X-RAY INTENSIFYING SCREENS

Abstract: X-ray intensifying screens which provide reduced mottle and noise, and improved image sharpness in photographic films associated therewith are produced by including small amounts of UV light absorbers in either the underlayer, an active layer containing a phosphor exhibiting narrow or line band emission, or the overcoat layer of the overall screen structure.

Description: X-RAY INTENSIFYING SCREENS This invention relates to X-ray intensifying screens used with photographic film elements to record X-ray images thereon, and particularly to screens having improved imaging characteristics. Still more particularly, this invention relates to X-ray screens having controllable speed and to radiographs produced therefrom which exhibit good image sharpness and reduced noise. X-ray intensifying screens made by incorporation of. a luminescent phosphor in a binder and coated on a suitable support are well-known in the prior art. These X-ray screens absorb X-rays and emit energy in the form cf light, which in turn exposes a photographic film associated therewith. The phosphors used in these screens can be generally segregated into three classes: (1) broad band emitters (e.g. Caws4); (2) narrow band emitters (e.g. BaFCl:Eu); and (3) line emitters (e.g. LaOBr:Tm). Conventionally, the X-ray screens made with these phosphors are used with radiographic, silver halide film to record X-ray images. In the field of medical X-ray the use of X-ray screens is particularly useful, since there is a pressing need to reduce patient dosage to potentially harmful X-rays. Screens serve to greatly increase the overall system speed, thus requiring less X-ray output to generate a satisfactory image. One method of increasing speed is to increase the thickness of the phosphor coating. As speed increases, however, there is usually a decrease in image sharpness and thus a decrease in the amount of medical information that can be generated by this system. Conventionally, the speed of an X-ray screen is reduced by lowering the thickness of the phosphor layer or by adding broad band absorbers (e.g. dyes or pigments) to the X-ray screen structure. Controlling screen speed by these prior art methods yields the desired higher sharpness but also increases the noise and mottle of the finished radiograph prepared therefrom. Radiographic noise is a grain or mottle pattern which results from the statistical nature of the imaging process. A source of noise is the distribution of scintillation efficiencies of the individual phosphor crystals. Those crystals with low efficiency may only contribute minimally to the imaging process while those with high efficiency lead to increased density. The fluctuations in developed density appear as noise. By narrowing the distribution of scintillation efficiencies, radiographic noise should be lowered, and broadening it will increase noise. As discussed above, the addition of dyes or pigments to the screen structure effectively broadens the distribution of scintillation efficiencies, thus increasing the noise. The X-ray screen structure conventionally consists of a phosphor-containing layer coated on a suitable support. It is also beneficial to use an underlayer (e.g. reflective or absorbing layer) coated between the phosphor and the support and to include a protective overcoat for the phosphor layer. It is an object of this invention to provide an improved X-ray screen prepared with phosphors exhibiting narrow band or line emissions. It is another object of this invention to provide X-ray screens which can be used with photographic film to achieve controlled speed, good image sharpness, and low noise and mottle. According to one aspect, the present invention provides an X-ray intensifying screen comprising, in order, (1) a support, (2) optionally an underlayer, (3) an active layer comprising a phosphor exhibiting narrow or line band emission dispersed in a binder, and (4) optionally an overcoat layer, characterized in that at least one of layers (2), (3) and (4) contains an effective amount of a nonfluorescing ultra-violet (W) light absorber having absorbance maxima at about 400 nm or below. An X-ray screen made according to the invention, used with a photographic film, will produce a sharper image with lower noise and mottle, compared with a screen made without the UV absorber While the use of a UV abosorber in an X-ray screen is not novel per se (see Martic et al, US-A-3,743,833), the use of a W absorber specifically with narrow or line band phosphor to greatly improve the image quality of these highly efficient materials has not been shown. There are a number of commercially available UV light absorbers that exhibit absorbance maxima at about 400 nm or below. All will function within the ambit of this invention. These include the following: Absorption Absorber Max. (nm) A 2-(2'-hydroxy-3',5'- ¯ 400 dialkylphenyl) benzotriazole B 2,2'-dihydroxy-5- ¯ 400 alkoxy-benzophenone C 2-(2'-hydroxy-3'-alkylphenyl) ¯ 400 benzotriazole D p-methoxybenzylidene ¯ 330 malonic acid, dimethyl ester Trade Name & Co. E essentially a Resyn 28-3307, ¯ 330 carboxylated vinyl National Starch acetate terpolymer Bridgewater, NJ which includes, as part of its chain, a copolymerized UV light absorber F oxalic anilide Sanduvor ALB ¯ 330 derivative Sandoz Inc., E. Hanover, NJ G oxalic anilide Sanduvor VSU, X 330 derivative Sandoz Inc. These absorbers can be dissolved in any convenient organic solvent (e.g. acetone) and added either to layer (2), (3) or (4) in amounts which depend on the effect desired and the layer to which they may be added. Layer (1) is a support. A number of elements can be used such as paper or metal foil, e.g., aluminum. It is preferred to use a macromolecular, hydrophobic organic polymer support. Suitable polymeric supports include cellulose derivatives, e.g., cellulose acetate, cellulose propionate, cellulose acetate butyrate, and ethyl cellulose; polyethylene, polyvinyl chloride, poly(vinyl chloride covinyl acetate), polyvinylidene chloride, polyvinyl acetate, polyacrylonitrile, polystyrene, and polyisobutylene; and polyesters, e.g., polyethylene terephthalates and homologous polyesters obtainable by the process described in Whinfield et al;, US 2,465,319. Polyethylene terephthalate films are particularly useful because of their dimensional stability. A particularly useful support is biaxially oriented polyethylene terephthalate coated with a subbing layer of vinylidene chloride-methylacrylate-itaconic acid copolymer as described in Alles et al., US-A-2,779,684. The thickness of the support may conveniently be anywhere from about 0.0025 in to 0.03 in (0.0064 cm to 0.0762 cm) with 0.01 in (0.0254 cm) being preferred. The support, moreover, may contain or be coated with dyes or finely divided pigments, e.g., titanium oxide, lithopone, magnesium carbonate, aluminum oxide, carbon black, and colored pigments, e.g. tartrazine (C.I. No. 640), Victoria Green W.B. Base (C.I. No. 800), and Nubian Resin Black (C.I. No. 864), as opacifying or light-absorbing agents. The above mentioned dyes and pigments are particularly useful in reflective layers to vary the light output or block unwanted wavelengths. In addition, the base support may be metallized to provide reflective action, e.g., the polyethylene terephthalate base can be coated with a thin layer of aluminum. It is preferred to apply a reflective layer comprising anatase grade TiO2, dispersed in chlorosulfonated polyethylene (e.g., Hypalon 20, E. I. du Pont de Nemours and Company, Inc., Wilmington, DE), on the support. A number of other reflective pigments can also be used in the reflective layer. These include the potassium titanates of Brixner, US-A-3,895,157, among others. There are a host of phosphors which exhibit narrow or line band emission. These include the yttrium tantalates described in Brixner, US-A 4,225,653, especially Y(Nb)TaO4; lanthanum oxyhalides (especially LaOCl:Bi and LaOBr:Tm), and the barium fluorohalides described in Joiner, US-A4,076,897 (especially BaFCl:Eu), among others. The phosphors are usually milled for several hours in a solvent/binder mixture and coated over the above referenced reflective layer. A number of binders can be used in the active layer (3) of this invention. It is preferred to use polyvinyl butyral resins but there are a host of suitable polymers used in the prior art that can be used here. For example, the polyurethane elastomeric binders are particularly effective because of their adhesion to the conventional supports described above. Polyurethanes are, however, subject to degradation by W light. Many of the conventional phosphors emit substantial amounts of W light which can further be reflected back into the active layer by the support; so steps must be taken to prevent this degradation (see Martic et al supra). Hence, it is preferred to use binders other than polyurethanes. A protective overcoat made according to any of the teachings of the prior art may be applied over the phosphor layer. These top coats can be cellulose nitrate or acetate or a mixture of resins comprising poly(methylmethacrylate), poly( isobutyl methacrylate), and vinyl chloride/vinyl acetate copolymers as taught by Patten, US-A-2,907,882. An overcoat similar to that described in Brixner et al in U.S. 3,895,157 is particularly efficacious. The overcoat layer can be from about 0.0001 in. to 0.005 in. (0.0003 cm. to 0.013 cm.) in thickness. It is preferred to used the fluoracrylate overcoats of Joiner, USSN 420,486, filed September 20, 1982, a copy of which application is on the European Patent Office file of the present application. A corresponding European Patent Application, No. 83109212.7, was filed on 17 September 1983 and was published as EP A. An X-ray screen prepared as described above, and in the examples that follow, is eminently useful in medical radiography, for example. The addition of the W light absorber permits one to control the speed of the screen in a predictable fashion. Thus, these screens, when exposed in conjunction with Xray photographic film, will exhibit excellent image sharpness and, importantly, noise and mottle is reduced. This allows more information to be determined from the image. This is a considerable and surprising finding over the prior art which teaches thickness reduction in the phosphor layer to improve image sharpness with a concurrent rise in noise and mottle. In a preferred aspect, the invention provides an X-ray intensifying screen composed of, in order, (1) a film-forming polyester support, (2) an underlayer comprising TiO2 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, whereinat least one of layers (2), (3) and (4) contains an effective amount of a nonfluorescing W light absorber having absorbance maxima below about 400 nm. The invention will now be illustrated by the the following non-limiting examples. EXAMPLE 1 Three X-ray intensifying screens were made comprising, in order, a support (1), a reflective layer (2), an active phosphor layer (3), and an overcoat layer (4). In this example, the UV light absorber was placed in layer (4). The reflective suspension was prepared by sandmilling a mixture of the following ingredients for about 4 hours: Ingredient wt. 8 Anatase TiO2 28.4 n-butyl acetate 35.1 Mixed petroleum naphtha, 23.5 initial B.P. 2470F (1190C) Dioctylester of sodium 1.1 sulfosuccinic acid Polymeric organic silicone fluid 0.6 (2% by weight in toluene) Chlorosulfonated polyethylene 11.3 (Hypalon 20) The milled suspension was filtered through filtering medium with an average pore-size of 30 microns. The suspension was deaerated and then coated on a sheet of biaxially oriented polyethylene terephthalate film provided with a substratum of vinylidene chloride-methylacrylate-itaconic acid copolymer prepared as described in US-A-2,698,240. reflective layer was obtained which gave a TiO2 coating weight of about 0.029 g/in2 (0.0045 g/cm2) After drying this layer at about 70-750F (21.1-23.90C), an active phosphor layer was applied on the reflective layer. The phosphor layer was prepared from the following composition, milled for about 12 hours: Ingredient Wt. % LaOBr:Tm 58.3 polyvinylbutyral(l) 5.6 n-butyl acetate 16.8 n-propanol 16.8 Polymeric organic silicone fluid 0.8 Potassium salt of monoethylphenylphenol 0.2 monosulfonic acid Glycerol monolaurate 1.4 (1) Granular, intrinsic visc. of 0.81 On the dried phosphor layer an overcoat layer was applied from the following solution: : Ingredient Wt. % Cellulose acetate (acetyl content 6.5 of 55.8%) Acetone 92.4 Urea-formaldehyde resin 0.9 2-heptadecyl-N,N-ethyl, 0.1 hydroxyethyl-isoimidazole ethyl sulfate salt Righ boiling waxy compound 0.1 (Acrawax 'C, Glyco Chemicals, Inc. Greenwich, Conn.) UV light absorber .04 Three screens were prepared. Screen I contained W light absorber (A), Screen II contained UV light absorber (B), and Screen III contained UV light absorber (C) at the levels shown above. For control, a screen was prepared without any W light absorber. The above referenced screens were tested by exposure with a high speed medical X-ray film to an X-ray unit at 80 KVp, 2 mAs through a 2 mm aluminum filter. A step wedge and a resolving power target were placed between the X-ray source and the film/screen (in direct contact therewith). Each exposed film was then developed, fixed and washed in a conventional X-ray developing system, and the following results were obtained: : Film Sensitometry Mottle (rms granu Screen(3) Rel. larity Sample Speed(1) Resolution at D=1.00)( (l.p./mm) Control 1.71 8.3 .014 Screen I .98 8.0 .011 ((A)-Tinuvin 328) Screen II .96 8.5 .012 ((B) Cyasorb UV-24) Screen III 1.05 7.7 .012 ((C)-Cyasorb -UV-5411) (1) In all cases the speed is relative to that obtained by exposure of Du Pont Hi-Plus screens and conventional blue-sensitive X-ray film where this speed = 1.00 (2) low numbers=low mottle (see US-A-4,028,550 at column 4, last paragraph). (3) W absorber is identified in brackets Thus, it is seen that addition of the UV light absorber to the overcoat layer (4) significantly reduces the mottle produced by the screen, improving the overall image quality of a film exposed thereto. EXAMPLE 2 A screen was prepared as in Example 1 except that W light absorber (B) was instead added to the active phosphor layer (3) at a level of 0.12% by weight of the phosphor (LaOBr:Tm). The following results were obtained: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle Control 1.76 8.8 .017 Of this 1.05 8.9 .014 invention Obviously the resolution was increased and the mottle significantly reduced when UV light absorber (B) was placed in the phosphor layer (3). EXAMPLE 3 To test the efficiency of this invention when a UV-light absorber was added to the reflective layer, a screen was made as in Example 1 except that UV light absorber (B) was instead added directly to the reflective suspension at a level of 0.18% by weight of the TiO2. The following results were obtained: Film Sensitometrv Screen Rel. Sample Speed Resolution Mottle Control 1.45 8.6 .012 Of this .82 8.6 .010 invention EXAMPLE 4 A W light absorber may also be used in conjunction with a light absorbing underlayer. To test this effect, an active phosphor suspension (see Example 1) containing 0.53% of UV light absorber (B) was coated directly on a polyethylene terephthalate film support which contained sufficient-carbon to yield an optical density greater than 8.0 (opaque base). This simulates the application of an absorbing underlayer (vs. reflective underlayer in the previous examples). The following results were obtained when this screen was used to expose a high speed X-ray film as taught in Example 1: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle Control 1.00 10.4 .014 Of this .65 10.4 .012 invention EXAMPLE 5 To compare screens prepared according to the teachings of this invention with prior art screens, three screens were made with the following structure: support (1) - same as Example 1 reflective layer (2) - same as Example 1 phosphor layer (3) - same as Example 1 except for coating weight and additives (see table, below) overcoat layer (4) - same as Example I excluding the W absorber In Screen I the phosphor layer was coated at a 0.022 inch (0.056 cm) thickness, and UV light absorber (B) at 0.52 wt.% on the phosphor was added. Screen II contained no additive and was coated at a 0.012 inch (0.030 cm) thickness. Screen III contained 1.7 ml of a 1% solution of solvent Yellow 3 Dye (C.I. No.11160:1), a monazo dye dissolved in acetone. The following results were obtained when these screens were used to expose samples of high speed medical X-ray film as described in Example 1: Film Sensitometry Phosphor Coat. Screen Thickess in Rel. Sample mils (mm) Speed Resolution Mottle I of this 22 (o.561 0.93 9.3 .011 invention II undyed 12 t0.3Q3 0.93 10.4 .015 III 1.7 ml 22 (0.5) 0.99 11.1 .013 dye soln. When coated at a thinner coating weight than the screen of this invention, the dyed screens (prior art elements) were ca. 408 higher in mottle, although somewhat sharper in resolution. When dye is added to a screen of comparable thickness, the screen is ca. 20% higher in mottle. Thus, the screen which embodies this invention can greatly improve the signal/mottle ratio and yield an overall improved image on an X-ray film exposed therewith. EXAMPLE 6 CaWO4 is an example of a broad band-emitting phosphor outside the scope of this invention. Three screens were made employing the following structure: The support (1) and reflective layer (2) were the same as described in Example 1. The phosphor layer (3) contained CaWO4 in place of LaOBr:Tm of Example 1. The overcoat layer (4) was the same as Example 1 except for the UV light absorber. Screen I contained no W light absorber. Screen II contained 0.52 weight % of W light absorber (B) and Screen III contained 0.52 weight % of W light absorber (A), based on cellulose acetate. These screens were used to expose high speed medical X-ray film as taught in Example 1, with the following results: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle I no UV absorber 0.51 8.4 .008 II W absorber (B) 0.45 8.6 .008 III UV absorber (A) .51 8.2 .008 As can be seen from these results, in comparison with Example 1, for instance, there is no effect on overall image quality when the UV light absorbers are used with broad band-emitting phosphors. EXAMPLE 7 Example 6 was repeated substituting Ba(Pb)SOo (another broad band-emitting phosphor) with the following results: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle I no UV absorber 0.33 9.1 .009 II UV absorber (B) 0.22 9.1 .009 III UV absorber (A) 0.21 9.0 .008 Here too, the tnT light absorber failed to significantly improve the image quality of the film exposed to a screen having the broad band-emitting Ba(Pb)SO4 phosphor. EXAMPLE 8 To test the effects of the W light absorber in yet another phosphor system, a screen was prepared as described in Example 1 using Y(Nb)TaO4 (narrow band emitter) phosphor in the active layer (3) and 0.52 wt. 8 of UV light absorber (B) in the overcoat layer (4). The following results were obtained: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle I control, 0.72 9.5 .012 no UV absorber II with UV 0.57 9.1 .009 absorber The improvement in image quality is noted here. EXAMPLE 9 Yet another phosphor having a narrow emission band is LaOCl:Bi. Two screens having this phosphor were prepared according to the teachings of Example 1. Screen I was prepared without a UV light absorber while Screen II contained 0.52 wt.% of tnT light absorber (A) in the overcoat layer. The screens were used to expose high speed medical X-ray film with the following results: Film Sensitometrv Screen Rel. Sample Speed Resolution Mottle I no W absorber 0.90 10.8 .017 II UV absorber (B) 0.71 12.2 .013 EXAMPLE 10 Example 9 was repeated using BaFCl:Eu (narrow band emitter) as the phosphor and with 1.04 wt. 8 of W light absorber (B) in the overcoat layer. The following results were noted: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle I control, 1.30 8.0 .013 no UV absorber II with UV 0.73 8.7 .011 absorber The improvement in image quality is noted here. EXAMPLE 11 Example 10 was repeated using LaOBr:Tb (line emitter) as the phosphor and with 0.52 wt.% of W light absorber (B) in the overcoat layer. The following results were noted: Film Sensitometry Screen Rel. Sample Speed Resolution Mottle I control, 1.61 7.7 .012 no tnT absorber II with UV 1.21 8.0 .011 absorber The improvement in image quality is noted here. As can be seen from the preceding examples the use of the UV light absorber within X-ray screens made according to the teachings of this invention, which are limited to those luminescent phosphors that have their primary emission as line or narrow bands, produces an X-ray intensifying screen which has greatly improved imaged characteristics in film used therewith. This was not possible in the prior art which taught that one must reduce the coating weight of the phosphor layer in order to achieve good resolution and subsequently found that the image was noisy. Images produced with the screens of this invention have excellent sharpness and, at the same time, have lower noise.

Claims: CLAIMS 1. An X-ray intensifying screen comprising, in order, (1) a support, (2) optionally an underlayer, (3) an active layer comprising a phosphor exhibiting narrow or line band emmission dispersed in a binder, and (4) optionally an overcoat layer, characterized in that at least one of layers (2), (3) and (4) contains an effective amount of a nonfluorescing ultra-violet (W) light absorber having absorbance maxima at 44out 400 nm or below. 2. A screen as claimed in claim 1 wherein the W light absorber is a member of the group consisting of a 2-(2'hydroxy-3',5'dialkyl-phenyl)benzotriazole, a 2,2'-dihydroxy-5-alkoxy-benzophenone, and an oxalic anilide derivative. 3. A screen as claimed in either of claims 1 and 2 wherein the phospher is LaOBr:Tm. 4. A screen as claimed in any one of claims 1 to 3 wherein the binder is polyvinyl butyral. 5. A screen as claimed in any one of claims 1 to 4 wherein an underlayer (2) is interposed between support (1) and active layer (3). 6. A screen as claimed in claim 5 wherein said under layer contains said nonfluorescing W light absorber having absorbance maxima below 400 nm. 7. A screen as claimed in any one of claims 1 to 6 wherein the support contains or is coated with opacifying dyes, reflective dyes, or finely divided pigments. 8. A screen as claimed in any one of the preceding claims wherein said support (1) is formed from a film forming polyester. 9. A screen as claimed in any one of the preceding claims wherein an underlayer (2) comprising TiO2 dispersed in chlorosulfonated polyethylene is interposed between support (1) and active layer (3). 10. A screen as claimed in any one of the preceding claims having an overcoat layer (4) comprising a fluoroacrylate polymer. 11. An X-ray intensifying screen composed of, in order, (1) a film-forming polyester support, (2) an underlayer comprising TiO2 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, wherein at least one of layers (2), (3) and (4) contains an effective amount of a nonfluorescing W light absorber having absorbance maxima below about 400 nm.

Inventors: ZEGARSKI, WILLIAM J.
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1984
Main classification: G21K4
IPC codes: G21K4