JPH1033520A - Cone beam x-ray tomography system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve picture quality and the quantitativeness of a CT value by finding the close of X-rays absorbed in a reagent and the thickness of an X-ray absorber plate for equalizing the dose of absorbed X-rays, comparing this thickness with predetermined absorber thickness, finding the ratio of scattered X-ray component and correcting the scattered X-rays of a two-dimensional X-ray image based on this ratio. SOLUTION: Offset level correction and diffused light correction processing is performed to a reagent projecting image C0, and a diffused light corrected image C3 is formed. On the other hand, a diffused light corrected image D3 is prepared from an air projecting image D0. Next, a reagent sensitivity distribution corrected image C5 is prepared by subtracting the logarithmic image of air from the logarithmic image of reagent. A distortion corrected image C6 is prepared by applying geometrical distortion correction to this image C5. A scattered X-ray component image C7 is found by convolving scattered X-ray distribution into this image C6. A scattered X-ray corrected image C8 is prepared by subtracting this image C7 from the distortion corrected image C6. Thus, distortion and sensitivity distribution or the like can be corrected and picture quality and CT value quantitativeness can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cone beam X-ray imaging apparatus, and more particularly to a technique for improving the quantitativeness of an image when three-dimensionally imaging the inside of a subject.

[0002]

2. Description of the Related Art Conventionally, as a technique for imaging a three-dimensional distribution inside a subject, an X-ray transmission image taken while irradiating cone-shaped X-rays from 360 degrees around the subject is reconstructed. There is a cone beam CT technique for imaging the inside of a subject.

In this case, a data correction processing method from acquisition of cone beam CT image data to three-dimensional image reconstruction is described in, for example, JAMIT Frontier '95, 23, Medical Imaging Technology Research Society, 23 -28 (1995).

In the method described in this document, an X-ray image intensifier is used as a two-dimensional X-ray image detector capable of performing continuous imaging, and the X-ray transmission of an object from multiple directions is first performed. An image is taken, then the distortion and sensitivity of the detector included in the X-ray transmission image are corrected, and furthermore, the protrusion of the subject from the detector is corrected, and then the three-dimensional cone beam CT is obtained. That is, three-dimensional image reconstruction is performed using an image reconstruction algorithm.

[0005] As described above, the most widely used two-dimensional X-ray image detector capable of performing continuous imaging is an X-ray image intensifier, an imaging optical system, and a television. It is a detector composed of a camera. However, in addition to this, a detector including a fluorescent plate, an imaging optical system, and a television camera, a fluorescent plate, a two-dimensional photodiode array, and a two-dimensional thin film transistor (T
There are two-dimensional X-ray image detectors such as a detector configured with an FT) array and a detector configured with a selenium film and a two-dimensional thin film transistor array.

[0006]

SUMMARY OF THE INVENTION As a result of studying the above prior art, the present inventor has found the following problems.

In a conventional cone-beam CT apparatus, diffuse light (Bering) generated on an output fluorescent screen of an X-ray image intensifier together with distortion of a detector, variation in sensitivity of the detector, and protrusion of an object from the detector. (Also referred to as glare) and scattered X-rays generated when X-rays pass through the inside of the subject, which results in a phenomenon of adding blur to the obtained X-ray transmission image, thereby lowering the image quality of the X-ray transmission image. As a result, the three-dimensional image obtained by the reconstruction also causes a phenomenon that the image quality is deteriorated.

[0008] Among the two-dimensional X-ray image detectors described above, those other than the selenium film convert an X-ray image into an optical image, and further convert the optical image into an electric signal. So that these detectors
Diffusion light is generated due to light diffusion inside the detector.

In particular, scattered X-rays are a cause of deterioration in image quality which cannot be avoided by using any two-dimensional X-ray image detector. When mixed in the X-ray, blur occurs in the X-ray transmission image.

Generally, in order to prevent the deterioration of the image quality of an X-ray transmission image due to the above-mentioned scattered X-rays, it is necessary to shield the scattered X-rays from the entire X-ray incident surface of the two-dimensional X-ray image detector. Are arranged. This X-ray grid
Limiting the direction of incidence of X-rays incident on the detector has the effect of blocking a large proportion of scattered X-rays, but the angle of incidence of scattered X-rays on a two-dimensional X-ray image detector is widely distributed. Therefore, it is impossible to completely shield the scattered X-rays from being mixed into the X-ray transmission image (actually measured image).

[0011] However, in a cone beam CT apparatus, unlike ordinary fluoroscopy and imaging, the X-ray absorption coefficient inside the subject is imaged, so that the image density is the same due to the influence of scattered X-rays. The problem is that the value of the image is biased in the power region.

Therefore, it is necessary to perform a correction process for removing the influence (blur) due to the scattered X-rays mixed from the X-ray transmission image. , Medical Physics, Vol. 20, 59-6.
Page 9 (1993) discloses a method of correcting diffuse light and scattered X-rays integrally.

Further, in the cone beam CT apparatus, when the width of the detector is smaller than the width of the subject, correction of projection protrusion is required separately. Must be estimated, there is an error in the correction, and the quantitativeness is degraded. Also,
Since data on the X-ray absorptance is required, it is desirable that a portion where the absorptivity is zero, that is, a portion where the subject does not exist, is included in the measurement image. That is, in the cone beam CT apparatus, it is desirable that a region where the absorber does not exist around the subject is captured in the measurement image (X-ray transmission image). Therefore, the two-dimensional X-ray image detector is increased in size. I have.

Therefore, it is a feature of the cone beam CT that a field where the thickness of the subject is zero is included in the visual field. Further, depending on the shape of the subject, there may be a case where the thickness of the absorber is zero even during fluoroscopy or imaging.

However, the above-described conventional method for correcting scattered X-rays does not consider the case where the field of view includes a portion where the thickness of the subject is zero in the field of view. There has been no method for properly correcting the deterioration of image quality due to lines.

Among the above-described two-dimensional X-ray image detectors, particularly, in the system using an X-ray image intensifier and a television camera, the distortion and sensitivity distribution of the detector are large, and these corrections are necessary. Scattered light and scattered X
A comprehensive correction algorithm is required, including line correction.

However, the above-mentioned Medical Physics magazine discloses a general method for correcting diffused light and scattered X-rays, but an algorithm for correcting scattered X-rays in a cone beam CT image generation method is not disclosed. Not disclosed.

A first object of the present invention is to easily correct a decrease in image quality due to scattered X-rays in an actually measured X-ray transmission image even when the field of view includes a portion where the thickness of the subject is zero. 3
An object of the present invention is to provide a cone beam CT apparatus capable of improving the quality of an image obtained by dimensional reconstruction and the quantification of CT values.

A second object of the present invention is to easily correct deterioration in image quality due to diffused light in a measurement system in an actually measured X-ray transmission image, and to improve image quality and CT value of an image obtained by three-dimensional reconstruction. An object of the present invention is to provide a cone beam CT apparatus capable of improving quantitative performance.

A third object of the present invention is to provide a method for measuring distortion, sensitivity distribution, diffused light, and scattered X-rays even when there is geometrical distortion and sensitivity distribution of an image in a measurement system in an actually measured X-ray transmission image. To provide a cone beam CT apparatus capable of easily correcting each of the above and improving the image quality of images obtained by three-dimensional reconstruction and the quantification of CT values.

The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0022]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

(1) An X-ray source for irradiating conical or pyramidal X-rays, two-dimensional X-ray image capturing means for capturing a two-dimensional X-ray image of the subject, and the two-dimensional X-ray image capturing means And a reconstructing means for reconstructing a three-dimensional X-ray CT image from a two-dimensional X-ray image picked up by a user. Measuring means for measuring in advance the relationship with the component ratio;
Absorber thickness calculation for calculating the thickness of the X-ray absorber when the X-ray absorption amount of the subject and the X-ray absorption amount of the X-ray absorber are equal from the two-dimensional X-ray image and the imaging conditions Means, the absorber thickness calculated by the absorber thickness calculating means is compared with a preset absorber thickness, and as a result of the comparison, the absorber thickness of the subject is the preset absorber thickness If the thickness is smaller than the thickness, the ratio of the scattered X-ray component corresponding to the preset absorber thickness is calculated as the ratio of the scattered X-ray component of the two-dimensional X-ray image captured by the two-dimensional X-ray image capturing means. And a scattered X-ray correction unit that corrects the scattered X-rays of the two-dimensional X-ray image based on the ratio of the scattered X-ray component.

(2) An X-ray source for irradiating conical or pyramid-shaped X-rays, optical image conversion means for converting a two-dimensional X-ray image of a subject into a two-dimensional optical image, and converting the two-dimensional optical image A cone-beam X-ray tomography apparatus comprising: a two-dimensional optical image capturing unit that captures an image; and a reconstructing unit that reconstructs a three-dimensional X-ray CT image from the two-dimensional optical image captured by the two-dimensional optical image capturing unit. A diffused light correction unit for correcting the diffused light of the two-dimensional optical image based on a previously measured ratio of the diffused light component and a point spread function; and a sensitivity distribution image captured without irradiating the X-ray. A sensitivity distribution correcting means for correcting the sensitivity distribution of the two-dimensional optical image corrected by the diffused light correcting means; and a two-dimensional optical image corrected by the sensitivity distribution correcting means based on a distortion conversion table measured in advance. Correction means for correcting distortion Scattered X-ray correction means for correcting scattered X-rays of the two-dimensional X-ray image corrected by the distortion correction means, and logarithmic conversion means for logarithmically converting the two-dimensional optical image corrected by the scattered X-ray correction means. And a protrusion correction means for performing correction of protrusion of the two-dimensional optical image after logarithmic conversion.
The reconstructing means reconstructs a three-dimensional X-ray CT image from the two-dimensional optical image after the protrusion correction.

(3) In the cone-beam X-ray tomography apparatus described in (2) above, measurement is performed to measure in advance the relationship between the thickness of the predetermined X-ray absorber and the ratio of the scattered X-ray component to the thickness. Means for calculating the thickness of the X-ray absorber when the X-ray absorption amount of the subject and the X-ray absorption amount of the X-ray absorber become equal from the two-dimensional X-ray image and the imaging conditions. Absorber thickness calculating means, comparing the absorber thickness calculated by the absorber thickness calculating means and the preset absorber thickness,
As a result of the comparison, when the absorber thickness of the subject is smaller than the preset absorber thickness, the ratio of the scattered X-ray component corresponding to the preset absorber thickness is set to 2
Comparing and replacing means for calculating the ratio of the scattered X-ray component of the two-dimensional X-ray image captured by the two-dimensional X-ray image capturing means;
A line correction unit corrects the scattered X-rays of the two-dimensional X-ray image based on the ratio of the scattered X-ray component.

(4) In the cone beam X-ray tomography apparatus described in (1) or (3) above, the scattered X-ray correction means calculates a product of the ratio measurement value and a point spread function. Means, a means for convolving the calculation result with the two-dimensional X-ray image to generate a scattered X-ray component image, and means for subtracting the scattered X-ray component image from the two-dimensional X-ray image. Have.

(5) In the cone beam X-ray tomography apparatus described in (2) or (3) above, the diffused light correction means calculates a product of the ratio of the diffused light component and the point spread function. Means for calculating, and calculating the result of the two-dimensional X
The apparatus includes means for performing a convolution operation on the line image to generate a diffused light component image, and means for subtracting the diffused light component image from the two-dimensional X-ray image.

(6) In the cone beam X-ray tomography apparatus according to the above (2) or (3), the diffused light correcting means includes a function of a product of a ratio of the diffused light component and a point spread function. Means for generating a diffused light correction filter in a Fourier space; Fourier transform means for performing a Fourier transform on the two-dimensional X-ray image; and a product operation of the diffused light correction filter on the X-ray image after the Fourier transform Means.

(7) In the cone beam X-ray tomography apparatus according to any one of the above (2) to (6), the edge portion of the X-ray shield is a two-dimensional array of two-dimensional optical image imaging means. At right angles to the row or column direction of
A cross-sectional profile of a two-dimensional X-ray image obtained by arranging an X-ray shield near the center of the field of view is converted into a Gaussian function component representing the spatial resolution of the two-dimensional optical image imaging means and the optical image conversion means, and diffused light. Means for performing a two-component fitting with an exponential function component representing a component and separating the component into a Gaussian function component and an exponential function component; a ratio of the diffused light component and a line image distribution from the separated Gaussian function component and the exponential function component Means for calculating a function, and means for approximating a point spread function from the line spread function.

(8) In the cone beam X-ray tomography apparatus according to any one of the above (2) to (6), the edge portion of the X-ray shield is a two-dimensional array of two-dimensional optical image capturing means. At right angles to the row or column direction of
An X-ray shield is arranged near the center of the visual field, and a cross-sectional profile of a two-dimensional X-ray image obtained by imaging a simulated body by simulating a subject simulated subject of various thicknesses on the X-ray shield is shown. Means for performing two-component fitting with a Gaussian function component representing the spatial resolution of the two-dimensional optical image capturing means and the optical image converting means and an exponential function component representing the scattered X-ray component, and separating the Gaussian function component and the exponential function component Means for calculating the ratio of the scattered X-ray component and the line spread function from the separated Gaussian function component and the exponential function component, and means for approximating a point spread function from the line spread function. Have.

(9) In the cone beam X-ray tomography apparatus according to any one of the above (1) to (8), the diffused light correction means may be a two-dimensional X-ray image taken under arbitrary imaging conditions. Means for calculating a ratio of a diffused light component and a point spread function from an image; and means for converting the point spread function into a function of a unit of a pixel of the two-dimensional optical image capturing means. Is changed, the diffused light is corrected based on the intensity ratio of the diffused light component and the point spread function.

(10) The cone beam X-ray tomography apparatus according to any one of the above (2) to (9), further comprising means for correcting an offset level of the two-dimensional X-ray image imaging means. Prior to the correction of the diffused light,
The offset level of the two-dimensional X-ray imaging unit is corrected.

According to the above-mentioned means (1), for example, the measuring means measures in advance the thickness of the predetermined X-ray absorber and the ratio of the scattered X-ray component to the thickness of the X-ray absorber.

When a two-dimensional X-ray image has been taken, first, the absorber thickness calculating means calculates the ratio of the scattered X-ray component to 2
The ratio of the scattered X-ray to the representative value of the thickness of the X-ray absorber of the subject is calculated based on the imaging conditions at the time of imaging the two-dimensional X-ray image.

Next, based on the result of the comparison and substitution means comparing the predetermined X-ray absorber thickness with the representative value of the X-ray absorber thickness of the subject, the scattered X-ray corresponding to the larger value is determined. The ratio of the component is output to the scattered X-ray correction unit, and the scattered X-ray correction unit corrects the scattered X-ray based on the ratio of the scattered X-ray component. Therefore, the minimum value of the thickness of the subject is zero. In some cases,
Scattering X under conditions when the thickness of the object absorber is large to some extent
Line correction can be performed.

That is, it is possible to prevent a decrease in correction accuracy when correcting scattered X-rays in a portion where the subject is thick when the image contains a thin portion of the subject.

Therefore, even when the minimum value of the thickness of the subject is zero, the quality of the image obtained by the three-dimensional reconstruction and the quantitativeness of the CT value can be improved.

According to the means (2) described above, correction (removal) is performed in the reverse order of the order in which the factors of blur are added.
The distortion, sensitivity distribution, diffused light, and scattered X-ray can be simply and accurately corrected for the reasons described in the examination of the correction order described later.

Therefore, the quality of the image obtained by the three-dimensional reconstruction and the quantitativeness of the CT value can be improved.

According to the above-mentioned means (3), the correction (removal) is performed in the reverse order of the order in which the factors of the blur are added, and the scattered X-ray correction means is used when correcting the scattered X-rays. Since the correction is performed based on the ratio of the line components, the distortion, the sensitivity distribution, the diffused light, and the scattered X-ray can each be easily corrected, and even when the minimum value of the thickness of the subject is zero, the Scattered X-ray correction can be performed under conditions when the absorber thickness is somewhat large.

According to the above-mentioned means (4), the means for calculating the product of the ratio of the scattered X-ray component and the point spread function converts the calculation result into a two-dimensional scattered X-ray component image. After performing the convolution operation on the X-ray image, the means for subtracting the scattered X-ray component image from the two-dimensional X-ray image performs the subtraction, so that the scattered X-rays can be corrected. The scattered X-ray can be corrected.

According to the above-mentioned means (5), first, the means for generating the diffused light distribution generates the diffused light distribution by calculating the product of the previously measured ratio of the diffused light component and the point spread function. Then, the means for performing the convolution operation can correct the diffused light of the two-dimensional X-ray image by convolving the diffused light distribution with respect to the two-dimensional X-ray image before the diffused light correction. In addition, the diffused light can be easily corrected.

According to the above-mentioned means (6), first, the means for generating the diffused light correction filter generates the diffused light correction filter in the Fourier space as a function of the product of the ratio of the diffused light component and the point spread function. I do. Next, the Fourier transform means Fourier-transforms the two-dimensional X-ray image before the diffuse light correction, and then performs a product operation of the diffuse light correction filter on the X-ray image after the Fourier transform to obtain a two-dimensional X-ray image. Can be corrected, so that in addition to the effects described above,
The diffused light can be corrected more easily than the above method.

According to the above-mentioned means (7), the edge portion of the X-ray shield is orthogonal to the row direction or the column direction of the two-dimensional array of the two-dimensional optical image pickup means, and is near the center of the visual field. The means for separating the cross-sectional profile of a two-dimensional X-ray image captured by arranging the X-ray shield is a Gaussian function component representing the spatial resolution of the two-dimensional optical image capturing means and the index representing the diffused light component of the optical image converting means. After performing a two-component fitting with a function component and separating it into a Gaussian function component and an exponential function component, a means for calculating the ratio of the diffused light component and the line spread function from the Gaussian function component and the exponential function component, and approximating means By approximating the point spread function from this line spread function, the point spread function and the ratio of the diffused light component that do not depend on the spatial resolution of the two-dimensional optical image capturing means and the optical image conversion means can be determined. It is required without using.

According to the above-mentioned means (8), the edge portion of the X-ray shield is orthogonal to the row direction or the column direction of the two-dimensional array of the two-dimensional optical image pickup means, and is located near the center of the visual field. A two-dimensional X-ray image cross-sectional profile obtained by arranging an X-ray shielding body and simulating a simulated object of various thicknesses on which a simulated subject is superimposed is used as a two-dimensional optical image imaging means and optical image conversion. A two-component fitting is performed between a Gaussian function component representing the spatial resolution of the means and an exponential function component representing the scattered X-ray component, and the separating means separates the components into a Gaussian function component and an exponential function component. The ratio of the scattered X-ray component and the line image distribution function are calculated from the function component, and the approximating means approximates the point spread function from the line image distribution function. The proportions and the point spread function of the scattered X-ray component which does not depend on the spatial resolution of the obtained without using a special subject.

According to the above-mentioned means (9), the ratio of the diffused light component and the point spread function are calculated from the two-dimensional X-ray image taken under arbitrary imaging conditions, and the point spread function is converted to the two-dimensional optical image. Even if the imaging condition is changed, the function of the image pickup unit is set as a unit, and even if the imaging condition is changed, the scattered light is corrected without changing the ratio of the diffused light component and the point spread function, so that the diffused light correction is performed. Therefore, the number of parameters that need to be obtained in advance can be reduced, so that the diffused light can be easily corrected.

According to the above-mentioned means (10), the means for correcting the offset level of the two-dimensional X-ray image pickup means corrects the offset level of the two-dimensional X-ray image pickup means prior to the correction of the diffused light. By performing the above, for example, the offset levels of the two-dimensional X-ray images captured from 360 degrees around the subject can be made uniform, so that the image quality of images obtained by the three-dimensional configuration and the quantitativeness of CT values can be further improved. .

Hereinafter, the correction order described above will be described in detail.

The causes of blur added to the X-ray transmission image in order are:
By removing in the reverse order of the addition order, theoretically accurate correction is possible. Hereinafter, a problem that occurs when the order of correction is changed will be described. In the following description, correction in the reverse order to the order in which the factors of blur are added is the normal order correction in the sense that it is theoretically correct, and correction is performed in the same order as the order in which the factors of blur are added. Performing the correction in the reverse order of the forward correction is referred to as reverse correction.

(Study 1) The order of offset correction and diffused light correction will be examined. However, if the offset component image is M
₁ , the image to which the offset component is added is M ₂ , the intensity of the diffused light component is av, and the distribution function is PSFv.

In the forward correction, diffused light correction is performed after offset correction. Therefore, the image after the forward correction is represented by the following equation 1. Hereinafter, the symbol ** indicates a two-dimensional convolution.

[0052]

M = (M ₂ −M ₁ ) − (M ₂ −M ₁ ) ** av · PSFv (1) In the reverse correction, offset correction is performed after diffused light correction.
The image M ′ after the reverse order correction is represented by the following Expression 2.

[0053]

M ′ = M ₂ −M ₂ ** av · PSFv−M ₁ = M−M ₁ ** av · PSFv (Equation 2) Reverse order correction from Equations 1 and 2 above In this case, overcorrection is performed with respect to forward correction. To compensate for over-correction amount, in addition to the calculation of the normal order correction, it requires the computation and sum operation convolving the diffuse light component distribution av · pSFV the offset component image M _1, the amount of computation increases.

(Examination 2) The order of diffused light correction and sensitivity distribution correction will be examined. The diffuse light component image is M ₃ , the image to which the diffuse light component is added is M ₄ , and the sensitivity distribution image is M ₅ . In the forward correction, sensitivity distribution correction is performed after diffused light correction. The calculation for obtaining the image M after the forward correction is represented by the following Expression 3.

[0055]

M = ln (M ₄ −M ₃ ) −ln (M ₅ ) (Equation 3) On the other hand, in reverse order correction, diffused light correction is performed after sensitivity distribution correction. In that case, after the sensitivity distribution correction in both the image M ₄ that joined the diffused light component and the diffused light component image M _3, it is necessary to perform a diffuse light correction. Further, since the sensitivity distribution correction is a logarithmic calculation and the diffused light correction is a real number calculation, the calculation for obtaining the image M ′ after the reverse correction is represented by the following equation (4).

[0056]

M ′ = ln [exp {ln (M ₄ ) −ln (M ₅ )} − exp {ln (M ₃ ) −ln (M ₅ )}] (Equation 4) According to the equation (4), the reverse order correction operation is more complicated than the forward order correction operation, and the amount of operation obviously increases.

(Study 3) The order of sensitivity distribution correction and geometric distortion correction will be examined. In the forward correction, geometric distortion correction is performed after sensitivity distribution correction. In this case, the geometric distortion correction operation is performed once. On the other hand, in the reverse correction, sensitivity distribution correction is performed after geometric distortion correction. In this case, since the sensitivity distribution depends on the position on the image, it is necessary to perform the sensitivity distribution correction after performing the geometric distortion correction on both the image before correction and the sensitivity distribution image. Therefore, in the reverse correction, the geometric distortion correction calculation is required twice, and the calculation amount is doubled compared to the normal correction.

(Examination 4) The geometric distortion correction and the scattered X-ray correction will be discussed. In the forward correction, scattered X-ray correction is performed after geometric distortion correction. In the reverse order correction, since the geometric distortion correction is performed after the scattered X-ray correction, the scattered X-ray correction is performed in a state where distortion is added. In this case, since the scattered X-ray correction is a blur generated before the geometric distortion occurs, it is necessary to add the influence of the geometric distortion to the scattered X-ray distribution function. Accordingly, since the geometric distortion depends on the position, the shape of the distribution function locally differs on the image. On the other hand, it is difficult to change the shape of the function according to the amount of distortion. Also, if the shape of the function changes in the image, batch processing using Fourier transform becomes impossible. Therefore, the correction calculation is more complicated in the reverse correction than in the normal correction.

As is apparent from the results of the examinations 1 to 4 described above, offset correction, diffused light correction, sensitivity distribution correction, geometric distortion correction, scattered X-ray By performing image correction in the order of correction, blur can be corrected accurately and simply.

[0060]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings together with embodiments (examples) of the invention.

In all the drawings for describing the embodiments of the present invention, components having the same functions are denoted by the same reference numerals, and their repeated description will be omitted.

FIG. 1 is a block diagram showing a schematic configuration of a cone-beam X-ray tomography apparatus according to an embodiment of the present invention. FIG. 1A shows a schematic configuration of the entire cone-beam X-ray tomography apparatus. FIG. 1B is a block diagram illustrating a schematic configuration of a two-dimensional X-ray image detector according to the present embodiment.

In FIG. 1, 1 is a photographing control device, 2 is X
3 is an X-ray grid, 4 is a two-dimensional X-ray image detector (two-dimensional X-ray imaging means), 5 is an image collection and processing device,
6 is a display device, 7 is a rotating plate, 8 is a couch top, 9 is a subject, 21 is an X-ray imaging intensifier, 22
Is an optical system, 23 is a television camera, 25 is an input phosphor screen, 2
Reference numeral 6 denotes an output phosphor screen.

In FIG. 1A, the photographing control means 1
For example, the unit is realized by a program executed by a known information processing apparatus, and controls the operation of the cone beam X-ray tomography apparatus according to the present embodiment.

The X-ray tube 2 is a well-known X-ray tube for irradiating conical or pyramidal X-rays, and the grid 3 is a well-known grid for blocking X-rays scattered inside the subject 9. The grid 3 according to the present embodiment is configured such that the directions of the inner septa all face the center of the X-ray tube 2.

The image collection and processing device 5 includes, for example, an image processing means realized by an image processing program executed by a known information processing device, a known storage means, a switching means, and a control means controlled by the image processing program. It is a means composed of signal conversion means and the like.
After performing a process described below including a correction process on the X-ray transmission image (two-dimensional X-ray image) captured by the line image detector 4, an image signal is output to the display device.

In particular, the above-mentioned switching means is a means for determining whether or not to execute the later-described correction on the X-ray transmission image, and includes, for example, a well-known switch, a well-known button, or a control screen. A switching method by selecting an icon or the like above can be considered. In this embodiment, when the switch is turned on, the correction processing is executed,
If it is turned off, no correction processing is executed. Therefore,
The operator of the cone-beam X-ray tomography apparatus according to the present embodiment can control the presence / absence of a correction process by controlling the switching means. Further, it is needless to say that whether or not the correction processing is performed can be confirmed by the switching unit.

The display device 6 is, for example, a well-known monitor, and converts a video signal output from the image collection and processing device 5 into a video.

The rotating plate 7 is driven by a driving means (not shown) controlled by the photographing control means 1 so that the X-ray tube 2, grid 3 and two-dimensional X-ray image detector 4 installed on the rotating plate 7 It is to rotate around.

The couch top 8 is a well-known couch top for arranging the subject 9 at a predetermined position, and the imaging position of the subject 9 is a supine position. However, it goes without saying that the photographing position, the angle between the couchtop 8 and the rotating plate 7 and the like can be arbitrarily changed.

Next, the operation of the cone beam X-ray tomography apparatus according to the present embodiment will be described with reference to FIG. 1. The X-ray tube 2 and the two-dimensional X-ray image detector 4 are fixed on a rotating plate 7. ,
X-ray transmission images from a plurality of directions are obtained by performing fluoroscopy or photographing while rotating around the subject 8.

For example, in the fluoroscopy mode, a captured X-ray transmission image (two-dimensional X-ray image) is displayed on the display device 6 immediately after being corrected by the image collection and processing device 5. On the other hand, in the imaging mode, the captured X-ray transmission image is corrected by the image collection and processing device 5, stored in a storage unit (not shown), and then displayed on the display device. The corrected X-ray transmission image is displayed on the display device 6 after being three-dimensionally reconstructed by the image acquisition and processing device 5.

At this time, the switching means (not shown) provided in the image acquisition and processing device 5 allows the operator to switch to determine whether or not to correct the photographed X-ray transmission image.

The X-ray image transmitted through the subject 9 is converted into an optical image by an X-ray image intensifier, then formed by an optical system, and read by a television camera. The output signal of the television camera is A / D-converted by an A / D converter (not shown) provided in the television camera, processed by the image collection and processing device 5, and displayed on the display device 6.

In FIG. 1B, an X-ray image intensifier 21 is a well-known X-ray image intensifier, and outputs an X-ray image input from an input fluorescent screen 25 to an output fluorescent screen 26 as an optical image. I do.

The optical system 22 is composed of, for example, a well-known lens group, and forms an optical image output from the output fluorescent screen 26 on the image sensor of the television camera 23.

The television camera 23 is a well-known television camera, and converts an optical image formed on an image sensor (not shown) into an analog electric signal.

The A / D converter is a well-known A / D converter for converting an analog electric signal into a digital signal.

Next, based on FIG. 1, the causes and the order of distortion, scattered X-rays, diffused light, and the like, which are factors that lower the image quality of an X-ray image, in the cone beam X-ray tomography apparatus of the present embodiment will be described. explain.

First, the X-rays emitted from the X-ray tube 2 are scattered inside the subject 9 when passing through the subject 9 to generate scattered X-rays. Is the input phosphor screen 25 of the X-ray image intensifier 21
(Hereinafter, abbreviated as an input surface) to be converted into an electronic image.

The electron image including the scattered X-rays is amplified inside the X-ray image intensifier 21 and formed on the output fluorescent screen 26 of the X-ray image intensifier 21.

The input fluorescent screen 25 of the X-ray image intensifier 21 has a spherical shape, while the output fluorescent screen 26 of the X-ray image intensifier 21 has a planar shape. Input phosphor screen 25
When the image obtained in step (1) is formed on the flat output phosphor screen 26, geometric distortion occurs in the X-ray image inside the X-ray image intensifier 21.

The shape of the distortion at this time is a so-called pincushion type in which the image is stretched toward the periphery of the image.

Further, since the image is stretched toward the periphery, a phenomenon occurs in which the sensitivity decreases toward the periphery of the image, and the sensitivity distribution becomes uneven in the electronic image. However, the distortion and the sensitivity distribution are locally different due to the distortion of the electron orbit during amplification and the unevenness of the sensitivity distribution of the input phosphor screen and the output phosphor screen itself.

The electronic image is converted into an optical image on the output phosphor screen 26. At this time, light diffusion occurs, but it is almost impossible to prevent the light diffusion. Next, the optical image is read by the television camera 23 (converted into an electric signal) after passing through the optical system 22. At this time, a level shift occurs by the offset level.

As described above, to the X-ray transmission image transmitted through the subject, scattered X-rays, geometric distortion, sensitivity distribution unevenness, diffused light, and offset image reduction factors are added in this order. An X-ray projection image is obtained in a state where the image reduction factor is added as a blur to the measurement image, that is, the X-ray projection image.

Therefore, the image quality of the X-ray projection image obtained by the cone beam X-ray tomography apparatus is the same as that of the scattered X-ray.
The three-dimensional reconstructed image reconstructed from these X-ray projection images is deteriorated in image quality and deteriorated in the quantitativeness of CT values, because the image is deteriorated by lines, sensitivity distribution unevenness, geometric distortion, and diffused light. Will occur.

FIG. 2 is a block diagram showing a schematic configuration of the image processing means of the image collecting and processing apparatus according to this embodiment.
Reference numeral 201 denotes a first offset level correcting unit, 202 denotes a second offset level correcting unit, 203 denotes a first diffused light correcting unit, 204 denotes a second diffused light correcting unit, 205 denotes a sensitivity distribution correcting unit, and 206 denotes a distortion. Correction means (geometric distortion correction means), 207 denotes a scattered X-ray correction means, 208 denotes a logarithmic conversion means, 209 denotes a protrusion correction means, and 210 denotes a cone beam image reconstruction calculation means.

However, each of the units 201 to 210 constituting the image processing unit of the image collection and processing unit 5 is realized by, for example, a program executed by a well-known information processing device as described in FIG. Is done.

In FIG. 2, a first offset level correcting means 201 is a means for correcting the offset level of an actually measured image, that is, an X-ray transmission image of the subject 9.
An offset corrected image in which the offset level of the X-ray transmission image is corrected is generated from an X-ray image and an X-ray transmission image captured in advance without X-ray irradiation.

Second offset level correcting means 202
Is a means for correcting the offset level of the detector sensitivity distribution image, and an X-ray image captured in advance without irradiating X-rays and an X-ray image captured only in air without arranging the subject 9 (air projection image) , An air image) to generate an offset corrected image in which the offset level of the detector sensitivity distribution image is corrected.

The first diffused light correction means 203 is a means for correcting the influence of diffused light contained in the offset corrected image of the X-ray transmission image after the offset level has been corrected,
The second diffused light correction unit 204 is a unit that corrects the influence of diffused light included in the offset corrected image of the detector sensitivity distribution image after the offset level has been corrected.

The sensitivity distribution correcting means 205 corrects a difference in sensitivity distribution included in the X-ray transmission image after the offset correction from the offset correction image of the X-ray transmission image and the offset correction image of the detector sensitivity distribution image. This is a means for generating a distribution correction image.

The distortion correction means 206 is a means for generating a geometric distortion corrected image in which the geometric distortion of the sensitivity distribution corrected image is corrected, based on a conversion table created based on the chart projection image measured in advance. .

The scattered X-ray correction means 209 corrects the influence of the scattered X-rays contained in the geometric distortion corrected image based on the scattered X-ray distribution created by the procedure described later, and generates a scattered X-ray corrected image. It is a means to do.

The logarithmic conversion means 208 is means for logarithmically converting the scattered X-ray corrected image, the protruding correction means 209 is a means for correcting the well-known visual field protruding, and the cone beam image reconstruction calculating means 210 is for performing the protruding correction. Is a means for generating a three-dimensional reconstructed image by back-projecting the X-ray image (projection image).

Next, FIG. 3 shows a flow for explaining the operation of the image processing means of the image collecting and processing apparatus of the present embodiment. Hereinafter, the operation of the image processing means will be described with reference to FIG. .

First, as step 0, imaging is performed in advance without irradiating X-rays to obtain an offset component image A. The intensity (proportion) of the diffused light component and the point spread function are obtained from the edge image, and the product of the two is taken to obtain the diffused light distribution B.

Next, an air projection image D is taken from the same direction as the direction in which the X-ray transmission image (projection image, X-ray image) C of the object 9 is taken without disposing the object 9.

A well-known chart for measuring the amount of geometric distortion in each pixel is arranged, and a projected image C of the subject 9 is arranged.
The chart projection image E of the aforementioned chart is photographed from the same direction as the photographing direction. Next, based on the chart projection image, a conversion table F indicating which pixel in the image without distortion corresponds to which pixel in the image with distortion.
Create

The measuring means (not shown) calculates the ratio of the scattered X-ray component to the thickness of several types of X-ray absorbers, obtains the relationship G between the thickness of the X-ray absorber and the ratio of the scattered X-ray component, and obtains the image maximum value. The thickness H of the X-ray absorber taking the following formula is estimated. Further, a point spread function I of scattered X-rays is obtained from an edge image for several types of X-ray absorber thicknesses.

Next, the processing of steps 1 to 6 is performed on each subject projected image C0.

First, as step 1, the object projection image C
For 0, the following offset level correction and diffused light correction processing are performed.

First offset level correcting means 201
Subtracts the offset component image A from the subject projection image C0 to create an offset component corrected image C1 (10
1).

The first diffused light correction means 203 convolves the diffused light distribution B with the offset component corrected image C1 to create a diffused light component image C2 (102).

The first diffused light correction means 203 subtracts the diffused light component image C2 from the offset component corrected image C1, and
A diffused light correction image C3 is created (103).

Next, as step 2, the subject projection image C
The next offset level correction and diffused light correction processing are performed on the air projected image D0 taken from the same direction as 0, as in the case of the subject projected image C0.

Offset component image A from air projection image D0
Is subtracted to create an offset component corrected image D1 (1
04).

Next, the diffused light distribution B is convolved with the offset component corrected image D1 to create a diffused light component image D2 (105).

The diffused light component image D2 is subtracted from the offset component corrected image D1 to create a diffused light corrected image D3 (106).

Next, as step 3, the sensitivity distribution correcting means 205 performs logarithmic conversion on the diffused light corrected subject image C3,
A logarithmic image C4 is created (107).

The diffused light corrected air image D3 is logarithmically converted to create a logarithmic image D4 (108).

The sensitivity distribution correcting means 205 subtracts the logarithmic image D4 of the air from the logarithmic image C4 of the subject (10
9) An antilogarithmic conversion of the subtracted image is performed to create a sensitivity distribution corrected image C5 of the subject (110).

However, when performing three-dimensional reconstruction, logarithmic conversion 208 is performed after sensitivity distribution correction 205. Therefore, if the antilogarithmic transformation 110 is omitted during the sensitivity distribution correction, the logarithmic transformation 208 can be omitted.

As step 4, the following geometric distortion correction is performed on the sensitivity distribution corrected image C5 of the subject using the conversion table F.

First, the geometric distortion correction means 206 determines which pixel of the sensitivity distribution corrected image C5 containing distortion corresponds to each pixel in the image without distortion, that is, the image C6 after distortion correction. Index from

Next, the geometric distortion correcting means 206 stores the indexed pixel value on the sensitivity distribution corrected image C5 as a pixel on the distortion corrected image C6 (111).

In step 5, a representative value J of the thickness of the subject is determined from the image measurement conditions and the image values for the distortion-corrected image C6 of the subject 9.

If the representative value J of the subject thickness is smaller than the thickness H of the X-ray absorber previously specified in step 0, the representative value J of the subject thickness is specified in advance. With the thickness H.

Using the relationship G between the thickness of the X-ray absorber and the ratio of the scattered X-ray component obtained in step 0 described above, the ratio K of the scattered X-ray component is obtained from the representative value J of the thickness of the subject. Further, using the point spread function I of scattered X-rays for several types of X-ray absorber thickness obtained in step 0, the average point spread function L of scattered X-rays for the representative value J of the thickness of the subject is calculated. Calculate by interpolation.

The product of the ratio K of the scattered X-ray component and the point spread function L is obtained, and the scattered X-ray distribution M with respect to the representative value I of the subject thickness is obtained.

The operation of step 5 will be described later in detail.

As step 6, the following scattered X-ray correction processing is performed on the distortion corrected image C6 of the subject.

First, the scattered X-ray correction means 207 convolves the scattered X-ray distribution M with the distortion corrected image C6 of the subject to create a scattered X-ray component image C7 (112).

Next, the scattered X-ray correction means 207 subtracts the scattered X-ray component image C7 from the distortion corrected image C6,
A line correction image C8 is created (113).

Here, in the case of the fluoroscopy mode, each corrected image C8 obtained by the above-described correction processing in steps 0 to 6 is displayed on the display device 6.

On the other hand, in the case of the photographing mode, each corrected image C8
Is used to perform three-dimensional reconstruction according to the following procedure.

First, the logarithmic conversion means 208 makes each corrected image C
After the logarithmic transformation of 8, the protrusion correction means 209 corrects the protrusion of the field of view, and the projection image C 9 obtained by convolving the filter.
Create

Next, cone beam image reconstruction calculating means 2
10 backprojects each projection image C9 to obtain a reconstructed image C10.

Note that the air projection image D described above is
Is photographed in all directions for photographing. In addition, in order to reduce the storage area of the air projection image D and simplify the correction process for the air projection image D, when the shooting direction is close, a representative air projection image or an average air projection image is used. Needless to say, it is good.

Further, a representative air projection image or an average air projection image may be used for all photographing directions.

For a well-known chart for measuring geometric distortion, for example, an X-ray shielding plate having a hole at the intersection of a lattice is used. Further, as the X-ray absorber, for example, a known acrylic plate is used.

In the present embodiment, the diffuse light component image C2,
Although the convolution is performed when creating the D2 and the scattered X-ray component image C7, a Fourier transform can be used instead of the convolution.

FIG. 4 is a diagram for explaining a measurement system of an edge image for measuring a diffused light distribution. Reference numeral 12 denotes an X-ray shielding plate. Note that arrows and X, Y, Z shown in FIG.
Indicates an X axis, a Y axis, and a Z axis, respectively.

In FIG. 4, the X-ray shielding plate 12 is, for example, a metal plate.
It is arranged at a distance of about 25 cm from the X-ray grid 3. At this time, the edge portion 14 of the X-ray shielding plate 12 is located at the center of the field of view of the two-dimensional X-ray image detector 4, and the edge portion 14 is arranged so as to be parallel to the Y axis, and the X-ray image ( Hereinafter, this will be referred to as an edge image).

Next, the edge portion 14 of the X-ray shielding plate 12 becomes the center of the visual field of the two-dimensional X-ray image detector 4, and
This edge portion 14 is arranged so as to be parallel to the X axis,
Take an edge image.

Next, FIG. 5 shows a flow for explaining the method of calculating the diffused light distribution, and the procedure of calculating the diffused light distribution will be described below with reference to FIG. In this embodiment,
It is assumed that the XY axis of the two-dimensional X-ray image detector and the XY axis of the captured edge image (projected image) match.

First, in step 11, the X-ray shielding plate 1 is placed so that the left half of the image is hidden at almost the same position as the subject 9.
2 and make an edge approximately parallel to the Y axis at the center of the image. At this time, the edge image P0 is photographed without placing the subject 9.

In step 12, profile data P1 which passes substantially through the center of the edge image P0 and is parallel to the X axis is obtained. Of the profile data P1, profile data P2 on the side where the X-rays are shielded (in the case of the present embodiment, on the left side of the edge image) with the edge position as the center is taken. However, the edge position described above is, for example, a peak position obtained by differentiating the profile data P1.

In step 13, profile data P3 is created by deleting data close to the edge by a width corresponding to the resolution of the two-dimensional X-ray image detector 4 from the profile data P2 on the side where X-rays are shielded. Next, the profile data P3 is fitted with a function to obtain an approximate expression P4. The function at this time is
For example, an exponential function is used. In addition, in order to improve the accuracy of fitting, the weight is assigned to data closer to the edge portion 14 at the time of fitting.

As step 14, according to the approximate expression P4,
The profile data P5 on the side where X-rays are shielded up to the edge position is created. Profile data P obtained by folding this profile data P5 point-symmetrically at the intersection with the edge
Create 6. Profile data P7 obtained by subtracting the profile data P6 from the profile data P1.
Create

In step 15, profile data P8 of the profile data P7 on the side where the X-ray is shielded from the edge position is taken.

In step 16, the profile data P8 on the side where X-rays are shielded is fitted with a function to obtain an approximate expression P9. As the above function, for example, an error function is used. Next, in order to improve the accuracy of fitting, for example, at the time of fitting,
The data closer to the edge is weighted. The maximum value of the profile data P7 is used as the maximum value of the function.

In step 17, the value at the edge position of the approximate expression P4 is divided by the value at the edge position of the approximate expression P9.
The result is defined as a ratio P10 of the diffused light component in the X-axis direction.
Next, the approximation formula P4 is differentiated, normalized so that its area becomes 1 and turned back to the edge at the edge position in line symmetry, and the result is defined as a diffused ray image distribution function P11 in the X-axis direction.

In step 18, the X-ray shielding plate 12 is placed so that the lower half of the image is hidden at substantially the same position as the subject 9, an edge parallel to the X-axis is formed substantially at the center of the image, and the Y-axis is set at 90 degrees. Assuming that the rotation has been made, steps 1 to 7 described above are executed, and the ratio P12 of the diffused light component in the Y-axis direction and the diffused ray image distribution function P13 in the Y-axis direction are obtained.

In step 19, the ratio P10 of the diffused light component in the X-axis direction and the ratio P12 of the diffused light component in the Y-axis direction are combined, and the result is set as the diffused light component ratio P14.
As a combining method, for example, an average value of both is taken.

Next, the diffuse ray image distribution function P1 in the X-axis direction
1 and the diffused ray image distribution function P13 in the Y-axis direction are synthesized,
The result is defined as a diffused light point spread function P15. In addition, as a synthesis method, for example, the product of both is taken.

Next, the product of the ratio P14 of the diffused light component and the diffused light point spread function P15 is obtained, and the result is defined as a diffused light distribution P16.

By obtaining the diffused light point spread function P15 in units of pixels of the image detector, the same diffused light point spread function P15 can be used regardless of the mode change of the X-ray image intensifier. In this case, the number of parameters obtained in advance for diffused light correction can be reduced. Furthermore, since the number of parameters required for the correction calculation is small, the correction calculation can be easily performed.

FIG. 6 is a diagram for explaining a measurement system of an edge image for measuring a scattered X-ray distribution. Reference numeral 13 denotes an X-ray absorber. Note that arrows and X, Y, Z shown in FIG.
Indicates an X axis, a Y axis, and a Z axis, respectively.

In FIG. 6, the X-ray absorber 13 is, for example, an acrylic plate.
3 together with the X-ray shielding plate 12 from the X-ray grid 3 to 25.
cm. At this time, the edge portion 14 of the X-ray shielding plate 12 is located at the center of the visual field of the two-dimensional X-ray image detector 4, and the edge portion 14 is arranged so as to be parallel to the Y axis, and an edge image is taken. I do.

Next, the edge portion 14 of the X-ray shielding plate 12 becomes the center of the visual field of the two-dimensional X-ray image detector 4, and
This edge portion 14 is arranged so as to be parallel to the X axis,
Take an edge image.

At this time, the edge image described above is blurred by scattered X-rays generated when X-rays emitted from the X-ray tube 2 pass through the X-ray absorber 13.

Next, FIG. 7 shows a flow for explaining the method of calculating the scattered X-ray distribution, and the calculation procedure of the scattered X-ray distribution will be described below with reference to FIG. In the present embodiment, it is also assumed that the XY axes of the two-dimensional X-ray image detector and the XY axes of the captured edge image match.

First, in step 21, the X-ray shielding plate 1 is placed so that the left half of the image is hidden at almost the same position as the subject 9.
2 and make an edge approximately parallel to the Y axis at the center of the image. The X-ray absorber 13 is arranged behind the edge when viewed from the X-ray source 2, and an edge image Q0 is taken. At this time, an edge image is taken for the X-ray absorbers 13 having several thicknesses. For the edge image Q0 at each X-ray absorber thickness,
The processing of steps 22 to 28 described below is performed.

In step 22, the diffuse light distribution P16 obtained in step 19 is convolved with the edge image Q0 to create a diffuse light component image Q1. The diffuse light component image Q1 is subtracted from the edge image Q0 to create a diffuse light corrected image Q2.

Subsequently, the correction of the sensitivity distribution and the correction of the image distortion are performed on the diffused light correction image Q2 in the same manner as in steps 3 and 4 described above, to generate a distortion correction image Q21.

At step 23, the distortion corrected image Q21
, Profile data Q3 that passes through the center of the image and is parallel to the X axis. Of the profile data Q3,
The profile data Q4 on the side where the X-ray is shielded (in the case of the present embodiment, the left side of the edge image) with the edge position as the center is obtained. However, the edge position is obtained by, for example, differentiating the profile data Q3 and setting the peak position at this time.

In step 24, profile data Q5 is created by deleting data close to the edge by a width corresponding to the resolution of the two-dimensional X-ray image detector 4 from the profile data Q4 on the side where X-rays are shielded. The profile data Q5 is fitted with a function to obtain an approximate expression Q6. The function at this time is, for example,
Use exponential function. In addition, in order to improve the fitting accuracy, in fitting, data that is closer to an edge is weighted.

In step 25, profile data Q7 on the side where X-rays are shielded up to the edge position is created according to the approximate expression Q6. The profile data Q8 is created by folding the profile data Q7 point-symmetrically at the intersection with the edge. Next, profile data Q9 is created by subtracting profile data Q8 from profile data Q3.

In step 26, profile data Q10 of the profile data Q9 on the side where the X-rays are shielded with the edge position as the center is obtained.

In step 27, the profile data Q10 on the side where the X-rays are shielded is fitted with a function to obtain an approximate expression Q11. Note that, for example, an error function is used as the function. Also, in order to improve the fitting accuracy, data is weighted closer to the edge at the time of fitting. Further, the maximum value of the profile data Q9 is used as the maximum value of the function.

In step 28, the value at the edge position of the approximate expression Q6 is divided by the value at the edge position of the approximate expression Q11, and the result is set as the ratio Q12 of the scattered X-ray component in the X-axis direction.

Next, the approximate expression Q6 is differentiated, and its area is 1
And folds back line-symmetrically with respect to the edge at the edge position, and the result is defined as a scattered X-ray image distribution function Q13 in the X-axis direction.

In step 29, the X-ray shielding plate 12 is placed so that the lower half of the image is hidden at substantially the same position as the subject 9, and an edge parallel to the X-axis is formed at substantially the center of the image. The X-ray absorber 13 is arranged behind the edge as viewed from the X-ray source, and an edge image Q14 is taken. At this time, several thicknesses of X
An edge image is taken of the line absorber plate 13.

Assuming that the Y-axis has been rotated by 90 degrees for each edge image Q14, the processing of the above-described steps 22 to 28 is performed to obtain the ratio X15 of the scattered X-ray component in the Y-axis direction and the scattered X-ray component in the Y-axis direction. A line image distribution function Q16 is obtained.

In step 30, the ratio X12 of the scattered X-ray component in the X-axis direction, obtained for the same X-ray absorber thickness
And a ratio X15 of the scattered X-ray component in the Y-axis direction, and the result of the synthesis is defined as a ratio Q17 of the scattered X-ray component. However, as a combining method, for example, an average value of both is taken. Next, the scattered X-ray image distribution function Q13 in the X-axis direction and the scattered X-ray image distribution function Q16 in the Y-axis direction for the same X-ray absorber thickness are synthesized, and the result is referred to as the scattered X-ray point image distribution. It is assumed that the function is Q18. As a combining method at this time, for example, a product of the two is taken.

The diffused light correction in step 22 described above can be simplified and performed on profile data.
That is, in the edge image Q0, the profile data Q20 which passes through substantially the center of the image and is parallel to the X-axis is taken, and the ratio P10 of the diffused light component in the X-axis direction obtained in the above-described step 19 is determined by the diffusion P-axis in the X-axis direction. Ray image distribution function P11
And diffused light component profile data Q2
Create 1. Next, the diffused light component profile data Q21 is subtracted from the profile data Q20 to create diffused light correction profile data Q22. This data is used as profile data Q3 parallel to the X axis in step 23 described above. The same can be obtained for profile data parallel to the Y axis.

Next, FIG. 8 is a diagram for explaining a method of generating a scattered X-ray distribution. Hereinafter, based on FIG. 8, the procedure for generating the scattered X-ray distribution shown in step 5 described above will be described in detail. explain.

First, a representative value rb of the thickness of the absorber of the subject 9 is obtained from the image before the correction of the scattered X-ray and the imaging conditions of the image (112). The method of obtaining this is shown separately. Next, a comparison is made between the representative value rb of the absorber thickness of the subject 9 and a predetermined representative value maximum limit value ra of the absorber thickness (113).

As a result of the comparison, when the representative value rb of the absorber thickness of the subject 9 is larger than the maximum limit value ra, the ratio of the scattered X-ray intensity corresponding to the representative value rb of the absorber thickness at this time is calculated. The scattered X-ray distribution function is applied to the scattered X-ray correction processing 114 (114) to generate a scattered X-ray corrected image.

On the other hand, as a result of the comparison, when the representative value rb of the absorber thickness of the subject 9 is smaller than the predetermined representative maximum limit value ra of the absorber thickness, the absorber thickness of the subject 9 is determined. The representative value maximum limit value ra of the absorber thickness is substituted for the representative value rb of (1) (116), and the scattered X-ray correction processing is performed (114). That is, the representative value of the absorber thickness is set to ra, and the scattered X-ray intensity ratio and the scattered X-ray distribution function corresponding to the ra are applied to the scattered X-ray correction, and the scattered X-ray corrected image 115 is generated.

However, the scattering X in step 114 described above
In the X-ray correction processing, a scattered X-ray image is generated by performing a convolution operation of the product of the scattered X-ray intensity ratio and the scattered X-ray distribution function on the image 110 before correction, and the scattered X-ray image is converted into an image before correction. This is a process of subtracting from.

FIG. 9 is a diagram for explaining the effect of the procedure for generating the scattered X-ray distribution according to the present embodiment, and FIG.
Is the maximum absorber thickness rb representing the subject 9
FIG. 9 is a diagram for explaining the effect when the value is larger than that of FIG.
(B) is a diagram for explaining an effect when the maximum limit value ra is larger than the absorber thickness rb representing the subject 9.

In FIG. 9, a curve S shows the relationship between the logarithm of the value of the direct X-ray image (horizontal axis) and the logarithm of the value of the scattered X-ray image (vertical axis). This curve S corresponds to grid 3,
The relationship between the thickness of the absorber and the actually measured value of the scattered X-ray intensity ratio at the same tube voltage and image intensifier mode applied to the X-ray tube 2 was determined by fitting.

The horizontal axis of FIG. 9, that is, the image level (image value) is related to the thickness of the absorber under predetermined photographing conditions by the following equation (5). Therefore, the scale of the absorber thickness below the horizontal axis indicates the calculated value based on Equation 5.

Note that the vertical and horizontal axes in FIG. 9 are logarithmic scales.

[0178]

L = (1 / μ) ln {(Io × Tp) / Ip} (Equation 5) where μ is the absorption coefficient of the absorber, and Io is the difference between the grid and the absorber under the imaging conditions. The calculated value of the image if there is no
Tp is the direct line transmittance of the grid, and Ip is the value of the direct line image.

In FIG. 9, the value of the direct X-ray image and the thickness of the absorber are such that the maximum value of the image corresponds to the thickness of the absorber of zero, and the value of the image of the scattered X-ray component at this time is Becomes zero.
The value of the image of the scattered X-ray component is about 5
Since the maximum value is taken at cm, the maximum limit value ra is set to a value of 5 cm at which the value of the scattered X-ray image becomes close to the maximum value in the present embodiment.

In the present embodiment, the thickness corresponding to the maximum value of the image is used as the absorber thickness rb representing the subject 9, and in the example shown in FIG. rb is 12 cm. In this case, the relationship between the straight line T passing through the value of the point on the curve S corresponding to rb and making the image values on the vertical axis and the horizontal axis proportional to the X-ray and the scattering X
Applied as a relationship of intensity with the line.

In this case, as shown in FIG.
As the straight line T moves away from the curve S, the relative error of the correction calculation increases. In this case, since the straight line T is always below the curve S, the scattered X-ray correction is insufficient correction, and the relative error at that time increases as the image value decreases, but the relative error increases when the image value decreases. The absolute error is small, and as the value of the image approaches zero, the absolute error also converges to zero, so it can be said that this is a relatively good approximation.

The example shown in FIG. 9B shows a case where the absorber thickness rb representing the subject, that is, the absorber thickness rb corresponding to the maximum value of the image is 1 cm. In this case, the thickness rb of the absorber representing the subject is smaller than the value (5 cm) of the limit value ra of the absorber, and therefore passes through the point on the curve S corresponding to the horizontal axis ra. The relationship of the straight line V in which the values of the image on the vertical axis and the horizontal axis are proportional to each other is directly applied as the relationship between the intensity of X-rays and the intensity of scattered X-rays.

In this case, the error is overcorrected when the thickness of the absorber is larger than rb and smaller than ra, and is undercorrected when the thickness of the absorber is larger than ra. Is done.

On the other hand, since the straight line U when the thickness rb representing the subject is used as it is is far from the curve S, as is clear from FIG. 9B, the relative error of the correction calculation increases. I do.

As is apparent from the results, the procedure for generating the scattered X-ray distribution according to the present embodiment is used, that is, the absorber thickness rb representing the subject and the representative maximum limit value ra of the absorber thickness are used. By comparing the magnitude relation with, and approximating the scattered X-ray distribution based on the larger value, ra becomes rb
Even when the X-ray transmission image captures a region around the subject 9 where the absorber does not exist (a region where the minimum thickness of the subject is zero) in the X-ray transmission image, Depending on the conditions when the sample absorber thickness is somewhat large,
A scattered X-ray correction operation with a small relative error can be performed.

As described above, according to the cone-beam X-ray tomography apparatus of the present embodiment, the causes of the blur added to the X-ray transmission image in order are removed in the reverse order of the addition. For this reason, the correction can be made by a simple calculation. Also, the calculation amount may be small.

Therefore, even if there is a geometric distortion and sensitivity distribution of the image in the measurement system in the actually measured X-ray transmission image, the distortion, sensitivity distribution, diffused light, and scattered X-ray are simply corrected. (3) The image quality of images obtained by three-dimensional reconstruction and the quantification of CT values can be improved.

Further, the magnitude relationship between the absorber thickness rb representing the subject and the representative maximum limit value ra of the absorber thickness is compared, and the scattered X-ray distribution is approximated based on the larger value. Thus, if ra is greater than rb, ie, X
Even in the case where an area around the subject 9 and no absorber is photographed in the line transmission image, a correction operation with a small relative error can be performed.

Therefore, even when the field of view includes a part where the thickness of the subject 9 is zero, the deterioration of the image quality due to the scattered X-rays in the actually measured X-ray transmission image can be easily corrected. Further, the image quality of images obtained by three-dimensional reconstruction and the quantification of CT values can be improved.

Further, the edge of the X-ray absorber is arranged near the center of the visual field at a position orthogonal to the row direction or the column direction of the two-dimensional array of the detectors. And measures a two-dimensional projected image, and calculates a cross-sectional profile of the Gaussian function component by a spatial resolution component of the detector,
It is obtained by two-component fitting with an exponential function component by a diffused light component, and a point spread function P1 is obtained from the line spread function.
By approximating 5, the intensity of the diffuse light component and the point spread function can be easily obtained as data that does not require a special subject and does not depend on the spatial resolution of the detector.

Therefore, it is possible to easily correct the deterioration of the image quality due to the diffused light in the measurement system in the actually measured X-ray transmission image, and to improve the image quality and the CT value of the image obtained by the three-dimensional reconstruction.

Similarly, the intensity of the scattered X-ray component and the point spread function are set such that the edge of the X-ray absorber is located at a position orthogonal to the row direction or the column direction of the two-dimensional array of the two-dimensional X-ray detector. It is placed near, and a two-dimensional projected image of a scatterer obtained by superimposing a subject simulated subject of various thicknesses on this is taken, and the cross-sectional profile of the projected image is calculated as , And the exponential function component of the scattered X-ray component, the
The intensity of the line component and the point spread function can be easily obtained as data that does not require a special subject and does not depend on the spatial resolution of the detector.

In the cone-beam X-ray tomography apparatus according to the present embodiment, when calculating the diffused light distribution and the scattered X-ray distribution, in order to reduce noise, the profile data used for fitting is parallel to the edge. Add several lines in the direction, or add several points in the direction perpendicular to the edge.

In the cone-beam X-ray tomography apparatus of the present embodiment, the profile data on the side where X-rays with less noise are shielded is used, but the profile data on the side irradiated with X-rays may be used. Needless to say. It goes without saying that fitting may be performed using all profile data regardless of occlusion.

In the cone beam X-ray tomography apparatus according to the present embodiment, it is assumed that the profile shape is substantially equal to the left and right or upper and lower edges, and the distribution formula is obtained by folding the approximate expression obtained from the profile on one side. Was. However, if the distribution differs depending on the direction of the edge, it is necessary to obtain a distribution function for the direction of each edge and combine them. Conversely, if the shapes of the profiles are substantially equal to the upper, lower, left, and right edges, any one of the profiles can be represented.

In the cone-beam X-ray tomography apparatus according to the present embodiment, the XY axes of the television camera and the XY axes of the photographed projection images are coincident with each other. In this case, it is needless to say that the distribution function is synthesized in consideration of the rotation angle.

In the cone beam X-ray tomography apparatus according to the present embodiment, the operation and effect of the case where an image intensifier and a television camera are used as the two-dimensional X-ray image detector 4 have been described. Without limitation, for example, an amorphous selenium film and a TFT
Two-dimensional X-ray detector using an array, a two-dimensional X-ray detector using a fluorescent plate and a two-dimensional photodiode array, a two-dimensional X-ray detector using a fluorescent plate and a TFT array, using a fluorescent plate, an optical system, and a television camera It goes without saying that a two-dimensional X-ray image detector such as a two-dimensional X-ray detector may be used.

In the two-dimensional X-ray detector described above,
In a two-dimensional X-ray detector using an amorphous selenium film, there is no process of generating diffused light.
The part related to the diffused light can be deleted.

Further, in the two-dimensional X-ray detector that does not use an optical system and an image intensifier, since there is no process of generating geometric distortion, a portion relating to geometric distortion can be deleted in the correction processing. .

In this embodiment, since the image intensifier and the television camera are used, the sensitivity distribution unevenness is large and needs to be corrected. However, when the sensitivity distribution unevenness is small, the correction need not be performed. .

As described above, the invention made by the present inventor
Although specifically described based on the embodiments of the present invention, the present invention is not limited to the embodiments of the present invention, and it is needless to say that various modifications can be made without departing from the gist of the present invention. .

[0202]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows.

(1) Even when the field of view includes a part where the thickness of the subject is zero, deterioration in image quality due to scattered X-rays can be easily corrected in an actually measured X-ray transmission image, and three-dimensional reconstruction can be performed. The image quality of the obtained image and the quantitativeness of the CT value can be improved.

(2) In an actually measured X-ray transmission image, deterioration of image quality due to diffused light in the measurement system can be easily corrected, and image quality of image obtained by three-dimensional reconstruction and CT
The quantification of the value can be improved.

(3) Even when there is a geometric distortion and sensitivity distribution of the image in the measurement system in the actually measured X-ray transmission image, the distortion, sensitivity distribution, diffused light, and scattered X-ray can be easily corrected respectively. In addition, the image quality of the image obtained by the three-dimensional reconstruction and the quantitativeness of the CT value can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a cone beam X-ray tomography apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of an image processing unit of the image collection and processing device according to the present embodiment.

FIG. 3 is a flowchart illustrating an operation of an image processing unit of the image collection and processing apparatus according to the present embodiment.

FIG. 4 is a diagram for explaining a measurement system of an edge image for measuring a diffused light distribution.

FIG. 5 is a flowchart for explaining a method of calculating a diffused light distribution.

FIG. 6 is a diagram illustrating a measurement system of an edge image for measuring a scattered X-ray distribution.

FIG. 7 is a flowchart for explaining a method of calculating a scattered X-ray distribution.

FIG. 8 is a diagram for explaining a method of generating a scattered X-ray distribution.

FIG. 9 is a diagram for explaining an effect obtained by a procedure for generating a scattered X-ray distribution according to the present embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Imaging control device, 2 ... X-ray tube, 3 ... X-ray grid, 4
... 2D X-ray image detector (2D X-ray imaging means), 5 ...
Image collection and processing device, 6 ... Display device, 7 ... Rotating plate, 8
... bed top, 9 ... subject, 13 ... X-ray absorber, 201 ...
First offset level correction means, 202: second offset level correction means, 203: first diffused light correction means, 204: second diffused light correction means, 205: sensitivity distribution correction means, 206: distortion correction means 207: Scattered X-ray correction means, 208: Logarithmic conversion means, 209: Protruding correction means, 210: Cone beam image reconstruction calculation means.

Claims (10)

[Claims] An X-ray source for irradiating conical or pyramid-shaped X-rays, a two-dimensional X-ray image capturing unit for capturing a two-dimensional X-ray image of a subject, and the two-dimensional X-ray image capturing unit A cone beam X-ray tomography apparatus having reconstruction means for reconstructing a three-dimensional X-ray CT image from a captured two-dimensional X-ray image, wherein a thickness of a predetermined X-ray absorber and a scattered X-ray component with respect to the thickness Measuring means for measuring the relationship with the ratio in advance, and the X-ray absorption amount of the subject and the X-ray absorption amount of the X-ray absorber are equal from the two-dimensional X-ray image and the imaging conditions. Absorber thickness calculating means for calculating the thickness of the X-ray absorber, comparing the absorber thickness calculated by the absorber thickness calculating means and a predetermined absorber thickness, the result of the comparison, If the absorber thickness of the subject is smaller than the preset absorber thickness, Comparing and replacing means for setting the ratio of the scattered X-ray component corresponding to the thickness of the absorber to be the ratio of the scattered X-ray component of the two-dimensional X-ray image captured by the two-dimensional X-ray image capturing means; Based on the ratio of the components, the two-dimensional X
A cone X-ray tomography apparatus comprising: a scattered X-ray correction unit configured to correct scattered X-rays of a line image. 2. An X-ray source for irradiating conical or pyramid-shaped X-rays, an optical image conversion means for converting a two-dimensional X-ray image of a subject into a two-dimensional optical image, and capturing the two-dimensional optical image A cone-beam X-ray tomography apparatus having two-dimensional optical image capturing means for performing a three-dimensional X-ray CT image reconstruction from a two-dimensional optical image captured by the two-dimensional optical image capturing means; A diffused light correcting unit that corrects the diffused light of the two-dimensional optical image based on the measured ratio of the diffused light component and the point spread function; and a sensitivity distribution image captured without irradiating the X-ray. A sensitivity distribution correcting means for correcting the sensitivity distribution of the two-dimensional optical image corrected by the diffused light correcting means, and a two-dimensional optical image corrected by the sensitivity distribution correcting means based on a distortion conversion table measured in advance. Distortion correction means for correcting distortion, 2D X corrected by Kiyugami correction means
Scattered X-ray correction means for correcting scattered X-rays of a line image, logarithmic conversion means for logarithmically converting the two-dimensional optical image corrected by the scattered X-ray correction means, and correction of protrusion of the two-dimensional optical image after logarithmic conversion A cone beam X-ray tomography apparatus, wherein the reconstruction unit reconstructs a three-dimensional X-ray CT image from the two-dimensional optical image after the protrusion correction. 3. A measuring means for preliminarily measuring a relationship between a thickness of a predetermined X-ray absorber and a ratio of a scattered X-ray component to the thickness, and the two-dimensional X-ray image and an imaging condition. X
Absorber thickness calculation means for calculating the thickness of the X-ray absorber when the X-ray absorption amount of the X-ray absorber is equal to the X-ray absorption amount of the X-ray absorber; and the absorption calculated by the absorber thickness calculation means. Compare the body thickness with a preset absorber thickness, and as a result of the comparison,
When the absorber thickness of the subject is smaller than the preset absorber thickness, the ratio of the scattered X-ray component corresponding to the preset absorber thickness is determined by the two-dimensional X-ray imaging. Means for comparing the ratio of the scattered X-ray component of the two-dimensional X-ray image picked up by the means, wherein the scattered X-ray correction unit determines the ratio of the scattered X-ray component based on the ratio of the scattered X-ray component. The cone beam X-ray tomography apparatus according to claim 2, wherein the scattered X-ray is corrected. 4. The scattered X-ray correction means calculates a product of the ratio measurement value and a point spread function, and convolves the calculation result with a two-dimensional X-ray image before diffused light correction. Means for generating a scattered X-ray component image;
The cone beam X-ray tomography apparatus according to any one of claims 1 to 3, further comprising: means for subtracting the scattered X-ray component image from the line image. 5. The diffused light correction unit calculates a product of the ratio of the diffused light component and the point spread function, and calculates the calculated result with respect to a two-dimensional X-ray image before diffused light correction. 5. The apparatus according to claim 2, further comprising: means for performing a convolution operation to generate a diffused light component image; and means for subtracting the diffused light component image from the two-dimensional X-ray image. Item 6. A cone beam X-ray tomography apparatus according to Item 1. 6. The diffused light correction unit includes: a unit configured to generate a diffused light correction filter in a Fourier space as a function of a product of the ratio of the diffused light component and a point spread function; 5. The image processing apparatus according to claim 2, further comprising: a Fourier transform unit for performing a Fourier transform on the X-ray image; and a unit for performing a product operation of the diffused light correction filter on the X-ray image after the Fourier transform. The cone beam X-ray tomography apparatus according to claim 1. 7. An image is taken by arranging the X-ray shield so that the edge portion of the X-ray shield is orthogonal to the row direction or the column direction of the two-dimensional array of the two-dimensional optical image capturing means and near the center of the visual field. The cross-sectional profile of the obtained two-dimensional X-ray image is subjected to two-component fitting using a Gaussian function component representing the spatial resolution of the two-dimensional optical image capturing means and the optical image converting means and an exponential function component representing the diffused light component. Means for separating the scattered light component and the line spread function from the separated Gaussian function component and the exponential function component, and a point image from the line spread function. The cone beam X-ray tomography apparatus according to any one of claims 2 to 6, further comprising means for approximating a distribution function. 8. The X-ray shield is arranged such that an edge portion of the X-ray shield is orthogonal to a row direction or a column direction of the two-dimensional array of the two-dimensional optical image pickup means and is near the center of the visual field. A cross-sectional profile of a two-dimensional X-ray image obtained by imaging an object simulated subject having various thicknesses on an X-ray shield to form a scatterer is calculated by using the spatial resolution of the two-dimensional optical image imaging unit and the optical image conversion unit. , And a two-component fitting with an exponential function component representing a scattered X-ray component,
Means for separating the Gaussian function component and the exponential function component, means for calculating the ratio of the scattered X-ray component and the line spread function from the separated Gaussian function component and the exponential function component, The cone beam X-ray tomography apparatus according to any one of claims 2 to 6, further comprising: means for approximating a point spread function from the function. 9. The diffused light correction unit calculates a ratio of a diffused light component and a point spread function from a two-dimensional X-ray image captured under an arbitrary imaging condition, and calculates the point spread function. Means for converting to a function in units of pixels of the three-dimensional optical image imaging means, even if the imaging conditions are changed, based on the intensity ratio of the diffused light component and the point spread function,
The cone beam X-ray tomography apparatus according to any one of claims 1 to 8, wherein the diffused light is corrected. 10. A device for correcting the offset level of the two-dimensional X-ray image capturing means, wherein the correction of the offset level of the two-dimensional X-ray image capturing means is performed prior to the correction of the diffused light. The cone-beam X-ray tomography apparatus according to any one of claims 2 to 9, wherein: