CN1751661A - Reconstruction method based on dual-source double-helix multi-slice helical CT - Google Patents

Abstract

A 3D image reconstructing method based on dual-source dual-spiral multi-layer spiral CT features that two x-ray sources, two structures of multi-row detectors and two spiral scan paths are used to increase the projection data acquisition speed. It includes such steps as calculating the projection data needed by reconstructing a planar image perpendicular to Z axis by using interpolation algorithm of the multi-layer projection data on two paths, reconstructing the planar image by 2D filter-reverse projection algorithm, and reconstructing a 3D image with a series of said planar images.

Description

Reconstruction method based on dual-source double-helix multi-slice helical CT

technical field

The invention relates to a method in the technical field of biomedical imaging, in particular to a reconstruction method based on dual-source double-helix multi-slice CT.

Background technique

The working process of single-source single-helical single-slice CT is as follows: the X-ray tube emits cone-beam X-rays at one point, and after passing through the object to be measured, the attenuated X-rays are detected by the single-row detector on the opposite side (single-row The detector is on the same plane as the X-ray source), and the X-rays are converted into electrical signals, sent to the computer's analog-to-digital converter and data collector, converted into digital signals and stored in the computer. This is just one exposure and acquisition process. In order to obtain the projection data needed to reconstruct the object under test, the X-ray source and the single-row detector need to move along a spiral trajectory relative to the object under test, and complete the process at each position. One exposure and data acquisition process. The trajectory of the X-ray source relative to the measured object is a single helix. A method to realize the helical trajectory movement of the X-ray source and the single-row detector relative to the measured object is: the X-ray source and the single-row detector are continuously rotated around a rotation axis, and the measured object moves in a straight line at a constant speed along the direction of the rotation axis. sports. A common Cartesian coordinate system is adopted, the Z axis coincides with the rotation axis, and the plane of the X and Y axes is parallel (including overlapping) to the plane of the single row of detectors and the X-ray source. On the helix, the distance between two adjacent helical turns along the Z axis is called the pitch. To reconstruct an image of a plane perpendicular to the Z-axis, at most the fan-beam projection data of each position on the spiral above and below the plane is required, and the projection data on the plane is interpolated through the Z-axis interpolation algorithm, and then two The image of the plane is reconstructed by a filtered back-projection algorithm.

Through literature search of the prior art, it is found that typical single-source single-helical multi-slice CT reconstruction methods such as (S. Part: Theory], IEEE Transactions on Medical Imaging [IEEE Medical Imaging Transactions], Vol.19, NO.9, pp.822-834). Single-source single-helical multi-slice CT is different from single-source single-helical single-slice CT: the X-ray tube emits cone-beam X-rays at one point, and after passing through the object to be measured, the attenuated X-rays are detected by multiple rows of detectors on the opposite side , the multi-row detectors are stacked along the Z-axis in the corresponding single-source single-helical single-slice CT. The trajectory of the X-ray source relative to the measured object is still a single helix. To reconstruct an image of a plane perpendicular to the Z axis, at most the multi-layer fan beam projection data of each position on the spiral above and below the plane is required, and the multi-layer fan beam projection data Z-axis interpolation algorithm is used to interpolate the image. Project the data on the plane, and then use the two-dimensional filtered back projection algorithm to reconstruct the image of the plane. In the plane passing through the X-ray source and the Z-axis, the angle between the center of a certain row of detectors and the X-ray source and the vertical line between the X-ray source and the Z-axis is called the cone angle. The Z-axis interpolation algorithm of multi-layer fan beam projection data requires a small cone angle, otherwise, the error of the reconstruction result will be unacceptably large. Compared with single-source single-helical multi-slice CT, single-source single-helical multi-slice CT has the advantages of large Z-axis coverage, fast data acquisition speed, and high imaging accuracy, but there may be low quality of reconstructed images and resolution of reconstructed images rate has not been improved.

Contents of the invention

The purpose of the present invention is to provide a reconstruction method based on dual-source double-helix multi-slice CT to improve the resolution of the reconstructed image and improve the quality of the image.

The present invention is achieved through the following technical solutions. The present invention adopts the structure of double X-ray sources and double multi-row detectors to scan with double helical paths, which improves the projection data collection speed, and utilizes the multi-layer projection data on the two paths, Through the double-source double-helical multi-layer Z-axis interpolation algorithm, the projection data required to reconstruct a plane image perpendicular to the Z-axis is calculated, and then the plane image is reconstructed by a two-dimensional filtered back-projection algorithm, and finally a series of such A three-dimensional image is reconstructed from the plane.

The present invention comprises the following steps:

(1) The first X-ray source emits cone-beam X-rays at one point. After passing through the measured object, the attenuated X-rays are detected by the first multi-row detector on the opposite side. The first multi-row detector is In the corresponding single-source single-helical single-slice CT, a single row of detectors is stacked along the Z axis. The second X-ray source emits cone-beam X-rays at one point. After passing through the measured object, the attenuated X-rays are detected by the second multi-row detector on the opposite side. The second multi-row detector is the corresponding single Single-row detectors are stacked along the Z-axis in the source single-helix single-slice CT.

(2) The plane perpendicular to the Z axis passing through the first X-ray source is parallel to the plane perpendicular to the Z axis passing through the second X-ray source. The distance from the first X-ray source to the Z-axis is equal to the distance from the second X-ray source to the Z-axis, and the distance from the center of the first multi-row detector to the Z-axis is the same as the distance from the center of the second multi-row detector to the Z-axis distances are equal. The first X-ray source and the second X-ray source are not on the same straight line parallel to the Z axis.

(3) The first X-ray source, the first multi-row detector, the second X-ray source, and the second multi-row detector rotate around the Z axis at the same speed, and perform a helical movement relative to the measured object. The trajectories of the first X-ray source and the second X-ray source relative to the measured object are double helixes.

(4) The first multi-row detector, the second multi-row detector and the corresponding projection data acquisition system respectively collect two sets of multi-layer projection data.

The projection data acquisition system refers to a system that records the data on the multi-row detectors and converts them into digital signals and transmits them to a computer for processing.

(5) Determine a reconstruction plane perpendicular to the Z axis, and use a dual-source double-helix multi-layer Z-axis interpolation algorithm, which may use 180° interpolation technology or 360° interpolation technology. The projection data on the plane is interpolated, and the projection data may be fan beam projection data or parallel beam projection data.

The described double-source double-helix multi-layer Z-axis interpolation algorithm refers to: to reconstruct the image of a certain plane perpendicular to the Z-axis, according to the two helical paths, the multi-layer sector of each position in each circle of the spiral above and below the plane Beam projection data, an algorithm that interpolates the projection data on the plane, participates in the projection data from the multi-layer projection data of the two helices.

The 180° interpolation technique refers to: when interpolating the projection data on the plane, two helical paths will be used for the multi-layer fan beams at each position on the helix of the upper and lower half circles of the plane plus the fan angle range. For projection data, the fan angle refers to the included angle between the two ends of a row of detectors and the line connecting the X-ray source in a group of X-ray source-multi-row detectors.

The 360° interpolation technique refers to: when interpolating the projection data on the plane, the multi-layer fan beam projection data of each position on the helix of two helical paths above and below the plane are used.

(6) Reconstruct the image on the plane with the two-dimensional filtered back projection algorithm.

The two-dimensional filtered back-projection algorithm refers to an algorithm that performs convolution operation on projection data and a filter function, and then performs back-projection.

(7) Repeat (5) and (6) to obtain a series of reconstructed images perpendicular to the Z-axis plane.

(8) Using the series of reconstructed images perpendicular to the Z-axis plane, a three-dimensional image of the measured object is generated, and the reconstructed three-dimensional image or two-dimensional tomographic image is displayed within a specified range.

The working principle of dual-source double-helical multi-layer CT: two sets of X-ray sources-multi-row detectors are placed in the same gantry at a certain angle, placed in the direction orthogonal to the gantry (Z-axis direction), and one can move along the Z-axis direction bed. The test subject is placed on the bed. Two sets of X-ray source-multiple rows of detectors rotate around the Z-axis at the same angular speed, and at the same time, the bed moves in a straight line at a uniform speed along the Z-axis. The trajectories of the two X-ray sources relative to the measured object are double helixes. When the pitch increases, only one set of X-ray source-multi-row detectors is used, and there is a gap in projection data collection. The projection data in this gap can be completed by another set of X-ray source-multi-row detectors. Helical multi-slice CT can only collect projection data in two turns, and dual-source double-helical multi-slice CT can collect projection data in one turn. We all know that when taking pictures, the camera shakes, and the photos taken will be blurred. Shorten the exposure time and increase the exposure to get clear photos. Similarly, dual-source double-helix multi-slice CT collects two sets of multi-slice projection data in different orientations at the same time, which shortens the scanning time and ensures that when the measured object has moving parts, the quality of the final reconstructed image is better than that of single-source single-helix High in multislice CT. The dual-source double-helical multi-layer Z-axis interpolation algorithm fuses the two sets of multi-layer projection data into a plane, and finally reconstructs a clear image on the plane with a two-dimensional filtered back projection algorithm.

The beneficial effects of the present invention are: (1) when the pitch is increased, the quality of the reconstructed image is improved compared with single-source single-helical multi-slice CT; (2) Z-axis coverage is increased than that of single-source single-helical multi-slice CT, thereby When all or part of the measured object is moving, a higher resolution in the Z-axis direction is obtained; (3) In one circle, more projection data are collected than single-source single-helical multi-slice CT, and the projection data acquisition speed Up to 2 times the original, thereby improving the resolution of the reconstructed image. The invention can be applied to dynamic imaging fields such as heart imaging, small animal imaging, infant imaging and the like.

Description of drawings

Fig. 1 is the schematic diagram of two groups of source scanning tracks of the present invention

Fig. 2 is the selection range figure of interpolation projection data of the present invention

Fig. 3 is the interpolation projection data selection figure of the present invention

Fig. 4 is an understanding diagram of the fan beam filter back projection reconstruction algorithm of the present invention

Detailed ways

In order to better understand the technical solution of the present invention, the following will be further described in conjunction with the accompanying drawings and specific embodiments, and the embodiments are implemented according to the following steps:

(1) Place the anesthetized living mouse on a bed that can perform uniform linear motion along the Z-axis direction. The first X-ray source emits cone beam X-rays at one point. After passing through the live mouse, the attenuated X-rays are transmitted to the opposite side The first 4 rows of detectors are detected, and each row of detectors is arc-shaped. The first 4 rows of detectors are stacked along the Z-axis in the corresponding single-source single-helical single-slice CT. The detectors are distributed symmetrically about the plane perpendicular to the Z axis where the emission source is located. In the triangle formed by each row of detectors and the emission source, the included angle between the emission source and the line connecting the two ends of the row of detectors is 50°. Each row of detectors consists of 512 detection units. The second X-ray source emits cone-beam X-rays at one point. After passing through the live mouse, the attenuated X-rays are detected by the second 4 rows of detectors on the opposite side. Each row of detectors is arc-shaped, and the second 4 A row of detectors is formed by stacking a single row of detectors along the Z-axis in the corresponding single-source single-helical single-slice CT, and the four rows of detectors are symmetrically distributed about the plane perpendicular to the Z-axis where the emission source is located. In the triangle formed by each row of detectors and the emission source, the included angle between the emission source and the line connecting the two ends of the row of detectors is 50°. Each row of detectors consists of 512 detection units. Viewed along the Z axis, the angle between the two sources and the center of the circle (rotation axis Z axis) is 90°.

(2) The plane perpendicular to the Z axis passing through the first X-ray source is parallel to the plane perpendicular to the Z axis passing through the second X-ray source. The distance from the first X-ray source to the Z-axis is equal to the distance from the second X-ray source to the Z-axis, and the distance from the center of the first 4-row detector to the Z-axis is the same as the distance from the center of the second 4-row detector to the Z-axis distances are equal. The first X-ray source and the second X-ray source are not on the same straight line parallel to the Z axis.

(3) The first X-ray source, the first 4-row detector, the second X-ray source, and the second 4-row detector rotate around the Z-axis at the same speed, and perform a spiral movement relative to the live mouse. The trajectories of the first X-ray source and the second X-ray source relative to the live mouse are a double helix. Figure 1 shows the respective motion trajectories of the two sources during the scanning process, forming a double helix. The solid line is the scanning trajectory of the first X-ray source, and the dotted line is the scanning trajectory of the second X-ray source. Only the direction of the Z axis is shown in the figure, and the actual position of the Z axis should be the rotation axis of the helix.

(4) During scanning, the data obtained by two sets of detectors are recorded by two sets of hardware respectively, and each rotation is 1 degree (recorded once. The first 4 rows of detectors, the second 4 rows of detectors and the corresponding projection data acquisition The system collects two groups of 4-layer projection data respectively. When recording the projection data, it should also record the spatial position of each group of scanning devices and other relevant information necessary for subsequent image reconstruction, including: Z of each X-ray emission source Axis position; Z-axis position of each row of detectors in each detector group at the time of data acquisition; angles through which each group of sources and receivers have been rotated, etc.

(5) Determine a reconstruction plane perpendicular to the Z axis, and the coordinate position ZR of the plane to be reconstructed on the Z axis.

The projection data segments necessary for interpolation are selected according to ZR. As shown in Fig. 2, the solid line is the scanning trajectory of the first X-ray source, and the dashed line is the scanning trajectory of the second X-ray source. Only the direction of the Z axis is shown in the figure, and the actual position of the Z axis should be the rotation axis of the helix. The scanning trajectory of a group of X-ray sources is randomly taken as a reference, and the scanning trajectory of the first X-ray source is taken as an example here. Through ZR and pitch d (the distance that the helix advances in the Z-axis direction every 360° rotation), the angle β that the first X-ray source has rotated can be calculated (set the angle at which the first X-ray source rotates initially is zero degrees), and then the position of the first X-ray source on the helical scanning trajectory at ZR (the position marked with an asterisk in Figure 2) can be determined.

The relationship between ZR and β is as follows:

$\mathrm{\β}=\frac{Z_R-Z_0}{d}\&Center\; Dot;360,$ Among them, Z ₀ represents the Z-axis coordinate of the initial position of the helical scanning track.

On another scanning track, select the corresponding position (the position marked by the hollow diamond in Figure 2), and calculate the angle β ₂ rotated by the second X-ray source when it travels to this position, β ₂ = β-90 . After obtaining β and β ₂ , for the first X-ray source, select its projection data in the range of [β-360, β+360]; for the second X-ray source, select its projection data in [β ₂ - 360, β ₂ +360] range projection data. The spatial relationship of the selected range of projection data is shown in Figure 2.

Interpolate the selected projection data. Suppose α∈[β, β+360], for the first X-ray source, select two sets of projection data obtained when it is located at α and α-360; for the first X-ray source, select its location at (α- 90) and (α-90)-360 two sets of projection data. The positions of the four emission sources corresponding to these four sets of data should be on the same straight line, as shown in Figure 3, the solid line is the scanning trajectory of the first X-ray source, and the dotted line is the scanning trajectory of the first X-ray source. Only the direction of the Z axis is shown in the figure, and the actual position of the Z axis should be the rotation axis of the helix. In this way, the projection data obtained by 4 groups of 16 rows of detectors in total are determined. It can be seen from the aforementioned scanning projection process that the Z-axis coordinates of these 16 rows of detectors are known. From them, two layers of projection data whose Z-axis coordinate positions are closest to ZR are selected (such as A and B in FIG. 3 ). The Z-axis coordinates of these two rows of data are respectively recorded as Z1 and Z2 (Z1 at A, Z2 at B), and the projection data of these two layers are respectively recorded as P1 and P2 (P1 at A and P2 at B) . Therefore, the interpolated projection data at α can be obtained by the following formula:

P(α)＝P ₁ +(P ₂ -P ₁ )(Z _R -Z ₁ )/(Z ₂ -Z ₁ ) (1)

Let α change continuously according to the previous scanning rotation interval between [β, β+360], calculate and record P(α) under the corresponding α, and obtain all interpolated projection data at ZR.

(6) Reconstruct the image on the plane with the two-dimensional filtered back projection algorithm.

Using the filter back projection reconstruction algorithm of equiangular fan beams, according to Fig. 4, in the figure, S represents the X-ray emission source; is negative); D represents the distance between the source and the center O; β represents the rotation angle of the source about the Y axis; r represents the line connecting the point (x, y) in the plane and the center O; φ represents the angle between r and the X axis. Note that f(x, y) is the value at the reconstruction point; p(β′, γ) is the projection data after interpolation (for the convenience of calculation, let ${\mathrm{\β}}^{\′}=\mathrm{\β}+\frac{\mathrm{\π}}{2}$ ), wherein, γ=nα, α is the angle between every two X-rays in the equiangular fan beam. L is the distance from S to the point (x, y) and L is the distance from S to the point (x, y).

First, calculate p′(β′,γ): p′(β′,γ)=p(β′,γ)Dcosγ (2)

Second, convolving p'(β', γ) and the filter g(γ) yields:

F(β′)=p′(β′,γ)*g(γ) (3)

in, $g\left(\mathrm{n\α}\right)=\frac{2{\mathrm{no}}^2}{\mathrm{\π}(4{\mathrm{no}}^2-1)\mathrm{the\; si}{\mathrm{no}}^2\mathrm{n\α}},\mathrm{no}-0,\±1,\±2,\±3,...\left(4\right)$

Finally, use the following formula to reconstruct the original image (beta' must be discretized).

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; L</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\underset{\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{<span\; class="notranslate">\; \&Integral;</span>}}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

(7) Repeat steps 5 and 6 to obtain a series of reconstructed images perpendicular to the Z-axis plane.

(8) Using the series of reconstructed images perpendicular to the Z-axis plane, a three-dimensional image of the live mouse is generated, and the reconstructed three-dimensional image or two-dimensional tomographic image is displayed within a specified range.

The implementation effect is shown in the table below: Single-turn Z-axis coverage scanning speed X-ray total dose Live mouse reconstruction image quality Dual-source double-helix 4-slice CT 2 2 1 clear Single-source single-helix 4-slice CT 1 1 1 Vague

The single-source single-helix 4-slice CT in the table removes a group of X-ray sources and 4 rows of detectors from the dual-source double-helix 4-slice CT, and other parameters remain unchanged. Quantitative indicators take the results of single-source single-helix 4-slice CT as a unit.

The table above shows the beneficial implementation effects.

Claims (8)

1, a kind of method for reconstructing based on double source Double helix multi-layer spiral CT, it is characterized in that, adopt the structure of two x-ray sources, two many detector row to scan with the Double helix path, improved the data for projection picking rate, utilize the multilamellar data for projection on two paths, by double source Double helix multilamellar Z axle interpolation algorithm, calculate and rebuild one perpendicular to the required data for projection of the plane picture of Z axle, filter back-projection algorithm by two dimension reconstructs this plane picture again, goes out 3-D view by a series of such planar reconstructions at last.

2, the method for reconstructing based on double source Double helix multi-layer spiral CT according to claim 1 is characterized in that, may further comprise the steps:

(1) first x-ray source sends the cone beam X ray on one point, after passing measurand, the X ray of decay is detected by first many detector row of offside, second x-ray source sends the cone beam X ray on one point, after passing measurand, the X ray of decay is detected by the detector row more than second of offside, and first and detector row more than second all are that single detector forms along the Z uranium pile is folded among corresponding single source single-screw monolayer CT;

(2) by first x-ray source perpendicular to the plane parallel of Z axle in by the plane of second x-ray source perpendicular to the Z axle, first x-ray source to the distance of Z axle equates with the distance of second x-ray source to the Z axle, first many detector row center equates with the distance of detector row center more than second to the Z axle to the distance of Z axle, and first x-ray source and second x-ray source be not on the same straight line parallel with the Z axle;

(3) first x-ray source, first many detector row, second x-ray source, detector row more than second with identical speed around Z axle turning cylinder, for the helical movement with respect to measurand, first x-ray source and second x-ray source are bifilar helixs with respect to the track of measurand;

(4) first many detector row, detector row more than second and corresponding data for projection acquisition system are gathered two groups of multilamellar data for projection respectively;

(5) determine a rebuilding plane perpendicular to the Z axle,, spend interpositionings or 360 ° of interpositioning interpolations with 180 and go out data for projection on this plane with double source Double helix multilamellar Z axle interpolation algorithm;

(6) reconstruct image on this plane with the two-dimensional filtering backprojection algorithm;

(7) repeat (5), (6), obtain a series of reconstructed images perpendicular to the Z axial plane;

(8), generate the 3-D view of measurand, three-dimensional image or two-dimensional ct image after in specified scope, demonstrating reconstruction with these a series of reconstructed images perpendicular to the Z axial plane.

3, the method for reconstructing based on double source Double helix multi-layer spiral CT according to claim 2, it is characterized in that, in the step (4), described data for projection acquisition system is meant and the data record on many detector row is got off and changes into the system that digital signal is passed to Computer Processing.

4, according to claim 1 or 2 described method for reconstructing based on double source Double helix multi-layer spiral CT, it is characterized in that, described double source Double helix multilamellar Z axle interpolation algorithm is meant: rebuild a certain planar image perpendicular to the Z axle, according to two spiral paths on this plane the multilamellar fladellum data for projection of each position in each circle spiral up and down, interpolation goes out the algorithm of the data for projection on this plane, participates in the multilamellar data for projection of data for projection from two spirals.

5, the method for reconstructing based on double source Double helix multi-layer spiral CT according to claim 2, it is characterized in that, described 180 ° of interpositionings are meant: when interpolation goes out data for projection on this plane, use two spiral paths on this plane up and down each half-turn add the multilamellar fladellum data for projection of each position on the spiral of segment angle scope, described segment angle is meant in one group of x-ray source-many detector row, the two-end-point of a detector row respectively with the angle of x-ray source line.

6, the method for reconstructing based on double source Double helix multi-layer spiral CT according to claim 2, it is characterized in that, described 360 ° of interpositionings are meant: when interpolation goes out data for projection on this plane, use two spiral paths on this plane the multilamellar fladellum data for projection of each position on spiral of each circle up and down.

7, according to claim 1 or 2 or 6 described method for reconstructing, it is characterized in that data for projection is the fladellum data for projection, or the parallel beam data for projection based on double source Double helix multi-layer spiral CT.

8, according to claim 1 or 2 described method for reconstructing, it is characterized in that described two-dimensional filtering backprojection algorithm is meant: a data for projection and a filter function are carried out convolution algorithm, carry out the algorithm of back projection then based on double source Double helix multi-layer spiral CT.

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