JP2006254934A - Irradiation field judgement device, irradiation field judgement method and its program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the recognition accuracy of irradiation field recognition processing relating to an irradiation field judgement device for recognizing the irradiation field of a radiographic image, an irradiation field judgement method and its program. <P>SOLUTION: A straight line ll to be the candidate of an irradiation field side is detected from the radiographic image P obtained by radiographing an object by using an irradiation field diaphragm and a reference point Q is set to the radiographic image P. The radiographic image P is divided into two regions by the detected straight line ll, and whether or not radiation is made incident in an external region not including the reference point Q of the two regions is evaluated. In the case of judging that the radiation is made incident on the external region, it is judged that the straight line is not the irradiation field side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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).

Alternatively, a method has been proposed in which a straight line is obtained by applying a Hough transform to these edge candidate points, and a region surrounded by the straight line is set as an irradiation field region (for example, Patent Document 2).
Japanese Unexamined Patent Publication No. 63-259538 JP-A-10-275213

  However, a method for recognizing an irradiation field contour by detecting a straight line based on candidate points on an edge obtained along the radial direction described above, or a method for recognizing an irradiation field contour by detecting a straight line using Hough transform. In this method, it is not always possible to obtain a straight line that matches the actual irradiation field outline, and the boundary between the subject and the X-ray irradiation unit may be detected directly. For this reason, as described above, a necessary image portion is blackened or a low density portion remains.

  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 determination device of the present invention includes a reference point setting unit that sets a reference point in a radiographic image obtained by photographing a subject using an irradiation field stop,
Straight line detection means for detecting a straight line that is a candidate for an irradiation field side from the radiographic image;
Evaluation for calculating an evaluation value by dividing the radiographic image by the detected straight line into two regions and evaluating whether radiation is incident on an external region that does not include the reference point in the two regions. A value calculating means;
When it is determined that the radiation is incident on the external region based on the evaluation value, the straight line includes a determination unit that determines that the straight line is not an irradiation field side. is there.

Further, the irradiation field determination method of the present invention includes a reference point setting step for setting a reference point in a radiographic image obtained by photographing a subject using an irradiation field stop,
A straight line detection step of detecting a straight line that is a candidate for an irradiation field side from the radiographic image;
Evaluation for calculating an evaluation value by dividing the radiographic image by the detected straight line into two regions and evaluating whether radiation is incident on an external region that does not include the reference point in the two regions. A value calculation step;
A determination step of determining that the straight line is not an irradiation field side when it is determined that the radiation is incident on the external region based on the evaluation value. is there.

Further, the program of the present invention provides a computer,
A reference point setting means for setting a reference point in a radiographic image obtained by photographing a subject using an irradiation field stop;
A straight line detection unit that detects a straight line that is a candidate for an irradiation field side from the radiographic image, and the radiographic image is divided into two regions by the detected straight line, and the external region that does not include the reference point in the two regions An evaluation value calculating means for calculating an evaluation value for evaluating whether or not radiation is incident on the region;
When it is determined that the radiation is incident on the external region based on the evaluation value, the straight line functions as a determination unit that determines that the line is not an 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.

  Moreover, it is desirable that the reference point is set at a substantially central portion.

  The “substantially central portion” refers to a range having a certain extent from the center point. The “substantially central portion” of the radiographic image obtained by radiography using the irradiation field stop is a range that is considered to be within the irradiation field of the radiographic image in comparison with a normal radiographic method. If a reference point is set for the part, "a reference point is set at a location that is within the irradiation field of the radiographic image.

The evaluation value is a ratio of the number of pixels having a pixel value corresponding to the direct X-ray irradiation unit existing in the external region to the total number of pixels existing in the external region,
The determination unit may determine that the straight line is not an irradiation field side when the evaluation value is higher than a predetermined reference value.

  The evaluation value may be a value calculated based on an image included in the external area.

The evaluation value is the intensity of the image component in the middle frequency band in the outer region,
The determination unit may determine that the straight line is not an irradiation field side when the evaluation value is higher than a predetermined reference.

  The “middle frequency band image component” includes many components of the image portion obtained by photographing the subject, and the “middle frequency band image component intensity” is the average pixel in the image obtained by extracting only the middle frequency band image component. Any value may be used as long as it is an index including image components in the middle frequency band.

  The evaluation value may be a value calculated based on an image included in the external region and an image included in the entire radiographic image.

The evaluation value is a ratio of a distribution width of pixel values appearing in all pixels in the external region to a distribution width of pixel values appearing in all pixels in the radiographic image.
If the evaluation value is distributed more widely than a predetermined reference value, the determination unit may determine that the straight line is not an irradiation field side.

  According to the present invention, a straight line that is a candidate for an irradiation field side is detected from a radiographic image obtained by photographing a subject using an irradiation field stop, and the radiographic image is divided into two regions by the detected straight line. Then, by evaluating whether or not radiation is incident on the external area corresponding to the outside of the straight line, it is possible to accurately detect the area outside the irradiation field, and to apply a blackening process to an appropriate area. it can.

  Further, if the discrimination is performed by looking at the ratio of the number of pixels corresponding to the direct X-ray irradiation unit existing in the external region, the discrimination accuracy can be determined without determining the region including the direct X-ray irradiation unit as the irradiation field region. Can be increased.

  In addition, if the discrimination is made by looking at the intensity of the image component in the middle frequency band in the outer area, the area including the portion where the subject is photographed is not determined as the irradiation field area, and the discrimination accuracy is improved. it can.

  Furthermore, if the distribution range of the whole pixel value in the radiographic image and the distribution range of the pixel value in the external region are determined, the region irradiated with X-rays having a wide distribution is determined as the irradiation field region. This makes it possible to improve the discrimination accuracy.

  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 X-ray imaging is performed using an irradiation field stop in 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 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. The region corresponding to the outside (irradiation field region) Pout is not irradiated with X-rays, while the region corresponding to the inner side (irradiation field region) Pin than the opening of the irradiation field stop is directly connected to the portion that has passed through the subject. The portion irradiated with X-rays is recorded. And the part corresponding to the opening part of an irradiation field stop becomes a shape substantially the same as an opening part, and the irradiation field outside area | region Pout becomes a bright area with a high brightness | luminance. At the boundary between the irradiation field region Pin and the irradiation field outside region Pout, an irradiation field contour PS composed of a plurality of edge lines whose density changes abruptly 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.

  The irradiation field recognition device 1 includes a straight line detection unit 10 that detects a straight line that is a candidate of an irradiation field side from a radiographic image P obtained by imaging a subject using an irradiation field stop, and a substantially central portion of the radiographic image P. The radiographic image P is divided into two regions by the reference point setting means 20 for setting a reference point and the detected straight line, and whether radiation is incident on an external region that does not include the reference point in the two regions. When it is determined that the radiation is incident on the external region based on the evaluation value and the evaluation value calculation means 30 that calculates the evaluation value that evaluates this, the straight line is determined not to be the irradiation field side And determining means 40 for performing.

  The straight line detection means 10 includes an edge candidate point detection means 11 that detects a large number of edge candidate points E that are considered to be points on the irradiation field contour PS based on the input image data S, and a large number of edge candidate points. And a candidate line detection means 12 that obtains an edge line L constituted by the edge candidate points E as a line (candidate line) that is a candidate for an irradiation field side by using the Hough transform based on E.

  As shown in FIG. 3A, the edge candidate point detection means 11 sets the center point K of the image P represented by the data S for the input image data S, and the radiographic image P from the center point K is set. For example, 240 radial straight lines are set at equal angular intervals facing the end portions, 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.

  Image data between pixels adjacent to each other in the direction along each of these 240 radial straight lines is compared, and two pixels having the largest difference are searched 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 manner, the edge candidate points E searched for each straight line direction are detected, and a total of 240 edge candidate points E are obtained (see FIG. 3B).

  The candidate line detection means 12 obtains a straight line connecting the edge candidate points E detected by the edge candidate point detection means 11 by Hough transformation. On the xy coordinate system shown in FIG. 3C, 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. 3C).

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. 4, when photographing is performed 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 means 20 sets a reference point Q at a substantially central part on the radiographic image P. When the radiographic image P is photographed, photographing is performed so that the subject is positioned substantially at the center. Therefore, if the approximate center of the radiographic image P is used as a reference point, the reference point Q is set in the irradiation field. Specifically, the center point K of the radiographic image P can be used, but if it is a point that seems to be in the irradiation field, it is a point other than the center point if it is determined in light of a normal imaging method. May be. Alternatively, if it is difficult to set automatically, the observer may set the reference point Q.

  As shown in FIG. 5, the evaluation value calculation means 30 divides the radiographic image P into two regions by the straight line extracted by the above method, and the external region that does not include the reference point Q among the two regions. Is set as Poutside, and it is evaluated whether or not the external region Poutside is an irradiation field region. When the external region Poutside 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 includes a method of evaluating and calculating only an image included in the external region Poutside, and a method of calculating and evaluating an image included in the external region Poutside and the entire image of the radiographic image P. Each method for obtaining the evaluation value will be described.

(1) Evaluation method using a histogram When the external area Poutside is outside the irradiation field, it becomes a high-luminance white area, and the pixel values are substantially constant. Therefore, the pixel values of the pixels included in the external area Poutside are distributed. The range to do is very narrow. On the other hand, the range in which the pixel values of all the pixels included in the radiographic image P are distributed is a fairly wide range.

  Therefore, first, the histogram Hall of pixel values included in the entire radiographic image P (here, the pixel value of a pixel having a high density is represented by a large value, and the pixel value decreases as the density decreases) will be described below. And the histogram Houtside of the pixel values in the outer area Poutside are calculated (see FIG. 6), the difference A between the maximum pixel value and the minimum pixel value in the histogram Hall, and the maximum pixel value and the minimum in the histogram Houtside. A difference B between pixel values is obtained, and B / A is obtained as an evaluation value.

  When the B / A is larger than the threshold value, the evaluation means 40 determines that the subject is photographed in the external region Poutside, and cancels it by determining that the straight line is not the irradiation field side. Since there are cases where the line irradiation section is present and absent, 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 part, it is determined that a straight line correctly detected with the outside of the straight line (outside region Poutside) coincided with the outside of the irradiation field is not the irradiation field side. The possibility to cancel increases. On the other hand, if the threshold is set too high, when the subject is photographed without the direct X-ray irradiation unit outside the straight line detected from the image where the direct X-ray irradiation unit exists (external region Poutside), Since the density width of the external region Poutside is relatively smaller than 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 extremities has a large density range because the region where the subject is photographed is relatively small and the direct X-ray irradiation unit is often present in the entire image. In addition, as shown in FIG. 7, 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 part R exists directly outside the straight line (outer region Poutside). When it can be determined from the imaging menu selected by the photographer that the image is an image of the extremities, the X-ray irradiation unit R is likely to exist directly outside the straight line (external region Poutside). May be set to about 0.9.

  On the other hand, since the density range of the whole image of an image obtained by photographing a hearing instrument, the lumbar spine, and the like is small, and it is difficult to evaluate a straight line using B / A as a threshold, straight lines are not canceled by this method.

  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, but a straight line appears at the boundary between the direct X-ray irradiation unit and the subject, depending on whether or not there is a direct X-ray irradiation unit depending on the part to be imaged. The threshold value to be set is changed in the range of 0.7 to 0.9 depending on whether or not.

  Alternatively, the pixel value variance σ1 in the external region Poutside and the pixel value variance σ2 included in the entire radiographic image P are obtained, and when the variance σ1 is not sufficiently small with respect to the variance σ2, the external region Poutside Since there is a high possibility that the subject has been photographed, it may be determined that the straight line is not the irradiation field side and canceled.

(2) Evaluation Method Using Pixel Values Since the external region Poutside is a region that has not been irradiated with radiation, a portion directly irradiated with X-rays is not included therein. That is, the out-of-field region hardly includes pixels having pixel values of the direct X-ray irradiation unit. Therefore, it is determined whether or not the external region Poutside is an irradiation field outside region from the ratio of pixels having pixel values of the direct X-ray irradiation unit in the external region Poutside.

  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.

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

(iii) As shown in FIG. 8, when taking a histogram of the entire image (Dh is a direct X-ray irradiator, Dm is a portion where a subject is imaged, and Dl is an irradiated field area), 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 portion 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 Poutside included in all the pixels is obtained as an evaluation value. If the determination means 40 is a pixel in which 0.1% or more of all the pixels in the external area Poutside have a pixel value of the direct X-ray component, there is a high possibility that the external area Poutside includes a region irradiated with radiation. Therefore, it is determined that the straight line is not the irradiation field side and is canceled.

(3) Method of evaluating using components in medium frequency band Alternatively, the external region Poutside 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 Poutside by extracting an image component in the middle frequency band from the external region Poutside and obtaining the intensity of the image component. Specifically, the intensity is calculated by the following expression using a 5 × 5 mask and a 3 × 3 mask with each pixel (point c in FIG. 9) in the outer region Poutside as the center.

A = average of pixels in 5 × 5 mask−average of pixels in 3 × 3 mask (3)
Strength = (Σ | A |) / N (4)
When the evaluation value is larger than the threshold value, the determination unit 40 determines that the 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 that intersects with a highly reliable straight line reference 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. 10, 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 a certain straight line having the highest certainty of being an irradiation field.

  As shown in FIG. 11, a straight line (parallel straight line) whose difference from the inclination of this straight line is within a range of ± α ° with reference to a reliable straight line with high reliability, or a difference from the inclination of this straight line. Other than the straight line within the range of 90 ° ± α ° (straight line), it is determined that it is not the irradiation field side and is 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. 12 (a), 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. 13, using a graph with the entropy H as the vertical axis and the difference value B as the horizontal axis, the line above the straight line L on the graph is canceled as having a low possibility of the irradiation field side, and the straight line l The lower line should be left with a high probability of irradiation field.

  When calculating the evaluation value by each of the above-described methods, the pixel values arranged at the end of the radiographic image may be values remaining on the recording medium, not the pixel values read from the IP. The pixel is preferably calculated so as not to be used for calculating the evaluation value.

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

  First, the center point K is automatically set as a reference point by the reference point setting means 20 (S100). Next, using this center point K, the edge candidate point detection means 11 of the straight line detection means 10 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 12 is detected. Then, using the Hough transform, an edge line L constituted by a large number of detected edge candidate points E is obtained as a straight line (candidate line) that is a candidate for an irradiation field side (S101).

  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 using the above-described evaluation means 40 (S102), and an extra straight line based on the calculated evaluation value. Is canceled (S103).

  S102 to S103 are repeated until the evaluation is completed for all the detected straight lines (S104).

  As described above in detail, it is possible to accurately determine whether or not the straight line detected by using each method is a straight line constituting the outline of the irradiation field, so that the irradiation field can be recognized correctly.

  In addition, it is possible to perform more accurate determination by evaluating a combination of several methods rather than using the above-described methods individually.

  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 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 a histogram of the entire radiographic image 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 explaining the method of detecting the irradiation field side from the straight line A figure for explaining how to find the gradient of pixel values The figure for demonstrating the method of determining edge likeness Flowchart for explaining operation of irradiation field recognition device

Explanation of symbols

1 Irradiation field recognition device
10 Straight line detection means
11 Edge candidate point detection means
12 Candidate line detection means
20 Reference point setting method
30 Evaluation value calculation means
40 Judgment means

Claims (9)

A reference point setting means for setting a reference point in a radiographic image obtained by photographing a subject using an irradiation field stop;
Straight line detection means for detecting a straight line that is a candidate for an irradiation field side from the radiographic image;
Evaluation for calculating an evaluation value by dividing the radiographic image by the detected straight line into two regions and evaluating whether radiation is incident on an external region that does not include the reference point in the two regions. A value calculating means;
Irradiation field, comprising: a determination unit that determines that the straight line is not an irradiation field side when it is determined that the radiation is incident on the outer region based on the evaluation value. Judgment device.   The irradiation field determination device according to claim 1, wherein the reference point is set at a substantially central portion. The evaluation value is a ratio of the number of pixels having a pixel value corresponding to the direct X-ray irradiation unit existing in the external region to the total number of pixels existing in the external region,
3. The irradiation field determination apparatus according to claim 1, wherein the determination unit determines that the straight line is not an irradiation field side when the evaluation value is higher than a predetermined reference value.   The irradiation field determination device according to claim 1, wherein the evaluation value is a value calculated based on an image included in the external region. The evaluation value is the intensity of the image component in the middle frequency band in the outer region,
The irradiation field determination apparatus according to claim 4, wherein the determination unit determines that the straight line is not an irradiation field side when the evaluation value is higher than a predetermined reference.   The irradiation field determination device according to claim 1, wherein the evaluation value is a value calculated based on an image included in the external region and an image included in the entire radiographic image. The evaluation value is a ratio of a distribution width of pixel values appearing in all pixels in the external region to a distribution width of pixel values appearing in all pixels in the radiographic image.
The irradiation field determination apparatus according to claim 6, wherein the determination unit determines that the straight line is not an irradiation field side when the evaluation value is distributed more widely than a predetermined reference value. A reference point setting step for setting a reference point on a radiographic image obtained by photographing a subject using an irradiation field stop;
A straight line detection step of detecting a straight line that is a candidate for an irradiation field side from the radiographic image;
Evaluation for calculating an evaluation value by dividing the radiographic image by the detected straight line into two regions and evaluating whether radiation is incident on an external region that does not include the reference point in the two regions. A value calculation step;
An irradiation field comprising: a determination step for determining that the straight line is not an irradiation field side when it is determined that the radiation is incident on the external region based on the evaluation value; Judgment method. Computer
A reference point setting means for setting a reference point in a radiographic image obtained by photographing a subject using an irradiation field stop;
Straight line detection means for detecting a straight line that is a candidate for an irradiation field side from the radiographic image;
Evaluation for calculating an evaluation value by dividing the radiographic image by the detected straight line into two regions and evaluating whether radiation is incident on an external region that does not include the reference point in the two regions. A value calculating means;
When determining that the radiation is incident on the external region based on the evaluation value, the program functions as a determination unit that determines that the straight line is not an irradiation field side.

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