Patent Publication Number: US-8995737-B2

Title: Image processing apparatus and image processing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image processing apparatus and an image processing method. 
     2. Description of the Related Art 
     An optical coherence tomography (OCT) technique using interference of multi-wavelength light enables acquisition of a high-resolution tomographic image of a sample (in particular, a fundus of the eye). 
     In recent years, an ophthalmologic OCT apparatus can acquire, in addition to a normal OCT image of a shape of a fundus tissue, a polarization-sensitive OCT image using a polarization parameter (i.e., retardation and orientation), which is an optical characteristic of the fundus tissue. 
     The polarization-sensitive OCT can form the polarization-sensitive OCT image using the polarization parameter, and perform distinguishing and segmentation of the fundus tissue. International Publication No. WO 2010/122118 A1 discusses a polarization-sensitive OCT which employs a light beam that has been modulated to a circularly-polarized beam as a measuring beam for examining the sample. Detection is then performed by splitting the interference beam into two linearly-polarized beams perpendicular to each other, so that the polarization-sensitive OCT image is generated. 
     However, International Publication No. WO 2010/122118 A1 does not discuss performing diagnosis support, i.e., providing easily-comparable images, which is the original objective of the polarization-sensitive OCT. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image processing apparatus capable of providing images so that a user can efficiently confirm the image indicating a polarization state. Such operational advantage is derived by each of configurations illustrated in exemplary embodiments of the present invention to be described below, and operational advantages that are not acquirable by conventional techniques are also included in the present invention. 
     According to an aspect of the present invention, an image processing apparatus includes a planar image acquisition unit configured to acquire a planar image of a subject, a tomographic image acquisition unit configured to acquire a tomographic image indicating a polarization state of the subject based on beams of different polarizations obtained by splitting a beam into which a return beam from the subject irradiated with a measuring beam and a reference beam corresponding to the measuring beam have been combined, and a display control unit configured to cause, in a case where a designation unit designates a portion of the tomographic image indicating the polarization state displayed on a display unit, position information indicating a position corresponding to the portion designated by the designation unit to be displayed on the planar image displayed on the display unit. 
     According to an exemplary embodiment of the present invention, the images can be displayed so that the user can effectively confirm the image indicating a polarization state. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic diagram illustrating an image processing apparatus according to an exemplary embodiment of the present invention. 
         FIGS. 2A ,  2 B,  2 C,  2 D, and  2 E illustrate examples of images generated by a signal processing unit. 
         FIG. 3  is a flowchart illustrating a process according to the present exemplary embodiment. 
         FIG. 4  illustrates a display example of a display screen on a display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 5  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 6  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 7  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 8  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 9  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 10  illustrates an example of a two-dimensional layer thickness map displayed on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 11  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 12  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
         FIG. 13  illustrates a display example of the display screen on the display unit in the image processing apparatus according to the present exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     An imaging apparatus according to an exemplary embodiment of the present invention is applicable to a subject such as a subject&#39;s eye, skin, and an internal organ. Further, the imaging apparatus according to the present exemplary embodiment may be an ophthalmologic apparatus or an endoscope. 
       FIG. 1  is a schematic diagram illustrating an ophthalmologic apparatus, which is an example of the imaging apparatus according to the present exemplary embodiment. At least a portion of a signal processing unit  190  to be described below may be considered as an image processing apparatus. In such a case, the ophthalmologic apparatus may be considered as an ophthalmologic system, or the imaging apparatus may be considered as an imaging system. 
     Referring to  FIG. 1 , the ophthalmologic apparatus includes a polarization-sensitive OCT (PS-OCT)  100 , a polarization-sensitive scanning laser ophthalmoscope (PS-SLO)  140 , an anterior segment imaging unit  160 , an internal fixation lamp  170 , and a control unit  200 . 
     The ophthalmologic apparatus is aligned by lighting and causing the subject&#39;s eye to gaze at the internal fixation lamp  170 , and using the image of the anterior segment of the subject captured by the anterior segment imaging unit  160 . After completing the alignment, the PS-OCT  100  and the PS-SLO  140  perform imaging of the fundus. 
     The configuration of the PS-OCT  100  will be described below. 
     A light source  101  made of a super luminescent diode (SLD), i.e., a low-coherent light source, emits light having a central wavelength of 850 nm and a bandwidth of 50 nm. Any light source that can emit low coherent light, such as an amplified spontaneous emission (ASE) light source, may be used as the light source  101 . 
     The light emitted from the light source  101  is guided by a single mode (SM) fiber  102  and a polarization controller  103  to a fiber coupler  104  having a polarization preserving function. The beam is then split into a measuring beam (hereinafter referred to as a “measuring beam for a tomographic image”, or an “OCT measuring beam”) and a reference beam corresponding to the measuring beam. 
     The polarization controller  103  adjusts the polarization state of the beam emitted from the light source  101 , and adjusts the beam to a linearly-polarized beam. A branching ratio of the fiber coupler  104  is 90 (reference beam):10 (measuring beam). 
     The measuring beam is output from a collimator  106  via a polarization maintaining (PM) fiber  105  as a parallel beam, and is converged by a lens  181 . The measuring beam which has passed through the lens  181  reaches a dichroic mirror  111  via an X scanner  107 , lenses  108  and  109 , and a Y scanner  110 . The X scanner  107  includes a galvano mirror that scans the measuring beam in a horizontal direction on a fundus Er, and the Y scanner  110  includes a galvano mirror that scans the measuring beam in a vertical direction on the fundus Er. The X scanner  107  and the Y scanner  110  are controlled by a drive control unit  180 , and are capable of scanning the measuring beam in a desired range on the fundus Er. The range in which the measuring beam is scanned on the fundus may be considered as an acquisition range of the tomographic image, an acquisition position of the tomographic image, and an irradiation position of the measuring beam. Further, the X scanner  107  and the Y scanner  110  are examples of a scanning unit for PS-OCT, and may be configured as a common XY scanner. The dichroic mirror  111  reflects light having wavelengths of 800 nm to 900 nm, and transmits light of other wavelengths. 
     The measuring beam reflected off the dichroic mirror  111  passes through, via a lens  112 , a λ/4 polarizing plate  113  arranged to be inclined at an angle of 45° from a P-polarization to an S-polarization with respect to an optical axis as a rotational axis. The phase of the beam is thus shifted by 90°, and is polarized to a circularly-polarized beam. The λ/4 polarizing plate  113  is an example of a polarization adjustment member for the measuring beam, which adjusts the polarization state of the measuring beam. If a PS-SLO optical system to be described below is to be applied, the λ/4 polarizing plate  113  may be disposed in a common optical path between a portion of the PS-OCT optical system and a portion of the PS-SLO optical system. As a result, variation in the polarization states generated in the images acquired by the PS-OCT optical system and the PS-SLO optical system can be reduced. In such a case, the scanning unit for the PS-SLO and the scanning unit for the PS-OCT are arranged in mutually-conjugate positions, and may be arranged to be conjugate with a pupil in the subject&#39;s eye. The inclination of the λ/4 polarizing plate  113  is an example of a state of the λ/4 polarizing plate  113 , and is an angle from a predetermined position in a case where the optical axis of a polarizing beam splitting surface of a fiber coupler  123  including a polarizing beam splitter is the rotating axis. 
     Further, the λ/4 polarizing plate  113  may be inserted to and removed from the optical path. For example, the λ/4 polarizing plate  113  may be mechanically configured to rotate around the optical axis or an axis parallel to the optical axis as the rotational axis. As a result, a compact apparatus capable of easily switching between the SLO optical system and the PS-SLO optical system can be realized. Further, a compact apparatus capable of easily switching between the OCT optical system and the PS-OCT optical system can be realized. 
     The beam incident on the subject&#39;s eye is thus polarized to a circularly-polarized beam by arranging the λ/4 polarizing plate  113  to be inclined at an angle of 45°. However, the beam may not become a circularly-polarized beam on the fundus Er due to the characteristic of the subject&#39;s eye. To solve such a problem, the drive control unit  180  can perform control to finely adjust the inclination of the λ/4 polarizing plate  113 . 
     A focus lens  114  mounted on a stage  116  focuses, on retinal layers in the fundus Er via an anterior segment Ea of the subject&#39;s eye, the measuring beam polarized to a circularly-polarized beam. The measuring beam irradiating the fundus Er is reflected and scattered by each retinal layer, and returns to the fiber coupler  104  via the above-described optical path. 
     On the other hand, the reference beam branched by the fiber coupler  104  is output as a parallel beam from a collimator  118  via a PM fiber  117 . The output reference beam is polarized by a λ/4 polarizing plate  119  arranged to be inclined at an angle of 22.5° from the P-polarization to the S-polarization with the optical axis as the rotational axis, similarly as the measuring beam. The λ/4 polarizing plate  119  is an example of the polarization adjustment member for the reference beam, which adjusts the polarization state of the reference beam. The reference beam is reflected via a dispersion compensation glass  120  by a mirror  122  mounted on a coherence gate stage  121 , and returns to the fiber coupler  104 . The reference beam passes through the λ/4 polarizing plate  119  twice, so that the linearly-polarized beam returns to the fiber coupler  104 . 
     The coherence gate stage  121  is controlled by the drive control unit  180  to deal with differences in an axial length of the subject&#39;s eye. The coherence gate is the position corresponding to an optical path length of the reference beam in the optical path of the measuring beam. According to the present exemplary embodiment, the optical path length of the reference beam is changeable. However, it is not limited thereto, as long as the difference in the optical path lengths of the measuring beam and the reference beam can be changed. 
     The return beam and the reference beam that have returned to the fiber coupler  104  are combined into an interference beam (hereinafter also referred to as a combined beam). The interference beam becomes incident on the fiber coupler  123  including the polarizing beam splitter, and is split at the branching ratio of 50:50 into a P-polarized beam and an S-polarized beam of different polarization directions. 
     The P-polarized beam is dispersed by a grating  131  via a PM fiber  124  and a collimator  130 , and is received by a lens  132  and a line camera  133 . The S-polarized beam is similarly dispersed by a grating  127  via a PM fiber  125  and a collimator  126 , and is received by a lens  128  and a line camera  129 . The gratings  127  and  131  and the line cameras  129  and  133  are arranged to match the direction of each polarization direction. 
     The beam received by each of the line cameras  129  and  133  is output as an electrical signal corresponding to the light intensity. The signal processing unit  190 , which is an example of a tomographic image generation unit, then receives the output electrical signals. 
     The inclinations of the λ/4 polarizing plates  113  and  119  can be automatically adjusted based on the inclination of the polarizing beam splitter. The inclinations of the λ/4 polarizing plates  113  and  119  can also be automatically adjusted with respect to a line connecting centers of an optic disk and a macula in the fundus. In such a case, it is desirable for an inclination detection unit (not illustrated) to detect the inclinations of the λ/4 polarizing plates  113  and  119 . The inclination detection unit can detect the current inclination and detect whether the inclination has reached a predetermined inclination. 
     The configuration of the PS-SLO  140  will be described below. 
     According to the present exemplary embodiment, a light source  141 , i.e., a semiconductor laser, emits a light beam having a central wavelength of 780 nm. The measuring beam emitted from the light source  141  (hereinafter also referred to as a measuring beam for a fundus image, or an SLO measuring beam) is polarized via a PM fiber  142  by a polarizing controller  145  to a linearly-polarized beam, and is output from a collimator  143  as a parallel beam. The output measuring beam then passes through a perforated portion of a perforated mirror  144 , and reaches, via a lens  155 , a dichroic mirror  154  via an X scanner  146 , lenses  147  and  148 , and a Y scanner  149 . The X scanner  146  includes a galvano mirror that scans the measuring beam in the horizontal direction on the fundus Er, and the Y scanner  149  includes a galvano mirror that scans the measuring beam in the vertical direction on the fundus Er. The X scanner  146  and the Y scanner  149  are controlled by the drive control unit  180 , and are capable of scanning the measuring beam in the desired range on the fundus Er. Further, the X scanner  146  and the Y scanner  149  are examples of a scanning unit for the PS-SLO, and may be configured as a common XY scanner. The dichroic mirror  154  reflects light having wavelengths of 760 nm to 800 nm, and transmits light of other wavelengths. 
     The linearly-polarized measuring beam reflected by the dichroic mirror  154  reaches the fundus Er via the optical path similar to that of the PS-OCT  100 . 
     The measuring beam irradiating the fundus Er is reflected and scattered by the fundus Er, and reaches the perforated mirror  144  via the above-described optical path. The beam reflected by the perforated mirror  144  is then split by a polarizing beam splitter  151  via the lens  150  into beams of different polarization directions (i.e., according to the present exemplary embodiment, split into a P-polarized beam and an S-polarized beam). The split beams are received by avalanche photodiodes (APD)  152  and  153 , converted into electrical signals, and received by the signal processing unit  190 , i.e., an example of the fundus image generation unit. 
     The position of the perforated mirror  144  is conjugate with the position of the pupil in the subject&#39;s eye. The perforated mirror  144  reflects the light that has passed through a peripheral region of the pupil among the light reflected and scattered by the fundus Er irradiated with the measuring beam. 
     According to the present exemplary embodiment, both the PS-OCT and the PS-SLO use an SM fiber. However, a similar configuration and effect may be acquired by using a PM fiber in a case where the polarizing controller controls polarization. Further, both the SM fiber and the PM fiber may be used together. For example, the SM fiber may be used to connect the light source  101  and the polarization controller  103 , and the PM fiber may be used to connect the polarization controller  103  and the fiber coupler  104 . Furthermore, the SM fiber may be used to connect the light source  141  and the polarization controller  145 , and the PM fiber may be used to connect the polarization controller  145  and the collimator  143 . 
     The anterior segment imaging unit  160  will be described below. 
     The anterior segment imaging unit  160  irradiates the anterior segment Ea using an irradiation light source  115  including light emitting diodes (LED)  115 - a  and  115 - b , which emit irradiation light having a wavelength of 1000 nm. The light reflected by the anterior segment Ea reaches a dichroic mirror  161  via the lens  114 , the polarizing plate  113 , the lens  112 , and the dichroic mirrors  111  and  154 . The dichroic mirror  161  reflects light having wavelengths of 980 nm to 1100 nm, and transmits light of other wavelengths. The light reflected by the dichroic mirror  161  is then received by an anterior segment camera  165  via lenses  162 ,  163 , and  164 . The light received by the anterior segment camera  165  is converted into an electrical signal and is received by the signal processing unit  190 . 
     The internal fixation lamp  170  will be described below. 
     The interior fixation lamp  170  includes an interior fixation lamp display unit  171  and a lens  172 . A plurality of LEDs arranged in a matrix shape is used as the interior fixation lamp display unit  171 . A lighting position of the LED is changed by control performed by the drive control unit  180  according to a region to be imaged. The light emitted from the interior fixation lamp display unit  171  is guided to the subject&#39;s eye via the lens  172 . The interior fixation lamp display unit  171  emits light having a wavelength of 520 nm, and the drive control unit  180  displays a desired pattern. 
     A control unit  200  for controlling the entire apparatus according to the present exemplary embodiment will be described below. 
     The control unit  200  includes the drive control unit  180 , the signal processing unit  190 , a display control unit  191 , and a display unit  192 . 
     The drive control unit  180  controls each unit as described above. 
     The signal processing unit  190  generates images based on the signals output from the line cameras  129  and  133 , the APD  152  and  153 , and the anterior segment camera  165 , analyzes the generated images, and generates visualization information of the analysis results. The image generation process will be described in detail below. 
     The display control unit  191  displays, on a display screen in the display unit  192 , the images generated by a tomographic image generation unit and a fundus image generation unit and acquired by a fundus image acquisition unit (not illustrated) and a tomographic image acquisition unit (not illustrated). The display unit  192  may be a liquid crystal display. The image data generated by the signal processing unit  190  may be transmitted to the display control unit  191  via wired or wireless communication. In such a case, the display control unit  191  may be considered as the image processing apparatus. Further, the fundus image acquisition unit may include the SLO optical system, and the tomographic image acquisition unit may include the OCT optical system, as the imaging system. According to the present exemplary embodiment, if the subject is other than the subject&#39;s eye, the fundus image (i.e., a fundus intensity image) may be indicated as a planar image (i.e., a plane intensity image), and the fundus image acquisition unit as a planar image acquisition unit. 
     The display unit  192  displays display forms indicating various types of information to be described below based on control performed by the display control unit  191 . Image data from the display control unit  191  may be transmitted to the display unit  192  via wired or wireless communication. Further, the display unit  192  is included in the control unit  200 . However, it is not limited thereto, and the display unit  192  may be separated from the control unit  200 . Furthermore, a tablet, which is an example of a portable device, configured by integrating the display control unit  191  and the display unit  192  may be used. In such a case, it is desirable to include a touch panel function in the display unit, so that the user can operate the touch panel to move the display position of the images, enlarge and reduce the images, and change the images to be displayed. 
     Image generation and image analysis processes performed in the signal processing unit  190  will be described below. 
     The signal processing unit  190  performs, on interference signals output from each of the line cameras  129  and  133 , reconfiguration processing employed in a common spectral domain (SD-) OCT. The signal processing unit  190  thus generates a tomographic image corresponding to a first polarized beam and a tomographic image corresponding to a second polarized beam, i.e., two tomographic images based on each polarization component. 
     More specifically, the signal processing unit  190  performs fixed pattern noise cancellation on the interference signals. The fixed pattern noise cancellation is performed by averaging a plurality of A-scan signals that has been detected and thus extracting the fixed pattern noise, and subtracting the extracted fixed pattern noise from the input interference signal. 
     The signal processing unit  190  then transforms the wavelength of the interference signal to a wave number, and performs Fourier transform, so that a tomographic signal indicating a polarization state is generated. 
     The signal processing unit  190  performs the above-described process for the interference signals of the two polarization components, and thus generates the two tomographic images. 
     Further, the signal processing unit  190  aligns the signals output from the APD  152  and  153  in synchronization with driving of the X scanner  146  and the Y scanner  149 . The signal processing unit  190  thus generates a fundus image corresponding to the first polarized beam and a fundus image corresponding to the second polarized beam, i.e., two fundus images based on respective polarization components. 
     Furthermore, the signal processing unit  190  generates a tomographic intensity image from the above-described two tomographic signals. 
     More specifically, the tomographic intensity image is basically the same as the tomographic image in the conventional OCT. A pixel value r of the tomographic intensity image is calculated from tomographic signals A H  and A V  acquired from each of the line cameras  129  and  133  using equation (1).
 
 r =√{square root over ( A   H   2   +A   V   2 )}  (1)
 
     Moreover, the signal processing unit  190  similarly generates a fundus intensity image from the two fundus images. 
       FIG. 2A  illustrates an example of the intensity image of the optic disk. 
     When the λ/4 polarizing plate  113  is removed from the optical path, the display control unit  191  may display, on the display unit  192 , the tomographic intensity image acquired employing the conventional OCT technique, or the fundus intensity image acquired employing the conventional SLO technique. 
     Further, the signal processing unit  190  generates a retardation image from the tomographic images of the polarization components that are perpendicular to each other. 
     A value δ of each pixel in the retardation image is a value indicating a ratio of the effects received by the vertical polarization component and the horizontal polarization component in the subject&#39;s eye, at the position of each pixel configuring the tomographic image. The value δ is calculated from each of the tomographic signals A H  and A V  using equation (2). 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     arctan 
                     ⁡ 
                     
                       [ 
                       
                         
                           A 
                           V 
                         
                         
                           A 
                           H 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
       FIG. 2B  illustrates an example of the retardation image of the optic disk generated as described above. The retardation image can be acquired by calculating equation (2) for each B-scan image. As described above, the retardation image is a tomographic image indicating the difference of the effect received by the two polarized beams in the subject&#39;s eye. Referring to  FIG. 2B , the values indicating the above-described ratio are displayed as a color tomographic image. A darker-shaded portion indicates that the value of the ratio is small, and a lighter-shaded portion indicates that the value of the ratio is large. As a result, generating the retardation image enables recognizing a layer in which there is birefringence. For further details, refer to “E. Gotzinger et al., Opt. Express 13, 10217, 2005”. 
     Furthermore, the signal processing unit  190  can similarly generate a retardation image in a planar direction of the fundus based on the outputs from the APD  152  and  153 . 
     The signal processing unit  190  generates a retardation map from the retardation images acquired with respect to a plurality of B-scan images. 
     More specifically, the signal processing unit  190  detects a retinal pigment epithelium (RPE) in each B-scan image. Since the RPE cancels polarization, the signal processing unit  190  searches for a retardation distribution of each A-scan along the depth direction in the range from an inner limiting membrane (ILM) without including the RPE. The signal processing unit  190  then sets a maximum value of the retardation as a representative value of the retardation in the A-scan. 
     The signal processing unit  190  performs the above-described process on all retardation images, and thus generates the retardation map. 
       FIG. 2C  illustrates an example of the retardation map of the optic disk. Referring to  FIG. 2C , the darker-shaded portion indicates that the value of the ratio is small, and the lighter-shaded portion indicates that the value of the ratio is large. The layer having birefringence in the optic disk is a retinal nerve fiber layer (RNFL), and the retardation map illustrates the value indicating the ratio caused by the birefringence in the RNFL and the thickness of the RNFL. As a result, the value indicating the ratio becomes large where the RNFL is thick, and becomes small where the RNFL is thin. The thickness of the RNFL for the entire fundus thus becomes recognizable using the retardation map, and can be used in the diagnosis of glaucoma. 
     The signal processing unit  190  performs linear approximation of the value of the retardation  5  in the range of ILM to RNFL in each A scan image of the previously generated retardation image. The signal processing unit  190  then determines the acquired slope as the birefringence at the position on the retina in the A-scan image. In other words, since retardation is a product of a distance and the birefringence in the RNFL, a linear relation is acquired by plotting the depth and the value of the retardation in each A-scan image. As a result, linear approximation is performed on the plot using a least square method, and the acquired slope becomes the value of the birefringence in the RNFL in the A-scan image. The signal processing unit  190  performs the above-described process on all of the acquired retardation images, and generates the map representing the birefringence. 
       FIG. 2D  illustrates an example of the birefringence map of the optic disk. The birefringence map directly maps the values of the birefringence. As a result, when a fibrous architecture of the RNFL changes even if the thickness of the RNFL does not change, the change can be visualized as the change in the birefringence. 
     The signal processing unit  190  calculates a Stokes vector S for each pixel from the acquired tomographic signals A H  and A V , and a phase difference ΔΦ between the tomographic signals A H  and A V , using equation (3). 
                   S   =       (         I           Q           U           V         )     =     (             A   H   2     +     A   V   2                   A   H   2     -     A   V   2                 2   ⁢     A   H     ⁢     A   V     ⁢   cos   ⁢           ⁢   ΔΦ               2   ⁢     A   H     ⁢     A   V     ⁢   sin   ⁢           ⁢   ΔΦ           )               (   3   )               
In such a case, ΔΦ is calculated from phases Φ H  and Φ V  of each signal acquired when calculating the two tomographic images, as ΔΦ=Φ V −Φ H .
 
     The signal processing unit  190  then sets, in each B-scan image, a window of the size that is proximately 70 μm in a main scanning direction of the measuring beam and 18 μm in a depth direction. The signal processing unit  190  then averages each element of the Stokes vector calculated for each pixel by a number C within each window, and calculates a degree of polarization uniformity (DOPU) within the window using equation (4).
 
DOPU=√{square root over ( Q   m   2   +U   m   2   +V   m   2 )}  (4)
 
In equation (4), Q m , U m , and V m  are values acquired by averaging the elements Q, U, and V in the Stokes vector within each window. The signal processing unit  190  performs the above-described process for all windows in the B-scan images, and generates a DOPU image of the optic disk as illustrated in  FIG. 2E . As described above, the DOPU image is a tomographic image indicating the degree of polarization uniformity of the two types of polarization.
 
     DOPU is a value indicating the uniformity of polarization, and becomes close to “1” when polarization is preserved, and smaller than “1” when polarization is cancelled or not preserved. Since the RPE in the structure of the retina cancels the polarization state, the value of the DOPU in the portion corresponding to the RPE in the DOPU image becomes lower than the values in the other portions. Referring to  FIG. 2E , the lighter-shaded portion indicates the RPE. The DOPU image visualizes the layer such as the RPE that cancels polarization, so that the image of the RPE can be firmly acquired as compared to the change in the intensity even when the RPE is deformed due to a disease. 
     Further, the signal processing unit  190  can similarly generate a DOPU image in the planar direction of the fundus based on the outputs from the APD  152  and  153 . 
     According to the present exemplary embodiment, the above-described tomographic images corresponding to the first and second polarized beams, the retardation image, and the DOPU image will be referred to as tomographic images indicating the polarization state. Further, according to the present exemplary embodiment, the above-described retardation map and the birefringence map will also be referred to as fundus images indicating the polarization state. 
     The signal processing unit  190  uses the above-described intensity image to perform segmentation of the tomographic image. 
     More specifically, the signal processing unit  190  applies, to the tomographic image to be processed, a median filter as a type of smoothing filter, and a Sobel filter as a type of an edge detection method. The signal processing unit  190  thus generates respective images (hereinafter referred to as a median image and a Sobel image). The signal processing unit  190  then generates a profile for each A-scan from the generated median image and Sobel image. The signal processing unit  190  generates the profile of the intensity value from the median image and the profile of a gradient from the Sobel image. The signal processing unit  190  detects peaks in the profiles generated from the Sobel image. Further, signal processing unit  190  extracts a boundary of each retinal layer by referring to the profiles of the median image corresponding to regions before and after the detected peaks and the regions between the detected peaks. 
     Further, the signal processing unit  190  measures each layer thickness in the direction of the A-scan line, and generates a layer thickness map of each layer. 
     The operation performed in the image processing apparatus according to the present exemplary embodiment will be described below. 
       FIG. 3  is a flowchart illustrating the operation performed by the image processing apparatus according to the present exemplary embodiment. 
     In step S 101 , the image processing apparatus and the subject&#39;s eye positioned on the image processing apparatus are aligned. The process unique to the present exemplary embodiment with respect to performing alignment will be described below. Since alignment of a working distance in X, Y, and Z directions, focusing, and adjustment of the coherence gate are common, description will be omitted. 
       FIG. 4  illustrates a window  400  displayed on the display unit  192  when performing adjustment. Referring to  FIG. 4 , a display area  410 , i.e., an example of a first display area, displays a fundus image  411  imaged by the PS-SLO  140  and generated by the signal processing unit  190 . A frame  412  indicating an imaging range (i.e., an acquisition range or an acquisition position) of the PS-OCT  100  is superimposed on the fundus image  411 . 
     An operator sets the imaging range under control of the drive control unit  180 , by clicking and dragging an instruction device (not illustrated), such as a mouse, and designating by a cursor displayed on the window  400 . In other words, the operator designates the frame  412  using the cursor, and moves the frame  412  by the dragging operation. As a result, the drive control unit  180  controls a drive angle of a scanner and sets the imaging range. According to the present exemplary embodiment, the mouse includes a sensor for detecting a movement signal when the user manually moves the mouse in two directions, left and right mouse buttons for detecting that the user has pressed the button, and a wheel mechanism between the two mouse buttons which is movable in front and back and left to right directions. Further, the instruction device may be realized by the display unit including a touch panel function, so that the operator designates the acquisition position on the touch panel. 
     The adjustment of the λ/4 polarizing plate  113  will be described below. 
     Referring to  FIG. 4 , indicators  413  and  414  are displayed for adjusting the angle of the λ/4 polarizing plate  113 . When the operator instructs using the instruction device, the angle of the λ/4 polarizing plate  113  is adjusted based on control of the drive control unit  180 . The indicator  413  is for instructing adjustment in a counterclockwise direction, and the indicator  414  is for instructing adjustment in a clockwise direction. A numerical value displayed besides the indicators  413  and  414  indicates the current angle of the λ/4 polarizing plate  113 . 
     The display control unit  191  may display on the display unit  192  the indicator for adjusting the angle of the λ/4 polarizing plate  119  and the indicator  413  side by side, or in place of the indicator  413 . 
     The operator gives, using the cursor by operating on the mouse, an instruction so that the intensities of the tomographic images of each polarized beam respectively displayed on a display area  430 , i.e., an example of a third display area, and a display area  440 , i.e., an example of a fourth display area, become the same. A peak intensity value may be displayed along with tomographic images  431  and  441  of each polarized beam, or a waveform of each interference signal may be displayed, so that the operator performs adjustment while viewing the peak intensity value or the waveform. The tomographic images  431  and  441  of each polarized beam are examples of tomographic images corresponding to the first polarized beam and the second polarized beam, respectively. It is desirable to display a type of each image on the tomographic images  431  and  441  of each polarized beam (or tomographic images  531  and  541  to be described below). For example, a letter “P” indicating the P-polarized beam and a letter “S” indicating the S-polarized beam may be displayed. As a result, such a display prevents the user from misrecognizing the image. The letters may be displayed above or besides the image instead of being superimposed on the image, as long as the display is arranged to be associated with the image. 
     Since the tomographic images  431  and  441  are tomographic images at predetermined positions within the frame  412 , display forms indicating the predetermined positions may be displayed within the frame  412 . The display forms indicating the predetermined positions within the frame  412  can be moved using a pointing device such as the mouse, and the tomographic images  431  and  441  also change in response to movement of the display forms indicating the predetermined positions. A drawing of the display form that indicate the predetermined positions within the image capturing range of the tomographic image will be omitted in the exemplary embodiments to be described below for simplification. However, the tomographic images are displayed corresponding to the positions of the display forms indicating the predetermined positions. 
     Further, it is not necessary at this point for a display area  420 , i.e., an example of a second display area, to display any information. If auto adjustment is to be performed, the current adjustment state, such as a message informing “adjusting λ/4 polarizing plate”, may be displayed on the display area  420 . Further, a display indicating patient information such as a left eye or a right eye, or image capturing information such as an image capturing mode may be performed on the window  400 . Furthermore, it is desirable to repeatedly insert and remove the λ/4 polarizing plate  113  with respect to the optical path for alternately acquiring the fundus intensity image and the tomographic image indicating the polarization state. As a result, the display control unit  191  even in a minimum-sized ophthalmologic apparatus can display the fundus intensity image on the display area  410  and the tomographic image indicating the polarization state on the display area  420 . 
     It is desirable to perform adjustment in the following order: alignment adjustment using the anterior segment image or a luminescent spot in a cornea; focus adjustment using the fundus image indicating the polarization state; coherence gate adjustment using the tomographic image indicating the polarization state; and adjustment of the λ/4 polarizing plate. Further, it is desirable to determine the acquisition position of the tomographic image indicating the polarization state before adjusting the coherence gate using the tomographic image indicating the polarization state. However, the acquisition position may be determined in an initial setting for acquiring a center region of the fundus image indicating the polarization state. Adjustment can thus be simply performed to accurately acquire the tomographic image indicating the polarization state that is finer and corresponding to a narrower range as compared to the fundus image indicating the polarization state. In such a case, the λ/4 polarizing plate may be automatically adjusted in response to completion of adjustment of the coherence gate, or in response to input of a signal for acquiring the image indicating the polarization state. Further, the λ/4 polarizing plate may be previously adjusted on an initial setting screen when activating the ophthalmologic apparatus, so that the λ/4 polarizing plate is not required to be adjusted for each image capturing. 
     Furthermore, if the λ/4 polarizing plate can be inserted and removed with respect to the optical path, it is desirable to perform adjustment in the following order: alignment adjustment using the anterior segment image or the luminescent spot in the cornea; focus adjustment using the SLO fundus image; coherence gate adjustment using the OCT tomographic image; and adjustment of the λ/4 polarizing plate after inserting the λ/4 polarizing plate in the optical path. Adjustment can thus be performed before acquiring the image indicating the polarization state, using the normal SLO fundus image and the OCT tomographic image that the user is intuitively used to. The coherence gate may also be adjusted using the tomographic image indicating the polarization state of the PS-OCT by inserting the λ/4 polarizing plate after performing focus adjustment. In such a case, the λ/4 polarizing plate may be automatically inserted in response to completion of adjustment of the coherence gate, or in response to input of the signal for acquiring the image indicating the polarization state. 
     Moreover, the focus may be finely adjusted using the OCT tomographic image after coarsely adjusting the focus using the SLO fundus image. 
     Further, all of such adjustments may be automatically performed in the above-described order, or by the user adjusting the cursor to a slider corresponding to each type of adjustment displayed on the display unit and performing dragging. Furthermore, if the λ/4 polarizing plate is to be inserted or removed, an icon instructing inserting or removing the λ/4 polarizing plate with respect to the optical path may be displayed on the display unit. 
     In step S 102 , step S 103 , and step S 104  illustrated in  FIG. 3 , each of the light sources  101  and  141  emits the measuring beam. The line cameras  129  and  133  and the APD  152  and  153  then receive the return beam, and the signal processing unit  190  generates and analyzes each image as described above. 
     The process for outputting the generated image and the analysis result performed in step S 105  will be described below. 
     After the signal processing unit  190  completes generating and analyzing each image, the display control unit  191  generates output information based on the result. The display control unit  191  then outputs to and displays on the display unit  192  the output information. 
       FIG. 5  illustrates a display example on the display unit  192  according to the present exemplary embodiment. 
     Referring to  FIG. 5 , a window  500  displayed on the display unit  192  includes display areas  510 ,  520 ,  530 , and  540 . 
     The display area  510 , i.e., an example of the first display area, displays a fundus image  511 , and a rectangular frame  512  indicating the position of the tomographic image is superimposed on the fundus image  511 . The fundus intensity image is displayed as the fundus image  511 . However, the fundus image may be generated based on a polarization signal. 
     The display area  520 , i.e., an example of the second display area, displays a tomographic image  521 . Further, the display area  520  displays buttons  522 ,  523 ,  524 , and  525 , i.e., examples of a selection unit for selecting the type of the tomographic image to be displayed. The user may select the type of the tomographic image from a menu instead of using the buttons  522 ,  523 ,  524 , and  525 . In the example illustrated in  FIG. 5 , the user has selected the button  522 . 
     The display area  530 , i.e., an example of a third display area, and the display area  540 , i.e., an example of a fourth display area, respectively display tomographic images  531  and  541  based on each polarization signal used in generating the tomographic image  521 . The display areas  530  and  540  may also display each fundus image based on each polarization signal from which the fundus image displayed on the display area  510  has been generated, according to an instruction by the operator via the menu. 
     The display areas  510 ,  520 ,  530 , and  540  illustrated in  FIG. 5  respectively display the fundus image  511 , the tomographic image  521 , i.e., the intensity image, and the tomographic images  531  and  541  generated from the polarization signals. However, it is not limited thereto. For example, the display areas  530  and  540  may instead display the tomographic images  531  and  541  only if the intensity value of the tomographic image  521  is less than a predetermined intensity value. As a result, the operator can recognize which of the beams of different polarizations has caused the low intensity value. If the intensity of the tomographic image  521  has decreased due to a change in the polarization state along with the usage of the apparatus, the operator can examine the tomographic images  531  and  541  and recognize how the polarization state has changed. The operator can perform calibration by adjusting the polarization state using the polarization controller so that the intensities of the tomographic images  531  and  541  become the same. 
     The display areas  530  and  540  are not limited to displaying the tomographic images  531  and  541  as described above in a case where the intensity value of the tomographic image  521  is less than the predetermined intensity value. The display areas  530  and  540  may also display the tomographic images  531  and  541  as described above in a case where the intensity value of a retardation image  621  or a DOPU image  721  to be described below is less than the predetermined intensity value. 
     It is desirable to display the tomographic intensity image  521 , the retardation image  621 , and the DOPU image  721  by superimposing the display form indicating the type of the image, such as “intensity”, “retardation”, and “DOPU” in characters. As a result, the user can be prevented from misrecognizing the image. 
     The type of image may be displayed above or besides the image instead of being superimposed on the image, as long as the characters are arranged to be associated with the image. 
     If the operator presses the button  523 , the tomographic image displayed on the display area  520  can be changed to the retardation image  621  as illustrated in  FIG. 6 . 
     Referring to  FIG. 6 , the display areas  530  and  540  respectively display the tomographic images  531  and  541  similarly as in  FIG. 5 . 
     If the operator then presses the button  524 , the tomographic image displayed on the display area  520  can be changed to a DOPU image  721  as illustrated in  FIG. 7 . 
     Referring to  FIG. 7 , the display area  530  displays the intensity image  521 , and the display area  540  displays the retardation image  621 . It is desirable to provide a button for selecting the image for each display area. The user thus becomes capable of easily selecting the images to be compared, such as a plurality of tomographic images indicating different polarization states. 
     If the operator presses the button  525 , the tomographic image displayed on the display area  520  can be changed to an image  821  indicating a segmentation result as illustrated in  FIG. 8 . Referring to  FIG. 8 , color line segments indicating the layer boundaries are superimposed on the tomographic image and displayed in the image  821 , and the RPE is highlighted. The layer selected by the operator using the cursor is highlighted. 
     The display area  540  displays a tomographic image  841 , used in performing segmentation, and buttons  842  and  843 . If the operator presses the buttons  842  and  843 , the intensity image  841  can be switched to a graph  941 , illustrated in  FIG. 9 , indicating the layer thickness of the highlighted layer. 
     Further, referring to  FIG. 9 , thickness information of the selected layer (e.g., a two-dimensional layer thickness map as illustrated in  FIG. 10 ) may be displayed on the display area  530 . Referring to  FIG. 10 , the thickness of the selected layer is expressed by a difference in color. An integration image may be displayed in place of the thickness of the selected layer illustrated in  FIG. 10 . The integration image may be generated based on a specific layer or on the entire PS-OCT. Further, according to the present exemplary embodiment, the image to be displayed is changed according to the instruction of the operator. However, the information on the disease to be diagnosed, such as the name of the disease, may be selected from the menu, and the image on which a priority order has been preset with respect to the disease may be displayed on each display area. 
     A display format of the display unit  192  in a case where the operator has performed operations with respect to the various images displayed on the display unit  192  will be described below. 
     Referring to  FIG. 11 , if the operator has designated a region of interest on a DOPU image  1021  using a cursor  1022 , i.e., an example of a designation unit, the display control unit  191  causes a change in the position on a fundus image  1011  corresponding to the designated position. Such a change indicates a highlighted display (e.g., circling, blinking, color change, and highlighting). In the example illustrated in  FIG. 11 , a circle  1013 - 1  is displayed at the position on the fundus image  1011  corresponding to the position on the DOPU image  1021  designated using the cursor  1022 . In other words, the display control unit  191  corresponds to a display control unit that causes, if the designation unit has designated a portion of a tomographic image indicating the polarization state displayed on the display unit, position information indicating a position corresponding to the portion designated by the designation unit to be displayed on a planar image displayed on a display unit. Further, the display control unit  191  causes the display unit  192  to display the fundus image  1011  and the DOPU image  1021  as illustrated in  FIG. 11 . The display control unit  191  thus causes the display unit to display the planar image and the tomographic image indicating the polarization state side by side. 
     Further, if the operator has sequentially designated a plurality of positions on the DOPU image  1021  using the cursor  1022 , the display control unit  191  sequentially displays circles  1013 - 2 ,  1013 - 3 , and  1013 - 4  at the positions on the fundus image  1011  which correspond to the designated positions. In such a case, the display control unit  191  may display all of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4 . In other words, the display control unit causes, if the designation unit has designated a plurality of portions on the tomographic image indicating the polarization state displayed on the display unit, a plurality of position information corresponding to the plurality of portions designated by the designation unit to be displayed on the planar image. 
     The display control unit  191  may instead display, on the fundus image  1011 , only the circle corresponding to the last position designated using the cursor  1022 . The display control unit  1901  thus causes, if the designation unit has designated a plurality of portions on the tomographic image indicating the polarization state displayed on the display unit, only the position information corresponding to the last designated portion among the plurality of portions designated by the designation unit to be displayed on the planar image. 
     Further, the display control unit  191  may cause the display unit  192  to display each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4  so that an order in which the positions are designated using the cursor  1022  is identifiable. For example, the display control unit  101  may cause the display unit  192  to display each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4  using a different color. Furthermore, the display control unit  101  may cause the display unit  192  to display the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4  using the same color, and to display the display form indicating the order, such as “1”, “2”, “3”, and “4”, near each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4 . Moreover, the display control unit  191  may change a blinking interval according to the order. For example, the display control unit  191  sets the blinking interval of the circle corresponding to the last position designated using the cursor  1022  to be shorter than the blinking intervals of the other circles. The circle corresponding to the last position designated using the cursor  1022  thus becomes noticeable. In other words, the display control unit  191  causes the plurality of display positions to be displayed in different display forms according to the order in which each of a plurality of portions on the tomographic image indicating the polarization state has been designated. 
     Further, the display control unit  191  may also cause a change in the position designated on the DOPU image  1021  by the cursor  1022  so that the point designated by the cursor  1022  becomes noticeable. For example, the display control unit  191  may cause a circle, similar to the circle  1013 - 1 , to be displayed on the position designated on the DOPU image  1021  by the cursor  1022 . 
     Furthermore, if the display control unit  191  has set different colors to each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4 , the display control unit  191  may perform control as follows. The display control unit  191  may set the colors of the circles displayed on each of the plurality of positions designated on the DOPU image  1021  by the cursor  1022 , to correspond to the colors of each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4 . 
     Moreover, if the display control unit  191  displays the display form indicating the order, such as “1”, “2”, “3”, and “4”, near each of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4 , the display control unit  191  may display a similar display form near each of the plurality of circles displayed on the DOPU image  1021 . 
     As a result, a correspondence relation between the DOPU image  1021  and the fundus image  1011  becomes easily recognizable, even when a plurality of points on the DOPU image  1021  has been designated using the cursor  1022 . 
     Further, the display control unit  191  may change the size of the circles  1013 - 1 ,  1013 - 2 ,  1013 - 3 , and  1013 - 4  according to a level of discontinuity of the layer existing in the position designated by the cursor  1022 . In such a case, the signal processing unit  190  analyzes the level of discontinuity of the layer existing in the position designated by the cursor  1022 . The level of discontinuity indicates the length in which the layer existing in the position designated by the cursor  1022  is continuous. For example, the size of the circle  1013 - 1  may be increased as the length in which the layer existing in the position designated by the cursor  1022  is continuous becomes shorter. Further, the size of the circle  1013 - 1  may be decreased as the length in which the layer existing in the position designated by the cursor  1022  is continuous becomes shorter. 
     The size of the circle displayed on the DOPU image  1021  may be similarly changed according to the level of discontinuity of the layer. 
     According to the above-described examples, the display control unit  191  causes the display unit  192  to display circles. However, it is not limited thereto, and the display unit  192  may display other shapes such as a square or a rectangle. Further, according to the above-described exemplary embodiment, the operator designates a point using the cursor  1022 . However, it is not limited thereto, and a range can be designated by the operator dragging the mouse. 
     Similarly, if the operator designates, using the cursor  1022 , a specific position on a tomographic intensity image  1031  and a retardation image  1041  illustrated in  FIG. 11 , a change is caused at the position on the fundus image  1011  corresponding to the specific position. Further, the shape of a mark, such as a circle displayed at the position on the fundus image  1011  corresponding to the position designated by the cursor  1022 , may be changed according to the type of tomographic image designated using the cursor  1022 . For example, if the operator has designated a specific position on the retardation image  1041  using the cursor  1022 , a circle is displayed at the position on the fundus image  1011  corresponding to the specific position. On the other hand, if the operator has designated a specific position on the DOPU image  1021  using the cursor  1022 , a triangle is displayed at the position on the fundus image  1011  corresponding to the specific position. As a result, the relation between the plurality of tomographic images indicating the polarization state of the subject&#39;s eye and the fundus image becomes easily recognizable. 
     Furthermore, the display control unit  191  may cause the point designated using the cursor  1022  to be displayed on the tomographic intensity image  1031  and the retardation image  1041 , similarly as on the DOPU image  1021 . 
     The display control unit  191  associates the position information of the DOPU image  1021 , the tomographic intensity image  1031 , and the retardation image  1041  with the position information of the fundus image  1011 . The display control unit  191  associates the position information when performing scanner control, or by employing feature points of the image. Other units may associate the position information as long as a similar effect can be acquired. 
     The operator may select the region of interest by operating the mouse or the touch panel, or the signal processing unit  190  may automatically select a diseased area. 
     If a fundus camera is employed in place of the PS-SLO  140 , the display control unit  191  displays on the display unit  192  a fundus image  1111  acquired by the fundus camera as illustrated in  FIG. 12 . A change is then caused in the portion of the fundus image  1111  corresponding to the portion designated on the DOPU image  1021 , similarly as in the case described with reference to  FIG. 11 . 
     Further, an image which is similar to the fundus image acquired by the fundus camera can be acquired by removing the λ/4 polarizing plate  113  from the optical path and performing image capturing. 
     If the operator designates the region of interest on a retardation image  1221  using a cursor  1222  as illustrated in  FIG. 13 , there is a change in a portion  1212  on a PS-SLO fundus image  1211  corresponding to the designated portion. Referring to  FIG. 13 , a retardation map  1213  is superimposed on the fundus image  1211 . 
     A similar effect can be acquired when using a fundus image generated from a three-dimensional image formed of a plurality of tomographic images, instead of using the above-described fundus image. More specifically, the signal processing unit  190  generates the three-dimensional image from a plurality of DOPU images or a plurality of retardation images acquired by the PS-OCT  100 . The signal processing unit  190  then generates from the three-dimensional image, a fundus image indicating the polarization state of the subject&#39;s eye. Alternatively, the signal processing unit  190  generates the three-dimensional image from a plurality of tomographic intensity images acquired by the PS-OCT  100 , and generates the fundus image from the three-dimensional image. 
     Further, the signal processing unit  190  can acquire the tomographic intensity image, based on the above-described equation (2), from the tomographic image indicating the polarization state of the subject&#39;s eye acquired by the PS-OCT  100 . The signal processing unit  190  can then generate the fundus image from the three-dimensional image generated based on the plurality of tomographic intensity images. In other words, the signal processing unit  190  obtains the planar image based on a plurality of tomographic images indicating the polarization state acquired by a tomographic image acquisition unit. 
     As a result, the PS-OCT  100  can singularly generate the fundus image and the tomographic image without using the PS-SLO  140  and the fundus camera. 
     Further, according to the present exemplary embodiment, the fundus images  1011 ,  1111 , and  1211  may be the DOPU image or the retardation image of a surface of the fundus. In such a case, the PS-SLO  100  acquires the DOPU image or the retardation image of the surface of the fundus. The PS-SLO  100  thus acquires, based on the beams of different polarizations obtained by splitting the return beam from the subject irradiated with light, the planar image indicating the polarization state of the subject. The display control unit  191  then causes the display unit  192  to display the DOPU image or the retardation image of the surface of the fundus, and the tomographic DOPU image or the tomographic retardation image. The display control unit  191  thus causes the display unit to display the planar image indicating the polarization state and the tomographic image indicating the polarization state side by side. In such a case, the display control unit  191  causes a circle to be displayed at the position on the DOPU image of the fundus corresponding to the position on the tomographic DOPU image  1021  designated using the cursor  1022 . In other words, the display control unit  191  causes, if the designation unit designates a portion of the tomographic image indicating the polarization state displayed on the display unit, position information indicating the position corresponding to the portion designated by the designation unit to be displayed on the planar image indicating the polarization state of the subject displayed on the display unit. 
     Furthermore, if the PS-OCT  100  is to function similarly as a normal OCT by removing the λ/4 polarizing plate  113  from the optical path, the PS-OCT  100 , i.e., an example of the tomographic image acquisition unit, performs control as follows. The PS-OCT  100  acquires the tomographic image of the subject, based on the combined beam into which the return beam from the subject irradiated with the measuring beam and the reference beam corresponding to the measuring beam have been combined. Moreover, if the PS-OCT  100  is to function similarly as a normal OCT by removing the λ/4 polarizing plate  113  from the optical path, the display control unit  191  can display, on the display unit  192 , the DOPU image or the retardation image of the surface of the fundus, and the tomographic image side by side. The display control unit  191  thus causes the display unit to display the planar image indicating the polarization state and the tomographic image side by side. In such a case, the display control unit  191  displays a circle at the position on the DOPU image of the fundus corresponding to the position of the tomographic intensity image designated using the cursor  1022 . The display control unit  191  thus causes, if the designation unit has designated a portion of the tomographic image displayed on the display unit, the position information indicating the position corresponding to the portion designated by the designation unit to be displayed on the planar image indicating the polarization state displayed on the display unit. 
     As described above, the portion corresponding to the region of interest in the tomographic image is displayed on the two-dimensional fundus image, so that the operator can easily recognize the corresponding portion. 
     According to the present exemplary embodiment, the images can be displayed so that the user can efficiently recognize the image indicating the polarization state. 
     Further, the operator can select the necessary image with easy operation. In particular, the operation becomes easier by previously associating the name of the disease with the image to be displayed. 
     Furthermore, the polarization of the measuring beam can be easily adjusted. 
     The positions of the display areas in which the above-described images are displayed are not limited thereto. For example, the fundus image may be displayed in the left display area in the display screen. Further, the number of images to be displayed is not limited thereto. For example, the fundus image and the tomographic image (i.e., two images) may be displayed side by side on the display screen when performing adjustment. The display method may then be changed after performing image capturing, and a plurality of tomographic images indicating different polarization states may be displayed side by side on the display screen along with the fundus image. Furthermore, the order and the positions in which the buttons  522 ,  523 ,  524 , and  525  are arranged are not limited thereto. 
     It is to be understood that the invention is not limited to the disclosed exemplary embodiments, and various modifications and changes may be made and executed without departing from the scope of the invention. For example, according to the present exemplary embodiment, the fundus intensity image is employed as the fundus image  511 . However, it is not limited thereto, and a retardation image in a planar direction of the fundus or a DOPU image in the planar direction of the fundus may be used instead of the fundus image  511 . 
     Further, the display control unit  191  may display, on one of the display areas in the display unit  192 , the retardation map or the birefringence map instead of the above-described images. Moreover, the display control unit  191  may superimpose and display the retardation map or the birefringence map on the fundus intensity image  511 . In such a case, it is desirable to superimpose and display the retardation map or the birefringence map on the area indicated by the frame  512 . 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment (s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2012-010275 filed Jan. 20, 2012, which is hereby incorporated by reference herein in its entirety.