JPH0690414B2 - Irradiation field recognition method and image processing condition determination method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention recognizes an irradiation field when radiation image information is recorded by narrowing the irradiation field on a recording medium such as a stimulable phosphor sheet. The present invention relates to a method and a method for determining an image processing condition for processing an image signal read from the recording medium, the method using the irradiation field recognition method.

(Prior Art) When a certain kind of phosphor is irradiated with radiation (X-rays, α-rays, β-rays, rays, electron beams, ultraviolet rays, etc.), a part of this radiation energy is accumulated in the phosphor and It is known that when a body is irradiated with excitation light such as visible light, the phosphor emits stimulated emission depending on the stored energy, and a phosphor having such a property is called a stimulable phosphor.

Using this stimulable phosphor, the radiation image information of a subject such as a human body is once recorded on a sheet-shaped stimulable phosphor, and then the stimulable phosphor sheet is scanned with excitation light such as laser light. The stimulated emission light is generated, the stimulated emission light is photoelectrically read to obtain an image signal, the image signal is subjected to image processing, and a radiation image of the subject is photographed based on the image signal subjected to the image processing. The applicant has already proposed a radiation image information recording / reproducing system for outputting a visible image on a recording material such as a photosensitive material or a display device such as a CRT.
(JP-A-55-12429, JP-A-56-11395, etc.) In addition, in the above system, in order to improve the observation and interpretation suitability of a visible image, when the above stimulated emission light is photoelectrically read, It is desirable to perform the reading based on the optimum reading condition determined according to the captured image. From this viewpoint, as one aspect of the above system, a stimulable phosphor sheet in which radiation image information of a subject is stored and recorded. Scanning with excitation light, prior to the "main reading" to obtain an electrical image signal for reproducing the diagnostic visible image by reading the stimulated emission light emitted from the sheet by this scanning by photoelectric reading means, Pre-reading is performed by scanning the sheet with excitation light of a lower level than the excitation light used for the main reading in advance and reading the outline of the image information accumulated and recorded on this sheet. The reading conditions when performing the main reading are determined based on the obtained image signal, the main reading is performed according to the reading conditions, and the image signal obtained by the main reading is input to the image processing means, and the image processing means There is known a system in which an image signal is processed so that an output image suitable for a diagnostic purpose is obtained according to a region to be imaged, a photographing method, and the like, and the image signal is reproduced as a visible output image on a photographic light-sensitive material. This is disclosed in Japanese Patent Application Laid-Open No. 58-67240, which was filed by the applicant of the present invention and which has already been published.

Here, the reading condition is a relationship between an input and an output in the reading unit, for example, in the above, an input (amount of stimulated emission light) and an output (electrical image signal level) in the photoelectric reading unit.
Is a general term for various conditions that affect the relationship with, for example, the read gain (sensitivity) that determines the relationship between input and output,
It means a scale factor (latitude) or the power of excitation light in reading.

Further, 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 unit area during pre-reading is that during the main reading. Means less than.

In this way, by grasping the outline of the image information accumulated and recorded on the sheet in advance of the main reading and performing the main reading in accordance with the reading condition determined based on the outline of the image information, the fluctuation of the subject or the imaging region or It is possible to eliminate the inconvenience caused by the fluctuation of the radiation energy level range accumulated and recorded on the sheet due to the fluctuation of the radiation exposure amount and the like, and it is possible to always obtain a visible image excellent in observation and interpretation.

(Problems to be Solved by the Invention) However, in the above system, in order to prevent radiation from being applied to a portion that is not necessary for humanitarian diagnosis, or when radiation is applied to a portion that is not necessary for diagnosis, the diagnosis is performed from that portion. Scattered rays enter the necessary part, which lowers the contrast resolution, so that the radiation field may be narrowed at the time of recording the radiation image information (at the time of imaging) for the purpose of preventing this.

When shooting is performed with the irradiation field narrowed down in this way, there will be a portion inside the irradiation field and a portion outside the irradiation field in the stimulable phosphor sheet. In that case, where is the portion inside the irradiation field, In other words, it would be convenient to know where the contour of the irradiation field exists.

This is because, for example, when pre-reading the stimulable phosphor sheet and trying to determine the reading conditions for the main reading based on the image signal obtained by this pre-reading, when the irradiation field is narrowed This is because it is preferable to determine the reading condition based only on the pre-reading image signal within the irradiation field range.

This point will be described in more detail below. That is, as a specific method of determining the reading conditions in the main reading based on the image signal obtained by the pre-reading,
For example, the last image of the level of the image signal obtained by prefetching is obtained, and the maximum image signal level Pmax and the minimum image signal level Pmin of the desired image signal range in this histogram are obtained from this histogram, and this Pm
ax and Pmin are respectively read in the main reading so as to correspond to the maximum signal level Qmax and the minimum signal level Qmin of the desired input signal range in the image processing means determined by the maximum density Dmax and the minimum density Dmin of the appropriate density range in the visible output image, respectively. A method for determining the reading conditions has been filed by the present applicant (Japanese Patent Laid-Open No. 60-156055).

However, when imaging is performed with the radiation field narrowed down as described above, normally, scattered radiation generated from a subject in the irradiation field is incident outside the irradiation field on the stimulable phosphor sheet, resulting in high sensitivity storage. Since the phosphor sheet also accumulates and records the scattered rays, the image signal level based on the scattered rays is also included in the histogram of the preread image signal level. Since the image signal level outside the irradiation field on the sheet based on this scattered radiation may be higher than the image signal level inside the irradiation field, the image signal levels inside and outside the irradiation field should be distinguished from the obtained histogram. It is difficult. Therefore, as described above, when Pmax and Pmin are obtained from the histogram, and when the reading conditions are determined from this, the minimum value of the image signal level in the irradiation field should originally be Pmin. It may happen that the minimum value is Pmin. When the minimum value of the image signal level outside the irradiation field is set to Pmin in this way, since that value is generally lower than the minimum value of the image signal level inside the irradiation field, the scattered radiation unnecessary for diagnosis in the main reading is reduced in concentration. Therefore, the density of the image of the portion necessary for diagnosis becomes too high, and as a result, the contrast decreases and it becomes difficult to make a satisfactory diagnosis.

That is, when shooting is performed with the irradiation field narrowed down, scattered rays generated from the subject are incident on the outside of the irradiation field on the sheet, and the pre-reading image signal includes those based on the scattered rays. Even if the reading conditions are determined based on such pre-read image signals, it is difficult to determine the optimum reading conditions, and as a result, it is difficult to obtain a visible image excellent in observation and interpretation suitability.

Therefore, when trying to determine the reading conditions based on the pre-reading image signal by the above method, the exposure field is accurately recognized when the image is taken with the irradiation field stopped down, and the pre-reading image signal in the irradiation field is detected. It is desirable to determine it based on this, and to eliminate the above-mentioned adverse effects of scattered radiation outside the irradiation field.

The above is an explanation of the necessity of irradiation field recognition when deciding the reading conditions when the stimulable phosphor sheet is used for imaging, but this irradiation field recognition is not limited to such a case, and is generally applied to the recording medium. This may be necessary due to various circumstances when the radiation image information is recorded by narrowing the field.

On the other hand, in the above system, image processing is performed on the read image signal as described above. This image processing is generally performed on the basis of image processing conditions determined on the basis of the imaged region or method so that an output image suitable for a diagnostic purpose can be obtained for each image. For example, it is conceivable that the determination is made based on the image signals obtained by the pre-reading or the actual reading instead of the imaged region or the imaged method, or based on both the image signals and the imaged region / imaged method.

However, when an image is taken with the irradiation field stopped down, even if the image processing conditions are determined based on the pre-reading or main-reading image signal as described above, the information due to scattered rays outside the irradiation field is included in the image signal as described above. Since (noise) is included, it is difficult to obtain a preferable image processing condition that is expected to be obtained by initially determining the image signal.

Therefore, when the image processing condition is determined based on the image signal as described above, when the image is captured with the irradiation field stopped, it is not determined simply based on the read image signal itself, but by some method. It is desirable to make the determination based on the image signal with less noise, which is obtained by eliminating the information due to.

The problem in determining such image processing conditions is not only in the case of photographing using the above-mentioned stimulable phosphor sheet, but generally in the case where radiation image information is recorded by narrowing the irradiation field on the recording medium. It can happen.

The image processing conditions collectively refer to various conditions that affect the relationship between input and output in the image processing means, and mean, for example, gradation processing conditions and spatial frequency processing conditions.

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

In view of the above circumstances, the second object of the present invention is to eliminate scattered radiation information outside the irradiation field when the radiation image information is recorded by narrowing the irradiation field on a recording medium such as a stimulable phosphor sheet. Another object of the present invention is to provide a method for determining an image processing condition based on an image signal with less noise.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the irradiation field recognition method according to the first aspect of the present invention is a stimulable phosphor sheet on which radiation image information has been captured by narrowing down the irradiation field. An image signal is read from a recording medium such as a recording medium, and a sample image signal relating to an arbitrary pixel row extending from the end of the recording area of the recording medium toward the center is extracted from the image signal, and the image density indicated by the sample image signal is extracted. The change is sequentially calculated from the end portion toward the center side for each pixel group of L ₁ , L ₂ , L ₃ ... Ln, and the approximation formulas F ₁ , F ₂ , F ₃ ... Is expressed as
The intersections of the straight lines indicated by these approximate expressions F ₁ , F ₂ , F ₃ ... Fn and the image density change curve in the region of the predetermined density or higher indicated by the sample image signal are P ₁ , P ₂ , P ₃ ... Pn, respectively. Then, among these intersections P ₁ , P ₂ , P ₃ ... Pn, the region closer to the sheet center than the intersection closest to the end is recognized as the irradiation field.

In order to achieve the above-mentioned object, the image processing condition determining method according to the second aspect of the present invention reads an image signal from a recording medium such as a stimulable phosphor sheet on which radiation image information is photographed by narrowing the irradiation field. , A sample image signal relating to an arbitrary pixel row extending from the end of the recording area of the recording medium toward the center side is extracted from the image signal, and the image density change indicated by the sample image signal is measured from the end to the center side. For each of the pixel groups L ₁ , L ₂ , L ₃ ... Ln, the approximation formulas F ₁ , F ₂ , F ₃ ...
, And the intersections of the straight lines represented by these approximate expressions F ₁ , F ₂ , F ₃ ... Fn and the image density change curve of the predetermined density or higher region indicated by the sample image signal are respectively P ₁ , P ₂ , P ₃ ... Pn, an image read from the recording medium by recognizing an area on the sheet center side of the intersection points P ₁ , P ₂ , P ₃ ... Pn closest to the end portion as the irradiation field. The image processing condition is determined based on the recognized image signal in the irradiation field among the signals.

The "recording medium" in the above means a medium capable of recording radiation image information, and the above-mentioned stimulable phosphor sheet can be mentioned as a specific example, but the recording medium is not necessarily limited thereto.

Further, the "image signal read from the recording medium" in the above means an image signal obtained by reading the image information recorded on the recording medium by some method, for example, in the above-mentioned stimulable phosphor sheet. It means an image signal obtained by pre-reading or main reading, 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 one.

In the image processing condition determining method, it does not matter whether the image signal for recognizing the irradiation field is the same as the image signal for determining the image processing condition. For example, the irradiation field may be recognized from the main reading image signal, and the image processing condition may be determined based on the main reading image signal in the irradiation field. Alternatively, the irradiation field may be recognized from the pre-reading image signal and the pre-reading or main reading image in the irradiation field may be recognized. The image processing condition may be determined based on the signal.

Further, the above-mentioned "determining the image processing condition based on the image signal in the irradiation field" means not only determining based on such an image signal but also such an image signal and other ones, for example, the above-mentioned It is also meant to include the case where the determination is made based on the imaged part, the imaged method, and the like.

The image processing condition may be determined based on the image signal in the irradiation field, that is, any method may be used as long as the image signal is used, and the specific method is not limited at all. .

Further, as the image processing condition to be determined, the gradation processing condition can be mentioned as a typical one, but it is not necessarily limited thereto.

(Effect of the Invention) As described above, the irradiation field recognition method according to the first aspect of the present invention extracts a sample image signal relating to an arbitrary pixel row extending from the end portion of the recording area of the recording medium toward the center side, and the sample image signal is extracted. the change in image density represented by the image signal, sequentially L ₁ from the end portion toward the center side, L _2, L ₃ ... respectively for each pixel group Ln, approximate expression F _1, F consisting essentially of linear equations ₂ , F ₃
... Fn, and the intersections of the straight lines represented by these approximate expressions F ₁ , F ₂ , F ₃ ... Fn and the image density change curve of the predetermined density or higher area indicated by the sample image signal are respectively P ₁ , P _2. , P ₃ … Pn
Then, among these intersections P ₁ , P ₂ , P ₃ ... Pn, the area closer to the sheet center than the intersection closest to the end is recognized as the irradiation field.

The image density indicated by the sample image signal extracted as described above is generally relatively high in the irradiation field and changes according to the image information, and is lower in the irradiation field portion than in the irradiation field. Then, the scattered radiation from the subject is also radiated to the outside portion of the irradiation field, and the scattered dose gradually decreases as the distance from the irradiation field increases. Therefore, the image density in the outside portion of the irradiation field becomes higher toward the center side, and moreover, at the end of the recording area. In the vicinity, it is relatively gentle and linearly increases, and as it approaches the irradiation field, it is convex toward the low concentration side and rapidly increases.

Therefore, in the sample image signal, the density change in each of the pixel groups L ₁ , L ₂ , L ₃ ... expressed as y = ax + b become approximate expression consisting equations _{F 1, F 2, F 3} ... Fn, these approximate expression F _1, F _2, and the straight lines indicated by F ₃ ... Fn, predetermined showing sample image signals If the intersection points P ₁ , P ₂ , P ₃ ... Pn with the image density change curve in the area of density d _T or more are obtained, the image density becomes sharply convex toward the low density side as it approaches the irradiation field as described above. Therefore, the intersection points P ₁ , P ₂ , P ₃ ... Pn are
The pixel corresponding to the intersection of each straight line indicated by F ₁ , F ₂ , F ₃ ... Fn and the image density change curve portion carrying the image in the irradiation field, and the intersection point closest to the end of the recording area among these intersections It can be said that X ₀ is a pixel existing at the edge portion of the irradiation field. Therefore, the intersections P ₁ , P ₂ , P ₃ ... Pn are located not on the density change curve portion near the end of the recording area showing the scattered radiation but on the image density change curve portion carrying the image in the irradiation field. As described above, if the predetermined density d _T is substantially and empirically obtained in advance, by using the d _T , the area inside (intermediate side) of the intersection closest to the end of the recording area among the intersections is determined. It can be recognized as the irradiation field.

Therefore, according to the present invention configured as described above, the irradiation field can be accurately recognized.

As described above, the image processing condition determining method according to the second aspect of the present invention recognizes the irradiation field by the irradiation field recognition method according to the first aspect of the present invention, and performs image processing based on the recognized image signal in the irradiation field. It determines the conditions.

If the irradiation field is obtained and only the image signal in the irradiation field is extracted, the extracted image signal is the image signal read from the entire recording medium from which the image signal due to scattered rays outside the irradiation field is excluded, that is, such scattering. It can be said that the image signal is a true image signal that does not include line noise.

Therefore, according to the present invention, the image processing condition can be determined based on a true image signal that does not include such scattered ray noise, and as a result, an appropriate image processing condition can be determined. You can

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

First, an embodiment of the irradiation field recognition method according to the first aspect of the present invention will be described. In this embodiment, the present invention is applied to the case of recognizing an irradiation field from a preread image signal in a stimulable phosphor sheet which is photographed with a rectangular irradiation field diaphragm.

FIG. 1 is a radiation image provided with a process for recognizing an irradiation field using one embodiment of the present invention, and the irradiation field recognized by this process is used when determining reading conditions and image processing conditions. 1 illustrates an example of an information recording / reproducing system. This radiation image information recording / reproducing system basically includes a radiation image capturing unit 20 and a read-ahead reading unit 3
0, a main reading reading unit 40, and an image reproducing unit 50. In the radiation image capturing unit 20, radiation 102 is emitted from a radiation source 100 such as an X-ray tube toward a subject (examinee) 101. At the position where the radiation 102 transmitted through the subject 101 is irradiated, the stimulable phosphor sheet 103 for accumulating the radiation energy is arranged as described above, and the transmitted radiation of the subject 101 is placed on the stimulable phosphor sheet 103. Image information is accumulated and recorded. Between the radiation source 100 and the subject 101, a diaphragm 10 that narrows down the irradiation field of the radiation 102.
4 are arranged.

The stimulable phosphor sheet 103 on which the radiation image information of the subject 101 is recorded in this manner is sent to the prereading reading unit 30 by the sheet transfer means 110 such as a transfer roller. The laser light 202 emitted from the laser light source 201 for pre-reading in the pre-reading reading section 30 passes through the filter 203 that cuts the wavelength region of the stimulated emission light emitted from the stimulable phosphor sheet 103 by the excitation of this laser light 202. After that, it is linearly deflected by an optical deflector 204 such as a galvanometer mirror, and is incident on the stimulable phosphor sheet 103 via a plane reflecting mirror 205. Here, the laser light source 201 is selected so that the wavelength range of the laser beam 202 as the excitation light does not overlap with the wavelength range of the stimulated emission light emitted by the stimulable phosphor sheet 103. On the other hand, the stimulable phosphor sheet 103 is transferred in the direction of arrow 206 by the sheet transfer means 210 such as a transfer roller to be sub-scanned, and as a result, the entire surface of the stimulable phosphor sheet 103 is irradiated with the laser beam 202. . Here, the emission intensity of the laser light source 201, the beam diameter of the laser light 202, the scanning speed of the laser light 202, the transfer speed of the stimulable phosphor sheet 103, the energy of the pre-read excitation light (laser light 202),
It is selected to be smaller than that of the main reading performed by the main reading reading unit 40 described later.

When the laser light 202 is irradiated as described above, the stimulable phosphor sheet 103 emits stimulated emission light of a light amount corresponding to the radiation energy stored and recorded in the stimulable phosphor sheet 103. Incident on. The stimulated emission light is guided through the light guide 207, emitted from the emission surface, and received by a photodetector 208 such as a photomultiplier.
On the light receiving surface of the photodetector 208, a filter that transmits only light in the wavelength range of stimulated emission light and cuts light in the wavelength range of excitation light is attached, and detects only stimulated emission light. It is possible. The detected stimulated emission light is converted into an electric signal (image signal) carrying stored and recorded information, and the amplifier 209
Is amplified by. The signal output from the amplifier 209 is A / D
The pre-read image signal Sp is digitized by the converter 211.
Is input to the main reading control circuit 314 of the main reading reading unit 40 and the irradiation field recognition circuit 220 described in detail later. The main reading control circuit 314 determines the read gain a, the recording scale factor b, and the image processing condition c based on the preread image signal in the irradiation field recognized by the irradiation field recognition circuit 220 among the preread image signals Sp.

The stimulable phosphor sheet that has been read ahead as described above
103 is transferred to the reading unit 40 for main reading. Book reader 4
At 0, the laser light 302 emitted from the main reading laser light source 301 passes through the filter 303 that cuts the wavelength region of the stimulated emission light emitted from the stimulable phosphor sheet 103 by the excitation of the cono laser light 302, and then the beam expander. The beam diameter is strictly adjusted by 304, is linearly deflected by an optical deflector 305 such as a galvanometer mirror, and the stimulable phosphor sheet 103 is passed through a plane reflecting mirror 306.
Incident on. An fθ lens 307 is arranged between the light deflector 305 and the plane reflecting mirror 306 so that the beam diameter of the laser light 302 scanning the stimulable phosphor sheet 103 becomes uniform. On the other hand, the stimulable phosphor sheet 103 is transferred in the direction of arrow 308 by the sheet transfer means 320 such as a transfer roller to be sub-scanned, and as a result, the stimulable phosphor sheet 103.
Is irradiated with laser light over the entire surface. When the laser light 302 is thus irradiated, the stimulable phosphor sheet 103 emits stimulated emission light of a light amount corresponding to the radiation energy stored and recorded in the stimulable phosphor sheet 103.
It is incident on 309. The stimulated emission light guided by repeating the total reflection in the main reading light guide 309 is emitted from the emission surface and is received by the photodetector 310 such as a photomultiplier. A filter that selectively transmits only the wavelength region of the stimulated emission light is attached to the light receiving surface of the photodetector 310,
The photodetector 310 detects only stimulated emission light.

The output of the photodetector 310, which photoelectrically detects the stimulated emission light indicating the radiation image recorded on the stimulable phosphor sheet 103, is appropriate by the amplifier 311 based on the reading gain a determined by the control circuit 314. It is amplified to a level electric signal. The amplified electric signal is input to the A / D converter 312, is also converted into a digital signal having an appropriate signal level width based on the recording scale factor b determined by the control circuit 314, and is input to the signal processing circuit 313. It In the signal processing circuit 313, the digital signal is subjected to signal processing (image processing) on the basis of the image processing condition c determined by the control circuit 314 so that a radiographic image excellent in observation and interpretation suitability can be obtained.

The read image signal (main read image signal) So output from the signal processing circuit 313 is input to the optical modulator 401 of the image reproducing unit 50. In the image reproducing section 50, the recording laser light source
The laser light 403 from 402 is modulated by the optical modulator 401 based on the main reading image signal So input from the signal processing circuit 313, deflected by the scanning mirror 404, and scanned on the photosensitive material 405 such as photographic film. . And photosensitive material 4
03 is transported in a direction (arrow 406 direction) orthogonal to the scanning direction in synchronization with the scanning, and a radiation image based on the main reading image signal So is output on the photosensitive material 405. As a method for reproducing the radiation image, various methods such as the above-described CRT display can be adopted in addition to such a method.

Here, when the radiation image information is stored and recorded (photographed) on the stimulable phosphor sheet 103, the aperture 104 is operated to apply an illumination field aperture as shown in FIG. An irradiation field (image recording portion) 103B is formed in a part of the recording area 103A. Then, as described above, the irradiation field recognition circuit 220 recognizes this radiation irradiation field 103B based on the look-ahead image signal Sp, sends information St indicating the irradiation field 103B to the control circuit 314, and this control circuit 314 performs the irradiation. Field information St
The read gain a, the recording scale factor b, and the image processing condition c are determined based on the preread image signal Sp in the irradiation field 103B.

Hereinafter, the irradiation field recognition by the irradiation field recognition circuit 220 will be described in detail. This irradiation field recognition circuit 220 is an A / D converter
Arbitrary pixels extending from the end portion of the recording area of the stimulable phosphor sheet 103 toward the center side, as indicated by the X ₁ -X ₁ line in FIG. 2A, from the entire pre-read image signal Sp input from the 211. An image signal for a column (for example, a pixel column in the main scanning direction) is extracted. The change in the image density d indicated by the sample image signal thus extracted is generally as shown in FIG. That is, the inside of the irradiation field 103B has a relatively high density, and changes corresponding to the image information are shown. The area outside the edge portion of this irradiation field 103B, that is, the outside area of the irradiation field becomes lower than the inside of the irradiation field 103B. . Here, the scattered radiation from the subject is also radiated to the outside of the irradiation field, and the scattered dose gradually decreases as the distance from the irradiation field 103B increases.
The image density d in the area outside the irradiation field changes as shown. FIG. 3 is an enlarged view of the density change in this portion. As shown in the figure, the image density d changes relatively gradually and linearly in the vicinity of the end of the recording area, and the irradiation field
As it approaches 103B, it becomes convex toward the low density side and increases sharply.

As shown in FIG. 3, the irradiation field recognition circuit 220 changes the density in each of the pixel groups L ₁ , L ₂ , L ₃ ... Ln each including a predetermined number N of pixels from the end of the recording area toward the center. In accordance with a known method, for example, it is represented by approximate expressions F ₁ , F ₂ , F ₃ ... Fn which are linear equations of y = ax + b. Note that this approximation formula may be another higher-order equation as long as it is a formula that substantially represents a straight line.

The irradiation field recognition circuit 220 then uses these approximate expressions F ₁ , F ₂ , F
₃ ... and the straight lines indicated by Fn, determine the intersection point _{P 1, P 2, P 3} ... Pn of the image density change curve of the sample image signal indicates. In this case, the image density change curve has a predetermined density d _T , for example, a predetermined value δ from the density of the pixel at the end of the recording area as a reference.
Constrained only to the curve in the region where the high concentration is exceeded. That is, the intersection of the straight line and the density change curve near the end of the recording area indicating the scattered radiation is not obtained, and the intersection of the straight line and the image density change curve carrying the image in the irradiation field is one for each straight line. You will be asked for each one. The above-mentioned predetermined value δ
The preferred value of can be determined substantially and empirically.

Next, the irradiation field recognition circuit 220 uses the intersection points P ₁ , P ₂ , P ₃ ... P described above.
Of n, the pixel X corresponding to the intersection closest to the end of the recording area
₀ is determined as the irradiation field edge point pixel. That is, as described above, the image density d becomes convex toward the low density side and rapidly increases as it approaches the irradiation field 103B, so that the intersection closest to the end of the recording area exists at the irradiation field edge. . By performing such an analysis also on the end portion on the opposite side of the recording area 103A, two irradiation field edge point pixels X ₀ can be detected in the pixel row X ₁ -X ₁ (see FIG. 2 (a)). .

The irradiation field recognition circuit 220 uses the above-mentioned analysis, for example, the pixel row X
Pixel row X ₂ -X ₂ parallel to ₁ -X ₁ and orthogonal pixel row Y _1-
Also for Y ₁ and Y ₂ -Y ₂ (see FIG. 2A), other irradiation field edge point pixels X ₀ and Y ₀ are obtained. Then, the portion outside the edge point pixels X ₀ and Y ₀ is recognized as the outside portion of the irradiation field and the inside portion is recognized as the irradiation field 103B, and the irradiation field information St indicating the recognized irradiation field 103B is controlled as described above. Circuit 31
Send to 4. If the control circuit 314 determines the reading gain a, the recording scale factor b, and the image processing condition c based on the preread image signal Sp in the irradiation field 103B indicated by the information St,
These conditions are not affected by the stored record information outside the irradiation field, and may be optimum for the radiation image information actually recorded in the irradiation field 103B.

The pixel columns for extracting the sample image signal are the example shown in FIG. 2, that is, the two columns (X in the horizontal direction of the stimulable phosphor sheet 103).
_1- X ₁ and X ₂ -X ₂ ) and two rows in the vertical direction (Y ₁ -Y ₁ and Y ₂ -Y ₂ ). The method of extracting the sample image signal shown in FIG. 2 is determined, for example, when the irradiation field 103B is narrowed down into a quadrangle on the stimulable phosphor sheet 103,
The irradiation field recognition circuit 220 is also effective when it is configured to recognize the irradiation field based on this premise. If the shape of the irradiation field is not determined, the sample image signal is extracted for a large number of pixel rows that are relatively close to each other, and the calculated large number of edge point pixels are determined. It suffices to recognize the inside of the bounded boundary line as the irradiation field, and therefore, it can be applied to the case of a circular or other irradiation field other than a quadrangle, or the direction of the quadrangle irradiation field is unknown.

Further, even in the case of so-called divided photographing in which one recording medium, in the above-described embodiment, one sheet is divided and photographing is performed in each division, for example, information that the divided photographing is performed is acquired in advance. Therefore, the present invention can be applied to each of the sections.

In the above embodiment, each pixel group L ₁ , L ₂ , L ₃
The area width of Ln is made constant by N pixels, but this area width may be changed for each pixel group.
For example, it may be a function of the order i from the sheet edge side of the pixel group. The preferable values of these region widths can be experimentally and empirically determined.

Further, the system shown in FIG. 1 has a reading system for main reading and a reading system for pre-reading separately.
As shown in No. 58-67242, the reading system for pre-reading is also used as the reading system for pre-reading.When the pre-reading is completed, the sheet transporting means returns the stimulable phosphor sheet to the reading system to perform the main reading. The excitation light energy may be adjusted by the excitation energy adjusting means so as to be smaller than that at the time of actual reading, and the present invention can also be adopted in such a system.

The irradiation field recognized as described above can be used for various purposes. For example, as described above, it can be used when only the image signal in the irradiation field is extracted from the pre-reading image signal and the reading condition is determined based on it. It can be used not only when extracting only the image signal of, and determining the image processing condition based on it, but also when extracting only the image signal in the irradiation field of the main reading image signal and determining the image processing condition based on it. And for other purposes,
For example, the irradiation field is recognized from the pre-read image signal, and during the main reading, the reading area is limited to the irradiation field as disclosed in Japanese Patent Application Laid-Open No. 60-120346 filed by the present applicant. Also available in case. In this way, by limiting the reading area of the main reading to the inside of the irradiation field, noise components due to scattered rays recorded outside the irradiation field of the stimulable phosphor sheet are not read, and an excellent final image can be obtained. Further, by narrowing the reading area, it is possible to shorten the reading time or increase the reading density.

Of course, the recognition of the irradiation field in the present invention is not limited to the pre-reading image signal as in the above-mentioned embodiment, but may be performed based on another image signal, for example, the main reading image signal.

Next, an example of the image processing condition determining method according to the second aspect of the present invention will be described.

As described in the radiation image information recording / reproducing system shown in FIG. 1, the present invention recognizes the irradiation field from the pre-reading image signal and determines the image processing condition based on the recognized pre-reading image signal in the irradiation field. However, the embodiment described below recognizes the irradiation field from the main reading image signal, and based on the recognized main reading image signal in the irradiation field, the gradation which is one of the image processing conditions. The present invention is applied when the processing conditions are determined.

In the present embodiment, first, the stimulable phosphor sheet that has been photographed by narrowing down the irradiation field is actually read to obtain an image signal.

The main reading to obtain an image signal means that the stimulable phosphor sheet is scanned by the pre-reading excitation light, and the stimulated emission light emitted from the sheet by the scanning is read by the photoelectric reading means. The obtained image signal is an electric signal corresponding to the stimulated emission light amount for each scanning point (that is, each pixel) on the sheet.

Next, a sample image signal relating to an arbitrary pixel row extending from the end portion of the recording area on the sheet toward the center side is extracted from the image signal thus obtained, and the image density change indicated by the sample image signal is calculated. sequentially _{L 1, L 2, L 3} ... respectively for each pixel group of Ln from the end toward the center side, the approximate expression F ₁ consisting essentially of linear equations, F _2, F ₃ ... Fn
, And the intersections of the straight lines represented by these approximate expressions F ₁ , F ₂ , F ₃ ... Fn and the image density change curve of the predetermined density or higher region indicated by the sample image signal are respectively P ₁ , P ₂ , P ₃ ... Pn, the area on the sheet center side of the intersection points P ₁ , P ₂ , P ₃ ... Pn closest to the end is recognized as the irradiation field.

The method for recognizing such an irradiation field is the same as the method for recognizing an irradiation field according to the first aspect of the present invention described above, and thus a detailed description thereof will be omitted.

Next, the gradation processing condition is determined based on the recognized image signal in the irradiation field among the actual reading image signals read from the recording medium. Any method may be used for this determination, but the following method can be given as an example.

Such a method obtains the histogram of the main reading image signal in the irradiation field (image signal level) and obtains the maximum image signal level Pmax and the minimum image signal level Pmin of the desired image signal range in this histogram from this histogram, and these Pmax and Pmin are The gradation processing conditions are determined so as to correspond to the maximum signal level Rmax and the minimum signal level Rmin of the desired input signal range in the image reproducing means, which are determined by the maximum density Dmax and the minimum density Dmin of the proper density range in the visible output image, respectively. To do.

This method will be described below in detail with reference to FIG. In FIG. 4, not the histogram of the electrical image signal obtained by photoelectrically reading the stimulated emission light by the photoelectric reading means, but the histogram of the stimulated emission light before the photoelectric reading is shown. As shown, this stimulated emission light is converted into an electrical image signal according to the fixed and linear reading conditions shown in the figure, so the amount of stimulated emission light and the image signal level are fixed. , And the histogram of the stimulated emission light is substantially the same as the histogram of the image signal. Therefore, in the following description, the stimulated emission light histogram shown in FIG. 4 is regarded as an image signal histogram.

It should be noted that in the present embodiment, the image signal used for the creation of the histogram is not necessarily limited to the one read under the linear reading condition as described above, and the point is that there is some correspondence with the stimulated emission light amount. Any image signal may be used as long as the image signal is included, and may be an image signal read under a non-linear reading condition, for example.

First, the histogram of the main reading image signal (image signal) in the irradiation field is obtained, and the range of the desired image signal (range of stimulated emission light amount) is obtained from this histogram. Since the pattern of the histogram is determined to some extent by the imaged region and the image capturing method, the desired image signal range is obtained from the histogram with reference to these imaged region and image capturing method. For example, the histogram in the case of chest radiography has a pattern as shown in FIG. 4, of which J is the mediastinum, K is the heart, L is the lung field, M is the skin and soft parts, and N is outside the subject. Therefore, it is possible to obtain the maximum image signal level Pmax (maximum stimulated emission light amount Smax) and the minimum image signal level Pmin (minimum stimulated emission light amount Smin) that are the range of the desired image signal from this histogram. . For example, in the case of FIG. 4, assuming that the information on the skin and the soft part M and the non-subject N is unnecessary, the range is from Pmax to Pmin as shown in the figure including the desired image signal ranges J, K and L. As a method of obtaining Pmax and Pmin, for example, a method of determining constant threshold values T ₁ and T ₂ according to a desired image signal range and then obtaining from these T ₁ and T ₂ is conceivable. It may be obtained from the histogram with.

On the other hand, in the radiation image information recording / reproducing system, an electric image signal is usually obtained from the stimulated emission light by the photoelectric reading means as described above, and various signal processing, especially gradation processing, is performed on this signal by the image processing means. Then, the signal is reproduced and recorded as a visible output image on the photographic light-sensitive material or the like by the image reproducing means. In this output image, there is a density range suitable for observation / interpretation, and in general, this appropriate density range (Dmax to Dmi
n) is predetermined, and the reproduction condition in the image reproducing means (condition that defines the relationship between the input to the reproducing means and the output from the reproducing means) is also predetermined. The input signal level range (Rmax to Rmin) to the image reproducing means corresponding to (Dmax to Dmin) is
It is uniquely determined according to this image reproduction condition.

Therefore, the gradation processing condition is determined so that the Pmax and Pmin obtained as described above correspond to the Rmax and Rmin determined as described above.

The gradation process is a process of converting the level of each image signal input to the image processing unit (gradation processing unit) according to a certain condition and then outputting the converted signal. The certain condition is called a gradation processing condition. , Usually represented by a non-linear tone curve.

Such gradation processing is intended to obtain a preferable visible output image suitable for a diagnostic purpose in accordance with imaging conditions such as an imaging site such as a head and chest and a simple imaging method such as contrast enhancement. Generally, a basic form of a non-linear gradation processing condition having a most preferable pattern for each photographing condition is set in advance, and appropriate for the gradation processing of each image according to the photographing condition of the image. It is preferable to select a basic form of different gradation processing conditions and perform gradation processing using the basic form.

Also in the present embodiment, in this way, an appropriate one is selected from the basic forms of the gradation processing conditions that are predetermined according to the image capturing conditions, and is corrected based on the irradiation field image signal. That is, as shown in the second quadrant of FIG. 4, that basic form is vertically shifted or rotated about a predetermined center point O so that Pmax, Pmin, etc.
Gradation processing conditions to be used by positioning are determined so as to correspond to max and Rmin.

It should be noted that the gradation processing condition is not limited to the non-linear one defined by the photographing condition as described above, and a linear one may be used. In that case, one predetermined straight line is used as in the above case. Similarly, rotate or shift
By positioning so that Pmax, Pmin, etc. correspond to Rmax, Rmin, the gradation processing conditions to be used are determined. The determination of the gradation processing condition by this method is based on only the image signal in the irradiation field, not on the imaging part or the imaging method as in the former case.

As described above, if the gradation processing of the image signal is performed according to the gradation processing condition set according to each captured image (image signal), for example, in the capturing of each image, the object arrow imaging site or the radiation Even if the radiation energy level range accumulated and recorded on each photographed sheet fluctuates due to fluctuations in the irradiation amount, the necessary subject image information is always observed and interpreted in any visible output image regardless of the fluctuation. It is convenient because it can be displayed in a suitable concentration range suitable for.

Further, in that case, according to the present invention, since the determination can be made based on the image signal that does not include the scattered radiation information noise, the desired image signal range can be more accurately obtained from the image signal histogram, and as a result, the necessary subject image can be obtained. The effect that information is displayed in the proper density range is more prominent.

Such a method of determining the gradation processing condition processes the image signal read based on the read condition determined without considering the variation of the recorded radiation energy level range in the individual imaging as described above. It is useful if used in some cases.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a radiation image information recording / reproducing system adopting the first and second embodiments of the present invention, and FIG. 2 is a state of irradiating a radiation field on a stimulable phosphor sheet in the above system, FIG. 3 is an explanatory view showing an example of an image density change of one pixel row on the sheet, FIG. 3 is an explanatory view explaining a method for recognizing an irradiation field in the first and second inventions, and FIG. It is explanatory drawing explaining an example of the method of determining a gradation process condition in this invention. 30 ... Read-ahead reading unit, 40 ... Main-reading reading unit 50 ... Image reproducing unit, 100 ... Radiation source 101 ... Subject, 102 ... Radiation ray 103 ... Accumulable phosphor sheet (recording medium) 104 ... Aperture, 201 ... Look-ahead Laser light source 202 ... Pre-reading laser light 204 ... Pre-reading light deflector 208 ... Pre-reading light detector 210 ... Pre-reading sheet transfer means 220 ... Irradiation field recognition circuit 301 ... Main reading laser light source 302 ... Main reading laser light 305 ... Main reading Light deflector 310 ... Main reading photodetector, 311 ... Amplifier 312 ... A / D converter, 314 ... Control circuit 320 ... Main reading sheet transfer means a ... Read gain b ... Recording scale factor c ... Image processing condition Sp ... Pre-read image signal, So ... Main read image signal St ... Irradiation field information

Claims (2)

[Claims] 1. A method of recognizing the irradiation field when radiation image information is recorded by applying an irradiation field stop on the recording medium, the method comprising recognizing an image signal read from the recording medium from the recording medium. A sample image signal relating to an arbitrary pixel column extending from the end of the recording area toward the center side is extracted, and the image density change indicated by the sample image signal is sequentially changed from the end toward the center side L ₁ , L ₂ , Approximate expression that is essentially a linear equation for each pixel group of L ₃ ... Ln
F _1, represented by F _2, F ₃ ... Fn, each line representing these approximate expression F _1, F _2, F ₃ ... Fn, the image density change curve of a predetermined concentration or more regions showing the sample image signal When the intersection points are P ₁ , P ₂ , P ₃ ... Pn, respectively, of these intersection points P ₁ , P ₂ , P ₃ ... Pn, the area closer to the sheet center than the intersection point closest to the end is the irradiation field. An irradiation field recognition method characterized by recognizing. 2. A method of determining an image processing condition when processing an image signal read from a recording medium on which radiation image information is recorded by narrowing an irradiation field, the image signal read from the recording medium. From the recording area of the recording medium to extract a sample image signal relating to an arbitrary pixel column extending toward the center side, the image density change indicated by the sample image signal, sequentially from the end portion toward the center side. For each pixel group of L ₁ , L ₂ , L ₃ ... Ln, approximation formulas F ₁ , F ₂ , F ₃ ...
, And the intersections of the straight lines represented by these approximate expressions F ₁ , F ₂ , F ₃ ... Fn and the image density change curve of the predetermined density or higher region indicated by the sample image signal are respectively P ₁ , P ₂ , P ₃ ... Pn, an image read from the recording medium by recognizing an area on the sheet center side of the intersection points P ₁ , P ₂ , P ₃ ... Pn closest to the end portion as the irradiation field. An image processing condition determining method, wherein the image processing condition is determined based on the recognized image signal in the irradiation field among signals.