Patent Publication Number: US-8985770-B2

Title: Ophthalmic imaging apparatus, method of controlling opthalmic apparatus and storage medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ophthalmic imaging apparatus which images an eye to be examined, a method of controlling the ophthalmic imaging apparatus, and a storage medium. 
     2. Description of the Related Art 
     Currently, various types of ophthalmic apparatuses using optical apparatuses are used. For example, various types of apparatuses such as an anterior ocular segment imaging apparatus, fundus camera, and scanning laser ophthalmoscope (SLO) are used as optical apparatuses for observing the eyes. Of these apparatuses, an optical tomography apparatus based on optical coherence tomography (OCT) using multiwavelength interference is an apparatus capable of obtaining a tomographic image of a sample with a high resolution. This apparatus has been becoming indispensable as an ophthalmic apparatus for out-patient clinics dedicated to retinal diseases. This apparatus will be referred to as an OCT apparatus hereinafter. 
     An OCT apparatus can measure a slice of an object to be examined by splitting measurement light which is low-coherent light into reference light and measurement light, irradiating the object with the measurement light, and making return light from the object interfere with the reference light. The OCT apparatus can obtain a high-resolution tomographic image by scanning measurement light on a sample. This apparatus therefore obtains a tomographic image of the retina of the fundus of the eye to be examined, and is widely used for ophthalmic diagnosis of the retina. If, however, the object to be examined is a living organism like the eye, the distortion of an image due to the movement of the eye poses a problem. Demands have therefore risen for high-speed, high-sensitivity measurement. 
     Japanese Patent Laid-Open No. 2009-523563 has proposed an OCT apparatus which obtains OCT images corresponding to scan patterns used for scanning of a plurality of portions. A scan pattern consists of a plurality of concentric circles and a plurality of radial lines. 
     Japanese Patent Laid-Open No. 2010-110392 has proposed an OCT apparatus which performs addition processing of a plurality of tomographic images obtained by imaging the same region and averaging pixel values to reduce the influence of noise which occurs irregularly. At this time, the OCT apparatus segments a captured image into a plurality of regions, and detects positional shift information between the respective captured images for each segmented region. The apparatus then performs correction for each segmented region based on the positional shift information, and averages the respective corrected images. 
     Although the OCT apparatus disclosed in Japanese Patent Laid-Open No. 2009-523563 has a scan pattern for scanning a plurality of areas obtained by segmenting a captured image, the apparatus does not perform averaging processing. This poses a problem: the obtained tomographic image is affected by irregular noise. 
     The OCT apparatus disclosed in Japanese Patent Laid-Open No. 2010-110392 averages a plurality of images captured in the same area to reduce the influence of irregular noise, and performs averaging processing of pixel values. The number of tomographic images to be obtained for the execution of this averaging processing is uniformly fixed. To perform averaging processing, it is necessary to capture a plurality of images in the same area. To obtain high-quality tomographic images, it is necessary to obtain a larger number of tomographic images. Although a captured image is segmented into a plurality of areas, averaging processing is not performed for each imaging area. In practice, therefore, unnecessary tomographic images may be obtained. This will lead to a longer imaging time and hence place a burden on an object to be examined. 
     In consideration of the above problems, the present invention provides a technique of shortening the time required for imaging and reducing the burden on a patient by decreasing the number of images to be obtained to obtain a high-quality tomographic image necessary for diagnosis. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided an ophthalmic imaging apparatus which obtains a tomographic image of an eye to be examined based on light obtained by combining return light from the eye irradiated with measurement light with reference light corresponding to the measurement light, the apparatus comprising: a scanning unit configured to scan the measurement light on the eye; and a control unit configured to control the number of times of scanning by the scanning unit in accordance with a scanning position of the scanning unit on the eye. 
     According to one aspect of the present invention, there is provided a method of controlling an ophthalmic imaging apparatus which includes a scanning unit and a control unit, and obtains a tomographic image of an eye to be examined based on light obtained by combining return light from the eye irradiated with measurement light with reference light corresponding to the measurement light, the method comprising: causing the scanning unit to scan the measurement light on the eye; and causing the control unit to control the number of times of scanning by the scanning unit in accordance with a scanning position of the scanning unit on the eye. 
     Further features of the present invention will be apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the arrangement of an OCT apparatus according to the first embodiment; 
         FIGS. 2A to 2B  are flowcharts showing a processing procedure for obtaining tomographic images using the OCT apparatus according to the first embodiment; 
         FIG. 3  is a flowchart showing the operation of a personal computer  125  according to the first embodiment; 
         FIGS. 4A to 4C  are views for explaining a method of obtaining tomographic images using the OCT apparatus according to the first embodiment; 
         FIG. 5  is a view showing the display screen of a monitor  124  according to the first embodiment; 
         FIG. 6  is a flowchart showing the operation of a personal computer  125  according to the second embodiment; 
         FIG. 7  is a view showing the arrangement of an OCT apparatus according to the third embodiment; and 
         FIG. 8  is a flowchart showing the operation of a personal computer  125  according to the third embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An exemplary embodiment(s) of the present invention will now be described in detail with reference to the drawings. It should be noted that the relative arrangement of the components, the numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. 
     First Embodiment 
     The first embodiment of the present invention will be described below with reference to  FIGS. 1 to 5 .  FIG. 1  shows the arrangement of an OCT apparatus (ophthalmic imaging apparatus) according to the first embodiment. The OCT apparatus splits light from a light source into measurement light and reference light, and obtains a tomographic image of the eye to be examined based on the wavelength spectrum of interference light between the reference light and return light returning from the eye upon irradiating the eye with the measurement light. Reference numeral  101  denotes a light source;  104 , exit light;  105 , reference light;  106 , measurement light;  142 , composite light;  107 , an eye to be examined;  108 , return light,  130 - 1  to  130 - 4 , single-mode fibers;  120 - 1 ,  120 - 2 , and  135 - 1  to  135 - 4 , lenses;  114 , a mirror;  115 , a dispersion-compensating glass;  117 , an electrically-driven stage;  119 , an XY scanner;  125 , a personal computer;  124 , a monitor;  126 , a cornea;  127 , a retina;  131 , an optical coupler;  139 , a line camera;  140 , a frame grabber;  141 , a transmission type diffraction grating; and  153 - 1  to  153 - 4 , polarization controllers. 
     As a whole, the OCT apparatus according to this embodiment forms a Michelson interferometer system. The exit light  104  emitted from the light source  101  passes through the polarization controller  153 - 1 . The optical coupler  131  splits the exit light  104  into the reference light  105  and the measurement light  106  at an intensity ratio of 50:50. The measurement light  106  returns as the return light  108  reflected or scattered by the retina  127  or the like of the eye  107  to be observed. The optical coupler  131  combines the return light  108  with the reference light  105  reflected by the mirror  114 . After the reference light  105  is combined with the return light  108 , the transmission type diffraction grating  141  spectrally separates the composite light for each wavelength. The resultant light strikes the line camera  139 . The line camera  139  converts the light intensity into a voltage for each position (wavelength), and forms a tomographic image of the eye  107  by using the resultant voltage signal. 
     The light source  101  and the like will be described next. The light source  101  is an SLD (Super Luminescent Diode) which is a low-coherent light source. This light source has a wavelength of 830 nm and a bandwidth of 50 nm. In this case, the bandwidth influences the resolution of an obtained tomographic image in the optical axis direction, and hence is an important parameter. As the type of light source, SLD is selected in this case. However, an ASE (Amplified Spontaneous Emission) light source or the like can be used as long as it can emit low-coherent light. In consideration of measurement on the eyes, a near-infrared wavelength is suitable for this embodiment. Furthermore, since a wavelength influences the resolution of an obtained tomographic image in the horizontal direction, the shorter the wavelength, the better. Assume that in this case, the wavelength is 830 nm. Depending on the measurement region to be observed, other wavelengths may be selected. 
     The optical path of the reference light  105  will be described next. The reference light  105  split by the optical coupler  131  passes through the polarization controller  153 - 2  and emerges from the lens  135 - 3  as nearly parallel light having a diameter of 1 mm. The emerging reference light  105  passes through the dispersion-compensating glass  115 . The lens  135 - 4  focuses the light onto the mirror  114 . The mirror  114  changes the direction of the reference light  105 , which then propagates to the optical coupler  131  through the same path. The reference light  105  whose direction is changed by the mirror  114  passes through the optical coupler  131  and is guided to the line camera  139 . 
     In this case, the dispersion-compensating glass  115  compensates for the dispersion caused when the measurement light  106  reciprocates in the eye  107  with respect to the reference light  105 . In this case, L 1 =23 mm, based on the premise of the diameter of the eyeball of the average Japanese person. An electrically-driven stage  117 - 1  can move in the directions indicted by the arrows in  FIG. 1 , and can adjust/control the position of the mirror  114 . This can adjust/control the optical path length of the reference light  105 . The personal computer  125  controls the electrically-driven stage  117 - 1  at high speed. 
     &lt;Arrangement of Measurement Optical Path&gt; 
     The optical path of the measurement light  106  will be described next. The measurement light  106  split by the optical coupler  131  passes through the polarization controller  153 - 4  and emerges from a lens  148  as nearly parallel light having a diameter of 1 mm. The light then strikes the mirror of the XY scanner  119 . For the sake of simplicity, in this case, the XY scanner  119  is presented as one mirror. In practice, however, two mirrors, that is, an X scan mirror and a Y scan mirror, are disposed near each other to raster-scan on the retina  127  in a direction perpendicular to the optical axis. In addition, the lenses  120 - 1  and  120 - 2  and the like are adjusted such that the center of the measurement light  106  coincides with the rotation center of the mirror of the XY scanner  119 . The lenses  120 - 1  and  120 - 2  constitute an optical system for scanning the measurement light  106  on the retina  127 , which serves to scan the retina  127  with a point near the cornea  126  being a fulcrum point. The measurement light  106  is configured to be formed into an image on the retina  127 . 
     An electrically-driven stage  117 - 2  can move in the directions indicted by the arrows in  FIG. 1 , and can adjust/control the position of the accompanying lens  120 - 2 . Adjusting the position of the lens  120 - 2  will focus the measurement light  106  onto a desired layer of the retina  127  of the eye  107 , thereby allowing observation. This technique can be applied to even a case in which the eye  107  has a refractive error. When the measurement light  106  strikes the eye  107 , the light reflected and scattered by the retina  127  becomes the return light  108 . The return light  108  passes through the optical coupler  131  along the same path and is guided to the line camera  139 . The personal computer  125  controls the electrically-driven stage  117 - 2  at high speed. 
     &lt;Arrangement of Spectroscopic Unit&gt; 
     The arrangement of the measurement system of the OCT apparatus according to this embodiment will be described next. The optical coupler  131  combines the reference light  105  with the return light  108  which is light reflected and scattered by the retina  127 . Composite light  142  emerges from a fiber end and passes through the polarization controller  153 - 3 . The lens  135 - 2  collimates the light into nearly parallel light. The transmission type diffraction grating  141  is irradiated with this nearly parallel light and spectrally separates the light for each wavelength. The lens  135 - 1  focuses the separated light. The line camera  139  converts the light intensity into a voltage for each position (wavelength). Interference fringes in a spectral area on the wavelength axis are observed on the line camera  139 . The spectroscopic unit will be described concretely below. 
     It is known that the OCT apparatus has the general characteristics that as the spectral width increases, the resolution of the OCT increases, whereas as the wavelength resolution in spectroscopy increases, the measurable width in the depth direction increases. These characteristics can be expressed by equations (1) and (2) given below: 
                   R   =     1     2   ⁢   Δ   ⁢           ⁢   K               (   1   )               D   =     N     2   ⁢   Δ   ⁢           ⁢   K               (   2   )               
where R is the resolution of the OCT, ΔK is the wavenumber width obtained by the line camera, D is the measurable width of the OCT in the depth direction, and N is the number of pixels of the line camera. Note that the spectral width is the range of wavelengths of light striking the N pixels of the line camera, and is a difference λ max −λ min  between a maximum wavelength λ max  and a minimum wavelength λ min . The wavenumber width ΔK is represented as ΔK=1/λ min −1/λ max . The resolution of the OCT is generally defined as half of the coherence length. This indicates that as ΔK increases, R decreases (the resolution of the OCT decreases), whereas when N is constant, as ΔK decreases (the wavelength resolution in spectroscopy increases), D increases (the measurable width in the depth direction increases). In this case, the wavelength resolution is the wavelength width obtained per pixel by dividing the spectral width by the number of pixels of the line camera. In general, the actual wavelength resolution is larger than the wavelength resolution defined in this case due to the optical aberrations of the lens.
 
     The frame grabber  140  converts the voltage signals, obtained by converting the light intensities into voltages using the line camera  139 , into digital values. The personal computer  125  forms a tomographic image by performing data processing of the digital values. In this case, the line camera  139  has 1024 pixels, and can obtain the intensity of the composite light  142  for each wavelength. 
     &lt;Method of Obtaining Tomographic Image&gt; 
     A method of obtaining a tomographic image (a plane parallel to the optical axis) of the retina  127  by using the OCT apparatus will be described next with reference to  FIGS. 4A to 4C .  FIG. 4A  shows how the eye  107  is observed with the OCT apparatus. The same reference numerals as in  FIG. 1  denote the same or corresponding constituent elements in  FIG. 4 , and a repetitive description will be omitted. As shown in  FIG. 4A , the measurement light  106  strikes the retina  127  through the cornea  126 . This light is reflected and scattered at various positions thereafter to become the return light  108 . The return light  108  reaches the line camera  139  with time delays at the respective positions. 
     In this case, since the bandwidth of the light source  101  is large and the spatial coherence length is short, when the optical length of reference light is almost equal to that of measurement light, the line camera  139  detects interference fringes. As described above, the interference fringes obtained by the line camera  139  are those in a spectral area on the wavelength axis. These interference fringes as information on the wavelength axis are converted into interference fringes on the optical frequency axis for each composite light  142  in consideration of the characteristics of the line camera  139  and transmission type diffraction grating  141 . In addition, performing inverse Fourier transform for the converted interference fringes on the optical frequency axis will obtain information in the depth direction. 
     As shown in  FIG. 4B , it is possible to obtain interference fringes for each X-axis position by detecting the interference fringes while driving the X-axis of the XY scanner  119 . That is, it is possible to obtain information in the depth direction for each X-axis position. As a result, a two-dimensional distribution of the intensities of the return light  108  on an X-Z plane is obtained. This two-dimensional distribution is a tomographic image  132  as shown in  FIG. 4C . The tomographic image  132  is basically an array of the intensities of the return light  108 , as described above, and is displayed by mapping the intensities on a grayscale. In this case, only the boundaries of the obtained tomographic image are emphasized and displayed. 
     A processing procedure in a method of obtaining tomographic images by using the OCT apparatus according to this embodiment will be described next with reference to  FIGS. 2A and 2B .  FIGS. 2A and 2B  are a flowchart showing processing operation performed by the CPU of the personal computer  125 . The following is a case in which the apparatus performs B-scan at each of five scanning regions L 1  to L 5  which are a plurality of scanning areas on the eye to be examined, as a position at which the eye is irradiated with measurement light, as indicated by “ 3001 ” in  FIG. 3 . 
     In step S 201 , the operator sets the number of tomographic images to be obtained at each scanning region on an input window  501  shown in  FIG. 5 . To set the number of tomographic images to be obtained is equivalent to setting the number of times of scanning. At first, the number of tomographic images to be obtained is set to a predetermined number in advance, and the operator can change the number of tomographic images to be obtained for each scanning region. In this embodiment, the numbers of tomographic images to be obtained are set as follows: L[1]: N, L[2] and L[3]: N-n1, and L[4] and L[5]: N-n2. That is, of the plurality of scanning areas on the eye  107 , a large number of tomographic images to be obtained are set at L[1] near the central position of the eye  107  (for example, a scanning position in the central portion), and a small number of tomographic images to be obtained are set at L[4] and L[5] near peripheral positions of the eye (for example, peripheral positions far from the scan center). 
     In step S 202 , the operator sets the number of tomographic images to be used for tomographic image averaging processing on the input window  501 . At first, the number of tomographic images to be used for averaging processing is set to a predetermined number in advance, and the operator can change the number of times of scanning for each scanning region with an input acceptance unit. In this embodiment, L[1], L[2], L[3], L[4], and L[5] are all set to “AUTO”. That is, the CPU automatically determines the quality of each of the number of tomographic images to be obtained which is set in step S 201 , and performs the processing of adding and averaging only the necessary tomographic images. 
     In step S 203 , the CPU determines whether setting of the number of tomographic images to be obtained for each scanning region and setting of the number of tomographic images to be used for averaging processing are complete. If the CPU determines that the setting of these numbers is complete (YES in step S 203 ), the process advances to step S 204 . If the CPU determines that the setting of these numbers is not complete (NO in step S 203 ), the process waits for the completion of the setting. 
     In step S 204 , the CPU detects the presence/absence of the input of an imaging start instruction by the operator to determine whether the operator has input an imaging start instruction. If the CPU determines that the operator has input an imaging start instruction (YES in step S 204 ), the process advances to step S 205 . If the CPU determines that the operator has not input an imaging start instruction (NO in step S 204 ), the process waits until the operator inputs an imaging start instruction. 
     In step S 205 , the CPU drives the XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at the scanning region L[1]. 
     In step S 206 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[1] in the X-axis direction (horizontal direction). 
     In step S 207 , the CPU determines whether the number of tomographic images to be obtained at the scanning region L[1] has reached a preset number N. As indicated by “ 3002 ” in  FIG. 3 , if the CPU determines that the number of tomographic image to be obtained has reached N (YES in step S 207 ), the process advances to step S 208 . If the CPU determines that the number of tomographic images to be obtained has not reached N (NO in step S 207 ), the process returns to step S 206 . 
     In step S 208 , the CPU drives the XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at the scanning region L[2]. 
     In step S 209 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[2] in the X-axis direction (horizontal direction). In step S 210 , the CPU determines whether the number of tomographic images to be obtained at the scanning region L[2] has reached a preset number N-n1. As indicated by “ 3002 ” in  FIG. 3 , if the CPU determines that the number of tomographic image to be obtained has reached N-n1 (YES in step S 210 ), the process advances to step S 211 . If the CPU determines that the number of tomographic images to be obtained has not reached N-n1 (NO in step S 210 ), the process returns to step S 209 . 
     In step S 211 , the CPU drives the XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at the scanning region L[3]. In addition, in step S 212 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[3] in the X-axis direction (horizontal direction). 
     In step S 213 , the CPU determines whether the number of tomographic images to be obtained at the scanning region L[3] has reached a preset number N-n1. As indicated by “ 3002 ” in  FIG. 3 , if the CPU determines that the number of tomographic images to be obtained has reached N-n1 (YES in step S 213 ), the process advances to step S 214 . If the CPU determines that the number of tomographic images to be obtained has not reached N-n1 (NO in step S 213 ), the process returns to step S 212 . 
     In step S 214 , the CPU drives the XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at the scanning region L[4]. In addition, in step S 215 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[4] in the X-axis direction (horizontal direction). 
     In step S 216 , the CPU determines whether the number of tomographic images to be obtained at the scanning region L[4] has reached a preset number N-n2. As indicated by “ 3002 ” in  FIG. 3 , if the CPU determines that the number of tomographic images to be obtained has reached N-n2 (YES in step S 216 ), the process advances to step S 217 . If the CPU determines that the number of tomographic images to be obtained has not reached N-n2 (NO in step S 216 ), the process returns to step S 215 . 
     In step S 217 , the CPU drives the XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at the scanning region L[5]. In addition, in step S 218 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[5] in the X-axis direction (horizontal direction). 
     In step S 219 , the CPU determines whether the number of tomographic images to be obtained at the scanning region L[5] has reached a preset number N-n2. As indicated by “ 3002 ” in  FIG. 3 , if the CPU determines that the number of tomographic image to be obtained has reached N-n2 (YES in step S 219 ), the process advances to step S 220 . If the CPU determines that the number of tomographic images to be obtained has not reached N-n2 (NO in step S 219 ), the process returns to step S 215 . 
     In step S 220 , the CPU extracts tomographic images which can be used for averaging processing from the tomographic images at the scanning regions L[1], L[2], L[3], L[4], and L[5] which correspond to the numbers of times of scanning. In this extraction processing, the CPU may extract all the tomographic images obtained at the scanning regions L[1], L[2], L[3], L[4], and L[5]. 
     In addition, the CPU may calculate S/N (Signal/Noise) ratios indicating the ratios between signal amounts (signal levels) and noise (noise levels) from the respective tomographic images at the scanning regions L[1], L[2], L[3], L[4], and L[5]. The CPU may extract some of the tomographic images of the respective scanning regions based on the calculated S/N ratios (S/N ratios). 
     If an S/N ratio is higher than a predetermined value, that is, noise is small, even a small number of tomographic images used for averaging processing allow obtainment of a high-quality tomographic image. It is therefore possible to decrease the number of tomographic images to be extracted. In contrast, if the S/N ratio is equal to or less than the predetermined value, that is, noise is large, it is not possible to obtain a high-quality tomographic image from a small number of tomographic images used for averaging processing. In this case, it is necessary to increase the number of tomographic images to be extracted. 
     For example, the CPU may quantify and classify the S/N values into 10 levels from the first to 10th levels, and decide the numbers of tomographic images to be extracted in accordance with the levels. 
     Assume that, as indicated by “ 3003 ” in  FIG. 3 , the extraction results in this embodiment are: L[1]: N-a images, L[2]: N-n1-b images, L[3]: N-n1-c images, L[4]: N-n2-d images, and L[5]: N-n2-e images. Note that when all the tomographic images are to be extracted,  a , b, c, d, and e are 0. In contrast, when some of the tomographic images are to be extracted, these values are decided in accordance with the above S/N values. 
     In step S 221 , the CPU performs averaging processing by using the tomographic images extracted in step S 220  with respect to each of the scanning regions L[1], L[2], L[3], L[4], and L[5]. In step S 222 , the CPU outputs the final tomographic images like those indicated by “ 3004 ” in  FIG. 3 . With the above operation, the CPU terminates the processing. 
     The manner in which this embodiment can shorten the time taken to obtain tomographic images will be described with reference to a concrete example. When the CPU obtains the same number N of tomographic images at each of all the five scanning regions L[1], L[2], L[3], L[4], and L[5] assuming that the data obtaining rate of the line camera  139  is set to R=70000 data/sec, number N of tomographic images to be obtained=10, and the number of times of scanning at each scanning region L is set to B=1024, a time T1 required for tomographic image obtaining processing is calculated by
 
 T 1 =B×N× 5 /R= 1024×10×5/70000=0.73 [sec]  (3)
 
     When changing the number of tomographic images to be obtained for each scanning region as in this embodiment with n1=4 and n2=6, a time T2 required to obtain tomographic images is calculated by 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     That is, according to this embodiment, the time required to obtain tomographic images is calculated by T2−T1. That is, the embodiment can shorten the time required to obtain tomographic images by about 0.3 sec. Assuming that the imaging time is fixed, and the overall imaging time is almost equal to the time taken to obtain tomographic images, the number of tomographic images to be obtained may be decided for each scanning region. 
     If imaging time=tomographic image obtaining time=T3 and T3≦1 sec, a total number M of the numbers of times tomographic images are obtained at the respective scanning regions is calculated by using T3, the data obtaining rate R, and the scan count B according to 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     A maximum 68 times of measurement may be assigned to the scanning regions L[1], L[2], L[3], L[4], and L[5] according to the calculation result of expression (5) as follows: L[1]: 20 images, L[2]: 13 images, L[3]: 13 images, L[4]: 11 images, and L[5]: 11 images. 
     As described above, the present invention can shorten the time required for imaging and reduce the burden on a patient by decreasing the number of images to be obtained to obtain a high-quality tomographic image necessary for diagnosis. 
     Second Embodiment 
     The second embodiment of the present invention will be described with reference to  FIG. 6 . The arrangement of an OCT apparatus according to the second embodiment is the same as that of the OCT apparatus shown in  FIG. 1  described in the first embodiment, and hence a description of the arrangement will be omitted.  FIG. 6  is a flowchart showing the processing operation performed by the CPU of a personal computer  125  according to the second embodiment. The processing in each of steps S 201  to S 204  described with reference to the flowcharts of  FIGS. 2A and 2B  is changed to that in each of steps S 601  to S 621 . 
     In step S 601 , the CPU drives an XY scanner  119  in the Y-axis direction to a position to obtain a tomographic image at a scanning region L[1]. 
     In step S 602 , the CPU drives the XY scanner  119  in the X-axis direction to obtain a tomographic image at the scanning region L[1] in the X-axis direction (horizontal direction). 
     In step S 603 , the CPU calculates an S/N (Signal/Noise) ratio from the tomographic image at the scanning region L[1] obtained in step S 602 . 
     In step S 604 , the CPU determines the number of tomographic images to be obtained at the scanning region L[1] based on the S/N ratio calculated in step S 603 . If the S/N ratio is higher than a predetermined value, that is, noise is small, even a small number of tomographic images used for averaging processing allows obtainment of a high-quality tomographic image. It is therefore possible to decrease the number of tomographic images to be obtained. In contrast, if an S/N ratio is equal to or less than the predetermined value, that is, noise is large, it is not possible to obtain a high-quality tomographic image from a small number of tomographic images used for averaging processing. In this case, it is necessary to increase the number of tomographic images to be obtained. 
     For example, the CPU may quantify and classify the S/N values into 10 levels from the first to 10th levels, and decide the numbers of tomographic images to be obtained in accordance with the levels. If the S/N ratio is higher than the predetermined value, it is possible to execute no averaging processing by setting the number of tomographic images to be obtained to 1. As in the case of the scanning region L[1], the CPU drives the XY scanner  119  in the Y-axis direction to obtain tomographic images at scanning regions L[2], L[3], L[4], and L[5] in steps S 605 , S 609 , S 613 , and S 617 . 
     In steps S 616 , S 610 , S 614 , and S 618 , the CPU drives the XY scanner  119  in the X-axis direction to obtain tomographic images at the scanning regions L[2], L[3], L[4], and L[5] in the X-axis direction (horizontal direction). 
     In steps S 607 , S 611 , S 615 , and S 619 , the CPU calculates S/N ratios from the tomographic images at the scanning regions L[2], L[3], L[4], and L[5] obtained in steps S 616 , S 610 , S 614 , and S 618 . 
     In steps S 608 , S 612 , S 616 , and S 620 , the CPU decides the numbers of tomographic images to be obtained at the scanning regions L[2], L[3], L[4], and L[5] based on the calculated S/N ratios. 
     When the CPU completes the processing in step S 620 , the process advances to step S 621 . In step S 621 , the CPU detects the presence/absence of the input of an imaging start instruction by the operator to determine whether the operator has input an imaging start instruction. If the CPU determines that the operator has input an imaging start instruction (YES in step S 621 ), the CPU terminates the processing. The process then advances to step S 205 . If the CPU determines that the operator has not input an imaging start instruction (NO in step S 621 ), the process returns to step S 601 . Note that this arrangement is configured to return to step S 601 . However, the arrangement may be configured to wait until an imaging start instruction is input. 
     In this embodiment, the CPU detects S/N ratios and decides the numbers of tomographic images to be obtained in accordance with the S/N values. In contrast, a normal eye database may be mounted in the apparatus in advance to detect a lesion portion of the eye to be examined instead of detecting S/N ratios and decide the numbers of tomographic images to be obtained at the scanning regions L[1], L[2], L[3], L[4], and L[5] in accordance with the state of the detected lesion portion. In this case, for example, the CPU may decide a large number of tomographic images to be obtained at a scanning region in which a lesion portion is detected, and may decide a small number of tomographic images to be obtained at a scanning region in which no lesion portion is detected. 
     As described above, the present invention can shorten the time required for imaging and reduce the burden on a patient by decreasing the number of images to be obtained to obtain a high-quality tomographic image necessary for diagnosis. 
     Third Embodiment 
     The third embodiment of the present invention will be described with reference to  FIGS. 7 and 8 .  FIG. 7  shows the arrangement of an OCT apparatus according to the third embodiment. Note that the same reference numerals as in  FIG. 1  denote the same constituent elements in  FIG. 7 . 
     An objective lens  302  is disposed to face an eye  107  to be examined. A perforated mirror  303  provided on the optical axis splits light into an optical path  351  and an optical path  352 . 
     The optical path  352  forms an illumination optical system which illuminates the fundus of the eye  107 . The illumination optical system includes a halogen lamp  316 , a strobe tube  314 , a lens  309 , a lens  311 , an optical filter  310 , a ring slit  312 , a condenser lens  313 , a condenser lens  315 , and a mirror  317 . The halogen lamp  316  is used to position the eye  107 . The strobe tube  314  is used to image the fundus of the eye  107 . The ring slit  312  forms illumination light from the halogen lamp  316  and the strobe tube  314  into a ring-like light beam. The perforated mirror  303  reflects the light beam to illuminate the fundus of the eye  107 . 
     On the other hand, the optical path  351  forms an imaging optical system which captures a tomographic image of the fundus of the eye  107  and a fundus image. Referring to  FIG. 7 , a focus lens  304  and an imaging lens  305  are disposed on the right side of the perforated mirror  303 . In this case, the focus lens  304  is supported to be movable in the optical axis direction. A personal computer  125  controls the position of the focus lens  304 . The optical path  351  is guided to an area sensor  321  through a quick return mirror  318 . In this case, the quick return mirror  318  is designed to reflect and transmit parts of infrared light and reflect visible light. Since the quick return mirror  318  is designed to reflect and transmit parts of infrared light, it is possible to simultaneously use a fixation lamp, the area sensor  321 , and an OCT imaging unit. A mirror  319  is designed to form reflected light into an image on the area sensor  321 . Light passing through the optical path  351  is guided to a dichroic mirror  405  through a mirror  306 , a field lens  322 , a mirror  307 , and a relay lens  308 . 
     The area sensor  321  is connected to the personal computer  125  to allow the personal computer  125  to capture a fundus image. 
     The dichroic mirror  405  splits the optical path  351  into an optical path  351 - 1  for tomographic image capturing operation and an optical path  351 - 2  for fundus image capturing operation. In this case, a relay lens  406  and a relay lens  407  are movably held. Finely adjusting the positions of the relay lenses  406  and  407  can adjust the optical axes of the optical path  351 - 1  and optical path  351 - 2 . In this case, for the sake of simplicity, the XY scanner  408  is presented as one mirror. In practice, however, two mirrors, that is, an X scan mirror and a Y scan mirror, are disposed near each other to raster-scan on a retina  127  of the eye  107  in a direction perpendicular to the optical axis. In addition, the personal computer  125  controls the XY scanner  408 . The optical axis of the optical path  351 - 1  is adjusted to coincide with the rotation center of the two mirrors of the XY scanner  408 . 
     A camera unit  500  is a digital single-lens reflex camera for capturing a fundus image, and forms a fundus image on the surface of an area sensor  501 . A collimate lens  409  is connected to a single-mode fiber  130 - 4 . Since other arrangements are the same as those in  FIG. 1 , a description of them will be omitted. 
     Illumination light from the halogen lamp  316  passes through the condenser lens  315  and the condenser lens  313  and is reflected by the mirror  317 . The ring slit  312  then forms the light into a ring-like light beam. This light beam passes through the lens  309  and the lens  311  and is reflected by the perforated mirror  303 . The light then passes through the objective lens  302  and illuminates the fundus of the eye  107 . 
     Reflected light from the retina  127  of the eye  107  passes through the objective lens  302  and passes through the hole portion of the perforated mirror  303 . The light is reflected by the quick return mirror  318  and the mirror  319  and passes through the focus lens  304  and the imaging lens  305  to be formed into an image as an image of the eye on the area sensor  321 . The personal computer  125  captures the fundus image formed on the area sensor  321 . 
     The operation of the personal computer  125  according to the third embodiment will be described next with reference to the flowchart of  FIG. 8 . The processing in each of steps S 201  to S 204  described with reference to the flowcharts of  FIGS. 2A and 2B  is changed to that in each of steps S 801  to S 803 . 
     In step S 801 , the CPU calculates S/N ratios at positions corresponding to scanning regions L[1], L[2], L[3], L[4], and L[5] from the fundus images captured by the personal computer  125  and output from the area sensor  321 . 
     In step S 802 , the CPU decides the numbers of tomographic images to be obtained at the scanning regions L[1], L[2], L[3], L[4], and L[5] based on the calculated S/N ratios. This embodiment uses the same determination criterion used for decisions as that in the second embodiment. 
     In step S 803 , the CPU detects the presence/absence of the input of an imaging start instruction by the operator to determine whether the operator has input an imaging start instruction. If the CPU determines that the operator has input an imaging start instruction (YES in step S 803 ), the process advances to step S 205 . If the CPU determines that the operator has not input an imaging start instruction (NO in step S 803 ), the process returns to step S 801 . With the above operation, the CPU terminates the processing shown in  FIG. 8 . 
     In this embodiment, the CPU detects S/N ratios and decides the numbers of tomographic images to be obtained in accordance with the S/N values. In contrast, a normal eye database may be mounted in the apparatus in advance to detect a lesion portion of the eye to be examined instead of detecting S/N ratios and decide the numbers of tomographic images to be obtained at the scanning regions L[1], L[2], L[3], L[4], and L[5] in accordance with the state of the detected lesion portion. In this case, for example, the CPU may decide a large number of tomographic images to be obtained at a scanning region in which a lesion portion is detected, and may decide a small number of tomographic images to be obtained at a scanning region in which no lesion portion is detected. 
     As has been described above, the present invention can shorten the time required for imaging and reduce the burden on a patient by decreasing the number of images to be obtained to obtain a high-quality tomographic image necessary for diagnosis. 
     Other Embodiments 
     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 (for example, computer-readable storage 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 such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-079801 filed on Mar. 31, 2011, which is hereby incorporated by reference herein in its entirety.