-dimensional butterfly wave filter mask of the present embodiment.
The size of original three-dimensional CT image and three-dimensional butterfly wave filter mask (actually should for 512 × 512 in the present embodiment
This is the size of every layer of two-dimensional CT image and two-dimentional butterfly wave filter mask).
(2) the three-dimensional butterfly wave filter mask obtained to step (1) using Hough transform method and mean filter method carries out pre-
Treatment, obtains pretreated three-dimensional butterfly wave filter mask.
(2-1) is modified using Hough transformation to described butterfly wave filter mask, specific as follows：
(2-11) determines identification butterfly using the circle in Hough transformation identification butterfly wave filter mask according to recognition result
The peripheral bright ring artifact region (including the circle for identifying) of wave filter mask；
Peripheral bright ring artifact region be butterfly wave filter mask in pixel gray value be more than 0.1mm-1。
During Hough transformation, with the region of the distance more than 200 pixels of central point only in traversal butterfly wave filter mask.
The gray value of all pixels point in peripheral bright ring artifact region is set to zero and then eliminates butterfly wave filter by (2-12)
Peripheral bright ring artifact region in mask, obtains peripheral bright ring artifact butterfly wave filter mask.
(2-2) has significant high frequency characteristics due to the ripple artifact, to removing peripheral bright ring artifact butterfly wave filter mask
Central area carry out mean filter, you can eliminate butterfly wave filter mask central area ripple artifact, pre-processed
Three-dimensional butterfly wave filter mask afterwards.
It to eliminate the center of butterfly wave filter mask is the center of circle that central area is, radius is the n region of pixel.This reality
It is 512 × 512 to apply the size of butterfly wave filter mask in example, now n=200.
The vicinity points number used when carrying out mean filter in the present embodiment is 9.
Three-dimensional butterfly wave filter mask is pre-processed in the present embodiment, is actually interpreted as to the two dimension on every layer
Butterfly wave filter mask is pre-processed.The two-dimentional butterfly wave filter of the pretreated three-dimensional butterfly wave filter mask for obtaining is covered
Shown in film such as Fig. 3 (a), the two-dimentional butterfly wave filter mask with the three-dimensional butterfly wave filter mask shown in Fig. 2 (b) compares, it is seen that
Outside bright ring artifact and the ripple artifact of zone line are eliminated, will not be by such artifact so in whole makeover process
Introduce original image.Simultaneously also without interference with histogrammic calculating (if bright ring artifact may have certain dry to histogram calculation
Disturb).
(3) a modifying factor is set for every layer of two-dimensional CT image, and is built based on described modifying factor, three-dimensional butterfly
The correction function of mode filter mask (being pretreated three-dimensional butterfly wave filter mask) and initial three-dimensional CT images, for right
Initial three-dimensional CT images are modified；
The correction function built in the present embodiment is as follows：
L=L0+[α1,α2... ..., αm]·[M1, M2... ..., Mm]；
L is revised three-dimensional CT image, L0It is initial three-dimensional CT images, αxIt is xth layer two dimension in initial three-dimensional CT images
CT images correspondence modifying factor, MxIt is the two-dimentional butterfly wave filter mask of xth layer in pretreated three-dimensional butterfly wave filter mask,
Wherein, x=1,2 ... ..., m.
Revised three-dimensional CT image L and initial three-dimensional CT images L in the present embodiment0Actually include m layers of two-dimensional ct
Image：
L=[I'1,I'2,……,I'm],L0=[I1,I2,……,Im], I'xAnd IxRespectively revised three dimensional CT figure
As the two-dimensional CT image of xth layer in L and initial three-dimensional CT images.
(4) modifying factor of image area is obtained using the method for mathematical optimization：
(4-1) builds majorized function, and majorized function includes optimization object function and optimization constraint function：
(4-11) is reflected in order that optimizing planarization to image in itself with this, is entered with using the correction function
The maximum of the histogram functions of the revised three-dimensional CT image L of row is used as optimization object function；
(4-12) is using the difference of the CT numbers of adjacent two layers two-dimensional CT image less than predetermined threshold value as optimization constraint function；
The threshold value of the predetermined threshold value is 0.005mm-1(i.e. CT numbers are more than 25HU).
(4-2) uses the initial value of each modifying factor of the determination of lax optimization, specific as follows：
It is 1 that the number of unknown number (modifying factor) relaxes in lax optimization object function：Make each layer of two-dimensional CT image pair
The modifying factor answered is identical.The initial value for solving and obtaining each modifying factor is optimized using described majorized function.
(4-3) is optimized using the initial value of step (4-2) each modifying factor by the majorized function of interior point method pair
Solve, obtain final product the optimal value of each modifying factor；
(5) it is to obtain revised three-dimensional CT image L the optimal value of each modifying factor to be substituted into correction function.
(6) the CT numbers to revised three-dimensional CT image L are demarcated：
(6-1) selects 3 kinds of setup action destination organizations from revised three-dimensional CT image L, is existed according to each destination organization
The CT numbers that CT numbers standard value under correspondence crest voltage is organized in revised three-dimensional CT image L to respective objects are carried out linearly
Fitting obtains calibration curve,
(6-2) is recovered to CT base lines the CT numbers of revised three-dimensional CT image L using described calibration curve, is obtained
Demarcate three-dimensional CT image.Wherein, CT bases line is CT numbers corresponding to human body composition material.
Crest voltage is 100kVp in the present embodiment, and corresponding CT standard values (i.e. CT criterion numerals) are for soft tissue
0.021mm-1。
The 3 d image data collected using CT machines (i.e. CT scanner) is (only to the two dimension of certain layer when displaying
Image is shown) significant degree of algorithm is tested.
Obtain being modified the initial three-dimensional CT images shown in Fig. 2 (a) in the present embodiment, the correction time for being spent is
2min, the correction result (i.e. revised three-dimensional CT image) that correspondence is obtained is such as shown in Fig. 3 (b), it is seen that by original after amendment
The Crape ring that scatter artefacts in three-dimensional CT image are caused has been basically eliminated, and normal Clinical practice will not be impacted.
Above-described specific embodiment has been described in detail to technical scheme and beneficial effect, Ying Li
Solution is to the foregoing is only presently most preferred embodiment of the invention, is not intended to limit the invention, all in principle model of the invention
Interior done any modification, supplement and equivalent etc. are enclosed, be should be included within the scope of the present invention.
Claims (10)
1. a kind of quick Scattering correction method in pyramidal CT image domain, it is characterised in that including：
(1) using filter back-projection algorithm according to actual measurement backprojection reconstruction three-dimensional butterfly wave filter mask and initial three-dimensional CT images；
Described initial three-dimensional CT images include m layers of two-dimensional CT image, and described three-dimensional butterfly wave filter mask includes m layers of two dimension
Butterfly wave filter mask；
Described actual measurement projection includes the data for projection of the data for projection of sweep object and butterfly wave filter, according to sweep object
Actual measurement projection reconstructs initial three-dimensional CT images using filter back-projection algorithm；Actual measurement projection according to butterfly wave filter is using filter
Ripple backprojection algorithm reconstructs three-dimensional butterfly wave filter mask；
Wherein, the data for projection that measurement is obtained when the data for projection of butterfly wave filter is unloaded CT scanner；
(2) a modifying factor is set for every layer of two-dimensional CT image, and is built based on described modifying factor, three-dimensional butterfly filter
The correction function of ripple device mask and initial three-dimensional CT images, for being modified to described initial three-dimensional CT images；
(3) majorized function is built for modifying factor, described majorized function includes optimization object function and optimization constraint function；
(4) optimal value that described majorized function obtains each modifying factor is solved, and by the optimal value generation of the modifying factor
Enter correction function and obtain revised three-dimensional CT image.
2. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 1, it is characterised in that step (2) structure
The correction function built is as follows：
L=L0+[α1,α2... ..., αm]·[M1, M2... ..., Mm]；
L is revised three-dimensional CT image, L0It is initial three-dimensional CT images, αxIt is xth layer two-dimensional ct figure in initial three-dimensional CT images
As correspondence modifying factor, MxIt is the two-dimentional butterfly wave filter mask of xth layer in three-dimensional butterfly wave filter mask, wherein, x=1,
2 ... ..., m.
3. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 1, it is characterised in that the step (3) is led to
Cross following method and build majorized function：
The maximum of the histogram functions of the three-dimensional CT image L of (3-1) after being modified using the correction function is used as excellent
Change object function；
(3-2) is using the difference of the CT numbers of adjacent two layers two-dimensional CT image less than predetermined threshold value as optimization constraint function.
4. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 3, it is characterised in that described predetermined threshold value
It is 30~50HU.
5. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 1, it is characterised in that the step (4) is led to
Cross following steps and solve described majorized function：
(4-1) determines the initial value of each modifying factor；
(4-2) is based on the initial value of the modifying factor, and solution is optimized to described majorized function, obtains final product each modifying factor
The optimal value of son.
6. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 5, it is characterised in that the step (4-1)
It is specific as follows：
After using the number of unknown number in the lax optimization object function of lax optimization to relax as 1, then solve the optimization after relaxing
Function is the initial value for obtaining each modifying factor.
7. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 5, it is characterised in that the step (4-2)
In solution is optimized to described majorized function by interior point method.
8. the quick Scattering correction method in pyramidal CT image domain as described in any one claim in claim 1~7, it is special
Levy and be, also obtain pretreated three-dimensional butterfly wave filter mask including to three-dimensional butterfly wave filter mask pre-process；
The peripheral bright ring artifact and ripple artifact of the three-dimensional butterfly wave filter mask are eliminated during pretreatment successively；
Corresponding three-dimensional butterfly wave filter mask is pretreated three-dimensional butterfly wave filter mask in the correction function.
9. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 8, it is characterised in that using suddenly during pretreatment
The peripheral bright ring artifact of husband's converter technique removal：
(S1-1) using the circle in Hough transformation identification butterfly wave filter mask, and identification butterfly filtering is determined according to recognition result
The peripheral bright ring artifact region of device mask；
(S1-2) gray value of all pixels point in peripheral bright ring artifact region is set to zero and then eliminates butterfly wave filter mask
In peripheral bright ring artifact region, obtain peripheral bright ring artifact butterfly wave filter mask；
Peripheral bright ring artifact region be butterfly wave filter mask in pixel gray value be more than 0.1mm-1Region；
During Hough transformation, with the region of the distance more than 200 pixels of central point only in traversal butterfly wave filter mask.
10. the quick Scattering correction method in pyramidal CT image domain as claimed in claim 9, it is characterised in that used during pretreatment
Mean filter method removes ripple artifact：
Mean filter is carried out to the central area for removing peripheral bright ring artifact butterfly wave filter mask, butterfly wave filter is eliminated and is covered
The ripple artifact of the central area of film.
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