Patent Publication Number: US-9839359-B2

Title: Fluorescence observation apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application PCT/JP2014/068701, with an international filing date of Jul. 14, 2014, which is hereby incorporated by reference herein in its entirety. This application claims the benefit of Japanese Patent Application No. 2013-173111, filed on Aug. 23, 2013, the content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fluorescence observation apparatus. 
     BACKGROUND ART 
     In the related art, with regard to fluorescence observation apparatuses with which an affected site is diagnosed by using a fluorescent agent, there is a known method in which a region in a white-light image of biological tissue from which fluorescence is detected is displayed by replacing the region with a marker having a predetermined color. By doing so, an observer can reliably recognize the presence of the affected site in a viewing field. On the other hand, there is a problem in that it is not possible to observe the morphology of the affected site to which the marker has been applied. Furthermore, there is a problem in that a marker that is different in nature from the rest of the white-light image appears unnatural to the observer. In particular, by displaying, also with the marker, fine noise that is erroneously detected as fluorescence, the observer experiences a sense of flickering. 
     Thus, there is a known method in which a fluorescence image is displayed by being superimposed on a white-light image by allocating the fluorescence image in which fluorescence from the affected site is captured to one of R, G, and B component images that constitute the white-light image (for example, see Patent Literature 1). The superimposed image generated by this method includes information about the white-light image also in the region that corresponds to a fluorescence region in the fluorescence image. Therefore, it is possible to display the fluorescence region without hindering morphological observation of the affected site, and also, without giving the observer an unnatural impression or a sense of flickering. However, with the method of Patent Literature 1, in the case in which the fluorescence intensity is low relative to a bright return-light image like the white-light image, the visibility of the fluorescence region in the superimposed image may be poor. 
     Thus, there is a known method in which analog signals that express a fluorescence image and an illumination-light image, respectively, are obtained by receiving the fluorescence image and the illumination-light image by using a photoelectric conversion device and by subjecting these images to photoelectric conversion, and in which the analog signals representing the fluorescence image are amplified by a greater gain than the analog signals representing the illumination-light image (for example, see Patent Literature 2). The photoelectric conversion device and an AD conversion circuit that processes the analog signals generated by the photoelectric conversion device contain dynamic ranges. Therefore, in the case in which the fluorescence intensity is low relative to a bright illumination-light image, in some cases, it is difficult to ensure a sufficient gain for analog signals representing a fluorescence image to achieve a sufficient visibility of a fluorescence region in a superimposed image. 
     In addition, a means for displaying an image, such as a monitor or the like, also contains a dynamic range related to displaying. Therefore, in the case in which an illumination-light image is displayed on a monitor with a certain level of brightness, even if it were possible to ensure a sufficiently large gain for analog signals representing a fluorescence image, in some cases, it is not possible to display a fluorescence region with a sufficient brightness to ensure sufficient visibility of the fluorescence region in a superimposed image. 
     CITATION LIST 
     Patent Literature 
     {PTL 1} PCT International Publication No. WO 2011/135992 
     {PTL 2} Japanese Unexamined Patent Application, Publication No. 2003-10101 
     SUMMARY OF INVENTION 
     The present invention provides a fluorescence observation apparatus including a light source that radiates illumination light and excitation light onto a subject; a return-light-image generating portion that generates a return-light image in which return light emitted from the subject due to the irradiation with the illumination light from the light source is captured; a fluorescence-image generating portion that generates a fluorescence image in which fluorescence emitted from the subject due to the irradiation with the excitation light from the light source is captured; a fluorescence detecting portion that detects a fluorescence region having gradation values equal to or greater than a predetermined gradation value threshold in the fluorescence image generated by the fluorescence-image generating portion; a return-light-image adjusting portion that adjusts gradation values of the return-light image; a superimposed-image generating portion that generates a superimposed image in which the return-light image, in which the gradation values have been adjusted by the return-light-image adjusting portion, and the fluorescence image are superimposed; and a coefficient setting portion that sets a degree-of-reduction, by which the gradation values of the return-light image are adjusted by the return-light-image adjusting portion, based on the detection result of the fluorescence detecting portion for the fluorescence region, wherein, in the case in which the fluorescence region is detected by the fluorescence detecting portion, the coefficient setting portion sets the degree-of-reduction so that gradation values of the return-light image are decreased as compared with the case in which the fluorescence region is not detected by the fluorescence detecting portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall configuration diagram of a fluorescence observation apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a flowchart for explaining image processing performed by an image-processing unit in  FIG. 1 . 
         FIG. 3  is an overall configuration diagram of a first modification of the fluorescence observation apparatus in  FIG. 1 . 
         FIG. 4  is an overall configuration diagram of a second modification of the fluorescence observation apparatus in  FIG. 1 . 
         FIG. 5  is an overall configuration diagram of a fluorescence observation apparatus according to a second embodiment of the present invention. 
         FIG. 6  is a graph of a function used in a coefficient setting portion in  FIG. 5 , in which the function shows a coefficient α with respect to a representative value n of gradation values of a fluorescence region. 
         FIG. 7  is a flowchart for explaining image processing performed by an image-processing unit in  FIG. 5 . 
         FIG. 8  is an overall configuration diagram of a fluorescence observation apparatus according to a third embodiment of the present invention. 
         FIG. 9  is a graph of a function used in a coefficient setting portion in  FIG. 8 , in which the function shows a coefficient β with respect to a representative value n of gradation values of a fluorescence region. 
         FIG. 10  is a flowchart for explaining image processing performed by an image-processing unit in  FIG. 8 . 
         FIG. 11  is an overall configuration diagram of a fluorescence observation apparatus according to a fourth embodiment of the present invention. 
         FIG. 12  is an overall configuration diagram of a fluorescence observation apparatus according to a fifth embodiment of the present invention. 
         FIG. 13  is a graph of a function used in a coefficient setting portion in  FIG. 12 , in which the function shows a coefficient α with respect to a ratio Z. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A fluorescence observation apparatus  100  according to a first embodiment of the present invention will be described below with reference to  FIGS. 1 to 4 . 
     As shown in  FIG. 1 , the fluorescence observation apparatus  100  according to this embodiment is an endoscope apparatus provided with an elongated inserted portion  2  that is inserted into a body, a light source  3 , an illumination unit  4  that radiates excitation light and illumination light coming from the light source  3  toward an observation subject (subject) X from the distal end of the inserted portion  2 , an image-acquisition unit  5  that is provided at the distal end of the inserted portion  2  and that acquires pieces of image information S 1  and S 2  about biological tissue, that is, the observation subject X, an image-processing unit  6  that is disposed at the base side of the inserted portion  2  and that processes the pieces of image information S 1  and S 2  acquired by the image-acquisition unit  5 , and a monitor  7  that displays an image G 3  processed by the image-processing unit  6 . 
     The light source  3  is provided with a xenon lamp  31 , a filter  32  that extracts the excitation light and the illumination light from light emitted from the xenon lamp  31 , and a coupling lens  33  that focuses the excitation light and the illumination light extracted by the filter  32 . The filter  32  selectively transmits light of a wavelength band from 400 nm to 740 nm, which corresponds to the excitation light and the illumination light. In other words, in this embodiment, near-infrared light (for example, the wavelength band from 700 nm to 740 nm) is used as the excitation light. Note that, instead of the xenon lamp  31 , other types of lamp light sources or semiconductor light sources such as an LED or the like may be employed. 
     The illumination unit  4  is provided with a light-guide fiber  41  that is disposed over nearly the entire length of the inserted portion  2  in the longitudinal direction thereof and an illumination optical system  42  that is provided at the distal end of the inserted portion  2 . The light-guide fiber  41  guides the excitation light and the illumination light focused by the coupling lens  33 . The illumination optical system  42  spreads out and radiates the excitation light and the illumination light guided thereto via the light-guide fiber  41  onto the observation subject X facing the distal end of the inserted portion  2 . 
     The image-acquisition unit  5  is provided with an objective lens  51  that collects light coming from the observation subject X, a dichroic mirror  52  that, of the light collected by the objective lens  51 , reflects excitation light and fluorescence and transmits white light having a shorter wavelength (wavelength band from 400 nm to 700 nm, return light) than the excitation light, two focusing lenses  53  and  54  that focus the fluorescence reflected by the dichroic mirror  52  and the white light transmitted through the dichroic mirror  52 , respectively, an image-acquisition device  55 , such as, a color CCD that captures the white light focused by the focusing lens  53 , and an image-acquisition device  56  like a high-sensitivity monochromatic CCD that captures the fluorescence focused by the focusing lens  54 . In the figures, reference sign  57  is an excitation-light cut filter that, of the light reflected by the dichroic mirror  52 , selectively transmits the fluorescence (for example, the wavelength band from 760 nm to 850 nm) and blocks the excitation light. 
     Although the image-acquisition device  55  for the white light and the image-acquisition device  56  for the fluorescence may be of different types from each other, as described above, they may be of the same type. 
     The image-processing unit  6  is provided with a white-light-image generating portion (return-light-image generating portion)  61  that generates a white-light image (return-light image) G 1  based on the white-light-image information S 1  acquired by the image-acquisition device  55 , a fluorescence-image generating portion  62  that generates a fluorescence image G 2  based on the fluorescence-image information S 2  acquired by the image-acquisition device  56 , a fluorescence detecting portion  63  that detects a fluorescence region F in the fluorescence image G 2  generated by the fluorescence-image generating portion  62 , a coefficient setting portion  64  that sets a coefficient α related to gradation values of the white-light image G 1  based on the detection result of the fluorescence detecting portion  63 , a white-light-image adjusting portion (return-light-image adjusting portion)  65  that generates an adjusted image G 1 ′ by adjusting the gradation values of the white-light image G 1  based on the coefficient α, and a superimposed-image generating portion  66  that generates a superimposed image G 3  by superimposing the fluorescence image G 2  on the adjusted image G 1 ′. 
     The fluorescence detecting portion  63  holds a predetermined threshold Th for the gradation values of the fluorescence image G 2 . The fluorescence detecting portion  63  compares the gradation values of individual pixels of the fluorescence image G 2  input from the fluorescence-image generating portion  62  with the threshold Th, and detects the pixels having gradation values equal to or greater than the threshold Th as the fluorescence region F. The fluorescence detecting portion  63  outputs, to the coefficient setting portion  64 , signals S 3  indicating whether or not the fluorescence region F have been detected. 
     The coefficient setting portion  64  holds two predetermined values α1 and α2 as the coefficient α, selects the value α1 or the value α 2  in accordance with the signals S 3  received from the fluorescence detecting portion  63 , and outputs the selected value to the white-light-image adjusting portion  65 . Here, α1=1 and 0&lt;α2&lt;1 (for example, α2=0.6). The coefficient setting portion  64  selects α2 in the case in which the signals indicating that the fluorescence region F have been detected are received from the fluorescence detecting portion  63 . On the other hand, the coefficient setting portion  64  selects α1 in the case in which the signals indicating that the fluorescence region F have not been detected are received from the fluorescence detecting portion  63 . 
     Expression (1) below expresses processing for generating the superimposed image G 3  by using the white-light image G 1  and the fluorescence image G 2 , which is performed by the white-light-image adjusting portion  65  and the superimposed-image generating portion  66 . In Expression (1), R, G, and B are gradation values of red (R) components, green (G) components, and blue (B) components of the individual pixels of the white-light image G 1 , FL is gradation values of the individual pixels of the fluorescence image G 2 , and R′, G′, and B′ are gradation values of R components, G components, and B components of the individual pixels of the superimposed image G 3 . The white-light-image adjusting portion  65  and the superimposed-image generating portion  66  apply the processing of Expression (1) to all pixels in the white-light image G 1  and the fluorescence image G 2 . 
     
       
         
           
             
               
                 
                   
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     Describing the processing expressed by Expression (1) more specifically, the white-light-image adjusting portion  65  multiplies the gradation values of the individual color components of the individual pixels of the white-light image G 1  by the coefficient α set by the coefficient setting portion  64 , thus calculating the gradation values of the individual color components of the individual pixels of the adjusted image G 1 ′. The superimposed-image generating portion  66  adds the monochromatic fluorescence image G 2  to one of the three color component images (i.e., the red (R) component image, the green (G) component image, and the blue (B) component image) that constitute the adjusted image G 1 ′ having colors. Then, the superimposed-image generating portion  66  reconstructs the superimposed image G 3  having colors by using the color component image to which the fluorescence image G 2  has been added and the other two color component images. 
     With the above-described processing, in the case in which the coefficient α is α1 (=1), the white-light image G 1  serves as the adjusted image G 1 ′ without modification. In other words, the superimposed image G 3  is generated based on the white-light image G 1  having the original gradation values. On the other hand, in the case in which the coefficient α is α2, a white-light image G 1  in which the gradation values are decreased is generated as an adjusted image G 1 ′, and the superimposed image G 3  is generated based on this adjusted image G 1 ′. With such processing, it is possible to adjust the gradation values of the white-light image G 1  by using only the computational processing, and thus, it is possible to simplify the apparatus configuration. In this superimposed image G 3 , the observer can perform observation by associating the morphology of the subject X and the fluorescence region F with each other. In addition, because the superimposed image G 3  includes information about the white-light image G 1  also in the region in which the fluorescence region F is superimposed, it is possible to observe the morphology of the subject X in the region corresponding to the fluorescence region F. 
     The superimposed image G 3  generated in accordance with Expression (1) is an image in which red, green, or blue fluorescence region F is displayed superimposed on the adjusted image G 1 ′. In this embodiment, because the hue of biological tissue, that is, the observation subject X, includes a large amount of R components, it is preferable that the color in which the fluorescence region F is displayed in the superimposed image G 3  be green, which is the complementary color of red. Therefore, the matrix in Expression (1) is set so that the fluorescence region F is allocated to the G component image of the adjusted image G 1 ′. 
     Next, the operation of the thus-configured fluorescence observation apparatus  100  will be described. 
     In order to observe the biological tissue, that is, the observation subject X, in the body by using the fluorescence observation apparatus  100  according to this embodiment, a fluorescent substance that accumulates in an affected site Y is administered to the observation subject X in advance. Then, the inserted portion  2  is inserted into the body, and the distal end of the inserted portion  2  is disposed so as to face the observation subject X. Subsequently, the light source  3  is activated to radiate the excitation light and the illumination light onto the observation subject X from the distal end of the inserted portion  2 . 
     In the observation subject X, the fluorescent substance contained in the affected site Y is excited by the excitation light, thus emitting fluorescence, and the white light is reflected at the surface of the observation subject X. A portion of the fluorescence emitted from the observation subject X and a portion of the white light reflected thereat return to the distal end of the inserted portion  2  and are collected by the objective lens  51 . 
     Of the light collected by the objective lens  51 , the white light is transmitted through the dichroic mirror  52 , is focused by the focusing lens  53 , and is acquired by the image-acquisition device  55  as the white-light-image information S 1 . On the other hand, the fluorescence collected by the objective lens  51  is reflected at the dichroic mirror  52 , is focused by the focusing lens  54  after the excitation light is removed therefrom by the excitation-light cut filter  57 , and is acquired by the image-acquisition device  56  as the fluorescence-image information S 2 . The pieces of image information S 1  and S 2  acquired by the individual image-acquisition devices  55  and  56  are transmitted to the image-processing unit  6 . 
       FIG. 2  shows a flowchart for explaining image processing performed by the image-processing unit  6 . 
     In the image-processing unit  6 , the white-light-image information S 1  is input to the white-light-image generating portion  61 , thus generating the white-light image G 1 , and the fluorescence-image information S 2  is input to the fluorescence-image generating portion  62 , thus generating the fluorescence image G 2  (step S 1 ). Subsequently, whether or not the fluorescence region F exists in the fluorescence image G 2  is judged in the fluorescence detecting portion  63  (step S 2 ). 
     Because the fluorescence region F is not detected in step S 2  (“NO” in step S 2 ) in the case in which the affected site Y does not exist in the viewing field, the coefficient α is set to α1 (=1) in the coefficient setting portion  64  (step S 3 ). Therefore, in the white-light-image adjusting portion  65 , the white-light image G 1  is output to the superimposed-image generating portion  66  as the adjusted image G 1 ′ without modification (step S 5 ), and, in the superimposed-image generating portion  66 , a superimposed image G 3  in which a fluorescence image G 2  having almost no gradation values is superimposed on the unprocessed white-light image G 1  is generated (step S 6 ). The superimposed image G 3  generated at this time is practically equivalent to a raw white-light image G 1 . 
     On the other hand, because the fluorescence region F is detected in step S 2  (“YES” in step S 2 ) in the case in which the affected site Y exists in the viewing field, the coefficient α is set to α2 (&lt;1) in the coefficient setting portion  64  (step S 4 ). Therefore, in the white-light-image adjusting portion  65 , the adjusted image G 1 ′ is generated by applying processing for decreasing the gradation values to the white-light image G 1  (step S 5 ), and thus, the superimposed image G 3  in which the fluorescence image G 2  having the original gradation values is superimposed on this adjusted image G 1 ′ is generated (step S 6 ). The superimposed image G 3  generated at this time is an image in which the fluorescence region F that is bright relative to the white-light image G 1  is superimposed on the white-light image G 1  having a lower overall brightness. Therefore, the brightness of the fluorescence region F stands out in the superimposed image G 3 . 
     As has been described above, with this embodiment, depending on whether or not the fluorescence region F on which the observer focuses exists in the viewing field, the brightness of the white-light image G 1  included in the superimposed image G 3  is changed. In other words, because an ordinary, clear white-light image G 1  is displayed on the monitor  7  in the case in which the fluorescence region F does not exist, the observer can clearly observe the morphology of the biological tissue. On the other hand, in the case in which the fluorescence region F exists, the superimposed image G 3  in which the brightness of the white-light image G 1  is decreased so that the brightness of the fluorescence region F stands out is displayed on the monitor  7 . Therefore, the observer can easily visually recognize the fluorescence region F and, at the same time, he/she can also sufficiently clearly observe the biological tissue that is displayed behind the fluorescence region F. 
     Note that, in this embodiment, when the fluorescence region F is detected by the fluorescence detecting portion  63 , it is preferable that the coefficient setting portion  64  set the coefficient α by taking into account also the number of pixels detected as the fluorescence region F. In other words, the fluorescence detecting portion  63  outputs, to the coefficient setting portion  64 , the number of pixels having the gradation values equal to or greater than the gradation value threshold Th. The coefficient setting portion  64  selects α2 only when the number of pixels received from the fluorescence detecting portion  63  is equal to or greater than a predetermined pixel number threshold. On the other hand, even if pixels having the gradation values equal to or greater than the gradation value threshold Th are detected, the coefficient setting portion  64  selects α1 when the number of pixels is less than the predetermined pixel number threshold. 
     Even in the case in which the affected site Y does not exist in the viewing field, noise having gradation values equal to or greater than the predetermined gradation value threshold Th could occur in the fluorescence image G 2 . In the case in which such noise is erroneously detected as the fluorescence region F, the brightness of the white-light image G 1  unnecessarily fluctuates in the superimposed image G 3 , thus bothering the observer. Therefore, by decreasing the gradation values of the white-light image G 1  only when there are a sufficient number of pixels that exceed the gradation value threshold Th, it is possible to prevent the gradation values of the white-light image G 1  from unnecessarily being decreased in response to noise. 
     Next, modifications of this embodiment will be described. 
     (First Modification) 
     As shown in  FIG. 3 , the fluorescence observation apparatus  100  according to a first modification of this embodiment is additionally provided with a threshold setting portion  67  that sets the gradation value threshold Th for the fluorescence detecting portion  63  based on the gradation values of the white-light image G 1 . 
     The threshold setting portion  67  receives the white-light image G 1  from the white-light-image generating portion  61 , and calculates a representative value m of the gradation values of the white-light image G 1 . The representative value m is, for example, an average value or a median value of the gradation values of all pixels of the white-light image G 1 . The threshold setting portion  67  calculates the gradation value threshold Th based on a predetermined function by using the representative value m. Here, the predetermined function is an increasing function in which the gradation value threshold Th increases with an increase in the representative value m. 
     The gradation values of the white-light image G 1  fluctuate overall, depending on the observation distance between the distal end of the inserted portion  2  and the observation subject X, and the overall brightness of the white-light image G 1  increases with a decrease in the observation distance. Similarly, the gradation values of the fluorescence image G 2  also fluctuate overall, depending on the observation distance, and, even if the intensity of the fluorescence emitted from the observation subject X is the same, the gradation values of the fluorescence region F in the fluorescence image G 2  decrease with an increase in the observation distance. 
     With this modification, the magnitude of the observation distance is judged based on the representative value m of the gradation values of the white-light image G 1 , the gradation value threshold Th is changed in accordance with the fluctuation of the gradation values of the fluorescence image G 2  caused by the changes in the observation distance, and thus, it is possible to enhance the detection precision for the fluorescence region F; as a result, there is an advantage in that a superimposed image G 3  in which the brightness of the white-light image G 1  is more appropriately adjusted can be presented to the observer. 
     (Second Modification) 
     As shown in  FIG. 4 , the fluorescence observation apparatus  100  according to a second modification of this embodiment is additionally provided with a division portion  68  that divides the fluorescence image G 2  by the white-light image G 1 . 
     The division portion  68  generates a fluorescence image (hereinafter, referred to as a corrected fluorescence image) G 2 ′ in which the gradation values are corrected by dividing the gradation values of the individual pixels of the fluorescence image G 2  input from the fluorescence-image generating portion  62  by the gradation values of the individual pixels of the white-light image G 1  input from the white-light-image generating portion  61 . Then, the division portion  68  outputs the generated corrected fluorescence image G 2 ′ to the fluorescence detecting portion  63  and the superimposed-image generating portion  66 . 
     The fluorescence detecting portion  63  detects the fluorescence region F in the corrected fluorescence image G 2 ′ instead of the fluorescence image G 2 . 
     The superimposed-image generating portion  66  generates the superimposed image G 3  by using the corrected fluorescence image G 2 ′ instead of the fluorescence image G 2 . In other words, the superimposed-image generating portion  66  uses the gradation values of the corrected fluorescence image G 2 ′ as FL in Expression (1). 
     The gradation values of the white-light image G 1  fluctuate depending on the observation angle between the distal end of the inserted portion  2  and the observation subject X in addition to the above-described observation distance. Similarly, the gradation values of the fluorescence image G 2  fluctuate depending on the observation distance and the observation angle. Therefore, by dividing the fluorescence image G 2  by the white-light image G 1 , the gradation values of the fluorescence image G 2  are normalized, and thus, a corrected fluorescence image G 2 ′ from which changes in the gradation values that depend on the observation distance and the observation angle are removed is obtained. By using such a corrected fluorescence image G 2 ′, it is possible to enhance the detection precision for the fluorescence region F, and thus, it is possible to provide a superimposed image G 3  having greater reliability. 
     Second Embodiment 
     Next, a fluorescence observation apparatus  200  according to a second embodiment of the present invention will be described with reference to  FIGS. 5 to 7 . In this embodiment, configurations differing from those of the first embodiment will mainly be described, and configurations that are the same as those of the first embodiment will be given the same reference signs and descriptions thereof will be omitted. 
     As shown in  FIG. 5 , the fluorescence observation apparatus  200  according to this embodiment mainly differs from that of the first embodiment in that a representative-value calculating portion  69  that calculates a representative value n of the gradation values of the fluorescence region F detected by the fluorescence detecting portion  63  is additionally provided and that the coefficient setting portion  64  sets the coefficient α in accordance with the representative value n calculated by the representative-value calculating portion  69 . 
     In this embodiment, in the case in which pixels having gradation values equal to or greater than the predetermined gradation value threshold Th exist, the fluorescence detecting portion  63  identifies the corresponding pixels, and outputs gradation values S 4  of the identified pixels to the representative-value calculating portion  69 . 
     As the representative value n of the gradation values S 4  of the pixels input from the fluorescence detecting portion  63 , the representative-value calculating portion  69  calculates, for example, an average value, and outputs the calculated representative value n to the coefficient setting portion  64 . 
     In this embodiment, the coefficient setting portion  64  calculates the value of the coefficient α based on a predetermined function by using the representative value n calculated by the representative-value calculating portion  69  and outputs the calculated value to the white-light-image adjusting portion  65 . Here, the predetermined function is an increasing function in which the coefficient α increases with an increase in the representative value n, and is, for example, a linear function, as shown in  FIG. 6 . The representative value n takes a value equal to or greater than the gradation value threshold Th but equal to or less than the maximum possible value that the gradation values of the fluorescence image G 2  could possibly take, and the coefficient α takes a value greater than zero but equal to or less than 1. 
     Next, the operation of the thus-configured fluorescence observation apparatus  200  will be described. 
     With the fluorescence observation apparatus  200  according to this embodiment, processing performed in step S 2  in the case in which the fluorescence region F is detected differs from that of the first embodiment. As shown in  FIG. 7 , in the case in which the fluorescence region F exists in the fluorescence image G 2  (“YES” in step S 2 ), the fluorescence region F is identified in the fluorescence detecting portion  63  (step S 21 ). Subsequently, the representative value n of the gradation values of the identified fluorescence region F is calculated in the representative-value calculating portion  69  (step S 22 ). Subsequently, the coefficient α in accordance with the representative value n is set in the coefficient setting portion  64  (step S 23 ). The white-light-image adjusting portion  65  generates the adjusted image G 1 ′ by using the coefficient α set in step S 23  (step S 5 ). 
     Here, the coefficient α set in the step S 23  is a value that reflects the brightness of whole fluorescence region F. Therefore, the degree-of-reduction when decreasing the gradation values of the white-light image G 1  in the white-light-image adjusting portion  65  is adjusted in accordance with the brightness of whole fluorescence region F. In other words, in the case in which the fluorescence region F as a whole is sufficiently bright, the coefficient α takes a value closer to 1, and an adjusted image G 1 ′ in which the brightness is hardly decreased relative to that of the white-light image G 1  is generated. On the other hand, in the case in which the fluorescence region F as a whole is sufficiently dark, the coefficient α takes a lower value, and an adjusted image G 1 ′ in which the brightness is sufficiently decreased relative to that of the white-light image G 1  is generated. 
     In the case in which the gradation values of the fluorescence region F are sufficiently high, sufficient visibility of the fluorescence region F is achieved relative to that of the white-light image G 1 . As has been described above, with this embodiment, the degree-of-reduction of the brightness of the white-light image G 1  is adjusted, in accordance with the brightness of the fluorescence region F, to an amount that is necessary and sufficient to achieve sufficient visibility of the fluorescence region F. Thus, there is an advantage in that it is possible to enhance the visibility of the fluorescence region F in the superimposed image G 3  and that it is also possible to minimize the degree-of-reduction of the brightness of the white-light image G 1  to prevent the image of the subject X in the superimposed image G 3  from becoming unclear. Because other operational advantages are the same as those of the first embodiment, descriptions thereof will be omitted. 
     Note that, in this embodiment, the coefficient setting portion  64  may output the calculated coefficient α to the monitor  7 , and the monitor  7  may display the coefficient α. 
     By doing so, as compared with the ordinary white-light image G 1  (in other words, when α=1), the observer can recognize the extent to which the brightness of the adjusted image G 1 ′ is decreased in the superimposed image G 3  currently displayed on the monitor  7 , and thus, there is an advantage in that it is possible to enhance the diagnosis precision based on the superimposed image G 3 . 
     Third Embodiment 
     Next, a fluorescence observation apparatus  300  according to a third embodiment of the present invention will be described with reference to  FIGS. 8 to 10 . In this embodiment, configurations differing from those of the first and second embodiments will mainly be described, and configurations that are the same as those of the first and second embodiments will be given the same reference signs, and descriptions thereof will be omitted. 
     As shown in  FIG. 8 , the fluorescence observation apparatus  300  according to this embodiment mainly differs from those of the first and second embodiments in that the representative-value calculating portion  69  described in the second embodiment and an exposure-level controlling portion (return-light-image adjusting portion)  70  that controls an aperture stop  8  provided in a stage preceding the image-acquisition device  55  for the white-light image G 1  are additionally provided, and that the coefficient setting portion  64  sets a coefficient β related to the aperture diameter of the aperture stop  8  instead of the coefficient α. 
     The coefficient setting portion  64  calculates the coefficient β based on a predetermined function by using the representative value n calculated by the representative-value calculating portion  69 , and outputs the calculated coefficient β to the exposure-level controlling portion  70 . Here, the predetermined function is an increasing function in which the coefficient β increases with an increase in the representative value n, and is, for example, a linear function, as shown in  FIG. 9 . The coefficient β takes a value greater than zero but equal to or less than 1. 
     The exposure-level controlling portion  70  controls the aperture diameter of the aperture stop  8  by transmitting signals S 5  that specify an aperture diameter φ to the aperture stop  8  based on the detection result of the fluorescence detecting portion  63  about the fluorescence region F and the coefficient β input from the coefficient setting portion  64 . Specifically, the exposure-level controlling portion  70  keeps the aperture diameter of the aperture stop  8  at a predetermined diameter φ during the normal operation in which the fluorescence region F is not detected by the fluorescence detecting portion  63 . On the other hand, in the case in which the fluorescence region F is detected by the fluorescence detecting portion  63 , the exposure-level controlling portion  70  controls the aperture diameter of the aperture stop  8  so that a diameter obtained by multiplying the diameter φ for the normal operation by the coefficient β is obtained. For example, in the case in which the coefficient β is 0.8, the aperture stop  8  is controlled so that the aperture diameter becomes 80% of that during the normal operation. In this case, the exposure level in the image-acquisition device  55  for the white light is consequently decreased to about 80% of that during the normal operation. 
     In this embodiment, the superimposed-image generating portion  66  generates the superimposed image G 3  by using, always without modification, the white-light image G 1  generated by the white-light-image generating portion  61  and the fluorescence image G 2  generated by the fluorescence-image generating portion  62 . In other words, in this embodiment, as described below, the white-light image G 1  generated by the white-light-image generating portion  61  serves as an adjusted image in which the gradation values have been adjusted. Therefore, regardless of the presence/absence of the fluorescence region F, the superimposed-image generating portion  66  generates the superimposed image G 3  in accordance with Expression (1) in which α is set to 1. 
     Next, the operation of the thus-configured fluorescence observation apparatus  300  will be described. 
     With the fluorescence observation apparatus  300  according to this embodiment, the processing after judging the presence/absence of the fluorescence region F in step S 2  differs from those of the first and second embodiments. As shown in  FIG. 10 , in the case in which the fluorescence region F is not detected in step S 2  (“NO” in step S 2 ), the aperture diameter of the aperture stop  8  is adjusted to the predetermined diameter φ (steps S 31  and S 36 ). By doing so, the white-light image G 1  to be generated next becomes an image possessing ordinary brightness. 
     On the other hand, in the case in which the fluorescence region F is detected in step S 2  (“YES” in step S 2 ), as in steps S 21  to S 23  of the second embodiment, the fluorescence region F is identified in the fluorescence detecting portion  63  (step S 32 ), the representative value n of the gradation values of the fluorescence region F is calculated in the representative-value calculating portion  69  (step S 33 ), and the coefficient β in accordance with the representative value n is set in the coefficient setting portion  64  (step S 34 ). Then, the aperture diameter of the aperture stop  8  is adjusted in accordance with the coefficient β so as to be smaller than that during the normal operation (steps S 35  and S 36 ). By doing so, the white-light image G 1  to be generated next becomes an image in which the brightness is decreased as compared with that during the normal operation. Specifically, by decreasing the degree-of-opening of the aperture stop  8  positioned at the stage preceding the image-acquisition device  55  for white light, while leaving an aperture stop (not shown) positioned at a stage preceding the image-acquisition device  56  for fluorescence with the degree-of-opening thereof unchanged, it is possible to decrease the relative brightness of the white-light image G 1  as compared with that of the fluorescence image G 2  by decreasing only the amount of the white light received from among the light (reflected/absorbed light) emitted from the same site. 
     As has been described above, with this embodiment, it is possible to adjust the brightness of the white-light image G 1  in accordance with the presence/absence of the fluorescence region F also by decreasing the exposure level in the image-acquisition device  55 , instead of decreasing the gradation values of the white-light image G 1  by means of computational processing, as in the first and second embodiments. By doing so, it is possible to achieve the same operational advantages as in the first embodiment. Furthermore, by adjusting the aperture diameter of the aperture stop  8  in accordance with the brightness of the fluorescence region F, there is an advantage in that, as with the second embodiment, it is possible to minimize the deterioration of the visibility of the white-light image G 1  in the superimposed image G 3  by adjusting the degree-of-reduction of the brightness of the white-light image G 1  to an amount that is necessary and sufficient to achieve sufficient visibility of the fluorescence region F. Furthermore, because the white-light image G 1  can be used without modification to generate the superimposed image, the computational processing can be simplified. 
     Note that, in this embodiment, although the gradation values of the white-light image G 1  are decreased by decreasing the amount of light that enters the image-acquisition device  55  by using the aperture stop  8 , alternatively, the exposure time in the image-acquisition device  55  may be decreased. The adjustment of the exposure time is performed by, for example, the exposure-level controlling portion  70  by controlling the amount of time by which an electronic shutter (not shown) provided in the image-acquisition device  55  is opened. By doing so also, it is possible to generate a white-light image G 1  in which the gradation values are decreased as compared with those during the normal operation by using the white-light-image generating portion  61 , as with the case in which the aperture stop  8  is used. 
     Fourth Embodiment 
     Next, a fluorescence observation apparatus  400  according to a fourth embodiment of the present invention will be described with reference to  FIG. 11 . In this embodiment, configurations differing from those of the first to third embodiments will mainly be described, and configurations that are the same as those of the first embodiment will be given the same reference signs and descriptions thereof will be omitted. 
     The fluorescence observation apparatus  400  according to this embodiment is the same as that of the third embodiment in that the gradation values of the white-light image G 1  are decreased by adjusting the exposure level in the image-acquisition device  55 . However, as shown in  FIG. 11 , the fluorescence observation apparatus  400  of this embodiment is provided with an aperture stop  81  at the light source  3 , instead of the stage preceding the image-acquisition device  55 , and the light-level controlling portion (exposure-level controlling portion, return-light-image adjusting portion)  71  changes the amount of illumination light radiated onto the observation subject X by controlling the aperture stop  81 . 
     In this embodiment, the light source  3  is additionally provided with another xenon lamp  311  and a beam splitter  34 . The beam splitter  34  combines the light emitted from the two xenon lamps  31  and  311  in the entrance optical axis leading to the light-guide fiber  41 . Note that the xenon lamps  31  and  311  may be other types of lamp light sources or semiconductor light sources, such as LEDs or the like. In addition, the xenon lamps  31  and  311  may be of the same type or of mutually different types. 
     The light source  3  is provided with a first filter  321  and a second filter  322  instead of the filter  32 . The first filter  321  extracts illumination light (for example, a wavelength band from 400 nm to 700 nm) from the light emitted from the first xenon lamp  31 . The second filter  322  extracts excitation light (for example, a wavelength band from 700 nm to 740 nm) from the light emitted from the second xenon lamp  311 . 
     The aperture stop  81  is disposed between the first filter  321  and the beam splitter  34  and changes the light level of only the illumination light in the light that enters the light-guide fiber  41 . 
     Except for controlling the aperture stop  81  instead of the aperture stop  8 , the light-level controlling portion  71  is the same as the exposure-level controlling portion  70  of the third embodiment. In this embodiment, by decreasing the aperture diameter of the aperture stop  81 , the amount of the illumination light radiated onto the observation subject X and the amount of the white light collected by the objective lens  51  are decreased, and, accordingly, the exposure level in the image-acquisition device  55  is decreased. 
     Descriptions of the operation and the operational advantages of the fluorescence observation apparatus  400  according to this embodiment will be omitted because they are the same as those of the fluorescence observation apparatus  300  of the third embodiment described above, except for the difference in terms of the aperture stop  81  in which the aperture diameter is adjusted in steps S 31 , S 35 , and S 36  in  FIG. 10 . 
     Fifth Embodiment 
     Next, a fluorescence observation apparatus  500  according to a fifth embodiment of the present invention will be described with reference to  FIGS. 12 and 13 . In this embodiment, configurations differing from those of the first to fourth embodiments will mainly be described, and configurations that are the same as those of the first embodiment will be given the same reference signs and descriptions thereof will be omitted. 
     As shown in  FIG. 12 , the fluorescence observation apparatus  500  according to this embodiment mainly differs from those of the first to fourth embodiments in that the coefficient setting portion  64  sets the coefficient α (0&lt;α≦1) by taking into account also the color components of the white-light image G 1 . 
     In this embodiment, the white-light-image generating portion  61  outputs the white-light image G 1  also to the coefficient setting portion  64  in addition to the white-light-image adjusting portion  65 . 
     Before setting the coefficient α, the coefficient setting portion  64  judges the color tone of the white-light image G 1  input from the white-light-image generating portion  61 . 
     Specifically, the coefficient setting portion  64  calculates an average value &lt;G′&gt; and an average value &lt;R&gt; by using the white-light image G 1 . The average value &lt;G′&gt; is the sum of an average value &lt;G&gt; of the gradations values of the G components in the white-light image G 1  and the representative value n calculated by the representative-value calculating portion  69 . The average value &lt;R&gt; is the average value of the gradation values of the R components in the white-light image G 1 . 
     Subsequently, the coefficient setting portion  64  calculates a ratio Z (=&lt;G′&gt;/&lt;R&gt;) between the average value &lt;G′&gt; and the average value &lt;R&gt;. Then, the coefficient setting portion  64  calculates the coefficient α based on a predetermined function by using the ratio Z, and outputs the coefficient α to the white-light-image adjusting portion  65 . Here, the predetermined function is an increasing function in which the coefficient α increases with an increase in the ratio Z, and is, for example, a linear function, as shown in  FIG. 13 . 
     The visibility of the fluorescence region F in the superimposed image G 3  also depends on the contrast between the hue of the biological tissue X and the color in which the fluorescence region F is displayed. 
     The above-described ratio Z expresses the contrast of the hue of the biological tissue relative to green, which is the color in which the fluorescence region F is displayed. Specifically, in the case in which the redness of the biological tissue is high mainly due to the effect of blood, the ratio Z and the coefficient α are decreased, and the degree-of-reduction of the gradation values of the white-light image G 1  is increased. On the other hand, in the case in which the redness of the biological tissue is low because the surface of the biological tissue is covered with fat or the like, the ratio Z and the coefficient α are increased, and the degree-of-reduction of the gradation values of the white-light image G 1  is decreased. 
     As has been described above, with this embodiment, by taking into consideration also the color tone of the white-light image G 1 , the degree-of-reduction of the brightness of the white-light image G 1  is adjusted to an amount that is necessary and sufficient to achieve sufficient visibility of the fluorescence region F, and thus, there is an advantage in that it is possible to enhance the visibility of the fluorescence region F in the superimposed image G 3 , and it is also possible to minimize the degree-of-reduction of the brightness of the white-light image G 1  to prevent the image of the subject X in the superimposed image G 3  from becoming unclear. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  200 ,  300 ,  400 ,  500  fluorescence observation apparatus 
           2  inserted portion 
           3  light source 
           31 ,  311  xenon lamp 
           32 ,  321 ,  322  filter 
           33  coupling lens 
           34  beam splitter 
           41  light-guide fiber 
           42  illumination optical system 
           51  objective lens 
           52  dichroic mirror 
           53 ,  54  focusing lens 
           55 ,  56  image-acquisition device 
           57  excitation-light cut filter 
           4  illumination unit 
           5  image-acquisition unit 
           6  image-processing unit 
           61  white-light-image generating portion (return-light-image generating portion) 
           62  fluorescence-image generating portion 
           63  fluorescence detecting portion 
           64  coefficient setting portion 
           65  white-light-image adjusting portion (return-light-image adjusting portion) 
           66  superimposed-image generating portion 
           67  threshold setting portion 
           68  division portion 
           69  representative-value calculating portion 
           70  exposure-level controlling portion (return-light-image adjusting portion) 
           71  light-level controlling portion (exposure-level controlling portion, return-light-image adjusting portion) 
           7  monitor 
           8 ,  81  aperture stop