Patent Number: 053409965
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a radiation image read-out apparatus wherein a stimulable phosphor sheet, on which a radiation image of an object has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected and converted into an image signal representing the radiation image. This invention also relates to a method for recording a radiation image on a stimulable phosphor sheet such that the range of the amount of light emitted by the stimulable phosphor sheet, on which a radiation image has been stored, can be compressed, and an apparatus for carrying out the method. This invention further relates to a stimulable phosphor sheet, which is used in the radiation image recording method and apparatus, and a cassette which houses the stimulable phosphor sheet. 2. Description of the Prior Art When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395 it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired. The processed image signal is used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced as a hard copy or may be displayed on a cathode ray tube (CRT). The radiation image recording and reproducing systems described above are applicable not only to medical diagnoses but also to detection of flaws in industrial products. Specifically, a radiation image of an industrial product, such as a round bar or a steel pipe, may be stored on a stimulable phosphor sheet and may then be reproduced as a visible image in the manner described above. In the visible image, even a flaw, which is present at a position deep from the surface of the industrial product, appears clearly as a pattern darker than the surrounding areas because of its lower radiation absorptivity. Industrial metal products, such as round bars, pipes (e.g. steel pipes), and die steel products, have markedly higher radiation absorptivity than human bodies, or the like. Therefore, when a radiation image of such an industrial product is to be stored on a stimulable phosphor sheet, it is necessary for the industrial product to be exposed to a markedly higher dose of radiation than when a radiation image of a human body, or the like, is to be recorded. As a result, the amount of energy stored on the stimulable phosphor sheet during its exposure to the radiation differ markedly for the region corresponding to the part of the industrial product having a comparatively high radiation absorptivity and for the region corresponding to the part of the industrial product having a comparatively low radiation absorptivity. When the radiation image of an industrial product is read out from the stimulable phosphor sheet, the dynamic range of the amount of light emitted by the stimulable phosphor sheet often becomes wider than 10.sup.4. However, the read-out range of a photoelectric read-out means, such as a photomultiplier, i.e. the range of the amount of light, which range the photoelectric read-out means can detect, is ordinarily approximately 10.sup.2 and is at most approximately 10.sup.4. Therefore, an ordinary photoelectric read-out means cannot accurately detect such a wide range of the amount of light emitted by the stimulable phosphor sheet. Even when a photoelectric read-out means is used which can detect such a wide range of the amount of light emitted by the stimulable phosphor sheet, because the wide range of the amount of emitted light must be detected, the change in the amount of emitted light per bit becomes large during A/D conversion of an image signal. Therefore, a small change in contrast cannot be detected easily. In general, the depth of a flaw occurring in a steep pipe, or the like, is very small, and therefore the difference between the flawed part and the surrounding parts in the radiation absorptivity is very small. In such cases, if a read-out image signal is obtained with a low contrast resolution, a difference between the flawed part and the surrounding parts in image density cannot be found easily in a visible radiation image reproduced from the read-out image signal. The problems described above can be eliminated by compressing the range of the amount of light emitted by the stimulable phosphor sheet. For this purpose, a specific stimulable phosphor sheet may be used which is disclosed in, for example, Japanese Unexamined Patent Publication No. 63(1988)-214700, and whose sensitivity to radiation varies locally in accordance with the radiation absorptivity characteristics of an object. However, such a specific stimulable phosphor sheet is applicable to only a specific object and is difficult to manufacture. Therefore, the disclosed technique is not suitable for practical use. SUMMARY OF THE INVENTION The primary object of the present invention is to provide a radiation image read-out apparatus which yields a read-out image signal such that part of an object, for example, a flawed part of an industrial product, having a radiation absorptivity slightly different from the surrounding parts can be accurately discriminated from the surrounding parts, and which is suitable for practical use. Another object of the present invention is to provide a radiation image recording method which is easy to carry out and wherein a radiation image is recorded on a stimulable phosphor sheet such that the range of the amount of light emitted by the stimulable phosphor sheet, on which the radiation image has been stored, can be compressed. A further object of the present invention is to provide an apparatus for carrying out the radiation image recording method. A still further object of the present invention is to provide a stimulable phosphor sheet, which is used in the radiation image recording method. The specific object of the present invention is to provide a cassette which houses the stimulable phosphor sheet. The first radiation image read-out apparatus is based on the findings that, basically, the radiation absorptivity of an object, such as a round bar or a steel pipe, is distributed with predetermined characteristics. Specifically, the present invention provides a first radiation image read-out apparatus for exposing a stimulable phosphor sheet, which has been exposed to radiation which has passed through an object whose radiation absorptivity is basically distributed with predetermined characteristics, to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, detecting the light, which is emitted by the portion of the stimulable phosphor sheet exposed to the stimulating rays, with a photoelectric read-out means, and thereby obtaining an image signal representing a radiation image of the object, wherein the improvement comprises the provision of an optical filter which is located between said stimulable phosphor sheet and said photoelectric read-out means such that the position of said optical filter corresponds to the position of the radiation image of said object stored on said stimulable phosphor sheet, the transmittance of said optical filter with respect to the light emitted by said stimulable phosphor sheet being distributed such that a basic difference in the radiation absorptivity of said object is compensated for. With the first radiation image read-out apparatus in accordance with the present invention, a basic difference in the radiation absorptivity of the object is accurately compensated for by the optical filter. Therefore, the intensity of light, which was emitted by the stimulable phosphor sheet and which has passed through the optical filter, distributes in an approximately flat pattern regardless of how the radiation absorptivity of the object is distributed. In cases where the purpose of the read-out operation is not to find the intrinsic shape of the object, or the like, but is to detect, for example, a flaw in the object, no problem occurs when the intensity of light, which was emitted by the stimulable phosphor sheet and which enters the photoelectric read-out means, distributes in an approximately flat pattern. The light, which has passed through the optical filter and which thus extends over a narrow dynamic range, is then detected by the photoelectric read-out means. During the detection, the read-out range of the photoelectric read-out means can be utilized broadly. Therefore, even a small difference in the amount of light emitted by the stimulable phosphor sheet, which difference is caused to occur by a flawed part of the object, or the like, can be enlarged and converted into a large difference in the image signal value. Accordingly, a visible image, in which a pattern of a flaw in the object, or the like, is illustrated clearly, can be reproduced from the image signal. The present invention also provides a second radiation image read-out apparatus for exposing a stimulable phosphor sheet, which has been exposed to radiation which has passed through an object whose radiation absorptivity is basically distributed with predetermined characteristics, to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, detecting the light, which is emitted by the portion of the stimulable phosphor sheet exposed to the stimulating rays, with a photoelectric read-out means, and thereby obtaining an image signal representing a radiation image of the object, wherein the improvement comprises the provision of a signal correction means which corrects said image signal with a correction signal having the characteristics that compensate for a basic difference in the radiation absorptivity of said object. With the second radiation image read-out apparatus in accordance with the present invention, a basic difference in the radiation absorptivity of the object is accurately compensated for by the signal correction means. Therefore, the level of the image signal, which has been corrected by the signal correction means, distributes in an approximately flat pattern regardless of how the radiation absorptivity of the object is distributed. In cases where the purpose of the read-out operation is not to find the intrinsic shape of the object, or the like, but is to detect, for example, a flaw in the object, no problem occurs when the level of the corrected image signal distributes in an approximately flat pattern. Also, in cases where the level of the corrected image signal distributes in an approximately flat pattern, a flaw in the object, or the like can be detected easily. The corrected image signal, which extends over a narrow dynamic range, should preferably be subjected to processing for enhancing the contrast. As a result, even a small difference in the level of the image signal, which difference is caused to occur by a flawed part of the object, or the like, can be enlarged. Accordingly, a visible image, in which a pattern of a flaw in the object, or the like, is illustrated clearly, can be reproduced from the image signal. The present invention further provides a radiation image recording method wherein a stimulable phosphor sheet provided with a layer of a stimulable phosphor is exposed to radiation, which carries information about an image, and the image is thereby stored on the stimulable phosphor sheet, said stimulable phosphor exhibiting such properties that, when it is exposed to radiation and is then exposed to stimulating rays, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation, the radiation image recording method comprising the steps of: i) locating a layer of a phosphor such that it faces the layer of said stimulable phosphor, said phosphor exhibiting such properties that, when it is exposed to the radiation, it produces the fluorescence having wavelengths falling within the stimulation wavelength range of said stimulable phosphor, and PA1 ii) thereafter irradiating the radiation to said stimulable phosphor sheet. The present invention still further provides a radiation image recording apparatus in which a stimulable phosphor sheet provided with a layer of a stimulable phosphor is exposed to radiation, which carries information about an image, and the image is thereby stored on the stimulable phosphor sheet, said stimulable phosphor exhibiting such properties that, when it is exposed to radiation and is then exposed to stimulating rays, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation, wherein the improvement comprises the provision of a member, which is provided with a layer of a phosphor and which is located such that the layer of said phosphor is close to or in close contact with said stimulable phosphor sheet located at the position that is exposed to the radiation, said phosphor exhibiting such properties that, when it is exposed to the radiation, it produces the fluorescence having wavelengths falling within the stimulation wavelength range of said stimulable phosphor. The present invention also provides a stimulable phosphor sheet provided with a layer of a stimulable phosphor which exhibits such properties that, when it is exposed to radiation and is then exposed to stimulating rays, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation, wherein the improvement comprises the provision of a layer of a phosphor, which is located such that it faces the layer of said stimulable phosphor and which exhibits such properties that, when it is exposed to the radiation, it produces the fluorescence having wavelengths falling within the stimulation wavelength range of said stimulable phosphor. By way of example, the phosphor which produces the fluorescence may be dispersed in a binder, which is used to hold the stimulable phosphor on a sheet-like substrate. Alternatively, the layer of the phosphor may be formed independently of the stimulable phosphor layer and located so that it is close to or in close contact with the stimulable phosphor layer. The present invention further provides a cassette for housing a stimulable phosphor sheet provided with a layer of a stimulable phosphor which exhibits such properties that, when it is exposed to radiation and is then exposed to stimulating rays, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation, wherein the improvement comprises the provision of a layer of a phosphor, which is located such that it faces the surface of said stimulable phosphor sheet housed in said cassette, said phosphor exhibiting such properties that, when it is exposed to the radiation, it produces the fluorescence having wavelengths falling within the stimulation wavelength range of said stimulable phosphor. With the radiation image recording method in accordance with the present invention, when the stimulable phosphor sheet is exposed to radiation, the stimulable phosphor layer of the stimulable phosphor sheet stores energy from the radiation. At the same time, the phosphor produces the fluorescence, and the stimulable phosphor layer is exposed to the fluorescence. The fluorescence has the effects of erasing the energy stored on the stimulable phosphor layer during its exposure to the radiation. Therefore, the erasing operation and the operation for recording a radiation image are carried out simultaneously. The amount of the fluorescence produced by the phosphor is larger at part exposed to a higher dose of radiation. Specifically, the erasing operation is effected to a higher extent at part exposed to a higher dose of radiation. Therefore, the dynamic range of the amount of energy stored on the stimulable phosphor sheet during its exposure to the radiation becomes narrower than when the phosphor which produces the fluorescence is not used. As a result, the range of the amount of light emitted by the stimulable phosphor sheet in proportion to the amount of energy stored thereon during its exposure to the radiation becomes compressed. Accordingly, with the radiation image recording method in accordance with the present invention, even when a stimulable phosphor sheet was exposed to a high dose of radiation, light emitted by the stimulable phosphor sheet can be detected accurately over the whole range of the amount of the emitted light. Also, part of the image, which was recorded with a low dose of radiation, can be read out at a high contrast resolution. Additionally, with the radiation image recording method in accordance with the present invention, a markedly high level of energy is not stored on the stimulable phosphor layer during its exposure to the radiation. Therefore, the problems can be prevented from occurring in that, when the radiation image is read out from the stimulable phosphor sheet, a markedly high level of light is emitted by part of the stimulable phosphor sheet, which was exposed to a high dose of radiation, due to flare of stimulating rays, or in that a markedly high level of after-glow is produced by such part of the stimulable phosphor sheet. Accordingly, with the radiation image recording method in accordance with the present invention, it is possible to eliminate the problems in that the contrast of an image part, which is to be viewed, is adversely affected by the light emitted due to flare of stimulating rays or due to the after-glow. Consequently, a visible radiation image can be reproduced which has good image quality and can serve as an effective tool in, for example, detection of a flaw in the object.