Patent Number: 056419686
Section: summary

FIELD OF THE INVENTION The present invention relates to a radiation image storage panel using a stimulable phosphor and a process for preparing a radiation image storage panel. BACKGROUND OF THE INVENTION As a method replacing a conventional radiography, a radiation image recording arid reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, was proposed and is practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis. The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. If the phosphor layer is self-supporting, however, the support may be omitted. Further, a transparent protective film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock. The phosphor layer generally comprises a binder and a stimulable phosphor (in the form of particles) dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when it is exposed to radiation such as X-rays and then excited with an electromagnetic wave (i.e., stimulating rays). Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the stimulable phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced on the panel in the form of a radiation energy-stored latent image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with the stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals. As described hereinbefore, the radiation image recording and reproducing method is a very advantageous radiation image reproducing method. Even in this method, it is desirable to give a reproduced radiation image of an improved quality such as high sharpness and good graininess with high sensitivity. The sensitivity of the radiation image storage panel essentially depends on the total amount of stimulated emission which is produced by the stimulable phosphor contained in the panel. The total amount of stimulated emission depends on the luminance of each stimulable phosphor particle as well as on the total amount of the stimulable phosphor particles contained in the phosphor layer of the panel. If the amount of stimulable phosphor in the phosphor layer is large, a large amount of radiation such as X-rays is absorbed by the phosphor layer, and hence a radiation image is obtained with a high sensitivity and a high graininess. It is also preferred to prepare the phosphor layer as thin as possible. This is because the thin phosphor layer reduces spread of stimulating rays in the phosphor layer by scattering and gives a reproduced radiation image of high sharpness. U.S. Pat. No. 4,910,407 discloses a compression treatment of a stimulable phosphor layer formed on a support. Thus compressed stimulable phosphor layer contains phosphor particles therein at a density higher than the uncompressed stimulable phosphor layer and shows increased sharpness. However, the compressed stimulable phosphor layer sometimes shows decreased sensitivity and graininess as compared with the uncompressed stimulable phosphor layer, probably because a portion of the phosphor particles are broken down by the compression treatment. U.S. Pat. Nos. 5,153,078 and 5,164,224 disclose a compression treatment of a stimulable phosphor layer comprising stimulable phosphor particles and a binder of a thermoplastic elastomer having a softening or melting point of 30.degree.-150.degree. C. at a temperature higher than the softening or melting point. Thus compressed stimulable phosphor layer shows increased sharpness with no decrease of graininess. In the radiation image recording and reproducing method, the radiation image storage panel is repeatedly employed in the steps of radiation of X-rays (recording of radiation image), irradiation of stimulating rays (reading out of the recorded radiation image), and exposure to erasing light (erasure of residual radiation image). Between these steps, the storage panel is transferred by conveyors such as belts and/or rollers within an apparatus for the radiation image recording and reproducing method. It has been now found that the radiation image storage panel having the compressed stimulable phosphor layer is apt to produce therein cracks in its repeated use in the radiation image recording and reproducing system. The production of cracks are still observed even in the compressed stimulable phosphor layer using as thermoplastic elastomer having a softening or melting point of 30.degree.-150.degree. C. as the binder. The radiation image storage panel having a cracked stimulable phosphor layer cannot reproduce a radiation image of high quality because X-rays or stimulating rays impinged on the cracked phosphor layer is scattered on the cracked portion. SUMMARY OF THE INVENTION Accordingly, the present invention has an object to provide a radiation image storage panel which is capable of giving a reproduced radiation image of high quality and further which shows high durability in the transferring steps in the radiation image reproducing apparatus. Further, the invention provides a process for preparing a radiation image storage panel which gives a reproduced radiation image of high quality and shows high durability. The present inventors have studied on the phenomenon of production of cracks on the stimulable phosphor layer of a radiation image storage panel and found that the cracks are easily produced when the radiation image storage panel is repeatedly bent or repeatedly encounters physical shock in its transfer operation within the radiation image reproducing apparatus. Tension stress produced in the phosphor layer by the bending and physical shock is considered to cause of the production of cracks in the phosphor layer. Based on the finding, the inventors have studied further to look for a binder material which is capable of releasing the tension stress produced in the transfer operation with no adverse effect in respect of the quality of radiation image to be obtained using the radiation image storage panel. The present invention resides in a radiation image storage panel having a phosphor layer comprising a stimulable phosphor and a binder, wherein the binder comprises a thermoplastic elastomer having a softening or melting point of 30.degree. to 150.degree. C. and a modulus of elasticity of not more than 0.3 kgf/mm.sup.2. The softening or melting point is equivalent to a Vicat softening point defined in ASTM D 1525. The radiation image storage panel of the invention preferably comprises a support and the phosphor layer using the above binder polymer, which is prepared by the steps of: coating on a temporary support a phosphor layer-forming coating dispersion which contains a stimulable phosphor and a binder comprising a thermoplastic elastomer having a softening or melting point of 30.degree. to 150.degree. C. and a modulus of elasticity of not more than 0.3 kgf/mm.sup.2 in a solvent to prepare a phosphor sheet; PA1 separating the phosphor sheet from the temporary support; and PA1 fixing the phosphor sheet onto the permanent support under a pressure of not lower than 50 kw/cm.sup.2 and at a temperature higher than the softening or melting point of the thermoplastic elastomer by 10.degree. to 50.degree. C. The preferred embodiments of the radiation image storage panel of the invention are described below: The radiation image storage panel wherein the modulus of elasticity of the thermoplastic elastomer is in the range of 0.001 to 0.1 kgf/mm.sup.2. The radiation image storage panel wherein the thermoplastic elastomer has a tensile strength in the range of 0.1 to 20 kgf/mm.sup.2. The radiation image storage panel wherein the thermoplastic elastomer has a tensile elongation in the range of 10 to 2,000%. The radiation image storage panel which further may have a protective layer and a cushioning layer showing an elongation at rupture of the cushioning layer is not less than 100% between the protective layer and the phosphor layer. The cushioning layer preferably is a layer comprising a polyurethane resin and shows an elongation at rupture in the range of 100 to 2,000%, preferably 300 to 2,000%.