CN113140016A - Metal artifact correction method and system of CBCT (Cone Beam computed tomography) equipment - Google Patents
Metal artifact correction method and system of CBCT (Cone Beam computed tomography) equipment Download PDFInfo
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Abstract
The invention provides a metal artifact correction method of CBCT equipment, which comprises the steps of extracting a metal target position from scanning data, projecting all pixels of the metal target position onto the surface of a detector, obtaining a coordinate value of the metal target on the surface of the detector, carrying out interpolation repair on the coordinate value, carrying out three-dimensional reconstruction on the repaired data, and adding the initially extracted metal target into a reconstructed three-dimensional image. The invention has the advantages that: after the metal target is separated from the original image, the metal target is mapped to the surface of the detector, then interpolation repairing is carried out, the area corresponding to the metal target is interpolated and subjected to three-dimensional reconstruction processing, and then the originally extracted metal target is added back, so that a three-dimensional image without artifacts generated by the metal target is obtained, and the correction of the metal target and the artifacts is conveniently and quickly realized.
Description
Technical Field
The invention relates to the technical field of image processing of CBCT equipment, in particular to a metal artifact correction method and a metal artifact correction system of CBCT equipment.
Background
Among oral medical devices, CBCT is a multidisciplinary cross-technology complex system medical device integrating mechanics, electrical, electronics, software and algorithms. The clarity of the CBCT image ultimately presented to the user is affected by a number of links in the system, with metal artifacts being one of the most significant contributing factors. The multi-energy property of X-ray in CBCT imaging is such that when X-ray passes through a high density material, such as a metal target, the low energy X-ray in the energy spectrum is attenuated, the physical process hardens the energy spectrum of X-ray, and the attenuation coefficient of the tissue is relatively reduced when X-ray passes through other tissues behind the high density target. This is the principle of the formation of hardening artifacts in CBCT and is also the cause of metal artifacts.
In order to clearly observe the details of the oral tissue structures such as teeth in clinical application, the CBCT imaging algorithm needs to perform metal artifact removal processing. The metal artifact removal process has two core processing steps, one of which is the positioning of the metal object in the projection, and the other is the interpolation process of the metal object. The former step is the basis of the latter step, so the positioning of the metal target is crucial in the metal artifact removal process flow.
The positioning of the metal object in the projection generally adopts a method that a metal object area is firstly segmented in a reconstructed CBCT image, then the CBCT image and the metal object image are respectively projected to a two-dimensional projection image in a forward direction, and the pixel position of the metal object is identified in the projection area. Another approach also requires first calculating the forward projections of both volume data, then transforming the projection data to the sinogram domain, and identifying the coordinate locations of the metal regions in the sinogram. Both methods need to use forward projection processing, and the forward projection method needs to perform interpolation in a three-dimensional volume image domain and then weight accumulation processing, which greatly increases the complexity of calculation and reduces the efficiency of three-dimensional reconstruction for removing metal artifacts.
For example, the invention patent application with publication number CN111815521A discloses a cone-beam CT metal artifact correction algorithm based on prior images, which performs forward projection on a metal image and a prior image, then performs interpolation restoration, and fuses the restored image with an image segmented by a threshold after reconstruction, thereby achieving the purpose of metal artifact correction. When the method is used for forward projection, the whole volume data of the prior volume image and the whole volume data of the metal volume image need to be respectively operated, and the data processing amount is large and time is consumed.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a metal artifact correction method only for calculating the pixel coordinates of a metal target, so that the calculation speed is increased.
The invention solves the technical problems through the following technical scheme: a metal artifact correction method for CBCT equipment comprises the steps of extracting a metal target position from volume data obtained by scanning, projecting all voxels of the metal target position onto the surface of a detector, obtaining a coordinate value of the metal target on the surface of the detector, carrying out interpolation repair on the coordinate value, carrying out three-dimensional reconstruction on repaired data, and adding an initially extracted metal target image into a reconstructed three-dimensional image.
After the metal target is separated from the original image, the metal target is mapped to the surface of the detector, then interpolation repairing is carried out, the area corresponding to the metal target is subjected to interpolation processing, then the three-dimensional image is reconstructed, and the originally extracted metal target image is added back, so that the three-dimensional image without the artifacts generated by the metal target is obtained, and the correction of the metal artifacts is conveniently and quickly realized.
Preferably, an image coordinate system is constructed by taking the rotation center O of the ray source S and the detector as the original points, a detector coordinate system is constructed by taking the central point D of the detector as the original point, the original point D of the detector coordinate system is collinear with the SO, the ray source S and the detector are respectively positioned at two sides of the rotation center O, the X direction of the image coordinate system points to the ray source S, the Z direction is the sky direction, and the Y direction is determined according to the right-hand rule; the X direction of the detector coordinate system points to the ray source S, the V direction is the sky direction, and the U direction is determined according to the right-hand rule;
for a point P (x, y, z) in the image coordinate system, after the source S has rotated an angle θ from the starting position, the coordinate of the point P in the detector coordinate system is Pθ(u,v,xθ) Then, then
[xθ,u,v,1]＝[x,y,z,1]*Rθ*R (1)
Wherein R isθIs a rotation matrix representing the coordinate transformation relationship of the same point in the image coordinate system before and after rotation, and R is a transformation matrix from the image coordinate system to the detector coordinate system
Wherein (x)c,uc,vc) Expressing the coordinates of the O point in the coordinate system of the detector, substituting the formulas (2) and (3) into the formula (1) to obtain
xθ＝xcosθ+ysinθ+xc (4)
u＝-xsinθ+ycosθ+uc (5)
v＝z+vc (6)
As can be seen from the definition of the coordinate system,
xc＝DSD-DsO (7)
uc＝0 (8)
vc＝0 (9)
wherein D isSDRepresenting the distance, D, of the source S from a point DSORepresents the distance of the source S from the point O; point Pθ(u,v,xθ) Projected point coordinates P on the detector surfaceθ,d(ud,vd) There is a relationship that the following is present,
su，svrepresenting the scale factor of spatial points in the detector coordinate system mapped to the detector surface, as can be seen from equation (4)
su＝sv＝DSD/(DSD-xθ) (11)
Substituting the equations (5), (6), (8), (9) and (11) into the equation (10) yields
ud＝(-xsinθ+ycosθ)*DSD/(DSO-xcosθ-ysinθ) (12)
vd＝z*DSD/(DSO-xcosθ-ysinθ) (13)
Equations (12) and (13) are the conversion equations for mapping the metal target locations to the detector surface.
Preferably, the method for extracting the metal target from the volume data acquired by scanning comprises the following steps:
reconstructing projection data by using FDK algorithm to obtain volume image VCBCTExtracting the volume image V by adopting a global threshold methodCBCTMetal target V inMetal。
Preferably, the pixel coordinates I are obtained by mapping the metal target onto the detector surfaceMetalFor pixel coordinate IMetalAnd (4) repairing by a bilinear interpolation method to obtain a repaired metal target.
Preferably, the three-dimensional reconstruction processing is performed on the repaired projection data through an FDK algorithm, and a metal target image extracted from the volume data obtained by scanning is added into the reconstructed image, so that a CBCT three-dimensional reconstructed image with metal artifacts removed is obtained.
The invention also provides a metal artifact correction system of the CBCT equipment, which comprises
A three-dimensional reconstruction module: reconstructing projection data by using an FDK algorithm to obtain a target volume image;
a metal extraction module: dividing the target volume image by using a global threshold method to obtain a metal target volume image;
a target mapping module: mapping the metal target to the surface of the detector to obtain a coordinate value of the metal target on the surface of the detector;
an interpolation patching module: carrying out bilinear interpolation operation on the coordinate value of the metal target on the surface of the detector to repair the metal target;
an artifact removal module: and carrying out three-dimensional reconstruction on the repaired projection data by using an FDK algorithm, and adding the metal target image extracted by the metal extraction module into the reconstructed image.
The metal artifact correction method and the metal artifact correction system for the CBCT equipment have the advantages that: after a metal target is separated from an original image, the metal target is mapped to the surface of a detector, then interpolation repairing is carried out, a region corresponding to the metal target is subjected to interpolation processing, then a three-dimensional image is reconstructed, and the original extracted metal target image is added back, so that a three-dimensional image without artifacts generated by the metal target is obtained, the correction of the metal artifacts is conveniently and quickly realized, the data operation amount and complexity are obviously reduced, the image processing speed is greatly improved, and a CBCT scanning image can be quickly obtained. The calculation process is simple, rapid and accurate, the effect of removing the metal artifacts can be effectively improved, the operation efficiency of the three-dimensional reconstruction algorithm for removing the metal artifacts is improved, the imaging efficiency can be obviously improved and the waiting time can be reduced in the applications of medical diagnosis, security inspection and the like.
Drawings
Fig. 1 is a flowchart of a metal artifact correction method of a CBCT apparatus according to an embodiment of the present invention;
fig. 2 is a simplified model diagram of a CBCT apparatus according to an embodiment of the present invention.
Detailed Description
To make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention are described below in detail and completely with reference to the accompanying drawings, and it is apparent that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, the present embodiment provides a metal artifact correction method for CBCT equipment, which extracts a metal target image from volume data obtained by scanning, maps the metal target onto a detector surface, obtains a coordinate value of the metal target on the detector surface, performs interpolation repair on the coordinate value, performs three-dimensional reconstruction on the repaired data, and adds the initially extracted metal target image into the reconstructed three-dimensional image.
In the embodiment, after the metal target is separated from the original image, the metal target is mapped to the surface of the detector, then interpolation repairing is performed, the area corresponding to the metal target is subjected to interpolation processing, then the three-dimensional image is reconstructed, and the originally extracted metal target image is added back, so that the three-dimensional image without the artifact generated by the metal target is obtained, and the correction of the metal target and the artifact is conveniently and quickly realized.
Assuming that the total number of pixels of the metal object after the segmentation is N, the method described in this embodiment only needs to calculate 2N values to determine the pixel coordinates of the metal object in a single projection. The forward projection domain method needs to traverse each pixel point in the projection, and the calculation of each pixel point needs to be calculated by utilizing all voxel interpolation values through which the ray passes, so that the calculation processing flow is greatly increased, and the calculation complexity is multiplied. The sinusoidal image domain method also needs to calculate forward projection, and the calculation amount is equivalent to the projection domain method. Therefore, compared with the prior art such as forward projection, the method for performing interpolation processing by mapping a metal target provided by the embodiment significantly reduces the computation amount and complexity of data, greatly improves the image processing speed, and can quickly acquire a CBCT scan image.
Fig. 2 shows a simplified CBCT system, in which point O is the rotation center of the radiation source S and the detector, the object to be scanned is placed between the radiation source S and the detector when in use, and the scanned image of the object can be obtained by rotating the radiation source and the detector more than one turn. In the implementation, an image coordinate system is established by taking an O point as an original point, a detector coordinate system is established by taking a central point D of a detector as an original point, the original point D of the detector coordinate system is collinear with the SO, a ray source S and the detector are respectively positioned at two sides of a rotation center O, in the working process, the X direction of the image coordinate system always points to the ray source S, the Z direction is the sky direction, and the Y direction is determined according to the right-hand rule; the X direction of the detector coordinate system points to the ray source S, the V direction is the sky direction, and the U direction is determined according to the right-hand rule;
for a point P (x, y, z) in the image coordinate system, after the source S has rotated an angle θ from the starting position, the coordinate of the point P in the detector coordinate system is Pθ(u,v,xθ) Then, then
[xθ,u,v,1]＝[x,y,z,1]*Rθ*R (1)
Wherein R isθIs a rotation matrix representing the coordinate transformation relationship of the same point in the image coordinate system before and after rotation, and R is a transformation matrix from the image coordinate system to the detector coordinate system
Wherein (x)c,uc,vc) Expressing the coordinates of the O point in the coordinate system of the detector, substituting the formulas (2) and (3) into the formula (1) to obtain
xθ＝xcosθ+ysinθ+xc (4)
u＝-xsinθ+ycosθ+uc (5)
v＝z+vc (6)
As can be seen from the definition of the coordinate system,
xc＝DSD-DSo (7)
uc＝0 (8)
vc＝0 (9)
wherein D isSDRepresenting the distance, D, of the source S from a point DSORepresents the distance of the source S from the point O; point Pθ(u,v,xθ) Projected point coordinates P on the detector surfaceθ,d(ud,vd) There is a relationship that the following is present,
su，svrepresenting the scale factor of spatial points in the detector coordinate system mapped to the detector surface, as can be seen from equation (4)
su＝sv＝DSD/(DSD-xθ) (11)
Substituting the equations (5), (6), (8), (9) and (11) into the equation (10) yields
ud＝(-xsinθ+ycosθ)*DSD/(DSO-xcosθ-ysinθ) (12)
vd＝z*DSD/(DSO-xcosθ-ysinθ) (13)
Equations (12) and (13) are the conversion equations for mapping the metal target locations to the detector surface.
Through the above relations, only the coordinates (x, y, z) of the target in the image coordinate system, the rotation angle theta of the detector and the imaging geometric parameter D need to be knownSDAnd DSOThe coordinates u, v of the object on the detector surface can be calculated. The calculation process is simple, rapid and accurate, the effect of removing the metal artifacts can be effectively improved, the operation efficiency of the three-dimensional reconstruction algorithm for removing the metal artifacts is improved, the imaging efficiency can be obviously improved and the waiting time can be reduced in the applications of medical diagnosis, security inspection and the like.
The method for extracting the metal target is the prior art, and the FDK algorithm is selected to perform three-dimensional reconstruction on the projection data to obtain a target volume image VCBCTExtracting the volume image V by adopting a global threshold methodCBCTMetal target V inMetal(ii) a The metal target V is then determined according to equations (12) and (13)MetalMapping to detector surface to obtain pixel coordinates IMetalFor pixel coordinate IMetalRepairing by a bilinear interpolation method to obtain repaired projection data; then, the three-dimensional reconstruction processing is carried out on the projection data after the repair through the FDK algorithm, no metal pixel and metal artifact information exist in the image at the moment, and then the original volume image V is obtainedCBCTThe metal target image V extracted from the imageMetalAnd adding the three-dimensional reconstruction image into the reconstructed image to obtain a CBCT three-dimensional reconstruction image with the metal artifacts removed.
The implementation also provides a metal artifact correction system of the CBCT equipment, which comprises
A three-dimensional reconstruction module: reconstructing projection data by using an FDK algorithm to obtain a target volume image;
a metal extraction module: dividing the target volume image by using a global threshold method to obtain a metal target volume image;
a target mapping module: mapping the metal target to the surface of the detector to obtain a coordinate value of the metal target on the surface of the detector;
an interpolation patching module: carrying out bilinear interpolation operation on the coordinate value of the metal target on the surface of the detector to repair the metal target;
an artifact removal module: and carrying out three-dimensional reconstruction on the repaired projection data by using an FDK algorithm, and adding the metal target image extracted by the metal extraction module into the reconstructed image.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A metal artifact correction method of CBCT equipment is characterized in that: extracting the position of a metal target from volume data obtained by scanning, projecting all voxels of the metal target onto the surface of a detector, obtaining the coordinate value of the metal target on the surface of the detector, carrying out interpolation repair on the coordinate value, carrying out three-dimensional reconstruction on the repaired data, and adding an initially extracted metal target image into a reconstructed three-dimensional image.
2. The metal artifact correction method of the CBCT apparatus according to claim 1, wherein: constructing an image coordinate system by taking the rotation center O of the ray source S and the detector as original points, constructing a detector coordinate system by taking the central point D of the detector as an original point, wherein the original point D of the detector coordinate system is collinear with the original point SO, the ray source S and the detector are respectively positioned at two sides of the rotation center O, the X direction of the image coordinate system points to the ray source S, the Z direction is the sky direction, and the Y direction is determined according to the right-hand rule; the X direction of the detector coordinate system points to the ray source S, the V direction is the sky direction, and the U direction is determined according to the right-hand rule;
for a point P (x, y, z) in the image coordinate system, after the source S has rotated an angle θ from the starting position, the point P is in the detector coordinate systemCoordinate in (B) is Pθ(u,v,xθ) Then, then
[xθ,u,v,1]＝[x,y,z,1]*Rθ*R (1)
Wherein R isθIs a rotation matrix representing the coordinate transformation relationship of the same point in the image coordinate system before and after rotation, and R is a transformation matrix from the image coordinate system to the detector coordinate system
Wherein (x)c,uc,vc) Expressing the coordinates of the O point in the coordinate system of the detector, substituting the formulas (2) and (3) into the formula (1) to obtain
xθ＝xcosθ+ysinθ+xc (4)
u＝-xsinθ+ycosθ+uc (5)
v＝z+vc (6)
As can be seen from the definition of the coordinate system,
xc＝DSD-DSO (7)
uc＝0 (8)
vc＝0 (9)
wherein D isSDRepresenting the distance, D, of the source S from a point DSORepresents the distance of the source S from the point O; point Pθ(u,v,xθ) Projected point coordinates P on the detector surfaceθ,d(ud,vd) There is a relationship that the following is present,
su，svspatial point mapping to probe, represented in the probe coordinate systemThe scale factor of the surface of the device can be known according to the formula (4)
su＝sv＝DSD/(DSD-xθ) (11)
Substituting the equations (5), (6), (8), (9) and (11) into the equation (10) yields
ud＝(-xsinθ+ycosθ)*DSD/(DSO-xcosθ-ysinθ) (12)
vd＝z*DSD/(DSO-xcosθ-ysinθ) (13)
Equations (12) and (13) are the conversion equations for mapping the metal target locations to the detector surface.
3. The metal artifact correction method of the CBCT apparatus according to claim 1, wherein: the method for extracting the metal target from the volume data acquired by scanning comprises the following steps:
reconstructing projection data by using FDK algorithm to obtain volume image VCBCTExtracting the volume image V by adopting a global threshold methodCBCTMetal target V inMetal。
4. The metal artifact correction method of the CBCT apparatus according to claim 1, wherein: mapping metal target to detector surface to obtain pixel coordinate IMetalFor pixel coordinate IMetalAnd (4) patching by a bilinear interpolation method to obtain patched projection data.
5. The metal artifact correction method of the CBCT equipment as recited in claim 4, wherein: and (3) carrying out three-dimensional reconstruction processing on the repaired projection data through an FDK algorithm, and adding a metal target image extracted from the volume data obtained by scanning into the reconstructed image to obtain a CBCT three-dimensional reconstructed image with metal artifacts removed.
6. A metal artifact correction system of a CBCT device is characterized in that: comprises that
A three-dimensional reconstruction module: carrying out three-dimensional reconstruction on the projection data to obtain a target volume image;
a metal extraction module: separating the target volume image to obtain a metal target volume image;
a target mapping module: mapping the metal target to the surface of the detector to obtain a coordinate value of the metal target on the surface of the detector;
an interpolation patching module: carrying out interpolation repair on the coordinate value of the metal target on the surface of the detector;
an artifact removal module: and performing three-dimensional reconstruction on the repaired projection data, and adding the metal target image extracted by the metal extraction module into the reconstructed image.
7. The metal artifact correction system of the CBCT apparatus of claim 6, wherein: constructing an image coordinate system by taking the rotation center O of the ray source S and the detector as original points, constructing a detector coordinate system by taking the central point D of the detector as an original point, wherein the original point D of the detector coordinate system is collinear with the original point SO, the ray source S and the detector are respectively positioned at two sides of the rotation center O, the X direction of the image coordinate system points to the ray source S, the Z direction is the sky direction, and the Y direction is determined according to the right-hand rule; the X direction of the detector coordinate system points to the ray source S, the V direction is the sky direction, and the U direction is determined according to the right-hand rule;
for a point P (x, y, z) in the image coordinate system, after the source S has rotated an angle θ from the starting position, the coordinate of the point P in the detector coordinate system is Pθ(u,v,xθ) Then, then
[xθ,u,v,1]＝[x,y,z,1]*Rθ*R (1)
Wherein R isθIs a rotation matrix representing the coordinate transformation relationship of the same point in the image coordinate system before and after rotation, and R is a transformation matrix from the image coordinate system to the detector coordinate system
Wherein (x)c,uc,vc) Expressing the coordinates of the O point in the coordinate system of the detector, substituting the formulas (2) and (3) into the formula (1) to obtain
xθ＝xcosθ+ysinθ+xc (4)
u＝-xsinθ+ycosθ+uc (5)
v＝z+vc (6)
As can be seen from the definition of the coordinate system,
xc＝DSD-DSO (7)
uc＝0 (8)
vc＝0 (9)
wherein D isSDRepresenting the distance, D, of the source S from a point DSORepresents the distance of the source S from the point O; point Pθ(u,v,xθ) Projected point coordinates P on the detector surfaceθ,d(ud,vd) There is a relationship that the following is present,
su，svrepresenting the scale factor of spatial points in the detector coordinate system mapped to the detector surface, as can be seen from equation (4)
su＝sv＝DSD/(DSD-xθ) (11)
Substituting the equations (5), (6), (8), (9) and (11) into the equation (10) yields
ud＝(-xsinθ+ycosθ)*DSD/(DSO-xcosθ-ysinθ) (12)
vd＝z*DSD/(DSO-xcosθ-ysinθ) (13)
Equations (12) and (13) are the conversion equations for mapping the metal target locations to the detector surface.
8. The metal artifact correction system of the CBCT apparatus of claim 6, wherein: the three-dimensional reconstruction module reconstructs projection data by using an FDK algorithm to obtain a volume image VCBCTThe metal extraction module adopts a global threshold value method to segment and extract the volume image VCBCTMetal object image V in (1)Metal。
9. The metal artifact correction system of the CBCT apparatus of claim 6, wherein: the target mapping module maps the metal target to the surface of the detector to obtain pixel coordinates IMetalInterpolation patching module to pixel coordinate IMetalAnd (4) patching by a bilinear interpolation method to obtain patched projection data.
10. The metal artifact correction system of the CBCT apparatus of claim 9, wherein: and the artifact removing module carries out three-dimensional reconstruction processing on the repaired projection data through an FDK algorithm, and adds a metal target image extracted from the volume data obtained by scanning into a reconstructed image to obtain a CBCT three-dimensional reconstructed image with the metal artifact removed.
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