Patent Publication Number: US-11033175-B2

Title: Endoscope system and operation method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2018/005789 filed on Feb. 19, 2018, which claims priority under 35 U.S.C § 119(a) to Japanese Patent Application No. 2017-038177 filed on Mar. 1, 2017. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an endoscope system for automatically detecting a predetermined detection target, such as a lesion portion, by using image processing, and also relates to an operation method for the endoscope system. 
     2. Description of the Related Art 
     In the medical field, diagnoses using an endoscope system including a light source device, an endoscope, and a processor device have widely been performed. In the endoscope system, illumination light emitted by the light source device is applied to an observation target through an endoscope, and the processor device generates an image of the observation target on the basis of image signals acquired by imaging the observation target illuminated with the illumination light. The image is displayed on a monitor, and thereby a medical practitioner is able to make a diagnosis while viewing the image on the monitor. 
     When making an endoscopic diagnosis, a medical practitioner attempts to constantly detect all predetermined detection targets that are to be carefully observed, such as a lesion or benign tumor in an organ. However, the accuracy of detecting the detection targets is influenced by the experiences and skills of the medical practitioner and is also influenced by the degree of fatigue of the medical practitioner. Thus, to reduce variation in diagnostic accuracy among medical practitioners, a technique has been developed for analyzing, using a computer, a large amount of endoscopic data acquired in daily diagnoses and extracting information helpful in diagnoses. For example, an automatic detection function of automatically detecting a portion with a disease by a computer instead of a medical practitioner at an endoscopic diagnosis makes it possible to prevent a detection target from being overlooked by a medical practitioner, and is expected to increase the confidence of an endoscopic diagnosis. 
     Specifically, according to JP2016-87370A, a screen displayed in a magnifying endoscope for the large intestine is divided into a plurality of images, and automatic detection and diagnosis are performed using pattern recognition to determine which of five surface structure patterns that are clinically classified corresponds to each of the images. According to JP2006-129950A, a portion suspected to have a lesion is automatically detected in a capsule endoscope, and a notification is made using a sound or the like when such a portion is detected. 
     SUMMARY OF THE INVENTION 
     Detecting of a detection target using the above-described automatic detection function is based on the assumption that the detection target is included in an image. However, a situation is assumed where, when it is difficult to photograph the detection target, for example, when the detection target is hidden behind a fold, the detection target is not photographed and is not included in an image. If the detection target is not included in an image, the automatic detection function of the computer does not work, and the detection target to be originally detected will be overlooked. 
     An object of the present invention is to provide an endoscope system capable of preventing a detection target from being overlooked in the case of using an automatic detection function for the detection target, and an operation method for the endoscope system. 
     An endoscope system according to the present invention includes an identification information acquisition unit that acquires first-diagnosis identification information at a first diagnosis and acquires second-diagnosis identification information at a second diagnosis that is different from the first diagnosis; a comparison processing unit that performs comparison processing of comparing the first-diagnosis identification information with the second-diagnosis identification information; and a notification unit that makes, if a determination is made that there is a difference in a detection target between the first diagnosis and the second diagnosis as a result of the comparison processing, a notification about an oversight of the detection target. 
     Preferably, the first-diagnosis identification information includes an image feature value of a detection target at the first diagnosis detected from an image acquired at the first diagnosis, the second-diagnosis identification information includes an image feature value of a detection target at the second diagnosis detected from an image acquired at the second diagnosis, and the identification information acquisition unit has an image feature value detection unit that automatically detects the image feature value of the detection target at the first diagnosis and automatically detects the image feature value of the detection target at the second diagnosis. 
     Preferably, the first-diagnosis identification information includes an image feature value of a detection target at the first diagnosis detected from an image acquired at the first diagnosis, the second-diagnosis identification information includes an image feature value of a detection target at the second diagnosis detected from an image acquired at the second diagnosis, and the notification unit makes the notification about the oversight of the detection target if a determination is made that the image feature value of the detection target at the first diagnosis does not match the image feature value of the detection target at the second diagnosis as a result of the comparison processing. 
     Preferably, the first-diagnosis identification information includes an image feature value of a detection target at the first diagnosis detected from an image acquired at the first diagnosis, and position information at the first diagnosis, the second-diagnosis identification information includes an image feature value of the detection target at the second diagnosis detected from an image acquired at the second diagnosis, and position information at the second diagnosis, and the notification unit makes the notification about the oversight of the detection target if a determination is made that the position information at the first diagnosis matches the position information at the second diagnosis and that the image feature value of the detection target at the first diagnosis does not match the image feature value of the detection target at the second diagnosis as a result of the comparison processing. 
     Preferably, the notification unit makes the notification about the oversight of the detection target if a determination is made that position information at the first diagnosis does not match position information at the second diagnosis and that an image feature value of a detection target at the first diagnosis does not match an image feature value of the detection target at the second diagnosis as a result of the comparison processing. 
     Preferably, acquisition of identification information by the identification information acquisition unit is switched from acquisition of the first-diagnosis identification information to acquisition of the second-diagnosis identification information. Preferably, the notification unit makes the notification using a warning message. Preferably, the notification unit makes the notification using a warning sound. 
     Preferably, the endoscope system includes a plurality of light sources having different wavelength characteristics, and the image feature value detection unit detects, from an image acquired by using at least one of the plurality of light sources, the image feature value of the detection target at the first diagnosis or the image feature value of the detection target at the second diagnosis. 
     An operation method for an endoscope system of the present invention includes a step of acquiring, with an identification information acquisition unit, first-diagnosis identification information at a first diagnosis and second-diagnosis identification information at a second diagnosis that is different from the first diagnosis; a step of performing, with a comparison processing unit, comparison processing of comparing the first-diagnosis identification information with the second-diagnosis identification information; and a step of making, with a notification unit, if a determination is made that there is a difference in a detection target between the first diagnosis and the second diagnosis as a result of the comparison processing, a notification about an oversight of the detection target. 
     According to the present invention, it is possible to prevent a detection target from being overlooked in the case of using an automatic detection function for the detection target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external appearance view of an endoscope system; 
         FIG. 2  is a block diagram illustrating the functions of the endoscope system according to a first embodiment; 
         FIG. 3  is a graph illustrating a spectrum of violet light V, blue light B, blue light Bx, green light G, and red light R; 
         FIG. 4  is a graph illustrating a spectrum of normal light according to the first embodiment; 
         FIG. 5  is a graph illustrating a spectrum of special light according to the first embodiment; 
         FIG. 6  is an explanatory diagram illustrating a method for calculating position information; 
         FIG. 7  is a block diagram illustrating the functions of a detection-target-oversight-prevention-mode processing unit; 
         FIG. 8  is a table illustrating determinations based on results of comparison processing; 
         FIG. 9  is an explanatory diagram illustrating a specific example of comparison processing; 
         FIG. 10  is an image diagram of a monitor that displays a main display image, sub display images, and detection-target-related information; 
         FIG. 11  is a flowchart illustrating a series of steps in a detection-target-oversight-prevention mode; 
         FIG. 12  is an explanatory diagram illustrating a case where detection targets match each other and a case where detection targets do not match each other in comparison processing; 
         FIG. 13  is an image diagram of a monitor that displays a main display image and a monitor that displays sub display images and detection-target-related information; 
         FIG. 14  is an image diagram of a detection target to which color is assigned in accordance with a lesion score and the surroundings thereof; 
         FIG. 15  is a block diagram illustrating the functions of an endoscope system according to a second embodiment; 
         FIG. 16  is a graph illustrating a spectrum of normal light according to the second embodiment; 
         FIG. 17  is a graph illustrating a spectrum of special light according to the second embodiment; 
         FIG. 18  is a block diagram illustrating the functions of an endoscope system according to a third embodiment; and 
         FIG. 19  is a plan view of a rotational filter. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     As illustrated in  FIG. 1 , an endoscope system  10  has an endoscope  12 , a light source device  14 , a processor device  16 , a monitor  18  (display unit), and a console  19 . The endoscope  12  is optically connected to the light source device  14  and is electrically connected to the processor device  16 . The endoscope  12  has an insertion section  12   a  to be inserted into a subject, an operation section  12   b  provided at a base end portion of the insertion section  12   a , and a bending portion  12   c  and a distal end portion  12   d  that are provided on a distal end side of the insertion section  12   a . Operating an angle knob  13   a  of the operation section  12   b  causes the bending portion  12   c  to perform a bending operation. The bending operation causes the distal end portion  12   d  to be directed in a desired direction. 
     The operation section  12   b  is provided with a still image acquisition unit  13   b  used for an operation of acquiring a still image, a mode switching unit  13   c  used for an operation of switching an observation mode, a zoom operation unit  13   d  used for an operation of changing zoom magnification, and an identification information switching unit  13   e , in addition to the angle knob  13   a . The still image acquisition unit  13   b  is capable of performing a freeze operation of displaying a still image of an observation target on the monitor  18  and a release operation of storing a still image in storage. 
     The endoscope system  10  has a normal mode, a special mode, and a detection-target-oversight-prevention mode as observation modes. When the observation mode is the normal mode, normal light generated by combining light beams of a plurality of colors at a light amount ratio Lc for the normal mode is emitted, and a normal image is displayed on the monitor  18  on the basis of image signals acquired by imaging an observation target illuminated with the normal light. When the observation mode is the special mode, special light generated by combining light beams of a plurality of colors at a light amount ratio Ls for the special mode is emitted, and a special image is displayed on the monitor  18  on the basis of image signals acquired by imaging an observation target illuminated with the special light. 
     When the observation mode is the detection-target-oversight-prevention mode, normal light and special light are alternately emitted. A normal image acquired by imaging an observation target illuminated with the normal light is displayed as a main display image on the monitor  18 , and a special image acquired by imaging the observation target illuminated with the special light is displayed as a sub display image on the monitor  18 . In the detection-target-oversight-prevention mode, whether there is an oversight of a detection target is determined, and a determination result is displayed in the main display image or the sub display image. Showing or hiding of the main display image and showing or hiding of the sub display image can be set as necessary. 
     The processor device  16  is electrically connected to the monitor  18  and the console  19 . The monitor  18  outputs and displays an image of an observation target, information accompanying the image, and so forth. The console  19  functions as a user interface that receives an input operation of designating a region of interest (ROI), setting a function, or the like. 
     As illustrated in  FIG. 2 , the light source device  14  includes a light source unit  20  that emits illumination light used to illuminate an observation target, and a light source control unit  22  that controls the light source unit  20 . The light source unit  20  is a semiconductor light source, such as light emitting diodes of a plurality of colors. The light source control unit  22  turns ON/OFF the LEDs or the like and adjusts driving currents and driving voltages for the LEDs or the like, thereby controlling the amount of illumination light to be emitted. In addition, the light source control unit  22  controls the wavelength range of the illumination light by, for example, changing an optical filter. 
     In the first embodiment, the light source unit  20  has LEDs of four colors: a violet light emitting diode (V-LED)  20   a , a blue light emitting diode (B-LED)  20   b , a green light emitting diode (G-LED)  20   c , and a red light emitting diode (R-LED)  20   d , and a wavelength cut filter  23 . As illustrated in  FIG. 3 , the V-LED  20   a  emits violet light V in a wavelength range of 380 nm to 420 nm. 
     The B-LED  20   b  emits blue light B in a wavelength range of 420 nm to 500 nm. Of the blue light B emitted by the B-LED  20   b , at least the long-wavelength side relative to a peak wavelength of 460 nm is cut off by the wavelength cut filter  23 . Accordingly, blue light Bx that has passed through the wavelength cut filter  23  is in a wavelength range of 420 nm to 460 nm. The light in the wavelength range on the long-wavelength side relative to 460 nm is cut off because the light in the wavelength range on the long-wavelength side relative to 460 nm is a factor in decreasing the contrast of blood vessels as an observation target. The wavelength cut filter  23  may decrease the amount of light in the wavelength range on the long-wavelength side relative to 460 nm instead of cutting off the light in the wavelength range on the long-wavelength side relative to 460 nm. 
     The G-LED  20   c  emits green light G in a wavelength range of 480 nm to 600 nm. The R-LED  20   d  emits red light R in a wavelength range of 600 nm to 650 nm. The light emitted by each of the LEDs  20   a  to  20   d  may have a center wavelength and a peak wavelength that are identical to or different from each other. 
     The light source control unit  22  controls ON/OFF of each of the LEDs  20   a  to  20   d  and the amount of light emission in an ON state independently from each other, thereby adjusting the emission timing, emission period, amount of light, and spectrum of illumination light. The ON/OFF control by the light source control unit  22  varies according to an observation mode. A reference brightness can be set by a brightness setting unit of the light source device  14 , the console  19 , or the like. 
     In the normal mode, the light source control unit  22  turns on all of the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d . At this time, as illustrated in  FIG. 4 , the light amount ratio Lc among the violet light V, the blue light Bx, the green light G, and the red light R is set such that the peak intensity of the blue light Bx is higher than the peak intensities of the violet light V, the green light G, and the red light R. Accordingly, in the normal mode, the light source device  14  emits, as normal light, multicolor light for the normal mode including the violet light V, the blue light Bx, the green light G, and the red light R. The normal light has a certain intensity or more in the blue range to the red range and is thus substantially white. 
     In the special mode, the light source control unit  22  turns on all of the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d . At this time, as illustrated in  FIG. 5 , the light amount ratio Ls among the violet light V, the blue light Bx, the green light G, and the red light R is set such that the peak intensity of the violet light V is higher than the peak intensities of the blue light Bx, the green light G, and the red light R and such that the peak intensities of the green light G and the red light R are lower than the peak intensities of the violet light V and the blue light Bx. Accordingly, in the special mode, the light source device  14  emits, as special light, multicolor light for the special mode including the violet light V, the blue light Bx, the green light G, and the red light R. The special light has a large proportion of the violet light V and is thus bluish. The special light does not need to include light of all the four colors, and may include light from at least one of the LEDs  20   a  to  20   d  of four colors. 
     In the detection-target-oversight-prevention mode, the light source control unit  22  controls the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d  so that normal light and special light are alternately emitted on a frame by frame basis. That is, the light source control unit  22  performs control to alternately switch the light amount ratio among the violet light V, the blue light Bx, the green light G, and the red light R between the light amount ratio Lc and the light amount ratio Ls on a frame by frame basis. 
     As illustrated in  FIG. 2 , the illumination light emitted by the light source unit  20  passes through a light path coupling unit (not illustrated) formed of a mirror, a lens, and the like and then enters a light guide  24  that is in the insertion section  12   a . The light guide  24  is built in the endoscope  12  and a universal cord, and causes the illumination light to propagate to the distal end portion  12   d  of the endoscope  12 . The universal cord is a cord that connects the endoscope  12  to the light source device  14  and the processor device  16 . A multimode fiber may be used as the light guide  24 . As an example, a small-diameter fiber cable with a core diameter of 105 μm, a clad diameter of 125 μm, and a diameter including a protective layer serving as an outer cover of ϕ0.3 mm to ϕ0.5 mm may be used as the light guide  24 . 
     The distal end portion  12   d  of the endoscope  12  is provided with an illumination optical system  30   a  and an imaging optical system  30   b . The illumination optical system  30   a  has an illumination lens  32 . An observation target is illuminated, via the illumination lens  32 , with illumination light that has propagated through the light guide  24 . The imaging optical system  30   b  has an objective lens  34 , a magnifying optical system  36 , and an image sensor  38 . Various types of light, such as reflected light, scattered light, and fluorescence from the observation target, enters the image sensor  38  through the objective lens  34  and the magnifying optical system  36 . Accordingly, an image of the observation target is formed on the image sensor  38 . 
     The magnifying optical system  36  includes a zoom lens  36   a  that magnifies an observation target, and a lens driving unit  36   b  that moves the zoom lens  36   a  in optical-axis directions CL. The zoom lens  36   a  is freely moved between a telephoto end and a wide end in accordance with zoom control by the lens driving unit  36   b , thereby magnifying or demagnifying the image of the observation target formed on the image sensor  38 . 
     The image sensor  38  is a color image sensor that performs imaging of an observation target irradiated with illumination light. Each pixel of the image sensor  38  is provided with a red (R) color filter, a green (G) color filter, or a blue (B) color filter. The image sensor  38  receives violet to blue light in a B pixel provided with the B color filter, receives green light in a G pixel provided with the G color filter, and receives red light in an R pixel provided with the R color filter. Also, the image sensor  38  outputs image signals of individual colors of RGB from the pixels of the individual colors. The image sensor  38  transmits the output image signals to a CDS circuit  40 . 
     In the normal mode, the image sensor  38  performs imaging of an observation target illuminated with normal light, thereby outputting a Bc image signal from the B pixel, outputting a Gc image signal from the G pixel, and outputting an Rc image signal from the R pixel. In the special mode, the image sensor  38  performs imaging of an observation target illuminated with special light, thereby outputting a Bs image signal from the B pixel, outputting a Gs image signal from the G pixel, and outputting an Rs image signal from the R pixel. In the detection-target-oversight-prevention mode, the image sensor  38  outputs a Bc image signal, a Gc image signal, and an Rc image signal from the B pixel, the G pixel, and the R pixel, respectively, when performing imaging of an observation target illuminated with normal light, and outputs a Bs image signal, a Gs image signal, and an Rs image signal from the B pixel, the G pixel, and the R pixel, respectively, when performing imaging of the observation target illuminated with special light. 
     A charge coupled device (CCD) image sensor, a complementary metal-oxide semiconductor (CMOS) image sensor, or the like can be used as the image sensor  38 . Instead of the image sensor  38  provided with color filters of the primary colors RGB, a complementary-color image sensor including complementary-color filters of cyan (C), magenta (M), yellow (Y), and green (G) may be used. In the case of using the complementary-color image sensor, image signals of four colors CMYG are output. Thus, by converting image signals of four colors CMYG into image signals of three colors RGB by using complementary color to primary color conversion, image signals of individual colors RGB similar to those in the image sensor  38  can be acquired. Alternatively, a monochrome sensor not provided with color filters may be used instead of the image sensor  38 . 
     The CDS circuit  40  performs correlated double sampling (CDS) on analog image signals received from the image sensor  38 . The image signals output from the CDS circuit  40  are input to an AGC circuit  42 . The AGC circuit  42  performs automatic gain control (AGC) on the image signals input thereto. An analog to digital (A/D) conversion circuit  44  converts the analog image signals output from the AGC circuit  42  into digital image signals. The A/D conversion circuit  44  inputs the digital image signals generated through the A/D conversion to the processor device  16 . 
     As illustrated in  FIG. 6 , measuring graduations  46  for measuring an insertion length of the insertion section  12   a  in a body are marked on an outer surface of the insertion section  12   a  of the endoscope  12 . The measuring graduations  46  are constituted by points marked at a predetermined interval (for example, an interval of 1 cm) in the longitudinal direction of the insertion section  12   a . The measuring graduations  46  are detected by a graduation detection sensor  48  provided near the mouth (in the case of upper endoscopy) or the anus (in the case of lower endoscopy) of a patient. The graduation detection sensor  48  is connected to the processor device  16  in a wired or wireless manner, and information detected by the graduation detection sensor  48  is transmitted to the processor device  16 . A position information calculation unit  70   a  of the processor device  16  calculates position information about the insertion section  12   a  in a body cavity on the basis of a detection result obtained by the graduation detection sensor  48 . 
       FIG. 6  illustrates that the graduation detection sensor  48  is provided on a mouthpiece MP held by the mouth of a patient. Here, position information is calculated by using the measuring graduations  46  and the graduation detection sensor  48 . Alternatively, a magnetic sensor (not illustrated) may be provided at the distal end portion  12   d  of the insertion section  12   a , and position information may be calculated by the position information calculation unit  70   a  on the basis of information acquired by the magnetic sensor. 
     As illustrated in  FIG. 2 , the processor device  16  includes an image signal acquisition unit  50 , a digital signal processor (DSP)  52 , a noise reduction unit  54 , an image processing unit  56 , and a display control unit  58 . 
     The image signal acquisition unit  50  acquires digital image signals corresponding to an observation mode from the endoscope  12 . In the normal mode, the image signal acquisition unit  50  acquires a Bc image signal, a Gc image signal, and an Rc image signal. In the special mode, the image signal acquisition unit  50  acquires a Bs image signal, a Gs image signal, and an Rs image signal. In the detection-target-oversight-prevention mode, the image signal acquisition unit  50  acquires a Bc image signal, a Gc image signal, and an Rc image signal of one frame during illumination with normal light, and acquires a Bs image signal, a Gs image signal, and an Rs image signal of one frame during illumination with special light. 
     The DSP  52  performs various signal processing operations, such as defect correction processing, offset processing, DSP gain correction processing, linear matrix processing, gamma conversion processing, and demosaicing processing, on the image signals acquired by the image signal acquisition unit  50 . The defect correction processing corrects a signal of a defective pixel of the image sensor  38 . The offset processing removes a dark current component from the image signal that has been subjected to the defect correction processing and sets an accurate zero level. The DSP gain correction processing multiplies the image signal that has been subjected to the offset processing by a specific DSP gain, thereby adjusting the signal level. 
     The linear matrix processing increases the color reproducibility of the image signal that has been subjected to the DSP gain correction processing. The gamma conversion processing adjusts the brightness and chroma of the image signal that has been subjected to the linear matrix processing. The image signal that has been subjected to the gamma conversion processing is subjected to demosaicing processing (also referred to as isotropic processing or synchronization processing), thereby generating, through interpolation, a signal of color insufficient in each pixel. The demosaicing processing enables all pixels to have signals of individual colors RGB. The noise reduction unit  54  performs noise reduction processing using, for example, a moving-average method, a median filter method, or the like, on the image signal that has been subjected to the demosaicing processing and so forth in the DSP  52 , thereby reducing noise. The image signal that has been subjected to the noise reduction is input to the image processing unit  56 . 
     The image processing unit  56  includes a normal-mode processing unit  60 , a special-mode processing unit  62 , and a detection-target-oversight-prevention-mode processing unit  64 . The normal-mode processing unit  60  operates when the normal mode is set, and performs color conversion processing, color enhancement processing, and structure enhancement processing on the Bc image signal, Gc image signal, and Rc image signal that have been received. In the color conversion processing, color conversion processing is performed on the RGB image signals by using 3×3 matrix processing, gradation transformation processing, three-dimensional look up table (LUT) processing, and the like. 
     The color enhancement processing is performed on the RGB image signals that have been subjected to color conversion processing. The structure enhancement processing is processing of enhancing the structure of an observation target and is performed on the RGB image signals that have been subjected to the color enhancement processing. The above-described various image processing operations enable a normal image to be acquired. The normal image is an image acquired on the basis of normal light including the violet light V, the blue light Bx, the green light G, and the red light R with a well-balanced ratio, and is thus an image with natural colors. The normal image is input to the display control unit  58 . 
     The special-mode processing unit  62  operates when the special mode is set. The special-mode processing unit  62  performs color conversion processing, color enhancement processing, and structure enhancement processing on the Bs image signal, Gs image signal, and Rs image signal that have been received. The processing performed in the color conversion processing, the color enhancement processing, and the structure enhancement processing is similar to that in the normal-mode processing unit  60 . The above-described various image processing operations enable a special image to be acquired. The special image is an image acquired on the basis of special light in which the amount of the violet light V having a high hemoglobin absorption coefficient of blood vessels is larger than the amount of the blue light Bx, the green light G, and the red light R, and thus the resolution of a blood vessel structure and a gland duct structure is higher than that of other structures. The special image is input to the display control unit  58 . 
     The detection-target-oversight-prevention-mode processing unit  64  operates when the detection-target-oversight-prevention mode is set. The detection-target-oversight-prevention-mode processing unit  64  automatically performs processing of detecting an image feature value from an image based on the Bs image signal, Gs image signal, and Rs image signal that have been received, and also performs processing of acquiring position information in a lumen. The image feature value and the position information correspond to identification information that is used to detect a detection target. The identification information includes first-diagnosis identification information acquired at a first diagnosis and second-diagnosis identification information acquired at a second diagnosis that is different from the first diagnosis. A switching operation for determining which of the first-diagnosis identification information and the second-diagnosis identification information is to be acquired is performed by the identification information switching unit  13   e.    
     The detection-target-oversight-prevention-mode processing unit  64  performs comparison processing of comparing the first-diagnosis identification information with the second-diagnosis identification information, and makes, if a determination is made that there is an oversight of a detection target at the first diagnosis and the second diagnosis as a result of the comparison processing, a notification about the fact. In addition, the detection-target-oversight-prevention-mode processing unit  64  generates a main display image from a Bc image signal, a Gc image signal, and an Rc image signal, and generates a sub display image from a Bs image signal, a Gs image signal, and an Rs image signal. The details of the detection-target-oversight-prevention-mode processing unit  64  will be described below. 
     The display control unit  58  performs display control for displaying an image and data received from the image processing unit  56  on the monitor  18 . When the normal mode is set, the display control unit  58  performs control to display a normal image on the monitor  18 . When the special mode is set, the display control unit  58  performs control to display a special image on the monitor  18 . When the detection-target-oversight-prevention mode is set, the display control unit  58  performs control to display a main display image or a sub display image on the monitor  18 , and also performs control to display a main display image or sub display image including guidance about an oversight of a detection target on the monitor  18  or to output a sound from the monitor  18 . 
     As illustrated in  FIG. 7 , the detection-target-oversight-prevention-mode processing unit  64  includes an identification information acquisition unit  70 , an identification information storage unit  72 , a comparison processing unit  74 , and a notification control unit  76 . The identification information acquisition unit  70  includes the position information calculation unit  70   a  that calculates position information on the basis of a detection result obtained by the graduation detection sensor  48 , an image feature value detection unit  70   b  that automatically detects an image feature value from at least one of the Bs image signal, the Gs image signal, or the Rs image signal, and a lesion determination unit  70   c  that determines whether or not the detected image feature value corresponds to an image feature value specific to a detection target. The lesion determination unit  70   c  stores in advance a plurality of template image feature values of detection targets as image information specific to the detection targets, for example, and determines, using artificial intelligence (AI) or the like, whether or not an extracted image feature value matches a template image feature value. Here, “match” includes a case where the image feature values compared with each other match and a case where the difference between the image feature values compared with each other is within a certain range. 
     When “acquisition of first-diagnosis identification information” is set by the identification information switching unit  13   e  and the lesion determination unit  70   c  determines that the detected image feature value corresponds to the image feature value specific to a detection target, the identification information acquisition unit  70  stores the image feature value and the position information at the point of time as first-diagnosis identification information in the identification information storage unit  72 . 
     On the other hand, when “acquisition of second-diagnosis identification information” is set by the identification information switching unit  13   e  and the lesion determination unit  70   c  determines that the detected image feature value does not correspond to the image feature value specific to a detection target, every time the position information calculation unit  70   a  calculates position information, the identification information acquisition unit  70  transmits the calculated position information as second-diagnosis identification information to the comparison processing unit  74 . When “acquisition of second-diagnosis identification information” is set by the identification information switching unit  13   e  and the lesion determination unit  70   c  determines that the detected image feature value corresponds to the image feature value specific to a detection target, the identification information acquisition unit  70  transmits the image feature value and the position information at the point of time as second-diagnosis identification information to the comparison processing unit  74 . 
     When “acquisition of second-diagnosis identification information” is set by the identification information switching unit  13   e , the comparison processing unit  74  performs comparison processing of comparing the first-diagnosis identification information stored in the identification information storage unit  72  with the second-diagnosis identification information. In the comparison processing, two processing operations are performed: position information comparison processing of comparing the position information at the first diagnosis included in the first-diagnosis identification information with the position information at the second diagnosis included in the second-diagnosis identification information; and image feature value comparison processing of comparing the image feature value at the first diagnosis included in the first-diagnosis identification information with the image feature value at the second diagnosis included in the second-diagnosis identification information. 
     As illustrated in  FIG. 8 , if the position information at the first diagnosis matches the position information at the second diagnosis in the position information comparison processing and if the image feature value at the first diagnosis matches the image feature value at the second diagnosis in the image feature value comparison processing, a determination “there is no oversight of a detection target” is made. On the other hand, if the position information at the first diagnosis matches the position information at the second diagnosis in the position information comparison processing and if the image feature value at the first diagnosis does not match the image feature value at the second diagnosis in the image feature value comparison processing, a determination “there is an oversight of a detection target at the second diagnosis” is made. “The image feature values match” includes a case where the image feature values compared with each other match and a case where the difference between the image feature values compared with each other is within a certain range. “The pieces of position information match” includes a case where the positions compared with each other match and a case where the difference between the positions compared with each other is within a certain range. 
     If the position information at the first diagnosis does not match the position information at the second diagnosis in the position information comparison processing and if the image feature value at the first diagnosis does not match the image feature value at the second diagnosis in the image feature value comparison processing, a determination “there is an oversight of a detection target at the first diagnosis” is made. 
     Referring to  FIG. 9 , a description will be given of a specific example of comparison processing in a case where the insertion section  12   a  of the endoscope  12  goes and returns along the same path in a lumen of the stomach, esophagus, large intestine, or the like, and where the former half corresponds to a first diagnosis and the latter half corresponds to a second diagnosis. At the first diagnosis in the former half, the identification information switching unit  13   e  sets “acquisition of first-diagnosis identification information”. At the first diagnosis in the former half, for example, the identification information acquisition unit  70  acquires pieces of identification information of four detection targets: a detection target K, a detection target L, a detection target M, and a detection target N, and stores the pieces of identification information in the identification information storage unit  72 . Here, the detection target K has position information XK and an image feature value YK as first-diagnosis identification information. The detection target L has position information XL and an image feature value YL as first-diagnosis identification information. The detection target M has position information XM and an image feature value YM as first-diagnosis identification information. The detection target N has position information XN and an image feature value YN as first-diagnosis identification information. 
     When the distal end portion  12   d  of the insertion section  12   a  reaches the terminal of the former half, the identification information switching unit  13   e  is operated, and the identification information switching unit  13   e  performs switching to “acquisition of second-diagnosis identification information”. Subsequently, at the second diagnosis in the latter half, the distal end portion  12   d  of the insertion section  12   a  is caused to return along the same path as the former half. At the second diagnosis, the comparison processing unit  74  performs comparison processing of comparing the first-diagnosis identification information with the second-diagnosis identification information not only when the identification information acquisition unit  70  detects an image feature value of a detection target but also every time the position information calculation unit  70   a  calculates position information. 
     For example, when the position information XM is acquired at the second diagnosis in the latter half and the image feature value YM of the detection target M is detected at the position, both the pieces of position information XM and the image feature values YM acquired at the first diagnosis and the second diagnosis match in the comparison processing. In this case, a determination “there is no oversight of a detection target” is made. Similarly, when the position information XK is acquired at the second diagnosis in the latter half and the image feature value YK of the detection target K is detected at the position, a determination “there is no oversight of a detection target” is made. 
     On the other hand, when the position information XN is acquired at the second diagnosis in the latter half and the image feature value of the detection target N is not detected at the position, the pieces of position information match but the image feature values do not match in the comparison processing. In this case, the detection target N detected at the first diagnosis is overlooked at the second diagnosis, and thus a determination “there is an oversight of a detection target at the second diagnosis” is made. When the position information XL is acquired at the second diagnosis in the latter half and the image feature value of the detection target L is not detected at the position, the detection target L detected at the first diagnosis is overlooked at the second diagnosis, and thus a determination “there is an oversight of a detection target at the second diagnosis” is made. 
     When position information Xp is acquired at the second diagnosis in the latter half and an image feature value Yp of a detection target P is detected at the position, none of the position information and the image feature value do not match in the comparison processing. In this case, the detection target P is overlooked at the first diagnosis, and thus a determination “there is an oversight of a detection target at the first diagnosis” is made. When position information Xq is acquired at the second diagnosis in the latter half and an image feature value Yq of a detection target Q is detected at the position, the detection target Q is overlooked at the first diagnosis, and thus a determination “there is an oversight of a detection target at the first diagnosis” is made. 
     The notification control unit  76  performs, via the display control unit  58 , control to make a notification about information related to a detection target, such as detection of the detection target or an oversight of the detection target. The notification control unit  76  generates a main display image from a Bc image signal, a Gc image signal, and an Rc image signal, and also generates a sub display image from a Bs image signal, a Gs image signal, and an Rs image signal. The generated main display image and sub display image are displayed on the monitor  18  via the display control unit  58  as illustrated in  FIG. 10 . In  FIG. 10 , the main display image is displayed in a larger size than the sub display images, but the sub display images may be displayed in a larger size than the main display image. Alternatively, either the main display image or the sub display images may be displayed. Although two sub display images are displayed side by side here, one sub display image or three or more sub display images may be displayed. 
     When the lesion determination unit  70   c  detects an image feature value of a detection target, the notification control unit  76  performs control to display an indicator, such as a square, circle, or arrow (in  FIG. 10 , squares are displayed as indicators) at the position of the detection target in the sub display image in a superimposed manner. This enables a user to recognize the position of the detection target. The indicator may be displayed on the main display image in a superimposed manner instead of or in addition to the sub display image. Alternatively, a warning sound for giving a notification indicating that the detection target has been detected may be output from a speaker of the monitor  18  instead of or in addition to the indicator. 
     The indicator may be displayed not only on a still image but also on a moving image. In that case, the indicator may be displayed to automatically follow a moving lesion on the basis of an image feature of the lesion that has once been detected. Furthermore, when a coloring agent of various types, such as indigo carmine or methylene blue, is applied to facilitate diagnosis, it is preferable to perform automatic detection on the basis of the surface structure or blood vessel structure of a predetermined detection target and to perform display using an indicator, although color information of an original lesion is lost. When a detection target such as a lesion is detected, a still image of the detection target may be automatically captured at each time and may be automatically stored as log information. 
     When the comparison processing unit  74  makes a determination “there is an oversight of a detection target at the first diagnosis” or “there is an oversight of a detection target at the second diagnosis”, the notification control unit  76  performs control to display the determination result as a warning message on the monitor  18 . In  FIG. 10 , “there is an oversight of a detection target at the first diagnosis” is displayed as a warning message. When a determination “there is no oversight of a detection target” is made, the notification control unit  76  may display guidance indicating the determination on the monitor  18 . The “notification unit” of the present invention corresponds to a configuration including at least the notification control unit  76  and the monitor  18  (display unit). 
     When the lesion determination unit  70   c  detects an image feature value of a detection target, the notification control unit  76  also displays information related to the detection target. The information related to the detection target is displayed in association with an indicator attached to the detection target by using a line or the like. Preferably, when a detection target is detected, a region including the detection target and a surrounding region is automatically magnified by electronic zoom, and the magnified image of the region is displayed as the information related to the detection target. It is also preferable to display a differentiation result of the detection target, such as a total score evaluating a probability of being a lesion region or a parameter score used to calculate the total score. As the total score, for example, a malignancy grade or stage of cancer may be displayed. As the parameter score, for example, the regularity of the pattern of blood vessels in the surface of cancer, or the regularity of the uneven pattern on the surface may be displayed. 
     Preferably, the differentiation result of the detection target is acquired by using artificial intelligence (AI) on the basis of the image feature value of the detection target. When a medical practitioner observes the detection target, such as a lesion, from a certain distance or at a certain magnification ratio using electronic zoom to perform differentiation, the light source mode may be automatically switched to the mode enabling easy differentiation, for example, to the special mode when the medical practitioner reaches a position at the certain distance or when the certain magnification ratio is reached in zooming. 
     Preferably, the range of the detection target is detected and the range of the detected lesion may be displayed as information related to the detection target by using a contour line LA such as a curve. Preferably, when it is determined that the detection target needs to be observed with special care because the total score or parameter score thereof acquired as a result of differentiation is greater than or equal to a certain value, the detection target is displayed with a predetermined region including the detection target being surrounded by a yellow or red indicator or the like so that particular attention is paid to the lesion. When a single detection target includes regions having different properties, the scores of the individual regions may be displayed by assigning total scores or parameter scores to a color map, or total scores or parameter scores may be displayed for the individual regions. Preferably, the depth of the detection target is detected and the detected depth of the detection target is displayed as information related to the detection target. Also, a subjective report related to the detection target may be displayed as information related to the detection target (for example, “this region corresponds to cancer for predetermined reasons”). 
     The above-described differentiation using AI or the like does not necessarily guarantee 100% accurate differentiation of a lesion. Thus, the confidence of the differentiation result may be displayed, or an “agreement button” for asking a medical practitioner whether or not he/she agrees to the differentiation result may be displayed on the monitor  18 , so that the medical practitioner is allowed to give final approval for the differentiation result. In this case, the “agreement button” is preferably operated by using the console  19 . 
     With use of the position information calculated by the position information calculation unit  70   a , a detection position of a detection target in a lumen or a current position of the distal end portion  12   d  of the endoscope  12  may be displayed as information related to the detection target. In  FIG. 10 , a detection position of a detection target in the entire large intestine and a current position of the distal end portion  12   d  are displayed. In addition, a previous image of the detection target detected in the former half or at a previous diagnosis, or one or a plurality of images of cases similar to the detected detection target may be displayed as information related to the detection target. 
     Preferably, various pieces of information illustrated in  FIG. 10 , such as an image, total score, parameter score, report, and detection position of a detection target such as a lesion, are automatically stored as a log. Furthermore, when retrieving and using various pieces of information previously recorded as a log, such as an image of a similar case or a previous image, during an examination, recognition means such as machine learning or AI may be used as means for identifying the image of a similar case or the previous image. 
     Next, a series of steps in the detection-target-oversight-prevention mode will be described with reference to the flowchart in  FIG. 11 . First, the detection-target-oversight-prevention mode is set, and the identification information switching unit  13   e  performs switching to “acquisition of first-diagnosis identification information”. Subsequently, insertion of the distal end portion  12   d  of the endoscope  12  into a lumen is started, and the distal end portion  12   d  is gradually moved in a pressing direction in the lumen. To detect a detection target by moving the distal end portion  12   d  from the entrance of the lumen to the terminal of an observable range in the lumen is referred to as detection of a detection target at the first diagnosis. 
     In the detection-target-oversight-prevention mode, an observation target is alternately illuminated with normal light and special light. A main display image is generated from a Bc image signal, a Gc image signal, and an Rc image signal that are acquired during illumination with the normal light, and a sub display image is generated from a Bs image signal, a Gs image signal, and an Rs image signal that are acquired during illumination with the special light. The main display image and the sub display image are displayed on the monitor  18 . 
     At the first diagnosis, an image feature value is detected from the image signals acquired during illumination with the special light, and whether or not the detected image feature value corresponds to the image feature value of a detection target is determined. If it is determined that the detected image feature value is the image feature value of a detection target (a detection target is detected), the detected image feature value and the position information at the point of time are stored as the first-diagnosis identification information in the identification information storage unit  72 . Subsequently, when the distal end portion  12   d  of the endoscope  12  reaches the terminal position of the observable range of the lumen, the identification information switching unit  13   e  is operated to perform switching to “acquisition of second-diagnosis identification information”. Subsequently, the distal end portion  12   d  is gradually moved in a pulling direction from the lumen to return along the same path as that of the first diagnosis. In this way, to detect a detection target by moving the distal end portion  12   d  from the terminal of the observable range of the lumen to the entrance of the lumen is referred to as detection of a detection target at the second diagnosis. 
     At the second diagnosis, an image feature value at the second diagnosis is detected from the image signals acquired during illumination with the special light, and the position information at the point of time is acquired as the second-diagnosis identification information. Every time the second-diagnosis identification information is acquired, the comparison processing unit  74  performs comparison processing of comparing the first-diagnosis identification information with the second-diagnosis identification information. In accordance with the result of the comparison processing, a determination “there is no oversight of a detection target”, “there is an oversight of a detection target at the first diagnosis”, or “there is an oversight of a detection target at the second diagnosis” is made. Subsequently, the notification control unit  76  makes a notification in accordance with the determination result. When a determination “there is an oversight of a detection target at the first diagnosis” or “there is an oversight of a detection target at the second diagnosis” is made, the notification control unit  76  makes a notification to make a user realize “an oversight of a detection target” by using a warning message or a warning sound. 
     In the above-described embodiment, when the comparison processing unit  74  performs image feature value comparison processing of comparing the image feature value at the first diagnosis with the image feature value at the second diagnosis, it is preferable to use, as an image feature value, a Gernika moment or Hu moment that is invariable with respect to a parallel shift, rotation, or scaling of an image, or SHIFT or SURF that is invariable with respect to a parallel shift, rotation, scaling, or change in illumination of an image. 
     As an example, the comparison processing unit  74  determines the degree of similarity on the basis of a matching method using Hu moments, and determines “an oversight of a detection target” on the basis of the determined degree of similarity. Here, as illustrated in  FIG. 12 , determination of the degree of similarity between the detection target A detected at the first diagnosis and the detection target B detected at the second diagnosis, and determination of the degree of similarity between the detection target A detected at the first diagnosis and the detection target C detected at the second diagnosis are illustrated. Preferably, the detection target includes not only the detection target itself but also a region including a normal portion around the detection target. 
     To determine the degree of similarity between the detection target A and the detection target B, a center moment in an image is calculated for each of the detection targets A and B by using the following equation (1). 
                     μ   pq     =       ∑   x     ⁢       ∑   y     ⁢         (     x   -     x   _       )     p     ⁢       (     y   -     y   _       )     q     ⁢     f   ⁡     (     x   ,   y     )                     (   1   )               
Here, x and y represent the coordinates of a pixel, and p, q=0, 1, 2, 3, and
 
               x   _     =       m   10       m   00                     y   _     =       m   01       m   00             
holds. However,
 
               m   pq     =       ∑   x     ⁢       ∑   y     ⁢       x   p     ⁢     y   q     ⁢     f   ⁡     (     x   ,   y     )                   
holds.
 
     Subsequently, a normalized center moment is calculated by using equation (2). 
                     η   pq     =       μ   pq       u   00   λ               (   2   )               
Here,
 
             λ   =         p   +   q     2     +   1           
holds. Finally, Hu moments, which are seven invariables, are calculated by using equation (3).
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The Hu moments calculated for the detection target A and the detection target B are represented by h i   A  and h j   B , respectively. Finally, the degree of similarity between the detection target A and the detection target B is calculated by using equation (4). 
                     I   ⁡     (     A   ,   B     )       =       ∑     i   =     1   ⁢   …7         ⁢           ⁢            m   i   A     -     m   i   B                      (   4   )               
Here,
 
 m   i   A =sign( h   i   A )·log  h   i   A  
 
 m   i   B =sign( h   i   B )·log  h   i   B  
 
holds. The following is a result of calculating the degree of similarity I(A, B) between the detection target A and the detection target B on the basis of the above.
 
 I ( A,B )=0.00001
 
     Here, the degree of similarity increases as the value of the degree of similarity I decreases. Thus, if
 
 I (α,β)≤0.000
 
is satisfied regarding the degree of similarity I(α,β) between the detection target α and the detection target β, the comparison processing unit  74  determines that the detection target α and the detection target β are the same lesion (match). Thus, the degree of similarity I(A, B) between the detection target A and the detection target B is expressed by
 
 I ( A,B )=0.0001≤0.0001
 
and thus the comparison processing unit  74  determines that the detection target A and the detection target B are the same lesion and that there is no oversight of a detection target at the first diagnosis. In this case, a determination “there is no oversight of a detection target” is made, and the fact is displayed on the monitor  18 .
 
     Subsequently, to determine the degree of similarity between the detection target A and the detection target C, the degree of similarity I(A, C) is calculated in accordance with a procedure similar to that for determining the degree of similarity between the detection targets A and B. As a result of calculating the degree of similarity I(A, C),
 
 I ( A,C )=0.04677&gt;0.0001
 
is obtained, and thus it is determined that the detection target A and the detection target C are lesions different from each other and that the detection target C is overlooked at the first diagnosis. In this case, a determination “there is an oversight of a detection target at the first diagnosis” is displayed on the monitor  18 .
 
     In the above-described embodiment, a main display image and sub display images are displayed side by side on the single monitor  18 . Alternatively, the main display image and the sub display images may be displayed by using a plurality of monitors  18 . Specifically, it is preferable to display the main display image on one of the plurality of monitors  18  and to display the sub display images on the other monitors. For example, in the case of using two monitors: a monitor  18   a  and a monitor  18   b , as illustrated in  FIG. 13 , only the main display image is displayed on the entire screen of the monitor  18   a , whereas only the sub display images and information related to a detection target are displayed on the monitor  18   b . The monitor  18   a  and the monitor  18   b  may be coupled to each other by a coupling arm or the like. 
     In the above-described embodiment, a notification is made by using an indicator or the like when a detection target is detected. In addition to this, the entire image including a detection target (the entire main display image or the entire sub display image) may be evaluated in terms of a probability of being a lesion region by using a lesion score, and the lesion score may be assigned to a color map and displayed. For example, in the case of  FIG. 14 , a color C 1  (for example, red) of a detection target and the surroundings thereof and a color C 2  (for example, blue) of a region away from the detection target are quite different from each other. 
     In the above-described embodiment, when the lesion determination unit  70   c  determines that the detected image feature value is the image feature value of a detection target (hereinafter “when a detection target is detected”), the detection target may be automatically magnified by using electronic zoom or optical zoom by the magnifying optical system  36 . When a detection target is detected, display for prompting a user to magnify the detection target by using optical zoom may be performed. Preferably, the magnified detection target is displayed as a main display image or a sub display image. 
     In the above-described embodiment, when a detection target is detected, a still image may be automatically stored. In the case of automatically storing a still image, it is preferable to acquire images of a plurality of frames as candidate still images to be stored, and to store, as a still image, the most tightly focused image among the candidate still images of the plurality of frames. A method for selecting a tightly focused image is, for example, to perform frequency analysis on the individual candidate still images to be stored and to select an image including a largest amount of high-frequency components as a still image to be stored. 
     In the above-described embodiment, a case is assumed where the distal end portion  12   d  of the endoscope  12  is caused to go and return in a lumen of the stomach, large intestine, esophagus, or the like in one diagnosis of a lesion, in which the former half corresponds to the first diagnosis and the latter half for returning the same path as the former half corresponds to the second diagnosis. The first diagnosis and the second diagnosis are not limited to the former half and the latter half of one diagnosis of a lesion, and another case may also be applied as long as the second diagnosis is performed after the first diagnosis in terms of time, and as long as the first diagnosis and the second diagnosis are performed along the same path to detect a lesion. For example, the first diagnosis and the second diagnosis may be performed on different dates to diagnose a lesion. In this case, the date of a first diagnosis, such as an initial diagnosis, may correspond to the time of the first diagnosis, and the date of a second diagnosis, such as follow-up, after the date of the first diagnosis may correspond to the time of the second diagnosis. In this case, it is preferable to automatically switch from “acquisition of first-diagnosis identification information” to “acquisition of second-diagnosis identification information” without operating the identification information switching unit  13   e.    
     In the above-described embodiment, whether or not there is an oversight of a detection target at the first diagnosis and the second diagnosis is determined by using both position information and an image feature value. Alternatively, whether or not there is an oversight of a detection target may be determined by using only an image feature value. In this case, comparison processing of comparing an image feature value of a detection target at the first diagnosis with an image feature value of a detection target at the second diagnosis is performed, and whether or not there is an oversight of a detection target is determined in accordance with the result of the comparison processing. 
     Second Embodiment 
     In a second embodiment, an observation target is illuminated by using a laser light source and a fluorescent body instead of the LEDs  20   a  to  20   d  of four colors according to the above-described first embodiment. Hereinafter, a description will be given of only a part different from that of the first embodiment, and a description will not be given of a part substantially the same as that of the first embodiment. 
     As illustrated in  FIG. 15 , in an endoscope system  100  according to the second embodiment, the light source unit  20  of the light source device  14  is provided with, instead of the LEDs  20   a  to  20   d  of four colors, a blue laser light source (referred to as “445LD”, LD stands for “laser diode”)  104  that emits blue laser light having a center wavelength of 445±10 nm and a blue-violet laser light source (referred to as “405LD”)  106  that emits blue-violet laser light having a center wavelength of 405±10 nm. The light emission from semiconductor light emitting elements of the light sources  104  and  106  is individually controlled by a light source control unit  108 , and the light amount ratio between the light emitted by the blue laser light source  104  and the light emitted by the blue-violet laser light source  106  is freely changed. 
     In the normal mode, the light source control unit  108  turns on the blue laser light source  104 . On the other hand, in the special mode, the light source control unit  108  turns on both the blue laser light source  104  and the blue-violet laser light source  106  and performs control so that the light emission rate of blue laser light is higher than the light emission rate of blue-violet laser light. In the detection-target-oversight-prevention mode, the light source control unit  108  alternately performs control to turn on only the blue laser light source  104  and control to turn on both the blue laser light source  104  and the blue-violet laser light source  106 . 
     Preferably, the half-width of the blue laser light or the blue-violet laser light is about ±10 nm. As the blue laser light source  104  and the blue-violet laser light source  106 , an InGaN-based laser diode of a broad area type can be used, and also an InGaNAs-based laser diode or a GaNAs-based laser diode can be used. Alternatively, a configuration using a light emitting body, such as a light emitting diode, may be used as the above-described light sources. 
     The illumination optical system  30   a  is provided with a fluorescent body  110  that the blue laser light or the blue-violet laser light from the light guide  24  enters, in addition to the illumination lens  32 . The fluorescent body  110  is excited by the blue laser light and emits fluorescence. Part of the blue laser light passes through the fluorescent body  110  without exciting the fluorescent body  110 . The blue-violet laser light passes through the fluorescent body  110  without exciting the fluorescent body  110 . The light from the fluorescent body  110  illuminates the inside of the body of an observation target through the illumination lens  32 . 
     Here, in the normal mode, the blue laser light mainly enters the fluorescent body  110 . Thus, wide-range light for the normal mode, generated by combining the blue laser light and fluorescence emitted by the fluorescent body  110  as a result of excitation caused by the blue laser light, as illustrated in  FIG. 16 , is applied as normal light to an observation target. The image sensor  38  performs imaging of the observation target illuminated with the normal light, and accordingly a normal image composed of a Bc image signal, a Gc image signal, and an Rc image signal is acquired. 
     On the other hand, in the special mode, both the blue-violet laser light and the blue laser light enter the fluorescent body  110 . Thus, wide-range light for the special mode, generated by combining the blue-violet laser light, the blue laser light, and fluorescence emitted by the fluorescent body  110  as a result of excitation caused by the blue laser light, as illustrated in  FIG. 17 , is applied as special light to an observation target. The image sensor  38  performs imaging of the observation target illuminated with the special light, and accordingly a special image composed of a Bs image signal, a Gs image signal, and an Rs image signal is acquired. 
     In the detection-target-oversight-prevention mode, the normal light illustrated in  FIG. 16  and the special light illustrated in  FIG. 17  are alternately applied to an observation target. A main display image is generated from the Bc image signal, Gc image signal, and Rc image signal that are acquired during illumination with the normal light, and a sub display image is generated from the Bs image signal, Gs image signal, and Rs image signal that are acquired during illumination with the special light. In addition, identification information of a detection target is detected from the Bs image signal, Gs image signal, and Rs image signal that are acquired during illumination with the special light. 
     Preferably, the fluorescent body  110  is made of a plurality of types of fluorescent materials that absorb part of the blue laser light and emit green to yellow light as a result of excitation (for example, a YAG-based fluorescent body, a BaMgAl 10 O 17  (BAM)-based fluorescent body, or the like). As in this configuration example, when a semiconductor light emitting element is used as an excitation light source of the fluorescent body  110 , high-intensity white light can be acquired at high emission efficiency, the intensity of the white light can be easily adjusted, and the change in color temperature and chromaticity of the white light can be reduced. 
     Third Embodiment 
     In a third embodiment, an observation target is illuminated by using a white light source, such as a xenon lamp, and a rotational filter, instead of the LEDs  20   a  to  20   d  of four colors. Imaging of the observation target may be performed by using a monochrome image sensor instead of the color image sensor  38 . Hereinafter, a description will be given of only a part different from that of the first embodiment, and a description will not be given of a part substantially the same as that of the first embodiment. 
     In an endoscope system  200  illustrated in  FIG. 18 , the light source device  14  is provided with a white light source unit  202 , a rotational filter  204 , and a filter switching unit  206 , instead of the LEDs  20   a  to  20   d  in the endoscope system  10 . The imaging optical system  30   b  is provided with a monochrome image sensor  208  that is not provided with color filters, instead of the color image sensor  38 . A diaphragm  203  is provided between the white light source unit  202  and the rotational filter  204 . The area of the opening portion of the diaphragm  203  is adjusted by a diaphragm control unit  205 . 
     The white light source unit  202  is a xenon lamp, a white LED, or the like, and emits white light having a wavelength range from blue to red. The rotational filter  204  includes a normal-mode filter  210  provided on the inner side closest to a rotational axis, and a special-mode filter  212  and a detection-target-oversight-prevention-mode filter  214  that are provided on the outer side of the normal-mode filter  210  (see  FIG. 19 ). 
     The filter switching unit  206  moves the rotational filter  204  in a diameter direction. Specifically, when the normal mode is set by the mode switching unit  13   c , the filter switching unit  206  inserts the normal-mode filter  210  into the light path of white light. When the special mode is set, the filter switching unit  206  inserts the special-mode filter  212  into the light path of white light. When the detection-target-oversight-prevention mode is set, the filter switching unit  206  inserts the detection-target-oversight-prevention-mode filter  214  into the light path of white light. 
     As illustrated in  FIG. 19 , the normal-mode filter  210  is provided with a Bb filter  210   a , a G filter  210   b , and an R filter  210   c  in a circumferential direction. The Bb filter  210   a  passes wide-range blue light Bb having a wavelength range of 400 to 500 nm of white light. The G filter  210   b  passes green light G of white light. The R filter  210   c  passes red light R of white light. Thus, in the normal mode, rotation of the rotational filter  204  causes the wide-range blue light Bb, the green light G, and the red light R to be sequentially emitted as normal light toward an observation target. 
     The special-mode filter  212  is provided with a Bn filter  212   a  and a Gn filter  212   b  in the circumferential direction. The Bn filter  212   a  passes blue narrow-range light Bn in 400 to 450 nm of white light. The Gn filter  212   b  passes green narrow-range light Gn in 530 to 570 nm of white light. Thus, in the special mode, rotation of the rotational filter  204  causes the blue narrow-range light and the green narrow-range light to be sequentially emitted as special light toward an observation target. 
     The detection-target-oversight-prevention-mode filter  214  is provided with a Bb filter  214   a , a G filter  214   b , an R filter  214   c , a Bn filter  214   d , and a Gn filter  214   e  in the circumferential direction. The Bb filter  214   a  passes the wide-range blue light Bb of white light. The G filter  214   b  passes the green light G of white light. The R filter  214   c  passes the red light R of white light. The Bn filter  214   d  passes the blue narrow-range light Bn of white light. The Gn filter  214   e  passes the green narrow-range light Gn of white light. Thus, in the detection-target-oversight-prevention mode, rotation of the rotational filter  204  causes the wide-range blue light Bb, the green light G, and the red light R to be sequentially emitted as normal light toward an observation target, and the blue narrow-range light and the green narrow-range light to be sequentially emitted as special light toward the observation target. 
     In the endoscope system  200 , in the normal mode, imaging of an observation target is performed by the monochrome image sensor  208  every time the observation target is illuminated with the wide-range blue light Bb, the green light G, and the red light R. Accordingly, a Bc image signal is acquired during illumination with the wide-range blue light Bb, a Gc image signal is acquired during illumination with the green light G, and an Rc image signal is acquired during illumination with the red light R. The Bc image signal, the Gc image signal, and the Rc image signal constitute a normal image. 
     In the special mode, imaging of an observation target is performed by the monochrome image sensor  208  every time the observation target is illuminated with the blue narrow-range light Bn and the green narrow-range light Gn. Accordingly, a Bn image signal is acquired during illumination with the blue narrow-range light Bn, and a Gn image signal is acquired during illumination with the green narrow-range light Gn. The Bn image signal and the Gn image signal constitute a special image. 
     In the detection-target-oversight-prevention mode, a main display image is generated on the basis of the Bc image signal acquired during illumination with the wide-range blue light Bb, the Gc image signal acquired during illumination with the green light G, and the Rc image signal acquired during illumination with the red light R. In addition, a sub display image is generated on the basis of the Bn image signal acquired during illumination with the blue narrow-range light Bn and the Gn image signal acquired during illumination with the green narrow-range light Gn, and a detection target is detected from the Bn image signal and the Gn image signal. 
     In the above-described embodiments, the hardware structure of a processing unit that executes various processing operations, such as the image processing unit  56 , may be various processors described below. The various processors include a central processing unit (CPU), which is a general-purpose processor executing software (program) and functioning as various processing units; a programmable logic device (PLD), which is a processor whose circuit configuration is changeable after manufacturing, such as a field programmable gate array (FPGA); a dedicated electric circuit, which is a processor having a circuit configuration designed exclusively for executing various processing operations, and the like. 
     A single processing unit may be constituted by one of these various processors or may be constituted by a combination of two or more processors of the same type or different types (for example, a combination of a plurality of FPGAs or a combination of a CPU and an FPGA). A plurality of processing units may be constituted by a single processor. Examples of constituting a plurality of processing units by a single processor are as follows. First, as represented by a computer of a client or server, a single processor is constituted by a combination of one or more CPUs and software, and the processor functions as a plurality of processing units. Secondly, as represented by a system on chip (SoC), a processor in which a single integrated circuit (IC) chip implements the function of an entire system including a plurality of processing units is used. In this way, various processing units are constituted by using one or more of the above-described various processors as a hardware structure. 
     Furthermore, the hardware structure of these various processors is, more specifically, electric circuitry including a combination of circuit elements, such as semiconductor elements. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  endoscope system 
               12  endoscope 
               12   a  insertion section 
               12   b  operation section 
               12   c  bending portion 
               12   d  distal end portion 
               13   a  angle knob 
               13   b  still image acquisition unit 
               13   c  mode switching unit 
               13   d  zoom operation unit 
               13   e  identification information switching unit 
               14  light source device 
               16  processor device 
               18 ,  18   a ,  18   b  monitor 
               19  console 
               20  light source unit 
               20   a  V-LED (light source) 
               20   b  B-LED (light source) 
               20   c  G-LED (light source) 
               20   d  R-LED (light source) 
               22  light source control unit 
               23  wavelength cut filter 
               24  light guide 
               30   a  illumination optical system 
               30   b  imaging optical system 
               32  illumination lens 
               34  objective lens 
               36  magnifying optical system 
               36   a  zoom lens 
               36   b  lens driving unit 
               38  image sensor 
               40  CDS circuit 
               42  AGC circuit 
               44  A/D conversion circuit 
               48  graduation detection sensor 
               50  image signal acquisition unit 
               52  DSP 
               54  noise reduction unit 
               56  image processing unit 
               58  display control unit 
               60  normal-mode processing unit 
               62  special-mode processing unit 
               64  detection-target-oversight-prevention-mode processing unit 
               70  identification information acquisition unit 
               70   a  position information calculation unit 
               70   b  image feature value detection unit 
               70   c  lesion determination unit 
               72  identification information storage unit 
               74  comparison processing unit 
               76  notification control unit 
               100  endoscope system 
               104  blue laser light source 
               106  blue-violet laser light source 
               108  light source control unit 
               110  fluorescent body 
               200  endoscopy system 
               202  white light source 
               204  rotational filter 
               205  diaphragm control unit 
               206  filter switching unit 
               208  image sensor 
               210  normal-mode filter 
               210   a  Bb filter 
               210   b  G filter 
               210   c  R filter 
               212  special-mode filter 
               212   a  Bn filter 
               212   b  Gn filter 
               214  detection-target-oversight-prevention-mode filter 
               214   a  Bb filter 
               214   b  G filter 
               214   c  R filter 
               214   d  Bn filter 
               214   e  Gn filter