JPH0517517B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an X-ray intensifying screen for use with a photographic film element on which an X-ray image is recorded, and in particular to a screen having improved image properties. For more details,
The present invention relates to an X-ray screen with controllable speed and to radiographs exhibiting good image clarity and reduced noise. Luminescent phosphor in the binder
X-ray intensifying screens prepared by incorporating phosphors and coated on suitable supports are well known in the art. These X-ray screens absorb X-rays and generate energy in the form of light, which in turn exposes the photographic film associated with it. There are generally three types of phosphors used in these screens: (1) broad band phosphors (e.g. CaWO ₄ ), (2) narrow band phosphors (e.g. BaFC:Eu) and (3) line phosphors (e.g. LaOBr). :Tm). X-ray screens manufactured using these phosphors are used in conjunction with radiographic silver halide films for recording X-ray images. The use of x-ray screens is particularly useful in the medical x-ray field since there is a critical need to reduce patient exposure to potentially harmful x-rays. The screen serves to greatly increase the overall system speed (sensitivity),
Therefore, less X-ray power is required for satisfactory image production. One way to increase speed is to increase the thickness of the phosphor coating. As speed is increased, however, there is usually a decrease in image sharpness and therefore a decrease in the amount of medical information that can be generated from this system. Normally the speed of an X-ray screen is reduced by decreasing the thickness of the phosphor layer or
It is reduced by the addition of broadband absorbers (eg dyes or pigments) to the line screen structure. Although these prior art screen speed controls provide the desired high sharpness, they also increase noise and mottle in the final radiographs produced therefrom. Radiographic noise is a grain or mottling pattern that results from the electrostatic nature of the imaging process. The source of noise is the dispersion of the scintillation effects of individual phosphor crystals. Crystals with low efficiency can contribute minimally to the imaging process, while those with high efficiency lead to increased density. Fluctuations in developer density appear as noise. Narrowing the dispersion of scintillation efficiency should reduce radiographic noise, and widening it increases noise. As discussed above, the addition of dyes or pigments to the screen structure effectively widens the dispersion of scintillation efficiency and thus increases noise. X-ray screen structures usually consist of a phosphor-containing layer coated on a suitable support. An underlayer (e.g. reflective or absorbing layer) coated between the phosphor and the support
It is also advantageous to use and include a protective overcoat over the phosphor layer. It is an object of the present invention to provide an improved X-ray screen manufactured using phosphors exhibiting narrow band or line emission. Another object of the present invention is that the X-ray film can be used with photographic film to achieve controlled speed, good image sharpness and low noise and mottling.
is to provide line screen. These and other objects sequentially include (1) a support, (2) an optional underlayer, and (3) an active layer containing a phosphor exhibiting a narrow or line band emitter dispersed in a binder. , and (4) an X-ray intensifying screen comprising an overcoat layer,
This improvement is achieved by incorporating in layer (2), (3) or (4) a small amount of a non-fluorescent ultraviolet (UV) light absorber having an absorption maximum below 400 nm. X-ray intensifying screens made according to the invention for use with photographic film provide sharp images with lower noise and mottling than screens made without UV absorbers. Although the use of UV absorbers in X-ray screens is not new per se (see U.S. Pat. No. 3,743,833), the use of specifically narrow band or line band phosphors to improve these highly efficient The material has not been shown to significantly improve image clarity. There are a number of commercially available UV light absorbers that exhibit absorption maxima below 400 nm. All work within the scope of this invention. These include the following:

[Table] Sol

[Table] Ndo Company Products〓
These absorbents can be dissolved in any convenient organic solvent (e.g. acetone) and added to layers (2), (3) or (4) in amounts depending on the desired effect and the layer to which they are added. I can do it. Layer (1) is the support. A number of elements can be used, such as paper or metal foil, such as aluminum. Preferably, a polymeric hydrophobic organic polymer support is used. Suitable polymeric supports include cellulose derivatives such as cellulose acetate, cellulose propionate, cellulose acetate butyrate and ethylcellulose, polyethylene, polyvinyl chloride, poly(vinyl chloride/vinyl acetate), polyvinylidene chloride, polyvinyl acetate, polyacrylonitrile. , polystyrene and polyisobutylene, and polyesters such as polyethylene terephthalate and homologous polyesters obtainable by the process described in US Pat. No. 2,465,319. Polyethylene terephthalate film is particularly useful because of its dimensional stability. A particularly useful support is biaxially oriented polyethylene terephthalate coated with a vinylidene chloride/methyl acrylate/itaconic acid copolymer subbing layer as described in U.S. Pat. No. 2,779,684. Support thickness is approximately 0.0025-0.03 inch (0.0064-0.0762
cm). 0.01 inch (0.0254
cm) is preferred. Additionally, the supports may contain dyes or finely divided pigments such as titanium oxide, lithopone, magnesium carbonate, aluminum oxide, carbon black and colored pigments such as tartrazine (CI No. 640), Victoria Green WB base (CI No. 800), and Nubian Resin Black (CI No. .864) as opacifiers or light absorbers or can be coated with them. The dyes and pigments described above are particularly useful in reflective layers to modify light generation or block unwanted wavelengths. Additionally, the base support can be metallized to provide a reflective effect. For example, a polyethylene terephthalate base can be coated with a thin aluminum layer. It is preferred to apply on the support a reflective layer comprising anatase grade _TiO2 dispersed in chlorosulfonated polyethylene (eg Hypalon 20, product of Dupont).A number of other reflective pigments are also reflective. These include, inter alia, potassium titanate, as described in U.S. Pat. No. 3,895,157. There are a number of phosphors that emit narrow or line emission bands. No. 4225653
Yztrium tantalate, especially Y
(Nb) TaO ₄ , lanthanum oxyhalide (especially
Mention may be made inter alia of LaOC:Bi and LaOBr:Tm) and barium fluorohalide (in particular BaFC:Eu) as described in US Pat. No. 4,076,897. These phosphors are usually milled in a solvent/binder mixture for several hours and coated onto the reflective layer referred to above. A large number of binders can be used in the active layer (3) of the present invention.
Although it is preferred to use polyvinyl butyral resins, there is also a group of suitable polymers used in the prior art that can be used herein. For example, polyurethane elastomeric binders are particularly effective because of their adhesion to the aforementioned conventional supports. However, polyurethane is subject to degradation by UV light. Many conventional phosphors produce substantial amounts of UV light. This can further be reflected back into the active layer by the support. Measures must therefore be taken to prevent this decomposition (see above-mentioned US Pat. No. 3,743,833). The use of binders other than polyurethane is therefore preferred. A protective overcoat prepared according to the teachings of the prior art can be applied over the phosphor layer. These topcoats can be cellulose nitrate or acetate or resin mixtures of poly(methyl methacrylate), poly(isobutyl methacrylate) and vinyl chloride/vinyl acetate copolymers (see US Pat. No. 2,907,882).
Overcoats similar to those described in US Pat. No. 3,895,157 are particularly useful. The overcoat layer is approximately 0.0001~0.005 inch (0.0003~0.013
cm) thick. Preferably, the fluoroacrylate overcoat described in US Pat. No. 4,204,87 is used. X-ray screens made as described in the examples above and below are clearly useful, for example, in medical radiography. The addition of UV light absorbers makes it possible to control the speed of the screen in a predictable manner. Thus, when exposed in combination with radiographic film, these screens provide excellent image sharpness and, importantly, reduced noise and mottling. This makes it possible to determine more information from the image. This is a significant and unexpected finding compared to the prior art which teaches that a reduction in phosphor layer thickness improves image sharpness only with a simultaneous increase in noise and mottle. The invention will now be illustrated by the following examples, of which Example 2 is considered the best mode. EXAMPLE 1 Three X-ray intensifying screens were produced, sequentially comprising a support (1), a reflective layer (2), an active phosphor layer (3) and an overcoat layer (4). In this example, UV
A light absorber was placed in layer 4. A reflective suspension was prepared by sand milling the following component mixture for about 4 hours. Ingredients wt% Anatase TiO ₂ 28.4 n-butyl acetate 35.1 Mixed petroleum naphtha (initial bp 247) 23.5 Sodium sulfosuccinate dioctyl ester 1.1 Polymeric organosilicone fluid (2 in toluene)
Weight%) 0.6 Chlorosulfonated polyethylene (Hypalon)
20) 11.3 The milled suspension was passed through a filter medium with an average pore size of 30μ. This suspension was degassed and then coated onto a biaxially oriented polyethylene terephthalate film sheet having a base layer of vinylidene chloride/methyl acrylate/itaconic acid copolymer prepared as described in U.S. Pat. No. 2,698,240. did. Approximately 0.029g/
A reflective layer was obtained giving a TiO ₂ coating weight of inches. After drying this layer at about 70-75°, an active phosphor layer was applied over the reflective layer. This phosphor was made from the following composition by milling for about 12 hours. Component weight% LaOBr: Tm 58.3 Polyvinyl butyral (granular, intrinsic viscosity
0.81) 5.6 n-butyl acetate 16.8 n-propanol 16.8 Polymeric organosilicone fluid 0.8 Monoethylphenylphenol monosulfonic acid potassium salt 0.2 Glycerol monolaurate 1.4 Over the dry phosphor layer is an overcoat layer from the following solutions: was applied. Ingredient weight% cellulose acetate (acetyl content 55.8
%) 6.5 Acetone 92.4 Urea-formaldehyde resin 0.9 n-heptadecyl-N,N-ethyl, hydroxyethyl-isoimidazole ethyl sulfate salt 0.1 High-boiling wax compound [Acrawax C, product of Glico Chemicals] 0.1 UV light Absorbent 0.04 Three screens were made. The screen is
The screen contained UV light absorber A, the screen contained UV light absorber B, and the screen contained UV light absorber C at the levels described above. As a control, a screen containing no UV light absorber was produced. The screen referenced above was tested by exposing it to an x-ray machine at 80 KVp, 2 mAs through a 2 mm aluminum filter with high speed medical x-ray film. A step wedge and resolution target were positioned between (in close contact with) the x-ray source and the film/screen. Each exposed film was then developed in a conventional X-ray development system, fixed and washed. The following results were obtained.

[Table] That is, overcoat layer 4 of UV light absorber
It is found that the addition into the screen significantly reduces the mottling produced by the screen and improves the overall image quality of the film exposed thereto. Example 2 As in Example 1 except that UV light absorber B was added to the active phosphor layer 3 at a level of 0.12% by weight phosphor (LaOBr:Tm). The following results were obtained.

[Table] When UV light absorber B was incorporated into the phosphor layer (3), the resolution was clearly increased and the mottling was significantly reduced. Example 3 To test the efficiency of the invention when a UV light absorber is added to the reflective layer, Example 1 is repeated but UV light absorber B is added directly to the reflective suspension at 0.18 wt% _TiO2. In addition screens were manufactured at the standard. The following results were obtained.

Table: Example 4 UV light absorbers may also be used in combination with light absorbing underlayers. 0.53 wt% to test this effect
Coating an activated phosphor suspension containing UV light absorber B (see Example 1) directly onto a polyethylene terephthalate film support (opaque base) containing sufficient carbon to produce an optical density of 8.0 or higher. did. This simulates the application of an absorbent underlayer (as opposed to a reflective underlayer in the previous example). Example 1 using this screen
When exposed to high-speed X-ray film as taught in , the following results were obtained.

TABLE EXAMPLE 5 Three screens were manufactured with the following construction to compare prior art screens and screens manufactured according to the teachings of the present invention. Support (1) - same as example 1 Reflective layer (2) - same as example 1 Phosphor layer (3) - same as example 1, but with different coating weights and additives (see table below) Overcoat layer 4 - Same as Example 1 In the screen the phosphor layer was coated to a thickness of 0.022 inches and UV light absorber B was added to the phosphor at 0.52% by weight. The screen contained no additives and was coated in a 0.012 inch layer. The screen was 1.7 ml of a 1% solution of monoazo dye Solvent Yellow 3 Dye (CI No. 11160:1) dissolved in acetone.
It contained. When these screens were used to expose high speed medical X-ray film samples as described in Example 1, the following results were obtained.

TABLE When coated with a thinner coating weight than the screen of the present invention, the dye-loaded screen (prior art element) had slightly sharper resolution, but it was about 40% higher in mottling. When dye was added to a screen of comparable thickness, the screen was approximately 20% more mottle-heavy. Thus, a screen incorporating the present invention can greatly improve the signal/pattern ratio and thereby produce an overall improved image of the exposed x-ray film. Example 6 CaWO ₄ is an example of a broadband emissive phosphor that is outside the scope of this invention. Three screens were manufactured using the following configuration. The support (1) and the reflective layer (2) were the same as described in Example 1. The phosphor layer (3) is LaOBr:Tm from Example 1.
It contained _CaWO4 instead. Overcoat layer (4) was the same as Example 1 except for the UV absorber.
The screen did not contain UV light absorbers. Screen contains 0.52% by weight of UV light absorber B
and the screen contains 0.52% UV
Light absorbent A was contained on a cellulose acetate basis. These screens were used to expose the high speed medical X-ray film taught in Example 1. The following results were obtained.

Table: As can be seen from these results, there was no effect on the overall image quality when a UV light absorber was used in conjunction with a broadband emitting phosphor, for example compared to Example 1. Example 7 Ba(Pb)SO ₄ (other broadband emitting phosphors)
Example 6 was repeated substituting . The following results were obtained.

[Table] Again, UV light absorbers are broadband emissive.
The image quality of films exposed to screens with Ba(Pb) _SO4 phosphors was not significantly improved. Example 8 To further test the effectiveness of UV light absorbers in other phosphor systems, active layers were prepared as described in Example 1.
A screen was made using Y(Nb)TaO ₄ (narrow band emitter) phosphor in (3) and 0.52% by weight of UV light absorber B in the overcoat layer (4). The following results were obtained.

[Table] Image quality improvement was observed here. Example 9 Still other phosphors with narrow emission bands are
LaOC: Bi. Two screens with this phosphor were made according to the teachings of Example 1. The screen was manufactured without UV light absorbers, while the screen contained 0.52% by weight in the overcoat layer.
It contained UV light absorber A. This screen was used for high speed medical X-ray film exposure and the following results were obtained.

[Table] Example 10 BaFC as phosphor: Eu (narrow band emitter)
Example 9 was repeated using UV light absorber B in the overcoat layer in an amount of 1.04% by weight. The following results were observed.

[Table] Image quality improvement was observed here. Example 11 Example 10 was repeated using LaOBr:Tb (line phosphor) as the phosphor and UV light absorber B in the overcoat layer in an amount of 0.52% by weight. The following results were observed.

[Table] Image quality improvement was observed here. As can be seen from the examples above, the use of UV light absorbers limited to luminescent phosphors with their primary emission as lines or narrow bands in X-ray screens produced according to the teachings of the present invention. Use produces an X-ray intensifying screen with greatly improved image properties in the film used therewith. This was not possible in the prior art, which taught that the coating weight of the phosphor layer had to be reduced in order to achieve good resolution, and the images were found to be noisy. Images produced using the screen of the invention have excellent clarity and at the same time lower noise.

Claims (1)

Claims: 1 Comprising in sequence: (1) a support, (2) an active layer comprising a phosphor exhibiting narrow or line band emission dispersed in a binder, and (3) an overcoat layer. and wherein layer (2) or (3) contains a small amount of a non-fluorescent ultraviolet (UV) light absorber having an absorption maximum below 400 nm. X-ray intensifying screen. 2 The UV light absorber is 2-(2'-hydroxy-3',
5′-dialkyl phenyl)benzotriazole,
An X-ray intensifying screen according to claim 1, which is from the group consisting of 2,2'-dihydroxy-5-alkoxy-benzophenones and oxalic acid anilide derivatives. 3. The X-ray intensifying screen according to claim 1, wherein the phosphor is LaOBr:Tm. 4. The X-ray intensifying screen according to claim 1, wherein the binder is polyvinyl butyral. 5. The X-ray intensifying screen according to claim 1, wherein the lower layer is inserted between the support 1 and the active layer 2. 6. An X-ray intensifying screen according to claim 1, wherein the support contains or is coated with an opacifying dye, a reflective dye or a finely divided pigment. 7. The X-ray intensifying screen according to claim 5, wherein the lower layer contains a small amount of a non-fluorescent UV light absorber having an absorption maximum below 400 nm. 8 Sequentially (1) a film-forming polyester support, (2) a bottom layer comprising TiO ₂ dispersed in chlorosulfonated polyethylene, (3) an active layer comprising LaOBr:Tm dispersed in polyvinyl butyral. , and (4) an overcoat layer consisting essentially of a fluoroacrylate polymer, such that layer (2), (3) or (4) contains a small amount of non-fluorescent material having an absorption maximum below 400 nm. An improved X-ray intensifying screen characterized in that it contains a photoluminescent UV light absorber.