JPH0466429B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a method for recording and reproducing radiographic image information, and more specifically, the present invention relates to a method for recording and reproducing radiographic image information, and more particularly, the present invention relates to a method for recording and reproducing radiographic image information, and more particularly, the present invention relates to a method for recording and reproducing radiographic image information, and more specifically, for recording and storing radiographic image information on a stimulable phosphor sheet that accumulates radiation energy. A radiation image in which the sheet is irradiated with excitation light, the stimulated luminescence light emitted from the sheet is thereby photoelectrically detected to obtain an image signal indicating the image information, and the radiation image is reproduced based on this image signal. The present invention relates to an information recording and reproducing method, and more particularly to a radiation image information recording cassette suitably used in this recording and reproducing method.

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

Using this stimulable phosphor, radiation image information of a subject such as a human body is recorded on a stimulable phosphor sheet, and this stimulable phosphor sheet is scanned with excitation light such as a laser beam to produce stimulated luminescence. Generate light, photoelectrically read the resulting stimulated luminescent light to obtain an image signal, and based on this image signal, a radiation image of the subject is displayed as a visible image on a recording material such as a photographic light-sensitive material or a display device such as a CRT. The present applicant has already proposed a radiation image information recording and reproducing system that outputs as follows. (JP-A No. 55-12429, No. 56-11395, No. 56
−11397 etc. ) 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 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 conditions, the amount of stimulated luminescence light 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. However, by using this electrical signal to output a 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, it is possible to obtain a radiation image that is not affected by fluctuations in radiation exposure. can.

In addition, according to this system, radiation image information stored 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 the radiographic image as a visible image on the display device, it is possible to obtain a very large effect that a radiographic image with excellent observation and interpretation suitability (diagnosis suitability) can be obtained.

However, in the above system, when radiation is irradiated to a subject in order to record radiographic image information of the subject, radiation confusion (Compton scattering and Thomoson scattering) occurs due to elastic collisions and electromagnetic interactions between the radiation and the subject material. The scattered radiation generated by this scattering propagates in three-dimensional random directions and also irradiates the stimulable phosphor sheet. In this way, when the stimulable phosphor sheet is irradiated with the above-mentioned scattered radiation in addition to the main transmitted radiation that should be irradiated (which is responsible for the transmitted radiation image of the subject), the stimulable phosphor sheet is recorded by the main transmitted radiation. The contrast and sharpness of the radiographic image will deteriorate.

Therefore, various attempts have been made to eliminate the influence of the scattered radiation. One such approach is to place a grid that absorbs scattered radiation between the subject and the stimulable phosphor sheet. For example, this grid is shown in Figure 3.
As shown in Fig. 4, lead plates 15a with a thickness of about 1 mm or less are combined in a grid or rows, and by arranging such grids as described above, a random pattern is created. Scattered radiation traveling in the direction becomes absorbed by the lead plate 15a. However, in general radiation image information recording (photography) of the human body, even if this grid is used to reduce scattered radiation, the amount of scattered radiation is approximately the same as the main transmitted radiation amount, which is a considerable level. Therefore, it is impossible to completely eliminate scattered radiation with the above grid.

In addition, first and second slit plates are arranged between the radiation source such as an X-ray tube and the subject, and between the subject and the stimulable phosphor sheet, respectively, to make the radiation into a fan beam. Attempts have also been made to move these slit plates synchronously to scan and record an object using fan beam radiation. In this way, the scattered radiation generated from the subject is
The slit plate prevents the light from reaching the stimulable phosphor sheet. However, with this method, it takes about 2 to 5 seconds to record radiation image information, and the subject tends to move during that time due to body movements, etc., so motion artifacts (false artifacts due to movement) may occur in the recorded image. images) are likely to occur.

(Objective of the Invention) Therefore, the present invention provides a radiation image information recording and reproducing method that can eliminate the influence of scattered radiation without causing the above-mentioned problems, and reproduce radiation images with excellent contrast and sharpness. Another object of the present invention is to provide a radiographic image information recording cassette that is suitably used in this method.

(Structure of the Invention) The radiation image information recording and reproducing method of the present invention stores and records transmitted radiation image information of a subject on a stimulable phosphor sheet as described above, and then irradiates the sheet with excitation light. In a method for recording and reproducing radiation image information, in which stimulated luminescence light emitted from the sheet is photoelectrically detected by a photodetector to obtain a read image signal, and radiation image information is reproduced as a visible image based on this image signal. , radiographic image information is accumulated and recorded on the first and second stimulable phosphor sheets with the aforementioned grid interposed therebetween, and the read image signal S ₁ obtained from the first stimulable phosphor sheet near the subject. Then, weighted subtraction is performed between the signals for corresponding pixels of the read image signal _S2 obtained from the second stimulable phosphor sheet as follows: Sp=αS ₁ −βS ₂ [α, β are weighting coefficients] , the radiation image information is reproduced based on this signal Sp.

Furthermore, the radiation image information recording cassette of the present invention, which is suitably used in the above method, includes first and second stimulable phosphor sheets and a scattered radiation absorbing grid disposed between these sheets. It is something that is stored inside the body.

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

FIG. 1 shows how radiation image information of a subject is recorded by a method according to an embodiment of the present invention. In this case, a recording cassette 10 according to the invention is used as an example. This cassette 10 is arranged so as to face a radiation source 11 such as an X-ray tube,
A subject (human body) 12 as a subject is placed between the cassette 10 and the radiation source 11. As shown in detail in FIG. 2, the cassette 10 includes a first stimulable phosphor sheet 13 as described above and a second stimulable phosphor sheet 14 similar to the one described above.
and a scattered radiation absorbing grid 15 arranged between these sheets 13 and 14 are housed in a storage body 18 consisting of a bottom plate 16 and a cover plate 17. As shown in FIG. 3, the grid 15 is made up of the aforementioned lead plates 15a combined in a lattice shape. Also this grid 15
Alternatively, as shown in FIG. 4, a structure in which lead plates 15a are combined in a row may be used. The cassette 10 is arranged so that the first stimulable phosphor sheet 13 faces the subject 12.

After the subject 12 and the cassette 10 are placed as shown in FIG. 1, the radiation source 11 is driven.
The subject 12 is irradiated with radiation 20 such as X-rays.
Then, the radiation (main transmitted radiation) 20a that has passed through the subject 12 is transmitted to the first and second rays in the cassette 10.
The stimulable phosphor sheets 13 and 14 are irradiated, and both sheets 13 and 14 store a part of the energy of this main transmitted radiation 20a. That is, on these sheets 13 and 14, transmitted radiation image information of the subject 12 carried by this main transmitted radiation 20a is stored and recorded. In addition, in this way, the patient 12 receives radiation 20
When irradiated with radiation 20, a portion of the radiation 20 is scattered in the manner described above. This scattered radiation 20b is 3
The light travels in a dimensionally random direction, and a portion of it also reaches the stimulable phosphor sheets 13 and 14. In this way, the stimulable phosphor sheets 13 and 14 generate scattered radiation 2 as a whole in addition to the main transmitted radiation 20a.
0b, the contrast and sharpness of the radiation images recorded on the stimulable phosphor sheets 13 and 14 by the main transmitted radiation 20a deteriorate.

Next, a mechanism for eliminating the influence of the scattered radiation 20b and reproducing a radiation image with excellent contrast and sharpness will be described. The first and second stimulable phosphor sheets 13 and 14
In the apparatus shown in the figure, image information is first read. stimulable phosphor sheet 13 or 1
4 is a sheet conveying means 21 such as an endless belt;
The paper is transported in the direction of arrow Y for sub-scanning. Further, the laser beam 23 as excitation light emitted from the laser light source 22 is deflected by a light deflector 24 such as a galvanometer mirror, and moves the stimulable phosphor sheet 13 or 14 in the sub-scanning direction Y.
Main scanning is performed in the direction of arrow X, which is substantially perpendicular to In this way, from the location of the sheet 13 or 14 irradiated with the laser beam 23, stimulated luminescence light 25 is emitted in an amount corresponding to the radiographic image information stored and recorded, and this stimulated luminescence light 25 is transmitted to the condenser 26. The light is collected by a photomultiplier (photomultiplier tube) 27 as a photodetector and photoelectrically detected.

The light collector 26 is made by molding a light guide material such as an acrylic plate, and the linear incident end surface 26a is formed by forming the stimulable phosphor sheet 13 or 1.
The light-receiving surface of the photomultiplier 27 is coupled to an annular output end surface 26b extending along the beam scanning line on the photomultiplier 4. The stimulated luminescent light 25 entering the light condenser 26 from the incident end surface 26a travels through the light condensing body 26 through repeated total reflection, exits from the output end surface 26b, and is received by the photomultiplier 27,
The photomultiplier 27 detects the amount of stimulated luminescence light 25 carrying the radiation image information. The preferred shape, material, and manufacturing method of the light condensing body 26 are described in, for example, U.S. Patent No.
It is described in detail in No. 4346295.

The output signal (read image signal) S ₀ of the photomultiplier 27 is digitized by the A/D converter 29 . In this way, a digital read image signal S ₁ is obtained from the first stimulable phosphor sheet 13, and a digital read image signal S ₂ is obtained from the second stimulable phosphor sheet 14. These read image signals
S ₁ and S ₂ each represent an image file 30 made of, for example, a magnetic disk, an optical disk, a magnetic tape, etc.
It is temporarily stored in A and 30B. Thereafter, these read image signals S ₁ and S ₂ are sent to the image files 30A and 30A, respectively.
30B and input to the digital subtracter 31. The subtracter 31 receives these image signals S ₁ and S ₂
A weighted subtraction is performed between the signals for the common pixels as follows: Sp=αS ₁ −βS ₂ [α, β are weighting coefficients]. The signal Sp obtained by this weighted subtraction is sent to the image processing circuit 32, where it undergoes processing such as logarithmic conversion processing, gradation processing, frequency processing, etc. It is input to the playback device 33. The signal Sp is the stimulated luminescence light 25 emitted from the first stimulable phosphor sheet 13 and the second stimulable phosphor sheet 14.
Since it is based on the read image signals S ₁ and S ₂ carrying a light amount of , by using this signal Sp, the transmitted radiation image of the subject 12 is reproduced as a visible image by the image reproduction device 33. By performing the weighted subtraction described above, this signal Sp carries radiation image information of the subject 12 based only on the main transmitted radiation 20a. Therefore, the image reproducing device 33 reproduces a radiation image with high contrast and sharpness while eliminating the influence of the scattered radiation 20b.

Here, by the above weighted subtraction, the scattered radiation 20
The mechanism by which the influence of b can be eliminated and how to obtain the weighting coefficients α and β will be explained in detail.

It is considered that the read image signal S ₁ from the first stimulable phosphor sheet 13 and the read image signal S ₂ from the second stimulable phosphor sheet 14 each consist of the following signal components.

S ₁ = Sp + Ss ………(1) S ₂ = Sp + Ss′ = Sp + kSs ………(2) Here, Sp is the signal component due to the main transmitted radiation, Ss,
Ss' is the signal component due to scattered radiation. Also k
is a constant, and in this case, the scattered radiation irradiated to the second stimulable phosphor sheet 14 is reduced by the grid 15 compared to the scattered radiation irradiated to the first stimulable phosphor sheet 13. Therefore, k<
It is 1. Therefore, from equations (1) and (2) above, Sp=k/k-1S ₁ -1/k-1S ₂ , and in this case, the weighting coefficients α and β mentioned above are set to k/(k-1) and 1, respectively. If weighted subtraction is performed as /(k-1), a signal component Sp due to only the main transmitted radiation will be obtained (that is, the scattered radiation component will be removed).

The simplest method for determining the weighting coefficients α and β is to attach a radiation absorber 50 on the subject 12 in advance and perform radiography, as shown in FIG. The radiation absorber 5
0 is a plate made of lead or the like (preferably cylindrical) with a thickness of several millimeters, and by attaching this to the subject 12 and photographing it, the radiation absorber 50 as shown in FIG. The main transmitted radiation 20a is in the shaded area.
will not reach the target, and only the scattered radiation 20b will reach the target. Image information as shown in FIG. 8 is recorded on the first stimulable phosphor sheet 13 and the second stimulable phosphor sheet 14 subjected to radiography in this manner. When the first and second stimulable phosphor sheets 13 and 14 are scanned with excitation light as shown in FIG. 8, the read image signals S ₁ and S ₂ obtained are as shown in FIG. 9, respectively. Become something. As shown in FIG. 9, scattered radiation 20
Compared to the read image signal S ₀ in an ideal state in which no b occurs at all, the read image signals S ₁ and S ₂ are each offset to the larger light amount side by the amount of stimulated luminescence due to the scattered radiation 20b. These offset amounts can be expressed as cf and bf, respectively. Here, b and c are coefficients corresponding to the content of scattered radiation in the radiation irradiated to the second stimulable phosphor sheet 14 and the first stimulable phosphor sheet 13, respectively, and f is arbitrarily normalized. The distribution function of the scattered radiation, Sp, is the component due to the main transmitted radiation 20a. Also, if the overall average luminescence amount of each image is A, B, and C, then S ₁ - S ₂ = (c - b) f ...... (3), and the distribution function f of scattered radiation is the coefficient (c −
It can be found by multiplying by b). Next, the above equation (3) is
Since it roughly corresponds to C-B in FIG. 9, the component Sp due to the main transmitted radiation 20a can be calculated from S ₂ using the following formula.
is required.

Sp=S ₂ -B-A/C-B (S ₁ -S ₂ ) =-B-A/C-BS ₁ + (1+B-A/C-B) S ₂ In other words, in this case, the weighting coefficient described above α, β
By performing weighted subtraction with α=-(B-A)/(C-B) and β=1+(B-A)/(C-B), respectively, the image signal Sp from which the scattered radiation component has been removed is obtained. can get. The above-mentioned average luminescence amounts A, B, and C can be determined interactively using a display terminal such as a CRT, or can be obtained by absorbing radiation on the first and second stimulable phosphor sheets 13 and 14 in advance. If the position of the body 50 has been identified, it can also be determined automatically.

Note that the signal Sp obtained by weighted subtraction may be inputted immediately to the image reproducing device 33 as described above, or may be temporarily recorded on a recording medium such as a magnetic disk or magnetic tape.

Further, in the above embodiment, the digitized read image signals S ₁ and S ₂ are subjected to weighted subtraction, but as shown in FIG. 6, the analog read image signals S ₁ and S ₂ are subjected to analog subtraction. The analog signal Sp (=αS ₁ −βS ₂ ) obtained as a result of weighted subtraction may be input to the circuit 40 and digitized by the A/D converter 29 . In addition, at this time, if the logarithmic conversion is performed by the log amplifier 28 and then input to the A/D converter 29, the dynamic range of Sp will be narrowed, band compression can be performed, and the Sp will become the radiation attenuation constant distribution of the subject. This is preferable because it corresponds to

In addition, in order to eliminate the influence of fluctuations in recording conditions or to obtain reproduced radiation images with excellent observability, it is necessary to check the recording state of the transmitted radiation image accumulated and recorded on the stimulable phosphor sheet 13 or 14, the properties of the subject, or The recording pattern determined by the recording method etc. is grasped before outputting a visible image for object observation, and the reading sensitivity of the photo multiplier 27 is adjusted to an appropriate value based on the grasped accumulated recording information. Alternatively, it is preferable to perform appropriate signal processing in the image processing circuit 32. Furthermore, in order to obtain a reproduced image with excellent observability, it is required to determine the recording scale factor during A/D conversion so that the resolution is optimized according to the contrast of the recorded pattern.

As a method of grasping the accumulated recorded information of the stimulable phosphor sheet 13 or 14 prior to outputting a visible image, for example, a method such as that shown in Japanese Patent Application Laid-open No. 89245/1983 can be used. . That is, prior to the actual reading, the stimulable phosphor sheet 13 is pre-heated using excitation light with an energy lower than that of the excitation light to be irradiated during the reading operation (main reading) to obtain a visible image for observation of the subject. Alternatively, a reading operation (read ahead) is performed to grasp the accumulated record information stored in sheet 14, and the accumulated record information of sheet 12 or 14 is grasped.
After that, the main reading is performed, and the reading gain can be appropriately adjusted, the recording scale factor can be determined, or appropriate signal processing can be performed based on the information obtained in the above-mentioned pre-reading. Furthermore, the above-mentioned weighting coefficients α and β for the subtraction can also be determined by using the pre-read image.

Furthermore, in the method of the present invention, the cassette 10 shown in FIG. 2 is not used, and the first and second stimulable phosphor sheets are placed facing the radiation source with a grid placed between them. Then, the transmitted radiation image information of the subject may be recorded on both of these sheets. However, the above cassette 1
If 0 is used, it becomes possible to easily set the first stimulable phosphor sheet 13, the second stimulable phosphor sheet 14, and the grid 15 in a fixed positional relationship. ) Work will be done more efficiently.

(Effects of the Invention) As explained in detail above, according to the radiation image information recording and reproducing method of the present invention, radiation image information of a subject is accumulated and recorded on two stimulable phosphor sheets, and images read from these sheets are stored. By reproducing the radiographic image of the subject using the signal obtained by weighted subtraction of the signal, it is possible to eliminate the influence of scattered radiation and reproduce the radiographic image, resulting in a radiographic image with high contrast and sharpness. You will be able to get it. Moreover, in the method of the present invention, the radiographic image information of the subject is recorded on the stimulable phosphor sheet using a general full-surface projection method without using radiation manipulation, so the radiographic image information can be recorded at high speed. Therefore, it is possible to prevent motion artifacts from occurring due to body movements of the subject. Further, by using the radiation image information recording cassette of the present invention, the above method can be easily carried out.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing how radiographic image information is recorded in the method of the present invention, FIG. 2 is a perspective view showing an example of the radiographic image information recording cassette of the present invention used in the above method, and FIG. FIG. 4 is a perspective view showing an example of a grid used in the above method, FIG. 5 is a schematic diagram showing an example of a radiation image information reading and reproducing apparatus for implementing the method of the present invention, and FIG. 6 is a diagram for implementing the method of the present invention. FIGS. 7, 8 and 9 are schematic diagrams showing another example of a radiation image information reading and reproducing apparatus, and are explanatory diagrams for explaining the method of the present invention. 10...Cassette, 11...Radiation source, 12...
...subject (subject), 13...first stimulable phosphor sheet, 14...second stimulable phosphor sheet,
15... Grid, 18... Storage body, 20... Radiation, 20a... Main transmitted radiation, 20b... Scattered radiation, 21... Sheet conveying means, 22... Laser light source, 23... Laser beam, 24... . . . Optical deflector, 25 _. . . Stimulated luminescent light, ₂₇ .

Claims (1)

[Claims] 1. A first stimulable phosphor sheet that stores radiation energy, a grid that absorbs scattered radiation, and a second stimulable phosphor sheet that stores radiation energy from the radiation source side. They are arranged in this order to face the radiation source, and a subject is placed between the radiation source and the first stimulable phosphor sheet, and the radiation emitted from the radiation source and transmitted through the subject is absorbed into the stimulable phosphor sheet. The radiation image information of the subject is accumulated and recorded on these stimulable phosphor sheets by irradiating the phosphor sheets, and then the excitation light is irradiated onto both stimulable phosphor sheets, thereby excitation light is emitted from the first and second stimulable phosphor sheets. The stimulated luminescence light emitted from the stimulable phosphor sheet is photoelectrically read by each photodetector to obtain read image signals S ₁ and S ₂ respectively, and then Sp=αS between the image signals for the common pixel. ₁ - βS ₂ [α, β are weighting coefficients] A weighted subtraction is performed, and the radiation image information is reproduced based on this signal Sp. 2 First and second which accumulate radiation energy
A radiation image information recording cassette comprising a stimulable phosphor sheet and a scattered radiation absorbing grid placed between these sheets.