Patent Number: 063296624
Section: summary

FIELD OF THE INVENTION The present invention relates to a radiation image forming system utilizing a combination which comprises a silver halide photographic material and a couple of radiographic intensifying screens. The invention also relates to a method for forming a radiation image or a radiographic image. BACKGROUND OF THE INVENTION In medical radiography such as X-ray photography, a "both-sided emulsion film type" silver halide photographic material (in which a photosensitive silver halide emulsion layer is provided on either side of a transparent support) is used and two radiographic intensifying screens are placed on both of the front and the back sides of the photographic material. A combination of the photographic material and the radiographic intensifying screens are then exposed to X-rays having passed through a patient. Such "both-sided photographic film system" is generally employed to obtain a radiation image of high quality with high sensitivity. However, the sharpness of the resultant image is often lowered by "cross-over light". The term of "cross-over light" means a visible light emitted by the radiographic intensifying screens which are placed on each side of the photographic material and then passes through the support of the photographic material to reach the photosensitive layer provided on the opposite side to deteriorate the sharpness. The deterioration thus caused by the cross-over light is referred to as "cross-over phenomenon". In order to reduce the cross-over light (hereinafter, often referred to as simply "cross-over"), various studies on a silver halide photographic material (herenafter often referred to as "photographic material" or "photosensitive material") and a radiographic intensifying screen (hereinafter often referred to as "intensifying scree" or "screen") have been made. As the studies on the photographic material, for example, Japanese Patent Provisional Publications No. H1-166031 and No. H1-172828 disclose a method using a mordant polymer and a method using a solid fine crystalline dye, respectively. In those methods, however, a considerably large amount of dye should be incorporated into the photographic material so as to satisfactorily reduce the cross-over light. Such large amount of dye is hardly removed rapidly in a development treatment. Therefore, into practically employable photographic materials, such large amount of dye cannot be incorporated. Further, if the dye is used in a large amount, an intensifying screen is liable to be stained with the dye transferred from the photographic material because the dye is insufficiently fixed to the photographic material. Although various dyes have been proposed to solve this problem, a satisfactory dye still has not been reported. A method using an intensifying screen which can intercept the cross-over light is also proposed in, for example, WO 93-01521, EP 650089 and EP 592724. In the method, luminescence of phosphor is shifted to the ultraviolet region so as to reduce the cross-over light. With respect to an intensifying screen containing Gd.sub.2 O.sub.2 S:Tb phosphor, various proposals are reported. Japanese Patent Provisional Publication No. 61-151534 discloses an intensifying screen in which a light-absorbing dye is contained and the phosphor is chosen so that a light emitted by the intensifying screen may comprise a green light component more than a blue light component. Each of Japanese Patent Provisional Publications No. 62-222200 and No. H4-155297 discloses a screen having differently colored phosphor layer to give an image of improved sharpness. RD83-22709 and RD82-218041 suggest that the sharpness of a resultant image can be improved by using a yellow dyesand/or a light-absorbing dye. Japanese Patent Publication No. 58-2640 teaches that the sharpness can be improved by applying a light-absorbing pigment onto the surface of phosphor layer. U.S. Pat. No. 4,362,944 proposes a surface protective layer which absorbs a part of the emitted light. Further, as an intensifying screen giving a low cross-over, an intensifying screen having a yellow-colored phosphor layer is commercially available (Eastman Kodak Co., Lanex Medium [trade name]). However, it appears to be indispensable that the dyes and/or pigments incorporated into an intensifying screen reduce sensitivity. With respect to an intensifying screen containing a fluorescent dye or pigment, EP 0595089 reports that a phosphor which emits a ultraviolet light improves in corporation with fluorescent dyes the sensitivity. Further, DE 2807398 and DE 3143810 disclose intensifying screens using s fluorescent pigment in the form of fine particles. Although various studies such as described above have been reported, it seems that there are no studies reporting that the cross-over of radiographic intensifying screens containing rare earth phosphors is well reduced using fluorescent dyes or pigments. Under the circumstances, a radiation image-forming system giving a radiation image (or radiographic image) of high quality with high sensitivity (i.e., high radiographic speed) is highly desired, and accordingly it is desired to further improve both radiographic intensifying screens and photographic materials. SUMMARY OF THE INVENTION It is an object of the present invention to provide a combination for radiation image formation and a radiation image-forming system giving a radiation image of high sharpness with high sensitivity. The inventors have studied on the radiographic intensifying screen utilizing a rare earth phosphor such as a terbium activated gadolinium oxysulfide phosphor, and have found that not only cross-over can be effectively reduced but also sensitivity (i.e., radiographic speed) can be remarkably enhanced by a combination of an intensifying screen containing a specific fluorescent dye or pigment and a specific photographic material. A typical rare earth phosphor used in the invention emits luminescence having a main peak at approx. 545 nm, and a silver halide photographic material used in combination with the intensifying screen containing said phosphor is beforehand sensitized with a dye so as to become highly sensitive to a light of a wavelength around 545 nm. The fluorescent dye or pigment in the intensifying screen absorbs a portion of luminescence of the phosphor in the wavelength region shorter than 500 nm (light in this wavelength region mainly causes the cross-over phenomenon because the photographic material absorbs almost no light in that wavelength region) and then emits light in the wavelength region near the main peak of the luminescence (i.e., approx. 545 nm). The dye contained in the photographic material selectively absorbs a light in the wavelength region of longer than 500 nm to prevent the light from crossing over the support of photographic material to reach the photographic emulsion layer on the reverse side. Accordingly, the present invention resides in a combination for radiation image formation which comprises a silver halide photographic material having a support and at least one silver halide emulsion layer provided on each side of the support, and two radiographic intensifying screens each having a support and at least one phosphor layer provided thereon, wherein the phosphor layer contains a rare earth phosphor represented by the following formula (I): EQU M.sub.w O.sub.w X:M' (I) PA1 in which M represents at least one rare earth atom selected from the group consisting of Y, La, Gd and Lu; X represents at least one chalcogen atom selected from the group consisting of S, Se and Te, or at least one halogen atom selected from the group consisting of F, Br, Cl and I; M' represents a rare earth atom which activates M; and w is 2 when X is a chalcogen atom or w is 1 when X is a halogen atom, PA1 the radiographic intensifying screen contains a fluorescent dye or pigment which absorbs a portion of luminescence emitted by the rare earth phosphor and then emits light in a visible region, and PA1 the photographic material shows a cross-over of 10% or less when it is exposed to radiation in combination with said intensifying screen. PA1 said photographic material shows a cross-over of 10% or less when it is placed between the two radiographic intensifying screens and exposed to radiation. PA1 the phosphor layer contains a rare earth phosphor represented by the above-mentioned formula (I), PA1 the radiographic intensifying screen contains a fluorescent dye or pigment which absorbs a portion of luminescence emitted by the rare earth phosphor and then emits light in a visible region, and PA1 the photographic material shows a cross-over of 106% or less when it is exposed to radiation in the system. PA1 forming a combination by placing a silver halide photographic material having a support and at least one silver halide emulsion layer provided on each side of the support between two radiographic intensifying screens each having a support and at least one phosphor layer provided thereon, said phosphor layer containing a rare earth phosphor represented by the aforementioned formula (I); PA1 imagewise exposing the combination to X-ray radiation; PA1 separating the exposed photographic material from the intensifying screens; and PA1 developing the exposed photographic material in a developing solution. The invention further resides in a radiation image-forming system comprising two radiographic intensifying screens each having a support and at least one phosphor layer provided thereon and a silver halide photographic material which is interposed between the two intensifying screens and has a support and at least one silver halide emulsion layer provided on each side of the support, wherein The invention furthermore resides in a method for forming a radiation image which comprises the steps of: In the invention, the rare earth phosphor in the radiographic intensifying screen preferably is a terbium activated gadolinium oxysulfide phosphor. The terbium activated gadolinium oxysulfide phosphor preferably contains terbium atom in an amount of 0.001 to 0.02 mol. per 1 mol. of Gd. The fluorescent dye or pigment preferably shows a light absorption peak in a wavelength region of shorter than 500 nm and an emission peak in the wavelength range of 450 to 600 nm under the condition that the wavelength of emission peak is longer than the wavelength of light absorption peak by at least 10 nm, preferably at least 20 nm. Also preferred is that the fluorescent dye or pigment shows a light absorption peak in a wavelength region of 400 to 490 nm and an emission peak in the wavelength range of 500 to 600 nm. The emission peak of the fluorescent dye or pigment preferably has a half-width of 100 nm or less. The fluorescent dye or pigment in the radiographic intensifying screen preferably is a carbocyanine dye, a xanthene dye, a triarylmethane dye, a coumarin dye, a phthalimide compound, a naphthalimide compound, a diketopyrrolopyrrole compound or a perylene compound. The fluorescent dye or pigment in the radiographic intensifying screen is preferably contained in the phosphor layer. In the combination of the invention, the silver halide photographic material preferably contains a dye which shows a light absorption peak in the wavelength region of 500 to 600 nm. The dye in the silver halide photographic material is preferably provided between the support and the silver halide emulsion layer, in the form of a dye layer. Preferably, this dye is decolorizable in a developing process. Also preferred is that the dye shows an absorption coefficient at 550 nm which is twice or more larger than that at 450 nm. The decolorizable dye preferably is in the form of solid fine particles.