JPH02239242A - Radiograph reading method - Google Patents

Abstract

PURPOSE:To improve the quality of a reproduced picture by irradiating the area other than the direct radiation part of an accumulating phosphor sheet with exciting light to read a radiograph. CONSTITUTION:An accumulating phosphor sheet 1 where a radiograph 2 having a direct radiation part 2b is accumulated and recorded is irradiated with exciting light, and the stimulated luminous light emitted from the sheet 1 is read to obtain a picture signal which expresses the radiograph 2. That is, the direct radiation part 2b is recognized, and thereafter, the area other than the direct radiation part 2b on the phosphor sheet 1 is irradiated with the exciting light, and the stimulated luminous light is read to obtain the picture signal. The area on the accumulating phosphor sheet directly irradiated with radiant rays which do not pass (transmitted or reflected) an object is called the direct radiation part. Thus, an influence of the direct radiation part having a great afterglow is eliminated, and the picture signal where this influence is reduced is obtained, and the occurrence of a trailing phenomenon or the like on the reproduced picture is suppressed to improve the picture quality.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention involves irradiating excitation light onto a stimulable phosphor sheet on which a radiographic image having a direct radiation area is stored, and by the irradiation, the stimulable phosphor sheet The present invention relates to a radiation image reading method for reading stimulated luminescence light emitted from a sheet to obtain an image signal representing the radiation image.

(Conventional technology) Radiation 1! When irradiated with (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays, etc.), a portion of this radiation energy is accumulated, and then when excitation light such as visible light is irradiated, it glows according to the accumulated energy. A stimulable phosphor (stimulable phosphor) that exhibits stimulable luminescence is known.

Using this stimulable phosphor, radiation image information of a subject such as a human body is recorded on a sheet of stimulable phosphor, and this stimulable phosphor sheet is scanned with excitation light such as a laser beam to stimulate Generate luminescent light, photoelectrically read the resulting stimulated luminescent light to obtain an image signal, and based on this image signal, output a radiation image of the subject as a visible image to a recording material such as a photographic light-sensitive material, CRT, etc. A radiation image recording and reproducing system was proposed (Japanese Patent Application Laid-open No. 12429/1983, 5B-11).
Publication No. 395, Publication No. 55-183472, Publication No. 5B-
104845, 55-118340, etc.) have been put into practical use.

This system has the practical advantage of being able to record images over a much wider range of radiation exposure compared to conventional radiographic systems using silver halide photography. In other words, in a stimulable phosphor, it is recognized that the amount of emitted light that is stimulated to emit light due to excitation after accumulation is proportional to the amount of radiation exposure over an extremely wide range.
Therefore, even if the amount of radiation exposure varies considerably due to various imaging conditions, the amount of stimulated luminescence emitted from the stimulable phosphor sheet can be read by the photoelectric conversion means by setting the reading gain to an appropriate value. By converting the radiation image into an electric signal and using this electric signal to output the radiation image as a visible image to a recording material such as a photographic light-sensitive material or a display device such as a CRT, a radiation image that is not affected by fluctuations in radiation exposure amount can be obtained. be able to.

FIG. 3 is a diagram showing an example of a radiation image stored and recorded on a stimulable phosphor sheet.

When photographing and recording a radiation image on the stimulable phosphor sheet 1, an irradiation field diaphragm may be used to irradiate only the subject and necessary portions of the stimulable phosphor sheet 1 with radiation. An irradiation field diaphragm is also used when photographing and recording this radiation image 2, and as a result, radiation is not irradiated to both ends 1a of the stimulable phosphor sheet 1, and the radiation image is captured at the center of the sheet 1. It is recorded only in 1b. Radiographic image 2 includes a subject area 2a where radiation that has passed through the subject is irradiated onto sheet 1 and an image of the subject is formed, and a direct radiation area where radiation is directly irradiated onto sheet 1 without passing through the subject. 2b. In order to read the radiation image 2 recorded on the stimulable phosphor sheet 1 and obtain an image signal, the stimulable phosphor sheet 1 is normally conveyed in the direction of arrow Y shown in the figure (sub. The entire surface of the sheet 1 is scanned by repeatedly scanning (main scanning) in the direction of the arrow X with the excitation light spot, and the stimulated luminescence light emitted from each scanning point is photoelectrically is read to obtain an image signal.

(Problems to be Solved by the Invention) Referring to FIG. 3, a case will be described in which the excitation light spot is scanned along the ξ axis in this figure.

Consider the moment when scanning is performed from the left side of the figure along the ξ axis, the scanning is directly transferred from the radiation section 2b to the object section 2a, and the scanning point 3 is irradiated with excitation light. This instantaneous excitation light is irradiated to the scanning point 3, and therefore, stimulated luminescence light is generated from the scanning point 3 in an amount corresponding to the intensity of the excitation light and the amount of radiation energy accumulated and recorded at the scanning point 3. .

However, stimulated luminescence light does not only occur while the excitation light is being irradiated, but afterglow occurs for a while even after the scanning point moves to the next one. The direct radiation area 2b is an area where the intensity of stimulated luminescence light emitted from this area is particularly strong,
For this reason, the amount of afterglow light is also particularly strong. Therefore, at the moment when the excitation light is irradiating the scanning point 3 in FIG. Even if the amount of radiation energy that is incident on the photodetector and accumulated and recorded at the scanning point 3 is small (corresponding to the low density of the reproduced image) in the same way as the emitted stimulated luminescence light, it is directly transmitted to the radiation section 2b. The amount of energy is large due to the effect of stimulated luminescence light emitted from (
In the reproduced image, a so-called tailing phenomenon occurs in which the density of the portion of the subject portion 2a directly adjacent to the radiation portion 2b increases along the main scanning line ξ. As a result, a problem has arisen in that the reproduced image becomes an image that is very difficult to view.

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a radiation image reading method that obtains an image signal in which the influence of afterglow is reduced.

(Means for Solving the Problem) FIG. 1 is a flowchart showing each step of the radiation image reading method of the present invention.

The present invention irradiates excitation light onto a stimulable phosphor sheet on which a radiographic image having a direct radiation area has been stored, and reads the stimulated luminescent light emitted from the stimulable phosphor sheet due to the irradiation. The present invention relates to a radiation image reading method for obtaining an image signal representing the radiation image.

In the radiation image reading method of the present invention, the direct radiation area is recognized (step a), and then the area other than the direct radiation area of the stimulable phosphor sheet is irradiated with excitation light to perform reading (step b). This is characterized in that an image signal is obtained by.

Here, the above-mentioned "direct radiation department" means, as mentioned above,
This refers to the area where the stimulable phosphor sheet is directly irradiated with radiation that does not pass through (transmit or reflect) the subject.

Further, the recognition of the direct radiation area in step a is not limited to a specific recognition method, but it can be recognized, for example, by obtaining a pre-read image signal and analyzing this pre-read image signal (Japanese Patent Laid-Open No. (Refer to IL-170178.) Here, the pre-read image signal is a pre-read image signal that is read in advance before obtaining the above-mentioned image signal by reading it under optimal reading conditions according to the dose of radiation irradiated to the stimulable phosphor sheet. A signal obtained by scanning a stimulable phosphor sheet with a low-level light beam and performing pre-reading to read the outline of the radiation image recorded on this sheet.After analyzing this pre-read image signal, The operation of scanning the radiation image by irradiating it with a high-level light beam and reading it under the optimal reading conditions for the radiation image to obtain the image signal is called main reading.

Here, reading conditions are a general term for various conditions that affect the relationship between the amount of stimulated luminescence light and the output of the reading device during reading, such as reading gain, scale factor, or , the power of excitation light during reading, etc.

Also, the high level and low level of the light beam are, respectively.
If the energy of the light beam irradiated per unit area of the sheet or the energy of stimulated luminescence light emitted from the sheet depends on the wavelength of the light beam (has a wavelength sensitivity distribution), It refers to the weighting energy after weighting the energy of the light beam irradiated per unit area of the sheet with the wavelength sensitivity.As a method of changing the level of the light beam, light beams of different wavelengths are used. methods used, methods that change the intensity of the light beam itself emitted from a laser light source, methods that change the intensity of the light beam by inserting or removing an ND filter, etc. on the optical path of the light beam, and methods that change the beam diameter of the light beam. Various known methods can be used, such as a method of changing the scanning density and a method of changing the scanning speed.

Further, the recognition of the direct radiation area in step a may be obtained by analyzing the instantaneous light emitted from the stimulable phosphor sheet when a radiation image is photographed and recorded on the sheet (Japanese Patent Laid-Open No.
1-238045). Further, in step a, a reproduced image based on the pre-read image signal may be displayed on, for example, a CRT display, and the operator may directly recognize the radiation area and designate the area.

In addition, in the reading in step b, reading may be performed by irradiating the entire area of the stimulable phosphor sheet with excitation light, excluding the direct radiation area. As shown in FIG. 3, reading may be performed by irradiating excitation light only to the area excluding the direct radiation part in the area where the radiation image 2 is recorded (referred to as the central part ib).

Note that the excitation light is not directly irradiated to the radiation area, and no image signal corresponding to this area can be obtained. However, since the direct radiation area does not carry any effective information, it is only necessary to display it with an appropriate uniform density in the reproduced image, and there is no problem in not being able to obtain an image signal for this area. It is something that cannot happen.

(Function) In the radiation image reading method of the present invention, after the direct radiation part is recognized as described above, excitation light is irradiated to the area other than the direct radiation part of the stimulable phosphor sheet to perform reading. The influence from the direct radiation area with extremely large afterglow is eliminated, so it is possible to obtain an image signal in which the influence of afterglow is extremely reduced, and the occurrence of phenomena such as so-called tailing in the reproduced image is suppressed, and the reproduced image image quality can be improved.

(Example) Examples of the present invention will be described below.

FIG. 2 is a perspective view showing an example of a radiation image reading device using an example of the radiation image reading method of the present invention. The radiation image reading device shown here uses a stimulable phosphor sheet and performs pre-reading.

A stimulable phosphor sheet 1 on which a radiation image has been recorded is first scanned with a weak light beam to emit only a portion of the radiation energy stored in this sheet 1, thereby performing a pre-reading process. 100 predetermined positions. The stimulable phosphor sheet 1 set at a predetermined position is transported by a sheet conveying means 7 such as an endless belt driven by a motor 6.
The paper is transported (sub-scanning) in the direction of arrow Y. On the other hand, a weak light beam 9 emitted from the laser light source 8 is transmitted to the motor 1.
Rotating polygon mirror l driven by 0 and rotating at high speed in the direction of the arrow
1 and is reflected and deflected by a focusing lens l such as an fθ lens.
2, the optical path is changed by a mirror 13, the light enters the sheet 1, and is main-scanned in the direction of arrow X, which is substantially perpendicular to the direction of sub-scanning (direction of arrow Y). From the part of the sheet 1 irradiated with this excitation light 9, stimulated luminescence light l4 is emitted in an amount corresponding to the radiographic image information stored and recorded, and this stimulated luminescence light l4 is transmitted by the light guide 5. and photoelectrically detected by a photomultiplier (photomultiplier tube) 1B. The light guide 15 is made by molding a light-guiding material such as an acrylic plate, and has a linear entrance end surface 15a extending along the main scanning line on the stimulable phosphor sheet 1. The light receiving surface of the photomultiplier 1B is coupled to the annularly formed exit end surface L5b. The stimulated luminescent light 14 that has entered the light guide 15 from the incident end surface 15a travels through the interior of the light guide 15 through repeated total reflection, exits from the exit end surface 15b, is received by the photomultiplier 1B, and is then received by the photomultiplier 1B. The amount of stimulated luminescent light l4 carrying image information is converted into an electrical signal by a photomultiplier l6.

The analog output signal S output from the photomultiplier 16 is logarithmically amplified by a logarithmic amplifier l7, and digitized by an A/D converter l8 to obtain a pre-read image signal Sp. The signal level of this pre-read image signal Sp is proportional to the logarithm of the intensity of stimulated luminescence light emitted from each pixel of sheet 1.

In the above-mentioned pre-reading, reading conditions such as the voltage value applied to the photomultiplier 16 and the amplification factor of the logarithmic amplifier 17 are set so that the radiation energy accumulated in the stimulable phosphor sheet 1 can be read over a wide range. is determined.

The obtained pre-read image signal Sp is sent to the calculation section I9, and the calculation section I9 directly determines the radiation area based on the pre-read image signal Sp, and also determines the reading conditions for main reading to be described later.

FIG. 3 is a diagram showing an example of a radiation image 2 stored and recorded on the stimulable phosphor sheet 1 shown in FIG. 2, and FIG. FIG. 3 is a diagram showing a histogram of a pre-read image signal obtained by reading the image signal;

The peak 1a' on the left side of FIG. 4 (the value of the pre-read image signal Sp is in the range of 81 to S3) corresponds to both ends La of the sheet 1 in FIG.
This is a set of pre-read image signals obtained from .

Since both end portions La of the sheet 1 are areas outside the irradiation area where radiation is not irradiated, the value of the pre-read image signal obtained from that area is small. The mountain 2a' in the center of FIG. 4 (the value of the pre-read image signal Sp is in the range of S3 to S4) corresponds to the subject part 2a shown in FIG. 3, and contains necessary information as image information. It is an area.

The peak 2b' on the right side of FIG. 4 (the value of the pre-read image signal Sp is 84
-S2) corresponds to the direct radiation section 2b shown in FIG. The pre-read image signal sp obtained from the direct radiation section 2b has a large value and therefore forms a peak 2b' on the right side of FIG. By analyzing the pre-read image signal Sp and finding the peak 2b' that directly corresponds to the radiation area,
Since it is already known from which part of sheet 1 the pre-read image signal Sp having a value in the range of 84 to S2, which is included in this mountain 2b', is obtained, - It is possible to know which part of the sheet 1 is the direct radiation part 2b.

In addition, the calculation unit l9 recognizes the area of the direct radiation section 2b based on the pre-read image signal Sp, and also recognizes the central area 1b (irradiation field) of the sheet 1 where the radiation image 2 is stored and recorded. Recognized. In the same way as the recognition method of the direct radiation part 2b, by knowing which region of the sheet 1 shown in FIG. 3 the pre-read image signal Sp included in the peak la' shown in FIG. 4 corresponds to, The area other than the area corresponding to the peak 1a' (both ends 1a of sheet 1 in FIG. 3) is the radiographic image 2.
It is recognized that this is the central part 1b (irradiation field) where the area is stored and recorded.

In this way, after the areas of the sheet 1 where the radiographic image 2 is not stored and the areas of the subject area 2a% and the direct radiation area 2b in the radiographic image 2 are separated, the subject area is separated during the main reading. In order to read the area 2a under the best conditions, that is, when the minimum value of the image signal SQ obtained in the main reading is set to S lx and the maximum value to 811aX, the pre-read image is read along the straight line G shown in Fig. 4. The reading conditions for the main reading are determined so that the image information included in the peak 2a' of the signal Sp is read as the image signal S0 within the range of Salx-Smax.

Note that the method for recognizing the irradiation field (the central portion 1b of sheet 1 where radiation iii image 2 is accumulated and recorded) is limited to the above-mentioned method that utilizes the fact that the value of the pre-read image signal Sp is small. For example, the outline of the irradiation field is determined based on image data corresponding to each pixel along a plurality of radial line segments connecting a predetermined point included in the irradiation field and the edge of sheet 1. Find the contour points on each of the above line segments,
A method of recognizing an area surrounded by lines along these outline points as an irradiation field (see Japanese Patent Laid-Open No. 133-259538) may be used.

Further, the method of recognizing the region of the direct radiation section 2b is not limited to the above-mentioned recognition method using the fact that the value of the pre-read image signal Sp is large;
The area of the radiation part 2b may be directly recognized by utilizing the fact that the change in the value of the pre-read image signal Sp obtained from the image signal Sp is small.
(See No. 7).

The stimulable phosphor sheet 1' whose pre-reading has been completed is set at a predetermined position in the main reading section 100' shown in FIG. ' direct radiation part 2b (see Figure 3)
Scanning is performed for other areas. Note that the configuration of the main reading section 100' is approximately the same as the configuration of the above-mentioned pre-reading section 100, so the components corresponding to the respective components of the pre-reading section 100 are the same as those used in the pre-reading section 100. The numbers are indicated with a dash and only the differences are explained.

Laser light? fi. The light beam 9' emitted from 8' is
An acousto-optic light modulator (AOM) 21 that can turn on/off (transmit/shield) the light beam 9' at high speed
After passing through, it is reversely deflected by a rotating polygon mirror 11'. The light beam 9' is directed to the direct radiation part 2b of the sheet 1' (
AOM2
1 controls ON/OFF of the light beam 9'. At this time, the stimulated luminescence light 14' emitted from the sheet 1' is read to obtain an analog signal S', which is logarithmically amplified by a logarithmic amplifier 17' and digitized by an A/D converter 18'. As a result, an image signal SQ is obtained. During this reading (main reading), the photo multibrader 18
The reading conditions such as the gain of ' and the amplification factor of the logarithmic amplifier 17' are set so as to provide the optimum reading conditions for the subject portion 2a based on the output from the calculation unit I9, as described above. This image signal SQ is sent to the calculation section 19'. The arithmetic unit 19' performs appropriate image processing on the image signal SO. The image signal subjected to this image processing is sent to the reproduction device 20, and a radiation image based on this image signal is reproduced and displayed.

In the above embodiment, the prefetch reading unit 100 and the main reading reading unit 1
00' are configured separately, but as described above, the configurations of the prefetch reading section l00 and the main reading reading section 100' are substantially the same, so the prefetching reading section 100 and the main reading reading section 100'
They may also be used together. In this case, after performing pre-reading by scanning with a weak light beam, the stimulable phosphor sheet IT may be moved back once and then scanned again with a strong light beam to perform main reading.

When the pre-reading section and the main reading section are used together, it is necessary to switch the intensity of the light beam for pre-reading and main reading. A method of switching the light intensity by inserting and removing an ND filter or the like on the optical path of the light beam, a method of changing the beam diameter of the light beam, a method of switching the speed of the main scanning and the speed of the sub-scanning, etc. Various known methods can be used.

In addition, during the above-mentioned reading, the AOM 21 is set so that the light beam 9' is irradiated to the entire area of the sheet 1' other than the direct radiation part 2b.
However, since the radiation image is stored and recorded only within the irradiation field, the light beam 9' is applied only to the area other than the direct radiation area within the irradiation field (i.e., the area of the subject part 2a shown in FIG. 3). The area may be read by illuminating the area.

Further, in the above embodiment, the radiation region is directly recognized by analyzing the pre-read image signal, but as described above, the present invention is not applicable only to a system that obtains the pre-read image signal. Furthermore, in the above embodiment, the AOM 21 is used to control the ON/OFF of the light beam 9';
N/OFF control is not limited to this, and for example, a light beam emitted from a semiconductor laser light source is used as excitation light, and the semiconductor laser is directly controlled to control the O of the light beam.
N/OFF may also be performed.

As described above, the present invention directly irradiates an excitation target onto a stimulable phosphor sheet on which a radiation image having a radiation area is accumulated and is recorded, and reads the stimulated luminescent light emitted from the stimulable phosphor sheet by the irradiation. It can be widely used when obtaining an image signal representing the radiation image.

(Effects of the Invention) As explained in detail above, the radiation image reading method of the present invention recognizes the direct radiation area and performs reading by irradiating excitation light to areas other than the direct radiation area of the stimulable phosphor sheet. By doing this, it is possible to obtain an image signal in which the influence of afterglow is extremely reduced, suppressing the occurrence of phenomena such as so-called tailing in the reproduced image, and improving the image quality of the reproduced image. can be done.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing each step of the radiation image reading method of the present invention; FIG. 2 is a perspective view showing an example of a radiation image reading device using an example of the radiation image reading method of the invention; Figure 3 shows an example of a radiation image stored and recorded on a stimulable phosphor sheet, and Figure 4 shows a pre-read image signal obtained from the entire surface of the stimulable phosphor sheet shown in Figure 3. It is a figure showing a histogram. L8. 18'... Arithmetic unit 20... Reproduction device 21... Acousto-optic light modulator (AOM) 100...
・Pre-reading section 30'... Pre-reading section 1... Stimulative phosphor sheet 2... Radiation image 2a... Subject section 2b.
... Direct radiation section 8, 8'... Laser beam i1iX 11, 11'.
... Rotating polygon mirror 14. 14' Stimulated luminescence light 15.
15'...Light guide 16. 16'... Logarithmic amplifier 17. 17'...A/D converter

Claims (1)

[Claims] A stimulable phosphor sheet on which a radiation image having a direct radiation area has been stored and recorded is irradiated with excitation light, and stimulated luminescence light emitted from the stimulable phosphor sheet by the irradiation is read to record the radiation image. In the radiation image reading method for obtaining a representative image signal, the direct radiation area is recognized, and the excitation light is irradiated to a region other than the direct radiation area of the stimulable phosphor sheet and the image signal is read. A radiation image reading method characterized in that: