JP3130389B2 - Radiation intensifying screen - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiographic intensifying screen (hereinafter simply referred to as "intensifying screen"). More particularly, the present invention relates to an intensifying screen particularly excellent in graininess.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for reducing exposure of a subject, and as a countermeasure, for example, in direct X-ray imaging using a combination of intensifying screen and film when performing a transmission inspection of a human body by X-rays. In the field, reduction of exposure by increasing the sensitivity of intensifying screens and films has been measured. There are various methods for increasing the sensitivity, such as the use of a high-sensitivity film, the use of a phosphor with high luminous efficiency, the increase in the phosphor coating weight, and the use of a reflective support for an intensifying screen. The use of a high-sensitivity film causes little decrease in sharpness, but deteriorates graininess. On the other hand, when the sensitivity of the intensifying screen is increased, deterioration of graininess or reduction of sharpness becomes a problem in any of the above cases. The discrimination ability of the subject is related to both the graininess and the sharpness, and there is a problem that the deterioration of the graininess lowers the discrimination ability of a subject having a particularly low contrast.

[0003] Further, the sharpness has been remarkably improved by the advancement of the film crossover reduction technology. Therefore, the necessity of improving the graininess has been further strengthened. In many cases, it is effective to significantly improve the graininess even if the content is reduced.

[0004]

In order to reduce the exposure and improve the sharpness, it is effective to use a phosphor having a high luminous efficiency. However, in this case, the granularity is inferior and the luminous efficiency is low. If a phosphor with a low phosphor is used, the graininess is good, but the sensitivity and sharpness are inferior.There is some reciprocity between the photographic properties, and intensifying screens with a single phosphor layer configuration It was difficult to satisfy these characteristics.

On the other hand, when a plurality of phosphors, particularly a plurality of phosphors having different luminous efficiencies, are used as a uniform mixture, the graininess is poor. Further, when these different kinds of phosphors are formed in layers, the balance of sensitivity, sharpness, and granularity changes depending on various factors such as the layer arrangement state and the particle diameter of the phosphors.

That is, in an intensifying screen composed of a plurality of phosphor layers made of different phosphors, a fluorescent material made of a phosphor having low luminous efficiency is provided on the outermost surface (the outermost surface on the side from which light is emitted, the same hereinafter). If the body layers are arranged, the granularity is improved, but the light emission from the phosphor layer having a higher luminous efficiency on the support side than the outermost phosphor layer is reduced by the outermost phosphor layer. The sharpness is reduced. In the case of the reverse arrangement, the sharpness is good but the graininess is inferior.
Thus, it has been difficult to improve the granularity by reducing the decrease in sensitivity and sharpness.

An object of the present invention is to provide an intensifying screen in which the reduction in sensitivity and sharpness is minimized and the graininess is improved.

[0008]

Means for Solving the Problems The present inventors have studied the construction of an intensifying screen for improving the graininess without lowering the sensitivity and sharpness as much as possible. As a result, in a radiographic intensifying screen having a support and a plurality of phosphor layers made of different kinds of phosphors sequentially formed on the support, the luminous efficiency of the phosphor in the topmost phosphor layer is reduced. The luminous efficiency of the phosphor in the next phosphor layer directly adjacent to the phosphor layer on the outermost surface is 2/3 or less, and the particle diameter of the phosphor in the phosphor layer on the outermost surface is 5 μm or less. Further, it has been found that the above object can be achieved when the thickness of the phosphor layer on the outermost surface corresponds to a phosphor application weight of 40 mg / cm ² or less.

In the present specification, the luminous efficiency of the phosphor in the phosphor layer is defined as a phosphor layer having a phosphor coating weight of 40 mg / cm ² using a phosphor of a specific type and particle size. From the X-ray absorptance (A) for X-rays at a voltage of 80 kV and the photographic sensitivity (S) when combined with a specific film, S / A
Means the value obtained by the following equation.

As a result of many trials and errors for improving graininess, the luminous efficiency of the phosphor in the phosphor layer on the outermost surface obtained by the above definition becomes lower than that of the next layer directly adjacent to the phosphor layer on the outermost surface. rather smaller than the emission efficiency of the phosphor in the phosphor layer, or
First, the particle size of the phosphor in the phosphor layer on the outermost surface should be 5 μm or less.
As a result , it was confirmed that the sensitivity and sharpness were less reduced and the graininess was improved. The luminous efficiency of the phosphor in the uppermost phosphor layer is 2/3 of the luminous efficiency of the phosphor in the next phosphor layer directly adjacent to the uppermost phosphor layer.
When the ratio exceeds, the effect of improving the graininess is small, but when the ratio is 2/3 or less, the effect of improving the graininess becomes sufficient.
By setting the ratio to 1/2 or less, the effect of improving graininess becomes remarkable.

The arrangement of the phosphor layers includes three or more phosphor layers, and a phosphor layer composed of the phosphor having the highest luminous efficiency among the plurality of phosphor layers is not adjacent to the outermost phosphor layer. A configuration for use on the body side is also conceivable, but this case is not preferable because sensitivity is greatly reduced due to light absorption by two or more phosphor layers on the outermost surface side of the layer. Therefore, when three or more phosphor layers are stacked, it is preferable that the phosphor layer composed of the phosphor having the highest luminous efficiency is adjacent to the phosphor layer on the outermost surface.

Since the luminous efficiency of the phosphor in the phosphor layer differs depending on the type of film used in combination, the case of an intensifying screen used in combination with a regular film and the case of an intensifying screen used in combination with an ortho film And the preferred phosphor as the phosphor in the phosphor layer on the outermost surface is different. Radiation intensifying screens used in combination with regular films include CaWO ₄ , Gd ₂ O ₂ S: Tb, BaSO ₄ : Pb, Hf
O ₂ : Ti, HfP ₂ O ₇ : Cu, CdWO ₄ , GdTaO ₄ : Tb, Gd ₂ O ₃・ Ta ₂ O
₅・ B ₂ O ₃ : Tb, (Gd, Y) ₂ O ₂ S: Tb, La ₂ O ₂ S: Tb, (Gd, Y) ₂ O
_At least one of 2S: Tb: Tm can be used, and CaWO ₄ , BaSO ₄ : Pb,
HfO ₂ : Ti, GdTaO ₄ : Tb, Gd ₂ O ₃・ Ta ₂ O ₅・ B ₂ O ₃ : Tb, YTaO
_{4, YTaO 4: Tm, YTaO} 4: Nb, HfO 2: Ti, HfP 2 O 7: Cu, CdWO
₄ , at least one of La ₂ O ₂ S: Tb can be used.

The light transmittance of the phosphor layer changes depending on the phosphor particle diameter and the thickness of the phosphor layer (phosphor coating weight), and the light transmittance is smaller when the small particle phosphor is used at the same film thickness. Thus, the thickness of the phosphor layer for obtaining a constant light transmittance can be reduced. In other words, when a small particle phosphor is used for the low-efficiency phosphor layer, the light transmittance of the low-efficiency phosphor layer becomes small. Effect of light emission from body layer
And the effect of improving graininess can be provided. In this case, since the low-efficiency phosphor layer is a thin film, the sharpness is hardly reduced, the X-ray absorption by the low-efficiency phosphor layer is small, the emission intensity of the high-efficiency phosphor layer is increased, and the sensitivity is advantageous. Become.

FIG. 1 is a graph illustrating the relationship between the phosphor coating weight and the light transmittance of the phosphor layer for an intensifying screen using Gd ₂ O ₂ S: Tb having different average particle diameters. The average particle size is 10.0 μm, the curve 2 is the average particle size of 5.0 μm, and the curve 3 is the average particle size of 3.8.
μm. As is clear from FIG. 1, as for the particle size of the phosphor in the phosphor layer on the outermost surface, the light transmittance at the same phosphor coating weight decreases as the particle size decreases, and the particle size of 5 μm or less decreases. By using the phosphor having a small thickness, that is, the phosphor coating weight 4
The graininess can be improved only by providing a low-efficiency phosphor layer having a thickness corresponding to 0 mg / cm ² or less, and a decrease in sharpness and sensitivity can be minimized. Therefore, it is preferable that the particle size of the phosphor used in the outermost phosphor layer is 4 μm or less, and the thickness of the outermost phosphor layer is a thickness corresponding to the phosphor coating weight of 20 mg / cm ² or less. Is preferred.

[0015] Te intensifying the smell of the invention, the total phosphor coating weight of the plurality of phosphor layers (after coating, the weight per unit area of the phosphor when dried) is practical photographic speed and sharpness In order to maintain the degree, generally 30 to 200 mg / cm ²
It is desirable that When the coating weight is less than 30 mg / cm ² , the photographic sensitivity decreases, and when the coating weight exceeds 200 mg / cm ² ,
The sensitivity is saturated and does not increase, while the sharpness tends to decrease.

As a general method for preparing the intensifying screen of the present invention, a phosphor is mixed with a binder in an appropriate amount, and an organic solvent is added thereto to prepare a phosphor coating solution having an appropriate viscosity. The coating liquid is applied onto the support with a knife coater, a roll coater or the like, and the drying step is repeated to form a plurality of phosphor layers. Some intensifying screens have a structure in which a light reflecting layer, a light absorbing layer or a metal foil layer is provided between the phosphor layer and the support. In this case, the light reflecting layer is previously provided on the support. Then, a light absorbing layer or a metal foil layer is provided, and the phosphor coating solution is applied thereon and dried to form a phosphor layer.

Examples of the binder include nitrified cotton, cellulose acetate, ethyl cellulose, polyvinyl butyral, linear polyester, polyvinyl acetate, vinylidene chloride-vinyl chloride copolymer, vinyl chloride-vinyl acetate copolymer, polyalkyl- (meth) acrylate, There is no particular limitation as long as it is conventionally known as a binder for intensifying screens, such as polycarbonate, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, gelatin, polysaccharides such as dextrin, and gum arabic.

The organic solvent used for preparing the phosphor coating solution includes, for example, ethanol, methyl ethyl ether, butyl acetate, ethyl acetate, ethyl ether, xylene and the like. In addition, a dispersant such as phthalic acid and stearic acid and a plasticizer such as triphenyl phosphate and diethyl phthalate can be added to the phosphor coating solution, if necessary.

On the phosphor layer formed as described above, a protective film is further formed as necessary. In its formation, cellulose acetate, nitrocellulose, cellulose derivatives such as cellulose acetate butyrate, polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, polycarbonate, polyvinyl butyral, polymethyl methacrylate, polyvinyl formal, polyurethane and the like A protective film coating solution having an appropriate viscosity is prepared by dissolving the resin in a solvent, and this is coated on the phosphor layer formed previously, and dried or dried, or a protective film formed in advance, for example, polyethylene terephthalate, polyethylene, A protective film is formed by laminating a transparent film such as polyvinylidene chloride or polyamide on the phosphor layer.

The support used in the intensifying screen of the present invention includes polyesters such as cellulose acetate, cellulose propionate, cellulose acetate butyrate and polyethylene terephthalate, polystyrene, polymethyl methacrylate, polyamide, polyimide, vinyl chloride-vinyl acetate. A resin such as a copolymer or polycarbonate molded into a film, baryta paper, resin coated paper, ordinary paper, aluminum alloy foil, or the like is used. When the above-mentioned plastic film or paper is used as the support of the intensifying screen of the present invention, a light absorbing substance such as carbon black or a light reflecting substance such as titanium dioxide or calcium carbonate is kneaded into these. For example, these may be mixed in advance.

The intensifying screen of the present invention can be manufactured by a method other than the above-described manufacturing method. That is, a protective film is formed on a smooth substrate in advance, and after a plurality of phosphor layers are sequentially formed thereon, this is peeled off from the substrate together with the protective film, and the support is again formed on the phosphor layer. It is a method of bonding.

[0022]

The invention can be better understood with reference to the following specific examples.

Example 1 and Comparative Examples 1 to 3 10 parts by weight of Gd ₂ O ₂ S: Tb having an average particle size of 9.5 μm as a phosphor, 1 part by weight of a vinyl chloride-vinyl acetate copolymer as a binder and an organic solvent Ethyl acetate was mixed to prepare a phosphor coating solution. This phosphor coating solution was uniformly coated on a 250 μm thick polyethylene terephthalate film support kneaded with carbon black using a knife coater so that the phosphor coating weight after drying was 50 mg / cm ^2. And dried to form a phosphor layer.

Next, a phosphor having an average particle diameter of 3.5 μm was used.
A phosphor coating solution was prepared by mixing 10 parts by weight of GdTaO ₄ : Tb, 1 part by weight of a vinyl chloride-vinyl acetate copolymer as a binder, and ethyl acetate as an organic solvent. The phosphor coating solution was dried on the phosphor layer, and the phosphor coating weight after drying was 10 mg / c.
The mixture was uniformly applied with a knife coater so as to obtain m ² and dried to form a phosphor layer.

Next, a transparent protective film made of a polyethylene terephthalate film having a thickness of 12 μm was laminated on the phosphor layer to prepare an intensifying screen (Example 1). Apart from this, the average particle diameter is 9.5 μm as a phosphor as a comparative example
Using only Gd ₂ O ₂ S: Tb, the phosphor coating weight after drying was 50
An intensifying screen (Comparative Example 1) was prepared in the same manner as in Example 1 except that a single phosphor layer of mg / cm ² was provided.

Further, the phosphor has an average particle diameter of 3.5 μm.
Instead of GdTaO ₄ : Tb, GdTaO ₄ : T with an average particle size of 8.5 μm
Example 1 except that the phosphor was applied uniformly using a knife coater so that the phosphor coating weight after drying was 10 mg / cm ² using b.
An intensifying screen (Comparative Example 2) was prepared in the same manner as described above. further,
Average the phosphor particle diameter 3.5 [mu] m of GdTaO _4: Instead of Tb, average particle diameter 8.5 [mu] m of GdTaO _4: using Tb, a knife coater so phosphor coating weight after drying of 20 mg / cm ² An intensifying screen (Comparative Example 3) was prepared in the same manner as in Example 1 except that the coating was performed uniformly.

The sensitivity of the radiographic intensifying screen of Example 1 and the intensifying screens of Comparative Examples 1 to 3 obtained as described above was measured using an orthotype film (Super HR-S manufactured by Fuji Photo Film Co., Ltd.). , Sharpness and graininess were measured.
The result shown in FIG. Comparative Example in which the intensifying screen of Example 1 of the present invention used Gd ₂ O ₂ S: Tb having an average particle diameter of 9.5 μm and provided a single phosphor layer having a phosphor coating weight of 50 mg / cm ² after drying. The intensifying screen had almost the same sensitivity and sharpness as the intensifying screen No. 1, and the graininess was greatly improved. GdT with an average particle size of 3.5 μm as a phosphor
GdTaO ₄ : Tb with average particle size 8.5 μm instead of aO ₄ : Tb
Although the intensifying screens of Comparative Examples 2 and 3 using have better graininess than the intensifying screens of Comparative Example 1, the intensifying screens are inferior to the intensifying screens of Example 1 of the present invention in graininess. Sharpness was also poor.

In Table 1, the relative luminous efficiency of the phosphor is obtained from the luminous efficiency obtained by S / A according to the above-described method, from the luminous efficiency of the phosphor using Gd ₂ O ₂ S: Tb having an average particle diameter of 9.5 μm. Is expressed as a relative value determined as 1.0. The photographic performance of the intensifying screen is as follows: photographic sensitivity, sharpness,
It shows the granularity, and each evaluation value is shown by the following values: Photo sensitivity: expressed as a relative value when the sensitivity of the intensifying screen of Comparative Example 1 is set to 100; Sharpness: Spatial frequency The MTF value at 2 lines / mm was determined, and expressed as a relative value when the MTF value of the intensifying screen of Comparative Example 1 at the spatial frequency was set to 100; graininess: photographic density 1.0, spatial frequency 0.1 to Displayed as RMS value at 0.5 lines / mm.

[0029]

[Table 1]

Example 2 and Comparative Examples 4 and 5 The intensifying screen (Example 2 and Comparative Example 4) was carried out in the same manner as in Example 1 except that the phosphor and the coating weight of the phosphor were changed as shown in Table 2.
And 5) were prepared. Table 2 reprints Comparative Example 1 for comparison.

The intensifying screen of Example 2 and the intensifying screens of Comparative Examples 1, 4 and 5 obtained as described above were obtained by using an orthotype film (Super HR-S manufactured by Fuji Photo Film Co., Ltd.). Table 2 shows the results of measurement of the sensitivity, sharpness, and granularity.
The result shown in FIG. In Table 2, the relative luminous efficiency of the phosphor was calculated from the luminous efficiency obtained by S / A according to the method described above, and the luminous efficiency of the phosphor using Gd ₂ O ₂ S: Tb having an average particle diameter of 9.5 μm was calculated as follows. It was indicated by a relative value obtained as 0. The photographic sensitivity and sharpness were expressed as relative values with the photographic sensitivity and sharpness of the intensifying screen of Comparative Example 1 set to 100, respectively. The intensifying screen of Example 2 of the present invention has Gd ₂ O ₂ S: Tb having an average particle size of 9.5 μm.
And the sensitivity and sharpness were almost the same as those of the intensifying screen of Comparative Example 1 provided with a single phosphor layer having a phosphor coating weight of 50 mg / cm ² after drying, and the graininess was greatly improved. . Instead of CaWO _{4 having} an average particle diameter of 4.1 μm as a phosphor, an average particle diameter of 10.0 μm was used.
The intensifying screens of Comparative Examples 4 and 5 using mWO of CaWO ₄ have higher graininess than the intensifying screen of Comparative Example 1, but have a lower graininess than the intensifying screen of Example 2 of the present invention. Inferiority and sharpness were also inferior.

[0032]

[Table 2]

Example 3 and Comparative Examples 6 to 8 Intensifying screens (Example 3 and Comparative Example 6) were made in the same manner as in Example 1 except that the phosphor and the coating weight of the phosphor were changed as shown in Table 3.
To 8).

The sensitivity and sharpness of the intensifying screen of Example 3 and the intensifying screens of Comparative Examples 6 to 8 obtained as described above were measured using a regular type film (New RX manufactured by Fuji Photo Film Co., Ltd.). When the graininess was measured, the results shown in Table 3 were obtained. In Table 3, the relative luminous efficiency of the phosphor was calculated from the luminous efficiency obtained by S / A according to the method described above, with the luminous efficiency of the phosphor using LaOBr: Tm having an average particle diameter of 8.0 μm as 1.0. Indicated by value. The photographic sensitivity and sharpness were expressed as relative values with the photographic sensitivity and sharpness of the intensifying screen of Comparative Example 6 being 100.

The intensifying screen of Example 3 of the present invention has an average particle size of 8.
Using 0 μm LaOBr: Tm, the phosphor coating weight after drying is 50
The sensitivity and the sharpness were almost the same as those of the intensifying screen of Comparative Example 6 provided with a single phosphor layer of mg / cm ² , and the graininess was greatly improved.
The intensifying screens of Comparative Examples 7 and 8 in which CaWO ₄ having an average particle diameter of 10.0 μm was used in place of CaWO _{4 having} an average particle diameter of 4.1 μm as the outermost phosphor had more granularity than the intensifying screen of Comparative Example 6. Although improved, the graininess and the sharpness were inferior to those of the intensifying screen of Example 3 of the present invention.

[0036]

[Table 3]

Example 4 and Comparative Examples 9 to 11 Intensifying screens (Example 4 and Comparative Example 9) were made in the same manner as in Example 1 except that the phosphor and the phosphor coating weight were as shown in Table 4.
To 11).

Using the intensifying screen of Example 4 and the intensifying screens of Comparative Examples 9 to 11 obtained as described above, an orthotype film (Super HR-S manufactured by Fuji Photo Film Co., Ltd.) was used.
When the sensitivity, sharpness, and granularity were measured, the results shown in Table 4 were obtained. In Table 4, the relative luminous efficiency of the phosphor was determined by comparing the luminous efficiency of the phosphor using Gd ₂ O ₂ S: Tb having an average particle diameter of 9.5 μm with 1.0 from the luminous efficiency obtained by S / A according to the method described above. The relative value was calculated. The photographic sensitivity and sharpness were expressed as relative values with the photographic sensitivity and sharpness of the intensifying screen of Comparative Example 9 set to 100, respectively.

The intensifying screen of Example 4 of the present invention had almost the same sensitivity and sharpness as the intensifying screen of Comparative Example 9, and the graininess was greatly improved. GdTa with an average particle size of 3.5 μm as a phosphor
The intensifying screens of Comparative Examples 10 and 11 using GdTaO ₄ : Tb having an average particle size of 8.5 μm instead of O ₄ : Tb have improved granularity compared with the intensifying screen of Comparative Example 9; Compared with the intensifying screen of Example 4, the intensifying screen of Comparative Example 10 was inferior in graininess,
The intensifying screen of Comparative Example 11 was inferior in sharpness.

[0040]

[Table 4]

[0041]

According to the present invention, it is possible to provide an intensifying screen with improved graininess eliminating only small as possible reduction in the sensitivity and sharpness.

[Brief description of the drawings]

FIG. 1 is a graph illustrating the relationship between the phosphor coating weight and the light transmittance of a phosphor layer for intensifying screens using Gd ₂ O ₂ S: Tb having different average particle diameters.

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masaaki Nakamura 1060 Narita, Odawara-shi, Kanagawa Kasei Optonics Co., Ltd. Odawara Plant (58) Field surveyed (Int.Cl. ⁷ , DB name) G21K 4/00 G03C 5/17

Claims (4)

(57) [Claims] 1. A radiation intensifying screen having a support and a plurality of phosphor layers each formed of a different kind of phosphor sequentially formed on the support, the outermost surface (the outermost surface on the side from which light is extracted) of the radiation intensifying screen. The luminous efficiency of the phosphor in the phosphor layer is not more than 2/3 of the luminous efficiency of the phosphor in the next phosphor layer directly adjacent to the phosphor layer on the outermost surface, and the phosphor layer on the outermost surface is A radiographic intensifying screen, wherein a phosphor particle diameter in the inside is 5 μm or less, and a thickness of the phosphor layer on the outermost surface corresponds to a phosphor application weight of 40 mg / cm ² or less. 2. The luminous efficiency of the phosphor in the phosphor layer on the outermost surface is not more than 1/2 of the luminous efficiency of the phosphor in the phosphor layer of the next layer directly adjacent to the phosphor layer on the outermost surface. The radiographic intensifying screen according to claim 1, wherein: 3. The phosphor particle diameter in the phosphor layer on the outermost surface is 4
The radiation intensifying screen according to claim 1, wherein the diameter is not more than μm. 4. The radiographic intensifying screen according to claim 1, wherein the thickness of the phosphor layer on the outermost surface is equivalent to the phosphor application weight of 20 mg / cm ² or less.