Abstract:
Disclosed herein is a method of inspecting influence of stray light which occurs in a radiation image reader. The inspection method comprises the steps of: preparing a storable fluorescent inspection sheet that has stored and recorded a radiation inspection image which has a density pattern in which one or more low-density and high-density regions having a contrast difference of at least 1:20 are arrayed in a horizontal scanning direction; obtaining an image inspection signal representing the radiation inspection image, by photelectrically reading the radiation inspection image from the storable fluorescent inspection sheet; and inspecting the influence of stray light, based on an image reproduced from the image inspection signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of inspecting stray light that occurs in a radiation image reader in which excitation light is illuminated to a storable fluorescent sheet having stored and recorded a radiation image, to obtain an image signal representing the radiation image, from the storable fluorescent sheet.  
           [0003]    2. Description of the Related Art  
           [0004]    Radiation recording-reproducing systems have been proposed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645, 55(1980)-116340, etc. These systems utilize a storable phosphor (stimulatable phosphor), which stores part of radiation energy when illuminated with radiation (such as X-rays, α-rays, β-rays, γ-rays, electron rays, ultraviolet rays, etc.) and emits photostimulated luminescent light in accordance with the quantity of the stored radiation energy when illuminated with excitation light (such as visible light, etc.). The radiation image of a subject, such as a human body, etc., is temporarily photographed and recorded on a storable fluorescent sheet. The storable fluorescent sheet with the radiation image emits photostimulated luminescent light when scanned with excitation light such as a light beam, etc. The photostimulated luminescent light obtained is photoelectrically read and converted to an image signal by photoelectric reading means such as a photomultiplier, etc. Based on the image signal, the radiation image of the subject is output as a visible image to a recording material such as photosensitive material, etc., or to a cathode-ray tube (CRT) display unit, etc.  
           [0005]    A radiation image reader used in the aforementioned radiation recording-reproducing system is equipped with a light source for emitting a light beam (excitation light), a rotating polygon mirror (deflection means) for reflecting and deflecting the light beam emitted from the light source, and a scanning optics system, which consists of an fθ lens, a cylindrical lens, and a cylindrical mirror. The light beam, reflected and deflected by the deflection means, is reflected by the cylindrical mirror to change the optical path and is scanned on the scan surface of the storable fluorescent sheet. Thus, the optical path of the light beam can be assured without increasing the size of the reader, by reflecting the light beam with the cylindrical mirror to change the optical path.  
           [0006]    In the above-mentioned scanning optics system, however, the light beam is reflected at the scan surface of the storable fluorescent sheet, and the reflected light beam is further reflected by the cylindrical mirror and returns to the cylindrical lens. Because of this, there is a possibility that the return light will be reflected at the surface of the cylindrical lens and will be incident again upon the scan surface as stray light. In certain cases, the light beam, reflected and deflected by the deflection means, is reflected by the scanning optics means and returns to the deflection means, and this return light is incident again on the scan surface as stray light. The stray light always occurs at the same position on the horizontal scanning line. Therefore, if stray light occurs in the radiation image reader mentioned above, the storable fluorescent sheet will be excited by this stray light and emit photostimulated fluorescent light, and consequently, noise in the form of lines extending in the vertical scanning direction will develop in the obtained image signal. Therefore, it is necessary to inspect stray light when shipping radiation image readers and to ship only readers having passed the inspection.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention has been made in view of the above-mentioned circumstances. Accordingly, it is the primary object of the present invention to provide an inspection method that is capable of inspecting stray light that occurs in a radiation image reader.  
           [0008]    To achieve this end and in accordance with an important aspect of the present invention, there is provided a method of inspecting influence of stray light which occurs in a radiation image reader equipped with horizontal scanning means for scanning excitation light on a storable fluorescent sheet, having stored and recorded a radiation image, in a horizontal scanning direction; vertical scanning means for scanning the storable fluorescent sheet in a vertical scanning direction approximately perpendicular to the horizontal scanning direction; and reading means for obtaining an image signal which represents the radiation image by photoelectrically reading the radiation image, stored and recorded in the storable fluorescent sheet, by the horizontal scanning of the excitation light. The inspection method comprises the steps of: preparing a storable fluorescent inspection sheet that has stored and recorded a radiation inspection image which has a density pattern in which one or more low-density and high-density regions having a contrast difference of at least 1:20 are arrayed in the horizontal scanning direction; obtaining an image inspection signal representing the radiation inspection image, by photelectrically reading the radiation inspection image from the storable fluorescent inspection sheet with the reading means; and inspecting the influence of stray light, based on an image reproduced from the image inspection signal.  
           [0009]    Note that it is desirable that the contrast difference be 1:20 or greater, preferably 1:50 or greater.  
           [0010]    In the inspection method, the boundary line, in the radiation inspection image, between the low-density and high-density regions may be constructed by a straight line and may be inclined with respect to the horizontal scanning direction so that it intersects both edges of the radiation inspection image which extend in the vertical scanning direction.  
           [0011]    The expression “the boundary line is inclined with respect to the horizontal scanning direction so that it intersects both edges of the radiation inspection image which extend in the vertical scanning direction” means that the boundary line is inclined with respect to the horizontal scanning direction and that the boundary line forms a density pattern by intersecting with the edges of the radiation inspection image which extend in the vertical scanning direction. Note that in cases where the radiation inspection image is rectangular in shape, the boundary line may be on the diagonal of the rectangular shape.  
           [0012]    In the inspection method, the density pattern in the radiation inspection image may include two high-density regions and one low-density region. The regions may be arrayed in the horizontal scanning direction in the order of one high-density region, the low-density region, and the other high-density region.  
           [0013]    In accordance with another important aspect of the present invention, there is provided a storable fluorescent inspection sheet having stored and recorded a radiation inspection image that has a density pattern in which one or more low-density and high-density regions having a contrast difference of at least 1:20 are arrayed in the horizontal scanning direction.  
           [0014]    In the storable fluorescent inspection sheet, the boundary line, in the radiation inspection image, between the low-density and high-density regions may be constructed by a straight line and may be inclined with respect to the horizontal scanning direction so that it intersects both edges of the radiation inspection image which extend in the vertical scanning direction. In this case, it is preferable that the density pattern in the radiation inspection image include two high-density regions and one low-density region. The regions may be arrayed in the horizontal scanning direction in the order of one high-density region, the low-density region, and the other high-density region.  
           [0015]    In accordance with still another important aspect of the present invention, there is provided a method of generating the aforementioned storable fluorescent inspection, comprising the steps of: disposing a radiation shielding member at a position corresponding to the density pattern on a storable fluorescent sheet; illuminating the storable fluorescent sheet, on which the shielding member has been disposed, with a dose of radiation that corresponds to the contrast difference; and storing and recording the radiation inspection image in the storable fluorescent sheet, by repeating the disposition of the radiation shielding member and the illumination of the radiation, with respect the storable fluorescent sheet until the density pattern is obtained.  
           [0016]    In accordance with yet still another important aspect of the present invention, there is provided a method of generating the aforementioned storable fluorescent inspection sheet, comprising the steps of: disposing a radiation transmittable member at a position corresponding to the density pattern on a storable fluorescent sheet, the radiation transmittable member having a radiation transmission factor which corresponds to the contrast difference; and storing and recording the radiation inspection image in the storable fluorescent sheet, by illuminating the storable fluorescent sheet, on which the radiation transmittable member has been disposed, with a dose of radiation that corresponds to the contrast difference.  
           [0017]    As described above, stray light always develops at the same position on the horizontal scanning direction. Therefore, if the storable fluorescent inspection sheet (which has stored and recorded a radiation inspection image that has a density pattern in which one or more low-density and high-density regions having a contrast difference of at least 1:20 are arrayed in the horizontal scanning direction) is read, and if stray light occurs in the high-density region during reading in the low-density region, noise in the form of a line extending in the vertical scanning direction will occur at a position being read because of the influence of the stray light. Therefore, in the case where a reproduced image is obtained by reproducing an image inspection signal obtained by reading the storable fluorescent inspection sheet, and noise in the form of a line extending in the vertical scanning direction occurs in the reproduced image, it can be judged that stray light has occurred in this radiation image reader. Thus, stray light in the radiation image reader can be inspected by employing the aforementioned storable fluorescent inspection sheet.  
           [0018]    In addition, in the case where the boundary line between the low-density and high-density regions is constructed by a straight line and is inclined with respect to the horizontal scanning direction, in the area where the low-density region is wide it is difficult to visually recognize noise resulting from stray light, even if the stray light occurs in the low-density region during reading in the low-density region. Since, however, the boundary line is inclined so that it intersects both edges of the radiation inspection image that extend in the vertical scanning direction, the stray light that occurs on any of the horizontal scanning lines during reading in the low-density region will always be illuminated on the high-density region, if the whole surface of the storable fluorescent inspection sheet is scanned in the horizontal scanning direction. Therefore, at the time stray light has been illuminated on the high-density region, noise resulting from stray light occurs in the low-density region. As a result, stray light can be inspected with reliability. Besides, the position at which stray light occurs can also be specified, because the intersection between the horizontal scanning line, which passes a point where noise develops or disappears, and the boundary line represents the position where stray light has occurred.  
           [0019]    Furthermore, even if stray light occurs at either a position where reading has finished, or a position where reading has not finished, during horizontal scanning at a certain horizontal scanning line, in the case where the density pattern is constructed of a high-density region, a low-density region, and a high-density region arrayed in the horizontal scanning direction in the recited order, if the whole surface of the storable fluorescent inspection sheet is scanned in the horizontal scanning direction the stray light occurring at any of the horizontal scanning lines during reading in the low-density region is always illuminated on the high-density region and noise resulting from the stray light occurs in the low-density region. Therefore, if the density pattern is constructed of a high-density region, a low-density region, and a high-density region arrayed in the horizontal scanning direction, stray light can be inspected regardless of the position at which the stray light occurs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention will be described in further detail with reference to the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a schematic diagram showing a radiation image reader in which an inspection of stray light is made by an inspection method in accordance with a first embodiment of the present invention;  
         [0022]    [0022]FIG. 2 is a plan view showing the construction of the optics unit shown in FIG. 1;  
         [0023]    [0023]FIGS. 3 through 5 are diagrams used for explaining stray light;  
         [0024]    [0024]FIG. 6 is a diagram showing a radiation inspection image, stored and recorded in the storable fluorescent inspection sheet generated according to the first embodiment of the present invention;  
         [0025]    [0025]FIG. 7 is a diagram showing the storable fluorescent inspection sheet;  
         [0026]    [0026]FIGS. 8A to  8 C are diagrams used for explaining how the storable fluorescent inspection sheet is generated;  
         [0027]    [0027]FIG. 9 is a diagram used to explain an inspection of stray light;  
         [0028]    [0028]FIG. 10 is a diagram showing an image obtained by reading the storable fluorescent inspection sheet;  
         [0029]    [0029]FIG. 11 is a diagram showing the positions in the storable fluorescent inspection sheet where reading is performed;  
         [0030]    [0030]FIG. 12 is a diagram showing a radiation inspection image, stored and recorded in a storable fluorescent inspection sheet generated according to a second embodiment of the present invention;  
         [0031]    [0031]FIG. 13 is a diagram showing how stray light is inspected by use of the storable fluorescent inspection sheet of the second embodiment;  
         [0032]    [0032]FIG. 14 is a diagram showing the positions in the storable fluorescent inspection sheet of the second embodiment where reading is performed;  
         [0033]    [0033]FIG. 15 is a diagram showing a radiation inspection image, stored and recorded in a storable fluorescent inspection sheet generated according to a third embodiment of the present invention; and  
         [0034]    [0034]FIG. 16 is a diagram showing how stray light is inspected by use of the storable fluorescent inspection sheet of the third embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings. As illustrated in FIG. 1, a radiation image reader  1  is equipped with conveyor rollers  9   a ,  9   b  for conveying a storable fluorescent sheet  4  in the direction of arrow y. The rollers  9   a ,  9   b  are rotated by a motor (not shown), and the storable fluorescent sheet  4  has stored and recorded a radiation image. Above the sheet  4  that is conveyed, an optics unit  10  is disposed for emitting laser light  11  being excitation light. The laser light  11  scans the sheet  4  in the horizontal scanning direction perpendicular to the paper surface. Above the position where the sheet  4  is scanned in the horizontal scanning direction with the laser light  11 , a collective guide  14 , for collecting the photostimulated luminescent light  13  emitted from the sheet  4  by the horizontal scanning of the laser light  14 , is disposed in proximity to the conveying passage of the sheet  4 . Near the collective guide  14 , a collective mirror  7  is disposed for reflecting the photostimulated luminescent light  13 , scattered and emitted from the storable fluorescent sheet  4 , toward the collective mirror  7 . The collective mirror  7  is supported by a mirror mount  6 . The collective guide  14  is connected with a photomultiplier  15 , which photoelectrically detects the photostimulated luminescent light  13 . This photomultiplier  15  is connected to a logarithmic amplifier  16 , which is in turn connected to an A/D converter  17 . The A/D converter  17  is connected to storage means  18 , which is in turn connected to image processing means  19 .  
         [0036]    [0036]FIG. 2 shows the construction of the optics unit  10 . As illustrated in the figure, the optics unit  10  is equipped with (1) a light source  32  for emitting laser light  11 ; (2) a collimator lens  34  for collimating the laser light  11 ; (3) a cylindrical lens  36  for converging the laser light  11  in the vertical scanning direction (i.e., the direction of arrow y shown in FIG. 1); (4) a reflection mirror  40  for reflecting the laser light  11 ; (5) a rotating polygon mirror  38 , which is driven by a motor (not shown), for reflecting and deflecting the laser light  11  reflected by the reflection mirror  40 ; (6) an fθ lens  46 , which consists of first and second spherical lenses  42  and  44 , for projecting the laser light  11 , reflected and deflected by the rotating polygon mirror  38 , onto the sheet  4 ; (7) a cylindrical mirror  48  for reflecting the laser light  11  toward the sheet  4 ; and (8) a cylindrical lens  50  which constitutes an imaging optics system for projecting the laser light  11  onto the sheet  4 , along with the cylindrical mirror  48 . These components are disposed within a housing  31 . Note that in FIG. 2 the optical axis of the laser light  11  is represented by X and the reflection surface of the cylindrical mirror  48  by  48 A.  
         [0037]    Now, the operation of the radiation image reader  1  will be described in detail. The storable fluorescent sheet  4  having stored and recorded the radiation image of a subject is set on the conveyor roller  9   a . Then, the set storable florescent sheet  4  is conveyed in the direction of arrow y (i.e., the vertical scanning direction) by the conveyor rollers  9   a ,  9   b . In the optics unit  10 , on the other hand, the laser light  11  emitted from the light source  32  is reflected and deflected by the rotating polygon mirror  38  and is further reflected toward the sheet  4  by the cylindrical mirror  48 . Next, the reflected laser light  11  is incident on the sheet  4  and scans the sheet  4  in the horizontal scanning direction approximately perpendicular to the vertical scanning direction. If the sheet  4  is scanned with the laser  11 , the photostimulated luminescent light  13 , which has a quantity of light corresponding to radiation image information being stored and recorded in the sheet  4 , is emitted from a position on the sheet  4  irradiated with the laser light  11 . The photostimulated luminescent light  13  emitted from the surface of the sheet  4  is scattering. However, the scattering light  13  is reflected by the collective mirror  7  and is collected by the collective guide  14 . The photostimulated luminescent light  13  incident within the collective guide  14  travels through the collective guide  14 , repeating total reflection. The photostimulated luminescent light  13  is received by the photomultiplier  15 , in which the light quantity of the photostimulated fluorescent light  13  representing the radiation image is converted to an analog image signal SA.  
         [0038]    The analog image signal SA output from the photomultiplier  15  is logarithmically amplified by the logarithmic amplifier  16  and input to the A/D converter  17 . The A/D converter  17  samples the amplified signal and converts the sampled signal to a digital image signal S 1 . The digital image signal S 1  is input to the storage means  18 . The digital image signal S 1  stored in the storage means  18  is input to the image processing means  19 , in which the signal S 1  is subjected to a predetermined image processing. In reproduction means (not shown), the processed digital image signal S 1  is used for reproduction of the radiation image. This reproduction means may be display means such as a CRT display, etc., or a recorder for performing optical scanning and recording operations on photosensitive film.  
         [0039]    Here, stray light will be described in detail with reference to FIGS. 3 and 4. Note that in FIG. 3, the laser light  11 , reflected toward the sheet  4  by the cylindrical mirror  48 , and the laser light  11  before reflection, are in the same plane for the purpose of explanation. As illustrated in FIGS. 3 and 4, the laser light  11  is reflected toward the sheet  4  by the cylindrical mirror  48  and is projected at position P 1  on the sheet  4 . The laser light  11  projected at the position P is reflected at the surface of the sheet  4  toward the cylindrical mirror  48  and is further reflected at the cylindrical mirror  48  toward the cylindrical lens  50 . Then, the laser light  11  reflected toward the cylindrical lens  50  is reflected as stray light  11 ′ at the surface of the cylindrical lens  50  toward the sheet  4 . Because the laser light  11  is incident on the sheet  4  at an angle to an optical axis X, as illustrated in FIG. 3, the stray light  11 ′ is projected at position P 2  away from the position P 1 .  
         [0040]    If the projected position P 1  of the laser light  11  differs from the projected position P 2  of the stray light  11 ′, as described above, the following problems will arise. That is, assuming the intensity of the laser light  11  is 1, the positive reflection component of the storable fluorescent sheet  4  is 5% and that the reflection factor of the surface of the cylindrical lens  50  is 0.5%, the intensity of the stray light  11 ′ at the position P 2  becomes 2.5×10 −4 . Therefore, if it is assumed that the intensities of radiation illuminated at the positions P 1  and P 2  are approximately the same when the radiation image reader  1  reads a radiation image from the storable fluorescent sheet  4  having stored and recorded the radiation image, the intensity of the signal obtained at the position P 1  becomes 1+2.5×10 −4  in view of the influence of the stray light  11 ′ and therefore there is little influence of the stray light  11 ′.  
         [0041]    However, when the radiation image recorded on the storable fluorescent sheet  4  is a human breast image such as that shown in FIG. 5, and the position P 1  is located at the lung field portion and the position P 2  at a position where radiation is directly illuminated without being passed through the human body, there are cases where the ratio of the intensities of radiation at the positions P 1  and P 2  on the storable fluorescent sheet  4  becomes 1:200. In such cases, the intensity of a signal obtained at the position P 1  becomes 1+2.5×10 −4 ×200=1.05 in view of the stray light  11 ′ and therefore there is a great influence of the stray light  11 ′.  
         [0042]    This embodiment provides a method of inspecting the aforementioned influence of the stray light  11 ′. For this purpose, this embodiment makes an inspection of stray light by employing a storable fluorescent inspection sheet. The storable fluorescent sheet has stored and recorded a radiation inspection image  20  that has a density pattern consisting of a low-density region  20 A and a high-density region  20 B, as shown in FIG. 6. Note that the radiation inspection image  20  shown in FIG. 6 is obtainable by illuminating a low dose of radiation (e.g., 1 mR) to a region  21 A on a storable fluorescent sheet and a high dose of radiation (e.g., 50 mR) to a region  21 B, as shown in FIG. 7. Here, a storable fluorescent sheet having stored and recorded the radiation inspection image  20  is taken to be a storable fluorescent inspection sheet  21 . Note that in this embodiment, the horizontal direction indicated by an arrow in FIG. 1 is assumed to be a horizontal scanning direction in reading a radiation image by the radiation image reader  1 .  
         [0043]    Now, a method of generating the storable fluorescent inspection sheet  21  will be described in detail with reference to FIG. 8. As illustrated in FIG. 8A, the region  21 A of a storable fluorescent sheet  21 ′ having stored and recorded no radiation image is shielded with a radiation shielding plate  71  (e.g., a lead plate of thickness 5 mm), and the unshielded region  20 B of the sheet  21 ′ is illuminated with radiation  72  with a dose of 50 mR, emitted from a radiation source  70 . Then, as illustrated in FIG. 8B, the region  21 B is shielded with the shielding plate  71 , and the unshielded region  20 A is illuminated with radiation  72  with a dose of 1 mR, emitted from the radiation source  70 . In this manner, the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  20  can be obtained as illustrated in FIG. 6.  
         [0044]    Note that the region  21 B may first be shielded with the shielding plate  71  and, after the illumination of the sheet  21 ′ with the radiation  72  of a dosage of 1 mR, the sheet  21 ′ may be illuminated with the radiation  72  of a dosage of 5 mR without employing the shielding plate  71 . Even in this case, the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  20  is obtainable. Furthermore, instead of the shielding plate  71 , the region  21 A may be shielded with a radiation transmittable plate  73 , constructed of a copper plate, etc., which has a transmission factor of 2%, and the sheet  21 ′ may be illuminated with radiation  72  of dosage 50 mR, emitted from the radiation source  70 , as illustrated in FIG. 8C. In this case, the region  21 A and the region  21 B are illuminated simultaneously with the 1-mR radiation  72  and the 50-mR radiation, respectively. Therefore, the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  20  can be obtained by a single photographing.  
         [0045]    The inspection of stray light is carried out by reading the aforementioned storable fluorescent inspection sheet  21  with the radiation image reader  1 . The inspection of stray light will be described with reference to FIG. 9. Assuming that, in the radiation image reader  1 , stray light develops at a position P 4  during reading at position P 3  shown in FIG. 9, a low-density region  22 A and a high-density region  22 B will develop in an image  22  obtained by reading the storable fluorescent inspection sheet  21 , and noise  23  in the form of a line extending in the vertical scanning direction will develop at the position corresponding to the position P 3  on the low-density region  22 A, as shown in FIG. 10.  
         [0046]    Therefore, by reading the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  20  that has the density pattern shown in FIG. 6, and by recognizing the image obtained by the reading, stray light in the radiation image reader  1  used in the reading can be inspected.  
         [0047]    Note that in the case where, in the radiation inspection image  20  such as that shown in FIG. 6, stray light develops at position P 6  during reading at position P 5  and develops at position P 8  during reading at position P 7 , as shown in FIG. 11, it is difficult to inspect the stray light because noise in the form of a line is inconspicuous in an image obtained. It is therefore preferable to employ the storable fluorescent inspection sheet  21  having stored and recorded a radiation inspection image  24  such as the one shown in FIG. 12. The radiation inspection image  24  has a density pattern such that a boundary line  24 C between a low-density region  24 A and a high-density region  24 B is inclined with respect to the horizontal scanning direction of the radiation inspection image  24  and that the boundary line  24 C intersects the vertical edges  25  and  26 , extending the vertical scanning direction, of the radiation inspection image  24 . A description will hereinafter be made of how stray light is inspected by use of the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  24 .  
         [0048]    [0048]FIG. 13 illustrates how an inspection of stray light is carried out by use of the storable fluorescent inspection sheet  21  shown in FIG. 12. Assume that in the radiation image reader  1 , stray light develops at the position P 6  shown in FIG. 11 during reading at the position P 5 . As illustrated in FIG. 13, a low-density region  27 A and a high-density region  27 B develop in an image  27 , obtained by reading the storable fluorescent inspection sheet  21 . In the case where the positions P 5  and P 6  on a certain horizontal scanning line are both in the low-density region  24 A of the radiation inspection image  24 , noise resulting from stray light is inconspicuous. However, in the case where the horizontal scanning line is moved by vertical scanning during reading of the storable fluorescent inspection sheet  21 , and the position P 5  is in the low-density region  24 A and the position P 6  in the high-density region  24 B, noise  23  in the form of a line extending in the vertical scanning direction will develop at the position in the image  27  that corresponds to the position P 5  in the low-density region  27 A. Therefore, using the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  24  that has the density pattern shown in FIG. 12, stray light can be inspected with reliability. In the image  27  obtained from a storable fluorescent inspection sheet  21  such as this, if the intersection between the horizontal scanning line, passing through point P 5 ′ where the noise  23  develops, and the boundary line  27 C (between the low-density region  27 A and the high-density region  27 B) is taken to be P 6 ′, the intersection P 6 ′ represents the position at which stray light develops. Therefore, the position at which stray light develops can also be found by use of the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  24  shown in FIG. 12.  
         [0049]    On the other hand, in the case where stray light develops at position P 7  during reading at position P 8  shown in FIG. 14 even when using the storable fluorescent inspection sheet  21  having storing and recorded the radiation inspection image  24 , stray light cannot be inspected, because, in the radiation inspection image  24  shown in FIG. 12, noise in the form of a line is inconspicuous in an obtained image even when the position P 7  is in the low-density region  24 A and the position P 8  in the high-density region  24 B, as well as when the positions P 7  and P 8  are both in the low-density region  24 A or high-density region  24 B. It is therefore preferable to employ a storable fluorescent inspection sheet  21  having stored and recorded a radiation inspection image  28  such as shown in FIG. 15. The radiation inspection image  28  has one low-density region  28 A and two high-density regions  28 B and also has a density pattern such that the low-density region  28 A is interposed between the two high-density regions  28 B, each boundary line  28 C between the regions  28 A and  28 B is inclined with respect to the horizontal scanning line, and each boundary line  28 C intersects the edges  51  and  52 , extending in the vertical scanning direction, of the radiation inspection image  28 . A description will hereinafter be given of how stray light is inspected by use of the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  28 .  
         [0050]    [0050]FIG. 16 illustrates how stray light is inspected by use of the storable fluorescent inspection sheet  21 . Assume that in the radiation image reader  1 , stray light develops at the position P 7  shown in FIG. 16 during reading at the position P 8 . As illustrated in FIG. 16, a low-density region  29 A and a high-density region  29 B develop in an image  29 , obtained by reading the storable fluorescent inspection sheet  21 . In the case where the positions P 7  and P 8  on a certain horizontal scanning line are both in the high-density region  28 B of the radiation inspection image  28 , noise resulting from stray light is inconspicuous. However, in the case where the horizontal scanning line is moved by vertical scanning during reading of the storable fluorescent inspection sheet  21 , and the position P 8  is in the low-density region  28 A and the position P 7  in the high-density region  28 B, noise  23  in the form of a line extending in the vertical scanning direction will develop at the position in the image  29  that corresponds to the position P 8  in the low-density region  29 A. Therefore, using the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  28  that has the density pattern shown in FIG. 15, stray light can be inspected no matter what position stray light develops at. In the image  29  obtained from a storable fluorescent inspection sheet  21  like this, if the intersection between the horizontal scanning line, passing through point P 8 ′ where the noise  23  disappears, and the boundary line  27 C (between the low-density region  29 A and the high-density region  29 B) is expressed in terms of P 7 ′, the intersection P 7 ′ represents the position at which stray light develops. Therefore, the position where stray light develops can also be found by use of the storable fluorescent inspection sheet  21  having stored and recorded the radiation inspection image  28  shown in FIG. 15.  
         [0051]    While, in the aforementioned embodiments, the inspection of stray light in the radiation image reader  1  has been carried out by use of the storable fluorescent inspection sheets  21  having stored and recorded the radiation inspection images  20 ,  24 , and  28 , the present invention is not limited to these radiation inspection images, but is also applicable to any radiation inspection image if it has a density pattern in which one or more low-density and high-density regions having a contrast difference of at least 1:20 are arrayed in the horizontal scanning direction.