JPH07101287B2 - Radiation field recognition method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention obtains a large number of image data corresponding to each pixel on a recording sheet on which a radiation image of a subject is recorded, and then obtains these image data. The present invention relates to a radiation field recognition method for recognizing a radiation field formed on a recording sheet.

(Prior Art) It is performed in various fields to read a recorded radiation image to obtain image data, perform appropriate image processing on the image data, and then reproduce and record the image. For example, an X-ray image is recorded using an X-ray film having a low gamma value designed so as to be compatible with the subsequent image processing, and this X-ray image is recorded.
Contrast and sharpness can be obtained by reading an X-ray image from a film on which a line image is recorded, converting it into an electric signal, subjecting this electric signal (image data) to image processing, and then reproducing it as a visible image on a copy photograph or the like. A system capable of obtaining a reproduced image with good image quality performance such as graininess has been developed (see Japanese Patent Publication No. 61-5193).

In addition, the applicant of the present application, radiation (X-ray, α-ray, β-ray, γ
Ray, electron beam, ultraviolet ray, etc.) causes a part of this radiation energy to be accumulated, and then irradiation with excitation light such as visible light causes stimulated emission depending on the accumulated energy. Radiation image of a subject such as a human body is temporarily photographed and recorded on a sheet-shaped stimulable phosphor by using a fluorescent phosphor, and this stimulable phosphor sheet is scanned with excitation light such as laser light to stimulate emission. Generates light, photoelectrically reads the resulting stimulated emission light to obtain image data, and based on this image data, outputs a radiation image of the subject as a visible image on a recording material such as a photographic light-sensitive material or CRT. A radiation image recording / reproducing system has already been proposed (JP-A-55-12429).
No. 56, No. 56-11395, No. 55-163472, No. 56-104645, No.
55-116340 etc.).

This system has the practical advantage of being able to record images over a very wide radiation exposure area compared to conventional radiographic systems using silver halide photography. That is, in the stimulable phosphor, it has been recognized that the amount of emitted light stimulated by excitation after storage is proportional to the radiation exposure amount over a very wide range,
Therefore, even if the radiation exposure amount fluctuates considerably due to various photographing conditions, the amount of stimulated emission light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means by setting the reading gain to an appropriate value. Obtaining a radiation image that is not affected by fluctuations in radiation exposure by converting it into an electrical signal and using this electrical signal to output a radiation image as a visible image on a recording material such as a photographic photosensitive material or a display device such as a CRT. You can

In the above system, the stimulable phosphor sheet is scanned with a low-level light beam in advance before the image data is obtained by reading under the optimum reading conditions according to the dose of radiation applied to the stimulable phosphor sheet. Pre-reading is performed to read the outline of the radiation image recorded on this sheet, the pre-reading image data obtained by this pre-reading is analyzed, and then the sheet is irradiated with a light beam of a higher level than the light beam at the time of pre-reading. There is also a system configured to perform the main scanning in which the radiation image is scanned and read under the optimum reading condition to obtain the image data (Japanese Patent Laid-Open Nos. 58-67240 and 58-58240).
-67241, 58-67242, etc.).

Here, the reading condition is a general term for various conditions that affect the relationship between the amount of stimulated emission light and the output of the reading device during reading, and for example, the reading gain, scale factor, or , And means the power of excitation light in reading.

Also, the high level / low level of the light beam is, respectively,
In the case where the intensity of the light beam irradiated per unit area of the sheet is high or low, or the intensity of the stimulated emission light emitted from the sheet depends on the wavelength of the light beam (has a wavelength sensitivity distribution), The intensity of the light beam emitted per unit area of the sheet is weighted by the above wavelength sensitivity. It is the large / small of the weighted intensity. As a method of changing the level of the light beam, light beams of different wavelengths are used. Method,
A method of changing the intensity of a light beam emitted from a laser light source or the like, a method of changing the intensity of a light beam by inserting or removing an ND filter or the like in the optical path of the light beam,
Various known methods such as a method of changing the scanning density by changing the beam diameter of the light beam and a method of changing the scanning speed can be used.

In addition, regardless of whether this system performs pre-reading or does not pre-read, the obtained image data (including pre-reading image data) is analyzed and optimum image processing conditions for performing image processing on the image data are determined. There is also a system that decides. The method of determining the optimum image processing conditions based on this image data is not limited to the system using the stimulable phosphor sheet, and the image data is obtained from the radiation image recorded on the recording sheet such as the conventional X-ray film. It is also applied to the system.

Various methods have been proposed for analyzing the above-mentioned image data (including pre-read image data) to obtain optimum reading conditions and image processing conditions. As one of the methods, a method of creating a histogram of image data is known. (For example,
JP-A-60-156055). By obtaining the histogram of the image data, it is possible to know, for example, the maximum value, the minimum value of the image data, the value of the image data at the point where the frequency is maximum, and the like. From these values, the stimulable phosphor sheet, X It is possible to grasp the characteristics of the radiation image recorded on the recording sheet such as the line film. Therefore, by finding the optimum reading conditions and image processing conditions based on this histogram,
It is possible to reproduce and output a radiation image that is excellent for observation.

On the other hand, when photographing and recording a radiation image on a recording sheet, in order to prevent irradiation of a portion that is not necessary for observing a subject, or to irradiate a portion that is unnecessary for observation, it is necessary to observe from that portion. Since scattered radiation enters the area and image quality performance deteriorates, shooting is performed using an irradiation field diaphragm that limits the irradiation area of the radiation so that the radiation is irradiated only to the necessary part of the subject and the recording sheet. Often.

However, in the case of analyzing the image data to obtain the reading condition and the image processing condition as described above, when the image data used for the analysis is the image data obtained from the recording sheet photographed using the irradiation field diaphragm, Even if the existence of this irradiation field is ignored and the image data is analyzed, the radiographic image captured and recorded is not correctly grasped, and incorrect reading conditions and image processing conditions are required, and a radiographic image excellent in observation appropriateness cannot be reproduced and recorded. A case arises.

In order to solve this, it is necessary to recognize the irradiation field and obtain the reading condition and the image processing condition based on the image data in the irradiation field before obtaining the reading condition and the image processing condition.

Among the methods of recognizing the irradiation field, as a versatile method that can accurately recognize the irradiation field even if the irradiation field has an irregular shape, for example, a predetermined point included in the irradiation field Based on the image data corresponding to each pixel along a plurality of radial line segments connecting the sheet and the sheet edge, contour points considered to be on the contour of the irradiation field are obtained for each of the line segments, and these contours are obtained. A method of recognizing an area surrounded by a line along a point as an irradiation field has already been proposed by the applicant (Japanese Patent Application No. 62-93633).

(Problems to be Solved by the Invention) First, the irradiation field is obtained as described above, and then by analyzing the image data corresponding to the obtained irradiation field,
Appropriate reading conditions and image processing conditions are required.

However, when a foreign substance other than the subject (for example, a lead protector for radiation protection or a fixture for fixing the subject) is photographed together with the subject during shooting, the calculation of the irradiation field may not be performed effectively. In this case, the reading condition and the image processing condition are determined based on the image data corresponding to the erroneously recognized irradiation field, and the radiation which is excellent in the observation appropriateness even though the calculation for recognizing the irradiation field is performed. There is a problem that an image may not be reproduced and recorded.

In view of the above problems, it is an object of the present invention to provide a radiation field recognition method capable of correctly recognizing a field even when a foreign substance other than the subject is photographed and recorded together with the subject at the time of photographing. It is a thing.

(Means for Solving the Problem) The radiation field recognition method of the present invention obtains a large number of image data corresponding to each pixel on a recording sheet on which a radiation image of a subject is recorded, and based on these image data In the radiation field recognition method for recognizing the radiation field formed on the recording sheet, a plurality of radial line segments connecting a predetermined point included in the irradiation field on the recording sheet and the edge of the recording sheet. For each of
Based on the image data corresponding to each pixel on each of these line segments, contour candidate points considered to be intersection points of the contour of the irradiation field on one or a plurality of recording sheets for each of these line segments and the line segment. Is calculated together with the candidate rank, and the contour candidate points on each line segment exist both in the predetermined peripheral area close to the edge of the recording sheet and in the central area inside the peripheral area, and the first edge in the peripheral area. When there are contour candidate points in the rank, the candidate rank of the contour candidate point in the highest rank among the contour candidate points in the central range is moved to the first rank, and the first rank for each line segment thus determined It is characterized in that the irradiation field is recognized based on the contour candidate points in the order of.

In a preferred embodiment of the present invention, the light representing the radiation image obtained from each pixel on the recording sheet is photoelectrically read to obtain a large number of image data corresponding to each pixel. The "light representing the radiation image obtained from each pixel on the sheet" includes stimulated emission light emitted from the stimulable phosphor sheet and light transmitted through the photographic film or reflected from the photographic film. Be done.

(Function) As a result of the statistical investigation of a large number of radiation images by the present inventors, the irradiation field is overwhelmingly placed near the center of the recording sheet, while the foreign matter photographed and recorded together with the subject is recorded on the recording sheet. It turned out that it is almost always present in the peripheral area.

The present invention has obtained a clue from the above-mentioned survey results, and one or more contour candidate points are provided as candidate ranks for each of a plurality of line segments connecting a predetermined point in the irradiation field and the end of the recording sheet. When there is a contour candidate point in the central range, even if there is a contour candidate point in the peripheral range, it is ignored. It will be removed and the irradiation field can be correctly recognized.

(Example) Hereinafter, the Example of this invention is described with reference to drawings.

FIG. 4 is a perspective view of an embodiment of the radiation image reading and reproducing apparatus, which illustrates an example of the radiation field recognition method of the present invention. This radiation image reproducing device is a device using the above-mentioned stimulable phosphor sheet.

The stimulable phosphor sheet 1 on which the radiation image is recorded is set at a predetermined position of the reading means 100. The stimulable phosphor sheet 1 set at this predetermined position is conveyed (sub-scanned) in the arrow Y direction by the sheet conveying means 15 such as an endless belt driven by a driving means (not shown). On the other hand, the light beam 17 emitted from the laser light source 16 is a motor.
A rotating polygon mirror 18 driven by 24 and rotating at high speed in the direction of the arrow.
After being reflected and deflected by the sheet 1 and passing through the focusing lens 19 such as the fθ lens, the optical path is changed by the mirror 20 and the sheet 1
X which is incident on and is substantially perpendicular to the sub-scanning direction (arrow Y direction).
Main scan in the direction. From the portion of the sheet 1 irradiated with the light beam 17, the stimulated emission light 21 having a light amount corresponding to the accumulated and recorded radiation image information is emitted, and the stimulated emission light 21 is guided by the light guide 22, It is detected photoelectrically by a photomultiplier (photomultiplier tube) 23. The light guide 22 is formed by molding a light guide material such as an acrylic plate, and is arranged so that the linear incident end face 22a extends along the main scanning line on the stimulable phosphor sheet 1. The photomultiplier 2 is attached to the emitting end face 22b formed in a ring shape.
3 light receiving surfaces are combined. Light guide from the incident end face 22a
The stimulated emission light 21 that has entered the inside 22 travels through the light guide 22 by repeatedly undergoing total reflection, is emitted from the emission end face 22b, is received by the photomultiplier 23, and is of the stimulated emission light 21 that represents a radiation image. The light quantity is converted into an electric signal by the photomultiplier 23.

The analog output signal S output from the photomultiplier 23 is logarithmically amplified by the log amplifier 26, and the A / D converter 27
And digitized to obtain image data S _Q corresponding to each pixel of the radiation image. The obtained image data S _Q is once stored in the storage means 28 and then read by the image processing means 29.

In the image processing means 29, regarding each of a plurality of radial line segments that connect a predetermined point included in the irradiation field and the end of the stimulable phosphor sheet 1, it is considered as an intersection of the irradiation field contour and the line segment. The contour points are determined, and the area surrounded by the line along these contour points is recognized as the irradiation field. When the irradiation field is recognized, the image data S _Q corresponding to this irradiation field is subjected to appropriate image processing.

The image data S _Q subjected to the image processing is sent to the reproducing means 30, and the reproducing means 30 reproduces and records the radiation image based on the image data S _Q.

Here, the irradiation field recognition method performed by the image processing means 29 to obtain the irradiation field based on the image data S _Q will be further described.

FIG. 1 is a diagram showing an example of a radiation image and image data S _Q obtained from this radiation image in order to explain one embodiment of the radiation field recognition method of the present invention.

A subject image 2 whose subject is the head of the human body is photographed and recorded on the stimulable phosphor sheet 1. An irradiation field diaphragm is used for photographing to irradiate only a portion of the head necessary for observation, and an irradiation field 3 is formed in the central portion of the stimulable phosphor sheet 1. Further, earpieces for fixing the head are used during photographing, and images 4 of the earpieces are formed on both left and right ends of the stimulable phosphor sheet 1. Here, the irradiation field diaphragm is used for the purpose of irradiating only the portion necessary for observing the subject with radiation, but scattered radiation is accumulated and recorded also outside the irradiation field 3, so An image is also formed outside the irradiation field 3.

Here, the center C of the stimulable phosphor sheet is selected as a predetermined point in the irradiation field 3, and each pixel on each line segment is aligned along each of the plurality of line segments 5 extending radially from this center C. The image data S _Q to be processed is subjected to a differential operation, and the point where the value of the image data S _Q suddenly drops is obtained as a contour candidate point.

Hereinafter, a case will be described in which the contour candidate points are obtained along the ξ axis among the plurality of line segments 15.

Clough A is image data obtained from each pixel along the ξ axis.
It is a graph showing the value of S _Q.

The value of the image data in the irradiation field 3 is the highest, and the value of the image data sharply decreases at the contour of the irradiation field 3. Further, the image data corresponding to each image in the image 4 of the ear pad further decreases in value because the radiation is blocked not only by the irradiation field diaphragm but also by the ear pad.

The graph B shows the image data shown in the graph A from the center C by ξ
Is a graph obtained by differentiating in the positive direction (right direction in the figure) and in the negative direction of ξ (left direction in the figure), and the extent to which the image data value sharply decreases is shown as each peak in graph B. There is.

In graph B, there are two main peaks a ₁ and a ₂ in the region from the center C toward the positive direction of the ξ axis, and a ₁ shows a larger peak than a ₂ . At this time, for the line segment extending from the center C in the positive direction of the ξ axis, the contour candidate points considered to be the contours of the irradiation field are a ₁ as the first candidate and a ₂ as the second candidate.
Asked as a candidate. In this case, since the contour candidate point a ₁ of the first candidate is within the predetermined central range D, the contour candidate point a ₁ of the first order is the contour candidate point on the line segment in the positive direction of the ξ axis. It is adopted as it is.

In the region from the center C toward the negative direction of the ξ axis in the graph B, three main peaks b ₁ , b ₂ , b ₃ are obtained, and the peak sizes are b ₁ , b ₂ , b _{3 in} descending order. Therefore, b ₁ is the first candidate, b ₂ is the second candidate, and b ₃ is the third candidate. Here, b ₁ is in the peripheral range E close to the end of the stimulable phosphor sheet 1, and since the contour candidate points b ₂ and b ₃ are also in the central range D, the contour candidate point b ₁ is Of the contour candidate points b ₂ and b ₃ that are excluded and are in the central range D, the first sequential contour candidate point b ₂ is moved up to the first candidate, and this contour candidate point is a line segment along the negative direction of the ξ axis. Is adopted as a candidate point for contour.

Note that there are contour candidate points only in the peripheral range E, and the central range D
If there is no contour candidate point inside, the contour of the irradiation field is considered to be within the peripheral range E, so the contour candidate points within the peripheral range E are adopted.

Note that the calculation for obtaining the contour candidate points along each line segment is not limited to finding a plurality of contour candidate points by one search as described above (for example, from the center C to the left in the figure along the arrow F in FIG. 1). It is not necessary to perform calculation such that _three contour candidate points b ₁ , b ₂ , b ₃ are obtained by one search (differential calculation) toward, for example, as shown by arrow G in FIG. Only _one contour candidate point b ₁ is obtained, and when the obtained contour candidate point b ₁ is within the peripheral range E, this time, the point b ₁ is searched toward the center C to find the next contour candidate point. When the contour candidate point b ₁ is found within the peripheral range E, the contour candidate point b ₁ of the first candidate is _first sought, as indicated by the arrow H in FIG. Again lowers, for example, the reference for obtaining contour candidate points, and the second line is drawn from the center C along the same line segment.
May be calculated so as to obtain the contour candidate points.

As described above, the center C and the stimulable phosphor sheet 1 are used.
Contour candidate points 6 for each of the plurality of line segments that connect to the end of the
(The contour candidate points determined as described above will be referred to as contour points 6 hereinafter). After these contour points 6 are obtained, if a line along these contour points 6 is obtained, that line becomes the contour of the irradiation field. The line along the contour point 6 is, for example, a method of connecting the remaining points after smoothing the points, a method of locally applying the least squares method to obtain a plurality of straight lines, and connecting them. Although it can be obtained by a method of applying a spline curve or the like, the image processing means 29 in this embodiment is configured to obtain a plurality of straight lines along the contour points by using the Hough transform. Hereinafter, the process of obtaining this straight line will be described in detail.

When the x-axis and the y-axis are obtained as shown in the figure with one end of the stimulable phosphor sheet 1 shown in FIG. 1 (the lower left corner of the figure) as the origin, the coordinates of each contour point are (x ₁ , y ₁ ), (x ₂ , y ₂ ), ...
, (X _n , y _n ), but these coordinates are represented here as coordinates (x ₀ , y ₀ ). Image processing means
29 (see FIG. 2) is a curve represented by ρ = x ₀ cos θ + y ₀ sin θ, where x ₀ and y ₀ are constants when the coordinates (x ₀ , y ₀ ) of the above contour points are used. Obtain the point coordinates (x ₀ , y ₀ ). This curve is as shown in FIG. 3, and there exist as many edge candidate point coordinates (x ₀ , y ₀ ).

Next, the image processing means 29 obtains an intersection (ρ ₀ , θ ₀ ) at which a predetermined number Q or more of the plurality of curves described above intersect each other. It should be noted that many curves rarely intersect at exactly one point due to an error in the contour point coordinates (x ₀ , y ₀ ), so in reality, for example, the intersections of two curves are separated from each other at intervals of a minute predetermined value or less. When they exist, the center of the intersection point group is defined as the intersection point (ρ ₀ , θ ₀ ). Next, the intersection (ρ ₀ , θ ₀ )
From this, a straight line defined by the following equation ρ ₀ = xcos θ ₀ + ysin θ ₀ in the xy orthogonal coordinate system can be obtained. This straight line is a straight line extending along a plurality of contour point coordinates (x ₀ , y ₀ ). When the contour points 6 are lined up as shown in FIG. 1, this straight line is a straight line L ₁ -L ₅ that extends each line segment forming the contour of the irradiation field 3 (see FIG. 1) as shown in FIG. Is required as. next,
A region surrounded by the plurality of straight lines L ₁ , L ₂ , L ₃ , ... L _n thus obtained is obtained, and this region is recognized as the irradiation field 3. This area is recognized in detail as follows, for example. In the image processing means 29 (see FIG. 2), line segments M ₁ , M ₂ , M ₃ , ... M connecting the corners and the center C of the stimulable phosphor sheet 1.
_m (4 if the stimulable phosphor sheet 1 is rectangular) is stored, and it is checked whether or not there is an intersection between each of the line segments M _{1 to} M _m and each of the straight lines L _{1 to} L _n . When this intersection exists, of the planes bisected by the straight line, the plane on the side including the sheet corner is cut off. This operation is all straight line L ₁
~ L _n , line segments M ₁ ~ M _m
A region surrounded by L _{1 to} L _n is left. The remaining area is the irradiation field 3 (see FIG. 1).

When the irradiation field is obtained in this way, the image data S _Q corresponding to this irradiation field is subjected to appropriate image processing.

In the embodiment described above, the point in the irradiation field which is the starting point in the direction of the differential processing is the center C of the stimulable phosphor sheet, but this point is not limited to the sheet center point, but any point existing in the irradiation field can be used. Any point may be used. For example, if the irradiation field is narrowed down to an extremely small value, the center point C of the sheet may be located outside the irradiation field. It is desirable to use the point that will always exist in the irradiation field, such as the center of gravity of the high-density region when binarized.

In addition, although the above-described embodiment has described the radiation image reading apparatus that does not perform pre-reading, pre-reading is performed to obtain pre-reading image data, an irradiation field is obtained based on the pre-reading image data, and a destination corresponding to the irradiation field is obtained. It goes without saying that the radiation field recognition method of the present invention can also be used in a system that obtains reading conditions for actual reading based on read image data.

Further, the present invention can be used not only in a device using a stimulable phosphor sheet but also in a device using a conventional X-ray film.

FIG. 5 is a perspective view of an embodiment of an X-ray image reading apparatus for reading an X-ray image recorded on an X-ray film.

The X-ray film 40 on which the X-ray image is recorded is set at a predetermined position, and the X-ray film 40 is conveyed by the film conveying means 41 to the arrow Y'shown in the figure.
Be transported to.

The reading light 43 emitted from the one-dimensionally elongated light source 42 is converged by the cylindrical lens 44 and linearly radiated on the X-ray film in the X'direction substantially perpendicular to the arrow Y'direction. Below the X-ray film 40 irradiated with the reading light 43, the X-ray film 40 is transmitted to the X-ray film 40, and the X-ray image intensity-modulated by the X-ray image recorded on the X-ray film 40 is received. A MOS in which a large number of solid-state photoelectric conversion elements corresponding to respective pixel intervals in the X'direction of a line image are linearly arranged.
A sensor 45 is provided. The MOS sensor 45 transmits the reading light transmitted through the X-ray film 40 to the X-ray film while the X-ray film is being conveyed in the direction of the arrow Y ′ while being irradiated with the reading light 43.
Light is received at a predetermined time interval corresponding to each pixel interval in the Y'direction of the line image.

FIG. 6 is a circuit diagram showing an equivalent circuit of the MOS sensor 45.

Signals due to photocarriers generated when a large number of solid-state photoelectric conversion elements 46 are exposed to the reading light 43 are stored in capacitors Ci (i = 1, 2, ... N) in the solid-state photoelectric conversion elements 46. The accumulated photocarrier signals are sequentially read by sequentially opening and closing the switch unit 48 controlled by the shift register 47, whereby a time-series image signal is obtained. This image signal is then amplified by the amplifier 49 and output from its output terminal 50.

The output analog image signal is sampled and converted into a digital image signal, and then the X-ray irradiation field is recognized based on this image signal in the same manner as in the above-described embodiment. Needless to say, in this embodiment, a CCD, a CPD (Charge Priming Device) or the like can be used instead of the MOS sensor 45. Also in the reading of the X-ray film, it is needless to say that the reading may be performed by two-dimensionally scanning with a light beam as in the case of reading the stimulable phosphor sheet described above. In the above embodiment, the light transmitted through the X-ray film 40 is received.
Of course, it can be configured to receive the light reflected from the line film 40.

As described above, the radiation field recognition method of the present invention reads a radiation image from a recording sheet on which a radiation image of a subject is recorded, obtains image data, and reproduces and outputs the radiation image based on the image data. It can be applied to playback devices in general.

(Effects of the Invention) As described in detail above, the method for recognizing a radiation field of the present invention is 1 for each of a plurality of radial line segments connecting a predetermined point included in the field and the end of the recording sheet. When one or more contour candidate points are obtained along with the candidate order, and when there is a contour area point in the central range, even if there are contour candidate points in the peripheral range, it is ignored. Even if recorded, the irradiation field can be correctly recognized.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a radiation image and image data obtained from this radiation image, and FIG. 2 is a graph for explaining a method for obtaining a straight line along an outline point, FIG. FIG. 4 is an explanatory diagram for explaining a method of extracting a region surrounded by a straight line along a contour point, and FIG. 4 shows an example of a radiation image reading / reproducing apparatus using an example of the radiation field recognition method of the present invention. 5 is a perspective view of an embodiment of an X-ray image reading apparatus for reading an X-ray image recorded on an X-ray film, and FIG. 6 is a circuit showing an equivalent circuit of a MOS sensor. It is a figure. 1 ... Accumulative phosphor sheet, 2 ... Subject image, 3 ... Irradiation field, 4 ... Image of earpiece, 5 ... Line segment, 6 ... Contour point, 21 ... Stimulated luminescent light, 23 ... Photomulti Pliers 26 …… Log amplifier, 27 …… A / D converter 28 …… Storing means, 29 …… Image processing means 30 …… Reproducing means, 40 …… X-ray film 45 …… MOS sensor, 100 …… Reading means

Claims (1)

[Claims] 1. A large number of image data corresponding to respective pixels on a recording sheet on which a radiation image of a subject is recorded are obtained, and based on these image data, a radiation irradiation field formed on the recording sheet. In the radiation field recognition method for recognizing the above, for each of a plurality of radial line segments that connect a predetermined point included in the radiation field on the recording sheet and an end of the recording sheet, Based on the image data corresponding to each pixel, a contour candidate point considered as an intersection of the contour of the irradiation field on the one or more recording sheets and the line segment is candidate for each of these line segments. Obtained together with the rank, the contour candidate points on each of the line segments exist both in a predetermined peripheral range near the edge of the recording sheet and in a central range inside the peripheral range, and When the contour candidate point having the first rank exists in the side range, the candidate rank of the contour candidate point having the highest rank among the contour candidate points in the central range is moved to the first rank. A radiation irradiation field recognition method characterized in that the irradiation field is recognized based on the contour candidate points in the first order for each of the line segments thus determined.