CN105997126B - A kind of cone-beam CT system geometric parameter calibration model and method - Google Patents
A kind of cone-beam CT system geometric parameter calibration model and method Download PDFInfo
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Abstract
The invention discloses cone-beam CT system geometric parameter bearing calibrations, comprising the following steps: the data for projection of calibration model under one projection angle of acquisition；The middle line equation that every linear target is determined according to model projection data, then finds out middle line intersection position, and then finds out the side length that four linear targets project the quadrangle of composition on the detector.Solve φ using the ratio between the left side and the right, top and it is the ratio between following solve θ, Δ D can be calculated according to the side length of any one projection quadrilateral.Using φ, θ, and projection coordinate of two adjacent straight-line intersections on the detector plane for offseting by Δ D there are φ, θ and along the x axis in calibration template, solve η, Δ u, Δ v.After acquiring geometric parameter, carries out band parameter FDK and rebuild, the reconstruction image after being corrected.The present invention improves on the basis of four point models, proposes wire-frame model, which had both remained four point models and found out 6 geometric parameters, the small advantage of operand by once projection, and improved noise immunity compared to dotted model.
Description
Technical field
The invention belongs to technical field of nondestructive testing, and in particular to a kind of to determine for the correction of cone-beam CT system geometric parameter
Mark model and method.
Background technique
Computerized tomography (Computed Tomography, CT) technology is a kind of important non-destructive testing technology, tool
There are high-resolution, high sensitivity and multi-level.The most representative algorithm for reconstructing of cone-beam CT system is that FDK rebuilds calculation
Method, and FDK algorithm requires cone-beam CT imaging system to must satisfy ideal imaging geometry, i.e. radiographic source, rotation center with
And the line at detector center intersects vertically perpendicular to detector plane, and with the shaft of turntable.And actual Cone-Beam CT system
System is difficult to meet the ideal relationship, will lead to reconstruction image and serious artifact occurs, reduces reconstruction precision.It therefore must be to Cone-Beam CT
System carries out the correction of geometric parameter, improves reconstructed image quality.
Yi Sun et al. is in " A Calibration Method for Misaligned Scanner Geometry in
A kind of bearing calibration based on point model is proposed in a Cone-beam Computed Tomography " text.This method be
Four dotted metal balls are inlayed on one poly (methyl methacrylate) plate, this four metal balls are located on four vertex of square, so
It can be obtained by the projected position of four metal balls on the detector afterwards.Pass through the opposite geometry between this four metal spheric projections
Positional relationship can successively calculate the geometric parameter of detector.But the dotted model obtains under the CT system there are geometric error
To reconstruction image in there is more serious distortion, this problem will lead to several in the very high micro-nano CT system of resolution requirement
What parametric solution accuracy is not high, so that artifact occurs in reconstruction image.
Summary of the invention
In view of this, the invention discloses a kind of cone-beam CT system geometric parameter calibration model and methods.
An object of the present invention is achieved through the following technical solutions: a kind of cone-beam CT system geometric parameter correction
Model, including calibration template, are provided with 4 disjoint linear targets in calibration template, 4 linear targets and respectively to two
The extended line at end surrounds square.
The second object of the present invention is to what is be achieved through the following technical solutions: a kind of cone-beam CT system geometric parameter correction
Method,
S1: the projection number of model is obtained using the data for projection that detector collects calibration template under a projection angle
According to；
S2: determining the middle line equation of every linear target according to the data for projection of model first, then according to adjacent two
Middle line finds out intersection position, and then finds out the side length that four linear targets project the quadrangle of composition on the detector；Finally, sharp
Solved with the ratio between the projection quadrilateral left side and the right and around central series rotate angle φ, using projection quadrilateral top with below it
Angle, θ is rotated around central row than solving；After obtaining φ and θ, Δ D is calculated according to the side length of any one projection quadrilateral；
S3: rotating angle φ around central series, rotate angle, θ around central row using what step S2 was acquired, and calibration template
Upper two adjacent straight-line intersections around central series rotate angle φ, angle, θ is rotated around central row and offset by along the x axis existing
Projection coordinate on the detector plane of Δ D can solve η, Δ u, Δ v；
S4: after acquiring six geometric parameters, carrying out band parameter FDK and rebuild, the reconstruction image after being corrected.
Advantageous effects:
The present invention improves on the basis of four point models, proposes a kind of wire-frame model, which both remains four
Point model finds out 6 geometric parameters, the small advantage of operand by once projection, and improves anti-noise compared to dotted model
Property.
Detailed description of the invention
To make the objectives, technical solutions, and advantages of the present invention clearer, below in conjunction with attached drawing to the present invention make into
The detailed description of one step, in which:
Fig. 1 wire-frame model structural schematic diagram；
Fig. 2 ideal geometrical structure cone-beam CT system perspective view；
The non-ideal geometry cone-beam CT system perspective view of Fig. 3；
Fig. 4 bearing calibration flow chart；
Front view under the non-ideal structure of Fig. 5；
Fig. 6 is the side view of Fig. 5；
Fig. 7 is the top view of Fig. 5；
Fig. 8 is the side view of the other side Fig. 5；
Relation schematic diagram between Fig. 9 angle Г and angle, θ；
Figure 10 plane P2With plane P3Relation schematic diagram；
Figure 11 plane P3With plane P4Relation schematic diagram.
Specific embodiment
Below with reference to attached drawing, a preferred embodiment of the present invention will be described in detail；It should be appreciated that preferred embodiment
Only for illustrating the present invention, rather than limiting the scope of protection of the present invention.
Fig. 1 wire-frame model structural schematic diagram；Fig. 2 ideal geometrical structure cone-beam CT system perspective view；Fig. 3 is non-ideal several
What structure cone-beam CT system perspective view；Fig. 4 bearing calibration flow chart；Front view under the non-ideal structure of Fig. 5；Fig. 6 is Fig. 5
Side view；Fig. 7 is the top view of Fig. 5；Fig. 8 is the side view of the other side Fig. 5；Relationship signal between Fig. 9 angle Г and angle, θ
Figure；Figure 10 plane P2With plane P3Relation schematic diagram；Figure 11 plane P3With plane P4Relation schematic diagram.Wherein L is indicated flat
The side length of face P coideal projection quadrilateral AA ' B ' B, R indicate radiographic source to the distance of template, D expression radiographic source to desired detection
The distance of device, l indicate the side length for the square that four extended lines are constituted, and L can be acquired according to R, D, l；Plane P is desired detection
Device plane, projection of four intersection points of the extended line of four linear targets on plane P is A ' AB ' B on calibration model；Plane P
Plane P is obtained after in-between column rotation angle φ1, four intersection points are in P1Projection in plane is DD ' C ' C；P1Plane is around wherein
Between row rotation θ angle after obtain P2Plane, while four intersection points are in P2EE ' F ' F is projected as in plane；Point S is radiographic source, point
G, G ', I ', I are the midpoint of side AB, A ' B ', DC, D ' C ' respectively；The angle of CD and EF is Г, and the angle and the function at the angle θ have
It closes, the angle of AS and GS are that the angle of α, BS and GS are also α；The angle of IO and GO and I ' O and G ' O be all φ, GS and SO with
And the angle of G ' S and SO are all β；The angle of D ' C ' and E ' F ' is that the angle of Г, A ' S and G ' S are α；D is plane P1On one
Point, DM is perpendicular to plane P2Point M is met at, the vertical line that M point makees EI excessively meets at point N, therefore, DM ⊥ EI, MN ⊥ EI, DN ⊥ EI.
Cone-beam CT imaging system is made of x-ray source, turntable and detector.In industrial cone-beam CT system, x-ray source
Fixed with detector, for turntable between radiographic source and detector, object to be detected follows turntable to rotate.The system
It must satisfy ideal imaging geometry, i.e., the line of radiographic source, rotation center and detector center is perpendicular to detector
Plane, and intersect vertically with the shaft of turntable.
Actual cone-beam CT system is difficult to meet ideal imaging relations, the geometric mismatch situation of system can be divided into three
Class: geometric mismatch situation, the geometric mismatch situation of radiographic source, the geometric mismatch situation of turntable of detector.In fact, latter two
Geometric mismatch situation can be equivalent by the geometric mismatch situation of detector, therefore the present invention only considered the geometric mismatch feelings of detector
Condition.The geometric mismatch situation of detector can be completely represented with six geometric parameters (φ, θ, η, Δ u, Δ v, Δ D).
Core of the invention is a wire frame calibration model, and structural schematic diagram is as shown in Figure 1.In order to guarantee reconstruction image
Contrast, calibrate template material selection N (100) type, Φ 100mm × 0.5mm, density 2.33g/cm3(18 DEG C), production side
Formula selection etching, cutting.In the ideal case, the projection extension of four linear targets on the detector should be constituted in the template
One square, perspective view are as shown in Figure 2.When imaging system geometries are in nonideality, template is calibrated
The projection on the detector of four lines will be enough at an arbitrary quadrilateral, perspective view is as shown in Figure 3.School of the invention
Positive process is as shown in figure 4, specific correction course is broadly divided into following three step:
S1 obtains the projection number of model using the data for projection that detector collects calibration model under a projection angle
According to.By general method by radiographic source, turntable and sensors' positioning on position as ideal as possible so that detector is flat
Face is vertical with the holding of turntable plane.Calibration model is fixed on turntable center by installs fixture, is guaranteed by conventional method
Model plane keeps as vertical as possible with turntable plane.
S2, pretreatment and geometrical parameter calibration.The middle line equation of every linear target is determined according to model projection first, so
Intersection position is found out according to adjacent two middle lines afterwards, and then finds out the quadrangle that four linear targets project composition on the detector
Side length.Wherein, radiographic source passes through measurement to the distance R of turntable planar central and the distance D of radiographic source to detector plane
It obtains.Since the error of distance R can be converted the error of distance D, and the error of distance D be six geometric error parameters it
One, therefore the measurement error of distance R will not influence last result.Finally, being asked using the ratio between the projection quadrilateral left side and the right
Solve φ, using projection quadrilateral top and the ratio between following solve θ.After obtaining φ and θ, according to any one projection quadrilateral
Side length can calculate Δ D.
S3 solves η, Δ u, Δ v.Using what second step acquired φ is rotated around central series, rotate θ, Yi Jiding around central row
In mark template two adjacent straight-line intersections exist around central series swing be φ, angle is rotated around central row is θ and along X-axis side
Projection coordinate on the detector plane for offseting by Δ D can solve η, Δ u, Δ v.
S4 after acquiring six geometric parameters, carries out band parameter FDK and rebuilds, the reconstruction image after being corrected.
Solving geometric parameter, specific step is as follows:
The first step calculates φ and θ:
The relationship on detector between each side of quadrangle is analyzed, two of rotationangleφ and θ respectively with projection quadrilateral are obtained
Group opposite side is related, and unrelated with other several geometric error parameters of system.Thus, it is supposed that Δ u=0, Δ v=0, Δ D=0, η
=0, simplified imaging relations are as shown in figure 5, the observation chart under the imaging relations different perspectives is as shown in Figure 6 to 8.
With reference to Fig. 6, can be obtained by sine:
Two formulas are divided by, and are obtained
With reference to Fig. 6 and Fig. 8 it can be concluded that the relational expression of SI and α and Г, can finally obtain:
It can similarly find out
Pass through the relationship of EF and E ' F ', it can be deduced that the relational expression of the geometric error parameter phi of imaging system:
Therefore, when carrying out geometric correction, projection quadrilateral is calculated by the coordinate of subpoint E, point F, point E ', point F '
Side length EF and E ' F ' may finally be obtained several in conjunction with the side length l of quadrangle on the distance R and template of radiographic source to template
What error parameter φ.
Similarly, pass through the relationship of EE ' and FF ', available Г
There are certain relationships with angle, θ by angle Г, as shown in Figure 9.
Cos Γ=cos γ cos θ (8)
By the coordinate position of fixed point E, point F, point E ', point F ', the side of available projection quadrilateral EE ' F ' F
Long EE ' and FF ' recycles above-mentioned formula combination R and D that can find out geometric error parameter θ.
Second step calculates Δ D.
Can offset Δ D directly be calculated using any one side length of projection quadrilateral.
Using two formula above, in conjunction with the available geometric error parameter, Δ D of φ and Г acquired.
Third step calculates η, Δ u, Δ v.
Figure 11 indicates plane P2Along U2Axle offset Δ u, along V2Axle offset Δ v finally obtains plane P3, Figure 11 expression P4Plane
By P3η angle is rotated around central point to obtain.Therefore in object under test certain point in plane P2, plane P3With plane P4Upper projection is sat
Mark relationship has:
u3=u- Δ u (11)
v3=v2-Δv (12)
u4=ucos η+v3Sin η=u2·cosη+v2·sinη-(Δu·cosη+Δv·sinη) (13)
v4=u3·sinη+v3Cos η=- u2·sinη+v2·cosη-(-Δu·sinη+Δv·cosη) (14)
(u4,v4)、(u3,v3)、(u2,v2) it is corresponding points respectively in plane P4、P3、P2On coordinate.If plane P4Upper four
The coordinate at any point is (u in subpoint4,v4), corresponding thereto in P2Point in plane is (u2,v2), coordinate (u2,v2)
Calculation formula is as follows:
If the point is in above calibration template center's row, v2> 0；If the point is in below calibration template center's row
When, v2< 0.We can be according to the intersection point of any two linear target extended line in plane P2Upper and plane P4On coordinate value,
(u2,v2) and (u4,v4), and equation (13) and equation (14) is combined to acquire η.Recycle (the u of any intersection point2,v2) and (u4,v4)
Coordinate value and expression formula (13) and expression formula (14) solve Δ u and Δ v.It should be noted that distance D is (D+ Δ D) at this time.
The above description is only a preferred embodiment of the present invention, is not intended to restrict the invention, it is clear that those skilled in the art
Various changes and modifications can be made to the invention by member without departing from the spirit and scope of the present invention.If in this way, of the invention
Within the scope of the claims of the present invention and its equivalent technology, then the present invention is also intended to encompass these to these modifications and variations
Including modification and variation.
Claims (1)
1. a kind of cone-beam CT system geometric parameter calibration model, it is characterised in that: template is calibrated including wire frame type, in calibration template
On be provided with 4 disjoint linear targets, 4 linear targets and respectively surround square to the extended line at both ends；
The bearing calibration of the model, comprising the following steps:
S1: the data for projection of model is obtained using the projection that detector collects calibration template under a projection angle；
S2: determining the middle line equation of every linear target according to the data for projection of model first, then according to adjacent two middle lines
Intersection position is found out, and then finds out the side length that four linear targets project the quadrangle of composition on the detector；Finally, using throwing
The ratio between the shadow quadrangle left side and the right, which are solved, around central series rotate angle φ, the top using projection quadrilateral with ask the ratio between below
Unwinding central row rotates angle, θ；After obtaining φ and θ, Δ D is calculated according to the side length of any one projection quadrilateral；
S3: rotating angle φ around central series, rotate angle, θ around central row using what step S2 was acquired, and two in calibration template
The adjacent straight-line intersection of item rotates angle φ, angle, θ is rotated around central row and offsets by Δ D along the x axis in presence around central series
Detector plane on projection coordinate, η, Δ u, Δ v can be solved；
S4: after acquiring six geometric parameters, carrying out band parameter FDK and rebuild, the reconstruction image after being corrected.
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| CN108122203B (en) * | 2016-11-29 | 2020-04-07 | 上海东软医疗科技有限公司 | Geometric parameter correction method, device, equipment and system |
| WO2018126335A1 (en) * | 2017-01-03 | 2018-07-12 | 苏州海斯菲德信息科技有限公司 | Method for evaluating and correcting geometric parameters of cone-beam ct system based on glomerulus motif |
| CN107233105B (en) * | 2017-05-24 | 2020-12-11 | 深圳先进技术研究院 | Correction method and correction system for CT image reconstruction |
| CN108201447A (en) * | 2017-11-21 | 2018-06-26 | 深圳先进技术研究院 | A kind of static state CT system geometric parameter bearing calibration |
| CN108663386B (en) * | 2018-05-09 | 2019-08-20 | 大连理工大学 | Cone-beam CT system probe angle bias measurement method based on feature texture template |
| CN111736235A (en) * | 2019-03-25 | 2020-10-02 | 同方威视技术股份有限公司 | Geometric parameter calibration piece and calibration method of CT (computed tomography) equipment |
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