Patent Publication Number: US-9414739-B2

Title: Imaging apparatus for controlling fluorescence imaging in divided imaging surface

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT international application Ser. No. PCT/JP2014/059473 filed on Mar. 31, 2014 which designates the United States, incorporated herein by reference, and which claims the benefit of priority from Japanese Patent Application No. 2013-160659, filed on Aug. 1, 2013, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an imaging apparatus that irradiates a subject with excitation light or visible light and receives reflected light from the subject by a plurality of pixels to perform photoelectric conversion, to thereby output image information. 
     2. Related Art 
     Conventionally, in medical fields, an endoscope system is used for observing an interior of an organ of a subject. In general, the endoscope system, which is a kind of an imaging apparatus, inserts an elongated and flexible insertion section into a body cavity of a subject such as a patient, and irradiates body tissues in the body cavity with white light through the inserted insertion section and receives reflected light by an imaging unit provided at a distal end of the insertion section, to thereby capture an in-vivo image. An image signal of the biological image taken by the endoscope system is transmitted to an image processing device outside the subject body through a transmission cable inside the insertion section and subjected to image processing in the image processing device and is, thereby, displayed on a monitor of the endoscope system. A user such as a doctor observes the interior of the organ of the subject through the in-vivo image displayed on the monitor. 
     As such an endoscope system, there is known technology capable of performing fluorescence observation that irradiates body tissues into which a fluorescence agent including a fluorescence marker is introduced with excitation light of a specific wavelength to capture fluorescence light or normal observation that irradiates body tissues with normal light in a visible wavelength range to capture reflected light (see Japanese Laid-open Patent Publication No. 2006-61435). In this technology, a brightness level of a fluorescence image is automatically adjusted with a brightness level of an image captured using the normal light set as a target value, whereby it is possible to display the fluorescence image with proper brightness without imposing burden on a user. 
     SUMMARY 
     In some embodiments, an imaging apparatus includes: an excitation light emission unit configured to irradiate a subject with excitation light for exciting a fluorescent substance introduced into the subject; a normal light emission unit configured to irradiate the subject with normal light including a visible wavelength range different from the excitation light; an imaging unit configured to form an optical image of the subject irradiated with the excitation light or normal light on an imaging surface to generate an image signal; a brightness signal generation unit configured to generate a brightness signal indicating brightness, based on the image signal generated by the imaging unit under irradiation with the normal light; and an amplification unit configured to set an amplification factor of the image signal to be generated by the imaging unit under irradiation with the excitation light, based on an amplification factor of the image signal according to the brightness signal generated by the brightness signal generation unit. 
     The above and other features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a schematic configuration of an endoscope system as an imaging apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view schematically explaining an internal configuration of a distal end section of an endoscope illustrated in  FIG. 1 ; 
         FIGS. 3A and 3B  are a block diagram illustrating a functional configuration of a main part of the endoscope system according to the first embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating an outline of processing that the endoscope system according to the first embodiment of the present invention executes; 
         FIGS. 5A and 5B  are a block diagram illustrating a functional configuration of a main part of an endoscope system according to a second embodiment of the present invention; 
         FIG. 6  is a flowchart illustrating an outline of processing that the endoscope system according to the second embodiment of the present invention executes; 
         FIG. 7  is a view schematically illustrating divided regions obtained by division of an imaging surface performed by a divided region setting unit of  FIG. 5B ; 
         FIGS. 8A and 8B  are a block diagram illustrating a functional configuration of a main part of an endoscope system according to a third embodiment of the present invention; 
         FIG. 9  is a flowchart illustrating an outline of processing that the endoscope system according to the third embodiment of the present invention executes; 
         FIG. 10  is a view schematically illustrating divided regions obtained by division of an imaging surface performed by a divided region setting unit of  FIG. 8B ; 
         FIG. 11  is a time chart of processing to be executed by an endoscope system according to a fourth embodiment of the present invention; 
         FIG. 12  is a view schematically illustrating divided regions obtained by division of an imaging surface performed by a divided region setting unit according to the fourth embodiment of the present invention; 
         FIG. 13  is a view illustrating a distal end surface of a distal end section of an endoscope system according to a fifth embodiment of the present invention; 
         FIG. 14  is a view illustrating a part of a cut-out surface obtained by cutting the distal end section along a line A-A of  FIG. 13 ; 
         FIG. 15  is a view illustrating a part of a cut-out surface obtained by cutting the distal end section along a line B-B of  FIG. 13 ; and 
         FIGS. 16A and 16B  are a block diagram illustrating a functional configuration of a main part of the endoscope system according to the fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a medical endoscope system that captures and display an image of an interior of a body cavity of a subject such as a patient will be described as modes for carrying out the invention (hereinafter, referred to as “embodiments”). The embodiments do not limit the invention. The same reference signs are used to designate the same elements throughout the drawings. Further, it is noted that each drawing is a schematic diagram, and a relationship between a thickness and a width, a dimensional ratio of each member, and the like may differ from those of actual ones. Furthermore, different drawings include elements which have different dimensional relations and ratios. 
     First Embodiment 
       FIG. 1  is a view illustrating a schematic configuration of the endoscope system as an imaging apparatus according to a first embodiment of the present invention. As illustrated in  FIG. 1 , an endoscope system  1  includes an endoscope  2  whose distal end is inserted into a body cavity of a subject to capture an in-vivo image of the subject, a control device  3  that performs specified processing on the in-vivo image captured by the endoscope  2  and totally controls operation of the endoscope system  1 , a light source device  4  serving as a light source unit for generating illumination light or excitation light to be emitted from the distal end of the endoscope  2 , and a display device  5  that displays the in-vivo image obtained as a result of the image processing applied by the control device  3 . 
     The endoscope  2  includes an elongated and flexible insertion section  21 , an operating unit  22  connected to a proximal end side of the insertion section  21  and configured to receive an input of various operation signals, and a universal cord  23  extending in a direction different from a direction in which the insertion section  21  extends from the operating unit  22  and incorporating various cables connected to the control device  3  and light source device  4 . 
     The insertion section  21  includes a distal end section  24  incorporating an imaging element to be described later, a freely bendable bending portion  25  constituted by a plurality of bending pieces, and a long flexible tube portion  26  connected to a proximal end side of the bending portion  25 . 
       FIG. 2  is a cross-sectional view schematically explaining an internal configuration of the distal end section  24 . As illustrated in  FIG. 2 , the distal end section  24  includes a light guide  241  constituted using glass fiber or the like and serving as a light guide for light generated by the light source device  4 , an illumination lens  242  provided at a distal end of the light guide  241 , an optical system  243  for light collection, an imaging element  244  provided at an image forming position of the optical system  243  and configured to receive light collected by the optical system  243 , perform photoelectric conversion to convert it into an electric signal, and apply specified signal processing to the obtained electric signal, and a treatment tool channel  245  through which a treatment tool for the endoscope  2  is passed. 
     The optical system  243  includes at least a lens  243   a  and a lens  243   b . A type or the number of lenses constituting the optical system  243  is not limited to that illustrated in  FIG. 2 . 
       FIGS. 3A and 3B  are a block diagram illustrating a functional configuration of a main part of the endoscope system  1 . A configuration of the imaging element  244  will be described with reference to  FIGS. 3A and 3B . As illustrated in  FIGS. 3A and 3B , the imaging element  244  includes a sensor unit  244   a  that photoelectric converts light from the optical system  243  to output an electric signal, an analog front end  244   b  (hereinafter, referred to as “AFE unit  244   b ”) that applies noise reduction and A/D conversion to the electric signal output from the sensor unit  244   a , a P/S converter  244   c  that parallel/serial converts a digital signal output from the AFE unit  244   b , a timing generator  244   d  that generates driving timing of the sensor unit  244   a  and pulses for various signal processing performed in the AFE unit  244   b  and P/S converter  244   c , and an imaging controller  244   e  that controls operation of the imaging element  244 . The imaging element  244  is a complementary metal oxide semiconductor (CMOS) image sensor. 
     The sensor unit  244   a  is connected to an IC circuit group  244 G through a substrate  244 S. The IC circuit group  244 G includes a plurality of IC circuits having functions corresponding respectively to the AFE unit  244   b , the P/S converter  244   c , the timing generator  244   d , and the imaging controller  244   e.    
     The sensor unit  244   a  includes a light receiving unit  244   f  in which a plurality of pixels each having a photodiode that accumulates electric charge in accordance with a light amount and an amplifier that amplifies the electric charge accumulated by the photodiode are arranged in two-dimensional matrix form and a reading unit  244   g  that reads, as image information, an electric signal generated by a pixel arbitrarily set as a readout target from among the plurality of pixels in the light receiving unit  244   f . In the light receiving unit  244   f , a color filter for each of RGB is provided for each pixel, enabling acquisition of a color image. 
     The AFE unit  244   b  includes a noise reduction unit  244   h  that reduces a noise component included in an electric signal (analog), an auto gain control (AGC) unit  244   i  that maintains a constant output level by adjusting an amplification factor (gain) of the electric signal, and an A/D converter  244   j  that applies A/D conversion to the electric signal output through the AGC unit  244   i . The noise reduction unit  244   h  performs noise reduction by using, for example, a correlated double sampling method. 
     The imaging controller  244   e  controls various operations of the distal end section  24  according to setting data received from the control device  3 . The imaging controller  244   e  is configured by using a central processing unit (CPU) and the like. Further, the imaging controller  244   e  sets a readout region that the reading unit  244   g  reads from the light receiving unit  244   f  according to the setting data received from the control device  3 . 
     An electrode  244 E provided on the substrate  244 S is connected with a cable assembly  246  in which a plurality of signal lines through which an electric signal is exchanged between the electrode  244 E and control device  3  are bundled. The plurality of signal lines include a signal line that transmits the image signal output from the imaging element  244  to the control device  3 , a signal line that transmits a control signal output from the control device  3  to the imaging element  244 , and the like. 
     The operating unit  22  includes a bending knob  221  that bends the bending portion  25  in up/down and left/right directions, a treatment tool insertion section  222  that inserts a treatment tool such as a biological forceps, a laser knife, and a test probe into a body cavity, and a plurality of switches  223  as an operation input section that inputs an operation command signal for peripheral equipment such as an air supply unit, a water supply unit, and a gas supply unit in addition to the control device  3  and the light source device  4 . The treatment tool inserted from the treatment tool insertion section  222  is extracted from an opening  245   a  through the treatment tool channel  245  of the distal end section  24 . 
     The universal cord  23  includes at least the light guide  241  and cable assembly  246 . The light guide  241  of the universal cord  23  is connected to the light source device  4 . The cable assembly  246  of the universal cord  23  is connected to the control device  3 . 
     Reference will be made below to a configuration of the control device  3 . The control device  3  includes an S/P converter  301 , an image processing unit  302 , a brightness detection unit  303 , a light controller  304 , a readout address setting unit  305 , a drive signal generation unit  306 , an input unit  307 , a storage unit  308 , a control unit  309 , and a reference clock generation unit  310 . 
     The S/P converter  301  applies serial/parallel conversion to the image signal (digital signal) received from the distal end section  24 . 
     The image processing unit  302  generates an in-vivo image to be displayed by the display device  5  based on the image signal of a parallel form output from the S/P converter  301 . The image processing unit  302  includes a synchronizer  302   a , a white balance (WB) adjustment unit  302   b , a gain adjustment unit  302   c , a γ correction unit  302   d , a D/A converter  302   e , a format change unit  302   f , and a sample memory  302   g  and a still image memory  302   h.    
     The synchronizer  302   a  inputs the image signal input as pixel information in three memories (not illustrated) provided for each pixel, sequentially updates and retains the values of the memories in association with the addresses of pixels of the light receiving unit  244   f  read by the reading unit  244   g , and synchronizes the image signals of the three memories as RGB image signals. The synchronizer  302   a  sequentially outputs the synchronized RGB image signals to the white balance adjustment unit  302   b  and outputs some RGB image signals as signals for image analysis such as brightness detection to the sample memory  302   g.    
     The white balance adjustment unit  302   b  automatically adjusts white balance of the RGB image signal. More specifically, the white balance adjustment unit  302   b  automatically adjusts the white balance of the RGB image signal based on a color temperature included in the RGB image signal. 
     The gain adjustment unit  302   c  adjusts a gain of the RGB image signal. The gain adjustment unit  302   c  outputs the RGB signal that has been subjected to the gain adjustment to the γ correction unit  302   d  and outputs some RGB signals as a signal for displaying a still image, a signal for displaying an enlarged image, or a signal for displaying an emphasized image to the still image memory  302   h.    
     The γ correction unit  302   d  performs gradation correction (γ correction) of the RGB image signal corresponding to the display device  5 . 
     The D/A converter  302   e  converts the RGB image signal that has been subjected to the gradation correction, which is output by the γ correction unit  302   d , into an analog signal. 
     The format change unit  302   f  converts the image signal converted into an analog signal into a moving image file format such as a high vision format and outputs the converted image signal to the display device  5 . 
     The brightness detection unit  303  detects a brightness level corresponding to each pixel from the RGB image signal retained by the sample memory  302   g  to record the detected brightness level in a memory provided inside the brightness detection unit  303  and to output the detected brightness level to the control unit  309 . In addition, the brightness detection unit  303  calculates an adjusted gain value and an amount of irradiation light based on the detected brightness level to output the adjusted gain value to the gain adjustment unit  302   c  and to output the amount of irradiation light to the light controller  304 . Further, the brightness detection unit  303  generates, based on the image signal generated by the imaging element  244  of the endoscope  2 , a brightness signal indicating the brightness and outputs the brightness signal to the control unit  309 . In the first embodiment, the brightness detection unit  303  functions as a brightness signal generation unit. 
     Under control of the control unit  309 , the light controller  304  sets a type, an amount, an emitting timing, and the like of the light generated by the light source device  4  based on the amount of irradiation light calculated by the brightness detection unit  303  and transmits a light source synchronization signal including the set conditions to the light source device  4 . 
     The readout address setting unit  305  has a function of setting pixels as reading targets and a reading order on a light receiving surface of the sensor unit  244   a . That is, the readout address setting unit  305  has a function of setting addresses of the pixels of the sensor unit  244   a  to be read by the AFE unit  244   b . In addition, the readout address setting unit  305  outputs set address information of the pixels as reading targets to the synchronizer  302   a.    
     The drive signal generation unit  306  generates a timing signal for driving the imaging element  244  and transmits the timing signal to the timing generator  244   d  through a specified signal line included in the cable assembly  246 . The timing signal includes address information of the pixels as reading targets. 
     The input unit  307  accepts an input of various signals such as an operation command signal that indicates operation of the endoscope system  1 . 
     The storage unit  308  is realized by using a semiconductor memory such as a flash memory or a dynamic random access memory (DRAM). The storage unit  308  stores therein various programs for operating the endoscope system  1  and data including various parameters and the like required for operation of the endoscope system  1 . 
     The control unit  309  is configured by using a CPU and the like. The control unit  309  performs drive control of components including the distal end section  24  and the light source device  4  and performs input and output control of information for the components. The control unit  309  transmits setting data for imaging control to the imaging controller  244   e  through a specified signal line included in the cable assembly  246 . The setting data includes: indicating information indicating an imaging speed (frame rate) of the imaging element  244  and a readout speed of the pixel information from arbitrary pixels of the sensor unit  244   a ; transmission control information of the pixel information that the AFE unit  244   b  reads; and the like. The control unit  309  includes an amplifier  309   a.    
     The amplifier  309   a  sets an amplification factor of the image signal that the imaging element  244  generates under irradiation with excitation light based on an amplification factor according to the brightness signal input from the brightness detection unit  303 . Specifically, the amplifier  309   a  sets, in the AGC unit  244   i , an amplification factor (gain) of the image signal that the imaging element  244  generates under irradiation with excitation light based on an amplification factor set in the AGC unit  244   i  according to the brightness signal that the brightness detection unit  303  generates based on the image signal that the imaging element  244  generates under irradiation with normal light (visible light). 
     The reference clock generation unit  310  generates a reference clock signal as a reference of operations of the components of the endoscope system  1  and supplies the generated reference clock signal to the components of the endoscope system  1 . 
     Reference will be made below to a configuration of the light source device  4 . The light source device  4  includes a white-light light source  41 , a special-light light source  42 , a light source controller  43 , and a light emitting diode (LED) driver  44 . 
     The white-light light source  41  is a white LED and generates white illumination light under control of the light source controller  43 . 
     The special-light light source  42  generates excitation light for exciting a fluorescent substance introduced into a subject. Specifically, the special-light light source  42  generates infrared light. Further, the special-light light source  42  may generate, as the special light, one of R-, G-, and B-component lights, each of which is a light different in wavelength band from the white illumination light and has been band-narrowed by a narrow band-pass filter. The special light that the special-light light source  42  generates may include narrow band imaging (NBI) illumination light of two kinds of bands, which are blue light and green light that have been band-narrowed so as to be easily absorbable by hemoglobin in blood. 
     The light source controller  43  controls an amount of current supplied to the white-light light source  41  or special-light light source  42  according to light source synchronization signal transmitted from the light controller  304 . 
     The LED driver  44  supplies current to the white-light light source  41  or special-light light source  42  under control of the light source controller  43  to make the white-light light source  41  or special-light light source  42  generate excitation light. The light generated by the white-light light source  41  or special-light light source  42  travels through the light guide  241  and is emitted outside from a distal end of the distal end section  24 . 
     The display device  5  has a function of receiving the in-vivo image generated by the control device  3  from the control device  3  through a video cable and displaying the in-vivo image. The display device  5  is provided with a liquid crystal display or an organic electro luminescence (EL) display. 
     Reference will be made below to processing to be executed for fluorescence observation of a subject in the thus configured endoscope system  1 .  FIG. 4  is a flowchart illustrating an outline of processing that the endoscope system  1  executes. In the following description, it is assumed that a fluorescence agent including a fluorescence marker has already been introduced into a living body of the subject by an operator using the treatment tool. 
     As illustrated in  FIG. 4 , the light source controller  43  drives the LED driver  44  under control of the control unit  309  to make the white-light light source  41  emit visible light (step S 101 ). 
     Subsequently, the imaging controller  244   e  drives the reading unit  244   g  at a given timing under control of the control unit  309  to output an image signal of visible reflected light received by the light receiving unit  244   f  to the control device  3  (step S 102 ). 
     Thereafter, the brightness detection unit  303  acquires the image signal of the visible light from the sample memory  302   g  and calculates an illuminance of each image in the light receiving unit  244   f  (step S 103 ), thereby generating a brightness signal of each pixel (step S 104 ). 
     Subsequently, based on the brightness signal generated by the brightness detection unit  303 , the amplifier  309   a  sets, in the AGC unit  244   i  of the AFE unit  244   b  through the imaging controller  244   e , an amplification factor of an image signal of visible light (normal light) output from each pixel of the light receiving unit  244   f  (step S 105 ). 
     Thereafter, the control unit  309  causes the image processing unit  302  to generate an image signal of while light and output the white light image signal to the display device  5  (step S 106 ). Specifically, when a normal observation mode based on the white light is set in the endoscope system  1 , the control unit  309  causes the image processing unit  302  to output the white light image signal to the display device  5 . This allows the operator to confirm the image displayed on the display device  5  and thereby to perform the normal observation of the subject. Further, the control unit  309  may make the image processing unit  302  store the white light image signal in the storage unit  308 . 
     Subsequently, according to the amplification factor of the visible light, the amplifier  309   a  sets, in the AGC unit  244   i  of the AFE unit  244   b  through the imaging controller  244   e , an amplification factor of a fluorescence image signal output from each image in the light receiving unit  244   f  (step S 107 ). 
     Thereafter, under control of the control unit  309 , the light source controller  43  drives the LED driver  44  to make the special-light light source  42  to emit excitation light (step S 108 ). 
     Subsequently, under control of the control unit  309 , the imaging controller  244   e  drives the reading unit  244   g  at a given timing to output the fluorescence image signal received by the light receiving unit  244   f  to the control device  3  (step S 109 ). 
     Thereafter, the control unit  309  causes the image processing unit  302  to generate the fluorescence image signal and output the generated fluorescence image signal to the display device  5  (step S 110 ). This allows the operator to confirm the fluorescence image displayed on the display device  5  and thereby to perform the fluorescence observation inside the subject. 
     Subsequently, the control unit  309  determines whether or not an instruction signal instructing end of the observation of the subject has been input from the input unit  307  (step S 111 ). When the control unit  309  determines that the instruction signal has been input (Yes in S 111 ), the endoscope system  1  ends this routine. On the other hand, when the control unit  309  determines that the instruction signal has not been input through the input unit  307  (No in S 111 ), the endoscope system  1  returns to step S 101 . 
     According to the first embodiment described above, the amplifier  309   a  sets the amplification factor of the fluorescence of each pixel based on the amplification factor of each image in the light receiving unit  244   f  according to the brightness signal of the image signal that the imaging element  244  generates under irradiation with the white light generated by the brightness detection unit  303 . This prevents a change in fluorescence intensity due to light distribution or influence of light absorption characteristics of an organ, thereby allowing the display device  5  to display a clear fluorescence image of the subject. 
     Further, according to the first embodiment, a region where reflected light obtained by irradiating body tissues with visible light is weak (i.e., dark region or bleeding region) is considered to be a region (location) where illumination light does not reach due to a long distance from the distal end section  24  or inadequate light distribution, and the same is applied to the excitation light. Thus, as described above, based on the amplification factor of each image in the light receiving unit  244   f  according to the brightness signal of the image signal that the imaging element  244  generates under irradiation with the white light generated by the brightness detection unit  303 , the amplifier  309   a  sets the amplification factor of the fluorescence of each pixel. As a result, it is possible to adequately perform observation of the fluorescence image without overlooking a fluorescence agent accumulation region. 
     Further, according to the first embodiment, even when agents having the same accumulation degree are observed, or even when a difference between intensity of the excitation light caused due to a distance from the distal end section  24  to the body tissues and intensity of the acquired fluorescence and a difference in optical characteristics among tissues surrounding the agent accumulation region are caused, the amplifier  309   a  optimizes an image-taking sensitivity with respect to each pixel such that a ratio between the amplification factor of the reflected light from the surrounding organ and amplification factor of the fluorescence is constant, thereby allowing observation of the fluorescence image to be adequately performed. In place of making the ratio constant, the amplifier  309   a  may multiply the amplification factor of the reflected light by a constant coefficient. 
     Further, in the first embodiment, the amplifier  309   a  sets the amplification factor of the fluorescence according to the amplification factor of the white light (visible light); alternatively, for example, when the fluorescence is infrared-ray (IR) light, a brightness signal corresponding to an image signal of red (R) included in the image signal may be used to set the amplification factor, followed by setting of the amplification factor of the fluorescence according to the set amplification factor. This reduces processing load of the control unit  309  to thereby increase a frame rate with which the endoscope  2  performs imaging operation. 
     Second Embodiment 
     Reference will be made below to a second embodiment of the present invention. An endoscope system according to the second embodiment only differs from the first embodiment in the configurations of the control unit of the control device and processing to be executed by the endoscope system. Specifically, in the above first embodiment, the amplification factor of each image is set upon the fluorescence observation; on the other hand, in the second embodiment, a region of the light receiving unit is divided into a plurality of regions, and the amplification factor upon the fluorescence observation is set for each divided region. Thus, hereinafter, a configuration of the control unit of the endoscope system according to the second embodiment is described first, followed by processing to be executed by the endoscope system. In the second embodiment, the same reference signs are given to the same elements as in the first embodiment. 
       FIGS. 5A and 5B  are a block diagram illustrating a functional configuration of a main part of an endoscope system  100  according to the second embodiment. As illustrated in  FIG. 5B , a control device  3  of the endoscope system  100  includes a control unit  101 . 
     The control unit  101  is configured by using a CPU and the like and performs drive control of components including the distal end section  24  and the light source device  4  and input and output control of information for the components. The control unit  101  transmits setting data for imaging control to the imaging controller  244   e  through a specified signal line included in the cable assembly  246 . The control unit  101  includes an amplifier  309   a  and a divided region setting unit  101   a.    
     The divided region setting unit  101   a  divides an imaging surface of the sensor unit  244   a  into specified regions to set a plurality of divided regions. Specifically, the divided region setting unit  101   a  divides the imaging surface of the sensor unit  244   a  through the readout address setting unit  305  to set a plurality of divided regions. The plurality of divided regions are used when the amplifier  309   a  to be described later sets the amplification factor in each divided region for the fluorescence observation. 
     Reference will be made below to processing to be executed by the endoscope system  100  having the above configuration.  FIG. 6  is a flowchart illustrating an outline of processing that the endoscope system  100  according to the second embodiment executes. 
     As illustrated in  FIG. 6 , the divided region setting unit  101   a  divides, through the imaging controller  244   e , a region of the light receiving unit  244   f  from which the reading unit  244   g  reads the image signal to set the divided regions (step S 201 ). For example, as illustrated in (a) of  FIG. 7 , the divided region setting unit  101   a  divides a light receiving surface of the sensor unit  244   a  by four to set divided regions A 1  to A 4 . 
     Steps S 202  and S 203  correspond respectively to steps S 101  and S 102  of above-mentioned  FIG. 4 . 
     Subsequently, the brightness detection unit  303  acquires the image signal of visible light from the sample memory  302   g , calculates an illuminance of each divided region of the light receiving unit  244   f  (step S 204 ), and generates a brightness signal of each divided region (step S 205 ). Specifically, as illustrated in (a) of  FIG. 7 , the brightness detection unit  303  calculates the illuminances of the respective divided regions A 1  to A 4  set by the divided region setting unit  101   a  and generates the brightness signals of the respective divided regions A 1  to A 4 . 
     Thereafter, based on the brightness signal of each divided region detected by the brightness detection unit  303 , the amplifier  309   a  sets, in the AGC unit  244   i  of the AFE unit  244   b  through the imaging controller  244   e , the amplification factor of visible light of the image signal output from each divided region of the light receiving unit  244   f  (step S 206 ). 
     Subsequently, the control unit  101  causes the image processing unit  302  to generate the image signal of the white light and output the generated image signal to the display device  5  (step S 207 ). 
     Thereafter, the divided region setting unit  101   a  further divides each divided region of the visible light to set small divided regions (step S 208 ). For example, as illustrated in (b) of  FIG. 7 , the divided region setting unit  101   a  further divides the divided region A 1  of the visible light to set small divided regions A 11  to A 14  of the excitation light. 
     Subsequently, the amplifier  309   a  sets the amplification factor of the fluorescence of each small divided region according to the amplification factor of each divided region of the visible light (step S 209 ). Specifically, the amplifier  309   a  sets the amplification factor of the fluorescence of the small divided regions A 11  to A 14  according to the amplification factor of the divided region A 1 . In this case, the amplifier  309   a  sets the amplification factor of the fluorescence of the small divided regions A 11  to A 14  based on an average value of the brightness signals of the respective pixels in the divided region A 1 . Thus, it is possible to set, in the divided regions A 1  to A 4 , an average value of intensities of reflected light of the visible light caused based on a distance from the endoscope  2  to an intra-body surface of the subject or optical characteristics of an organ and thereby to adjust a gain for making constant a brightness ratio between the average value and fluorescence in the small divided regions A 11  to A 14 , A 21  to A 24 , A 31  to A 34 , and A 41  to A 44 . 
     Steps S 210  to S 213  correspond respectively to steps S 108  and S 111  of  FIG. 4 . 
     According to the second embodiment described above, the amplifier  309   a  sets, for the divided regions set by the divided region setting unit  101   a , the amplification factor of the image signal that the imaging element  244  generates under irradiation with the excitation light. Thus, it is possible to set, in the divided regions, the average value of intensities of reflected light of the visible light caused based on a distance from the endoscope  2  to an intra-body surface of the subject or optical characteristics of an organ and thereby to adjust a gain for making constant a brightness ratio between the average value and fluorescence in the divided regions. 
     Further, according to the second embodiment, the divided region setting unit  101   a  further divides each divided region of the imaging surface of the sensor unit  244   a  to set the small divided regions, and the amplifier  309   a  sets the amplification factor of the image signal of the fluorescence corresponding to each small divided region, so that it is possible to adjust the gain of the fluorescence more finely. Thus, even when a distance between the distal end section  24  and body tissues is changed in the range of capturing, it is possible to adequately and evenly amplify the image signal on the imaging surface of the imaging element  244 , thereby allowing a smooth fluorescence image to be captured. 
     Further, according to the second embodiment, it is possible to obtain an image in which the fluorescence image emitted by irradiation of the excitation light is superimposed on the visible light (white light) image with a sufficient dynamic range or contrast even for a subject having a light distribution or an absorption distribution of the image. 
     In the second embodiment, the amplifier  309   a  sets the amplification factor of the image signal of the fluorescence corresponding to each divided region according to the amplification factor of the visible light; however, it is not necessary to set amplification factor for a divided region with a specified fluorescence intensity or more. This makes a surrounding region of the fluorescence corresponding to an affected part easy to see and reduces noise caused due to the amplification. 
     Further, in the second embodiment, the amplification factor of the image signal upon the fluorescence observation is set by adjusting the gain in the AGC unit  244   i . Alternatively, however, it is possible to adjust sensitivity or amplification factor by multiple readout without resetting each pixel of the sensor unit  244   a.    
     Further, in the second embodiment, the divided region setting unit  101   a  divides the imaging surface of the sensor unit  244   a  by four; however, the number of divisions may be set arbitrarily according to need. For example, the divided region setting unit  101   a  may divide the imaging surface of the sensor unit  244   a  into specified divided regions, e.g., nine divided regions according to a size of the imaging surface of the sensor unit  244   a  or depending on organ or site of the subject. Similarly, the divided region setting unit  101   a  may arbitrarily set the number of divisions of each divided region. 
     Third Embodiment 
     Reference will be made below to a third embodiment of the present invention. An endoscope system according to the third embodiment only differs in the configurations of the control unit of the control device and processing to be executed by the endoscope system. Specifically, in the third embodiment, only pixels in which the fluorescence is detected are subjected to exposure, and other pixels are exposed to visible light at a low gain. Thus, hereinafter, a configuration of the control unit of the endoscope system according to the third embodiment is described first, followed by processing to be executed by the endoscope system. In the third embodiment, the same reference signs are given to the same elements as in the above embodiments. 
       FIGS. 8A and 8B  are a block diagram illustrating a functional configuration of a main part of an endoscope system  110  according to the third embodiment. As illustrated in  FIG. 8B , a control device  3  of the endoscope system  110  includes a control unit  111 . 
     The control unit  111  is configured by using a CPU and the like and performs drive control of components including the distal end section  24  and the light source device  4  and input and output control of information for the components. The control unit  111  transmits setting data for imaging control to the imaging controller  244   e  through a specified signal line included in the cable assembly  246 . The control unit  111  includes an amplifier  309   a , a divided region setting unit  101   a , and a fluorescence determination unit  111   a.    
     The fluorescence determination unit  111   a  determines presence or absence of a fluorescence region that emits the fluorescence in an image corresponding to the image signal generated by the imaging element  244 . Specifically, the fluorescence determination unit  111   a  determines, upon the fluorescence observation, presence or absence of a region including a certain level of brightness in the image generated by the imaging element  244 . The fluorescence determination unit  111   a  may determine an address of the pixel corresponding to the fluorescence region that emits the fluorescence in the image. 
     Reference will be made below to processing to be executed by the endoscope system  110  having the above configuration.  FIG. 9  is a flowchart illustrating an outline of processing that the endoscope system  110  according to the third embodiment executes. 
     As illustrated in  FIG. 9 , steps S 301  to S 308  correspond respectively to steps S 201  to S 207  and step S 209  of  FIG. 6 . Further, steps S 309  to S 311  correspond respectively to steps S 108  to S 110  of  FIG. 4 . 
     After step S 311 , the fluorescence determination unit  111   a  determines whether or not there exists any divided region where the fluorescence is detected in the fluorescence image corresponding to the fluorescence image signal (step S 312 ). Specifically, the fluorescence determination unit  111   a  determines whether or not a divided region including a certain level of brightness in the fluorescence image. When the fluorescence determination unit  111   a  determines that there exists any divided region where the fluorescence is detected in the fluorescence image corresponding to the fluorescence image signal (Yes in step S 312 ), the endoscope system  1  shifts to step S 313  to be described later. On the other hand, when the fluorescence determination unit  111   a  determines that there exists no divided region where the fluorescence is detected in the fluorescence image corresponding to the fluorescence image signal (No in step S 312 ), the endoscope system  1  shifts to step S 317  to be described later. 
     In step S 313 , the divided region setting unit  101   a  divides the divided region where the fluorescence is detected into predetermine regions to set small divided regions. Specifically, as illustrated in  FIG. 10 , the divided region setting unit  101   a  divides a divided region A 3  where fluorescence S 1  is detected into specified regions to set small divided regions A 31  to A 34  ( FIG. 10 : (a)→(b)). Subsequently, the divided region setting unit  101   a  sets, through the imaging controller  244   e , a region of the light receiving unit  244   f  to be subjected to exposure. Specifically, the divided region setting unit  101   a  sets, through the imaging controller  244   e , a region corresponding to the small divided region where the fluorescence S 1  is detected as a region corresponding to the pixel of the light receiving unit  244   f  that receives the fluorescence for exposure. 
     Thereafter, the light source controller  43  drives the LED driver  44  under control of a control unit  111  to make the special-light light source  42  emit the excitation light (step S 314 ), and amplifier  309   a  causes only the pixels constituting the small divided region set by the divided region setting unit  101   a  to receive the fluorescence for exposure (step S 315 ). Thus, the sensor unit  244   a  only needs to expose only the pixel constituting the small divided region, enabling long-time exposure, which can increase the amplification factor (sensitivity) of the fluorescence. As a result, the endoscope system  1  exposes only the necessary pixels of the light receiving unit  244   f  for a long time to thereby allow the fluorescence image signal to be output continuously and smoothly as compared to a case where all the pixels constituting the light receiving unit  244   f  are subjected to long time exposure, without decreasing a set frame rate and without prolonging a time required to perform a series of imaging (exposure) operations. 
     Subsequently, the image processing unit  302  applies specified image processing to the image signal output from the imaging element  244  of the distal end section  24  to generate the fluorescence image signal and outputs the generated fluorescence image signal to the display device  5  (step S 316 ). 
     Thereafter, the control unit  111  determines whether or not an end instruction signal has been input to the endoscope system  1  (step S 317 ). When the control unit  111  determines that the end instruction signal has been input (Yes in S 317 ), the endoscope system  1  ends this routine. 
     On the other hand, when the control unit  111  determines that the end instruction signal has not been input (No in S 317 ), the endoscope system  1  returns to step S 302 . In this case, as illustrated in  FIG. 10 , the divided region setting unit  101   a  may further divide the small divided region where the fluorescence S 1  is detected into specified regions until the end instruction signal is input to set minute divided regions A 321  to A 324  ( FIG. 10 : (b)→(c)), and then the amplifier  309   a  may set the amplification factor (sensitivity) for imaging of the fluorescence or long time exposure. 
     According to the third embodiment described above, the amplifier  309   a  causes only the pixels constituting the small divided region set by the divided region setting unit  101   a  to receive the fluorescence for exposure. Thus, the sensor unit  244   a  only needs to expose only the pixel constituting the small divided region, enabling long-time exposure, which can increase the amplification factor (sensitivity) of the fluorescence. As a result, the endoscope system  1  exposes only the necessary pixels of the light receiving unit  244   f  for a long time to thereby allow the fluorescence image signal to be output continuously and smoothly as compared to a case where all the pixels constituting the light receiving unit  244   f  are subjected to long time exposure, without decreasing a set frame rate and without prolonging a time required to perform a series of imaging (exposure) operations. 
     Further, in the third embodiment, a region where the fluorescence is detected by previous irradiation of the excitation light may be divided into specified regions to set the divided regions. That is, a configuration may be adopted in which high-sensitivity imaging is performed in a small region of interest. This allows the fluorescence image to be captured with a sufficient dynamic range or contrast. 
     Fourth Embodiment 
     Reference will be made below to a fourth embodiment. An endoscope system according to the fourth embodiment has the same configuration as those according to the above respective embodiments and only differs in processing to be executed by the endoscope system. Specifically, in the above embodiments, each pixel included in the divided region where the fluorescence is detected is subjected to exposure for a long time to increase imaging sensitivity of the fluorescence; on the other hand, in the fourth embodiment, a region where the amplification factor is increased is made gradually smaller from the entire region toward the region where the fluorescence is detected. Thus, hereinafter, processing to be executed by the endoscope system according to the fourth embodiment will be described. In the following description, the region where the fluorescence is detected is referred to as a region of interest (hereinafter, abbreviated as “ROI”). 
       FIG. 11  is a time chart of processing to be executed by the endoscope system  1  according to the fourth embodiment. In  FIG. 11 , (a) of  FIG. 11  illustrates a timing of regions to be output from the light receiving unit  244   f , and (b) of  FIG. 11  illustrates a timing at which reset is instructed for the pixels constituting the light receiving unit  244   f . In  FIG. 11 , a horizontal axis indicates time. 
     As illustrated in  FIG. 11 , based on a determination result from the fluorescence determination unit  111   a , the divided region setting unit  101   a  causes the imaging controller  244   e  to read out and output the regions of the light receiving unit  244   f  in the order of the entire region, ROI 1 , ROI 2 , ROI 3 , and ROI 4 . Specifically, as illustrated in  FIG. 12 , based on a determination result from the fluorescence determination unit  111   a , the divided region setting unit  101   a  causes the imaging controller  244   e  to read out and output the regions of the light receiving unit  244   f  in the order of the entire region, R 1 , ROI 1 , ROI 2 , ROI 3 , and ROI 4  such that the region where the fluorescence S 1  corresponding to an affected part is detected becomes gradually smaller in an image W 1 . At this time, the images corresponding to the ROIs have the same center coordinates and sizes thereof are gradually reduced (ROI 1 &gt;ROI 2 &gt;ROI 3 &gt;ROI 4 ). As a result, the number of times of exposure becomes maximum at a superimposed portion of the ROIs, resulting in high sensitivity. However, the sensitivity is gradually decreased, eliminating feeling of difference from a surrounding image. 
     According to the fourth embodiment described above, based on a determination result from the fluorescence determination unit  111   a , the divided region setting unit  101   a  makes a region of the light receiving surface of the light receiving unit  244   f  where the amplification factor is increased gradually smaller toward the region where the fluorescence S 1  is detected. Thus, it is possible to obtain a smooth image in which the sensitivity is naturally increased without decreasing a frame rate of the imaging element  244 . 
     Further, according to the fourth embodiment, the amplifier  309   a  sets, in a gradient manner, the sensitivity of a boundary of the region having the fluorescence, so that a feeling of strangeness of the image can be reduced. 
     Further, according to the fourth embodiment, even when there is any unnecessary high brightness region other than the ROI region, it is possible to reduce a region where the sensitivity is saturated and to prevent occurrence of malfunction, such as blackout, due to automatic control of brightness. 
     Fifth Embodiment 
     Reference will be made below to a fifth embodiment. In an endoscope system according to the fifth embodiment, two imaging elements are provided at the distal end section of the endoscope. Thus, hereinafter, only a configuration different from those of the endoscope systems according to the above respective embodiments will be described. In the fifth embodiment, the same reference sings are given to the same elements as in the above embodiments. 
       FIG. 13  is a view illustrating a distal end surface of a distal end section of the endoscope system according to the fifth embodiment.  FIG. 14  is a view illustrating a part of a cut-out surface obtained by cutting the distal end section along a line A-A of  FIG. 13 .  FIG. 15  is a view illustrating a part of a cut-out surface obtained by cutting the distal end section along a line B-B of  FIG. 13 .  FIGS. 16A and 16B  are a block diagram illustrating a functional configuration of a main part of the endoscope system according to the fifth embodiment. 
     As illustrated in  FIGS. 13 to 16 , in the distal end surface of a distal end section  6  of an endoscope  2 , a washing nozzle  61 , an observation window  62 , an observation window  63 , an illumination lens  242 , and a treatment tool channel  245  are provided. 
     The observation windows  62  and  63  are closed. Light incident from outside to the observation window  62  is incident to a first optical system  621 A constituted by a plurality of lenses  621   a  and  621   b , collected, and incident to a first light receiving unit  622 A. Light incident to the observation window  63  is incident to a second optical system  631 B constituted by a plurality of lenses  631   c  and  631   d , collected, and incident to a second light receiving unit  632 B. 
     The first light receiving unit  622 A has a plurality of imaging pixels arranged in a two-dimensional matrix and is disposed such that light emitted from the first optical system  621 A is incident thereto. The first light receiving unit  622 A receives light incident thereto through the first optical system  621 A to capture a body cavity. A cover glass  623 A is provided on a light receiving surface side of the first light receiving unit  622 A. An on-chip filter  624 A in which red (R), green (G), or blue (B) filters are arranged corresponding to an arrangement of pixels constituting the first light receiving unit  622 A is provided between the cover glass  623 A and first light receiving unit  622 A, whereby the first light receiving unit  622 A captures a color image. The on-chip filter  624 A may be a complementary color filter in which cyan, magenta, yellow, and green filters are arranged. 
     The second light receiving unit  632 B has a plurality of imaging pixels arranged in a two-dimensional matrix and is disposed such that light emitted from the second optical system  631 B is incident thereto. A spectral filter  633  that transmits only light of a specified wavelength band and a cover glass  634 B are provided on a light receiving surface side of the second light receiving unit  632 B. The second light receiving unit  632 B has characteristics of capturing an image for fluorescence observation corresponding to fluorescence of a specified wavelength band as a monochrome image. 
     The first light receiving unit  622 A and second light receiving unit  632 B are implemented on a circuit substrate  66  together with a driver  64  that instructs an imaging timing to the first light receiving unit  622 A and second light receiving unit  632 B and supplies power thereto, a conversion circuit  65  that reads out an image signal of the first light receiving unit  622 A and second light receiving unit  632 B and converts the image signal into an electric signal, and the like. The first and second light receiving units  622 A and  632 B are implemented on the circuit substrate  66  with their light receiving surfaces juxtaposed left and right. An electrode  67  is provided on the circuit substrate  66 . The electrode  67  is connected to a signal line  68  for exchanging an electric signal with the control device  3  through, e.g., an anisotropic conductive resin film. A cable assembly is formed by a plurality of signal lines including a signal line  67   a  for transmitting an image signal which is an electric signal output from the first and second light receiving units  622 A and  632 B and a signal line for transmitting a control signal from the control device  3 . 
     A synchronizer  121  sets pixels to be read out from the first and second light receiving units  622 A and  632 B such that first and second light receiving units  622 A and  632 B alternately read out image information. The synchronizer  121  controls timings of exposure processing performed in the first and second light receiving units  622 A and  632 B and pixel information readout processing from the first and second light receiving units  622 A and  632 B by the timing generator  244   d  and AFE unit  244   b  in correlation with each other. The pixel information read out from the first and second light receiving units  622 A and  632 B is transmitted by the same transmission path. 
     In an endoscope system  120  thus configured, the brightness detection unit  303  generates the brightness signal based on the image signal captured by the first light receiving unit  622 A, and the amplifier  309   a  sets, based on the amplification factor according to the brightness signal generated by the brightness detection unit  303 , the amplification factor of the image signal that the second light receiving unit  632 B generates under irradiation with the excitation light. This allows the endoscope system  120  to substantially simultaneously adjust sensitivity of imaging of the excitation light with imaging operation of the normal light (visible light). 
     According to some embodiments, based on the amplification factor according to the brightness signal of the image signal generated by the imaging element under irradiation with the normal light, the brightness signal being generated by the brightness signal generation unit, the amplifier sets the amplification factor of the fluorescence of each pixel. This allows observation of the fluorescence image to be adequately performed without overlooking emitted fluorescence. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.