BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a radiation image read-out apparatus in a radiation image recording and reproducing system using a stimulable phosphor sheet. 
     2. Description of the Prior Art 
     A radiation image recording and reproducing system using a stimulable phosphor sheet is disclosed, for example, in U.S. Pat. Nos. 4,258,264 and 4,315,318, and Japanese Unexamined Patent Publication No. 56(1981)-11395. The system was also announced at meetings of the radiation society, in &#34;Nikkan Kogyo Shinbun&#34; (Daily Industrial Newspaper), June 23, 1981 edition, page 16, and elsewhere, and attracted attention in various fields. 
     The aforesaid radiation image recording and reproducing system is capable of providing various radiation images useful for viewing, particularly for diagnostic purposes, which could not be obtained by conventional radiography using a silver halide photographic material. Moreover, the radiation image recording and reproducing system can further provide improved diagnostic effects when combined with an up-to-date medical image recording technique such as computed tomography (CT) or introduction of radioisotope into the human body. Therefore, the system is very effective for viewing purposes, particularly for medical diagnosis. 
     The aforesaid radiation image recording and reproducing system generally carries out the steps of (i) recording (storing a radiation image in a stimulable phosphor sheet), (ii) read-out (converting the stored image into an image signal, and sometimes storing the read-out image signal on a conventional medium such as magnetic tape), and (iii) reproducing (converting the image signal into a visible image, i.e. displaying the visible image on a display device such as a cathode ray tube (CRT) or permanently recording the visible image on a recording medium such as photographic film). The present invention is concerned with the read-out step in the aforesaid radiation image recording and reproducing system. 
     FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system. 
     In the apparatus of FIG. 1, a laser beam 1a of a predetermined intensity is emitted as stimulating rays from a laser beam source 1 to a galvanometer mirror 2. The laser beam 1a is deflected by the galvanometer mirror 2 to form a laser beam 1b impinging upon a stimulable phosphor sheet 3 positioned below the galvanometer mirror 2 so that the stimulable phosphor sheet 3 is scanned by the laser beam 1b in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam 1b impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1b. As the stimulable phosphor sheet 3 is scanned by the laser beam 1b, the portion of the sheet 3 exposed to the laser beam 1b emits light having an intensity proportional to the stored radiation energy. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its light input face 4a positioned close to the sheet 3 in parallel to the main scanning line. A light guiding mirror 10 having a plane parallel to the main scanning direction is positioned to stand face to face with the light input face 4a. The light guiding mirror 10 reflects the light, which is emitted by the stimulable phosphor sheet 3 towards the light guiding mirror 10, to the light input face 4a, thereby improving the light guiding efficiency of the light guide member 4. 
     The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its light input face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5. A filter (not shown) transmitting the light emitted by the stimulable phosphor sheet 3 and absorbing the stimulating rays is positioned between the rear end portion 4c of the light guide member 4 and the photomultiplier 5, thereby intercepting the stimulating rays entering the light guide member 4 by being reflected by the stimulable phosphor sheet 3 after impinging thereupon. Therefore, only the light emitted by the stimulable phosphor sheet 3 is guided to the photomultiplier 5. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or is stored in a magnetic tape 8. 
     However, in the radiation image read-out apparatus arranged as described above, a problem arises when the stimulable phosphor sheet 3 is scanned by the laser beam (stimulating rays) 1b. Namely, the laser beam 1b impinging upon the stimulable phosphor sheet 3 is reflected by the sheet 3 to the light input face 4a and the light guiding mirror 10, and is further reflected by the light input face 4a and the light guiding mirror 10 to portions of the sheet 3 that have not yet been scanned, thereby stimulating the non-scanned portion and causing it to emit light. When light is emitted by the non-scanned portion of the stimulable phosphor sheet 3 outside of the scanned portion thereof, the light is guided by the light guide member 4 together with the light emitted by the scanned portion of the sheet 3, and is processed as if it were image information from the scanned portion. Therefore, the reproduced image thus obtained becomes incorrect. 
     FIG. 2 is an enlarged perspective view showing the section near the point 3a of FIG. 1. The aforesaid problem will further be described with reference to FIG. 2 by using as an example the laser beam 1c at a given instant in the scanning by the laser beam 1b in FIG. 1 as an example. The point 3a on the stimulable phosphor sheet 3 exposed to the laser beam 1c emits light in proportion to the radiation energy stored therein upon stimulation by the laser beam 1c. At the same time, the laser beam 1c impinging upon the point 3a is partially reflected and scattered by the point 3a. A part of the laser beam reflected and scattered is further reflected by the light input face 4a of the light guide member 4 and the reflection plane of the light guiding mirror 10 as indicated, by way of example, by arrows 11a, 11b, and 11c, and impinges upon non-scanned portions of the stimulable phosphor sheet 3 outside of the point 3a, thereby stimulating the non-scanned portions and causing them to emit light. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide a radiation image read-out apparatus in which stimulating rays impinging upon a stimulable phosphor sheet and reflected thereby toward the reflection plane of the light guiding mirror are not reflected by the reflection plane. 
     Another object of the present invention is to provide a radiation image read-out apparatus realizing a reproduced visible image of high image quality. 
     The present invention provides a radiation image read-out apparatus including a scanning optical system for scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays, a light guide member having a light input face positioned in the vicinity of a portion of said stimulable phosphor sheet scanned by said stimulating rays, said light guide member guiding light, which is emitted by said stimulable phosphor sheet in proportion to the stored radiation energy when said stimulable phosphor sheet is scanned by said stimulating rays, from said light input face, a light guiding mirror positioned in the vicinity of said light input face and said portion of said stimulable phosphor sheet scanned by said stimulating rays, said light guiding mirror reflecting the light emitted by said scanned portion to said light input face, and a photoelectric converter for receiving the light guided by said light guide member and photoelectrically converting it into an electric signal, 
     wherein the improvement comprises the provision of: 
     (i) a stimulating ray reflection preventing means positioned between said portion of said stimulable phosphor sheet scanned by said stimulating rays and said light guiding mirror, and 
     (ii) a means for preventing transmission of said stimulating rays and transmitting the light emitted by said stimulable phosphor sheet in proportion to the stored radiation energy, said means positioned between said light guiding mirror and said photoelectric converter. 
     In the present invention, the stimulating ray reflection preventing means may be constituted as a reflection plane of the light guiding mirror which reflects the light emitted by the stimulable phosphor sheet and which does not reflect the stimulating rays, and the means for preventing transmission of stimulating rays and transmitting the light emitted by the stimulable phosphor sheet may be a filter positioned at the photoelectric converter for selectively transmitting the light emitted by the stimulable phosphor sheet. Alternatively, the stimulating ray reflection preventing means and the means for preventing transmission of stimulating rays and transmitting the light emitted by the stimulable phosphor sheet may be constituted by a single filter for absorbing the stimulating rays and transmitting the light emitted by the stimulable phosphor sheet. In this case, the filter is positioned between the portion of the stimulable phosphor sheet scanned by the stimulating rays and the light guiding mirror. In addition to the filter, another filter of the same type may also be positioned between the portion of the stimulable phosphor sheet scanned by the stimulating rays and the light input face of the light guide member. 
     In the present invention, since the stimulating ray reflection preventing means is positioned between the scanned portion of the stimulable phosphor sheet and the light guiding mirror, no light is emitted by the non-scanned portion of the stimulable phosphor sheet outside of the scanned portion thereof, and it is possible to obtain a reproduced visible image of high image quality. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing the conventional radiation image read-out apparatus, 
     FIG. 2 is an enlarged perspective view showing the section near the point 3a of FIG. 1, 
     FIG. 3 is an enlarged sectional view showing a part of an embodiment of the radiation image read-out apparatus in accordance with the present invention, which corresponds to a sectional view taken along line II--II of FIG. 1, 
     FIG. 4 is a plan view showing another embodiment of the radiaiton image read-out apparatus in accordance with the present invention, 
     FIG. 5 is an enlarged side view showing a part of the apparatus of FIG. 4, which is taken in the direction as indicated by the arrow D of FIG. 4, and 
     FIGS. 6A to 6D are side views showing further embodiments of the radiation image read-out apparatus in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinbelow be described in further detail with reference to the accompanying drawings. 
     The embodiments of the radiation image read-out apparatus in accordance with the present invention as shown in FIGS. 3 to 6D have a configuration similar to the configuration of the conventional apparatus shown in FIG. 1 except that in FIG. 3 a stimulating ray reflection preventing film is overlaid on the light input face of the light guide member and the light guiding mirror is provided with a reflection plane which reflects the light emitted by the stimulable phosphor sheet and which does not reflect the stimulating rays, and that in FIGS. 4 to 6D a filter for absorbing the stimulating rays and transmitting the light emitted by the stimulable phosphor sheet is positioned between the portion of the stimulable phosphor sheet scanned by the stimulating rays and the light guiding mirror. Therefore, in FIGS. 3 to 6D, similar elements are numbered with the same reference numerals with respect to FIG. 1. 
     Referring to FIG. 3, the light input face 4a of the light guide member 4 positioned along the portion of the stimulable phosphor sheet 3 scanned by the stimulating rays 1b is provided with a stimulating ray reflection preventing film 13, and a light guiding mirror 10 having a reflection plane 10a is positioned along the scanned portion of the stimulable phosphor sheet so that the reflection plane 10a stands face to face with the light input face 4a of the light guide member 4. The stimulating ray reflection preventing film 13 is used for preventing the stimulating rays from being reflected by the light input face 4a, thereby further improving the image quality of the reproduced visible image. However, the stimulating ray reflection preventing film 13 need not necessarily be provided. 
     The light guiding mirror 10 having the reflection plane 10a should be such that it reflects the light emitted by the stimulable phosphor sheet 3 and impinging upon the reflection plane 10a and that, owing to transmission, absorption or the like, it does not reflect the stimulating rays. In order to fabricate the light guiding mirror 10 so that it exhibits such optical property, a surface of metal aluminium may be subjected to alumite coloration treatment, thereby forming a colored alumite film as the reflection plane 10a. In this case, the surface of metal aluminium is subjected to anodic oxidation, and at the same time colored by a dye which reflects the light emitted by the stimulable phosphor sheet 3 and which absorbs the stimulating rays. In this manner, the colored alumite film having the aforesaid optical property is formed as the reflection plane 10a on the surface of metal aluminium. Or, the light guiding mirror 10 may be fabricated as a dichroic mirror by overlaying a dichroic coating as the reflection plane 10a on the surface of a substrate of glass, a plastic material, or the like. Of course, in this case, the dichroic coating overlaid on the substrate surface must exhibit the optical property of reflecting the light emitted by the stimulable phosphor sheet 3 and transmitting the stimulating rays. The stimulating rays of a comparatively long wavelength (e.g. 633 nm) impinging upon the reflection plane 10a are absorbed by the reflection plane 10a (in the case of the colored alumite film) or are transmitted therethrough (in the case of the dichroic coating). Therefore, the stimulating rays are not reflected by the light guiding mirror 10, and only the light having a comparatively short wavelength (e.g. 400 nm) emitted by the stimulable phosphor sheet 3 is reflected by the light guiding mirror 10. 
     The stimulating ray reflection preventing film 13 should be such that it prevents reflection of the stimulating rays 1b and reflection of the light emitted by the stimulable phosphor sheet 3, thereby efficiently guiding the light emitted by the stimulable phosphor sheet 3 into the light guide member 4. As the stimulating ray reflection preventing film 13 exhibiting such optical property, it is possible to use, for example, a deposited thin film of a material exhibiting a refractive index smaller than that of the material of the light guide member 4. In the case where the deposited thin film is provided, reflection of the stimulating rays 1b becomes minimum when the optical thickness of the deposited thin film is a quarter of the wavelength of the stimulating rays 1b. Reflection of the stimulating rays 1b is completely eliminated when the condition n1=√n2 holds between the refractive index n1 of the deposited thin film material and the refractive index n2 of the light guide member material. When the light guide member 4 is fabricated of a plastic material or glass, the material of the deposited thin film positioned on the light input face of the light guide member 4 may be MgF 2 , CaF 2 , cryolite, or the like. The deposited thin film prevents reflection of the stimulating rays 1b and the light emitted by the stimulable phosphor sheet 3. Therefore, the light emitted by the stimulable phosphor sheet 3 is efficiently guided into the light guide member 4. The stimulating rays 1b prevented from reflection by the deposited thin film and guided into the light guide member 4 are absorbed and eliminated by the filter positioned between the light guide member 4 and the photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 and allowed to enter the light guide member 4 since it is not reflected by the deposited thin film passes through the filter and is received by the photomultiplier 5. 
     The light guiding mirror 10 is not limited to the one as described above, and may be of any type insofar as it reflects the light emitted by the stimulable phosphor sheet 3 and does not reflect the stimulating rays. 
     FIGS. 4 and 5 show another embodiment of the radiation image read-out apparatus in accordance with the present invention. In this embodiment, a plate-like filter 15 for absorbing the stimulating rays 1d reflected by the stimulable phosphor sheet 3 and transmitting the light emitted by the stimulable phosphor sheet 3 is positioned between the position 3a, where the stimulating rays 1b impinge upon the sheet 3, and a light guiding mirror 14 for reflecting the light emitted by the stimulable phosphor sheet 3 to the light guiding mirror 4a of the light guide member 4. 
     The filter 15 may, for example, be a wavelength selection transmitting filter which absorbs the reflected stimulating rays 1d and transmits the light emitted by the stimulable phosphor sheet 3 on the basis of the difference in the wavelength distribution therebetween. As the filter 15, it is possible to use, for example, filter NO. B-380 supplied by Hoya Garasu K.K. when the wavelength of the reflected stimulating rays 1d is 633 nm and the wavelength of the light emitted by the stimulable phosphor sheet 3 is 390 nm. Of course, absorption of the reflected stimulating rays 1d and transmission of the light emitted by the stimulable phosphor sheet 3, which are effected by the filter 15, need not necessarily be perfect. 
     The filter 15 may be closely contacted with the whole front end face of the light guiding mirror 14 including a curved reflection plane as shown in FIG. 6A, or may be closely contacted with only the curved reflection plane as shown in FIG. 6B. Also, the filter may be positioned in a flat plate form near the curved reflection plane as shown in FIG. 6C, or may be closely contacted with the whole surface of a straight reflection plane as shown in FIG. 6D. 
     In the embodiments of FIGS. 4 to 6D, the filter 15 is positioned to stand face to face with the light guiding mirror 14 or is closely contacted therewith approximately over the entire length of the light guiding mirror 14 so that all the stimulating rays 1d reflected in such a direction as to impinge upon the reflection plane of the light guiding mirror 14 are absorbed by the filter 15. However, the filter 15 may also be positioned only at a part of the light guiding mirror 14. 
     Also, some filters may exhibit a relatively low transmittance of the light emitted by the stimulable phosphor sheet or slight reflection of the emitted light from the filter surface. When such filters are used, the efficiency of guiding the light emitted by the stimulable phosphor sheet is decreased more or less. In such a case, it is possible to position the filter 15 vertically moveably, for example, in the embodiment of FIG. 4. Normally, the filter 15 is maintained in the upper position to avoid interference with the light emitted by the stimulable phosphor sheet 3. When decrease in the flare phenomenon caused by the reflected stimulating rays is more important than the light guiding efficiency, for example, when a slight difference in contrast should be discriminated as in the case where a tumor image near a portion of the stimulable phosphor sheet exposed to a high radiation dose outside of the object is diagnosed or minute calcification is diagnosed in mammography, the filter 15 is moved down to absorb the reflected stimulating rays.
A radiation image read-out apparatus comprises an optical system for scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays, a light guide member for guiding light emitted by the stimulable phosphor sheet upon exposure to stimulating rays, a light guiding mirror for reflecting the emitted light to the light guide member, and a photoelectric converter for detecting the light guided by the light guide member. The light guiding mirror is provided with a reflection plane which reflects the light emitted by the stimulable phosphor sheet and which does not reflect stimulating rays. Or, a filter for absorbing the stimulating rays reflected by the stimulable phosphor sheet and transmitting the light emitted by the stimulable phosphor sheet is positioned between the scanned portion of the stimulable phosphor sheet and the light guiding mirror.