Patent Publication Number: US-2022218190-A1

Title: Endoscope system and method of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2020/036987 filed on 29 Sep. 2020, which claims priority under 35 U.S.C § 119(a) to Japanese Patent Application No. 2019-183849 filed on 4 Oct. 2019. The above application 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 that obtains the oxygen saturation of an object to be observed and a method of operating the endoscope system. 
     2. Description of the Related Art 
     An endoscope system, which picks up the image of an object to be observed in a living body, is widely used in a medical field. Since a correlation between oxygen saturation and a lesion area has been known, a diagnosis using oxygen saturation has also been made in recent years. In the diagnosis using oxygen saturation, the oxygen saturation of the object to be observed is calculated on the basis of a plurality of spectral images, which are obtained in a case where an affected area is irradiated with illumination light for oxygen saturation observation of which the light absorption coefficient is changed depending on oxygen saturation, and an oxygen saturation image in which the oxygen saturation is shown as an image is displayed on a monitor. 
     In a case where oxygen saturation is calculated, signal ratios between different spectral images are used as calculation values obtained from calculation processing based on the plurality of spectral images. Since the slight variation of the signal ratio to be input becomes the noise of oxygen saturation to be output, not only the amount of light emitted from a light source unit, which emits illumination light for oxygen saturation observation, needs to be strictly controlled in terms of hardware but also a technique for correcting the individual difference of an endoscope or the light source unit is also important. For example, in JP2014-76375A, pre-image pickup is performed before main image pickup for calculating oxygen saturation and oxygen saturation is accurately calculated using a correction amount obtained in the pre-image pickup. 
     SUMMARY OF THE INVENTION 
     With regard to the signal ratio used for the calculation of oxygen saturation, a difference in signal ratio may occur even though the image of the same object to be observed is picked up since the spectral characteristics of light source units, which emit illumination light for oxygen saturation observation, individually vary. Further, a difference may occur in the signal ratio obtained from the same object to be observed due to a combination of a light source device that includes a light source unit and an endoscope that picks up the image of an object to be observed. In contrast, it is conceivable that white balance correction processing (for example, WO2015/194422A (corresponding to US2017/085851A1)) is performed to correct a difference in signal ratio. However, white balance correction processing is to correct a white portion so that an individual difference is removed, and not only a white portion but also other color portions are used in the calculation of oxygen saturation. For this reason, it is difficult to sufficiently correct a difference in signal ratio, which is caused by an individual difference, by only white balance correction processing. 
     An object of the present invention is to provide an endoscope system that can accurately calculate oxygen saturation regardless of an individual difference of a light source unit, which emits illumination light for oxygen saturation observation, or the like and a method of operating the endoscope system. 
     An endoscope system according to an aspect of the present invention comprises: a plurality of semiconductor light sources that emit illumination light having a plurality of wavelength ranges; an endoscope that obtains a plurality of spectral images from image pickup of an object to be observed illuminated on the basis of the illumination light; a first correction parameter storage memory that stores first correction parameters to be used to correct differences in spectral characteristics of the respective semiconductor light sources; a white balance correction data storage memory that stores white balance correction data; and an image control processor that calculates oxygen saturation of the object to be observed on the basis of the plurality of spectral images, the first correction parameters, and the white balance correction data. 
     It is preferable that the image control processor acquires specific spectral images subjected to white balance correction by performing white balance correction processing, which is based on white balance correction data, on specific spectral images, which are required for calculation of the oxygen saturation, among the spectral images, calculates calculation values for calculation of the oxygen saturation on the basis of the specific spectral images subjected to the white balance correction, calculates correction amounts, which are used to correct the calculation values, by calculation processing based on the calculation values and the first correction parameters, and calculates the oxygen saturation on the basis of corrected calculation values that are obtained from correction of the calculation values using the correction amounts. 
     It is preferable that the image control processor acquires second correction parameters, which are used to correct differences in the calculation values caused by combinations of the semiconductor light sources and the endoscope and calculates the correction amounts on the basis of the second correction parameters in addition to the calculation values and the first correction parameters in a case where the correction amounts are to be calculated. It is preferable that the semiconductor light source emits specific illumination light to be used to acquire the second correction parameters. It is preferable that the specific illumination light is green light. 
     It is preferable that the endoscope system comprises a light source processor that acquires third correction parameters to be used to correct differences in the specific spectral images based on changes in wavelength characteristics of the illumination light caused by changes in amounts of light of the semiconductor light sources, and the image control processor calculates the correction amounts on the basis of the third correction parameters in addition to the calculation values and the first correction parameters. It is preferable that each of the first correction parameters is obtained on the basis of a difference value between a peak wavelength of each semiconductor light source and a peak wavelength of a reference light source. 
     It is preferable that the plurality of semiconductor light sources include a BS light source that emits first blue light of which a central wavelength includes a wavelength of 450 nm, a BL light source that emits second blue light of which a central wavelength includes a wavelength of 470 nm, a green light source that emits green light, and a red light source that emits red light. It is preferable that the plurality of spectral images include a B1 image that corresponds to first blue light of which a central wavelength includes a wavelength of 450 nm, a B2 image that corresponds to second blue light of which a central wavelength includes a wavelength of 470 nm, a G1 image that corresponds to green light, and an R1 image that corresponds to red light. 
     According to another aspect of the present invention, there is provided a method of operating an endoscope system that includes a plurality of semiconductor light sources emitting illumination light having a plurality of wavelength ranges. The method comprises: a step of causing an endoscope to obtain a plurality of spectral images from image pickup of an object to be observed illuminated on the basis of the illumination light; and a step of causing a processor to calculate oxygen saturation of the object to be observed on the basis of the plurality of spectral images, first correction parameters to be used to correct differences in spectral characteristics of the respective semiconductor light sources, and white balance correction data. 
     According to the present invention, it is possible to accurately calculate oxygen saturation regardless of an individual difference of a light source unit, which emits illumination light for oxygen saturation observation, or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the appearance of an endoscope system. 
         FIG. 2  is a block diagram showing the functions of the endoscope system. 
         FIG. 3  is a graph showing the emission spectra of first blue light, second blue light, green light, and red light. 
         FIG. 4  is a block diagram showing the functions of a third correction parameter acquisition section. 
         FIG. 5  is a graph showing a relationship between first blue light and the amount of wavelength variation. 
         FIG. 6  is a diagram illustrating a light emission pattern in an oxygen saturation observation mode of a first embodiment. 
         FIG. 7  is a graph showing the spectral characteristics of color filters. 
         FIG. 8  is a block diagram showing the functions of a special processing section of the first embodiment. 
         FIG. 9  is a diagram showing a relationship between oxygen saturation and signal ratios X and Y. 
         FIG. 10  is a flowchart showing the flow of a calibration mode. 
         FIG. 11  is a diagram illustrating a calibration device. 
         FIG. 12  is a diagram illustrating a correction data acquisition region AR. 
         FIG. 13  is a flowchart showing the flow of an oxygen saturation observation mode. 
         FIG. 14  is a diagram illustrating a light emission pattern in an oxygen saturation observation mode of a second embodiment. 
         FIG. 15  is a block diagram showing the functions of a special processing section of the second embodiment. 
         FIG. 16  is a block diagram showing the functions of a second correction parameter acquisition section. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     As shown in  FIG. 1 , an endoscope system  10  according to a first embodiment includes an endoscope  12 , a light source device  14 , a processor device  16 , a monitor  18  serving as a display unit, and a user interface  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  includes an insertion part  12   a  that is to be inserted into an object to be examined, an operation part  12   b  that is provided at the proximal end portion of the insertion part  12   a , and a bendable part  12   c  and a distal end part  12   d  that are provided on the distal end side of the insertion part  12   a . The bendable part  12   c  is bent in a case where angle knobs  12   e  of the operation part  12   b  are operated. As a result of the bending of the bendable part  12   c , the distal end part  12   d  faces in a desired direction. The distal end part  12   d  is provided with a jet port (not shown) that jets air, water, and the like toward an object to be observed. 
     Further, the operation part  12   b  is provided with a mode changeover switch  13   a , a freeze switch  13   b , and a zoom operation part  13   c  in addition to the angle knobs  12   e . The mode changeover switch  13   a  is used for an operation for switching an observation mode. The endoscope system  10  has a normal mode, a special mode, and a calibration mode. The normal mode is an observation mode in which an image having a natural color tone (hereinafter, referred to as a normal image) obtained from the image pickup of the object to be observed using white light as illumination light is displayed on the monitor  18 . 
     The special mode is an oxygen saturation observation mode in which the oxygen saturation of the object to be observed is calculated and displayed. In the oxygen saturation observation mode, the oxygen saturation of the object to be observed is calculated using a plurality of spectral images obtained from the image pickup of the object to be observed and an image in which the value of the calculated oxygen saturation is shown using a pseudo color (hereinafter, referred to as an oxygen saturation image) is generated and displayed on the monitor  18 . White balance correction data to be used for white balance correction processing are acquired in the calibration mode. 
     The freeze switch  13   b  is a switch that is used to give a static image-acquisition instruction to the processor device  16 , and the processor device  16  stores static images according to the static image-acquisition instruction. 
     The processor device  16  is electrically connected to the monitor  18  and the user interface  19 . The monitor  18  outputs and displays the image of each observation mode, image information accessory to the image, and the like. The user interface  19  includes a keyboard and the like, and receives input operations, such as function settings. A mouse and the like may be provided as the user interface  19  in addition to the keyboard. An external recording unit (not shown) in which images, image information, and the like are recorded may be connected to the processor device  16 . 
     As shown in  FIG. 2 , the light source device  14  comprises a light source unit  20  that emits illumination light and a light source controller  22  that controls the drive of the light source unit  20 . 
     The light source unit  20  comprises four semiconductor light sources, that is, a BS light source  20   a , a BL light source  20   b , a G light source  20   c , and an R light source  20   d . In this embodiment, all of the BS light source  20   a , the BL light source  20   b , the G light source  20   c , and the R light source  20   d  are light emitting diodes (LEDs). A combination of a laser diode (LD), a phosphor, and a band-limiting filter; a combination of a lamp, such as a xenon lamp, and a band-limiting filter; and the like can be used in the light source unit  20  instead of these LEDs. 
     As shown in  FIG. 3 , the BS light source  20   a  is a blue light source that emits first blue light BS of which the central wavelength (peak wavelength) is in the range of about 450±10 nm and the wavelength range is in the range of about 420 nm to 500 nm. The BL light source  20   b  is a blue light source that emits so-called blue narrow-band light (hereinafter, referred to as second blue light BL) of which the central wavelength (peak wavelength) and the wavelength range are in the range of about 470±10 nm. The G light source  20   c  is a green light source that emits green light G of which the central wavelength (peak wavelength) is in the range of about 540±20 nm and the wavelength range is in the range of about 480 nm to 600 nm. The R light source  20   d  is a red light source that emits red light R of which the central wavelength (peak wavelength) is in the range of about 620±20 nm and the wavelength range is in the range of about 600 nm to 650 nm. 
     The light source device  14  is provided with a first correction parameter storage memory  23  that stores first correction parameters to be used in the oxygen saturation observation mode. The first correction parameter is a parameter that is used for the calculation of a correction value for a calculation value, and is a parameter that is used to correct an individual difference (variation) in the optical characteristics of each of the BS light source  20   a , the BL light source  20   b , the G light source  20   c , and the R light source  20   d . The light sources  20   a  to  20   d  vary (have variations) in spectral characteristics, such as a peak wavelength value or a central wavelength. 
     Specifically, the first correction parameters include a parameter Δ1(B450) that is obtained on the basis of a difference value between a peak wavelength CBS of first blue light BS of the BS light source  20   a  and the peak wavelength of a reference light source. In addition, the first correction parameters include a parameter Δ1(B470) that is obtained on the basis of a difference value between a peak wavelength CBL of second blue light BL of the BL light source  20   b  and the peak wavelength of the reference light source, a parameter Δ1(G) that is obtained on the basis of a difference value between a peak wavelength CG of green light G of the G light source  20   c  and the peak wavelength of the reference light source, and a parameter Δ1(R) that is obtained on the basis of a difference value between a peak wavelength CR of red light R of the R light source  20   d  and the peak wavelength of the reference light source. It is preferable that the spectral characteristics of each individual light source device  14  are measured and parameterized at the time of shipment from the factory with regard to the first correction parameters. 
     Further, the light source device  14  is provided with a third correction parameter acquisition section  24  (see  FIG. 2 ) that acquires third correction parameters to be used in the calibration mode. The third correction parameter is a parameter related to a difference in a specific spectral image (a spectral image for the calculation of oxygen saturation) based on a change in the wavelength characteristics (mainly, wavelength variation) of each illumination light caused by a change in the amount of light emitted from each of the BS light source  20   a , the BL light source  20   b , the G light source  20   c , and the R light source  20   d . As shown in  FIG. 4 , the third correction parameter acquisition section  24  comprises a light amount-wavelength variation relationship table  24   a  and a third correction parameter calculation section  24   b  in order to acquire the third correction parameters. In the light source device  14 , programs related to various types of processing are incorporated into a program memory. The programs incorporated into the program memory are executed by a central controller (not shown) for a light source formed of a light source processor, so that the functions of the third correction parameter acquisition section are realized. 
     A relationship between the amount of each illumination light and the wavelength variation of each illumination light is stored in the light amount-wavelength variation relationship table  24   a . A table for each of the BS light source  20   a , the BL light source  20   b , the G light source  20   c , and the R light source  20   d  is provided in the light amount-wavelength variation relationship table  24   a . For example, as shown in  FIG. 5 , a relationship between the amount LMBS of first blue light BS and the amount Δλ(B450) of wavelength variation (for example, the amount of central wavelength variation) of first blue light BS, which is caused by a change in the amount of first blue light BS, is stored in the light amount-wavelength variation relationship table  24   a  to be used for the BS light source  20   a . In a case where the amount of first blue light BS is relatively large, a relationship between the amount of first blue light BS and the amount Δλ of wavelength variation is linear. However, in a case where the amount of first blue light BS is relatively small, a relationship between the amount of first blue light BS and the amount Δλ of wavelength variation is non-linear. 
     In a case where the amount of light emitted from the light source unit  20  is changed, the third correction parameter calculation section  24   b  specifies the amount W. of wavelength variation with reference to the light amount-wavelength variation relationship table  24   a  corresponding to a light source from which the amount of light emitted is changed. Then, the third correction parameter calculation section  24   b  calculates third correction parameters corresponding to the amount Δλ of wavelength variation on the basis of a predetermined relationship between the amount Δλ of wavelength variation and the third correction parameters. The third correction parameters include four parameters, that is, a third correction parameter Δ3(B450) for the wavelength variation of the BS light source  20   a , a third correction parameter Δ3(B470) for the wavelength variation of the BL light source  20   b , a third correction parameter Δ3(G) for the wavelength variation of the G light source  20   c , and a third correction parameter Δ3(R) for the wavelength variation of the R light source  20   d.    
     The light source controller  22  independently controls the timings of the turning on or off of the respective light sources  20   a  to  20   d  of the light source unit  20 , the amounts of light emitted at the times of turning on, and the like. Due to the control of the light source controller  22 , the light source unit  20  emits illumination light for normal observation to be used in the normal mode and illumination light for oxygen saturation observation to be used in the oxygen saturation observation mode. 
     In the case of the normal mode, the light source controller  22  simultaneously turns on the BS light source  20   a , the G light source  20   c , and the R light source  20   d . For this reason, illumination light for normal observation is white light that includes first blue light BS, green light G, and red light R. In this embodiment, in the case of the normal mode, the light source unit  20  constantly emits the white light but may emit the white light at an image pickup timing (hereinafter, referred to as an image pickup frame) of the object to be observed. 
     In the case of the oxygen saturation observation mode or the calibration mode, the light source controller  22  alternately and repeatedly turns on or off the respective light sources  20   a  to  20   d  in a first pattern and a second pattern. The first pattern is a pattern in which the BS light source  20   a , the G light source  20   c , and the R light source  20   d  are simultaneously turned on. In the case of the first pattern, white light including first blue light BS, green light Q and red light R is emitted as illumination light. On the other hand, the second pattern is a pattern in which the BL light source  20   b , the G light source  20   c , and the R light source  20   d  are simultaneously turned on. For this reason, illumination light for oxygen saturation observation including second blue light BL, green light Q and red light R is emitted in the case of the second pattern. In the oxygen saturation observation mode, as shown in  FIG. 6 , white light and illumination light for oxygen saturation observation are alternately and repeatedly emitted for, for example, each image pickup frame in accordance with an image pickup frame. 
     As shown in  FIG. 2 , illumination light emitted from the light source unit  20  is incident on a light guide  41 . The light guide  41  is built in the endoscope  12  and a universal cord (a cord connecting the endoscope  12  to the light source device  14  and the processor device  16 ), and transmits illumination light to the distal end part  12   d  of the endoscope  12 . A multimode fiber can be used as the light guide  41 . For example, a thin fiber cable of which a total diameter of a core diameter of 105 μm, a cladding diameter of 125 μm, and a protective layer forming a covering is in the range of φ 0.3 to 0.5 mm can be used. 
     The distal end part  12   d  of the endoscope  12  is provided with an illumination optical system  30   a  and an image pickup optical system  30   b . The illumination optical system  30   a  includes an illumination lens  45 , and the object to be observed is irradiated with illumination light through the illumination lens  45 . The image pickup optical system  30   b  includes an objective lens  46 , a zoom lens  47 , and an image sensor  48 . The image sensor  48  picks up the image of the object to be observed using the reflected light and the like (including scattered light, fluorescence emitted from the object to be observed, fluorescence caused by a medicine given to the object to be observed, or the like) of illumination light returning from the object to be observed through the objective lens  46  and the zoom lens  47 . The zoom lens  47  is moved by the operation of the zoom operation part  13   c  and increases or reduces the size of the object to be observed of which the image is to be picked up by the image sensor  48 . 
     The image sensor  48  is a primary color sensor, and comprises three types of pixels, that is, B pixels (blue pixel) including blue color filters, G pixels (green pixels) including green color filters, and R pixels (red pixels) including red color filters. As shown in  FIG. 7 , the blue color filter mainly transmits light of a blue light wavelength range, specifically, light of which the wavelength range is in the range of 380 to 560 nm. The transmittance of the blue color filter BF has a peak near a wavelength of 460 to 470 nm. The green color filter GF mainly transmits light of a green light wavelength range, specifically, light of which the wavelength range is in the range of 460 to 470 nm. The red color filter RF mainly transmits light of a red light wavelength range, specifically, light of which the wavelength range is in the range of 580 to 760 nm. 
     In a case where the image of the object to be observed is picked up by the image sensor  48 , three types of spectral images, that is, a B image (blue image) obtained from the image pickup in the B pixel, a G image (green image) obtained from the image pickup in the G pixel, and an R image (red image) obtained from the image pickup in the R pixel can be obtained at the maximum in a single image pickup. Since illumination light for normal observation to be used is white light in the case of the normal mode, Bc images, Gc images, and Rc images are obtained. The Bc image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of first blue light BS, and the Gc image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of green light G. Likewise, the Rc image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of red light R. 
     On the other hand, in the oxygen saturation observation mode or the calibration mode, B1 images, G1 images, and R1 images are acquired as spectral images in a frame where white light is emitted, and B2 images, G2 images, and R2 images are acquired as spectral images in a frame where illumination light for oxygen saturation observation is emitted. The B1 image, the G1 image, and the R1 image are the same as the Bc image, the Gc image, and the Rc image. The B2 image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of second blue light BL. The G2 image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of green light G. Likewise, the R2 image is an image that is obtained from the image pickup of the object to be observed mainly using the reflected light and the like of red light R. 
     A charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor can be used as the image sensor  48 . Further, the image sensor  48  of this embodiment is a primary color sensor, but a complementary color sensor can also be used as the image sensor  48 . The complementary color sensor includes, for example, cyan pixels provided with cyan color filters, magenta pixels provided with magenta color filters, yellow pixels provided with yellow color filters, and green pixels provided with green color filters. In a case where the complementary color sensor is used, images obtained from the respective color pixels described above can be converted into the B image, the G image, and the R image by complementary color-primary color conversion. Further, a monochrome sensor not provided with color filters can be used as the image sensor  48  instead of the color sensor. In this case, the images having the respective colors can be obtained from the sequential image pickup of the object to be observed using illumination light having the respective colors, such as B, G, and R. 
     As shown in  FIG. 2 , the processor device  16  includes a general controller  52 , an image acquisition unit  54 , an image processing unit  61 , a static image storage controller  64 , a static image storage unit  65 , and a display controller  66 . 
     The general controller  52  controls each unit provided in the processor device  16 . The general controller  52  performs a control corresponding to each mode on the basis of a mode changeover signal output from the mode changeover switch  13   a . Further, the general controller  52  controls the endoscope  12  and the light source device  14 . The general controller  52  controls the irradiation timing of illumination light by controlling the light source controller  22  of the light source device  14 . Furthermore, the general controller  52  controls an image pickup timing by controlling the image sensor  48  of the endoscope  12 . 
     In the processor device  16 , programs related to various types of processing are incorporated into the program memory. The programs incorporated into the program memory are executed by the general controller  52  formed of an image control processor, so that the functions of the image acquisition unit  54 , the image processing unit  61 , the static image storage controller  64 , the static image storage unit  65 , and the display controller  66  are realized. 
     The image acquisition unit  54  acquires the image of the object to be observed from the image sensor  48 . In the case of the normal mode, the image acquisition unit  54  acquires Bc images, Gc images, and Rc images for each image pickup frame. In the case of the oxygen saturation observation mode or the calibration mode, the image acquisition unit  54  acquires B1 images, G1 images, and R1 images in an image pickup frame where white light is used as illumination light and acquires B2 images, G2 images, and R2 images in an image pickup frame where illumination light for oxygen saturation observation is used as illumination light. 
     Further, the image acquisition unit  54  includes a digital signal processor (DSP)  56 , a noise-reduction section  58 , and a conversion section  59 , and performs various types of processing on the acquired images using these. 
     The DSP  56  performs various types of processing, such as defect correction processing, offset processing, gain correction processing, matrix processing, gamma conversion processing, demosaicing processing, and YC conversion processing, on the acquired images as necessary. 
     The defect correction processing is processing for correcting the pixel values of pixels corresponding to defective pixels of the image sensor  48 . The offset processing is processing for reducing dark current components from the images subjected to the defect correction processing and setting an accurate zero level. The gain correction processing is processing for multiplying the images, which have been subjected to the offset processing, by gains to adjust the signal level of each image. The matrix processing is processing for improving the color reproducibility of the images subjected to the offset processing, and the gamma conversion processing is processing for adjusting the brightness and chroma saturation of the images subjected to the matrix processing. The demosaicing processing (also referred to as equalization processing or demosaicing) is processing for interpolating the pixel values of missing pixels, and is performed on the images subjected to the gamma conversion processing. The missing pixels are pixels that do not have pixel values since other color pixels are arranged in the image sensor  48  due to the arrangement of color filters. For example, since the B image is an image obtained from the image pickup of the object to be observed in the B pixel, pixels arranged at the positions of the G pixel and the R pixel of the image sensor  48  do not have pixel values. In the demosaicing processing, the B image is interpolated to generate the pixel values of pixels arranged at the positions of the G pixel and the R pixel of the image sensor  48 . The YC conversion processing is processing for converting images, which have been subjected to the demosaicing processing, into luminance channel Y, color difference channels Cb, and color difference channels Cr. 
     The noise-reduction section  58  performs noise-reduction processing on the luminance channel Y, the color difference channels Cb, and the color difference channels Cr, using, for example, a moving average method, a median filtering method, or the like. The conversion section  59  converts the luminance channel Y, the color difference channels Cb. and the color difference channels Cr, which have been subjected to the noise-reduction processing, into images having the respective colors of B, Q and R, again. 
     The image processing unit  61  includes a normal processing section  62 , a special processing section  63 , and a calibration processing section  68 . The normal processing section  62  operates in the normal mode; performs color conversion processing, color enhancement processing, and structure enhancement processing on the Bc images, the Gc images, and Rc images for one image pickup frame, which have been subjected to the above-mentioned various types of processing; and generates normal images. In the color conversion processing, 3×3-matrix processing, gradation transformation processing, and three-dimensional look up table (LUT) processing, and the like are performed on the images having the respective colors of B, G and R. The color enhancement processing is processing for enhancing the color of an image, and the structure enhancement processing is processing for enhancing, for example, the tissue or structure of the object to be observed, such as blood vessels or a pit pattern. The display controller  66  sequentially acquires normal images from the normal processing section  62 , converts the acquired normal images into a format that is suitable to be displayed, and sequentially outputs and displays the converted normal images on the monitor  18 . Accordingly, in the case of the normal mode, a medical doctor or the like can observe the object to be observed using the video of the normal images. 
     The special processing section  63  operates in the oxygen saturation observation mode, and calculates oxygen saturation on the basis of the first correction parameters and white balance correction data, which are acquired in the calibration mode, in addition to the B1 images, the G1 images, the R1 images, and the B2 images that are specific spectral images required for the calculation of oxygen saturation. Further, the special processing section  63  creates an oxygen saturation image subjected to coloring processing or the like according to the calculated oxygen saturation. 
     As shown in  FIG. 8 , the special processing section  63  comprises a white balance correction processing section  70 , a calculation value-calculation section  71 , a correction amount calculation section  72 , an oxygen saturation calculation section  73 , and an image creation section  74 . The white balance correction processing section  70  performs white balance correction processing on the B1 images, the G1 images, the R1 images, and the B2 images, which are specific spectral images, using the white balance correction data stored in a white balance correction data storage memory  70   a . The white balance correction data are data that are obtained from the image pickup of a reference white plate WP and are used to correct a difference in a white portion of a spectral image caused by a combination of the endoscope  12  and the light source device  14 . The white balance correction data include white balance correction data NB1 for B1 image, white balance correction data NG1 for G1 image, white balance correction data NR1 for R1 image, and white balance correction data NB2 for B2 image. 
     The B1 image is divided by the white balance correction data NB1 for B1 image in the white balance correction processing section  70 , so that a B1 image subjected to white balance correction (B1*=B1/NB1) is obtained. Likewise, the G1 image is divided by the white balance correction data NG1 for G1 image, so that a G1 image subjected to white balance correction (G1*=G1/NG1) is obtained. Further, the R1 image is divided by the white balance correction data NR1 for R1 image, so that an R1 image subjected to white balance correction (R1*=R1/NR1) is obtained. Furthermore, the B2 image is divided by the white balance correction data NB2 for B2 image, so that a B2 image subjected to white balance correction (B2*=B2/NB2) is obtained. 
     In order to correct a difference in the specific spectral image based on the wavelength variation of each semiconductor light source of the light source unit  20 , it is preferable that the B1 image, the G1 image, the R1 image, and the B2 image subjected to white balance correction (B*, G1*, R1*, and B2) are further corrected in the white balance correction processing section  70  using the third correction parameters Δ3(B450), Δ3(B470), Δ3(G), and Δ3(R). 
     Specifically, it is preferable that the B1 image subjected to white balance correction is multiplied by a correction coefficient α(B1) for B1 image based on the third correction parameters Δ3(B450), Δ3(B470), Δ3(G), and Δ3(R) (B1*×α(B1)). Likewise, it is preferable that the G1 image subjected to white balance correction is multiplied by a correction coefficient α(G1) for G1 image based on the third correction parameters Δ3(B450), Δ3(B470), Δ3(G), and Δ3(R) (G1*×α(G1)). Further, it is preferable that the R1 image subjected to white balance correction is multiplied by a correction coefficient α(R1) for R1 image based on the third correction parameters Δ3(B450), Δ3(B470), Δ3(G), and Δ3(R) (R1*×α(R1)). Furthermore, it is preferable that the B2 image subjected to white balance correction is multiplied by a correction coefficient (B2) for B2 image based on the third correction parameters Δ3(B450), Δ3(B470), Δ3(G), and Δ3(R) (B2*×α(B2)). 
     The calculation value-calculation section  71  calculates calculation values for the calculation of oxygen saturation by calculation processing based on the B1 image, the G1 image, the R1 image, and the B2 image (B1*, G1*, R1*, and B2* or B1*×α(B1), G1*×α(G1), R1*×α(R1), and B2*×α(B2)) that are the specific spectral images subjected to white balance correction. Specifically, the calculation value-calculation section  71  calculates a signal ratio X (=ln(R1*/G1*)) as the calculation value by performing calculation, which logarithmizes the R1 image subjected to white balance correction by dividing the R1 image subjected to white balance correction by the G1 image subjected to white balance correction, as the calculation processing. Further, the calculation value-calculation section  71  calculates a signal ratio Y (=ln(B2*/G1*)) as the calculation value by performing calculation, which logarithmizes the B2 image subjected to white balance correction by dividing the B2 image subjected to white balance correction by the G1 image subjected to w % bite balance correction, as the calculation processing. 
     The correction amount calculation section  72  calculates correction amounts, which are used to correct the signal ratios X and Y, on the basis of the signal ratios X and Y, which are calculation values, and the first correction parameters. The reason to correct the signal ratios X and Y even though white balance correction processing is performed as described above is as follows. A first signal ratio and a second signal ratio show the same value (“0”) in a white portion due to white balance correction processing regardless of a combination of the endoscope  12  and the light source device  14 , but are not corrected in color portions other than the white portion. In a case where the first signal ratio and the second signal ratio are not corrected in other color portions, it is difficult to accurately calculate oxygen saturation. 
     Accordingly, the correction amount calculation section  72  acquires first correction parameters Δ1(B450), Δ1(B470), Δ1(G), and Δ1(R), which are used to correct differences in the spectral characteristics of the respective semiconductor light sources of the light source unit  20 , from the light source device  14  and calculates the correction amounts ΔX and ΔY of the signal ratios X and Y using these first correction parameters Δ1(B450), Δ1(B470), Δ1(G), and Δ1(R). Specifically, the correction amount calculation section  72  calculates the correction amounts ΔX and ΔY according to the following equation (1). 
       Δ X=M 11× X+M 12× Y  
 
       Δ Y=M 21× X+M 22× Y   Equation (1)
 
     Here, M_ij is a matrix coefficient and “M_ij=b450_ij×Δ1(B450)+b470_ij×Δ1(B470)+g_ij×Δ1(G)+r_ijΔ1(R)” is satisfied. b450_ij, b470_ij, g_ij, and r_ij are arbitrary coefficients. “i” of subindices is 1 or 2 and “j” of subindices is 1 or 2. 
     The oxygen saturation calculation section  73  corrects the signal ratios X and Y using the correction amounts ΔX and ΔY and calculates oxygen saturation on the basis of the corrected signal ratios X* and Y*. Specifically, the oxygen saturation calculation section  73  calculates the corrected signal ratios X* and Y* by the following equation (2). 
     
       
      
       X*=X+ΔX  
      
     
         Y*=Y+ΔY   Equation (2)
 
     The oxygen saturation calculation section  73  calculates oxygen saturation corresponding to the corrected X* and Y* for each pixel with reference to a data storage section  73   a  for oxygen saturation in which a correlation between the signal ratios X and Y and oxygen saturation is stored. With regard to the correlation stored in the data storage section  73   a  for oxygen saturation, isolines, which connect points where oxygen saturation has the same value, are formed in a substantially horizontal direction as shown in  FIG. 9  in a feature space of which the vertical axis represents the signal ratio Y and the horizontal axis represents the signal ratio X. Further, the isoline is positioned on the lower side in the direction of the vertical axis as oxygen saturation is larger. For example, an isoline  75  on which oxygen saturation is 100% is positioned below an isoline  76  on which oxygen saturation is 0%. 
     For example, referring to the correlation stored in the data storage section  73   a  for oxygen saturation, oxygen saturation corresponding to the corrected signal ratios X* and Y* is “40%”. Accordingly, the oxygen saturation calculation section  70  calculates the oxygen saturation of a pixel corresponding to the corrected signal ratios X* and Y* as “40%”. 
     The image creation section  74  creates an oxygen saturation image using oxygen saturation. Specifically, the image creation section  74  creates a normal image on the basis of the B1 image, the G1 image, and the R1 image and changes the color tone of the normal image according to oxygen saturation. For example, it is preferable that the image creation section  74  does not change the color tone of a pixel of the normal image where oxygen saturation exceeds 70% and displays the pixel as it is, but changes the color tone of a pixel of the normal image where oxygen saturation is 70% or less and displays the pixel. In a case where a color tone is to be changed, it is preferable that the color tone is closer to a cold color (for example, blue) as oxygen saturation is lower. The image creation section  74  may create an oxygen saturation image using oxygen saturation and a specific color image without creating a normal image. In this case, it is preferable that the image creation section  74  creates an oxygen saturation image using the luminance channel Y and the color difference channels Cr and Cb. For example, it is preferable that the luminance channel Y is assigned to G1 image signals and the color difference channels Cr and Cb are assigned according to oxygen saturation. 
     A series of flows of the calibration mode in which white balance correction data are acquired will be described with reference to a flowchart shown in  FIG. 10 . The calibration mode is executed at a facility (a hospital or the like) where the endoscope system  10  is installed. First, the endoscope system  10  is switched to the calibration mode by the operation of the mode changeover switch  3   a  of the endoscope  12 . In a case where the endoscope system  10  is switched to the calibration mode, white light and illumination light for oxygen saturation observation are emitted alternately. Then, the endoscope  12  is inserted into the calibration device  80  including the reference white plate WP as shown in  FIG. 11 . The image of the reference white plate WP is picked up in a state where the distal end part  12   d  of the endoscope faces the reference white plate WP. In a case where the image pickup state of the reference white plate WP satisfies a certain condition, the static images of the reference white plate WP are acquired automatically or manually (by the operation of the freeze switch  13   b ). The static images of the reference white plate WP are transmitted to the calibration processing section  68 . 
     The static images of the reference white plate WP include a B1p image, a G1p image, and an R1p image that are the spectral images of white light and a B2p image, a G2p image, and an R2p image that are the spectral images of illumination light for oxygen saturation observation. The calibration processing section  68  sets a rectangular correction data acquisition region AR shown in  FIG. 12  in each of the B1p image, the G1p image, the R1p image, and the B2p image among the static images of the reference white plate WP, and calculates average values of the pixel values of the correction data acquisition regions AR. The average values of the pixel values of the correction data acquisition regions AR of the B1p image, the G1p image, the R1p image, and the B2p image are defined as the white balance correction data NB1, NG1, NR1, and NB2. The white balance correction data are stored in the white balance correction data storage memory  70   a . The shape of the correction data acquisition region AR may be a circular shape in addition to a rectangular shape. 
     Next, a series of flows of the oxygen saturation observation mode will be described with reference to a flowchart shown in  FIG. 13 . The endoscope system  10  is switched to the oxygen saturation observation mode by the operation of the mode changeover switch  13   a  of the endoscope  12 . In a case where the endoscope system  10  is switched to the oxygen saturation observation mode, white light and illumination light for oxygen saturation observation are emitted alternately. The B1 image, the G1 image, and the R1 image are acquired in the case of the image pickup frame of white light. The B2 image, the G2 image, and the R2 image are acquired in the image pickup frame of illumination light for oxygen saturation observation. The B1 image, the G1 image, the R1 image, and the B2 image serve as specific spectral images to be used for the calculation of oxygen saturation. 
     The white balance correction processing section  70  performs white balance correction processing on the B1 image, the G1 image, the R1 image, and the B2 image that are specific spectral images. Accordingly, the B1 image, the G1 image, the R1 image, and the B2 image subjected to white balance correction are acquired. Next, the calculation value-calculation section  71  calculates the signal ratios X and Y as calculation values for the calculation of oxygen saturation by calculation processing based on the B1 image, the G1 image, the R1 image, and the B2 image subjected to white balance correction. 
     The correction amount calculation section  72  calculates correction amounts ΔX and ΔY, which are used to correct the signal ratios X and Y, on the basis of the signal ratios X and Y and the first correction parameters that are used to correct differences in the spectral characteristics of the respective semiconductor light sources of the light source unit  20 . The oxygen saturation calculation section  73  corrects the signal ratios X and Y using the correction amounts ΔX and ΔY and calculates oxygen saturation on the basis of the corrected signal ratios X* and Y*. The image creation section  74  creates an oxygen saturation image using the calculated oxygen saturation. The created oxygen saturation image is displayed on the monitor  18 . 
     Second Embodiment 
     In a second embodiment, correction amounts of the signal ratios X and Y are calculated using second correction parameters, which are used to correct differences in calculation values caused by combinations of the respective semiconductor light sources of the light source unit  20  and the endoscope  12 , in addition to the first correction parameters. In an oxygen saturation observation mode of the second embodiment, specific illumination light to be used to acquire the second correction parameters is emitted in addition to white light and illumination light for oxygen saturation observation. For example, it is preferable that green light G emitted from the green light source  20   c  is used as the specific illumination light. In this case, it is preferable that white light, illumination light for oxygen saturation observation, and green light are alternately emitted as shown in  FIG. 14 . B3 images, G3 images, and R3 images are obtained in the image pickup frame of green light as spectral images. The B3 images, the G3 images, and the R3 images may be subjected to white balance correction processing. Further, the correction amounts may be calculated on the basis of the first correction parameters, the second correction parameters, and the third correction parameters. Furthermore, the correction amounts may be calculated on the basis of at least one of the first correction parameters, the second correction parameters, or the third correction parameters. 
     A special processing section  63  of the second embodiment is provided with a second correction parameter acquisition section  90  that acquires the second correction parameters. As shown in  FIG. 16 , the second correction parameter acquisition section  90  comprises a reference signal ratio storage section  90   a  that stores reference signal ratios Xs and Ys related to signal ratios X and Y, and the like in advance, a second correction parameter-calculation-signal ratio calculation section  90   b , and a second correction parameter calculation section  90   c  that calculates the second correction parameters. 
     The reference signal ratio storage section  90   a  includes a reference signal ratio Z1s related to a signal ratio Z1 (=B1/G1) that is obtained in a case where the B1 image is divided by the G1 image, a reference signal ratio Z2s related to a signal ratio Z2 (=B3/G1) that is obtained in a case where the B3 image is divided by the G1 image, a reference signal ratio Z3s related to a signal ratio Z3 (=G3/G1) that is obtained in a case where the G3 image is divided by the G1 image, a reference signal ratio Z4s related to a signal ratio Z4 (=G2/G1) that is obtained in a case where the G2 image is divided by the G1 image, and a reference signal ratio Z5s related to a signal ratio Z5 (=R3/G1) that is obtained in a case where the R3 image is divided by the G1 image, in addition to the reference signal ratios Xs and Ys. The above-mentioned signal ratios Z1 to Z5 are signal ratios for the calculation of second correction parameters that are used for the calculation of the second correction parameters. 
     The second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratios for the calculation of second correction parameters. That is, the second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratio Z1 on the basis of the B1 image and the G1 image as the signal ratios for the calculation of second correction parameters. Likewise, the second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratio Z2 on the basis of the B3 image and the G1 image. Further, the second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratio Z3 on the basis of the G3 image and the G1 image. Furthermore, the second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratio Z4 on the basis of the G2 image and the G1 image. The second correction parameter-calculation-signal ratio calculation section  90   b  calculates the signal ratio Z5 on the basis of the R3 image and the G1 image. 
     The second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(X) by a difference between the signal ratio X and the reference signal ratio Xs. Likewise, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Y) by a difference between the signal ratio Y and the reference signal ratio Ys. Further, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Z1) by a difference between the signal ratio Z1 and the reference signal ratio Z1s. Furthermore, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Z2) by a difference between the signal ratio Z2 and the reference signal ratio Z2s. Moreover, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Z3) by a difference between the signal ratio Z3 and the reference signal ratio Z3s. Further, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Z4) by a difference between the signal ratio Z4 and the reference signal ratio Z4s. Furthermore, the second correction parameter calculation section  90   c  calculates a second correction parameter Δ2(Z5) by a difference between the signal ratio Z5 and the reference signal ratio Z5s. 
     A correction amount calculation section  72  of the second embodiment also calculates the correction amounts ΔX and ΔY of the signal ratios X and Y by Equation (1) as in the first embodiment. However, a matrix coefficient M_ij of Equation (1) in the second embodiment is different from that in the first embodiment. The matrix coefficient M_ij of the second embodiment is as follows. As in the first embodiment, “i” of subindices is 1 or 2 and “j” of subindices is 1 or 2. 
         M _ ij=b 450_ ij ×Δ( B 450)+ b 470_ ij×Δ 1( B 470)+ g _ ij×Δ 1( G )+ r _ ijΔ 1( R )+ B 1_ ij×A 2( Z 1)+ B 2_ ijΔ 2( Y )+ B 3_ ij×Δ 2( Z 2)+ G 2_ ij×Δ 2( X )+ G 3_ ij×Δ 2( Z 3)+ R 2_ ij×Δ 2( Z 4)+ R 3_ ij - A 2( Z 5)
 
     Here, B1, B2, B3, G2, R3, R2, and R3 are arbitrary coefficients. 
     In the above-mentioned embodiments, an example of an endoscope system for a lumen using the endoscope  12 , which is a soft endoscope, has been described as the endoscope system according to the embodiment of the present invention. However, the endoscope system according to the embodiment of the present invention can also be applied to an endoscope system for an abdominal cavity that uses a surgical rigid endoscope. 
     The hardware structures of the processing units, which execute various types of processing in the above description, such as the first correction parameter storage memory  23 , the third correction parameter acquisition section  24 , the image acquisition unit  54 , the image processing unit  61 , the normal processing section  62 , the special processing section  63 , the static image storage controller  64 , the static image storage unit  65 , the display controller  66 , the calibration processing section  68 , the white balance correction processing section  70 , the white balance correction data storage memory  70   a , the calculation value-calculation section  71 , the correction amount calculation section  72 , the oxygen saturation calculation section  73 , the data storage section  73   a  for oxygen saturation, the image creation section  74 , the second correction parameter acquisition section  90 , the reference signal ratio storage section  90   a , the second correction parameter-calculation-signal ratio calculation section  90   b , and the second correction parameter calculation section  90   c , are various processors to be described below. The various processors include: a central processing unit (CPU) that is a general-purpose processor functioning as various processing units by executing software (program); a programmable logic device (PLD) that is a processor of which the circuit configuration can be changed after manufacture, such as a field programmable gate array (FPGA); a dedicated electrical circuit (graphical processing unit: GPU)) that is a processor having circuit configuration designed exclusively to perform various types of processing; and the like. 
     One processing unit may be formed of one of these various processors, or may be formed of a combination of two or more same type or different types of processors (for example, a plurality of FPGAs, a combination of a CPU and an FPGA, or a combination of GPU and CPU). Further, a plurality of processing units may be formed of one processor. As an example where a plurality of processing units are formed of one processor, first, there is an aspect where one processor is formed of a combination of one or more CPUs and software as typified by a computer, such as a client or a server, and functions as a plurality of processing units. Second, there is an aspect where a processor fulfilling the functions of the entire system, which includes a plurality of processing units, by one integrated circuit (IC) chip as typified by a system-on-chip (SoC) or the like is used. In this way, various processing units are formed using one or more of the above-mentioned various processors as hardware structures. 
     In addition, the hardware structures of these various processors are more specifically electrical circuitry where circuit elements, such as semiconductor elements, are combined. Further, the hardware structure of the storage unit is a storage device, such as a hard disc drive (HDD) or a solid state drive (SSD). 
     EXPLANATION OF REFERENCES 
     
         
         
           
               10 : endoscope system 
               12 : endoscope 
               12   a : insertion part 
               12   b : operation part 
               12   c : bendable part 
               12   d : distal end part 
               12   e : angle knob 
               13   a : mode changeover switch 
               13   b : freeze switch 
               13   c : zoom operation part 
               14 : light source device 
               16 : processor device 
               18 : monitor 
               19 : user interface 
               20 : light source unit 
               20   a : BS light source 
               20   b : BL light source 
               20   c : G light source 
               20   d : R light source 
               22 : light source controller 
               23 : first correction parameter storage memory 
               24 : third correction parameter acquisition section 
               24   a : light amount-wavelength variation relationship table 
               24   b : third correction parameter calculation section 
               30   a : illumination optical system 
               30   b : image pickup optical system 
               41 : light guide 
               45 : illumination lens 
               46 : objective lens 
               47 : zoom lens 
               48 : image sensor 
               52 : general controller 
               54 : image acquisition unit 
               56 : DSP 
               58 : noise-reduction section 
               59 : conversion section 
               61 : image processing unit 
               62 : normal processing section 
               63 : special processing section 
               64 : static image storage controller 
               65 : static image storage unit 
               66 : display controller 
               68 : calibration processing section 
               70 : white balance correction processing section 
               70   a : white balance correction data storage memory 
               71 : calculation value-calculation section 
               72 : correction amount calculation section 
               73 : oxygen saturation calculation section 
               73   a : data storage section for oxygen saturation 
               74 : image creation section 
               75 : isoline of 100% 
               76 : isoline of 0% 
               80 : calibration device 
               90 : second correction parameter acquisition section 
               90   a : reference signal ratio storage section 
               90   b : second correction parameter-calculation-signal ratio calculation section 
               90   c : second correction parameter calculation section 
             CBS: peak wavelength of first blue light 
             CBL: peak wavelength of second blue light 
             CG: peak wavelength of green light 
             CR: peak wavelength of red light 
             BF: blue color filter 
             GF: green color filter 
             RF: red color filter 
             WP: reference white plate 
             AR: correction data acquisition region