JPH0882880A - Method for deciding corresponding point for aligning radiograph - Google Patents

Abstract

PURPOSE: To obtain a suitable corresponding point for aligning as for a method for deciding the corresponding point used for aligning a radiograph. CONSTITUTION: The reference corresponding points 40A-40D are set on the radiograph 4a and template areas 41A-41D with the reference corresponding points as a center are set. By using the areas 41A-41D, template matching operation is executed on the radiograph 4a. Besides, the plural corresponding points are set on the radiograph 4a. As for the plural corresponding points, such evaluation that a correlation value between the template areas and one part of the other radiograph obtained by the template matching operation is equal to or over a prescribed value, the distribution of the correlation value near the plural corresponding points does not become a ridge state and the ratio of a distance between two optional reference corresponding points and a distance between two corresponding points being on the other radiograph and corresponding to two reference corresponding points is within the range of a fixed value is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting misalignment of a plurality of radiation images to obtain corresponding points used when aligning the images, and more particularly to overlaying or subtracting radiation images. The present invention relates to a method for obtaining corresponding points when aligning radiation images.

[0002]

2. Description of the Related Art A stimulable phosphor is used to temporarily record radiation image information of an object such as a human body on a stimulable phosphor sheet (hereinafter referred to as a stimulable phosphor sheet), and this is recorded by excitation light. A method is known in which scanning is performed to cause stimulated emission of light, the stimulated emission of light is photoelectrically read to obtain an image signal, and the image signal is processed to obtain a radiation image of a subject having good diagnostic suitability. This final image can be played as a hard copy or on a CRT.

On the other hand, overlay processing of radiation images has been conventionally known (see, for example, JP-A-56-11399).
Generally, radiation images are used for diagnostic purposes and other purposes, but in using them, it is required to satisfactorily detect minute radiation absorption differences of a subject. The degree of this detection in the radiographic image is called contrast detectability or simply detectability. The higher the detectability, the higher the diagnostic performance.
It can be said that the radiographic image has high practical value. Therefore, in order to improve the diagnostic performance, it is desired to increase this detectability, but the biggest obstacle factor is various noises. The superposition processing is a method of significantly reducing the various noises, making it possible to clearly observe even a slight radiation absorption difference of the subject in the final image, and greatly improving the detectability. That is, a radiation image is captured (accumulated and recorded) on a plurality of stacked stimulable phosphor sheets, and a plurality of image signals obtained by subjecting the plurality of sheets to a reading process are subjected to an addition process. Is to reduce.

Conventionally, in order to actually carry out this superimposing processing, for example, two stimulable phosphor sheets are put in a cassette in an overlapping manner to photograph an object, and the two stimulable phosphor sheets are attached to the cassette. A method is used in which normal reading processing is sequentially performed to obtain two sets of image signals, and the two sets of image signals are added.

On the other hand, subtraction processing of radiation images has been conventionally known. This subtraction of a radiation image means that two radiation images captured under different conditions are photoelectrically read out to obtain digital image signals, and then these digital image signals are subjected to subtraction processing in correspondence with each pixel of both images. This is a method of obtaining a difference signal for extracting a specific structure in a radiation image. By using the difference signal thus obtained, a radiation image in which only the specific structure is extracted can be reproduced.

There are basically the following two methods for this subtraction processing. That is, (1) a specific structure is extracted by subtracting (subtracting) the image signal of the radiation image in which the contrast agent is not injected from the image signal of the radiation image in which the specific structure is emphasized by the injection of the contrast agent. So-called temporal subtraction processing, which (2)
Radiation having different energy distributions is radiated to the same subject, or radiation after passing through the subject is radiated to the two radiation detecting means by changing the energy distribution state, whereby images having different specific structures are displayed. This is a so-called energy subtraction process in which an image of a specific structure is extracted by allowing it to exist between two radiographic images, and then performing an appropriate weighting between the image signals of the two radiographic images and performing subtraction. .

Since this subtraction processing is an extremely effective method especially for medical diagnosis, it has received a great deal of attention in recent years, and its research and development have been actively promoted by making full use of electronic technology.

However, the following problems occur in the radiation image superposition processing method and subtraction processing method using the above-described stimulable phosphor sheet.

That is, in each of the above-described processing methods using the stimulable phosphor sheet, two (or sometimes three or more) stimulable phosphor sheets are sequentially or simultaneously inserted into the photographing table and overlapped with each other. The radiation image to be subtracted is taken, and then the stimulable phosphor sheet is individually inserted into the reading device, and each time the stimulable luminescent light emitted by irradiating the stimulable phosphor sheet with excitation light is detected. The radiographic image is read out by this, but in this process, even if the mechanical accuracy of all devices involved in imaging and reading is increased, positional deviation and rotational deviation occur between the images to be superposed or subtracted. Becomes As a result, although various noises are averaged and reduced in the superposition processing by this processing, blurring occurs in the entire image including the edge portion of the structure in the image, and the image to be observed becomes unsuitable for observation, In addition, in the subtraction process, the image to be erased is not erased, or conversely, the image to be extracted is erased to generate a false image, which makes it impossible to obtain an accurate subtraction image. As described above, it has been found that the positional deviation and the rotational deviation described above cause serious trouble in diagnosis.

When such a deviation occurs between the radiation image information stored and recorded on the stimulable phosphor sheet, the radiation image is stored and recorded in the stimulable phosphor as a latent image.
Unlike the case of an X-ray photo film capable of capturing an X-ray image as a visible image, it is impossible to visually match two X-ray images, and it is extremely difficult to correct the deviation.

Further, even if the positional deviation and the rotational deviation that occur between the two radiation images can be detected by some means, if the conventionally known arithmetic processing is performed to correct the data of the read radiation image, especially the rotational deviation occurs. A great amount of time is spent in the correction, which is a very serious problem in practical use.

Therefore, the applicant of the present invention has proposed a subtraction processing method for a radiation image using a marker having a shape which provides a reference point or a reference line in Japanese Patent Application Laid-Open No. 58-163338. This method records a marker on two stimulable phosphor sheets at a fixed position with respect to the radiographic image, detects the marker when reading the radiographic image,
Positional deviation and rotational deviation are calculated to rotate and / or move one of the deviations of the radiation image to be subtracted on the digital data, and the image data is subtracted between the corresponding pixels of this radiation image. . The alignment step in the subtraction processing method for a radiation image using this marker can also be applied to the above-described overlay processing method. In that case, after the alignment is performed, the addition processing of the image data may be performed between the corresponding pixels of the radiation image.

However, in this method, each time a radiographic image is taken, the above-mentioned marker must be stored and recorded together with the subject in the stimulable phosphor sheet. Then, there is a problem that the image information of the subject cannot be obtained from the portion of the accumulated and recorded radiation image that overlaps the position of the marker.

Therefore, the applicant of the present application has proposed a method of aligning a radiation image without using a marker or the like for alignment (Japanese Patent Application No. 4-318533). In this method, a template region is set in one of the plurality of radiation images to be aligned, template matching is performed for other radiation images using the template region, and at least two markers are provided for each radiation image. In the method to find the corresponding points instead of, affine-transform each corresponding point so that the corresponding points of each radiographic image match, and perform rotation movement correction, enlargement or reduction ratio correction and parallel movement correction for each radiographic image. is there.

According to this method, since the corresponding point which is a substitute for the marker is found out from the image and used, the marker or the like is not recorded together with the subject for alignment, and it is quick and highly accurate. Can be aligned.

[0016]

However, in the above-described alignment method, the corresponding points of each radiation image are obtained by template matching. For example, when noise in the radiation image is obtained as the corresponding points. However, such a corresponding point is not a corresponding point that is positionally common to the radiographic images, and the alignment cannot be performed accurately. Further, when the corresponding points are obtained at the position where the distribution of the correlation value on the edge of the radiographic image is ridge-shaped, the image is displaced along this ridge at the time of alignment, and as a result, the alignment can be performed accurately. There were times when it couldn't be done.

In view of the above circumstances, the present invention provides a method for determining corresponding points for alignment of a radiographic image, which can obtain corresponding points suitable for alignment in order to perform accurate alignment. It is intended.

[0018]

A method for determining corresponding points for alignment of radiation images according to the present invention is a method for determining corresponding points for aligning a plurality of radiation images,
A template region is set on one of the radiation images, and the template region is moved on a radiation image other than the one radiation image, and the template region and a part of the other radiation image. By performing template matching to find the correlation value with
A plurality of corresponding points corresponding to each of the radiographic images are obtained, and the corresponding points of one radiographic image among the radiographic images are set as reference corresponding points, and (1) the template matching is performed for the plurality of corresponding points. The correlation value between the template region and a part of the other radiographic image obtained by the above is greater than or equal to a predetermined value, and (2) the distribution of the correlation values near the plurality of corresponding points does not have a ridge shape. (3) The condition that the ratio between the distance between any two reference corresponding points and the distance between two corresponding points on another radiographic image corresponding to the two reference corresponding points is within a certain range. It is characterized in that at least one of the above conditions is evaluated, and corresponding points that match the evaluation are used as the corresponding points for alignment.

When the number of corresponding points that match the evaluation is less than a predetermined number, the template area is set and the template is set on the radiation image that is a predetermined distance or more from the corresponding points that do not match the evaluation. matching,
And it is preferable to repeat the evaluation until the corresponding points matching the evaluation reach the predetermined number.

Here, the distribution of the correlation value having a ridge shape is obtained by template matching in the vicinity of the edge of another radiation image when the template region including the edge portion of the radiation image is set, for example. The correlation value has a ridge shape as shown in FIG. 8 when the distribution of the correlation value is viewed three-dimensionally. Such a state is referred to as a state in which the distribution of correlation values is ridge-shaped.

Here, the template matching is a process of finding a best matching place by moving the template region on another radiation image when the template region is set on one radiation image as described above. Then, the point representing the place gives the coordinates of the corresponding point.

In such a template matching, the correlation method and the SSDA (Sequential Similarity Detection Algoli) are used as evaluation scales indicating the matching degree.
thms).

The correlation method is a method in which a product is calculated for each corresponding pixel, and a value obtained by standardizing the sum of the products (hereinafter referred to as a standardized value) is used as a measure of superposition. In this standardization, the sum of the products (squares) of the pixels themselves in each area is calculated, the product of each sum is further calculated, and the square root of this product is calculated as the denominator of the sum of the products for each corresponding pixel. It is carried out by When the superposition is perfect, the numerator products are not all sums of squares due to noise, etc.
Therefore, it is considered that the standardized value does not become 1, but becomes the maximum value closest to 1. Therefore, it is considered that the template region is moved variously on the radiographic image and the superposition is achieved with the above-described movement that maximizes the standardized value. However, the movement having the maximum standardized value cannot be determined unless all the movements are completed. Details of this method can be found in, for example, Smith et al.
cloud tracking using precisely aligned digital ATS
pictures ”ibid. , July 1972, volume c-21, pages 715-729.

The SSDA is a measure of superimposing the sum (residual difference) of absolute values of differences for each pixel.
When the superposition is perfect, it is considered that the residual is minimized even if it is not zero due to noise or the like. Therefore, it is considered that the template region is moved variously on the image and the overlay is achieved with the movement that minimizes the residual error. At this time, if the overlay is deviated, the residual error rapidly increases when the pixels are sequentially added. Therefore, if the residual exceeds a certain threshold in the middle of the addition, the method of aborting the addition immediately and moving to the next movement is this SSD.
It is A. Since the calculation used is only addition, and in many cases the calculation is aborted halfway, the calculation time is greatly shortened. For details of this method, see, for example, Barnea et al. "A class.
of algorithms for fast digital image registration
IEEE. Trans. , Vol. C-21, February 1972, pp. 179-186.

[0025]

The method of determining corresponding points for alignment of a radiation image according to the present invention is suitable for alignment like noise in a radiation image by evaluating whether or not the condition (1) described above is satisfied. Corresponding points that do not exist are excluded, and corresponding points that are likely to shift along the edge, such as the corresponding points found in the edge portion of the radiation image, are excluded by the evaluation of whether or not the condition (2) is satisfied. Further, when the radiographic images are completely coincident with each other, the distance between the corresponding two corresponding points in each radiographic image has a constant value, and therefore each radiographic image is evaluated by whether or not the condition (3) is satisfied. Corresponding points that are not common to will be eliminated. In this way, by evaluating whether or not any one of the three conditions is satisfied, corresponding points that are not suitable for alignment are eliminated, and only corresponding points that are more suitable for alignment are selected. The Rukoto.

When a plurality of recording media are superposed and imaged, the distance between any two corresponding points on the radiation image near the radiation source and the radiation corresponding to these two corresponding points. With respect to the distance between the two corresponding points on the side farther from the source, the latter is larger because the latter is irradiated with the radiation. Therefore, in the evaluation of (3), by setting the ratio of the former to the latter within the range of a constant value, a plurality of recording media are overlapped and radiographed to obtain a plurality of radiographic images. However, it is possible to accurately obtain corresponding points suitable for alignment.

Further, since none of the conditions (1), (2) and (3) is satisfied, the number of corresponding points may be too small to perform accurate alignment. There is. In such a case, that is, when the number of corresponding points that match the evaluation is less than a predetermined number, on the radiographic image separated by a predetermined distance or more from the corresponding points that do not match the evaluation, template region setting, template matching and By performing the evaluation repeatedly until the number of corresponding points that match the evaluation reaches a predetermined number or more, more corresponding points for alignment can be obtained. Therefore, by using these corresponding points, alignment can be performed more accurately. It can be carried out.

Here, the above-mentioned evaluation and the like are repeatedly performed on the radiation image which is separated from the corresponding point which does not conform to the evaluation by a predetermined distance or more. For example, at the corresponding point which does not conform to the evaluation, the distribution of the correlation value is a ridge. If the above-mentioned evaluation is performed again in the vicinity of this corresponding point, the distribution of the correlation value becomes a ridge at the obtained corresponding point, which does not meet the condition of (2) above. Sometimes. For this reason, the above evaluation and the like are repeated again on the radiation image that is separated by a predetermined distance or more from the corresponding point that does not match the evaluation.

[0029]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram of a radiation imaging apparatus which is an embodiment of an apparatus for recording a radiation image used in the present invention. The radiation image obtained by this imaging is used for energy subtraction processing.

The stimulable phosphor sheets 5 and 7 are stacked with the sheet 7 facing down with the filter 6 sandwiched therebetween. On this, a radiation source 2 that emits radiation 3 through a subject 4
Is arranged. The radiation imaging apparatus 1 is configured as described above.

Radiation 3 emitted from the radiation source 2 is applied to the subject 4. The radiation 3a that has passed through the subject 4 is applied to the first stimulable phosphor sheet 5, and a part of the energy of the radiation 3a is recorded in the first stimulable phosphor sheet 5, whereby the sheet 5 is exposed. Radiation images are accumulated and recorded. The radiation 3b transmitted through the sheet 5 is further transmitted through the filter 6, and the radiation 3c transmitted through the filter 6 is second.
The stimulable phosphor sheet 7 is irradiated. As a result, the radiation image of the subject 4 is also stored and recorded on the sheet 7.

FIG. 2 shows each stimulable phosphor sheet 5 and 7.
It is the figure which represented typically the radiographic image accumulated and recorded in. The radiation images 4a and 4b of the subject 4 are accumulated and recorded on substantially the entire surfaces of the stimulable phosphor sheets 5 and 7. That is, the radiation image 4a is obtained from the stimulable phosphor sheet 5 on the upper side,
The radiation image 4b is a radiation image obtained from the lower stimulable phosphor sheet 7.

FIG. 3 shows an embodiment of a radiation image reading apparatus which is an embodiment of a reading unit for reading a radiation image used in the present invention and an embodiment of an arithmetic unit which carries out the alignment method of the present invention and performs subtraction processing. It is a perspective view of an image processing display device.

After radiography is performed by the radiographic apparatus 1 shown in FIG. 1, the first and second stimulable phosphor sheets 5 and 7 are set one by one at a predetermined position of the radiographic image reading apparatus 10. Here, the case of reading the first radiation image accumulated and recorded in the first stimulable phosphor sheet 5 will be described.

The stimulable phosphor sheet 5 in which the first radiation image is stored and recorded, which is set at a predetermined position, is moved in the arrow Y direction by the sheet conveying means 15 such as an endless belt driven by a driving means (not shown). It is transported (sub-scan). On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotating polygon mirror 19 driven by a motor 18 and rotating at a high speed in the arrow Z direction, and after passing through a focusing lens 20 such as an fθ lens, an optical path is passed by a mirror 21. Incident on the stimulable phosphor sheet 5 by changing the direction, and the sub-scanning direction (arrow Y direction)
The main scanning is performed in the direction of the arrow X, which is substantially vertical. From the place where the light beam 17 of the stimulable phosphor sheet 5 is irradiated, stimulated emission light 22 having a light amount corresponding to the stored and recorded radiation image information is emitted. And is photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is made by molding a light guide material such as an acrylic plate, and is arranged so that the linear incident end face 23a extends along the main scanning line on the stimulable phosphor sheet 5. , Injection end face 23 formed in an annular shape
The light receiving surface of the photomultiplier 24 is coupled to b. The photostimulated luminescent light 22 that has entered the light guide 23 through the incident end face 23a travels through the inside of the light guide 23 after repeating total reflection, and then exits from the exit end face 23b to exit from the photomultiplier 24.
The photostimulated luminescent light 22 representing the radiation image is converted into an electric signal by the photomultiplier 24. The analog signal S output from the photomultiplier 24 is logarithmically amplified by the log amplifier 25, is then input to the A / D converter 26, is sampled, and is a digital image signal SO.
Is obtained. This image signal SO represents the first radiation image accumulated and recorded on the first stimulable phosphor sheet 5, and is referred to as the first image signal SO ₁ . The first image signal SO ₁ is temporarily recorded in the internal memory in the image processing display device 30.

The image processing display device 30 includes a keyboard 31 for inputting various instructions, a CRT display 32 for displaying a visible image based on auxiliary information for the instructions and an image signal,
Floppy disk drive device 33 in which a floppy disk as an auxiliary storage medium is loaded and driven, and a CPU
A main body 34 having a built-in memory and an internal memory is provided.

Then, in the same manner as described above, the second image signal SO ₂ representing the second radiation image accumulated and recorded on the second stimulable phosphor sheet 7 is obtained, and the second image signal SO ₂ is obtained.
_{2 is} also temporarily stored in the internal memory in the image processing display device 30.

When the two image signals SO ₁ and SO ₂ to be subjected to the subtraction operation are stored in the internal memory in this way, these two image signals SO ₁ and SO ₂ are read out and these two images are read. The images are aligned so that a corresponding subtraction operation is performed between the pixels of the radiation images carried by the signals SO ₁ and SO ₂ .

Here, in the present embodiment, the image signal SO
_A method of aligning two radiation images represented by ₁ and SO ₂ will be described.

In the method of aligning a radiation image according to the present invention, the radiation image 4a obtained from the stimulable phosphor sheet 5 located near the radiation source 2 in FIG. 1 is divided into four equal parts as shown in FIG. Four areas 14A, 14B, 14C, 14
Determine D. Then, for each area, an image is filtered using a cross-type filter to detect a point that gives the maximum output value of the filter.

Here, the filter matrix is Aij (i =
1,2, ... n, j = 1,2, ... n), the filter expression is

[0043]

[Equation 1]

However, the cross type filter is the element a.
= Ai, j (i = j or i = nj-1), b = Ai, j (i ≠ j and i ≠ n-
j-1) means a matrix such that a ≠ b (a> b). In the present embodiment, for example,

[0045]

[Equation 2]

The following matrix is used.

By using such a cross-type filter, as shown in FIG. 4, a complex portion having a structure such as a cross edge where ribs intersect with each other, that is, the density is extremely high when viewed from the surrounding area. Can be detected, and in this embodiment, four points 40 can be detected.
A, 40B, 40C, 40D can be detected. In addition,
A plurality of such cross edges exist in the four areas 14A to 14D, but in the present embodiment, the points 40A, 40B, 40C and 40D at which the output of the filter has the maximum value in each area.
Shall be detected.

When the four points 40A to 40D are detected in this way, these four points are set as reference corresponding points, and a template region centered on each reference corresponding point is set on the radiation image 4a. That is, as shown in FIG. 5, each area divided into four equal parts.
Template regions 41A to 41D centering on the corresponding reference points 40A to 40D are set in 14A to 14D.

Next, the template regions 41A to 41D are moved within a predetermined range on the radiation image 4b to perform template matching. Here, template matching is performed using the above-mentioned correlation method or SSDA. In the correlation method, as described above, the point having the maximum standardized value gives the coordinates of the corresponding points described below. Also, SSDA
As described above, the point where the sum of residuals is the minimum gives the coordinates of the corresponding point.

By performing template matching in this way, a plurality of corresponding points can be obtained in the radiation image 4b as shown in FIG.

Next, the plurality of corresponding points thus obtained are evaluated as to whether or not the following conditions are satisfied. That is, (1) the correlation value between the template regions 41A to 41D and a part of the radiation image 4b obtained by the template matching is equal to or more than a predetermined value, and (2) the distribution of the correlation value near the corresponding points is ridge-like. (3) Evaluation that the ratio between the distance between any two reference corresponding points and the distance between the two corresponding points on the radiographic image 4b corresponding to the two reference corresponding points is within a certain range. I do. This evaluation will be described in detail below.

First, the evaluation of (1) will be described.

For a plurality of corresponding points obtained on the radiation image 4b shown in FIG. 6, a correlation value is calculated when template matching is performed at each corresponding point, and the value of the correlation value at each corresponding point is less than 0.84. Eliminate corresponding points.
That is, the correlation value at each corresponding point is CORR (i,
j) (i, j are the coordinates of the corresponding points), the corresponding points that satisfy CORR (i, j) <0.84 (3) are excluded from the plurality of corresponding points shown in FIG. As a result, of the plurality of corresponding points found in the radiation image 4b, the corresponding points such as noise that are not suitable for alignment are eliminated.

Next, the above-mentioned evaluation (2) will be described.

First, with respect to a plurality of corresponding points obtained on the radiation image 4b shown in FIG. 6, the inclinations of the correlation values at the eight points in the vertical and horizontal directions and the 45 ° diagonal direction with respect to each corresponding point are obtained. For example, the pixel 50 that is the corresponding point shown in FIG.
The inclination of the correlation value at the time of template matching in the eight pixels 51A to 51H is obtained with the center of

That is, Ak = (CORR (i, j) -CORRk (i, j)) / 10 (4) where CORR (i, j): Correlation value at corresponding point CORRk (i, j): k The correlation value after 10 pixels in the direction Ak: The inclination is obtained from the inclination in the k direction, and if the angular inclination Ak is Ak <Th (Th = 0.00075) (5), the pixel is determined from the plurality of corresponding points shown in FIG. Exclude.

As described above, when the slope of the correlation value is small in a certain direction, it is determined that the pixel exists in the portion where the distribution of the correlation value shown in FIG. 8 has a ridge shape. In this way, if the corresponding points are present in the portion where the distribution of the correlation value is ridge-shaped, the image will be displaced along this ridge when the alignment is performed. Therefore, as shown in FIG. 8, when the distribution of correlation values in the vicinity of a certain pixel 52 has a ridge shape and the position of the corresponding point tends to shift in the direction of arrow A, whether or not the condition (2) is satisfied is determined. According to the evaluation, the corresponding points are unsuitable for alignment,
Excluded from multiple corresponding points.

Next, the above-mentioned evaluation (3) will be described.

First, four reference corresponding points on the radiation image 4a
The distance between any two points of 40A to 40D and the distance between two corresponding points on the radiation image 4b corresponding to these two points are obtained. For example, as shown in FIG. 9, the distance AB between the reference corresponding points 40A and 40B on the radiation image 4a and the radiation image 4b
The distance A'B 'between the corresponding points 40A' and 40B 'above is obtained. That is, the coordinates of the reference corresponding points 40A and 40B are (U
i, Vi), (Uj, Vj), and corresponding points 40A ', 40
When the coordinates of B'are (Xi, Yi), (Xj, Yj)

[0060]

(Equation 3)

The distances AB and A'B 'are obtained by

Then, the ratio of the distances AB and A'B 'is obtained, and it is checked whether or not this ratio is within a predetermined threshold range. That is, Th1 <A'B '/ AB <Th2 (7) is evaluated, and only corresponding points satisfying the condition of Expression (7) are used as corresponding points suitable for alignment.

Here, the ratio A'B '/ AB in equation (7)
Is supposed to be 1 when the positions of the points truly correspond, but when the radiation image shown in FIG. 1 is taken,
The radiation image accumulated and recorded on the stimulable phosphor sheet 7 is
The image is slightly larger than the radiation image accumulated and recorded on the stimulable phosphor sheet 5. This is a phenomenon caused by the radiation being spread and irradiated because the stimulable phosphor sheet 7 is located farther from the sheet 5 as viewed from the radiation source 2. Therefore, A'B 'in equation (7)
It is desirable that the condition (3) is evaluated by setting the condition of / AB as 1 and setting it within the range of threshold values Th1 to Th2 slightly larger than 1.

In this way, the plurality of corresponding points shown in FIG. 6 are evaluated, and finally, as the corresponding points suitable for alignment, the corresponding points shown in FIG.
As shown in, maximum four corresponding points 44A to 44D are four areas.
Selected within 43A-43D.

Then, the coordinates of the reference corresponding points 40A to 40D on the radiation image 4a are set to (ui, vi) (i = 1 to 4), and affine transformation is performed.

[0066]

[Equation 4]

However, m is the enlargement / reduction ratio, θ is the rotation amount,
A and B are the first radiation image by converting the coordinates of each corresponding point (xi, yi) according to the coefficient indicating the rotational movement correction and the enlargement or reduction ratio correction, and C and D according to the coefficient indicating the parallel movement correction. The 4a and the second radiation image 4b are superposed. Here, in the coordinate transformation based on the equation (3), the entire second radiographic image is enlarged or reduced independently in the X direction and the Y direction, the second radiographic image is rotationally moved, and the second radiographic image is rotated. The parallel translation of the radiation image in X direction and Y direction is performed at the same time.

Here, the coefficients A, B, and
A method of obtaining C and D will be described.

In the present invention, the coefficients A, B, C and D are determined by the method of least squares. First, the positional relationship between the reference corresponding points (ui, vi) and the corresponding points (xi, yi) is determined. The error E is represented by E = Σ (u−ui) ² + Σ (v−vi) ² = Σ (A · xi −B · yi + C−ui) ² + Σ (B · xi + A · yi + D−vi) ² ... 9) and solving equation (9) to minimize the squared error of the error E,

[0070]

(Equation 5)

It becomes Therefore, when the linear equation is solved from the equation (10), the coefficients A, B, C and D are obtained as follows.

A = [d · (e + f) −b · i−c · j] / Δ B = [d · (g + h) + c · i−b · j] / Δ C = [− b · (e + f) + c・ (−g + h) + a · i] / ΔD = [− c · (e + f) −b · (−g + h) + a · j] / Δ where Δ = a · d−b ² −c ² a = Σ ( xi ² + yi ² ), b = Σxi, c = Σyi, d = Σ e = Σxi · ui, f = Σyi · vi, g = Σyi · ui, h = Σxi · vi, i = Σui, j = Σvi ... 1 1) By using the coefficients A, B, C, and D obtained in this way to perform coordinate conversion using equation (8), each corresponding point 44
The second radiographic image 4b can be approximately equalized to the first radiographic image 4a so that the error between A to 44D and the reference corresponding points 40A to 40D is minimized. That is, as shown in FIG. 11, when looking at the data obtained by aligning 55 sets of radiation images by the alignment method of the present invention, the reference level is | R.
When | = 4.5 and max | Ri | = 1.00, all data are within the reference level, and | R | = 3.15, max | R
It is within i | = 0.7, and it can be seen that the alignment is performed with extremely high precision as compared with the alignment result by the conventional method shown in FIG.

[0073] After this manner affine transformation is performed, subtraction processing, i.e. the image signal after the alignment of the image signal SO ₂ SO ₂ 'when _{the, S1 = Wa · SO 1 -Wb} · SO 2' + C (12) where Wa and Wb are weighting coefficients and C is a bias component.

The weighted subtraction is performed by the image signal S1 corresponding to the image of the difference between the two radiation images.
Is generated. This image signal S1 is used for the image processing display device 30.
Sent to the CRT display 32, and a visible image (energy subtraction image) based on the image signal S1 is reproduced and displayed on the CRT display 32. The main body
The function (combination of hardware and software) for performing the above subtraction processing executed in 34 is considered as an example of the arithmetic unit of the present invention.

In the above embodiment, the corresponding points detected by the template matching are affine-transformed from the radiation image obtained from the stimulable phosphor sheet 7 located far from the radiation source 2 in FIG. However, this is due to the following reasons. That is, when a radiation image is stored and recorded on two stimulable phosphor sheets by so-called one shot as shown in FIG. 1, the two sheets 5 and 7 are spatially different from each other with the filter 6 interposed therebetween. Therefore, the distance between the two sheets is different from the radiation source 2 and the subject 4, and therefore the size of the radiation image recorded on each sheet 5 and 7 is different, and the sheets are far from the radiation source 2. The radiation image 4b accumulated and recorded on the sheet 7 at the position becomes an image with more blurring and more scattered rays than the radiation image 4a accumulated and recorded on the sheet 5. Further, when the above-mentioned affine transformation is performed, the image quality of the transformed image deteriorates to some extent because it is necessary to perform some kind of interpolation between pixels. Therefore, rather than affine-transforming the high-quality radiation image 4a and degrading the image quality, the radiation image 4a
By converting the radiation image 4b, which is inferior in image quality as compared to, the image quality of the resulting subtraction image is guaranteed. Therefore, the radiation image 4b obtained from the stimulable phosphor sheet 7 located far from the radiation source 2 is affine-transformed.

Further, in the above-mentioned embodiment, it is used for alignment of corresponding points satisfying all the above-mentioned three conditions (1), (2) and (3). Corresponding points that satisfy at least one of the three conditions may be used for alignment. However, the more the conditions are, the more corresponding points are selected for the alignment. Therefore, the number of conditions may be determined according to the required alignment accuracy.

Further, in the above-mentioned embodiment, 0.84 is used as the threshold value of the correlation value in the condition (1), but this threshold value may be any value.
As shown in 13, the threshold value may be set to increase as the dose of the radiation image increases. The reason why the threshold value is changed according to the dose is that when the radiation dose is low, noise increases, the output value of the filter decreases, and it becomes difficult to obtain the feature point.

In addition, in the above-described embodiment, when the condition (2) is evaluated, the pixel 50 which is the corresponding point is calculated.
Eight pixels 51A to 51H in the up, down, left, right and diagonal directions of 45 degrees
The slope of the correlation value at
Four pixels 51A, 51C, 51E, 51G in the vertical and horizontal directions of 50
It is also possible to obtain the slope of the correlation value at. However, as the number of this direction increases, the corresponding point more suitable for the alignment is selected. Therefore, the number of pixels for which the slope of the correlation value is obtained may be determined according to the required alignment accuracy.

By performing the evaluation on the condition (2) described above, even if a corresponding point is obtained on the edge of the irradiation field, as in the case where a radiation image is captured using the irradiation field diaphragm, Since these corresponding points are eliminated, the images will not be misaligned and aligned along the irradiation field edge.

Further, in the above-described embodiment, the alignment is performed by the affine transformation after the corresponding points are selected. However, the alignment method is not limited to the affine transformation, and the above-mentioned method is used. In the method of position alignment using the marker disclosed in Japanese Patent Laid-Open No. 58-163338, the position shift may be corrected by using corresponding points instead of the marker.

Further, in the above-mentioned embodiment, when the number of corresponding points is 2 or less as a result of the evaluation,
That is, when there are too few corresponding points for accurate alignment, the conditions in the evaluation described above may be relaxed and the evaluation may be performed again to find the corresponding points.

In such a case, the above (1) and (2)
From the corresponding points that do not satisfy the conditions, that is, at a position separated by a predetermined distance or more from the corresponding points that do not satisfy any of the conditions of (3), that is, in the region including the corresponding points that do not match the evaluation as shown in FIG. At a position separated by a radius r or more,
The above-described template region setting, template matching, and evaluation may be repeatedly performed until the corresponding points reach a predetermined number or more with which accurate alignment can be performed.

That is, as shown in FIG. 14, when the reference corresponding point 40A obtained in the area 14A is a corresponding point that does not match the evaluation, the radiation image 4a is generated in the shaded area separated by a radius r from the corresponding point 40A. For the above, the filtering process described above is performed to set a template region, and the newly set template region is moved within a predetermined range on the radiation image 4b to perform template matching. Then, after a new corresponding point is obtained by this template matching, at least one of the conditions (1), (2) and (3) described above for this corresponding point.
Evaluate whether or not one of the conditions is satisfied. As a result, the number of corresponding points for alignment is equal to or greater than a predetermined number (for example, 3
If more than one), the corresponding points are used for alignment. On the other hand, when the number of corresponding points to be aligned does not exceed the predetermined number, the setting of the template area, template matching, and evaluation are repeated until the number of corresponding points to be aligned exceeds the predetermined number. In this way, by repeatedly setting the template area and the like until the number of corresponding points becomes equal to or greater than the predetermined number, it is possible to reliably obtain a plurality of corresponding points for alignment. It becomes possible to perform the alignment more accurately by performing the alignment using.

Here, the above-mentioned evaluation and the like are repeatedly performed on the radiographic image separated by a radius r or more from the corresponding points which are not suitable for the evaluation. If the above-mentioned evaluation is performed again in the vicinity of this corresponding point, the distribution of the correlation value may become ridge-shaped again in the area near the obtained new corresponding point. , It may not meet the above condition (2). For this reason, the above-described evaluation and the like are repeated again on the radiation image that is separated by a radius r or more from the corresponding point that does not match the evaluation. The radius r needs to be set so that the ridge-like portion can be avoided.

When the evaluation is repeated in this way, it is preferable to limit the number of repetitions in order to shorten the calculation time. According to the experiment of the radiographic image of the chest of the applicant, when the number of corresponding points for alignment in the database of about 500 cases is 4, the average number of corresponding points is repeated. When not performed, it was 2.8, and when the number of repetitions was limited to 3, it was 3.8. Therefore, when limiting the number of repetitions, it is preferable to limit the number of times to about three.

Further, in the above-mentioned embodiment, the two radiation images are aligned in order to perform the energy subtraction processing, but the two radiation images are aligned in order to perform the superposition processing. You may do it. That is, as shown in FIG. 15, the radiation image of the subject 4 is accumulated and recorded on the two stimulable phosphor sheets 5'and 7'without using the filter 6 in the radiation image capturing apparatus 1 similar to that shown in FIG. Fluorescent phosphor sheet 5 ', 7'
From the radiation image reading apparatus shown in FIG. 3, image signals SO ₁ and SO ₂ representing two radiation images are obtained from the radiation image obtained from the two stimulable phosphor sheets 5 ′ and 7 ′. After performing the affine transformation on the radiographic image obtained from ′ in the same manner as in the above-described embodiment and performing alignment,
Superimposing process, that 'when _{a, S2 = Wc · SO 1 +} Wd · SO 2' image signal SO ₂ after alignment of the image signal SO ₂ ... (13) However, Wc, Wd by weighting factor, weighting addition Is performed, and thereby, the image signal S2 corresponding to the image of the sum of the two radiation images is generated.

Further, in the above-described embodiment, the radiation image is filtered by the cross filter to detect the point that gives the maximum value of the filter output. Any point may be used, such as a point representing the maximum value in each area or a point having a certain value or more.

Further, in the above-mentioned embodiment, the alignment of two radiation images is explained, but the number of radiation images to be aligned may be three or more. In this case, of the three or more radiation images, one of the radiation images has the above-mentioned reference corresponding point set, the other radiation images have corresponding points set by template matching, and the corresponding points of each radiation image are set as reference points by affine transformation. It may be adapted to the corresponding points.

In the above-described embodiment, the alignment is performed by the affine transformation shown in the equation (8), which is a general equation.

[0090]

(Equation 6)

(However, u and v are the coordinates of the reference corresponding point, x,
y is the coordinate of the corresponding point to be converted, a, b, c, d are coefficients indicating rotational movement correction and enlargement or reduction correction, and e, f
Is any coefficient as long as it performs the affine transformation represented by (coefficient indicating parallel movement correction).

[0092]

As described in detail above, the method for determining corresponding points for alignment of a radiation image according to the present invention has been described with respect to a plurality of corresponding points obtained by template matching (1), (2) and (). Since at least one of 3) is evaluated, corresponding points suitable for alignment can be more accurately obtained. If alignment is performed using the thus obtained corresponding points, more accurate alignment can be achieved. Alignment can be performed quickly.

When the number of corresponding points that match the evaluation is less than the predetermined number, the template area is set, the template matching and the evaluation are repeated until the number of corresponding points that match the evaluation reaches a predetermined number. Since more corresponding points for alignment can be obtained, the alignment can be performed more accurately by using these corresponding points.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a radiographic image recording apparatus that obtains a radiographic image that performs subtraction according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a radiation image accumulated and recorded on each stimulable phosphor sheet.

FIG. 3 is a schematic diagram of an image reading apparatus including an apparatus for performing a radiation image registration method according to the present invention.

FIG. 4 is a diagram showing a state in which a radiographic image is divided and reference corresponding points are set.

FIG. 5 is a diagram showing a state in which a template region is set in a radiation image.

FIG. 6 is a diagram showing a state in which corresponding points are set in another radiation image.

FIG. 7 is a diagram for explaining evaluation (2).

FIG. 8 is a diagram showing a ridge-like distribution of correlation values.

FIG. 9 is a diagram for explaining evaluation (3).

FIG. 10 is a diagram showing selected corresponding points.

FIG. 11 is a diagram showing a result of registration performed by the radiation image registration method according to the present invention.

FIG. 12 is a diagram showing a state in which registration is performed by a conventional radiation image registration method.

FIG. 13 is a diagram showing a relationship between radiation and a threshold value.

FIG. 14 is a diagram in which a region distant by a radius r or more from a corresponding point that does not match the evaluation is represented by diagonal lines.

FIG. 15 is a schematic diagram of a radiographic image recording apparatus that obtains a radiographic image for superimposing according to an embodiment of the present invention.

[Explanation of symbols]

 1 Radiography apparatus 2 Radiation source 3 Radiation 4 Subject 5 First stimulable phosphor sheet 6 Filter 7 Second stimulable phosphor sheet 10 Radiation image reader 15 Sheet conveying means 16 Laser light source 17 Light beam 18 Motor 19 rotation Polygonal mirror 20 Focusing lens 21 Mirror 22 Excited emission light 23 Light guide 24 Photomultiplier 25 Log amplifier 26 A / D converter 30 Image processing display device 31 Keyboard 32 CRT display 33 Floppy disk drive device 34 Main body 40A-40D Standard Corresponding points 41A to 41D Template area 44A to 44D Corresponding points

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G06T 7/00 G06F 15/62 390 A 9061-5H 15/70 330 P

Claims (2)

[Claims] 1. A method for determining corresponding points for aligning a plurality of radiation images, wherein a template region is set on one radiation image among the radiation images, and a template region other than the one radiation image is set. By performing template matching to find the correlation value between the template region and a part of the other radiation image by moving the template region on the radiation image of The corresponding points of one radiation image among the radiation images are set as reference corresponding points, and (1) the template region and the other radiation obtained by the template matching are obtained for the plurality of corresponding points. The correlation value with a part of the image is equal to or greater than a predetermined value, and (2) the distribution of the correlation values near the plurality of corresponding points is a ridge. (3) The ratio between the distance between any two reference corresponding points and the distance between two corresponding points on another radiographic image corresponding to the two reference corresponding points is within a certain range. Of at least one of the above conditions is evaluated, and corresponding points that match the evaluation are determined as corresponding points for the alignment, and corresponding points for alignment of the radiographic image are determined. Method. 2. When the number of corresponding points that match the evaluation is less than a predetermined number, the template region is set and the template matching is performed on the radiation image that is a predetermined distance or more from the corresponding points that do not match the evaluation. And the evaluation is repeated until the number of corresponding points conforming to the evaluation reaches the predetermined number.