JP2704288B2 - Method for determining image points in subject image - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an image processing apparatus, based on an image signal obtained from each pixel on a recording sheet on which a radiation image including a subject image is recorded.
The present invention relates to a method for obtaining an image point (one point on an image) in the subject image.

2. Description of the Related Art In various fields, reading a recorded radiation image to obtain an image signal, performing appropriate image processing on the image signal, and reproducing and recording the image have been performed in various fields. For example,
An X-ray image is recorded using an X-ray film having a low gamma value designed to be compatible with the subsequent image processing, and the X-ray image is read from the film on which the X-ray image is recorded and converted into an electric signal. By subjecting the electric signal (image signal) to image processing and reproducing it as a visible image in a copy photograph or the like, a system capable of obtaining a reproduced image with good image quality performance such as contrast, sharpness, and graininess is provided. It has been developed (see JP-B-61-5193).

In addition, the applicant (X-ray, α-ray, β-ray,
Irradiation with gamma rays, electron beams, ultraviolet rays, etc. accumulates a part of this radiation energy, and then irradiation with excitation light, such as visible light, causes a stimulable phosphor (luminous) to emit stimulated emission according to the accumulated energy. Using a photostimulable phosphor), a radiation image of a subject such as a human body is once photographed and recorded on a sheet-shaped stimulable phosphor, and the stimulable phosphor sheet is scanned with excitation light such as a laser beam to radiate. Generates emitted light, obtains an image signal by photoelectrically reading the obtained stimulated emission light, and outputs a radiation image of the subject as a visible image to a recording material such as a photographic photosensitive material or a CRT based on the image signal. There has already been proposed a radiation image recording / reproducing system (Japanese Patent Laid-Open No. 55-12429).
Nos. 56-11395, 55-163472, 56-104645
No. 55-116340, etc.).

This system has the practical advantage of being able to record images over a very large radiation exposure area compared to conventional radiographic systems using silver halide photography. That is, in the case of the stimulable phosphor, it has been recognized that the amount of emitted light that is stimulated by excitation after accumulation is proportional to the radiation exposure amount over an extremely wide range. Even if fluctuates considerably, the amount of the stimulating light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means with the reading gain set to an appropriate value and converted into an electric signal. By outputting a radiation image as a visible image on a recording material such as a photographic light-sensitive material or a display device such as a CRT using, a radiation image which is not affected by a change in radiation exposure can be obtained.

In the above-described system, before obtaining an image signal by reading under optimal reading conditions according to the dose of radiation applied to the stimulable phosphor sheet, the stimulable phosphor sheet is scanned with a low-level light beam in advance. A pre-read for reading the outline of the radiation image recorded on this sheet is performed, a pre-read image signal obtained by this pre-read is analyzed, and then the sheet is irradiated with a light beam having a higher level than the light beam at the time of the pre-read. There is also a system which is configured to perform a main reading in which an image signal is obtained by reading the radiation image under the optimum reading conditions (see Japanese Patent Application Laid-Open Nos. 58-67240 and 58-6).
Nos. 7241 and 58-67242).

The reading condition is a general term for various conditions that affect the relationship between the amount of stimulated emission light in reading and the output of the reading device. For example, a reading gain, a scale factor, or a scale factor that determines an input / output relationship. , And the power of the excitation light in reading.

Further, the high level / low level of the light beam means that the intensity of the light beam applied per unit area of the sheet is large / small, or the intensity of the stimulating light emitted from the sheet is the light beam. (Having a wavelength sensitivity distribution) means the magnitude / smallness of the weighted intensity after weighting the intensity of the light beam irradiated per unit area of the sheet by the wavelength sensitivity. Methods for changing the beam level include using a light beam of a different wavelength, changing the intensity of the light beam emitted from a laser light source, etc., and 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 beam intensity, 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 the system performs the pre-reading or the system which does not perform the pre-reading, the obtained image signal (including the pre-reading image signal) is analyzed, and an optimum image processing condition when performing image processing on the image signal is determined. Some systems let you decide. The method for determining the optimum image processing conditions based on this image signal is not limited to the system using the stimulable phosphor sheet, but obtains the image signal from a radiation image recorded on a recording sheet such as a conventional X-ray film. It is also applied to the system.

Various methods have been proposed for analyzing the image signal (including the pre-read image signal) to obtain optimum reading conditions and image processing conditions. One of the methods is to create a histogram of the image signal. (For example, Japanese Patent Application No. 59-12658 (Japanese Patent Publication No. 03-22968)) By obtaining a histogram of an image signal, for example, the maximum value, the minimum value of the image signal, and the point where the frequency becomes maximum can be determined. The value of the image signal can be known, and from these values, the characteristics of the radiation image recorded on the recording sheet such as the stimulable phosphor sheet and the X-ray film can be grasped. By finding the optimal reading conditions and image processing conditions, it is possible to reproduce and output a radiation image with excellent observation suitability.

On the other hand, when capturing and recording a radiographic image on a recording sheet, it is necessary to prevent radiation from irradiating a part not necessary for observation of the subject, or to irradiate a part unnecessary for observation with radiation from that part. Since scattered radiation enters the area and image quality performance deteriorates, use a field stop that limits the irradiation area of the radiation so that the radiation is irradiated only to the necessary part of the subject and a part of the recording sheet. Often.

However, when the image signal is analyzed as described above to determine the reading conditions and the image processing conditions, the image signal used for the analysis is an image signal obtained from a recording sheet photographed using an irradiation field aperture. Even if the image signal is analyzed ignoring the presence of this irradiation field, the captured and recorded radiation image is not correctly grasped, and incorrect reading conditions and image processing conditions are required. Not be performed.

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 signal in the irradiation field before obtaining the reading condition and the image processing condition.

Among the methods for recognizing an irradiation field, a versatile method capable of accurately recognizing the irradiation field even when the irradiation field has an irregular shape includes, for example, a predetermined point included in the irradiation field. Based on the image signals corresponding to each pixel along a plurality of radial directions toward the sheet edge from, a plurality of contour points considered to be on the contour of the irradiation field are obtained, and a line along these contour points is obtained. A method of recognizing a region surrounded by a field as an irradiation field has already been proposed by the present applicant (Japanese Patent Application No. 62-93333 (Japanese Patent Application Laid-Open No. 63-259538)).

In this method, the predetermined point needs to be a point in the irradiation field, and more preferably a point in the subject image in the irradiation field. However, since the irradiation field diaphragm is used to photograph only a necessary part of the subject, in photographing using the irradiation field diaphragm, almost all image points in the irradiation field (especially at the approximate center of the irradiation field) are hardly formed. In the case of, the image point is within the subject image.

(Problems to be Solved by the Invention) As described above, first, an irradiation field is obtained, and then, by analyzing an image signal corresponding to the obtained irradiation field, appropriate reading conditions and image processing conditions are obtained. .

However, in order to recognize an irradiation field by the method proposed in Japanese Patent Application No. 62-93633, it is necessary to find an image point in the irradiation field (preferably in a subject image). The reason is that this method is a method of recognizing an irradiation field by radially scanning from a certain point in the irradiation field to obtain irradiation field edge candidate points, and the image point which is the starting point of scanning is in the irradiation field. It is possible to obtain the irradiation field edge for the first time, and even when the radiation starts scanning from the so-called plain area where the radiation directly enters the recording sheet without penetrating the subject even in the irradiation field, only the plain area is irradiated. This is because it is necessary to start scanning from an image point located in the subject image since there is a possibility that the scanning is performed.

A method is also conceivable in which the center point of the recording sheet is set as an image point in the irradiation field (subject image). For example, as shown in FIG. 5A, when the irradiation field 2 is located at the center of the recording sheet 1, the center point C of the recording sheet 1 becomes an image point in the irradiation field 2, and as described above, in most cases, the subject image 3 are the image points.

However, when the irradiation field is deviated from the center C of the recording sheet 1 as shown in FIG.
Cannot be determined. In the case where the photographing is performed without using the irradiation field stop, all the points including the center point of the recording sheet 1 are image points in the irradiation field 2 as shown in FIG. 5C. Is not used, and the probability that the subject image 3 is not substantially at the center of the irradiation field 2 (in this case, the same as the center C of the recording sheet 1) is determined by using the irradiation field aperture as shown in FIGS. 5A and 5B. Therefore, the center point C of the recording sheet is often not an image point in the subject image.

In addition, obtaining the image points in the subject image is not only recognizing the irradiation field as described above, but also, for example, when reproducing and outputting a part of the radiation image as a visible image by enlarging it, This is also effective when determining whether to output a visible image centered on.

In view of the above circumstances, an object of the present invention is to provide a method of obtaining an image point in a subject image of a radiation image including a subject image obtained by shooting with or without using an irradiation field diaphragm. Is what you do.

(Means for Solving the Problems) One of the methods for determining an image point in a subject image according to the present invention is that each pixel on a recording sheet such as a stimulable phosphor sheet or a photographic film on which a radiation image including the subject image is recorded. The image signal value corresponding to each pixel or the reciprocal thereof is weighted to obtain the center of gravity of the recording sheet based on the image signal obtained from the image signal, and the image signal at the center of gravity represents an image representing a substantial image. It is characterized in that it is determined whether or not the signal is within the range of the signal, and if so, the center of gravity is set as an image point in the subject image.

Another method of determining an image point in a subject image according to the present invention is to obtain an image signal in the same manner as described above, and then, based on this image signal, an image signal value corresponding to each pixel or this image signal value. When the reciprocal of is associated with each corresponding pixel, an image signal value or the reciprocal of the image signal value or the reciprocal thereof is accumulated in each of the two directions on the recording sheet to obtain a cumulative distribution. For each of the cumulative distributions, a coordinate point in each direction corresponding to a value approximately half of the maximum cumulative value is obtained, and a position on the recording sheet determined by these coordinate points is set as a temporary image point in the subject image, It is determined whether or not the image signal at the temporary image point falls within the range of the image signal representing the substantial image, and if so, the temporary image point is determined as an image point in the subject. It is an feature.

In the above two methods, the following method can be used as to which of the image signal value and its reciprocal is used. That is, after obtaining an image signal, based on the image signal, a first representative value representing an image signal value corresponding to a peripheral portion of the recording sheet, and an image signal corresponding to all or substantially the central portion of the recording sheet. Calculating a second representative value representing the value, comparing the first representative value with the second representative value, and selecting one of the image signal value and the reciprocal thereof according to the comparison result. Can be.

Here, the "image signal obtained from each pixel on the recording sheet" includes, for example, an image signal obtained by photoelectrically reading the stimulable light emitted from the stimulable phosphor sheet or a photographic film. Or an image signal obtained by photoelectrically reading light reflected from a photographic film.
However, the present invention is not necessarily limited to these.

The “image signal value” refers to a value of each pixel of the image signal.

Further, the “first representative value representing the image signal value corresponding to the peripheral portion of the recording sheet” does not necessarily need to be based on all the image signals corresponding to the peripheral portion of the recording sheet. The value may vary within a range that can be considered to be representative of the value. That is, the first representative value may be obtained based on the image signal corresponding to a part of the peripheral portion within a range that can be regarded as representing the image signal value corresponding to the peripheral portion.

Further, the "second representative value representing the image signal value corresponding to the entire or substantially central portion of the recording sheet" is necessarily the same as the peripheral portion, but not necessarily the entire image signal corresponding to the entire recording sheet or substantially the central portion. It is not necessary to use the value based on the image signal value, and the value may vary within a range that can be regarded as representing the image signal value corresponding to the entire or substantially central portion of the recording sheet. That is, the second representative value may be obtained based on an image signal corresponding to an area having a predetermined spread around substantially the entire area or substantially the center of the recording sheet.

The first representative value and the second representative value are, for example, an average value, a median value, and (maximum value +
(Minimum value) / 2, an image signal corresponding to a predetermined cumulative value obtained from a cumulative histogram (see graph B in FIG. 2C described later) plotted by adding the number of image signals from the image signal having a smaller value. Are used, but the first representative value and the second representative value need not be obtained by the same calculation method (for example, both are average values), and appropriate representative values are selected respectively.

Further, the "range of the image signal representing the substantial image"
Means, for example, in the histogram of the image signal value corresponding to each pixel, a predetermined ratio from the upper limit (Smax) and the lower limit (Smin), for example, 10% to 10% of the range of the entire histogram.
It can be set as a range between two values set inside by 20%.

(Operation) One of the image point determining methods in the subject image according to the present invention is to determine the center of gravity of the recording sheet by weighting each corresponding pixel with the image signal value corresponding to each pixel or the reciprocal of the image signal value thereof. This is to determine whether or not the image signal value at the center of gravity falls within the range of the image signal representing the substantial image. Therefore, the center of gravity is determined as an image point in the subject image with a sufficiently high probability for practical use. Can be

Further, another method of determining an image point in a subject image according to the present invention is such that when an image signal value corresponding to each pixel or a reciprocal of this image signal value is associated with each corresponding pixel, the method differs from each other on a recording sheet. For each of the two directions, the image signal value or the reciprocal is accumulated in each of the directions to obtain a cumulative distribution, and a cumulative distribution is obtained. For each of these cumulative distributions, a direction corresponding to a value approximately half of the maximum cumulative value is obtained. Is determined, and further, it is determined whether or not the image signal value at the coordinate point falls within the range of the image signal representing the substantial image. Therefore, on the recording sheet determined by these coordinate points The position is obtained as an image point in the subject image with a practically high probability, similarly to the center of gravity.

Here, in the above two methods, regarding whether to use the image signal value or the reciprocal, the irradiation field aperture is used at the time of imaging and the radiation image is recorded only in a narrow range of the recording sheet, Alternatively, it differs depending on whether a radiation image is recorded over a wide range of the recording sheet without using or using the irradiation field stop, and whether the radiation image recorded on the recording sheet is a so-called negative image or positive image. For this reason, it is conceivable to directly use these pieces of information and select one of the image signal value or the reciprocal each time shooting or reading is performed, but a system that handles only one of the negative image and the positive image, or a recording sheet Such as a system using a stimulable phosphor sheet,
In a system where there is no distinction between negative and positive, a first representative value representing the image signal value corresponding to the peripheral portion of the recording sheet and an image signal value corresponding to the entire or substantially central portion of the recording sheet are represented. By obtaining a second representative value and comparing the magnitude of the first representative value with the second representative value, a radiation image is used and a radiation image is recorded only in a narrow area of the recording sheet. It is possible to determine whether the radiation image is recorded over a wide range of the recording sheet without using or using the irradiation field stop, and according to the comparison result, the image signal value or the reciprocal is used. And using the selected image signal value or the reciprocal, an image point in the subject image can be determined by any of the methods described above.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of an embodiment of a radiation image reading and reproducing apparatus using an example of the method for determining an image point in a subject image according to the present invention. This radiation image reading and reproducing apparatus is an apparatus using the above-described stimulable phosphor sheet.

The stimulable phosphor sheet 11 on which the radiation image is recorded is set at a predetermined position of the reading means 100. The stimulable phosphor sheet 11 set at the predetermined position is conveyed (sub-scanning) in the direction of arrow Y by 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
Rotating polygon mirror 18 driven by 24 and rotating at high speed in the direction of the arrow
After passing through a converging lens 19 such as an fθ lens, the optical path is changed by a mirror 20 so that the sheet 11
And an arrow X substantially perpendicular to the sub-scanning direction (arrow Y direction)
Main scan in the direction. From the portion of the sheet 11 irradiated with the light beam 17, a stimulating luminescent light 21 having an amount corresponding to the radiation image information stored and recorded is diverged, and the stimulating luminescent light 21 is guided by a light guide 22, Photoelectrically detected by a photomultiplier (photomultiplier tube) 23. The light guide 22 is formed by molding a light-guiding material such as an acrylic plate, and is arranged such that a linear incident end face 22a extends along a main scanning line on the stimulable phosphor sheet 11. The photomultiplier 2 is formed on the emission end face 22b formed in an annular shape.
The three light receiving surfaces are connected. Light guide from entrance end face 22a
The stimulated emission light 21 entering into the light guide 22 repeats total reflection inside the light guide 22 and is emitted from the emission end face 22b to be received by the photomultiplier 23, and the stimulated emission light 21 representing a radiation image is emitted. Is converted by the photomultiplier 23 into an electric signal.

The analog output signal S output from the photomultiplier 23 is logarithmically amplified by the log amplifier 26, and the A / D converter 2
Digitization is performed in step 7, and an image signal _SQ is obtained. The obtained image signal _SQ is temporarily stored in the storage means 28 and then read out by the image processing means 29.

In the image processing means 29, an image point in the subject image is obtained, and irradiation is performed based on image signals corresponding to pixels along a plurality of radial directions from the image point to the end of the stimulable phosphor sheet 11. A plurality of contour points considered to be on the contour of the field are obtained, and a region surrounded by a line along these contour points is recognized as an irradiation field. When the irradiation field is recognized, appropriate image processing is performed on the image signal S _Q corresponding to the irradiation field.

The image signal S _Q, which has been subjected to image processing is sent to the playback unit 30, the radiation image is reproduced recorded based on the reproduction unit 30 in the image signal S _Q.

Here, it performed by the image processing unit 29, further describes a method of determining based on an image point in an object image into an image signal S _Q.

In the image processing unit 29, similarly to the case of representing the image signal may represent an image signal value for the image signal value S _Q (Easy S _Q
Is used. ) Or respectively by weighting each pixel corresponding with the inverse 1 / S _Q of the image signal values, the stimulable phosphor sheet
Eleven centroids are obtained, and these centroids are used as image points in the subject image.

To do this calculation, first, whether to perform an arithmetic operation using the image signal value S _Q, or it is necessary to determine whether to perform an arithmetic operation using the reciprocal 1 / S _Q. In the system shown in this embodiment, there is no distinction between the negative image and the positive image described above.
Since the stimulated emission light of the light amount substantially proportional to the radiation energy stored in the stimulable phosphor sheet 11 is obtained, the image signal value S _Q or the reciprocal 1 / S _Q is used, An irradiation field diaphragm is used during imaging, and a radiation image is recorded only in a narrow area of the stimulable phosphor sheet 11, or the stimulable phosphor sheet 11 is used without or with the irradiation field diaphragm. Is determined based on whether a radiation image is recorded over a wide range of The information on the irradiation field may be input to the image processing means 29 by an input means such as a keyboard (not shown) each time an image is taken, but in the present embodiment, the image signal value is generated by the apparatus itself by a method described below. Use S _Q or reciprocal of the above
It is determined whether 1 / _SQ is used.

FIG. 2A is a view showing a state in which radiation is irradiated only to a narrow area on the stimulable phosphor sheet 11 using the irradiation field stop, and FIG. 2B is a view showing the state without using the irradiation field stop. FIG. 6 is a diagram illustrating a state where radiation is applied to the entire surface of the stimulable phosphor sheet.

Area outside the irradiation field 12 shown in Figure 2A, the radiation because the poorly illuminated, image signals S _Q obtained from each pixel of the region has a very small value. Fig. 2A,
In the irradiation field 12 shown in FIG. 2B, a region other than the subject image 13 is a region where the radiation is directly irradiated on the stimulable phosphor sheet 11 without passing (transmitting or reflecting) through the subject (a so-called plain portion). ), and the image signal S _Q obtained from the pixels of this region has a very large value. Image signals S _Q obtained from the pixels corresponding to the object image 13, since it corresponds to a region where the radiation after having passed through the subject is irradiated to the stimulable phosphor sheet, perforated intermediate values doing.

2A and 2B, a first representative value representing the area 14 is obtained based on the image signals corresponding to the respective pixels of the peripheral portion 14. As shown in FIG. As the first representative value, an average value of the image signals corresponding to the area 14 is adopted. Further, based on an image signal corresponding to the entirety of the stimulable phosphor sheet 11, a second representative value representing the entire radiation image of this sheet 11 is obtained.

A method for obtaining the second representative value will be described with reference to FIG. 2C.

The 2C Figure is a graph showing an example of a histogram of the image signal value S _Q corresponding to the entire surface of the sheet 11 (Graph A) and the cumulative histogram (graph B). The horizontal axis is the image signal value S _Q
The vertical axis represents the image signal S _Q for the graph A.
The graph B shows the frequency of occurrence of each of the values, and the cumulative value (%) obtained by sequentially accumulating the frequency of the graph A.

In this manner, a cumulative histogram as shown in FIG. 2C is obtained in order to obtain the second representative value, and the image signal value S ₃₀ corresponding to the cumulative value (%) 30% is obtained from the cumulative histogram.
Is required, this S ₃₀ is a second representative value.

When the first representative value and the second representative value are obtained in this way, the magnitude of the two representative values is determined. The first representative value is the image signal S _Q corresponding to the area 14 as described above.
Is the average value. In the case of the radiation image shown in FIG. 2A, since the region 14 is a region in which radiation is blocked by the irradiation field stop, the average value of the image signal corresponding to this region 14 shows a small value. In the case of the radiation image shown in FIG. 2B, since the area 14 is an area directly irradiated with radiation without passing through (passing or reflecting) the subject, the average value of the image signal corresponding to this area 14 is obtained. Indicates a large value. In addition, as described above, a value approximately at the center between the first representative value corresponding to FIG. 2A and the first representative value corresponding to FIG. 2B is selected as the second representative value.

Therefore, the first representative value is compared with the second representative value, and when the first representative value <the second representative value, the irradiation field diaphragm is used at the time of photographing, and the stimulable phosphor sheet 11 corresponds to the case where among the radiation image only in a narrow range has been recorded, in this case the determination of the effect to be obtained an image point in an object image by using the image signal value S _Q as described later is performed. When the first representative value is larger than the second representative value, a radiation image is recorded over a wide range of the stimulable phosphor sheet 11 even when the irradiation field diaphragm is not used or the irradiation field diaphragm is used. corresponds to a case where there, in this case the determination of the effect to be obtained an image point in an object image by using the reciprocal 1 / S _Q of the image signal value S _Q as described later is performed.

Figures 3A, Figure 3B, the Figure 2A is a diagram showing an example with a radiation image of the graph of 2B image signal values respectively corresponding to Figure S _Q, the inverse 1 / S _Q.

FIG. 3A corresponds to FIG. 2A and shows a radiation image in which the irradiation field 12 is formed only in a narrow area of the stimulable phosphor sheet 11. The plot of the image signal value S _Q corresponding to each pixel along a straight line in this case, as shown in the graph C, the image signal corresponding to each pixel in the radiation field 12 has a large value. Therefore, in this case, by obtaining the center of gravity by weighting each pixel corresponding with the image signal value S _Q, the center of gravity is obtained as an image point of the temporary in the irradiation field 12, and stop the irradiation field as described above Is used for photographing only a necessary portion of the subject, and therefore, the center of gravity is obtained as a temporary image point in the subject image 13 with practically sufficient accuracy.

FIG. 3B corresponds to FIG. 2B and shows a radiation image in which the entire surface of the stimulable phosphor sheet 11 corresponds to the irradiation field 12.
In this case, the image signal value corresponding to each pixel along the straight line
Plotting the reciprocal 1 / S _Q of S _Q, as shown in the graph D, the reciprocal 1 / S _Q corresponding to each pixel in the subject image has a large value. Therefore, in this case, by obtaining the center of gravity by weighting each pixel corresponding with the reciprocal 1 / S _Q, the center of gravity is determined as the image point of the temporary in the object image.

Here, to obtain the center of gravity by weighting each pixel corresponding with the image signal value S _Q or its inverse 1 / S _Q refers to performing an operation described below.

The coordinates of pixels arranged in the x direction are represented by i (positive integer), and the coordinates of pixels arranged in the y direction are represented by j (positive integer).
the image signal values corresponding to the j) S _Q or its inverse 1 / S _Q expressed by P (i, j). Also, And the centroids in the x and y directions are x _c and y _c , , The coordinates (x _c , y _c ) of the center of gravity are obtained.

The calculation performed by the image processing means 29 shown in FIG. 1 is replaced with the above-described calculation for obtaining the center of gravity, and the image signal value _SQ or its reciprocal 1 / _SQ is used to correspond to each of the pixels corresponding thereto. when the said for each of two different directions on the recording sheet (where x and y directions), the image signal value S _Q or its inverse 1 / S _Q in the x-direction, the cumulative respectively the y-direction The plotted cumulative distributions are obtained, and coordinate points in the x and y directions corresponding to approximately half of the maximum cumulative value for each of these cumulative distributions.
x _c ′, y _c ′ may be obtained. In this case, a position (x _c ′, y _c ′) on the stimulable phosphor sheet 11 determined by these coordinate points x _c ′, y _c ′ is obtained as a temporary image point in the subject image.

FIG. 4 is a diagram illustrating an example of a distribution of image signal values in the x and y directions and a cumulative distribution of those distributions for explaining the calculation.

The image signal value S _Q or its inverse 1 / S _Q selected by the above-described method so as to correspond to each pixel (i, j) expressed by P (i, j).

At this time, the graph E is Is plotted in the x direction (i direction), graph G
Is Is plotted in the y direction (j direction), and graphs F and H are obtained by accumulating Py (i) and Px (j) shown in graphs E and G in the x direction and y direction, respectively. Values (%). That is, graph F and graph G are respectively Is shown.

Here, the coordinate points x _c ′ and y _c ′ in the x and y directions are respectively Is required. The coordinate points x _c ′, y _c ′ are the points x _c ′, y _c ′ at which the graph F and the graph H in FIG. 4 reach 50%.

By determining the coordinate point (x _c ′, y _c ′) in this way, the coordinate point (x _c ′, y _c ′) is used as a temporary image in the subject image in the same manner as when the center of gravity is determined. Points.

After the provisional image point in the subject image is obtained in this manner, it is determined whether or not this provisional image point is a true image point by determining that the density at that image point falls within the range of the density of the substantial image. It is determined by looking at whether or not the That is, it is determined whether or not the image signal at the temporary image point falls within the range of the image signal representing the substantial image. This determination is made, for example,
In the histogram of FIG. 2C, the maximum value (Sma
15% smaller Th ₁ from x), the minimum value (whether a comparator or the like contains the value of the image signal in the image points of the provisional range sandwiched from Smin) to the 20% larger value Th ₂ The temporary image point is determined to be an image point in the subject image if it is used and included.

After the image points in the subject image have been obtained in this way, as described above, the image points corresponding to the pixels along a plurality of directions on the radiation from the image points toward the end of the stimulable phosphor sheet 11 are obtained. Based on the image signal to be obtained, a plurality of contour points considered to be on the contour of the irradiation field are obtained, and a region surrounded by a line along these contour points is recognized as the irradiation field, and the area corresponding to this irradiation field is recognized. appropriate image processing is performed on the image signal S _Q.

In the above embodiment, the case where the obtained image point in the subject image is used for recognition of the irradiation field has been described. However, obtaining the image point in the subject image is not used only for recognition of the irradiation field. For example, when a visible image is obtained by the reproducing means 30 shown in FIG. 1, the image processing means 29 obtains an image point in the subject image as described above, so that the subject image is located at the end of the stimulable phosphor sheet 11. Even if it is recorded, it is possible to obtain a visible image in which the subject image is arranged at the center.

In the above embodiment, the radiation image reading apparatus that does not perform prefetching is described. However, prefetching is performed to obtain a prefetching image signal, and an irradiation field is obtained based on the prefetching image signal. It is needless to say that the method for determining an image point in a subject image according to the present invention can also be used in a system for obtaining reading conditions at the time of main reading based on a read image signal.

In addition, the present invention can be used not only for an apparatus using a stimulable phosphor sheet but also for an apparatus using a conventional X-ray film.

FIG. 6 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.

An X-ray film conveying means 41 on which an X-ray image including a subject image set at a predetermined position is recorded is conveyed in the direction of arrow Y 'shown in the figure.

Further, the reading light 43 emitted from the light source 42 extending one-dimensionally long is converged by the cylindrical lens 44 and
The linear film is irradiated linearly 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 is located at a position where the reading light 43 which has passed through the X-ray film 40 and is intensity-modulated by the X-ray image recorded on the X-ray film 40 is received. MO in which a large number of solid-state photoelectric conversion elements corresponding to each pixel interval in the X 'direction of a line image are linearly arranged
An S sensor 45 is provided. The MOS sensor 45 converts the reading light transmitted through the X-ray film 40 into X-rays 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. 7 is a circuit diagram showing an equivalent circuit of the MOS sensor 45.

A signal by a photocarrier generated when the reading light 43 irradiates a large number of solid-state photoelectric conversion elements 46 is a signal generated by the solid-state photoelectric conversion elements 46.
Are stored in capacitors Ci (i = 1, 2,..., N). The stored photocarrier signals are sequentially read out by sequentially opening and closing the switch unit 48 controlled by the shift register 47, whereby time-sequential image signals are 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. Thereafter, based on the image signal, an image point in the subject image is determined in the same manner as in the above-described embodiment. Is used to recognize the irradiation field. In this embodiment, it is needless to say that a CCD, a CPD (Charge Priming Device) or the like can be used instead of the MOS sensor 45. In reading an X-ray film, it is a matter of course that reading may be performed by two-dimensionally scanning with a light beam similarly to the above-described reading of the stimulable phosphor sheet. In the above embodiment, the light transmitted through the X-ray film 40 is received. However, it is needless to say that the light reflected from the X-ray film 40 can be received.

As described above, the method for determining an image point in a subject image according to the present invention includes:
The present invention can be applied to a general radiation image reading / reproducing apparatus that reads light representing a radiation image obtained from a recording sheet on which a radiation image of a subject is recorded, obtains an image signal, and reproduces and outputs a radiation image based on the image signal. .

(Effects of the Invention) As described in detail above, one of the image point determining methods in the subject image according to the present invention is to weight each pixel corresponding to each pixel by an image signal value corresponding to each pixel or a reciprocal thereof and to a recording sheet. The center of gravity of the subject is determined, and it is determined whether or not the image signal at the center of gravity falls within the range of the image signal representing the substantial image. , An image point in the subject image can be obtained. Another method of determining an image point in a subject image according to the present invention is such that when an image signal value corresponding to each pixel or a reciprocal of the image signal value is associated with each corresponding pixel, each other on the recording sheet. For each of the two different directions, the image signal value or the reciprocal is accumulated in each direction to obtain a cumulative distribution, and a cumulative distribution is obtained for each of these directions. Are determined based on whether or not the corresponding image signal falls within the range of the substantial image signal in the same manner as in the above-described method. The position is obtained as an image point in the subject image with high precision, similarly to the center of gravity.

Further, when each of the above methods is applied to a system that handles only one of a negative image and a positive image, a system that uses a stimulable phosphor sheet as a recording sheet, and a system in which there is no distinction between negative and positive, a recording method is used. A first representative value representing the image signal value corresponding to the peripheral portion of the sheet and a second representative value representing the image signal value corresponding to the entire or substantially central portion of the recording sheet are obtained. By comparing the magnitude of the representative value with the second representative value, the radiation field diaphragm is used and the radiation image is recorded only in a narrow area of the recording sheet, or without using the radiation field diaphragm or It is possible to determine whether a radiation image is recorded over a wide range of the recording sheet even when used, and select one of the image signal value or the reciprocal according to the comparison result. The selected the image signal values or the reciprocal can be defined image point in an object image by one of the methods described above with reference.

As described above, among the methods of determining the image point in the subject image using the position of the center of gravity, the method of using the position of the center of gravity obtained by weighting with the image signal value is a large irradiation field aperture (the irradiation field is small). The method of using the position of the center of gravity obtained by weighting with the reciprocal of the image signal value is performed when a small irradiation field aperture (the irradiation field is large) or no irradiation field aperture is performed. The coordinate points corresponding to about half of the maximum cumulative value in each cumulative distribution when the image signal value or the reciprocal thereof is accumulated in each of two different directions. The method of determining the image point in the subject image using the position determined by the above can be suitably used, for example, when the aperture shape is complicated.

Even when the blank portion is located at the center, if there is some variation in the image signal value distribution (density distribution) in the surrounding area (unless the image signal value distribution in the surrounding area is uniform) ), A position determined by a coordinate point corresponding to the position of the center of gravity or a value approximately half of the maximum cumulative value can also be located in the subject image. Further, when the image signal distribution around the blank portion is uniform, the position of the center of gravity determined by the coordinate point corresponding to about half of the maximum cumulative value is also located in the blank portion. However, even in that case, according to the present invention, when it is determined whether or not the image signal at those positions falls within the range of the image signal representing the substantial image, the positions are included in the subject image. Since the image point is used as an image point, if the position is located within the blank portion, the image signal at that position does not fall within the range of the image signal that substantially represents the image, so that the position is determined by the above determination. It is unlikely that the image point in the subject image will be determined as an image point in the subject image.

[Brief description of the drawings]

FIG. 1 is a perspective view of an embodiment of a radiation image reading and reproducing apparatus using an example of a method for determining an image point in a subject image according to the present invention. FIG. 2A is a view showing only a small area on a stimulable phosphor sheet. FIG. 2B is a diagram showing a state where the radiation is irradiated, FIG. 2B is a diagram showing a state where the radiation is irradiated over the entire surface of the stimulable phosphor sheet, and FIG. 2C is an image corresponding to the entire surface of the stimulable phosphor sheet. graph showing an example of a signal S _Q cumulative histogram and the histogram (graph a) of (graph B), figures 3A, Figure 3B, the image signal value S _Q, an example of a graph of the reciprocal 1 / S _Q, radiation FIG. 4 shows an example of the distribution of image signal values in the x and y directions and the cumulative distribution of those distributions. FIG. 5A shows an irradiation field at the center of the recording sheet. Figure 5B shows the radiation image. FIG. 5C shows a radiation image in which the entire recording sheet is an irradiation field by photographing without using an irradiation field diaphragm. FIG. 5C shows an X-ray film. FIG. 7 is a perspective view of an embodiment of an X-ray image reading apparatus for reading a recorded X-ray image. FIG. 7 is a circuit diagram showing an equivalent circuit of a MOS sensor. 1 ... recording sheet, 2, 12 ... irradiation field 3,13 ... subject image, 11 ... stimulable phosphor sheet 14 ... peripheral part, 12 ... stimulating luminescence 23 ... photomultiplier 26 ... ... Log amplifier, 27 ... A / D converter 28 ... Storage means, 29 ... Image processing means 30 ... Reproduction means, 40 ... X-ray film 45 ... MOS sensor

Claims (3)

(57) [Claims] An image signal obtained from each pixel on a recording sheet on which a radiation image including a subject image is recorded,
The image signal value corresponding to each pixel or the reciprocal of the image signal value weights each corresponding pixel to determine the center of gravity of the recording sheet, and the image signal at this center of gravity falls within the range of the image signal representing a substantial image. A method for determining an image point in a subject image, wherein it is determined whether or not the subject image is included, and when the subject is included, the center of gravity is set as an image point in the subject image. 2. A method according to claim 1, wherein said image signal is obtained from each pixel on a recording sheet on which a radiation image including a subject image is recorded.
When the image signal value corresponding to each pixel or the reciprocal of the image signal value is associated with each corresponding pixel,
For each of the two different directions on the recording sheet, an image signal value or a reciprocal of the image signal value is accumulated in each of the directions to obtain a cumulative distribution, and a maximum cumulative value of each of these cumulative distributions is obtained. Coordinate points in each of the directions corresponding to substantially half the values are obtained, and a position on the recording sheet determined by these coordinate points is set as a temporary image point in the subject image, and an image signal at the temporary image point is obtained. Is determined to be within the range of an image signal representing a substantial image, and if so, the tentative image point is set as an image point within the subject. Method. 3. After obtaining the image signal, a first representative value representing the image signal value corresponding to a peripheral portion of the recording sheet and all or substantially all of the recording sheet based on the image signal. A second representative value representing the image signal value corresponding to the central portion is obtained, the magnitudes of the first representative value and the second representative value are compared, and the image signal value or the second representative value is determined according to the comparison result. 3. An image point in the subject image is determined by selecting one of the reciprocals of the image signal value and using the image signal value or the reciprocal of the image signal value thus selected. The method for determining an image point in a subject image described in the above.