JPS62104263A - Method for recognizing irradiation field - Google Patents

Abstract

PURPOSE:To always recognize an accurate irradiation field by obtaining a characteristic value and a picture data at a position where a value differentiating the picture data at every line in a prescribed direction exceeds a prescribed value and recognizing the inside of a profile line tying positions where the picture data and the characteristic value are coincident as the irradiation field. CONSTITUTION:Since the picture data corresponds to the magnitude of the energy of the radiant ray incident in a sheet, the picture data at the outside of the irradiation field has a low quantization level in general and the picture data in the irradiation field has a high quantization level in general. Thus, the difference among the picture data (differentiation value) where the profile of the irradiation field exists have in general a larger quantization level than the difference (differentiation value) of the picture data of the other part. Then it is decided that a point whose differentiation value is a prescribed value T0 or over is a point in the inside of the profile of irradiation field. As a result, the point is used as an irradiation field profile candidate point.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is applicable to the case where radiation image information is recorded on a recording medium such as a stimulable phosphor sheet (
) regarding how to recognize the radiation field.

(Conventional technology) Certain types of phosphors are exposed to radiation (X-rays, α-rays, β-rays.

When irradiated with γ-rays, electron beams, ultraviolet rays, etc., a part of this radiation energy is accumulated in the phosphor, and when this phosphor is irradiated with excitation light such as visible light, the phosphor changes depending on the accumulated energy. is known to exhibit stimulated luminescence, and phosphors exhibiting this property are called stimulable phosphors.

Using this stimulable phosphor, radiation image information of a subject such as a human body is partially recorded on a sheet of stimulable phosphor, and then this stimulable phosphor sheet is exposed to excitation light such as a laser beam. Scanning is performed to generate stimulated luminescence light, this stimulated luminescence light is read photoelectrically to obtain an image signal 19, image processing is performed on this image signal, and this image processing JjI! Based on the image signal that has been subjected to
The applicant has already proposed 12 radiation image information recording and reproducing systems that output visible images on a display device such as a CRT (Japanese Patent Laid-Open Nos. 55-12429 and 56-11).
395 etc.).

In the above system, in order to improve the suitability for observation and interpretation of 7fl@, when photoelectrically reading the stimulated luminescence light or when performing image processing on the image signal, a It is desirable to perform the reading and both 9ffi processing based on the optimum reading line f1 and image processing conditions.

As a method for optimally determining the reading conditions and image processing conditions such as gradation processing conditions as described above according to each photographed image, it is disclosed in Japanese Patent Application Laid-open No. 1988-G7240, etc., which the present applicant previously filed. In other words, the MVI phosphor sheet on which radiation image information is stored and recorded is scanned with excitation light, and the stimulated luminescence emitted from the sheet is read by a photoelectric reading means for diagnosis. Obtain an electrical image signal that does not harm the visual image [Prior to book reading J, the sheet is scanned in advance with excitation light of a lower level than the excitation light used for this book reading, and the image is accumulated and recorded on this sheet. "Read ahead" to read the outline of information
There is a method of determining the reading conditions (reading conditions when performing actual reading here) and image processing conditions based on the image information obtained by this pre-reading.

As a specific method for determining the reading conditions for main reading based on the image information obtained by such pre-reading, for example, there is
As described in the publication, a histogram of the image information (image signal level) obtained by (q) is obtained by pre-reading, and the n-large image signal level 3 max of the desired image information range is calculated from this histogram. and the minimum image signal level Sm1n, and this 5IIl
ax and 5IIlin are respectively the maximum signal level Q max and minimum signal level Qmin of the desired input signal range in the image processing means determined by the maximum density [) max and minimum density [) win in the appropriate a degree range in the visible output image. A method of determining reading conditions for main reading may be considered in a corresponding manner.

Further, as a specific method for determining image processing conditions, such as gradation processing conditions, based on the image information obtained by pre-reading, for example, the same method as above, that is, the image information obtained by pre-reading (image signal level ), the maximum image signal level 3 max and the minimum image signal level S min of the desired image information range in this histogram are determined, and this Smax and 3 m
The maximum signal level Rmax and the minimum signal level of the desired input signal range in the image reproduction means (visible image output means) where in is determined by the maximum a degree 1) max and the minimum density DIIlin of the appropriate density range in the visible output image, respectively. RI! A possible method is to determine the gradation processing conditions so as to correspond to lin.

J3, image processing conditions such as this gradation processing condition can be determined based not only on the image information q'd by the above-mentioned pre-reading, but also based on the +Q-added image information by the main reading, and even in that case, For example, as in the previous case, create a histogram of the image information (image signal level) obtained from the main reading, and use this histogram to
A possible method is to obtain max and S sin and determine the gradation processing conditions so that 5IIlax and 5w1n correspond to Rmax and Rmin, respectively.

In the above, reading conditions refer to conditions that affect the relationship between the input and output of the reading means, for example, in the above, the relationship between the input (stimulated luminescence light m) and the output (electrical image signal level) of the photoelectric reading means. It is a general term for various conditions, and means, for example, reading gain (sensitivity, scale factor (latitude)) that determines the relationship between input and output, or the power of excitation light in reading.

In addition, in the above, the image processing condition is a general term for various conditions that affect the relationship between input and output in the image processing means, and includes, for example, gradation processing conditions, spatial frequency processing conditions, etc. .

Furthermore, in the above, the fact that the excitation light used for pre-reading is at a lower level than the excitation light used for main reading means that the effective energy of the excitation light that the stimulable phosphor sheet receives per area of This means that the value is smaller than that when reading a book.

(Problem to be Solved by the Invention) By the way, in order to avoid irradiating 11i rays to parts that are not necessary for humane diagnosis, or to apply tIl rays to parts that are not necessary for diagnosis, the parts necessary for diagnosis are removed from that part. In order to prevent this, the radiation illumination (
g) In many cases, it is preferable to narrow down the field. However, when the radiation irradiation field is narrowed down in this way, the scattered radiation generated from the subject in the irradiation field usually enters the irradiation field above the stimulable phosphor sheet 1, and the highly sensitive stimulable phosphor Since the sheet also accumulates and records these scattered rays, the histogram of image information (image signal level) obtained by pre-reading also includes the image signal level based on these scattered rays. Since the image signal level outside the irradiation field on the sheet that is surrounded by these scattered rays may be higher than the image signal level inside the irradiation field, the obtained histogram is used to distinguish between the image signal levels inside and outside the irradiation field. That is difficult. Therefore, when calculating Smax and Sm1n from the histogram and determining the reading conditions from the histogram as described above, the minimum value of the image signal level within the irradiation field should be S n+in, but the image signal level due to the scattered radiation outside the irradiation field is A case may occur where the minimum value of is set to Sm1n. In this way, when the minimum value of the image signal level outside the irradiation field is set to 5IIlin, that value is generally lower than the minimum value of the image signal level within the irradiation field. Therefore, the density of the image in the area necessary for diagnosis is too high, resulting in a decrease in contrast, making it difficult to make a satisfactory diagnosis.

In other words, when capturing images by narrowing down the irradiation field, scattered rays generated from the subject enter the irradiation field from sea 1 to above, and the image information that is 1!7 by pre-reading is based on this scattered ray. Even if the reading conditions are determined based on such pre-read image information, it is difficult to determine the optimal reading conditions.As a result, it is difficult to determine the optimal reading conditions. It becomes difficult to obtain.

In addition, such problems occur not only when determining reading conditions based on prefetched image information, but also when determining image processing conditions such as gradation processing conditions based on the above-mentioned prefetched image information or prefetched image information. can also exist.

Therefore, when trying to determine reading conditions and image processing conditions based on pre-read or main-read image information using the method described above, it is necessary to accurately recognize the irradiation field when the irradiation field is diaphragm-shaded. It is desirable to determine these conditions based only on the image information within the irradiation field in the pre-read or main-read image information to eliminate the adverse effects of the above-mentioned scattered radiation outside the irradiation field.

The above is an example of the necessity of irradiation field recognition when deciding the reading conditions etc. for the 11th phosphor sheet publication, but this irradiation field recognition is not limited to such cases. This is also necessary in the case of .

In view of the above circumstances, an object of the present invention is to provide a method for recognizing an irradiation field when radiation image information is recorded on a recording medium such as a stimulable phosphor sheet by restricting the irradiation field. be.

(Means for Solving the Problems) In order to achieve the above object, the irradiation field recognition method according to the present invention uses image information read from a recording medium such as a stimulable phosphor sheet that has been imaged to be recorded on the recording medium. Find the image data falling at each position, define each position lined up in a predetermined direction on the recording medium as one line, perform differentiation processing on the image data in the line, and calculate the absolute value of the differential value as a predetermined value. A position exceeding 11flTo is set as a candidate point for the irradiation field contour on that line, and the image data on the line at that candidate point is obtained,
Characteristics of the image data 1a T h from the image data
, the position where the image data has its characteristic value Th on the line is detected as an irradiation field contour point on that line, and the detection of the irradiation field contour point is performed for each line in a predetermined range on the recording medium. The method is characterized in that the inside of the line connecting the irradiation field contour points on each line is recognized as the irradiation field.

Note that the above-mentioned recording medium J refers to a medium capable of recording radiation image information, and a specific example thereof includes the above-mentioned stimulable phosphor sheet, but is not necessarily limited thereto.

Furthermore, the term "image information read from a recording medium" as used above means image information recorded on the recording medium that has been read by some method, such as pre-reading or actual reading on the stimulable phosphor sheet mentioned above. This does not mean the obtained image information, but is not necessarily limited thereto.

Of course, the method of using the irradiation field recognized by the above method is not limited to any particular method.

(Effects of the Invention) The method according to the present invention performs a differential process on image data for each line in a predetermined direction, and the differential value is set to a predetermined value TO.
Obtain the image data at a position beyond , obtain the characteristic value from the image data, and set the position where the image data is the same as the characteristic value as the irradiation field contour point on each line,
The contour points are connected to obtain a contour line, and the area inside the contour line is recognized as the irradiation field.

Generally, the differential value of image data at the position where the irradiation field contour exists is larger than the differential value of image data at other positions, so the differential value is set to an appropriately predetermined value T.
It can be determined that a position where the value is equal to or more than o is a point where the contour of the irradiation field exists. Therefore, the entire contour of the irradiation field can usually be detected by checking whether the differential value is greater than or equal to the predetermined value To.

However, for various reasons, there are cases where the differential value at the position does not exceed the predetermined value of 10 even though the irradiation field contour exists. In such a case, it is not possible to detect a pause in the contour of the irradiation field by simply examining the differential value, and therefore the irradiation field cannot be recognized accurately.

As described above, the method according to the present invention does not directly derive from the differential value, but uses the differential value to obtain the characteristic value Th of the image data at the position where the irradiation field contour is expected to exist (where the irradiation field contour exists). The position image data is estimated to be such a value) and the position where the irradiation field contour is likely to be found (irradiation field contour point) is determined from this characteristic value Th. The irradiation field can be detected by connecting the irradiation field contour points obtained by
It is possible to reliably obtain the contour of the irradiation field, which cannot be determined directly from the differential value, and therefore the advantage is that the irradiation field can always be accurately recognized.

The method according to the invention also has the advantage that its field detection algorithm is relatively simple.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In the embodiment described below, as shown in FIG. 1
The present invention is applied to the case where the irradiation field is recognized from the pre-read image information in 10-10.

In this method, first, jq
Digital image data at each position on the stimulable phosphor sheet is obtained from the image information obtained.

The image information 1q obtained by the above-mentioned pre-reading refers to the image information for each scanning point (that is, each pixel) on the stimulable phosphor sheet obtained by reading the stimulated luminescence light emitted by the pre-reading excitation light scanning using a photoelectric conversion means. Information consisting of electrical signals corresponding to the amount of stimulated luminescence. This information, of course, corresponds to the IIi ray image information stored and recorded on the sheet.

FIG. 2(a) shows the stimulable phosphor sheet 10 in FIG.
This figure does not show an enlarged view of part G of
．． 1>, f(1,2), . . . indicate the digitized image information at each pixel (1, 1), (1, 2), . . . . In this embodiment, as shown in FIGS. 1 and 2, the X-axis and y-axis are set to be perpendicular to each other, and the X-axis direction is aligned with the main scanning direction, and the y-axis direction is aligned with the sub-scanning direction. We are doing so. FIGS. 2(b) and 2(C) will be explained later.

In order to obtain digital image data at each position on the sheet from the above image information, it is first necessary to set the position on the sheet. Setting this position is li! It may be done row by line, or it may be a single position for white pixels having a certain relationship, for example, 3 to 5 pixels lined up in a certain direction. In the former case, the digital image data at each position means the digitized image information of the pixel corresponding to that position, and in the latter case, the digital image data at each position means the digital image data at each position. It means something determined based on the image information of one or more pixels included, for example, digital image data obtained by averaging image information of a plurality of pixels.

In this embodiment, this position setting is performed pixel by pixel.

After the positions are set in this way and the digital image data at each position is obtained, each position aligned in a predetermined direction on the sheet 10 is then set as one line. This line may be set in only one direction or may be set in two directions. Of course, depending on the situation, it may be set in more directions.

In this embodiment, lines are set in the X-axis direction and the y-axis direction, which are orthogonal to each other. That is, each position lined up in the X-axis direction (1,1), (2,1), (3°1), (
4,1), (5,1), river... is the first in the X-axis direction
Line LXI, each position (1,2), (2,2),
(3,2), (4,2), (5,2),...
... is set as the second line LX2 in the X-axis direction,
Thereafter, in the same manner, the third line Lx3 and the fourth line Lx4 in the X-axis direction are formed. ...... is set, and each position aligned in the y-axis direction < 1. +), (1,2>, (
1,3>, (1°4), ...... is the first line Lyl in the y-axis direction, each position (2,1), (2,2)
, (2,3), (2,4),... are set as the second line LV2 in the yIII11 direction,
Similarly, the third line LV3 in the y-axis direction. 4th line Ly4. ...... is set.

Once the lines are set in this manner, a differential process or the like is performed on each line to detect the field contour points on the line. The method for detecting the contour points of the irradiation field will be explained by taking as an example the case of the first line LXI in the X-axis direction shown in FIG.

FIG. 3 is a diagram showing the size of digital image data at each position on the line 1-xn, and FIG. 4 is a diagram showing the size of the digital image data on the line lxn.
FIG. 3 is a diagram showing differential values at each position.

Then, the digital image data on the line xn is subjected to differential processing to obtain the 43-fold differential (Oδ) at each position on the line.The method of differentiation may be either first-order differentiation or higher-order differentiation. In the case of an image, differentiation is equivalent to finding the difference between image data that exist in the vicinity. Existing in the vicinity does not only mean that they are adjacent to each other; for example, it can also be the case that they exist every other time. It means to include.

In this embodiment, −th order differentiation is performed to obtain the differential value δ for each position.
seek. As described above, this δ corresponds to the difference between image data at adjacent positions in the X-axis direction, and is expressed as in the following equation.

δ(1,n)=f(1,n>-f(2,n)δ(2,n
)=f(2,n)-f(3,n) Once the differential value δ at each position of the n-th line 1xn-L is obtained in this way, the absolute value of the differential value is the predetermined value T. To the position above,
Let C and D be the irradiation field contour candidate points on the line 1-xn.

Since the image data corresponds to the magnitude of the energy of the radiation incident on the sheet, image data outside the irradiation field generally has a low suspension level, and image data within the irradiation field generally has a high suspension level. Therefore, the difference (differential value) between image data in a portion where the contour of the irradiation field exists is generally a larger proton level than the difference (differential value) between image data in other portions. Therefore, the differential value is the predetermined value T. It can be determined that the above position is a point where the outline of the irradiation field exists, and as a result, that point is selected as a candidate point for the irradiation field outline.

However, even if it is possible to determine that a position where the differential value is greater than or equal to the predetermined value To is a point where an irradiation field contour exists, the differential value of the point where the contour exists does not necessarily exceed the predetermined value To. That is, for example, the left radiation field 1 in this embodiment
Despite the existence of a contour line such as the right side contour line 12b in Figure 2, the image data changes slowly (see Figure 3), and as a result, as shown in Figure 4, the right side contour line 12b
Even though the position B exists, the differential value of the position B may not exceed the predetermined value T0 (19).

Therefore, the above image data is subjected to differential processing, and the differential value is subjected to threshold processing! It is not always possible to find the irradiation field contour point only by 'p'. Therefore, in the present invention, in order to find all contour points even in such a case, the following processing is further performed.

First, L in 1 at contour candidate points Δ, C, and O generated by threshold processing using the above-mentioned predetermined value To.
- the digital image data T^, Tc, To on xn
Find (see Figure 3). Next, these image data TA
, TC, Tol,: the characteristic value T11 of the digital image data TA, TC, TO is determined.

The characteristic value Th may be any value as long as it corresponds to the image data T^, Tc, To, for example, TA, TC.
, the minimum value, average value, median value, maximum value, etc. of TO can be used as the characteristic value. In this example, TA, Tc, TO
TA, which is the minimum value in , is adopted as the characteristic value Th. If the minimum value is taken as the characteristic value Th in this way, the finally detected irradiation field can be made relatively wide,
As a result, it is possible to reduce the possibility that the detected irradiation field will be narrower than the actual irradiation field.

- After finding the characteristic value Th, as shown in FIG.
Determine C=, D=, and connect the A device to the above line lx, respectively.
In other words, the position (8-~[3'', C-~D') where the digital image data is greater than or equal to this characteristic value Th is detected as the irradiation field contour point on the line 1-xn. Determine the range.

Then, the work of detecting the irradiation field contour points on the line of one tree is performed for each line L× in the X-axis direction.
Execute over the entire axial direction, connect the irradiation field contour points on each line Lx, and make left and right Terutano 12.14 (7)
Axial contour lines 12a, 12b.

Find 14a and 14b.

Next, detect the irradiation field contour point on any line Lyn in the X-axis direction in the same manner as above.
(This detection of contour points is performed for each line LV in the X-axis direction over the entire area in the X-axis direction, and the contour points on each line Ly are connected to form a contour line 12c in the X-axis direction,
12d, 14c, +4d are determined, and the area inside these contour lines 12c, 12d, +4c, 14d and the above-mentioned contour lines 12a, 12b, 14a, 14b, that is, the range surrounded by these contour lines, is recognized as the irradiation field.

The detection of the contour points may be performed for each line within a predetermined range on the sheet 10. In the above example, sheet 10. [Although this is done for each line over the entire range of Execute the contour line position in the y-axis direction A-, B=, C
', D-, the next line Ly in the y'tl+ direction
Regarding this, it may be carried out only for those existing within the range of A'' to B- and C- to D'''.

In the above embodiment, only the differential processing of the image data and the threshold processing of the differential 1+lJ result in a contour line 1 of the tree.
2b cannot be determined, but the present invention can be used even in cases where all outline positions can be determined only by the above-mentioned differentiation and threshold processing. In the case of the method of determining the contour line position by differentiation and threshold processing (it is necessary to judge whether the contour line position is detected or not by that method,
If there is a contour that has not been detected, it is necessary to perform some kind of processing to further detect that contour, which complicates the algorithm for final irradiation field detection. According to the method, all contour positions can always be automatically detected regardless of whether all contour positions can be detected in the initial differentiation and threshold processing, and the algorithm is simple. It has the advantage of being right. In the above embodiment, one rectangular field was taken, but even if it is a circle or other irradiation field other than a rectangle, the present invention can recognize the irradiation field.

Further, although the above embodiment deals with the case where there are two irradiation fields due to the divided dance, the present invention is also applicable to the case where there is only one irradiation field. In that case, for example, there are only two contour lines in the y-axis direction, so the threshold processing described above
There may also be a case where there is only one contour candidate point.

In that case, the digital image data at the contour candidate point is also 1
In that case, for example, the image data itself may be used as the characteristic value Th. In addition, even in the case of the above-mentioned divided dance imaging, the present invention can be applied to each section (in this case, each section can be considered as one sheet having one irradiation field) by obtaining information in advance that it is a minute υ1 imaging. It is also possible to apply

Also, 1. In the above embodiment, contour point detection is performed for each of the lines in the X-axis direction and the y-axis direction, but it is not necessarily necessary to perform the detection for lines in two directions. For example, as shown in Figures 5(a) and 5(b), if the shape of the irradiated field is a circle or a diagonal rectangle, the irradiated field can be contoured by simply detecting the contour points for each line in the X-axis direction. can be detected in its entirety.

Furthermore, in the above embodiment, the digital image data was obtained by setting the position on the sheet 10 pixel by pixel, but for example, when it is known in advance that the irradiation field is a rectangle, it is possible to set the position on the sheet 10 in pixel units. Select the y-axis and y-axis, and first select the y-axis.
One position is set for every three pixels lined up in the axial direction, that is, position (1, 2) == three pixels (1, 1) + (1, 2) as shown in Figure 2 (b).
+ (1, 3) position (2, 2) = -3 pixels (2, 1) ÷ (2, 2)
+ (2, 3) position (1, 5) ′ = 3 pixels (1, 4) + (1, 5
) + (1, e) position (2,5)” = 3 pixels (2,4) + (2,5)
+ (2, G), and calculate the digital image data F at each position using the following formula, F (1, 2) - = (r (1, 1) + f (L 2 )+f
(1,3) )/3 F(2,2)==(f'(2,1)+f(2,2)+f
(2,3) )/3 F(1,5)−=(f (1,4)+f (1,5
)+f (1,6) )/3 F(2,5)==(f(2,4>+f(2,5)+f
(2, (1) )/3 This image data F is differentiated for each line in the X-axis direction to find the differential value δ at each position as shown in Figure 2 (C),
Using this differential value δ, the irradiation field contour line in the y-axis direction is detected in the same manner as described above, and then one position is set for every three pixels lined up in the X-axis direction, that is, each position is Set as below, position (2, 1) - = 3 pixels (1, 1)
+(2,1>+(3,1) position (2,2) ==3 pixels (1,2)+(2,2
)+(3,2) digit δ(5,i)”=3 pixels (4,1)+(5,1)
+ (G, 1) position < 5.2) - = 3 pixels (4, 2) + (5, 2)
+<6. 2) The digital image data F at each position is converted to the position (1, 2
)−, (2,2)=・・・・・・ Add i
This image data F is differentiated for each line in the y-axis direction to obtain a differential value δ, and using this differential value δ, the X@force direction irradiation field contour is determined by the same method as described above. It may also be detected.

Setting FP1 in this way means preprocessing the image information for each pixel before performing subsequent differential processing, etc. By performing this preprocessing, noise contained in the image information can be reduced. Since the influence can be eliminated and the number of image data to be processed thereafter can be reduced, the contour of the irradiation field can be detected more accurately and at high speed.

The irradiation field recognized as described above can be used for various purposes. For example, extract only the image information within the irradiation field from the pre-reading or main reading image information as mentioned above, and then identify the reading conditions and image processing conditions (if you want to recognize the irradiation field based on the main reading image information, only the image processing conditions). ), it can of course be used for other purposes, such as recognizing the irradiation field from pre-read image information, and during actual reading, using the Japanese Patent Laid-Open Publication No. 1983-1995, which the present applicant previously filed. As disclosed in No. 120346,
It can also be used to limit the reading area within the irradiation field. By limiting the reading area of the main reading to the irradiation field in this way, noise components due to scattered radiation recorded outside the stimulable phosphor sheet can be read. You can get a great final image without any problems. Furthermore, by narrowing down the reading area, it is possible to shorten the reading time or increase the reading density.

Further, the method of setting reading conditions and image processing conditions based on image information within the irradiation field is not limited to the method described above, and various methods can be used.

Note that the reading conditions and image processing conditions are determined not only based on the image information within the above-mentioned irradiation field, but also based on the dark areas of the subject that will be the counterstain for removal, such as the head, chest, and abdomen. It can also be determined by taking into consideration imaging methods such as simple imaging, contrast imaging, tomography, enlarged recirculation, or diagnostic purposes.

The present invention can be modified in various ways without departing from the spirit thereof, and is not limited to the embodiments described above.

[Brief explanation of drawings]

Figure 1 shows the stimulable phosphor sheet and the irradiation field;
Figure (a) is an enlarged view of part G in Figure 1, Figure 2 (b) is a diagram that does not show digital image data at each position, Figure 2 (C) is a diagram showing differential values at each position, Figure 3 shows the line Lxn
A diagram showing the digital image data above, Figure 4
Diagrams showing differential values of digital image data on It is a diagram. 10... Recording medium (stimulable phosphor sheet) 12, 14
...Irradiation fields 12a, 12b, 12c, 12d, 1
4a, 14b, 14c. 14d...Irradiation field outline Figure 1 Figure 3 Figure 4 Shi1

Claims (2)

[Claims] (1) A method for recognizing the irradiation field when radiation image information is recorded on the recording medium by applying an irradiation field aperture, the method comprising: determining each position on the recording medium from image information read from the recording medium; Find the image data at , define each position lined up in a predetermined direction on the recording medium as one line, perform differential processing on the image data in the line, and calculate the position where the absolute value of the differential value exceeds a predetermined value T_0. is the irradiation field contour candidate point on the line, the image data on the line at the candidate point is determined, the characteristic value Th of the image data is determined from the image data, and the characteristic value Th of the image data on the line is determined. A position with a value Th is detected as an irradiation field contour point on that line, and the irradiation field contour point is detected for each line in a predetermined range on the recording medium, and the irradiation field contour points on each line are connected. An irradiation field recognition method characterized by recognizing the inside of the line as the irradiation field. (2) An x-axis and a y-axis that are perpendicular to each other are set on the recording medium, and the detection of the irradiation field contour point on the line is performed using the x-axis.
2. The irradiation field recognition method according to claim 1, wherein the irradiation field recognition method is performed for both axial lines and y-axis lines.