CN101987021A - Scattering correction method of CT system and CT system - Google Patents

Abstract

A scatter correction method for a CT system, comprising the following steps: acquiring a bright-field image; placing a scatter corrector between an object to be scanned and a detector, performing equiangular circular scanning to obtain an attenuation projection image; The calibrator performs an equiangular circular scan to obtain a projection image set and a scatter correction image; a scattering intensity distribution map is generated according to the bright field image, a scatter correction image, and an attenuation projection image; and the difference between the projection image set and the scatter intensity distribution map is obtained Corrected set of projected images. In the scatter correction method of the above-mentioned CT system and in the CT system, the influence of scatter is eliminated by placing the scatter corrector between the scanned object and the detector, which greatly reduces the scanning time and the amount of data processed, and effectively improves the efficiency and precision.

Description

Scatter Correction Method of CT System and CT System

【Technical field】

The invention relates to a simulation imaging technology, in particular to a scattering correction method of a CT system and a CT system.

【Background technique】

CT imaging technology (computed tomography, electronic computer X-ray tomography technology) has multiple functions of X-ray fluoroscopy, photographing and volume imaging of the treatment position. A ray consists of two components, the initial ray and the scattered ray. The initial rays generate signals and then generate images, and the scattered rays generate noise and image artifacts, such as cupping artifacts and streak artifacts, which reduce the contrast of the projected image and result in inaccurate reconstructed CT values. Linear array detectors are used in fan-beam CT systems, which are only affected by one-dimensional scattering and will not generate a large amount of scattered rays. A very good scattering suppression effect can be achieved by installing a collimator. However, the cone-beam CT system is due to the detector The area array detector is used, so that the scattering is distributed in two dimensions, and the irradiation of scattered rays is much larger, so it is necessary to try to eliminate the influence of scattering.

In the cone-beam CT system, since the scattering is distributed in two dimensions, it is difficult to achieve the desired effect by a single hardware correction or software correction method. Therefore, a hybrid correction method combining hardware and software has emerged. The current correction methods include the following three :

(1) Frequency modulation method. The frequency modulation method is to place a grid with different penetrations between the scanned object and the detector. The direct primary photon and the scattered photon have different frequency response characteristics for the modulator, that is, the direct photon is modulated into a high-frequency signal, while the scattered photon is modulated into a low-frequency signal, and then the scattered component is converted from the projected image by using a nonlinear frequency domain filter. separated in the spectrum. This method has the advantage of simple scattering correction equipment, but for different scanned objects and different imaging precision, it is necessary to adjust the appropriate grid spacing.

(2) Primary modulation method. In the primary modulation method, a calibration sheet needs to be made. A piece of aluminum plate with a thickness of 2mm is evenly covered with square protrusions, the side length of the protrusions is 2mm, the spacing is 2mm, and the thickness is 1mm. As a modulation encoder, the calibration sheet is placed between the ray source and the object to be scanned. The process of the primary modulation method can be divided into the following steps: the primary ray emitted by the ray source is "modulated and coded" by the calibration sheet, and then passes through the object to be scanned. In the frequency domain, the transmitted ray received by the detector carries coded information, and the scattered ray is low-frequency noise. The high-pass filter is used to filter the projection to deduct the influence of scattering, but the primary modulation method requires very high requirements in the actual application process. high and difficult to achieve.

In addition, after setting the scanning parameters, the air projection image and the beam attenuation grid projection image can be collected, and the object to be detected can be scanned circularly, and the projection image set I with the beam attenuation grid and the object to be detected can be collected and the object to be scanned II. Calculate the projection position of the center of each metal ball in the beam attenuation grid, use the beam attenuation grid correction method to calculate the scattered field distribution image corresponding to the projection image in the projection image set I, and convert the projection image set to I subtracted the corresponding scattering field distribution image to obtain the projection image set III after scattering correction, and then reconstructed the serial slice sequence after scattering correction through the FBP (Filtered Back Projection) algorithm. In this method, the beam attenuation grid is placed between the ray source and the scanned object, close to the scanned object, since the outgoing light first passes through the beam attenuation grid, the spectrum of the outgoing light changes. In addition, for the same scanned object, it needs to be scanned twice, the scanning time is doubled, and the amount of processed data is also doubled, which is very inefficient.

【Content of invention】

Based on this, it is necessary to provide a scatter correction method for a CT system that can improve efficiency.

In addition, there is a need to provide a CT system that can improve efficiency.

A scatter correction method for a CT system, comprising the following steps: acquiring a bright-field image; placing a scatter corrector between an object to be scanned and a detector, performing equiangular circular scanning to obtain an attenuation projection image; The calibrator performs an equiangular circular scan to obtain a projection image set and a scatter correction image; a scattering intensity distribution map is generated according to the bright field image, a scatter correction image, and an attenuation projection image; and the difference between the projection image set and the scatter intensity distribution map is obtained Corrected set of projected images.

Preferably, the scattering corrector is a small ball embedded in a thin plate, the absorption coefficient of the thin plate is smaller than that of the small ball.

Preferably, said pellets are distributed in a checkerboard manner in said sheet.

Preferably, the step of generating a scattering intensity distribution map according to the bright field image, the scattering correction image and the attenuation projection image is: obtaining the center coordinates of each projection circle from the scattering correction image through a genetic algorithm; obtaining The initial ray intensity corresponding to each projection circle, and obtain the ray intensity corresponding to each projection circle after passing through the scattering corrector and the corrected total ray intensity from the scatter correction image; obtain from the projection image set The total ray intensity of the object; the scatter value distribution in the attenuation projection image is obtained by the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity and the total ray intensity of the object; the scatter value distribution is binary Dimensional interpolation and angle interpolation are used to obtain the distribution map of scattering intensity.

Preferably, after the step of obtaining the corrected projection image set through the difference between the projection image set and the scattering intensity distribution map, the step further includes: performing image reconstruction on the corrected projection image set.

A CT system at least includes: a scanning module, configured to acquire a bright-field image, perform equiangular scanning on an object to be scanned and a scatter corrector placed between the object to be scanned and a detector to obtain an attenuated projection image, and respectively Perform equiangular circular scanning on the scanned object and the scatter corrector to obtain a projection image set and a scatter correction image; a processing module is used to generate a scatter intensity distribution map according to the bright field image, the scatter correction image and the attenuation projection image; the correction module, A corrected projection image set is obtained by using the difference between the projection image set and the scattering intensity distribution map.

Preferably, the scattering corrector is a small ball embedded in a thin plate, the absorption coefficient of the thin plate is smaller than that of the small ball.

Preferably, said pellets are distributed in a checkerboard manner in said sheet.

Preferably, the processing module includes: a position acquisition unit, used to obtain the center coordinates of each projection circle from the scatter correction image through a genetic algorithm; an intensity acquisition unit, used to obtain the center coordinates of each projection circle from the bright field image The corresponding initial ray intensity, and obtain the ray intensity corresponding to each projection circle after passing through the scatter corrector and the corrected total ray intensity from the scatter correction image, and obtain the total ray intensity of the object from the projection image set; the calculation unit , used to obtain the scatter value distribution in the attenuation projection image through the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity and the total ray intensity of the object; the interpolation unit is used to calculate the scatter value Two-dimensional interpolation and angle interpolation are performed on the distribution to obtain the distribution map of scattering intensity.

Preferably, it further includes: a reconstruction module, configured to perform image reconstruction on the corrected projection image set. In the scatter correction method of the above-mentioned CT system and in the CT system, the influence of scatter is eliminated by placing the scatter corrector between the scanned object and the detector, which greatly reduces the scanning time and the amount of data processed, and effectively improves the efficiency and precision.

The scatter correction method for the CT system and the scatter corrector in the CT system are simple in structure, low in manufacturing cost, easy to implement, and suitable for environments where the properties of scanned objects vary greatly.

【Description of drawings】

Fig. 1 is a scatter correction method of a CT system in an embodiment;

Figure 2 is a schematic diagram of a scatter corrector in one embodiment;

Fig. 3 is a flow chart of generating a scattering intensity distribution map according to a bright field image, a scattering correction image and an attenuation projection image in one embodiment;

Fig. 4 is a schematic diagram of scanning bright field in an embodiment;

Fig. 5 is a schematic diagram of placing an object to be scanned and adding a scatter corrector in one embodiment;

Fig. 6 is a schematic diagram of scanning only the scanned object in one embodiment;

Figure 7 is a schematic diagram of a scanning-only scatter corrector in one embodiment;

Figure 8 is a detailed block diagram of the CT system in one embodiment;

Figure 9 is a schematic diagram of a processing module in one embodiment.

【Detailed ways】

Fig. 1 shows a scatter correction method of a CT system in an embodiment, including the following steps:

In step S10, a bright field image is acquired. In this embodiment, no object is placed in the scanning field of view, and the bright field image is obtained by scanning the bright field. The scanned object is not placed in the imaging field of view, and the scanned image obtained by turning on the light source is the current image.

In step S20, the scattering corrector is placed between the object to be scanned and the detector, and an equiangular circular scan is performed to obtain an attenuation projection image. In this embodiment, as shown in FIG. 2 , the scatter correction device (SCD) is embedded with small balls in the thin plate, and the absorption coefficient of the thin plate is smaller than that of the small balls. The pellets are distributed in a checkerboard pattern in the sheet. The size of the scatter corrector can be between the scanned object and the detector. In order to make the actual operation easier, the size of the scatter corrector can be consistent with the size of the detector to meet the needs, that is, preferably 50mm*50mm ~500mm*500mm range. The thickness of the scatter corrector must be greater than the radius of the ball so that the sphere can be fixed. Therefore, the thickness of the scatter corrector is preferably in the range of 1 mm to 50 mm. In a specific embodiment, a scattering corrector with a size of 50mm*50mm and a thickness of 1mm-5mm is selected. In another embodiment, the balls can also be distributed in other ways, and the diameter depends on the actual situation. For example, for the current industrial and medical CT systems, the parameters of the balls are: the diameter range is 0.5 mm to 20 mm; The range of hole depth is 0.25mm to 10mm; the range of center distance is 1mm to 40mm. In a preferred embodiment, the diameter ranges from 0.5 mm to 1.5 mm; the hole depth ranges from 0.25 mm to 1 mm; the center-to-center distance ranges from 1 mm to 4 mm. The thin plate is preferably a polyethylene plastic plate, but other materials with low absorption coefficient can also be used, such as cardboard, wood veneer, etc. The pellets are preferably metal pellets, but can also be other high absorption materials, such as lead balls.

In the process of X-rays passing through the scanned object and entering the detector, if the ray does not react with the scanned object, this ray can be called the initial ray. If there is a reaction, the X-ray energy ranges from 0.1 to 1 MeV. Mainly due to the Compton effect, this kind of ray can be called scattered ray. Therefore, the radiation absorbed by the detector consists of two parts, namely the initial radiation and the scattered radiation. For industrial CT and medical CT, the scattered radiation in the energy range of 0kv ~ 450kv is mostly produced by the Compton effect, while the cone beam CT Scattered rays also exist in the low-energy system with an energy range of 20kv to 90kv in the system, and the influence of scattered rays is eliminated by the function of the scattering corrector during the scanning process.

As for the position of the scatter corrector, there are four positions for the scatter corrector to be placed in the field of view: (1) the scatter corrector is placed close to the light source; (2) the scatter corrector is placed between the scanned object and the detector (3) place the scatter corrector between the scanned object and the detector close to the scanned object; (4) place the scatter corrector between the scanned object and the detector close to The position of the detector. For position (1) and position (2), the scatter corrector is placed in front of the object to be scanned along the propagation direction of the ray, at this time the ray first passes through the scatter corrector, and the scattering distribution has not yet formed; for position (4), due to The scatter corrector is close to the detector, and the ball in the scatter corrector will absorb the initial ray and the scattered ray, thus changing the intensity distribution of the scatter; only position (3) after the ray passes through the scanned object, the scatter distribution has been formed, At this time, it passes through the scattering corrector, and the ball absorbs the initial ray, that is, the ray that has not changed its direction, so what is obtained at the corresponding position on the detector is the scattered ray and the attenuated initial ray. It can be seen that, during the scanning process, the scattering corrector is placed between the object to be scanned and the detector, and as close as possible to the object to be scanned.

Since the spatial variation of the scattering intensity distribution is not large, even if the scanning angle interval is increased, the variation of the scattering intensity distribution is still small. After adding the scattering corrector, only 60 equiangular circular scans are required to obtain 60 images.

In step S30, the scanned object and the scatter corrector are scanned at equal angles to obtain a projected image set and a scatter corrected image. In this embodiment, only the scanned object is placed in the field of view, and the scanned object is scanned 360 times at equal angular intervals to obtain a projection image set, and then the scanned object is removed, and only the scattering corrector is placed for scanning to obtain the scattering Correct the image.

In step S40, a scattering intensity distribution map is generated according to the bright field image, the scattering correction image and the attenuation projection image. In this embodiment, data are extracted from bright field images, scatter correction images, and attenuation projection images respectively, and a scattering intensity distribution map is generated by genetic algorithm and Lambert's law, so as to correct the projection image set.

In a specific embodiment, as shown in FIG. 3, the steps of generating a scattering intensity distribution map according to the bright field image, the scattering correction image and the attenuation projection image are:

In step S401, the center coordinates of each projection circle are obtained from the scatter-corrected image through a genetic algorithm. In this embodiment, a series of small ball projection circle images are obtained from the scatter correction image, so as to obtain the center coordinates of each projection circle through a genetic algorithm. For example, Matlab can be used to write a program, and a genetic algorithm can be used to find One point and one radius, so that the sum of the squares of the distance from each point to this point and the radius is the smallest, and then the center coordinates of each projection circle in the scatter-corrected image can be obtained.

In step S402, the initial ray intensity corresponding to each projection circle is obtained from the bright field image, and the ray intensity corresponding to each projection circle after passing through the scattering corrector is obtained from the scatter correction image, and the attenuation projection image The corrected total ray intensity in . In this embodiment, as shown in Figure 4, the initial ray intensity I ₀ of the corresponding point of the center coordinates of each projection circle can be measured separately from the bright field image, and the corresponding ray intensity I 0 can be obtained from the scattering correction image according to the center coordinates of each projection circle. The ray intensity I ₃ after passing through the scattering corrector, as shown in FIG. 5 , is measured in the attenuation projection image to obtain the corrected total ray intensity D ₁ . The corrected total ray intensity _D1 is the total ray intensity received on the detector when the scatter corrector is placed between the scanned object and the detector.

In step S403, the total ray intensity of the object is obtained from the projection image set. In this embodiment, as shown in Figure 6, the total ray intensity _D2 of the object measured in the projected image set is the value received by the detector when only the scanned object is placed and the scanned object is scanned 360 times at equal angular intervals. total ray intensity.

In step S404, the scatter value distribution in the attenuation projection image is obtained through the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity, and the total object ray intensity. In the present embodiment, according to Lambert's law, as shown in Figures 5 to 7, formula (1) can be obtained by Figure 6, formula (2) and formula (3) can be obtained by Figure 5, formula (3) can be obtained by Figure 7 4):

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

I ₂ =D ₁ -S...(3)

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Among them, μ ₁ is the linear attenuation coefficient of the scanned object, μ ₂ is the linear attenuation coefficient of the small ball in the scattering corrector, l ₁ is the path length through the scanned object, l ₂ is the diameter of the small ball, and S is the scattered ray Intensity, I ₁ is the intensity of rays passing through the scanned object, and I ₂ is the intensity of rays passing through the ball.

Put formula (1) into (2) to get:

将公式(2)带入公式(5)得： By formula (2)

Put formula (2) into formula (5) to get:

According to the formula (6), the scattered ray intensity S of each projection circle in the attenuation projection image can be obtained, and then the scattering value distribution of each small ball in all attenuation projection images can be obtained.

In step S405, two-dimensional interpolation and angle interpolation are performed on the distribution of scattering values to obtain a distribution diagram of scattering intensity. In this embodiment, since the spatial variation of scattered ray intensity is slow and the spatial frequency is low, two-dimensional interpolation can be performed on each image in the scatter value distribution to obtain the same number of scatter intensity distribution maps as the attenuation projection images. At this time, angle interpolation is performed on points at the same pixel position in the scattering intensity distribution map after two-dimensional interpolation to obtain a set of scattering intensity distribution maps corresponding to the projection image set.

In step S50, the corrected projection image set is obtained by the difference between the projection image set and the scattering intensity distribution map. In this embodiment, the corrected image is obtained by sequentially subtracting the scattering intensity distribution map from the projected image set.

In other embodiments, the scatter correction method for the above CT system further includes the step of performing image reconstruction on the corrected projection image set. In this embodiment, a BPF algorithm (Backprojection Filtration, back-projection filtering reconstruction algorithm) may be used for image reconstruction.

In addition, it is also necessary to provide a CT system to which the above scatter correction method is applied. As shown in FIG. 8 , the system at least includes a scanning module 10 , a processing module 20 and a correction module 30 .

The scanning module 10 is used to acquire bright-field images, scan the scanned object and the scatter corrector placed between the scanned object and the detector at an equal angle to obtain an attenuation projection image, and separately scan the scanned object and the scatter corrector Equiangular circular scans yield projected image sets and scatter-corrected images. In this embodiment, no object to be scanned is placed in the imaging field of view, and the scanned image obtained by turning on the light source is the field image. Scatter correctors refer to the embedding of small spheres in thin plates whose absorption coefficient is smaller than that of the small spheres. The pellets are distributed in a checkerboard pattern in the sheet. The size of the scatter corrector can be between the scanned object and the detector. In order to make the actual operation easier, the size of the scatter corrector can be consistent with the size of the detector to meet the needs, that is, preferably 50mm*50mm ~500mm*500mm range. The thickness of the scatter corrector must be greater than the radius of the ball so that the sphere can be fixed. Therefore, the thickness of the scatter corrector is preferably in the range of 1 mm to 50 mm. In a specific embodiment, a scattering corrector with a size of 50mm*50mm and a thickness of 1mm-5mm is selected. In another embodiment, the balls can also be distributed in other ways, and the diameter depends on the actual situation. For example, for the current industrial and medical CT systems, the parameters of the balls are: the diameter range is 0.5 mm to 20 mm; The range of hole depth is 0.25mm to 10mm; the range of center distance is 1mm to 40mm. In a preferred embodiment, the diameter ranges from 0.5 mm to 1.5 mm; the hole depth ranges from 0.25 mm to 1 mm; the center-to-center distance ranges from 1 mm to 4 mm. The thin plate is preferably a polyethylene plastic plate, but other materials with low absorption coefficient can also be used, such as cardboard, wood veneer, etc. The pellets are preferably metal pellets, but can also be other high absorption materials, such as lead balls.

In the process of X-rays passing through the scanned object and entering the detector, if the ray does not react with the scanned object, this ray can be called the initial ray. If there is a reaction, the X-ray energy ranges from 0.1 to 1 MeV. Mainly due to the Compton effect, this kind of ray can be called scattered ray. Therefore, the radiation absorbed by the detector consists of two parts, namely the initial radiation and the scattered radiation. For industrial CT and medical CT, the scattered radiation in the energy range of 0kv ~ 450kv is mostly produced by the Compton effect, while the cone beam CT Scattered rays also exist in low-energy systems with medium energy ranging from 20kv to 90kv. During the scanning process, the effect of scattered rays is eliminated by the function of the scattering corrector.

As for the position of the scatter corrector, there are four positions for the scatter corrector to be placed in the field of view: (1) the scatter corrector is placed close to the light source; (2) the scatter corrector is placed between the scanned object and the detector (3) place the scatter corrector between the scanned object and the detector close to the scanned object; (4) place the scatter corrector between the scanned object and the detector close to The position of the detector. For position (1) and position (2), the scatter corrector is placed in front of the object to be scanned along the propagation direction of the ray, at this time the ray first passes through the scatter corrector, and the scattering distribution has not yet formed; for position (4), due to The scatter corrector is close to the detector, and the ball in the scatter corrector will absorb the initial ray and the scattered ray, thus changing the intensity distribution of the scatter; only position (3) after the ray passes through the scanned object, the scatter distribution has been formed, At this time, it passes through the scattering corrector, and the ball absorbs the initial ray, that is, the ray that has not changed its direction, so what is obtained at the corresponding position on the detector is the scattered ray and the attenuated initial ray. It can be seen that, during the scanning process, the scattering corrector is placed between the object to be scanned and the detector, and as close as possible to the object to be scanned.

No object is placed in the scanning field of view, and the scanning module 10 scans the bright field to obtain a bright field image. Since the spatial variation of the scattering intensity distribution is not large, even if the scanning angle interval is increased, the variation of the scattering intensity distribution is still small. After the scattering corrector is added, the scanning module 10 only needs to perform equiangular circular scanning 60 times to obtain 60 images. .

Only the scanned object is placed in the field of view, and the scanning module 10 scans the scanned object 360 times at equal angular intervals to obtain a projection image set, and then removes the scanned object, and only places the scattering corrector for scanning to obtain a scattering corrected image .

The processing module 20 is configured to generate a scattering intensity distribution map according to the bright field image, the scattering correction image and the attenuation projection image. In this embodiment, the processing module 20 extracts data from bright-field images, scatter-corrected images, and attenuation projection images, and generates a scattering intensity distribution map through genetic algorithm and Lambert's law, so as to correct the projection image set.

In a specific embodiment, as shown in FIG. 9 , the processing module 20 includes a position acquisition unit 201 , an intensity acquisition unit 202 , a calculation unit 203 and an interpolation unit 204 .

The position obtaining unit 201 is configured to obtain the center coordinates of each projected circle from the scatter correction image through a genetic algorithm. In this embodiment, the position acquisition unit 201 obtains a series of small ball projection circle images with high absorption coefficients from the scatter correction image, thereby obtaining the center coordinates of each projection circle through a genetic algorithm. For example, the position acquisition unit 201 can use Matlab Write a program and use the genetic algorithm to find a point and a radius in the projection circle, so that the sum of the squares of the distance from each point to this point and the radius is the smallest, and then the center coordinates of each projection circle in the scatter-corrected image can be obtained.

The intensity acquisition unit 202 is configured to obtain the initial ray intensity corresponding to each projection circle from the bright field image, and obtain the ray intensity corresponding to each projection circle after passing through the scattering corrector and the corrected ray intensity from the scatter correction image. Total ray intensity, obtain the total ray intensity of the object from the projection image set. In this embodiment, the intensity acquisition unit 202 can respectively measure the initial ray intensity I ₀ of the corresponding point of the center coordinates of each projection circle from the bright field image, and obtain the corresponding penetration intensity I 0 from the scattering correction image according to the center coordinates of each projection circle. The ray intensity I ₃ after passing through the scattering corrector is measured in the attenuation projection image to obtain the corrected total ray intensity D ₁ . The corrected total ray intensity _D1 is the total ray intensity received on the detector when the scatter corrector is placed between the scanned object and the detector. The total ray intensity _D2 measured by the intensity acquisition unit 202 in the projection image set is the total ray intensity received by the detector when only the scanned object is placed and the scanned object is scanned 360 times at equal angular intervals.

The calculation unit 203 is configured to obtain the distribution of scattering values in the attenuation projection image through the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity, and the total object ray intensity. In this embodiment, the calculation unit 203 calculates the distribution of scattering values in the attenuation projection image by the following formula according to Lambert's law.

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

I ₂ =D ₁ -S...(3)

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Among them, μ ₁ is the linear attenuation coefficient of the scanned object, μ ₂ is the linear attenuation coefficient of the small ball in the scattering corrector, l ₁ is the path length through the scanned object, l ₂ is the diameter of the small ball, and S is the scattered ray Intensity, I ₁ is the intensity of rays passing through the scanned object, and I ₂ is the intensity of rays passing through the ball.

Put formula (1) into (2) to get:

将公式(2)带入公式(5)得：

By formula (2)

Put formula (2) into formula (5) to get:

According to the formula (6), the scattered ray intensity S of each projection circle in the attenuation projection image can be obtained, and then the scattering value distribution of each small ball in all attenuation projection images can be obtained.

The interpolation unit 204 is configured to perform two-dimensional interpolation and angle interpolation on the scatter value distribution to obtain a scatter intensity distribution map. In this embodiment, since the scattered ray intensity varies slowly in space and the spatial frequency is low, the interpolation unit 204 can perform two-dimensional interpolation on each image in the scatter value distribution to obtain a scatter intensity distribution map with the same number of attenuation projection images . At this time, angle interpolation is performed on points at the same pixel position in the scattering intensity distribution map after two-dimensional interpolation to obtain a set of scattering intensity distribution maps corresponding to the projection image set.

The correction module 30 is configured to obtain a corrected projection image set through the difference between the projection image set and the scattering intensity distribution map. In this embodiment, the correction module 30 sequentially subtracts the scattering intensity distribution map from the projected image set to obtain the corrected image.

In other embodiments, the above-mentioned CT system further includes a reconstruction module, which is used to perform image reconstruction on the corrected projection image set. In this embodiment, the reconstruction module can apply the BPF algorithm to perform image reconstruction.

The scatter correction method of the above-mentioned CT system and the application process of the CT system will be described below in conjunction with a detailed embodiment. In this embodiment, a series of small ball projection circle images can be obtained from the scatter correction image, and a point and a radius R are found in the projection circle using a genetic algorithm, so that the sum of the squares of the distances from each point to this point and R The minimum coordinates (u _i , v _i ) of each projected circle center in the scatter-corrected image can be obtained.

在衰减投影图像和投影图像集中分别测量(u_i，v_u)对应点的校正的总射线强度D₁和物体总射线强度D₂，由公式(6)可以得出衰减投影图像中的散射值分布。In the bright-field image, the value of the initial ray intensity I ₀ corresponding to the point (u _i , v _i ) can be measured separately, and the value of I ₃ corresponding to (u _i , v _i ) can be measured in the scatter-corrected image . It can be calculated by formula (4)

Measure the corrected total ray intensity D ₁ of the corresponding point (u _i , v _u ) and the total ray intensity D ₂ of the object in the attenuation projection image and the projection image set respectively, and the scattering value in the attenuation projection image can be obtained from the formula (6) distributed.

Two-dimensional interpolation is performed on the distribution of scattering values, and interpolation by angle is obtained to obtain the distribution map of scattering intensity. The difference between the projection image set and the scattering intensity distribution map is the corrected projection image set.

进而得到散射值分布。For example, assuming that the coordinates of the center of a projected circle calculated from the scatter-corrected image are (800, 500), in the bright-field image, the measured I ₀ is 10000. In the scatter-corrected image, the I ₃ is 5000, then the formula (4 )have to The D ₁ of the (800, 500) position obtained in the attenuation projection image is 2000, and the D ₂ of the (800, 500) position obtained from the projection image set is 3000, then the (800, 500) position can be obtained by formula (6) The scattered ray intensity

Then the distribution of scatter values is obtained.

The scattering correction method of the above CT system and the detector in the CT system adopt an area array detector.

In the scatter correction method of the above-mentioned CT system and in the CT system, the influence of scatter is eliminated by placing the scatter corrector between the scanned object and the detector, which greatly reduces the scanning time and the amount of data processed, and effectively improves the efficiency and precision.

The scatter correction method for the CT system and the scatter corrector in the CT system are simple in structure, low in manufacturing cost, easy to implement, and suitable for environments where the properties of scanned objects vary greatly.

The above-mentioned embodiments only express several implementation modes of the present invention, and the description thereof is relatively specific and detailed, but should not be construed as limiting the patent scope of the present invention. It should be pointed out that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention should be based on the appended claims.

Claims (10)

1. A scatter correction method for a CT system, comprising the following steps: Get a bright field image; Place the scatter corrector between the scanned object and the detector, and perform equiangular circular scanning to obtain attenuated projection images; Perform equiangular circular scanning on the scanned object and the scatter corrector respectively to obtain the projected image set and the scatter corrected image; generating a scatter intensity distribution map based on the bright field image, the scatter corrected image, and the attenuated projection image; A corrected projection image set is obtained through the difference between the projection image set and the scattering intensity distribution map. 2 . The method for scatter correction of a CT system according to claim 1 , wherein the scatter corrector is to embed small balls in a thin plate, and the absorption coefficient of the thin plate is smaller than that of the small balls. 3 . 3 . The method for scatter correction of a CT system according to claim 2 , wherein the small balls are distributed in a chessboard in the thin plate. 4 . 4. The scatter correction method of the CT system according to claim 1, wherein the step of generating a scatter intensity distribution map according to the bright field image, the scatter correction image and the attenuation projection image is: The center coordinates of each projected circle are obtained from the scatter-corrected image by genetic algorithm; Obtain the initial ray intensity corresponding to each projection circle from the bright field image, and obtain the ray intensity after passing through the scattering corrector and the corrected total ray intensity corresponding to each projection circle from the scatter correction image ; Obtain the total ray intensity of the object from the projection image set; obtaining the distribution of scattering values in the attenuation projection image through the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity, and the total ray intensity of the object; Two-dimensional interpolation and angle interpolation are performed on the distribution of scattering values to obtain a distribution diagram of scattering intensity. 5. The scatter correction method of the CT system according to claim 1, characterized in that, after the step of obtaining the corrected projection image set through the difference between the projection image set and the scattering intensity distribution map, it also includes: Image reconstruction is performed on the corrected projection image set. 6. A CT system, characterized in that it comprises at least: The scanning module is used to obtain bright-field images, scan the scanned object and the scatter corrector placed between the scanned object and the detector at an equal angle to obtain an attenuation projection image, and separately scan the scanned object and the scatter corrector Perform equiangular circular scans to obtain projection image sets and scatter-corrected images; A processing module, configured to generate a scattering intensity distribution map according to the bright field image, the scattering correction image and the attenuation projection image; A correction module, configured to obtain a corrected projection image set from the difference between the projection image set and the scattering intensity distribution map. 7. The CT system according to claim 6, wherein the scatter corrector is a small ball embedded in a thin plate, and the absorption coefficient of the thin plate is smaller than that of the small ball. 8. The CT system according to claim 7, wherein the spheres are distributed in a checkerboard in the thin plate. 9. The CT system according to claim 6, wherein the processing module comprises: A position acquisition unit, configured to obtain the center coordinates of each projected circle from the scatter correction image through a genetic algorithm; An intensity acquiring unit, configured to obtain initial ray intensities corresponding to the respective projection circles from the bright field image, and obtain ray intensities corresponding to the respective projection circles after passing through the scattering corrector from the scatter correction image And the corrected total ray intensity, the total ray intensity of the object is obtained from the projection image set; A calculation unit, configured to obtain the distribution of scattering values in the attenuation projection image through the initial ray intensity, the ray intensity after passing through the scatter corrector, the corrected total ray intensity, and the total object ray intensity; The interpolation unit is configured to perform two-dimensional interpolation and angle interpolation on the distribution of scattering values to obtain a distribution diagram of scattering intensity. 10. The CT system according to claim 6, further comprising: A reconstruction module, configured to perform image reconstruction on the corrected projection image set.

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