JP2006288467A - Device and method for judging irradiation field and its program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for recognizing an irradiation field of radiographic images in which the recognition accuracy of irradiation field recognition processing is improved, an irradiation field recognition method, and its program. <P>SOLUTION: The division pattern of the radiographic image P radiographed in division is set, and the radiographic image P is divided into a plurality of sections according to the division pattern. An irradiation field area P<SB>in</SB>is recognized from the images inside the respective divided sections, the recognized irradiation field area P<SB>in</SB>of the respective sections and the boundaries n of the respective divided sections are compared, and whether or not the division is appropriate is judged. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an irradiation field determination device that recognizes an irradiation field of a radiographic image, an irradiation field determination method, and a program thereof.

  Conventionally, there are various cases in which a recorded radiographic image is read to obtain image data, an appropriate image processing is performed on the image data, and then a visible image suitable for interpretation is reproduced based on the processed image data. In the field.

  By the way, when radiographic images are captured and recorded in X-ray imaging, in order to minimize the influence on the living body due to radiation irradiation, and to prevent deterioration in image quality performance due to scattered light from parts unnecessary for observation, etc. In many cases, an irradiation field stop made of lead or the like is used to limit the irradiation area so that the radiation is irradiated only to a necessary part of the subject.

  When photographing is performed using an irradiation field stop, an image of the subject or the like is recorded in an internal region (irradiation field region) of the opening contour of the irradiation field stop on a recording medium such as a stimulable phosphor sheet. Radiation does not reach the outer region (irradiation field region) and is in an unexposed state. That is, the irradiation field contour of the image corresponding to the opening contour is an edge line.

  Then, when image processing is performed by reading image data from a recording medium in which an image is recorded only in the irradiation field region in this way, if gradation processing or the like is performed only on the image data in the irradiation field region, The processing load can be reduced and the processing speed can be improved.

  On the other hand, since the irradiation field area is in an unexposed state, in a negative image such as a medical X-ray film, it becomes the lowest density area (area having a low pixel value). When observing a transmitted image with light, the minimum density area is very bright, so that the brightness of the irradiation field area, especially the area close to the irradiation field area, is affected. Interpretation performance is reduced. Similarly, when the image is displayed on an image display device such as a CRT, the area outside the irradiation field is a high-luminance area, which causes an obstacle to interpretation of the image in the irradiation field.

  Therefore, in the radiographic image recording / reproducing system, processing for forcibly replacing each image data for such an irradiation field region uniformly with a value corresponding to the highest density (or lowest luminance) is performed. This process is generally called a blackening process. To perform this blackening process, it is very important to accurately recognize the irradiation field contour.

  For example, it is well known to obtain an irradiation field contour by searching for a portion where the change in image data is steep, utilizing the fact that the irradiation field contour becomes an edge line in which the density change of the image changes sharply. However, as a specific method for obtaining the edge line, a plurality of radial straight lines from a predetermined point of the image (for example, the center point of the image) to the edge of the image are set, and the direction of each of these straight lines is set. A method has been proposed in which candidate points on an edge having a large difference in data for each direction are detected based on the image data along the line, and a line serving as an edge is detected based on these candidate points (for example, Patent Documents). 1).

  In addition, when capturing and recording a radiographic image, so-called divided imaging is often performed. The radiographic image is often divided into a plurality of predetermined predetermined divided sections, and shooting is performed by irradiating each section with radiation. Even in such divided shooting, an image is obtained using the irradiation field stop and the sections are separated. In such a case, it is impossible to accurately recognize the irradiation field on the assumption that only one irradiation field exists. Therefore, for example, when divided shooting is performed in four divisions, a dividing line for dividing each section often appears at the center in the vertical direction or the center in the horizontal direction of the image. There is one that searches for an edge and determines whether or not the image has been divided and photographed based on whether or not the edge is aligned on a straight line (for example, Patent Document 2).

Alternatively, prepare a binarization mask corresponding to the division pattern in advance, binarize the divided photographed image to separate the irradiation field and the irradiation field, and compare the prepared binarization mask with the pattern. A method of recognizing a division pattern depending on whether or not they match is proposed (for example, Patent Document 3).
Japanese Unexamined Patent Publication No. 63-259538 Japanese Unexamined Patent Publication No. 63-257879 Japanese Unexamined Patent Publication No. 1-212065

  However, in Patent Document 1, since an object is photographed at the center of an image and an edge is searched for, the irradiation field cannot be recognized well when divided photographing is performed. Therefore, when divided shooting is performed, it is necessary to recognize the irradiation field for each section after recognizing each section that has been divided and shot.

  However, each section that was shot separately is not necessarily divided at the center in the vertical direction or the center in the left-right direction of the image. May be photographed with an inclination, and there are cases in which accurate recognition is not possible even if an attempt is made to recognize a section based on a standard division pattern as in the methods of Patent Documents 2 and 3.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an irradiation field recognition apparatus, an irradiation field recognition method, and a program thereof for a radiographic image, which have improved recognition accuracy over conventional irradiation field recognition processing. It is what.

The irradiation field recognition device of the present invention includes an input unit for inputting a radiographic image obtained by photographing a subject using an irradiation field stop,
Division pattern setting means for setting a division pattern of the radiographic image obtained by division imaging;
A dividing unit that divides the radiographic image input by the input unit into a plurality of sections according to the division pattern;
Irradiation field recognition means for recognizing an irradiation field region from the image in each of the divided sections;
A determination means for comparing the irradiation field area of each section recognized by the irradiation field recognition means and the boundary of each divided section to determine whether or not the division is appropriate,
Until the determination unit determines that the division is appropriate, a division pattern different from the division pattern already set by the division pattern setting unit is set, and the radiographic image is converted into the different division pattern by the division unit. Then, after dividing into a plurality of sections, the process for recognizing the irradiation field in each section by the irradiation field recognition means is repeated.

The irradiation field recognition method of the present invention includes an input step of inputting a radiographic image obtained by photographing a subject using an irradiation field stop,
A division pattern setting step for setting a division pattern of the radiographic image obtained by the division imaging;
A division step of dividing the radiographic image input by the input step into a plurality of sections according to the division pattern;
An irradiation field recognition step for recognizing an irradiation field region from the image in each of the divided sections;
A determination step of determining whether or not the division is appropriate by comparing the irradiation field area recognized by the irradiation field recognition step and the boundaries of the plurality of divided sections;
Until the division step is determined to be appropriate, a division pattern different from the division pattern already set by the division pattern setting step is set, and the radiographic image is changed to the different division pattern by the division step. Then, the process of recognizing the irradiation field in each section is repeated in the irradiation field recognition step.

Further, the program of the present invention provides a computer,
An input means for inputting a radiographic image obtained by photographing a subject using an irradiation field stop;
Division pattern setting means for setting a division pattern of the radiographic image obtained by division imaging;
A dividing unit that divides the radiographic image input by the input unit into a plurality of sections according to the division pattern;
Irradiation field recognition means for recognizing an irradiation field region from the image in each of the divided sections;
Compare the irradiation field area recognized by the irradiation field recognition means and the boundaries of the plurality of divided sections to function as a determination means for determining whether or not the division is appropriate,
Until the determination unit determines that the division is appropriate, a division pattern different from the division pattern already set by the division pattern setting unit is set, and the radiographic image is converted into the different division pattern by the division unit. Then, after the process is divided into a plurality of sections, the irradiation field recognition means functions to repeat the process of recognizing the irradiation field in each section.

  The “division pattern” is a division pattern used when the radiographic image is divided and photographed, and includes a plurality of division patterns such as two divisions in the vertical direction, two divisions in the horizontal direction, and four divisions in both the vertical and horizontal directions.

  “Section” refers to one obtained by dividing a radiographic image, and “Dividing a radiographic image into a plurality of sections” means that a subject is divided into a plurality of parts to be captured separately. When images are taken together as a single radiographic image, this means that a plurality of images in the radiographic image are divided into individual images so as to match the division pattern at the time of division imaging.

  “Recognizing the irradiation field area” means that a radiographic image taken using the irradiation field stop is discriminated to find the irradiation field area irradiated with the radiation. “Recognizing the irradiation field area” means that the irradiation field area is recognized from each section of the radiographic image divided into sections.

  “Determining whether or not the division is appropriate” means determining whether or not one section obtained by dividing the radiographic image is coincident with the division at the time of the division imaging.

  In “repeat process”, the dividing means (dividing step), the irradiation field recognizing means (irradiation field recognizing step), and the determining means (determining step) are repeated or determined in advance until it is determined that the dividing is appropriate. For all of the plurality of divided patterns, the dividing means, the irradiation field recognizing means, and the determining means are repeatedly performed to adopt an optimum divided pattern, or the dividing means and the irradiation are applied to all of the plurality of predetermined divided patterns. This includes a case where after the field recognition means is repeatedly performed, the determination is performed by the determination means and the optimum division pattern is adopted.

The irradiation field recognition means includes
Straight line detection means for detecting a straight line that is a candidate for the irradiation field side of the irradiation field region from the image in the section;
A reference point setting means for setting a reference point in the section;
Evaluation that divides the image of the section into two regions by the detected straight line and calculates an evaluation value that evaluates whether radiation is incident on an external region that does not include the reference point among the two regions. A value calculating means;
Based on the evaluation value, when it is determined that the radiation is incident on the external region, the straight line has irradiation field side determination means for determining that the line is not an irradiation field side,
It is preferable to recognize the irradiation field region based on the remaining straight lines excluding the straight line determined not to be the irradiation field side from among the straight lines that are candidates for the irradiation field side.

  The “reference point” only needs to be within the range considered to be within the radiation field of the radiographic image in comparison with the usual imaging method, and “reference point setting” can be set automatically, even if it is set automatically. An instruction input by a person or the like may be used.

  The “region where radiation is incident” is a region irradiated with X-rays transmitted through the subject or a region directly irradiated with X-rays. The “region where radiation is incident” includes scattered radiation. A region where radiation is incident is not included in a region where X-rays are not originally irradiated.

  According to the present invention, the divided radiographic image is divided into a plurality of sections in accordance with the division pattern, the irradiation field area is recognized from the images in each divided section, and the boundary between the recognized irradiation field area and the section is recognized. And determining whether or not the division is appropriate, it becomes possible to accurately recognize the irradiation field region according to the pattern at the time of the division imaging.

  Furthermore, when recognizing an irradiation field region, if it is evaluated whether or not radiation is incident on the outside of the irradiation field region, an irradiation field region extending over a plurality of sections can be identified within each section. It will not be mistaken for an irradiation field region that is divided into two.

  Hereinafter, a specific embodiment of an irradiation field recognition apparatus for radiographic images of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration of an embodiment of an irradiation field recognition apparatus according to the present invention. FIG. 2 shows a radiographic image capturing apparatus using an irradiation field stop (see FIG. 2A) and the irradiation field stop. It is a figure (refer FIG.2 (b)) which shows the stimulable fluorescent substance sheet in which the irradiation field outline corresponding to an opening outline was formed.

When performing X-ray imaging using an irradiation field stop with an imaging apparatus, first, an image obtained by imaging a subject is arranged so as to be at the center of a radiographic image P, and as shown in FIG. An image is taken with an irradiation field stop having an opening such as a rectangle between the radiation source and the subject. The portion outside the opening is a lead plate that prevents the X-rays from reaching the subject and the stimulable phosphor sheet. When X-rays are irradiated from the X-ray source to the subject in this state, an opening of the irradiation field stop is displayed on the radiographic image P obtained from the stimulable phosphor sheet, as shown in FIG. moiety in the corresponding area (irradiation field area) P _out outside without X-rays are irradiated, whereas, in the irradiation field stop region corresponding to the inner side of the opening of the (radiation area) P _in which has passed through the subject of The portion directly irradiated with X-rays is recorded. Then, a portion corresponding to the opening of the diaphragm irradiation field becomes the opening substantially the same shape, the irradiation field area P _out becomes high brightness dazzling region. The boundary of the irradiation field region P _in the irradiation field area P _out, the irradiation field contour PS comprising a plurality of edge lines density changes sharply is formed. The opening of the irradiation field stop is usually rectangular, and the irradiation field contour PS is composed of four straight irradiation field sides.

Further, when the divided-captured, segmented radiation area in each section was exists, it appears more radiation area P _in. Normally, in divided shooting, as shown in FIG. 3, the image is divided into 2 parts in the vertical direction (FIG. 3B), 2 parts in the horizontal direction (FIG. 3C), and 4 parts in both the vertical and horizontal directions (FIG. 3A). ))

  Therefore, a method for recognizing the irradiation field of the radiographic image P that has been divided and photographed using the irradiation field recognition device 1 of the present invention will be described. As shown in FIG. 1, the irradiation field recognition apparatus 1 of the present invention includes an input means 10 for inputting a radiographic image P obtained by imaging a subject using an irradiation field stop, and radiographic images obtained by division imaging. From the division pattern setting means 20 for setting the division pattern of P, the division means 30 for dividing the radiographic image P inputted by the input means 10 into a plurality of sections according to the division pattern, and the images in each divided section The irradiation field recognition means 40 for recognizing the irradiation field area and the irradiation field area of each section recognized by the irradiation field recognition means and the boundary of each divided section are compared to determine whether or not the division is appropriate. And determining means 50 for performing.

  The division pattern is a division pattern used when the radiographic image is divided and photographed. As shown in FIG. 3, there are two divisions in the vertical direction, two divisions in the horizontal direction, and four divisions in both the vertical and horizontal directions. The pattern setting means 20 selects and sets one of them.

  The dividing unit 30 divides the radiographic image P into a plurality of sections according to the division pattern set by the division pattern setting unit 20. Here, a case will be described in which a division pattern of two divisions is set in the vertical direction as shown in FIG.

  First, a candidate division line to be divided in the vertical direction is searched from the radiographic image P. In this embodiment, one pixel corresponds to 1.8 mm, and the radiographic image P having a horizontal size of 170 pixels and a vertical size of 140 pixels is taken as an example. This will be specifically described.

  First, when a pixel value of an X-ray irradiation region irradiated with X-rays is examined from a histogram of the entire image, as shown in FIG. 4, a histogram of pixel values (here, density is included) included in the entire radiographic image P. In the following description, it is assumed that the pixel value of a high pixel is expressed as a large value, and the pixel value decreases as the density decreases. Each peak corresponds to the direct X-ray irradiation unit Dh, the region Dm where the subject is imaged, and the irradiation field region D1, and the pixel values of the X-ray irradiation region irradiated with the X-ray are Dm and Dh.

  Therefore, as shown in FIG. 5, the pixel values of the pixels arranged on the vertical line of the radiographic image P are shifted little by little from the center line m toward the left and right ends (arrows in FIG. 5A). However, a vertical line in which no pixel having the pixel value of the X-ray irradiation region exists on the vertical line is searched, and a portion in which there is no pixel having the pixel value of the X-ray irradiation region, that is, X Find the part D where the line is not irradiated.

  Next, the center portion M is a range of 20 pixels on the left and right with respect to the vertical line m passing through the left and right centers of the image, and a common portion of the portion D and the center portion M not irradiated with X-rays is It is assumed that the range where the candidate division line exists (shaded area in FIG. 5B).

  Within the range where the candidate dividing lines exist, the pixel values of the pixels arranged on the vertical line are averaged, a vertical line with the average of the minimum value a is found, and each vertical line in the vertical direction of this line is found. Search for a vertical line whose average pixel value on the direction line exceeds a + 7, and divide the vertical line existing in the middle of the range between the left and right vertical lines exceeding a + 7 for the first time as candidate lines Let n.

The radiographic image P is divided into two sections with the division candidate line n as a boundary. In the case of dividing into two in the horizontal direction and in the case of dividing into four in both the vertical and horizontal directions, the candidate division line n is similarly searched and divided into images P _{sec of} each section as shown in FIG.

As shown in FIG. 7, the irradiation field recognizing unit 40 detects a straight line that is a candidate for the irradiation field side from the image P _sec in each section obtained by photographing the subject using the irradiation field stop. When, the reference point setting means 44 for setting a reference point in the substantially central portion of the image P _sec in each compartment, by dividing the image P _sec in each compartment into two regions by the detected straight line, the two regions Evaluation value calculating means 45 for calculating an evaluation value for evaluating whether or not radiation is incident on an external region that does not include a reference point, and determining that the radiation is incident on the external region based on the evaluation value If so, the irradiation field side determination means 46 for determining that the straight line is not the irradiation field side is provided.

Further, the straight line detection means 41 is based on the inputted image P _sec in each section, candidate point detection means 42 for detecting candidate points E of a large number of edges considered as points on the irradiation field contour PS, Candidate line detecting means 43 for obtaining edge lines L constituted by candidate points E as lines (candidate lines) that are candidates for the irradiation field side using Hough transform based on a large number of candidate points E It is a configuration.

Candidate point detecting means 42, with the image P _sec in each compartment is input, as shown in FIG. 8 (a), to set the center point K of the image P _sec in that each compartment, each from the center point K For example, 240 radial straight lines are set at equal angular intervals respectively directed to the ends of the image P _{sec in} the section, for example, at intervals of 1.5 degrees. The set number and interval of the radial lines, the set position of the center of the radial line group, and the like can be changed as appropriate.

  The pixel values between pixels adjacent to each other in the direction along each of these 240 radial straight lines are compared, and two pixels having the largest difference are searched for on the line. There is a large difference in pixel value between the two searched pixels, and this is an edge candidate point E that is highly likely to be a point existing on the edge. In this way, the edge candidate points E searched for each linear direction are detected, and a total of 240 edge candidate points E are obtained (see FIG. 8B).

  The candidate line detection means 43 obtains a straight line connecting the edge candidate points E detected by the candidate point detection means 42 by Hough transformation. In the xy coordinate system shown in FIG. 8 (c), when the coordinates of the i-th edge candidate point Ei are (xi, yi) (i = 1, 2,..., 240), these xi and yi are constants. Then, a curve LSi represented by the following equation (1) is obtained for each edge candidate point.

ρ = xi cos θ + y i sin θ (1)
This expression (1) represents an expression of a straight line passing through the coordinates (xi, yi) with the center coordinates fixed at (xi, yi) in the XY space, and ρ passes through the coordinates (xi, yi). A distance θ between the straight line LL and the origin O of the xy coordinate system, θ represents an angle formed by a perpendicular line connecting the straight line LL and the origin O of the xy coordinate system and the x axis. Each straight line LL passing through the coordinates (xi, yi) is obtained by gradually changing ρ and θ in the equation (1). In the θρ space (Hough space), these straight lines LL passing through the coordinates (xi, yi) are obtained. The straight line LL is expressed as one curve shown in the following formula (2) (see FIG. 8C).

ρj = xcos θj + ysin θj (2)
When the same operation is performed on each edge candidate point, 240 curves LSi (i = 1, 2,..., 240) are represented in the Hough space. Each intersection position (ρj, θj) where the 240 curves LSi intersect is obtained, and the number of curves intersecting at each intersection position (ρj, θj) is counted. On the Hough space, intersections α and β with a large number of intersecting curves (count values) represent straight lines passing through many edge candidate points E.

  Therefore, the above-described count is performed for all edge candidate points E, and the intersection positions (ρj, θj) are extracted in order from the highest one until the preset number is reached, and these are returned to the real space and returned to XY. A straight line on the plane is obtained, and the obtained plural straight lines are candidate lines LL. Since the shape of the irradiation field is a rectangle surrounded by four straight lines, at least four or more candidate lines LL are extracted.

  However, the candidate line LL may be a straight line that represents the outer shape of the subject in addition to the straight line that forms the shape of the irradiation field. For example, as shown in FIG. 9, when photographing from the chest of the subject to the arm, there is a possibility of detection including the straight line ll constituting the shape of the arm. Therefore, the extracted straight line 11 is evaluated, and straight lines other than the straight lines constituting the shape of these irradiation fields are canceled.

The reference point setting unit 44 sets a reference point Q at a substantially central portion on the image _Psec in each section. When photographing the image P _sec in each section, photographing is performed so that the subject is positioned substantially at the center. Therefore, if the approximate center of the image P _sec in each section is used as the reference point, the reference point Q is set in the irradiation field. Specifically, the center point K of the image P _sec in each section can be used, but any point other than the center point can be used as long as the point is considered to be in the irradiation field. It may be a point. Alternatively, if it is difficult to set automatically, the observer may set the reference point Q.

As shown in FIG. 10, the evaluation value calculation means 45 divides the image P _sec in each section into two regions by the straight line extracted by the above method, and includes the reference point Q in the two regions. A non-external region is defined as P _outside, and it is evaluated whether or not the external region P _outside is an irradiation field _outside region. When the external region P _outside is an irradiation field region, the region is a high-luminance region where no radiation is incident. Therefore, an evaluation value is obtained by evaluating whether or not a region where a subject is imaged or a region where radiation is incident, such as a direct X-ray irradiation unit, exists in the external region _Poutside . The evaluation value is calculated by evaluating only an image included in the outer region P _outside and a method of calculating and evaluating the image included in the outer region P _outside and the entire image P _sec in each section. However, each method for obtaining the evaluation value will be described below.

(1) Method of Evaluation Using Histogram When the external region P _outside is _{outside the} irradiation field, it becomes a high-intensity white region, and the pixel value becomes substantially constant. Therefore, the pixel values of the pixels included in the external region P _outside The distribution range is very narrow. On the other hand, the range in which the pixel values of all the pixels included in the image P _sec in each section are distributed is a fairly wide range.

Therefore, first, a histogram H _{all of} pixel values included in the entire image P _sec in each section and a histogram H _outside of pixel values in the outer region P _outside are calculated (see FIG. 11), and within the histogram H _all A difference A between the maximum pixel value and the minimum pixel value and a difference B between the maximum pixel value and the minimum pixel value in the histogram H _outside are obtained, and B / A is obtained as an evaluation value.

When the B / A is larger than the threshold value, the evaluation value calculation unit 45 determines that the subject is photographed in the external region P _outside and cancels it by determining that the straight line is not the irradiation field side. Depending on the case, there may be a case where the direct X-ray irradiating part is present or not, and therefore, it is preferable to change the threshold value.

The pixel with the lowest density value (maximum luminance value) in the image is often outside the irradiation field. If a high-density area appears outside the irradiation field due to the influence of scattered radiation, the range of density values that appear outside the irradiation field (density) (Width) also becomes wider. Therefore, in an image where a direct X-ray irradiation part exists, the density width appearing in the entire image is large, but in an image where there is no direct X-ray irradiation part, the density width appearing in the entire image is small and appears outside the irradiation field. The density width becomes relatively larger than the density width of the entire image. For this reason, if the threshold value is too small in an image where there is no direct X-ray irradiation unit, it is determined that the straight line correctly detected by the outside of the straight line (outer region P _outside ) coincides with the _outside of the irradiation field is not the irradiation field side. Will be more likely to cancel. On the other hand, if the threshold is set too high, when the subject is photographed without the direct X-ray irradiator _outside the straight line (external region P _outside ) detected from the image with the direct X-ray irradiator present. Since the density width of the external region P _outside is relatively small compared to the density width of the entire image, it is impossible to cancel such a straight line that is not the irradiation field side.

In addition, an image obtained by photographing the limb has a large density range because the region where the subject is photographed is relatively small and there are many direct X-ray irradiators in the entire image P _sec in the section. In addition, as shown in FIG. 12, for example, the X-ray irradiation part R exists directly around the arm and is often widely distributed along the irradiation field contour PS. When a straight line serving as the contour of the arm is detected as an irradiation field side candidate from such an image, the X-ray irradiation unit R exists directly _{outside the} straight line (external region P _outside ). When it can be determined from the imaging menu selected by the photographer that the image is obtained by imaging the extremity in this way, there is a high possibility that the X-ray irradiation unit R exists directly _{outside the} straight line (external region P _outside ). The threshold value may be set high to about 0.9.

  On the other hand, an image obtained by photographing a hearing instrument, a lumbar spine, or the like often has a small density width of the entire image, and it is difficult to evaluate a straight line using B / A as a threshold value.

  In the case of photographing other than the above-mentioned limbs, hearing instruments, and lumbar spine, the threshold is generally set to about 0.8.

  As can be seen from the above, it is most preferable to set the threshold to 0.8 to 0.9. However, depending on the part to be imaged, there are many or few images in which the direct X-ray irradiation unit exists, and the boundary between the direct X-ray irradiation unit and the subject. Depending on whether there are many or few linear structures, the threshold value to be set is changed in the range of 0.7 to 0.9.

Alternatively, the variance σ1 of the pixel values in the external area P _outside and the variance σ2 of the pixel values included in the entire image P _sec in each section are obtained, and when the variance σ1 is not sufficiently small with respect to the variance σ2, the external Since there is a high possibility that the subject is photographed in the area P _outside , it may be determined that the straight line is not the irradiation field side and canceled.

(2) Evaluation Method Using Pixel Value Since the external region P _outside is a region that has not been irradiated with radiation, a portion directly irradiated with X-rays is not included therein. That is, the irradiation field region hardly includes pixels having pixel values of the direct X-ray irradiation unit. Therefore, the proportion of pixels having a pixel value of the direct X-ray irradiation unit in the external region _{P outside outside,} external area _{P outside outside} to the determination of whether the irradiation field region.

  Since the pixel value of the direct X-ray irradiation unit varies depending on the amount of radiation, a procedure for determining a threshold value for distinguishing between the pixel value of the direct X-ray irradiation unit and the pixel values of other portions will be described below. Here, the gradation of the image is represented by 8 bits (0 to 255).

(i) First, the maximum pixel value histmax is obtained from the histogram of the entire image P _sec in each section.

(ii) The direct X-ray irradiator is a high-luminance region and usually has a pixel value exceeding at least 100. Therefore, it is determined according to histmax whether the image has a direct X-ray irradiation unit.

  (A) In the case of histmax ≦ 100, it is determined that the image does not have a direct X-ray irradiation unit, and the threshold value is not searched.

  (A) If histmax> 100, go to (iii) to calculate a threshold value for distinguishing the pixel value of the direct X-ray irradiation unit from other regions.

 (iii) As shown in FIG. 13, when taking a histogram of the whole image (Dh is a direct X-ray irradiation unit, Dm is a region where a subject is imaged, and Dl is an irradiation field region), there are roughly three types. Although there is a peak, the pixel value of the direct X-ray irradiation part is included in the part Dh having the peak on the highest density side. Therefore, a search is made for a boundary histmax2 between the portion Dh on the highest density side and the region Dm where the subject having a peak around the middle is photographed.

  First, j is set to histmax-5, and the ratio of the number of pixels having the pixel value j to the total number of pixels is calculated while j is decreased by 5, and for the first time, j that is less than 0.2% is set to histmax2.

(iv) A pixel having a pixel value between histmax and histmax2 is a direct X-ray irradiation unit, but since the direct X-ray irradiation unit never has a pixel value of 100 or less, when histmax2 is 100 or less It is determined that the search for histmax2 has failed. Therefore,
(A) When histmax2 ≦ 100, it is empirically found that most of the pixel values of the direct X-ray irradiation part exist between histmax and histmax −20.
Th = histmax − 20
And decide.

  (B) If histmax2> 100, go to (v).

(v) So, if histmax2> 100, set the threshold Th
Th = histmax2 −15
And decide. However, since the pixel value directly appearing in the X-ray irradiation unit is a region with little variation, the histmax-Th is larger than 45 (that is, the pixel value of the direct X-ray irradiation unit is distributed over a considerably wide range) Usually not possible. Therefore,
(A) If histmax − Th> 45, set the threshold Th
Th = histmax − 20
And decide.

  (B) If histmax − Th ≦ 45, go to (vi).

(vi) Further, when the ratio of the number of pixels having a pixel value of histmax2 −15 or more to the total number of pixels is 95% or more, a clear peak does not appear. Therefore, the threshold value Th is determined as follows.

(A) In the case of 95% or more, the threshold value Th is determined from the range of pixel values appearing in the direct X-ray irradiation unit obtained empirically,
Th = histmax − 20
And decide.

(B) If it is less than 95%, the threshold value Th is a place slightly lower than the pixel value range of the searched direct X-ray irradiation part,
Th = histmax2 − 15
And decide.

The pixel value exceeding the threshold value Th obtained as described above is the pixel value of the direct X-ray component. Using this pixel value, the ratio of the direct X-ray component pixels in the external area P _outside included in all the pixels is obtained as an evaluation value. The irradiation field side determination means 46 includes a region irradiated with radiation in the outer region P _outside if 0.1% or more of all the pixels in the outer region P _outside are pixels having a direct X-ray component pixel value. Since there is a high possibility, it is determined that the straight line is not the irradiation field side and is canceled.

(3) Method of evaluating using components in middle frequency band Alternatively, the external region P _outside is a region where no radiation is irradiated, and does not include a region where the subject is imaged. The area outside the irradiation field and the direct X-ray irradiation area are images in the low frequency band, and the noise component is the image in the high frequency band, but the area where the subject is photographed is the image in the medium frequency band. Therefore, by extracting only the image component in the middle frequency band, it is possible to extract only the image portion where the subject is photographed.

Therefore, it is possible to evaluate whether or not a subject is included in the external region P _outside by extracting an image component in the middle frequency band from the external region P _outside and _obtaining the intensity of the image component. Specifically, the intensity is obtained by the calculation of the following equation using a 5 × 5 mask and a 3 × 3 mask around each pixel (point c in FIG. 14) in the outer region P _outside .

A = average of pixels in 5 × 5 mask−average of pixels in 3 × 3 mask (3)
Strength = (Σ | A |) / N (4)
When this evaluation value is larger than the threshold value, the irradiation field side determination means 46 determines that this straight line is not the irradiation field side and cancels it. A rough structure such as a subject's bone or body contour is an image component included in a frequency band of 0.056 to 0.093 cycle / mm. Therefore, in order to extract an image component composed of this frequency component, it is optimal to use a 5 × 5 mask having a side of 5 mm to 20 mm, and a 3 × 3 mask having a side of 3 mm to 12 mm. For example, if a 5 × 5 mask has a side of 9 mm, a 3 × 3 mask having a side of 5.4 mm is used. Further, the size of the mask is not limited to 5 × 5 and 3 × 3, and a mask having an appropriate size may be adopted according to the resolution of the image.

(4) Method of evaluation using an angle intersecting with a confidence line with high reliability The line that becomes a division between the irradiation field region and the irradiation field region is also a place where the pixel value changes rapidly. Therefore, there is a high possibility that an irradiation field side has a sharp change in shading from the straight lines that are candidates for the irradiation field side. Therefore, the following calculation is performed in a region in the vicinity of 3 × 3 pixels centering on each pixel on the straight line.

  As shown in FIG. 15, the difference value B is obtained by the following equation with the pixel (point e) existing at the center in the 3 × 3 pixel region as the center.

B = | (ag) + (bh) + (cl) | + | (ca) + (fd) + (ig) | (5)
This difference value B is calculated for all points on the straight line, and an index value ΣB / N is obtained by dividing the sum ΣB of the difference values B at each point by the total number N of pixels on the straight line. The largest straight line is selected from a plurality of straight lines as the confidence line having the highest reliability as the irradiation side.

  As shown in FIG. 16, with reference to a confidence line with high reliability, a straight line (parallel straight line) whose difference from the inclination of this straight line is within a range of ± α °, or a difference from the inclination of this straight line is The lines other than the straight line within the range of 90 ° ± α ° (orthogonal straight line) are determined to be not the irradiation field side and are canceled. Empirically, good results are obtained when α ° is about 10 °.

(5) A method for evaluating a straight line using the edge-likeness The line that becomes the division between the irradiation field area and the irradiation field area is sharply shaded as described above, and further, the direction in which the pixel values change are aligned. It was an edge. Using the above-described index value ΣB / N, the reliability that is the irradiation field side is obtained, and further, the entropy of each straight line is calculated to evaluate whether the direction of the change in light and shade is aligned.

  First, in order to calculate entropy, as shown in FIG. 17A, the gradient θ of the pixel value at each pixel on the straight line using a 5 × 5 mask with each pixel on the straight line as the center (point O). Is calculated.

y = (a + b + c + d + e) − (l + m + n + p + q) (6)
x = (e + g + i + k + q) − (a + f + h + j + i) (7)
When y ≧ 0 θ = tan ⁻¹ (y / x) (8)
When y <0 θ = −tan ⁻¹ (y / x) (9)
It becomes.

  The obtained gradient θ is distributed to one of the closest directions among the gradient directions 1 to 8 shown in FIG.

  The entropy is calculated by looking at which one of the gradient directions 1 to 8 is assigned to the gradient θ at each point on the straight line. From the total number Total of points on the straight line and the number Zi of points having the same gradient direction i on the straight line, the occurrence probability P (i) at which the same gradient direction i occurs is obtained.

P (i) = Zi / Total (10)
From this, the entropy H is
H = −ΣP (i) ln (P (i)) (11)
It becomes. As shown in FIG. 18, a graph with the entropy H as the vertical axis and the difference value B as the horizontal axis is used, and the line above the straight line L on the graph is canceled as being less likely to be an irradiation field side, and the straight line l The lower line should be left with a high probability of irradiation field.

When the evaluation value is calculated by each of the above-described methods, the pixel values arranged at the end of the image P _sec in each section may not be the pixel values read from the IP but the values remaining on the recording medium. It is preferable that several pixels from the end are calculated so as not to be used for calculating the evaluation value.

Determining unit 50, whether or not divided by comparing the boundary of each zone divided in the radiation area P _in the dividing means 40 of each partition recognized by the irradiation field recognition means 40 (candidate dividing line) is appropriate Determine whether. Therefore, when the following conditions are met, it is determined that the divided sections are not appropriate. Here, the case of dividing into two in the vertical direction will be described.

(1) When one irradiation field region extends over multiple sections
(i) As shown in FIG. 19, it is determined whether or not the division candidate line n and the irradiation field contour PS intersect at least one of the divided sections.
(ii) if they intersect, the difference between the recognized maximum and minimum values of the longitudinal coordinate of the irradiation field area b - a (= maximum value minimum value), recognized irradiation field P _in If a / b> 0.5 holds when the difference a (= maximum value−minimum value) between the maximum value and the minimum coordinate value in the vertical direction of the portion where the division candidate line n overlaps, the division candidate line n is because of the high set potential across the radiation area P _in which skewed determines that division is not appropriate.

(2) When the two irradiation field regions extend over a plurality of sections (part 1)
(i) As shown in FIG. 20, it is determined whether the division candidate line n and the irradiation field outline PS intersect each other in the divided sections adjacent to each other.
(ii) if intersect, field sides constituting the two irradiation field P _in recognized in each compartment, a combination l ¹ _pair of field sides aligned in one straight _line, l ² _pair 2 one when present, it is highly likely that those were recognized in compartments divided what is supposed to be one radiation area P _in two, determines that division is not appropriate.

(3) When the two irradiation field regions extend over a plurality of sections (part 2)
(i) As shown in FIG. 21, it is determined whether the division candidate line n and the irradiation field outline PS overlap each other in the divided sections adjacent to each other.
(ii) in the case of overlap, among the field sides constituting the two irradiation field P _in recognized in each compartment, one of the field sides of each exposure field region arranged on one straight line field sides In the case where there is one combination l ³ _pair of, since it is highly likely that the irradiation field region is recognized in a section obtained by dividing one originally the irradiation field region into two, Judge that it was not appropriate.

  The case of dividing into two in the horizontal direction and the case of dividing into four in both the vertical and horizontal directions can also be determined in the same manner as the methods (1) to (3).

  Next, operation | movement of the irradiation field recognition apparatus 1 is demonstrated according to the flowchart of FIG.

  First, a radiographic image P is input from the input means 10 (S100). First, the division pattern setting unit 20 sets four divisions in the vertical and horizontal directions (S101), and the division unit 30 sets the radiographic image P according to the division pattern in the vertical division candidate lines and the horizontal division candidates. The line is searched to divide the radiographic image P into four sections (S102).

Therefore, the irradiation field recognition means 40 recognizes the irradiation field from the image P _sec in each section (S103). As shown in FIG. 23, the irradiation field recognition unit 40 first sets the center point K automatically as a reference point by the reference point setting unit 44 (S120). Next, using this center point K, the candidate point detection means 42 of the straight line detection means 41 detects a number of edge candidate points E that are considered to be points on the irradiation field contour PS, and the candidate line detection means 43. Then, using the Hough transform, an edge line L constituted by a large number of detected candidate points E is obtained as a straight line (candidate line) that is a candidate for an irradiation field side (S121).

  Since more straight lines as irradiation field side candidates are detected than the number of irradiation field sides, an evaluation value is calculated for each straight line by using the above-described evaluation value calculation means 45 (S122), and an extra is calculated based on the calculated evaluation value. A straight line is canceled (S123).

S122 to S123 are repeated until evaluation is completed for all the detected straight lines (S124). Based on the remaining straight lines excluding the straight lines determined not to be the irradiation field side from among the straight lines that are candidates for the irradiation field side, recognizing the irradiation field area _{P in} from the image _{P sec} in the compartment.

This recognition process is repeated to recognize the irradiation field area Pin _in all four sections (P105). It determines whether division by comparing the candidate dividing line n as the border of each section obtained by dividing the radiation area P _in each compartment is suitable in determining means 50 (S106). From the determination result obtained from the determination means 50, if the division is appropriate, the process is terminated as if the image was shot in four divisions.

  When it is determined that the division is not appropriate (S106), the division pattern setting unit 20 sets the vertical division into two as the division pattern (S107), and the processes of S102 to S105 are performed again. Also, when the irradiation field is not recognized in the recognition process of the irradiation field recognition means 40, it is determined that the division pattern is not correct, and the process proceeds to S107.

  Therefore, it is determined whether the divided pattern is appropriate even in the case of the vertically divided pattern. If it is determined that the divided pattern is not appropriate even in the vertically divided pattern, the determination is further performed with the horizontally divided pattern. If any division pattern is not appropriate, the irradiation field is further recognized without division.

  Furthermore, as shown in FIG. 24, the determination may be performed for a three-division pattern. When the determination is made with this three-divided pattern, the vertical three-divided pattern, the horizontal two-divided pattern, the vertical and horizontal four-divided pattern, the vertical two-divided pattern, the horizontal two-divided pattern, and the divided Judgment is made in the order of no pattern.

  As described above in detail, the radiographic image is divided in accordance with the division pattern, the irradiation field area is recognized for each divided section, and it is determined whether or not the recognized irradiation field area is appropriate. By doing so, it becomes possible to accurately recognize the irradiation field region from the radiographic images taken by division.

  Moreover, it can be made to operate | move as an irradiation field determination apparatus by installing in a computer using the medium which recorded the program which functions each said means on a computer. This program can also be provided via a network.

Schematic diagram of irradiation field determination device The figure for explaining the method of photographing with the irradiation field stop Example of divided shooting Example of histogram of whole radiographic image (part 1) Diagram for explaining how to find the candidate division line An example of divided sections Configuration diagram of irradiation field recognition means Diagram for explaining a straight line detection method An example of false detection of a straight line that is a candidate for an irradiation field Diagram for explaining the relationship between external area and reference point Example of histogram of whole radiographic image and histogram of irradiation field An example of a radiographic image of an extremity Example of histogram of whole radiographic image (2) An example of a mask for extracting image components in the middle frequency band The figure for explaining the calculation method of the difference value The figure for demonstrating the method to detect a straight line orthogonal or parallel to a certain straight line as an irradiation field side A figure for explaining how to find the gradient of pixel values The figure for demonstrating the method of determining edge likeness The figure for demonstrating the appropriate determination of the divided division (the 1) The figure for demonstrating the appropriate determination of the divided division (the 2) The figure for demonstrating the appropriate determination of the division (Part 3) Flowchart for explaining operation of irradiation field recognition device Flowchart for explaining operation of irradiation field recognition processing Example of division pattern

Explanation of symbols

1 Irradiation field recognition device
10 Input means
20 Division pattern setting method
30 Dividing means
40 Irradiation field recognition means
41 Straight line detection means
42 Candidate point detection means
43 Candidate line detection means
44 Reference point setting method
45 Evaluation value calculation means
46 Irradiation field judgment means
50 Judgment means

Claims (4)

An input means for inputting a radiographic image obtained by photographing a subject using an irradiation field stop;
Division pattern setting means for setting a division pattern of the radiographic image obtained by division imaging;
A dividing unit that divides the radiographic image input by the input unit into a plurality of sections according to the division pattern;
Irradiation field recognition means for recognizing an irradiation field region from the image in each of the divided sections;
A determination means for comparing the irradiation field area of each section recognized by the irradiation field recognition means and the boundary of each divided section to determine whether or not the division is appropriate,
Until the determination unit determines that the division is appropriate, a division pattern different from the division pattern already set by the division pattern setting unit is set, and the radiographic image is converted into the different division pattern by the division unit. The irradiation field recognition apparatus is characterized by repeating the process of recognizing an irradiation field in each section by the irradiation field recognition means after dividing into a plurality of sections according to the above. The irradiation field recognition means
Straight line detection means for detecting a straight line that is a candidate for the irradiation field side of the irradiation field region from the image in the section;
A reference point setting means for setting a reference point in the section;
Evaluation that divides the image of the section into two regions by the detected straight line and calculates an evaluation value that evaluates whether radiation is incident on an external region that does not include the reference point among the two regions. A value calculating means;
Based on the evaluation value, when it is determined that the radiation is incident on the external region, the straight line has irradiation field side determination means for determining that the line is not an irradiation field side,
An irradiation field recognizing apparatus for recognizing an irradiation field region based on the remaining straight lines excluding a straight line determined not to be the irradiation field side from among the straight lines that are candidates for the irradiation field side. An input step for inputting a radiographic image obtained by photographing a subject using an irradiation field stop;
A division pattern setting step for setting a division pattern of the radiographic image obtained by the division imaging;
A division step of dividing the radiographic image input by the input step into a plurality of sections according to the division pattern;
An irradiation field recognition step for recognizing an irradiation field region from the image in each of the divided sections;
A determination step of determining whether or not the division is appropriate by comparing the irradiation field area recognized by the irradiation field recognition step and the boundaries of the plurality of divided sections;
Until the division step is determined to be appropriate, a division pattern different from the division pattern already set by the division pattern setting step is set, and the radiographic image is changed to the different division pattern by the division step. The irradiation field recognition method is characterized by repeating the process of recognizing an irradiation field in each section in the irradiation field recognition step after dividing into a plurality of sections. Computer
An input means for inputting a radiographic image obtained by photographing a subject using an irradiation field stop;
Division pattern setting means for setting a division pattern of the radiographic image obtained by division imaging;
A dividing unit that divides the radiographic image input by the input unit into a plurality of sections according to the division pattern;
Irradiation field recognition means for recognizing an irradiation field region from the image in each of the divided sections;
Compare the irradiation field area recognized by the irradiation field recognition means and the boundaries of the plurality of divided sections to function as a determination means for determining whether or not the division is appropriate,
Until the determination unit determines that the division is appropriate, a division pattern different from the division pattern already set by the division pattern setting unit is set, and the radiographic image is converted into the different division pattern by the division unit. The program is made to function so as to repeat the process of recognizing the irradiation field in each section by the irradiation field recognizing means after being divided into a plurality of sections according to the above.

Images