CN106204673B - CT image metal artifact bearing calibration based on energy spectral filter and Image Residual re-projection - Google Patents
CT image metal artifact bearing calibration based on energy spectral filter and Image Residual re-projection Download PDFInfo
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- G06T11/00—2D [Two Dimensional] image generation
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Abstract
The present invention relates to a kind of CT image metal artifact bearing calibrations based on energy spectral filter and Image Residual re-projection, and filter thickness is calculated according to initial ray power spectrum and the relevant substance attenuation coefficient change rate curve of energy first；And optimal optical filter is determined compared with effective high-energy ray transmitance of different materials optical filter by calculating ratio；It is extracted and is deducted by clutter noise outside original reconstruction image target area, effectively inhibited the scatter artefacts in original image, improve picture contrast.The present invention removes shot noise by residual error re-projection method in original projection, collimator or scatter removal after complicated detector are not depended on, the priori conditions such as Imaging physics are not needed yet, a large amount of shot noise in former projected image can be removed, the image quality of detected target inside and outside contour is effectively improved.
Description
Technical field
The invention belongs to industry CT technical field of imaging, in particular to a kind of based on energy spectral filter and Image Residual re-projection
CT image metal artifact bearing calibration.
Background technique
Industrial computed tomography (Industrial Computed Tomography, ICT) utilizes different angles
The Transmission Attenuation information for spending lower X-ray carries out tomographic imaging to object, can obtain object in non-contact, undamaged situation
Internal structural information.Compared with the conventional nondestructiving detecting means such as radiography, ultrasound detection, eddy detection technology, industry CT
Have many advantages, such as that fast detection speed, space and density resolution are high, can intuitively show structure, group inside object to be detected
At and defective eigenpairs, be widely used in the numerous areas such as Aeronautics and Astronautics, defence and military.In industrial application, CT inspection
It is in the majority with metallics to survey object.Since metallics can make X-ray that beam hardening occur and generate a large amount of scattered photons, cause
The characteristics of image not being consistent with physical object structure, referred to as metal artifacts can be generated in CT image, seriously affect interior of articles
Identification, extraction and the judgement of effective information.Since intravascular stent, metal artificial tooth etc. are in clinical a large amount of uses, metal artifacts are asked
Topic is also generally existing in Medical CT, and imageable target shows as the metallics in nonmetallic substance comprising finite region.
Influence of the metal artifacts to medical image, which is mainly reflected in, observes difficulty to metallics surrounding tissue, brings very to clinical diagnosis
Big puzzlement.It therefore, is also always the research hotspot and difficult point in the field CT to the research of metal artifacts reduction method.
Current metal artifacts reduction method can be mainly divided into three classes: interpolation correction method, iterative corrections method x and mixing school
It executes；Wherein, interpolation correction method is also referred to as sinogram restorative procedure, is most common method in metal artifacts removal, such side
Method is split and positions to metal firstly the need of in image area, is then determined in the corresponding sinogram of metal and is thrown by orthographic projection
Shadow zone domain, and the data in the region are regarded as contaminated and are rejected, recycle the method reparation of interpolation to reject the data in region,
Finally complete to rebuild using the data after repairing；Iterative method is also referred to as the method for successive approximation, and this method also needs to determine metal just
View field in string figure, and the partial data is rejected, certain priori knowledge is then introduced, the method for iteration is passed through
It is rebuild using the projection of missing data, essence is to belong to missing data reconstruction；Composed correction method is interpolation method and iteration
The mixing of method is mainly characterized by not completing not instead of once in interpolation repair process, gradually be repaired by the process of continuous iteration
The data for the metallic region rejected again finally complete reconstruction.These methods require first to be split positioning to metallic region, and
The contaminated data removal navigated to, then passes through or the method for interpolation or iteration or interpolation and iterative mixing is to removal
Data carry out reconstruction.Therefore, it is in nonmetallic substance that these algorithms more detect target suitable for Medical CT
The case where including limited regional metal substance, then cannot effectively fit for detecting the case where target is all-metal substance in industry CT
With.
Summary of the invention
In order to overcome the shortcomings in the prior art, the present invention provides a kind of CT based on energy spectral filter and Image Residual re-projection
Image metal artifact bearing calibration, for two main elements for causing metal artifacts: beam hardening and photon equilibrium state are ground
Study carefully, according to estimation power spectrum and filbtercharacteristic, designs optimal optical filter, protected to the full extent while filtering out low energy ray
The effective X-ray for remaining for imaging is avoided using collimator after complicated detector, by spuious outside target area
Noise extracts and re-projection, removes a large amount of scattered signal in former projected image, effectively increases image quality and is detected
Survey the contrast of target inside and outside contour.
According to design scheme provided by the present invention, a kind of CT image gold based on energy spectral filter and Image Residual re-projection
Belong to artifact correction method, comprise the following steps:
Step 1, according to attenuation rate change curve and effective rays transmitance, design optimal optical filter；
Step 2 estimates shot noise outside original reconstruction image target area, by residual error re-projection method in original
Begin to remove shot noise in projection, obtains amendment projection；
Step 3 projects backprojection reconstruction again according to amendment, obtains correction of a final proof image.
Above-mentioned, step 1 specifically includes following content: according to attenuation rate change curve and Beer law formula I=I0e-μl, the thickness of optical filter is calculated；Pass through formula:The effective of different materials optical filter is calculated
Radiolucent rate, analysis are compared, and optimal optical filter is designed, wherein L is the thickness of optical filter, I0It respectively indicates with I and initially penetrates
Line and penetrate transmitted intensity, μ indicate homogeneous substance attenuation coefficient, l indicate substance thickness.
Above-mentioned, step 2 specifically includes following content:
Step 2.1 carries out backprojection reconstruction to original projection, original reconstruction image is obtained, to target in original reconstruction image
Profile extracts, and the voxel value except objective contour in original reconstruction image is set as zero, keeps the voxel in objective contour
It is worth constant, obtains revised target image；
Step 2.2 subtracts revised target image using original reconstruction image, obtains image area residual error；
Step 2.3 carries out low-pass filtering to image area residual error, and carries out scattered signal estimation；
Step 2.4 carries out re-projection according to scattered signal estimated result, and scattered signal is transformed into projection from image area
Domain；
Step 2.5, the re-projection that deduction step 2.4 obtains in original projection are as a result, obtain amendment projection.
Beneficial effects of the present invention:
1, the present invention is for two main elements for causing metal artifacts: beam hardening and photon equilibrium state are studied, root
Power spectrum and filbtercharacteristic according to estimates design optimal optical filter, retain use to the full extent while filtering out low energy ray
In effective X-ray of imaging, avoid using collimator after complicated detector, by clutter noise outside target area
It extracts and re-projection, removes a large amount of scattered signal in former projected image, effectively increase image quality and detected mesh
Mark the contrast of inside and outside contour.
2, the present invention is calculated according to initial ray power spectrum and the relevant substance attenuation coefficient change rate curve of energy first
To filter thickness；And optimal optical filter is determined compared with effective high-energy ray transmitance of different materials optical filter by calculating ratio；
It is extracted and is deducted by clutter noise outside original reconstruction image target area, can effectively inhibit the scattering in original image pseudo-
Shadow improves picture contrast；The design method of optical filter is suitable for a variety of situations, robustness with higher；Lead in the present invention
It crosses residual error re-projection method and removes shot noise in original projection, do not depend on collimator or scattering after complicated detector
Correcting plate does not need the priori conditions such as Imaging physics yet, can remove a large amount of shot noise in former projected image, effectively improve
It is detected the image quality of target inside and outside contour.
Detailed description of the invention:
Fig. 1 is flow diagram of the invention；
Fig. 2 is the flow chart of the invention for removing shot noise in original projection by residual error re-projection method；
Fig. 3 is aircraft engine turbo blade schematic diagram used in confirmatory experiment in embodiment two；
Fig. 4 is the corresponding effective rays transmitance table of different optical filters；
Turbo blade reconstructed results schematic diagram when Fig. 5 is free of light filter；
Fig. 6 is using turbo blade reconstructed results schematic diagram when different optical filters；
Fig. 7 is to use three kinds of copper, lead, tungsten filter plate reconstructed results image signal-to-noise ratio computation chosen area schematic diagrames；
Fig. 8 is selected trizonal average signal-to-noise ratio in Fig. 7；
Fig. 9 is to use image area residual sum projection domain residual error schematic diagram in the correction course of tungsten optical filter；
Figure 10 is the correction of a final proof image schematic diagram obtained using the present invention.
Specific embodiment:
The generation of metal artifacts is that the comprehensive of many factors embodies, and can substantially be divided into ray hardened effect, photon equilibrium state
Effect, electronics starvation effect and partial volume effect, wherein beam hardening is principal element.It is all-metal for detection target
The case where substance, when detecting voltage and reaching several hundred kev, X-ray and matter interaction are leading, and golden with Compton scattering
Belonging to again is strong scattering source, at this point, metal artifacts can be regarded as beam hardening effect and photon equilibrium state effect is leading total
The result of same-action.
Wherein, the beam hardening origin cause of formation is known as single energy ray by the molecular ray of the light of identical energy.In general, CT is rebuild
Algorithm, parsing and Class of Iterative algorithm including the overwhelming majority, being all based on X-ray is single energy.And in practice, current commercialization
X-ray is all continuous multi-power spectrum.Wherein, sigmatron penetrate object ability it is strong, low energy X ray penetrates the ability of object
Relatively weak, after this X-ray for allowing for continuous multi-power spectrum passes through object, the X-ray of low energy is easier to be attenuated, to make
The X-ray energy spectrum that detector receives is changed compared to incident X-rays power spectrum.This variation shows as received X-ray
Middle high-energy component ratio increases, i.e., so-called beam hardening phenomenon.
When ray is single energy, by Beer law formula (1) it is found that ray when ray passes through homogeneous substance meets:
I=I0e-μl, (1)
Wherein, I0Initial ray is respectively indicated with I and through transmitted intensity, μ indicates that homogeneous substance attenuation coefficient, l indicate
Substance thickness.By formula (1) it is found that data for projection meets:
That is, projection value and penetration thickness l meet linear relationship.
When ray is multipotency, Beer law formula can be expressed as
I=I0∫S(E)e-∫u(E)dldE, (3)
It is linearly closed as can be seen that projection value p and penetration thickness l no longer meets when u (E) is not constant, when multipotency ray
System.Therefore, the essential reason that beam hardening artifact generates is: incident X-rays are continuous multi-power spectrums, and same substance is not to
The damping capacity of co-energy X-ray is different.
In addition, the scatter artefacts origin cause of formation, when scattering is that ray passes through object, the electron outside nucleus hair of incident X-rays and material atom
Raw collision, to reduce energy and change the process of transmission direction.When scattering, detector visits the X-ray that member receives
Not only there is the photon of direct incidence, but also has the photon of scattering.These scattered photons can make to visit first signal deviation true value, cause
There are scatter artefacts in CT image.Under normal conditions, scattered photon be random directive from all directions, scattering cross section is relatively flat
It is sliding.When many rays scatter, it is equivalent to and has been superimposed one layer of low-frequency noise in original signal.Scattered photon is to projection value
It must influence state are as follows:
Wherein, IsIndicate scattered photon intensity.
Therefore, scattered ray can make image generate shade, cup-shaped as an important noise source in CT imaging process
Artifact and strip artifact etc. cause soft edge, degradation in contrast, picture quality are seriously reduced, especially in industry
When metallics being imaged in CT, Scattering Factors specific gravity shared in metal artifacts is significantly increased, it is necessary to scattering
Artifact is inhibited.
The present invention is described in further detail with technical solution with reference to the accompanying drawing, and detailed by preferred embodiment
Describe bright embodiments of the present invention in detail, but embodiments of the present invention are not limited to this.
Embodiment one, a kind of shown in Figure 1, CT image metal artifact based on energy spectral filter and Image Residual re-projection
Bearing calibration comprises the following steps:
Step 1, according to attenuation rate change curve and effective rays transmitance, design optimal optical filter；
Step 2 estimates shot noise outside original reconstruction image target area, by residual error re-projection method in original
Begin to remove shot noise in projection, obtains amendment projection；
Step 3 projects backprojection reconstruction again according to amendment, obtains correction of a final proof image.
The present invention passes through the optimal optical filter of design first, filters out the part for being easy to harden in original power spectrum, while retaining most
More effective rays；Then, shot noise outside reconstruction image target area is estimated, is existed by the method for residual error re-projection
Shot noise is deducted in original projection；Finally, projecting backprojection reconstruction again using amendment, correction of a final proof image is obtained, realizes CT
The metal artifacts reduction of image can effectively inhibit metal artifacts caused by beam hardening and photon equilibrium state to influence, and improve image
Contrast and improvement image quality.
Embodiment two, referring to shown in Fig. 1~10, it is a kind of based on can spectral filter and Image Residual re-projection CT image metal
Artifact correction method includes following content:
Step 1, according to attenuation rate change curve and effective rays transmitance, design optimal optical filter, substance attenuation rate becomes
Changing curve is material mass attenuation coefficient to the absolute value of photon energy partial derivative, has reacted substance attenuation coefficient with photon energy
The severe degree for changing and changing indicates substance attenuation coefficient not with the change of photon energy when attenuation coefficient becomes cancellation
And change, i.e., attenuation coefficient has constant value in continuum；And when attenuation coefficient change rate increases, indicate that substance declines
Subtracting the degree that coefficient changes with photon energy and changed to increase, i.e. otherness of the attenuation coefficient under different photon energies increases,
For attenuation coefficient change rate close to zero energy section, substance attenuation coefficient is approximately equal, and projection value meets as follows at this time:, when S (E) indicate normalization can time spectrum, ∫ S (E) dE=1, therefore, p ≈ ul, i.e. projection value with wear
Saturating length l approximation meets linear relationship, meet based on it is single it can be assumed that algorithm for reconstructing requirement, use the ray of this section of power spectrum to carry out
Beam hardening artifact can effectively be inhibited when imaging, can determine that needs filtered out is easy to harden by attenuation coefficient change rate curve
Part, and according to attenuation rate change curve and Beer law formula I=I0e-μl, the thickness of optical filter is calculated；Effective rays
Transmitance refers to the ratio that initial ray is accounted for through the ray after optical filter to be imaged, and optical filter is to filter out low energy X ray same
When, it can also filter sigmatron；In CT imaging, the size of statistical noise and the size of effective X-ray intensity are closely related,
Statistical noise can effectively be inhibited by increasing transmitted intensity, improve at image signal-to-noise ratio, pass through formula:
The effective rays transmitance of different materials optical filter is calculated, analysis is compared, and optimal optical filter is designed, so as to be imaged
Transmitted intensity highest, inhibit hardening artifact while, improve the signal-to-noise ratio of image, wherein L be optical filter thickness, I0And I
It respectively indicates initial ray and through transmitted intensity, μ indicates homogeneous substance attenuation coefficient.
Step 2 estimates shot noise outside original reconstruction image target area, by residual error re-projection method in original
Begin to remove shot noise in projection, obtains amendment projection, based on two physics facts in scattering imaging, 1) original reconstruction image
Nonzero value except middle objective contour should be artifact, this part artifact is mainly derived from the shot noise in original projection；2) it dissipates
Penetrating signal is a kind of low frequency signal, and it includes as follows for removing shot noise specifically in original projection by residual error re-projection method:
Step 2.1 carries out backprojection reconstruction to original projection, original reconstruction image is obtained, to target in original reconstruction image
Profile extracts, and the voxel value except objective contour in original reconstruction image is set as zero, keeps the voxel in objective contour
It is worth constant, obtains revised target image, conventional image procossing mode can be used to carry out the extraction of objective contour, for example,
Use the image partition method based on threshold value；
Step 2.2 subtracts revised target image using original reconstruction image, obtains image area residual error；
Step 2.3 carries out low-pass filtering to image area residual error, and carries out scattered signal estimation；
Step 2.4 carries out re-projection according to scattered signal estimated result, and scattered signal is transformed into projection from image area
Domain；
Step 2.5, the re-projection that deduction step 2.4 obtains in original projection are as a result, obtain amendment projection.
Step 3 projects backprojection reconstruction again according to amendment, obtains correction of a final proof image.
The present invention does not depend on collimator or scatter removal after complicated detector, does not need Imaging physics etc. first yet
Test condition, a large amount of shot noise in former projected image can be removed, effectively improve detected target inside and outside contour at image quality
Amount.
Below by specific experiment, the present invention will be further described:
Engine turbine blade is imaged using actual industrial CT system, in actual industrial CT system, radiographic source is
U.S. Varian HPX-450-11, detector be Germany PerkinElmer-XRD-1621, aircraft engine turbo blade, such as
Shown in Fig. 3, and the picture quality of correction front and back is compared, is filtered respectively in no filter plate and using aluminium, copper, lead and four kinds of tungsten
Image Acquisition and reconstruction have been carried out to engine turbine blade in the case where wave plate.By calculate, different optical filters it is most munificent
Degree is respectively 150mm, 16mm, 1mm and 0.8mm, and corresponding effective rays transmitance is as shown in figure 4, compared by the numerical value of Fig. 4 can
It is successively tungsten, lead, copper, aluminium, i.e. having for imaging from high in the end to X-ray retention characteristic effective in initial power spectrum to find out
Imitating X-ray intensity is successively tungsten, lead, copper, aluminium from big to small.Therefore, based on the effective X-ray intensity in front and imaging signal to noise ratio
It analyzes, in four kinds of materials, tungsten is the optical filter of override selection, what it can hold up to while filtering out low energy X ray
Better image signal-to-noise ratio is able to maintain while effective sigmatron, i.e. correction hardening artifact.
Experiment condition is tube voltage 300kv, 1 degree is divided between projection acquisition, 360 width of acquired projections number, image reconstruction uses
FDK algorithm, reconstructed results when no filter plate are as shown in figure 5, using the reconstructed results of four kinds of optical filters respectively as shown in fig. 6, Fig. 6
In: (a) indicate that the reconstructed results using aluminium filter plate, (b) indicate that the reconstructed results using copper filter plate, (c) indicate to use lead
The reconstructed results of filter plate, (d) indicate to use the reconstructed results of tungsten filter plate, as seen from the figure, original when without using optical filter
A large amount of metal artifacts are contained in reconstruction image, picture contrast is poor, and edge is very fuzzy, especially blade concave edges
Image by artifact is the most serious.As seen from Figure 6 using after optical filter, hardening artifact has obtained very great Cheng in original image
The removal of degree, blade edge are still become apparent to enhancing is compared by artifact effects concave edges the most serious, but
It is that good apparently without other optical filters using the correction result of aluminium optical filter, this is primarily due to aluminium and is filtering out low energy X ray
A large amount of sigmatron has been filtered out simultaneously, has caused effective X-ray to be imaged to greatly reduce, so intuitively going to see image
Noise is maximum, this is consistent with analysis herein above；And the calibration result of copper, lead and tungsten is more preferable, this is primarily due to these three
Optical filter can retain more sigmatron when filtering out low energy X ray.Intuitive observation, compares lead and tungsten optical filter, uses
Noise becomes apparent in the result of copper optical filter.For the function and effect for further comparing verifying copper, three kinds of lead, tungsten filter plates, lead to
The signal noise ratio (snr) of image for comparing three kinds of filter plate reconstructed results is crossed, the selection in signal-to-noise ratio computation region is as shown in fig. 7, trizonal
Average signal-to-noise ratio as shown in figure 8, the signal noise ratio (snr) of image obtained when using copper filter plate be 5.14, obtained using lead filter plate
Signal noise ratio (snr) of image is 6.56, and the signal noise ratio (snr) of image obtained using tungsten filter plate is 6.64, is illustrated in above 4 in material, tungsten
Effect is best.
Corrected by hardening artifact, original reconstructed image quality is obviously improved, blade edge obtained restore and
Contrast is promoted, and still, is influenced by scatter artefacts, has a large amount of clutter noise outside image target area, so that edge contrast
It spends still not ideal enough.Therefore, the correction based on tungsten optical filter is herein corrected scatter artefacts as a result, further using,
Wherein, image area residual sum projection domain residual error in correction course is respectively as shown in figure 9, (a) indicates image area residual error in Fig. 9,
(b) projection domain residual error is indicated, (a) can be seen that in Fig. 9, and a large amount of noise signal is contained in original image, uses the side this paper
Method is extracted these noises and is transformed into the result such as Fig. 9 of projection domain shown in (b), and final correction result images are by Figure 10 institute
Show, by being deducted in original projection after noise signal and largely clutter noise is efficiently removed in the result rebuild again,
Picture quality has clear improvement, and picture contrast further increases, and blade outer edge is more clear.
The present invention is directed to two main elements for causing metal artifacts: beam hardening and photon equilibrium state are studied, according to
Estimate power spectrum and filbtercharacteristic, designs optimal optical filter, be preserved for the full extent while filtering out low energy ray
Effective X-ray of imaging, avoids using collimator after complicated detector, by clutter noise outside target area into
Row extracts and re-projection, removes a large amount of scattered signal in former projected image, effectively increases image quality and detected target
The contrast of inside and outside contour.
The invention is not limited to above-mentioned specific embodiment, those skilled in the art can also make a variety of variations accordingly,
But it is any all to cover within the scope of the claims with equivalent or similar variation of the invention.
Claims (2)
1. a kind of CT image metal artifact bearing calibration based on energy spectral filter and Image Residual re-projection, it is characterised in that: packet
Containing following steps:
Step 1, according to attenuation rate change curve and effective rays transmitance, design optimal optical filter；
Step 2 estimates shot noise outside original reconstruction image target area, by residual error re-projection method in original throwing
Shot noise is removed in shadow, obtains amendment projection；Specifically include following content:
Step 2.1 carries out backprojection reconstruction to original projection, original reconstruction image is obtained, to objective contour in original reconstruction image
It extracts, the voxel value except objective contour in original reconstruction image is set as zero, the voxel value in holding objective contour is not
Become, obtains revised target image；
Step 2.2 subtracts revised target image using original reconstruction image, obtains image area residual error；
Step 2.3 carries out low-pass filtering to image area residual error, and carries out scattered signal estimation；
Step 2.4 carries out re-projection according to scattered signal estimated result, and scattered signal is transformed into projection domain from image area；
Step 2.5, the re-projection that deduction step 2.4 obtains in original projection are as a result, obtain amendment projection；
Step 3 projects backprojection reconstruction again according to amendment, obtains correction of a final proof image.
2. the CT image metal artifact correction side according to claim 1 based on energy spectral filter and Image Residual re-projection
Method, it is characterised in that: the step 1 specifically includes following content: according to attenuation rate change curve and Beer law formula I=
I0e-μl, the thickness of optical filter is calculated；Pass through formula:Having for different materials optical filter is calculated
Radiolucent rate is imitated, analysis is compared, and optimal optical filter is designed, wherein L is the thickness of optical filter, I0It is respectively indicated initially with I
Ray and penetrate transmitted intensity, μ indicate homogeneous substance attenuation coefficient, l indicate substance thickness, S (E) indicate normalization power spectrum.
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| CN109540930B (en) * | 2018-09-17 | 2021-05-11 | 南京航空航天大学 | Multi-energy spectrum X-ray imaging scattering estimation and correction method |
| CN111096761B (en) * | 2018-10-29 | 2024-03-08 | 上海西门子医疗器械有限公司 | Method, device and related equipment for correcting scattering of wedge-shaped filter |
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| CN111476856B (en) * | 2020-04-08 | 2023-06-06 | 中北大学 | Multispectral CT imaging method |
| CN112508808B (en) * | 2020-11-26 | 2023-08-01 | 中国人民解放军战略支援部队信息工程大学 | CT double-domain combined metal artifact correction method based on generation countermeasure network |
| CN113177991A (en) * | 2021-04-29 | 2021-07-27 | 中国科学技术大学 | Method for correcting scattering artifacts in CBCT (cone beam computed tomography) based on planned CT (computed tomography) |
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