JPH0525427B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention is directed to irradiating excitation light to a stimulable phosphor sheet on which radiation image information is stored and recorded, thereby causing radiation image information to be emitted from the stimulable phosphor sheet. The present invention relates to a method for determining optimal reading conditions in a radiation image information reading method in which the radiation image information is read by photoelectrically detecting stimulated luminescence light.

(Technical Background and Prior Art of the Invention) Certain phosphors are exposed to radiation (X-rays, α-rays, β-rays,
When irradiated with γ-rays, electron beams, ultraviolet rays, etc., a portion of this radiation energy is accumulated in the phosphor, and when this phosphor is irradiated with excitation light such as visible light, it It is known that phosphors exhibit stimulated luminescence, and phosphors exhibiting this property are called stimulable phosphors.

Using this stimulable phosphor, radiation image information of the human body, etc. is recorded on a sheet of stimulable phosphor,
This stimulable phosphor sheet is scanned with excitation light such as a laser beam to generate stimulated luminescent light, and the resulting stimulated luminescent light is read photoelectrically to obtain an image signal, and based on this image signal, a photograph is taken. Recording materials such as photosensitive materials,
The applicant has already proposed a radiation image information recording and reproducing system that outputs a radiation image as a visible image on a display device such as a CRT. (Unexamined Japanese Patent Publication 1983)
-12492, 56-11395, etc.).

This method has the practical advantage that images can be recorded over a much wider range of radiation exposure compared to conventional radiographic systems using silver halide photography. In other words, in stimulable phosphors, it is recognized that the amount of emitted light that is stimulated and emitted by excitation after accumulation is proportional to the amount of radiation exposure over an extremely wide range. Even if the amount of radiation exposure varies considerably depending on the imaging conditions, the amount of stimulated luminescence emitted from the stimulable phosphor sheet is read by the photoelectric conversion means by setting the reading gain to an appropriate value and converted into an electrical signal. This electrical signal is used to output the radiation image as a visible image to a recording material such as a photographic material or a display device such as a CRT, thereby obtaining a radiation image that is not affected by fluctuations in radiation exposure. be able to.

In addition, according to this system, radiation image information accumulated and recorded on a stimulable phosphor sheet is converted into an electrical signal, then subjected to appropriate signal processing, and this electrical signal is used to produce recording materials such as photographic light-sensitive materials, CRTs, etc. By outputting a radiographic image as a visible image on a display device such as the above, it is possible to obtain a very large effect that a radiographic image with excellent observation and interpretation suitability (diagnosis suitability) can be obtained.

In this way, in a radiation imaging system that uses a stimulable phosphor sheet, the reading gain can be set to an appropriate value to photoelectrically convert the stimulated luminescence light and output it as a visible image, so the radiation source Fluctuations in radiation exposure due to fluctuations in tube voltage or MAS values,
Accumulation occurs in the stimulable phosphor due to factors such as variations in the sensitivity of the stimulable phosphor sheet, variations in the sensitivity of the photodetector, changes in exposure due to subject conditions, or differences in radiation transmittance depending on the subject. Even if the stored energy differs, and even if the amount of radiation exposure is reduced, it is possible to obtain a radiation image that is not affected by fluctuations in these factors.

However, in order to eliminate the effects of fluctuations in imaging conditions or to obtain radiographic images that are suitable for observation and interpretation, it is necessary to change the recording state of the radiographic image information accumulated and recorded on the stimulable phosphor sheet, or the chest, Recording patterns (hereinafter collectively referred to as "accumulated recorded information") determined by the body part of the subject such as the abdomen and the imaging method such as plain radiography or contrast radiography are used for observation and interpretation. This information is determined prior to the output of a visible image, and the reading gain is adjusted to an appropriate value based on the acquired recorded information, and the recording scale is adjusted to optimize the resolution according to the contrast of the recorded pattern. It is necessary to determine the actors.

In this way, Japanese Patent Laid-Open No. 58
The method disclosed in No.-67240 is known. This method uses excitation light of a lower level than the excitation light that should be irradiated during the reading operation (hereinafter referred to as "main reading") to obtain a visible image for observation and interpretation. Perform reading operations (hereinafter referred to as "read ahead") to grasp the accumulated record information of radiographic images accumulated and recorded on the stimulable phosphor sheet, grasp the outline of accumulated records of radiographic images, and read the main reading. When performing this, reading conditions such as the reading gain and recording scale factor are optimally determined based on this pre-read information.

Note that the excitation light used for pre-reading is at a lower level than the excitation light used for main reading because the effective energy of the excitation light that the stimulable phosphor sheet receives per unit area during pre-reading is lower than the excitation light used for main reading. This means that it is smaller than the actual value. As a method to make the excitation light of the pre-reading at a lower level than the excitation light of the main reading, there is a method of reducing the output of the excitation light source such as a laser light source, and a method of passing the excitation light emitted from the light source to an ND filter, AOM, etc. in its optical path. There are methods to attenuate the excitation light, and methods to provide a light source for pre-reading and a light source for main reading separately and make the output of the former smaller than the output of the latter.Furthermore, there are methods of increasing the beam diameter of the excitation light. Examples include a method of increasing the scanning speed of excitation light, and a method of increasing the transport speed of the stimulable phosphor sheet.

According to the above method, the recording state and recording pattern of the radiographic image information accumulated and recorded on the stimulable phosphor sheet can be grasped in advance before the actual reading, so reading with an exceptionally wide dynamic range is possible. Even without using the system, by appropriately adjusting the reading gain and appropriately determining the recording scale factor based on this recorded information, it is possible to obtain radiographic images with excellent suitability for observation and interpretation.

On the other hand, when recording radiation image information (photography),
To prevent radiation from being irradiated to areas that are not necessary for diagnosis, or because if radiation is irradiated to areas that are not necessary for diagnosis, scattered rays will enter the areas necessary for diagnosis from that area, reducing contrast resolution. For this purpose, the entire recording area of the stimulable phosphor sheet is often imaged by narrowing the radiation irradiation field.

However, when the radiation irradiation field is narrowed down in this way, the scattered radiation generated from the subject in the irradiation field usually enters the irradiation field on the stimulable phosphor sheet, and the highly sensitive stimulable phosphor sheet The sheet also accumulates and records these scattered rays. Therefore, the image information obtained by pre-reading includes information from these scattered rays, so even if the above-mentioned reading conditions are determined based on such pre-read image information, optimal reading will not be possible. The conditions cannot be determined, and as a result, it becomes impossible to obtain reconstructed radiographic images with excellent suitability for observation and interpretation.

(Object of the Invention) The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to read radiation image information from a stimulable phosphor sheet that has been photographed with an irradiation field aperture. The purpose of this invention is to provide a method for determining reading conditions for radiation image information, which can eliminate the influence of accumulated information outside the radiation irradiation field and determine optimal reading conditions for image information within the irradiation field. be.

(Structure of the Invention) The method for determining reading conditions for radiation image information of the present invention performs pre-reading on a stimulable phosphor sheet on which radiation image information has been recorded (photographed) by applying irradiation field aperture as described above. , in a radiation image information reading method in which reading conditions for main reading are determined based on the information obtained by this pre-reading, from the pre-read image signal obtained by pre-reading,
A sample image signal related to an arbitrary pixel column extending from the edge of the recording area of the sheet toward the center is extracted, and the image density change indicated by this sample image signal is calculated as follows:
From the above end toward the center, sequentially L ₁ , L ₂ , L ₃
...Effectively 1 for each pixel group of Ln
It is represented by approximate expressions F ₁ , F ₂ , F ₃ ...Fn consisting of the following equations, and each straight line indicated by these approximate expressions F ₁ , F ₂ , F ₃ ...Fn and the area of a predetermined density or higher indicated by the above sample image signal are The intersection points with the image density change curve are respectively P ₁ and
When P ₂ , P ₃ ...Pn, these intersection points P ₁ , P ₂ ,
P ₃ ...The area of Pn closer to the center of the sheet than the intersection closest to the edge of the sheet is recognized as the radiation irradiation field, and the main reading is performed based on the pre-read image information regarding the area recognized as the radiation irradiation field in this way. This feature is characterized in that the reading conditions at the time are determined.

(Embodiments) Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 shows a radiation image information recording and reproducing system in which reading conditions for main reading are determined by a method according to an embodiment of the present invention. This radiation image information recording and reproducing system basically includes a radiation image capturing section 20, a pre-reading reading section 30, a main reading reading section 40, and an image reproducing section 50. In the radiographic image capturing unit 20, radiation 102 is emitted from a radiation source 100 such as an X-ray tube toward a subject (subject) 101, for example. The stimulable phosphor sheet 103 that accumulates radiation energy is placed at the position where the radiation 102 that has passed through the subject 101 is irradiated, as described above. Transmission radiation image information is accumulated and recorded. Note that an aperture 104 that narrows down the irradiation field of the radiation 102 is arranged between the radiation source 100 and the subject 101.

The stimulable phosphor sheet 103 on which the radiation image information of the subject 101 has been recorded in this manner is sent to the pre-reading reading section 30 by sheet transport means 110 such as a transport roller. A laser beam 202 emitted from a pre-reading laser light source 201 in the pre-reading reading unit 30 is passed through a filter that cuts out the wavelength range of stimulated luminescence light emitted from the stimulable phosphor sheet 103 by excitation of the laser beam 202. 2
03, the light is linearly deflected by a light 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 light 202 as excitation light does not overlap with the wavelength range of the stimulated luminescence light emitted by the stimulable phosphor sheet 103. On the other hand, the phosphor sheet 103 is transferred to a sheet transport means 210 such as a transport roller.
The phosphor sheet 103 is moved in the direction of the arrow 206 for sub-scanning, and as a result, the entire surface of the phosphor sheet 103 is irradiated with the laser light 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, and the transport speed of the stimulable phosphor sheet 103 are determined by the energy of the pre-read excitation light (laser light 202), which will be described later. It is selected to be smaller than that of the book reading performed in the book reading reading section 40.

When the laser beam 202 is irradiated as described above,
The stimulable phosphor sheet 103 emits stimulated luminescent light in an amount corresponding to the radiation energy stored and recorded therein, and this luminescent light is transmitted to the pre-reading light guide 2.
It is incident on 07. This light guide 20 is the stimulable luminescent light.
7, exits from the exit surface, and is received by a photodetector 208 such as a photomultiplier. A filter is attached to the light-receiving surface of the photodetector 208, which transmits only light in the wavelength range of stimulated luminescence light and cuts out light in the wavelength range of excitation light, and detects only stimulated luminescence light. It is becoming possible to do so. The detected stimulated luminescence light is converted into an electrical signal carrying accumulated recording information and amplified by an amplifier 209. The signal output from the amplifier 209 is digitized by the A/D converter 211 and inputted to the main reading control circuit 314 of the main reading reading section 40 as a pre-read image signal Sp. This book reading control circuit 3
14 is a reading gain setting value a, a recording scale factor setting value b, and a reproduction image processing condition setting value c based on the accumulated recording information indicated by the prefetched image signal Sp.
Determine. The pre-read image signal Sp is also input to an irradiation field recognition circuit 220, which will be described in detail later.

The stimulable phosphor sheet 103 that has undergone pre-reading as described above is transferred to the main reading reading section 40. In the reading unit 40 for main reading, a laser beam 302 emitted from a laser light source 301 for main reading is passed through a filter that cuts out the wavelength range of stimulated luminescence light emitted from a stimulable phosphor sheet 103 by excitation of this laser light 302. After passing through 303, the beam diameter is strictly adjusted by a beam expander 304, linearly deflected by an optical deflector 305 such as a galvanometer mirror, and then reflected by a plane reflector 306 to form an accumulative fluorescent light. body sheet 103
incident on the top. Optical deflector 305 and plane reflector 30
An fθ lens 307 is disposed between the stimulable phosphor sheet 103 and the stimulable phosphor sheet 103 so that the beam diameter of the laser beam 302 that scans the stimulable phosphor sheet 103 is uniform. On the other hand,
The stimulable phosphor sheet 103 is transported in the direction of an arrow 308 by a sheet transport means 320 such as a transport roller to perform sub-scanning, and as a result, the entire surface of the stimulable phosphor sheet 103 is irradiated with laser light. Ru. When irradiated with the laser beam 302 in this manner, the stimulable phosphor sheet 103 emits stimulated luminescence light with an amount of light corresponding to the radiation energy stored and recorded therein, and this luminescent light is directed to the main reading light guide 309. incident. Stimulated luminescent light guided through the main reading light guide 309 while undergoing repeated total reflection is emitted from its exit surface and is received by a photodetector 310 such as a photomultiplier. A filter that selectively transmits only the wavelength range of the stimulated luminescent light is attached to the light receiving surface of the photodetector 310, so that the photodetector 310 detects only the stimulated luminescent light.

The output of the photodetector 310 that photoelectrically detects the stimulated luminescent light representing the radiation image recorded on the stimulable phosphor sheet 103 is read based on the read gain setting value a determined by the control circuit 314. An amplifier 311 with a set gain amplifies the electrical signal to an appropriate level. The amplified electrical signal is input to the A/D converter 312, and based on the recording scale factor setting value b, it is converted into a digital signal by a recording scale factor suitable for the signal fluctuation range, and is input to the signal processing circuit 313. be done. The digital signal is subjected to signal processing (image processing), such as gradation processing, in the signal processing circuit 313 based on the reproduction image processing condition setting value c, so that a radiation image with excellent observation and interpretation suitability is 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 reproduction section 50. This image reproduction section 5
0, the laser beam 403 from the recording laser light source 402 is transmitted by the optical modulator 401 to the actual reading image signal input from the signal processing circuit 313.
The light-sensitive material 405, such as a photographic film, is modulated based on So and deflected by a scanning mirror 404.
Scan above. The photosensitive material 405 is then transported in a direction perpendicular to the scanning direction (arrow 406 direction) in synchronization with the scanning, and a radiation image based on the actual reading image signal So is output onto the photosensitive material 405. In addition to this method, there are other ways to reproduce radiographic images, such as the above-mentioned CRT display, etc.
Various methods can be adopted.

When recording (photographing) radiographic image information on the stimulable phosphor sheet 103, for the reason mentioned above, the aperture 104 is operated and the irradiation is performed as shown in FIG. 2a, for example. Noshibori may be applied. That is, in this case, a radiation irradiation field (image recording portion) 103B is formed in a part of the recording area 103A of the stimulable phosphor sheet 103. Based on the pre-read image signal Sp from the entire recording area 103A of the stimulable phosphor sheet 103, the reading gain and recording scale factor as the main reading conditions are determined, and as described above, those reading conditions are , irradiation field 10
The accumulated recorded image in 3B becomes inappropriate. Therefore, the irradiation field recognition circuit 220
is the radiation irradiation field 1 based on the look-ahead image signal Sp.
03B and information St indicating the irradiation field 103B.
is sent to the control circuit 314, and the control circuit 314 determines the reading gain setting value a and recording scale factor setting value b only from the pre-read image signal Sp for the inside of the irradiation field 103B indicated by the irradiation field information St. ing.

Irradiation field recognition by the irradiation field recognition circuit 220 will be described in detail below. The irradiation field recognition circuit 220 detects the edge of the recording area of the stimulable phosphor sheet 103 from the entire pre-read image signal Sp input from the A/D converter 211, as shown by the line _X1 - _X1 in FIG. An image signal for an arbitrary pixel column (for example, a pixel column in the main scanning direction) extending from the center toward the center is extracted. Generally, the change in image density d shown by the sample image signal extracted in this way is b in Fig. 2.
It will look like the one shown below. That is, the irradiation field 103
The density inside B is relatively high and shows a change depending on the image information, and the density is lower in the area outside the edge portion of this irradiation field 103B, that is, in the outside area of radiation irradiation than in the irradiation field 103B. here,
Scattered radiation from the object is also applied to the outside irradiation area, and the amount of scattered radiation gradually decreases as the distance from the irradiation field 103B increases, so the image density d in the outside irradiation area changes as shown. FIG. 3 shows an enlarged view of the density change in this part. As shown in the figure, the image density d changes relatively slowly and linearly near the edge of the recording area, and as it approaches the irradiation field 103B. It becomes convex roughly toward the low concentration side and increases rapidly.

As shown in FIG. 3, the irradiation field recognition circuit 220
A predetermined number N from the edge of the recording area toward the center.
Pixel groups L ₁ , L ₂ , L ₃ ...
The concentration changes in Ln are each expressed by approximate expressions F ₁ , F ₂ , F ₃ . . . Fn, which are linear equations such as y=ax+b, using a known method. Note that other higher-order equations may be used as this approximate equation as long as they substantially represent a straight line.

Then, _the irradiation field _recognition circuit 220 determines the intersection points _{P 1 ,} P ₂ ,
P ₃ ...Find Pn. In this case, the image density change curve is limited to a curve in an area exceeding a predetermined density d _T , for example, a density higher by a predetermined value than the density of a pixel at the end of the recording area. In other words, the intersection of the density change curve near the edge of the recording area indicating the scattered rays with each straight line cannot be found, and the intersection of the above straight line with the image density change curve that carries the image within the irradiation field is determined once for each straight line. Each will be required. Note that a preferable value for the predetermined value δ can be determined experimentally or empirically.

Next, the irradiation field recognition circuit 220 detects the above intersection point.
Among P ₁ , P ₂ , P ₃ . . . Pn, the pixel X ₀ corresponding to the intersection closest to the edge of the recording area is determined to be the irradiation field edge point pixel. In other words, as mentioned above, the image density d
As it approaches the irradiation field 103B, it becomes convex roughly toward the low density side and increases rapidly, so the intersection closest to the edge of the recording area exists at the edge of the irradiation field. By performing such an analysis on the opposite end of the recording area 103A, two irradiation field edge point pixels _X0 can be detected for the pixel row _X1 - _X1 (see FIG. 2a).

The irradiation field recognition circuit 220 also performs the above-mentioned analysis on, for example, the pixel row X ₂ −X ₂ parallel to the pixel row X ₁ −X ₁ and the pixel rows Y ₁ −Y ₁ and Y 2 −Y ₂ perpendicular to the pixel _{row X 1} −X 1 . (See Figure 2a), find the other irradiation field edge point pixels X ₀ and Y ₀ . The area outside these edge point pixels X ₀ , Y ₀ is the outside irradiation area,
The inner portion is recognized as the irradiation field 103B, and irradiation field information St indicating the recognized irradiation field 103B is sent to the control circuit 314 as described above. The control circuit 314
If the reading gain setting value a and the recording scale factor setting value b are determined based only on the pre-read image signal Sp in the irradiation field 103B indicated by the information St, these conditions will eliminate the influence of the accumulated recorded information in the irradiation field area. It can be optimal for the radiation image information actually recorded within the irradiation field 103B.

In this embodiment, the image processing condition setting value c is also determined based only on the pre-read image signal Sp for the inside of the irradiation field 103B, and
It is designed to be optimal for radiation image information within 3B.

The pixel row for extracting the sample image signal is the example shown in FIG. 2, that is, the stimulable phosphor sheet 103.
It is not limited to two rows in the horizontal direction (X 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 based on the assumption that the irradiation field 103B is narrowed down to a rectangular shape on the stimulable phosphor sheet 103, and the irradiation field recognition circuit 220 is also based on this premise. This is effective when the system is configured to recognize the irradiation field. If the shape of the irradiation field is not determined, sample image signals are extracted from a large number of relatively dense pixel columns, and the obtained irradiation field edge point pixels are What is necessary is to recognize the area inside the boundary line connecting the irradiation field as the irradiation field.

Furthermore, in the above embodiment, each pixel group
The area widths of L ₁ , L ₂ , L ₃ ...Ln are each kept constant at N pixels, but this area width may be changed for each pixel group. For example, from the sheet edge side of the pixel group It may also be a function for the order i. Note that preferable values of these region widths can be determined experimentally or empirically. Furthermore, the apparatus shown in FIG. 1 has separate reading systems for main reading and reading systems for pre-reading.
As shown in No. 58-67242, the reading system for main reading and the reading system for pre-reading are used together, and when the pre-reading is completed, the stimulable phosphor sheet is returned to the reading system by the sheet transport means to perform the main reading, and the pre-reading is performed. Sometimes, the excitation light energy may be adjusted by an excitation light energy adjustment means so that the excitation light energy is smaller than that during main reading, and the method of the present invention can also be applied when reading radiation image information using such a device. It is.

(Effects of the Invention) As explained in detail above, according to the method for determining reading conditions for radiation image information of the present invention, in pre-reading, the influence of the portion outside the radiation irradiation area is eliminated, and accumulated records recorded in the radiation irradiation field are It is possible to understand information correctly and appropriately set reading conditions for book reading. Therefore, according to the method of the present invention, even from a stimulable phosphor sheet that has been photographed using a radiation aperture, it is possible to always reproduce a radiographic image that is highly suitable for observation and interpretation.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a radiation image information recording and reproducing system that determines reading conditions using a method according to an embodiment of the present invention, and FIG. 2 is a diagram showing a radiation field narrowing state on a stimulable phosphor sheet according to the present invention; FIG. 3 is an explanatory diagram showing an example of image density change in one pixel row of the sheet, and FIG. 3 is an explanatory diagram illustrating the method of the present invention. 30... Reading section for pre-reading, 40... Reading section for main reading, 50... Image reproduction section, 100... Radiation source, 10
DESCRIPTION OF SYMBOLS 1...Subject, 102...Radiation line, 103...Storage phosphor sheet, 104...Aperture, 201...Laser light source for pre-reading, 202...Laser light for pre-reading, 204
... Optical deflector for pre-reading, 208... Photodetector for pre-reading, 210... Sheet transport means for pre-reading, 220...
Irradiation field recognition circuit, 301...Laser light source for book reading,
302... Laser light for main reading, 305... Optical deflector for main reading, 310... Photodetector for main reading, 311... Amplifier, 312... A/D converter, 314... Control circuit, 320... Sheet transport means for main reading, a... Reading gain setting value, b...Recording scale factor setting value, Sp...Pre-read image signal, So...Actual reading image signal,
St…Irradiation field information.

Claims (1)

[Scope of Claims] 1. Irradiating excitation light onto a stimulable phosphor sheet on which radiation image information is accumulated and recorded by applying irradiation field aperture,
Prior to main reading, the stimulated luminescence light emitted from the sheet by this excitation light irradiation is photoelectrically read by a photodetector to obtain an image signal for visible image reproduction. The sheet is irradiated with low-level excitation light to perform pre-reading to read the outline of the image information accumulated and recorded on this sheet, and based on the information obtained by this pre-reading, the reading conditions for performing the main reading are determined. In the radiation image information reading method, the main reading is performed according to the reading conditions, from the pre-read image signal obtained by the pre-reading, a pre-read image signal extending from the edge of the recording area of the sheet toward the center side. Extract a sample image signal related to an arbitrary pixel column, and calculate the image density change indicated by this sample image signal by
Sequentially L ₁ , L ₂ , L ₃ from the end toward the center
...Effectively 1 for each pixel group of Ln
It is expressed by approximate expressions F ₁ , F ₂ , F ₃ ...Fn consisting of the following equations, and each straight line indicated by these approximate expressions F ₁ , F ₂ , F ₃ ...Fn and the area of a predetermined density or higher indicated by the sample image signal are The intersection points with the image density change curve are respectively P ₁ and
When P ₂ , P ₃ ...Pn, these intersection points P ₁ , P ₂ ,
P ₃ ...Recognizes the region of Pn closer to the center of the sheet than the intersection closest to the edge of the sheet as the radiation irradiation field, and determines the reading conditions based on pre-read information regarding the region recognized as the radiation irradiation field. A method for determining reading conditions for radiographic image information.