CN101603810A - Quick positioning method of cone-beam CT system and special measuring tool thereof - Google Patents

Abstract

The invention discloses a kind of quick positioning method of cone-beam CT system, the round bar measuring tool is placed on the rotary table center of cone-beam CT system, perhaps keep the round bar measuring tool motionless, with the past radiographic source direction translation h of rotary table, perhaps with the round bar measuring tool edge direction translation D/5～D/3 parallel with the flat panel detector imaging plane, perhaps keep the round bar measuring tool motionless, with rotary table Rotate 180 degree, obtain several projected images, calculate the cone-beam CT system positional parameter, comprise the distance D so of ray source focus to rotation center, ray source focus is to the distance D sd of flat panel detector, the irradiation position of central beam on flat panel detector (x, y) and flat panel detector rotation angle γ.Also disclose above-mentioned quick positioning method of cone-beam CT system special measuring tool, be characterized in comprising round bar and base, round bar and base are connected as a single entity and have relief groove on round bars.Since adopt special measuring tool to scan, simple to operate; Can measure 5 main positional parameter Dso, Dsd, x, y and γ of cone-beam CT system simultaneously.

Description

Quick positioning method of cone-beam CT system and special measuring tool thereof

Technical field

The present invention relates to a kind of method for rapidly positioning, particularly quick positioning method of cone-beam CT system.Also relate to this quick positioning method of cone-beam CT system special measuring tool.

Background technology

Existing cone-beam CT system mainly is made of x-ray source, rotary table and flat panel detector, its installing and locating parameter comprises: ray source focus is to distance D so at rotary table center, ray source focus is to the distance D sd of flat panel detector, irradiation position (the x of central beam (ray vertical that radiographic source sends) on flat panel detector with the rotary table turning axle, y), and flat panel detector respectively around rotation angle α, β and the γ of X, Y and Z axle.Wherein, Dso, Dsd, x, y and γ have material impact to rebuilding sectioning image quality and subsequent applications.The cone-beam CT system that adopts conventional means to install can only guarantee the relative position relation that parts are basic, and can not directly obtain high-precision positional parameter.

Document " the cone-beam CT system installation parameter is determined technical research, computer engineering and application, 2006, Vol.20, p170-173 " discloses a kind of golden section iterative search method based on thin string reconstructed image assessment and has measured the cone-beam CT system positional parameter.This method construct a sectioning image gray scale density function as objective function, adopt golden section search to adjust each positional parameter respectively, finally make objective function reach minimum value, thereby determine the positional parameter of system.The CT that this method relates in the golden section iterative search repeatedly rebuilds, and calculated amount is big, and measuring once needs a few hours; Need before the iterative computation to adopt other method to measure initial value,, may cause this method to lose efficacy if the initial value error is too big; Studies show that further Dso and Dsd can not make the objective function convergence, therefore can not carry out iterative search to these two parameters.

Summary of the invention

In order to overcome prior art location many deficiencies consuming time, the invention provides a kind of quick positioning method of cone-beam CT system, adopt homemade round bar measuring tool to carry out the several simple scanning, can calculate main positional parameter Dso, Dsd, x, y and the γ of cone-beam CT system fast.

The technical solution adopted for the present invention to solve the technical problems: a kind of quick positioning method of cone-beam CT system is characterized in comprising the steps:

(a) the round bar measuring tool is placed on the rotary table center of cone-beam CT system, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G1, and carry out cutting by rebuilding required projection picture size;

(b) keep the round bar measuring tool motionless, with the past radiographic source direction of rotary table or flat panel detector direction translation h=40～80mm, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G2, and carry out cutting by rebuilding required projection picture size;

(c) with the round bar measuring tool edge direction translation D/5～D/3 parallel with the flat panel detector imaging plane, remember that this position is 0 degree, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G3, and carry out cutting by rebuilding required projection picture size;

(d) keep the round bar measuring tool motionless,, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G4, and carry out cutting by rebuilding required projection picture size with rotary table Rotate 180 degree;

(e) utilize the projected image of above-mentioned collection, calculate Dso, Dsd and y,

1. will ask the projected image G1 behind the logarithm to carry out the stack of projection pixel gray scale, obtain 1 row projection by row;

2. the mid point with this row projection is a starting point, upwards searches first local gray level maximal value picture element by pixel, and this point is labeled as P _Max, judgment criterion is that the gray scale of adjacent 2～5 pixels of this pixel is all less than the gray scale of this pixel;

3. from P _MaxUpwards search first local gray level minimum value picture element by pixel, this point is labeled as P _Min, judgment criterion is that the gray scale of adjacent 2～5 pixels of this pixel is all greater than the gray scale of this pixel;

4. calculate P _MaxAnd P _MinThe mean value of pixel gray scale;

5. at P _MaxWith P _MinBetween search immediate two picture elements of average gray therewith;

6. according to average gray these two picture elements are carried out linear interpolation, obtain an inferior picture element, this point is labeled as P ₁

7. adopting the method that 2.～6. goes on foot all fours with above-mentioned the, is starting point with the mid point of this row projection, searches downwards and calculate by pixel to obtain P ₂The point;

8. to asking the projected image G2 behind the logarithm, adopt with above-mentioned the method that 1.～7. goes on foot all fours and can obtain P ₃Point and P ₄The point;

9. according to the correlativity of obtaining twice scanning projection of projected image G1, projected image G2, obtain system of equations:

$\left\{\begin{array}{c}\frac{a}{b}=\frac{\mathrm{Dso}}{\mathrm{Dsd}}=\frac{L}{\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}}\\ \frac{a}{b+\stackrel{\&OverBar;}{P_2{P}_4}}=\frac{\mathrm{Dso}-h}{\mathrm{Dsd}}=\frac{L}{\stackrel{\&OverBar;}{P_3{P}_4}}\end{array}\right.$

Wherein, a is that relief groove is to the distance of central beam down in the scanning for the first time, and b is the projected length of a on flat panel detector, P ₁P ₂, P ₃P ₄And P ₂P ₄Calculate by above-mentioned measurement; The group of solving an equation can get $\mathrm{Dso}=\frac{h\&CenterDot;\stackrel{\&OverBar;}{P_3{P}_4}}{\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}},$ $\mathrm{Dsd}=\frac{h\&CenterDot;\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}\&CenterDot;\stackrel{\&OverBar;}{P_3{P}_4}}{(\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2})\&CenterDot;L},$ $b=\frac{\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}\&CenterDot;\stackrel{\&OverBar;}{P_2{P}_4}}{\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="A2009100231370009H1.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}},$ Then the value of central beam ordinate is y=P ₂+ b;

Dso is the distance of ray source focus to rotation center; Dsd is the distance of ray source focus to flat panel detector; (x y) is the irradiation position of central beam on flat panel detector;

Calculate the step of x and γ:

1. 0 degree and 180 is spent the projected image G3 of positions and projected image G4 and subtract each other by corresponding pixel gray-scale value and take absolute value, obtain 1 width of cloth image G5;

2. projected image G5 is divided into 10～20 parts on short transverse, gets every part middle row pixel, obtain equally spaced 10～20 row pixels;

3. each the row pixel that respectively 2. step is obtained is searched the minimum gradation value pixel in the round bar view field;

4. be the center with the minimum gradation value pixel, on corresponding line, get 2～7 adjacent pixels of its left and right sides respectively, obtain 5～15 continuous pixels;

5. be independent variable with the pixel location, adopt the least square quadratic polynomial that these pixel gray scales are carried out match, obtain a plurality of quadratic polynomials identical with the line number amount;

6. calculate the minimum value of each quadratic polynomial respectively, obtain the X value corresponding,, promptly obtain the coordinate points on a plurality of flat panel detectors identical with the line number amount in conjunction with the Y coordinate of being expert at minimum value;

7. adopt these coordinate points of least-squares line match, the straight-line equation that obtains is the mathematic(al) representation of projection straight line on flat panel detector of rotary table turning axle;

8. calculate the slope of this straight line, i.e. flat panel detector rotation angle γ;

9. with central beam ordinate value y substitution straight-line equation, calculate central beam abscissa value x.

A kind of above-mentioned quick positioning method of cone-beam CT system special measuring tool is characterized in comprising round bar and base, and round bar and base are connected as a single entity, and the diameter D of round bar is 10～20mm, the relief groove that to open two width on round bar be 2～4mm.

Described withdrawing groove depth 2～3mm, and lay respectively at flat panel detector imaging window center both sides up and down.

The invention has the beneficial effects as follows: owing to adopt homemade round bar measuring tool to scan, simple to operate; Can measure 5 main positional parameter Dso, Dsd, x, y and γ of cone-beam CT system simultaneously; Measuring accuracy can reach inferior pixel level; Whole position fixing process drops to several minutes by a few hours of prior art.

Below in conjunction with drawings and Examples the present invention is elaborated.

Description of drawings

Fig. 1 is a quick positioning method of cone-beam CT system special measuring tool structural representation of the present invention.

Fig. 2 is quick positioning method of cone-beam CT system of the present invention and special measuring tool position view.

Fig. 3 is the projection imaging geometric relationship figure that quick positioning method of cone-beam CT system of the present invention obtains projected image G1 and projected image G2.

Steel wire is rebuild the section gray-scale map for the 200th layer in Fig. 4 embodiment.

Steel wire is rebuild the section gray-scale map for the 512nd layer in Fig. 5 embodiment.

Steel wire is rebuild the section gray-scale map for the 800th layer in Fig. 6 embodiment.

Among the figure, 1-round bar, 2-relief groove, 3-base.

Embodiment

Following examples are with reference to Fig. 1～6.To a cone-beam CT system, the step of implementing the inventive method is as follows:

(1) the round bar measuring tool is placed on the rotary table center of cone-beam CT system, gather 20 width of cloth projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G1, and be cut into 1024 * 1024 by rebuilding required projection picture size.The round bar measuring tool that is adopted is made of even aluminium matter round bar and disc base.Round bar diameter D=20mm, relief groove diameter d=16mm, two relief groove width are 4mm, and relief groove is at a distance of L=60mm and lay respectively at flat panel detector imaging window center both sides up and down.

(2) keep the round bar measuring tool motionless, rotary table toward radiographic source direction translation h=50mm, is gathered 20 width of cloth projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G2, and be cut into 1024 * 1024 by rebuilding required projection picture size.

(3) with the round bar measuring tool edge direction translation 5mm parallel with the flat panel detector imaging plane, remember that this position is 0 degree, gather 20 width of cloth projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G3, and be cut into 1024 * 1024 by rebuilding required projection picture size;

(4) keep the round bar measuring tool motionless, with rotary table Rotate 180 degree, gather 20 width of cloth projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G4, and be cut into 1024 * 1024 by rebuilding required projection picture size;

(5) utilize the projected image of above-mentioned collection, calculate the cone-beam CT system positional parameter, comprise the distance D so of ray source focus to rotation center, ray source focus is to the distance D sd of flat panel detector, the irradiation position of central beam on flat panel detector (x, y) and flat panel detector rotation angle γ.

At first calculate Dso, Dsd and y, adopt following treatment step:

1) will ask the projected image G1 behind the logarithm to carry out the stack of projection pixel gray scale, obtain 1 row projection by row;

2) mid point with this row projection is a starting point, upwards searches first local gray level maximal value picture element by pixel, and this point is labeled as P _Max, judgment criterion is that the gray scale of adjacent 3 pixels of this pixel is all less than the gray scale of this pixel;

3) from P _MaxUpwards search first local gray level minimum value picture element by pixel, this point is labeled as P _Min, judgment criterion is that the gray scale of adjacent 3 pixels of this pixel is all greater than the gray scale of this pixel;

4) calculate P _MaxAnd P _MinThe mean value of pixel gray scale;

5) at P _MaxWith P _MinBetween search immediate two picture elements of average gray therewith;

6) according to average gray these two picture elements are carried out linear interpolation, obtain an inferior picture element, this point is labeled as P ₁, P ₁=233.245101;

7) adopting method with above-mentioned the 2nd～6 step all fours, is starting point with the mid point of this row projection, searches downwards and calculate by pixel to obtain P ₂Point, P ₂=817.660577;

8), adopt the method with above-mentioned the 1st～7 step all fours can obtain P to asking the projected image G2 behind the logarithm ₃Point and P ₄Point, P ₃=203.3431227, P ₄=846.659914;

9) according to the correlativity of obtaining twice scanning projection of G1, G2, calculating can get: Dso=1092.19, Dsd=1348.81, y=529.931.

Next calculates x and γ, adopts following treatment step:

1) 0 degree and 180 is spent the projected image G3 of positions and G4 and subtract each other by corresponding pixel gray-scale value and take absolute value, obtain 1 width of cloth image G5;

2) G5 is divided into 10 parts on short transverse, gets every part middle row pixel, then obtain equally spaced 10 row pixels;

3) each the row pixel that respectively previous step is obtained is searched the minimum gradation value pixel in the round bar view field;

4) be the center with the minimum gradation value pixel, on corresponding line, get 4 adjacent pixels of its left and right sides respectively, obtain 9 continuous pixels;

5) with the pixel location be independent variable, adopt the least square quadratic polynomial that these pixel gray scales are carried out match, obtain a plurality of quadratic polynomials identical with the line number amount, wherein the 153rd row fitting result is g=1.9068x ²-1932.771x+489792;

6) calculate the minimum value of each quadratic polynomial respectively, obtain the X value corresponding,, promptly obtain the coordinate points on a plurality of flat panel detectors identical with the line number amount in conjunction with the Y coordinate of being expert at minimum value.For g=1.9068x ²-1932.771x+489792, the x=506.81 of its minimum value correspondence then is (506.81,153) by the coordinate that this row obtains;

7) adopt these coordinate points of least-squares line match, the straight-line equation that obtains is the mathematic(al) representation y=-342.6993x+173850.6722 of projection straight line on flat panel detector of rotary table turning axle;

8) calculating the slope of this straight line, promptly is flat panel detector rotation angle γ=89.83281 °;

9) with the central beam ordinate value y=529.931 substitution straight-line equation of previous calculations gained, can calculate central beam abscissa value x=505.751664.

Accuracy for the main positional parameter of cone-beam CT system of verifying above-mentioned gained, adopt these parameters to rebuild to a steel wire by this system scan, get the 200th layer, the 512nd layer and the 800th layer of its steel wire reconstruction gray scale of observation of rebuilding section, as seen the centrality of steel wire reconstruction gray scale is all fine, shows that the main positional parameter of cone-beam CT system that adopts the inventive method to obtain is accurately and reliably.

Claims (3)

1, a kind of quick positioning method of cone-beam CT system is characterized in that comprising the steps:

(a) the round bar measuring tool is placed on the rotary table center of cone-beam CT system, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G1, and carry out cutting by rebuilding required projection picture size;

(b) keep the round bar measuring tool motionless, with the past radiographic source direction of rotary table or flat panel detector direction translation h=40～80mm, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G2, and carry out cutting by rebuilding required projection picture size;

(c) with the round bar measuring tool edge direction translation D/5～D/3 parallel with the flat panel detector imaging plane, remember that this position is 0 degree, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G3, and carry out cutting by rebuilding required projection picture size;

(d) keep the round bar measuring tool motionless,, gather several projected images, then these images are carried out gray scale stack and average by corresponding pixel, obtain 1 width of cloth projected image G4, and carry out cutting by rebuilding required projection picture size with rotary table Rotate 180 degree;

(e) utilize the projected image of above-mentioned collection, calculate Dso, Dsd and y,

1. will ask the projected image G1 behind the logarithm to carry out the stack of projection pixel gray scale, obtain 1 row projection by row;

2. the mid point with this row projection is a starting point, upwards searches first local gray level maximal value picture element by pixel, and this point is labeled as P _Max, judgment criterion is that the gray scale of adjacent 2～5 pixels of this pixel is all less than the gray scale of this pixel;

3. from P _MaxUpwards search first local gray level minimum value picture element by pixel, this point is labeled as P _Min, judgment criterion is that the gray scale of adjacent 2～5 pixels of this pixel is all greater than the gray scale of this pixel;

4. calculate P _MaxAnd P _MinThe mean value of pixel gray scale;

5. at p _MaxWith P _MinBetween search immediate two picture elements of average gray therewith;

6. according to average gray these two picture elements are carried out linear interpolation, obtain an inferior picture element, this point is labeled as P ₁

7. adopting the method that 2.～6. goes on foot all fours with above-mentioned the, is starting point with the mid point of this row projection, searches downwards and calculate by pixel to obtain P ₂The point;

8. to asking the projected image G2 behind the logarithm, adopt with above-mentioned the method that 1.～7. goes on foot all fours and can obtain P ₃Point and P ₄The point;

9. according to the correlativity of obtaining twice scanning projection of projected image G1, projected image G2, obtain system of equations:

$\left\{\begin{array}{c}\frac{a}{b}=\frac{\mathrm{Dso}}{\mathrm{Dsd}}=\frac{L}{\stackrel{\&OverBar;}{P_1{P}_2}}\\ \frac{a}{b+\stackrel{\&OverBar;}{P_2{P}_4}}=\frac{\mathrm{Dso}-h}{\mathrm{Dsd}}=\frac{L}{\stackrel{\&OverBar;}{P_3{P}_4}}\end{array}\right.$

Wherein, a is that relief groove is to the distance of central beam down in the scanning for the first time, and b is the projected length of a on flat panel detector, P ₁P ₂, P ₃P ₄And P ₂P ₄Calculate by above-mentioned measurement; The group of solving an equation can get $\mathrm{Dso}=\frac{h\&CenterDot;\stackrel{\&OverBar;}{P_3{P}_4}}{\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{P}_2}},$ $\mathrm{Dsd}=\frac{h\&CenterDot;\stackrel{\&OverBar;}{P_1{P}_2}\&CenterDot;\stackrel{\&OverBar;}{P_3{P}_4}}{(\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{P}_2})\&CenterDot;L},$ $b=\frac{\stackrel{\&OverBar;}{P_1{P}_2}\&CenterDot;\stackrel{\&OverBar;}{P_2{P}_4}}{\stackrel{\&OverBar;}{P_3{P}_4}-\stackrel{\&OverBar;}{P_1{P}_2}},$ Then the value of central beam ordinate is y=P ₂+ b;

Dso is the distance of ray source focus to rotation center; Dsd is the distance of ray source focus to flat panel detector; (x y) is the irradiation position of central beam on flat panel detector;

Calculate the step of x and γ:

1. 0 degree and 180 is spent the projected image G3 of positions and projected image G4 and subtract each other by corresponding pixel gray-scale value and take absolute value, obtain 1 width of cloth image G5;

2. projected image G5 is divided into 10～20 parts on short transverse, gets every part middle row pixel, obtain equally spaced 10～20 row pixels;

3. each the row pixel that respectively 2. step is obtained is searched the minimum gradation value pixel in the round bar view field;

4. be the center with the minimum gradation value pixel, on corresponding line, get 2～7 adjacent pixels of its left and right sides respectively, obtain 5～15 continuous pixels;

5. be independent variable with the pixel location, adopt the least square quadratic polynomial that these pixel gray scales are carried out match, obtain a plurality of quadratic polynomials identical with the line number amount;

6. calculate the minimum value of each quadratic polynomial respectively, obtain the X value corresponding,, promptly obtain the coordinate points on a plurality of flat panel detectors identical with the line number amount in conjunction with the Y coordinate of being expert at minimum value;

7. adopt these coordinate points of least-squares line match, the straight-line equation that obtains is the mathematic(al) representation of projection straight line on flat panel detector of rotary table turning axle;

8. calculate the slope of this straight line, i.e. flat panel detector rotation angle γ;

9. with central beam ordinate value y substitution straight-line equation, calculate central beam abscissa value x.

2, the described quick positioning method of cone-beam CT system special measuring tool of a kind of claim 1, it is characterized in that: comprise round bar and base, round bar and base are connected as a single entity, the relief groove that to open two width on round bar be 2～4mm.

3, according to the described quick positioning method of cone-beam CT system special measuring tool of claim 2, it is characterized in that: described withdrawing groove depth 2～3mm, and lay respectively at flat panel detector imaging window center both sides up and down.

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