WO2016009604A1 - Appareil de traitement d'image, procédé de traitement d'image et programme - Google Patents

Appareil de traitement d'image, procédé de traitement d'image et programme Download PDF

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Publication number
WO2016009604A1
WO2016009604A1 PCT/JP2015/003298 JP2015003298W WO2016009604A1 WO 2016009604 A1 WO2016009604 A1 WO 2016009604A1 JP 2015003298 W JP2015003298 W JP 2015003298W WO 2016009604 A1 WO2016009604 A1 WO 2016009604A1
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light
subject
model
image processing
polarization
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PCT/JP2015/003298
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English (en)
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Toshiharu Sumiya
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Canon Kabushiki Kaisha
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
    • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
    • A61B3/102—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for optical coherence tomography [OCT]
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/0059—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence
    • A61B5/0062—Arrangements for scanning
    • A61B5/0066—Optical coherence imaging

Definitions

  • the present invention relates to an image processing apparatus, an image processing method, and a program.
  • optical coherence tomography capable of imaging optical characteristics and motion of fundus tissues
  • a polarization sensitive OCT which is one of such OCT performs imaging using a polarization parameter (retardation and orientation) which is one of the optical characteristics of fundus tissues.
  • the polarization sensitive OCT may form a polarization sensitive OCT image using the polarization parameter and perform detection and segmentation of fundus tissues.
  • the polarization sensitive OCT divides interfering light into two linearly-polarized light beams which are orthogonal to each other and detects the linearly-polarized light beams using light modulated into circularly-polarized light as measurement light used to observe a sample so as to generate a polarization sensitive OCT image (refer to PTL 1).
  • the polarization sensitive OCT controls S-polarized light using a unit for controlling a polarization state, such as a polarization controller or a polarizer, so as to obtain a predetermined polarization state.
  • a polarization state may be changed with use of an apparatus.
  • a light guiding member such as an optical fiber
  • a bending load is applied to optical fibers, birefringence is generated, and therefore, a polarization state is changed.
  • Birefringence of optical elements has temperature dependency, and therefore, when a temperature of the apparatus is changed, a polarization characteristic of the optical fibers used in the light path may be changed.
  • PTL 2 proposes an apparatus including a control unit which reflects or scatters measurement light and a polarization control unit in order to correct changed polarized state.
  • the apparatus controls polarization of the measurement light using the polarization control unit in accordance with an intensity of the polarization obtained when the measurement light is irradiated to the control unit.
  • the control unit which reflects or scatters measurement light does not have birefringence, and performs correction on samples having retardation of 0°. Furthermore, the apparatus disclosed in PTL 2 performs correction using different polarization states for different cases, that is, a case where a subject is measured and a case where correction is performed.
  • a fundus tissue which is a subject, in a nerve fiber layer, for example, has a value of a retardation of approximately 10° to approximately 50°.
  • Optical transmissivities of members included in the light path of the OCT are different depending on a polarization component. Therefore, correction suitable for the subject may not be performed, and accordingly, there is a demand for more precise correction.
  • the present invention provides correction of a polarization state suitable for a subject.
  • the present invention provides an image processing apparatus that obtains an image representing a polarization state of a subject based on interfering light obtained from reference light and reflection light of measurement light which is irradiated to the subject.
  • the image processing apparatus includes a detector configured to detect the interfering light obtained from the reference light and the reflection light of the measurement light irradiated to the subject or a model having a predetermined polarization characteristic, a correction value calculation unit configured to calculate a correction value based on a value of the light from the model detected by the detector and the predetermined polarization characteristic of the model, and a polarization characteristic calculation unit configured to calculate a polarization characteristic of the subject based on the correction value and a value of the light from the subject detected by the detector.
  • Fig. 1 is a diagram illustrating an imaging apparatus serving as an image processing apparatus according to a first embodiment.
  • Fig. 2 is a diagram illustrating a model.
  • Fig. 3A is a diagram illustrating a luminance image of an optic disk portion.
  • Fig. 3B is a diagram illustrating a retardation image of the optic disk portion.
  • Fig. 3C is a diagram illustrating a DOPU image of the optic disk portion.
  • Fig. 3D is a diagram illustrating a retardation map of the optic disk portion.
  • Fig. 3E is a diagram illustrating a birefringence map of the optic disk portion.
  • Fig. 4 is a flowchart illustrating a correction process performed by the imaging apparatus.
  • Fig. 5 is a flowchart illustrating a measurement process.
  • Fig. 6 is a diagram illustrating an imaging apparatus according to a second embodiment.
  • Fig. 7 is a flowchart illustrating a correction process according to the second embodiment.
  • Fig. 8 is a diagram illustrating an imaging apparatus according to a third embodiment.
  • Fig. 9 is a diagram illustrating an imaging apparatus according to a fourth embodiment.
  • Fig. 10 is a diagram illustrating an imaging apparatus according to a fifth embodiment.
  • Fig. 1 is a diagram illustrating an imaging apparatus serving as an image processing apparatus according to a first embodiment.
  • an imaging apparatus an ophthalmic apparatus which obtains an image of a subject is described while setting a subject's eye as the subject.
  • the subject is not limited to the subject's eye.
  • skin, an internal organ, or the like may be set as a subject and an imaging apparatus may capture an image of skin, an internal organ, or the like.
  • an endoscope may be used as the imaging apparatus.
  • the imaging apparatus is polarization sensitive OCT (hereinafter referred to as "PS-OCT").
  • the imaging apparatus includes an optical interferometer 100, an anterior eye segment imaging unit 160, an internal fixation lamp 170, and a control device 180. Alignment of the apparatus is performed using an image of an anterior eye segment of the subject observed by the anterior eye segment imaging unit 160. After the alignment is completed, the internal fixation lamp 170 is turned on so that the optical interferometer 100 captures an image of an ocular fundus while the subject's eye gazes the internal fixation lamp 170.
  • a light source 101 is a super luminescent diode (SLD) which is a low coherent light source and emits light having a center wavelength of 850 nm and a bandwidth of 50 nm.
  • SLD super luminescent diode
  • any light source including amplified spontaneous emission (ASE) may be employed as long as the light source is capable of emitting low coherent light.
  • the light emitted from the light source 101 is guided into a polarization maintaining fiber coupler 104 through a polarization maintaining fiber 102 and a polarization controller 103 and is divided into measurement light and reference light.
  • the polarization controller 103 controls a state of polarization of the light emitted from the light source 101 so as to obtain linearly-polarized light.
  • the polarization controller 103 performs polarization control in an orthogonal direction relative to directions of polarized light beams divided by a fiber coupler 123 which will be described hereinafter.
  • an inline polarization controller is used as the polarization controller 103 in this embodiment, the present invention is not limited to this.
  • the polarization controller 103 may be a paddle polarization controller having a plurality of paddles, for example.
  • the polarization controller 103 may be a polarization controller configured by combining a ⁇ /4 wave plate and a ⁇ /2 wave plate with each other.
  • a division ratio of the polarization maintaining fiber coupler 104 is 90:10 ((reference light):(measurement light)).
  • the divided measurement light is emitted from a collimator 106 through a polarization maintaining fiber 105 as parallel light.
  • the emitted measurement light reaches a dichroic mirror 111 through an X scanner 107, lenses 108 and 109, and a Y scanner 110.
  • the X scanner 107 is constituted by a galvanometer mirror which performs scanning using the measurement light in a horizontal direction in an ocular fundus Er.
  • the Y scanner 110 is constituted by a galvanometer mirror which performs scanning using the measurement light in a vertical direction in the ocular fundus Er.
  • the X scanner 107 and the Y scanner 110 are controlled by a driving controller 181 so that a region of the ocular fundus Er is scanned by the measurement light.
  • the dichroic mirror 111 is characterized by reflecting light having a wavelength of 800 nm to 900 nm and allowing other light to pass.
  • the measurement light reflected by the dichroic mirror 111 is transmitted through a lens 112 to a ⁇ /4 wave plate 113 which is inclined by 45° so that a phase of the measurement light is shifted by 90° and the measurement light is subjected to polarization control so that circularly-polarized light is obtained.
  • light which is incident on the subject's eye is subjected to polarization control so as to obtain circularly-polarized light by disposing the ⁇ /4 wave plate 113 in an inclination manner by 45°.
  • the circularly-polarized light may not be obtained in the ocular fundus Er due to characteristics of the subject's eye. Therefore, the inclination of the ⁇ /4 wave plate 113 may be finely controlled by the driving controller 181.
  • a mirror 210 serving as a reflector is detachably disposed, that is, movably disposed.
  • the mirror 210 is controlled by the driving controller 181 so as to retract from the light path of the measurement light when an image of a subject is captured.
  • the driving controller 181 inserts the mirror 210 into the light path of the measurement light and causes the measurement light to be reflected by a mirror 211.
  • the mirror 211 reflects the measurement light reflected by the mirror 210 toward a model 200. A configuration of the model 200 will be described in detail later.
  • the measurement light is reflected by the model 200 toward the mirror 211 and returns to the polarization maintaining fiber coupler 104 through the light path described above.
  • the measurement light which has been subjected to the polarization control so that circularly-polarized light is obtained is focused on a retina layer of the ocular fundus Er through an anterior eye segment Ea of an eye which is the subject by a focus lens 114 disposed on a stage 116.
  • the measurement light which irradiates the ocular fundus Er is reflected and scattered by retina layers and returns to the polarization maintaining fiber coupler 104 through the light path described above.
  • the driving controller 181 performs a light path control process of guiding the light path of the measurement light to the model 200 or the subject by moving the mirror 210 serving as the reflector.
  • the reference light divided by the polarization maintaining fiber coupler 104 is emitted as parallel light from a collimator 118 through a polarization maintaining fiber 117.
  • the emitted reference light is subjected to polarization control in a ⁇ /4 wave plate 119 disposed in an inclination manner by 22.5°.
  • the reference light is reflected by a mirror 122 disposed on a coherence gate stage 121 through a dispersion compensation glass 120 and returns to the polarization maintaining fiber coupler 104.
  • the reference light passes the ⁇ /4 wave plate 119 twice so that linearly-polarized light returns to the polarization maintaining fiber coupler 104.
  • polarization control is performed so that linearly-polarized light which is inclined by 45° relative to directions of polarized light beams divided by the fiber coupler 123 which will be described below is obtained.
  • the coherence gate stage 121 is controlled by the driving controller 181 so as to cope with differences among lengths of eyeballs of examiners.
  • the reflection light of the measurement light which returns to the polarization maintaining fiber coupler 104 and the reference light are combined with each other so as to obtain interfering light to be incident on the fiber coupler 123 incorporating a polarization beam splitter, and the interfering light is divided into P-polarized light and S-polarized light which have different polarization directions (a division process).
  • the P-polarized light is transmitted through a polarization maintaining fiber 124 and a collimator 130, divided by a grating 131, and received by a lens 132 and a line camera 133.
  • the S-polarized light is transmitted through a polarization maintaining fiber 125 and a collimator 126, divided by a grating 127, and received by a lens 128 and a line camera 129.
  • the gratings 127 and 131 and the line cameras 129 and 133 are disposed so as to correspond to directions of the S-polarized light and the P-polarized light, respectively.
  • the light beams received by the line cameras 129 and 133 are output as electric signals corresponding to light intensities and received by a signal processor 182.
  • Fig. 2 is a diagram illustrating the model 200.
  • the model 200 includes a retarder 200a and a reflector 200b.
  • the retarder 200a and the reflector 200b may be in contact with each other or a gap may be interposed between the retarder 200a and the reflector 200b.
  • a lens (not illustrated) which collects light or which attains focus may be disposed in front of the model 200.
  • the retarder 200a is also referred to as a "phase shifter" since the retarder 200a shifts a phase of transmitted light.
  • the retarder 200a is a birefringent member or an optically-anisotropic body in which a retardation and a direction of an optical axis are predetermined. Since a predetermined value is set to a retardation, a material in which a thickness and birefringence thereof are easily defined and are hardly changed is preferably used. Examples of the retarder 200a include quartz, calcite, magnesium fluoride crystal, an organic matter having anisotropy, and liquid crystal.
  • the retarder 200a may be a single sheet of a birefringent material or obtained by combining two or more sheets of materials.
  • the retardation of an optic nerve fiber layer is approximately 10° to approximately 50°, and therefore, a retardation of the retarder 200a is also preferably a predetermined value within a range from 10° to 50°.
  • a calculated retardation is inverted every 90°, and therefore, a range of the retardation may be obtained by adding an integer multiple of 90°.
  • the retardation and the optical axis of the retarder 200a are measured in advance by a general birefringence measurement apparatus, such as a polarization microscope, or corrected PS-OCT.
  • the reflector 200b is a material which reflects the measurement light transmitted through the retarder 200a.
  • Examples of the reflector 200b include an optical mirror which is coated with metal.
  • a scattering body which is formed of metal microparticles, such as TiO 2 may be used.
  • a length of the light path of the measurement light is defined in a position of a reflection surface of the reflector 200b.
  • the model 200 is held inside the imaging apparatus in a predetermined position so as to have a predetermined optical axis.
  • the position of the model 200 is within a range in which a length of a light path of the reference light is controllable.
  • the model 200 is preferably disposed in a position in which a length of the light path of the measurement light at a time when the subject is measured is substantially the same as a length of the light path of the measurement light at a time when the model 200 is measured.
  • an incident angle of the measurement light relative to a surface of the retarder 200a is set such that a predetermined retardation is obtained.
  • the direction of the optical axis of the retarder 200a is maintained so as to have a predetermined arbitrary angle. Since the predetermined angle is set, correction may be performed even when orientation is shifted.
  • the anterior eye segment imaging unit 160 irradiates the anterior eye segment Ea with illumination light having a wavelength of 1000 nm emitted from an illumination light source 115 including LEDs 115a and 115b.
  • the light reflected by the anterior eye segment Ea is transmitted through the focus lens 114, the ⁇ /4 wave plate 113, the lens 112, and the dichroic mirror 111 to a dichroic mirror 161.
  • the dichroic mirror 161 is characterized by reflecting light having a wavelength of 980 nm to 1100 nm and allowing light having other wavelengths to pass.
  • the light reflected by the dichroic mirror 161 is transmitted through lenses 162, 163, and 164 and received by an anterior eye segment camera 165.
  • the light received by the anterior eye segment camera 165 is converted into an electric signal to be supplied to the signal processor 182.
  • the internal fixation lamp 170 will be described.
  • the internal fixation lamp 170 includes a display unit 171 and a lens 172.
  • the display unit 171 is formed by a plurality of light emitting diodes (LDs) arranged in a matrix. Lighting positions of the light emitting diodes are changed depending on a portion to be captured under control of the driving controller 181.
  • Light emitted from the display unit 171 is guided to the subject's eye through the lens 172.
  • the light emitted from the display unit 171 has a wavelength of 520 nm and displays a desired pattern by the driving controller 181.
  • the control device 180 includes the driving controller 181, the signal processor 182, a controller 183, and a display unit 184.
  • the driving controller 181 controls the various units as described above.
  • the driving controller 181 controls the mirror 210 so as to switch a target of irradiation of the measurement light between the subject and the model 200.
  • the signal processor 182 generates an image in accordance with signals output from the line cameras 129 and 133 and the anterior eye segment camera 165.
  • the signal processor 182 further performs analysis of the generated image and generation of visualization information of a result of the analysis. The generation of an image and the like will be described in detail later.
  • the controller 183 controls the entire imaging apparatus and displays an image and the like generated by the signal processor 182 in a display screen of the display unit 184.
  • the display unit 184 displays various information under control of the controller 183 as described below.
  • the display unit 184 is a liquid crystal display, for example.
  • image data generated by the signal processor 182 may be transmitted to the controller 183 in a wired manner or a wireless manner.
  • the controller 183 may be regarded as an image processing apparatus.
  • the control device 180 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). Functions and processes of the control device 180 described below are realized when the CPU reads programs stored in the ROM or the like and executes the programs.
  • the signal processor 182 generates a tomographic signal by performing a reconfiguration process used in spectral domain (SD)-OCT on interfering signals supplied from the line cameras 129 and 133.
  • the signal processor 182 removes fixed pattern noise from the interfering signals.
  • the removal of the fixed pattern noise is performed by extracting the fixed pattern noise by averaging a plurality of detected A-scans and subtracting the extracted fixed pattern noise from the input interfering signals.
  • the signal processor 182 converts wavelengths of the interfering signals into wavenumbers and generates tomographic signals by performing Fourier transform. By performing the process on the two interfering signals having different polarization components, two tomographic signals based on the polarization components are generated.
  • the signal processor 182 generates tomographic luminance images from the two tomographic signals described above.
  • the signal processor 182 generates two tomographic images based on the polarization components (which are also referred to as a "tomographic image corresponding to first polarized light” and a “tomographic image corresponding to second polarized light”) by aligning the tomographic signals in synchronization with driving of the X scanner 107 and the Y scanner 110.
  • the luminance images are basically the same as tomographic images generated by general OCTs, and pixel values r of the luminance images are calculated from tomographic signals A H and A V obtained by the line cameras 129 and 133 in accordance with Expression 1.
  • Fig. 3A is a diagram illustrating a luminance image of an optic disk portion.
  • the signal processor 182 generates a retardation image using the tomographic signals having the polarization components which are orthogonal to each other.
  • Values ⁇ of pixels of the retardation image are obtained by digitizing phase differences (presence or absence (a thickness) of a fiber layer from an intensity ratio) between vertical polarization components and horizontal polarization components and are calculated using the tomographic signals A H and A V in accordance with Expression 2.
  • ⁇ arctan[A V /A H ] Expression 2
  • Fig. 3B is a diagram illustrating a retardation image (which is also referred to as a "tomographic image" representing a phase difference of polarized light beams) of the optic disk portion generated as described above, and the image is obtained by calculating Expression 2 on B scan images.
  • a portion in which a phase difference is generated in the tomographic image is displayed, and a deep color portion (denoted by negatively-sloped hatched line in Fig. 3B) corresponds to a large phase difference whereas a light color portion (denoted by positively-sloped hatched line in Fig. 3B) corresponds to a small phase difference. Therefore, by generating the retardation image, a layer having birefringence may be recognized.
  • the signal processor 182 calculates a Stokes vector S for each pixel using the obtained tomographic signals A H and A V and a phase difference ⁇ between the tomographic signals A H and A V in accordance with Expression 3.
  • the signal processor 182 sets windows having a length of approximately 70 ⁇ m in a main scanning direction of the measurement light and a length of approximately 18 ⁇ m in a depth direction in the B scan images. Thereafter, the signal processor 182 averages elements of the Stokes vectors calculated for individual pixels in accordance with Expression 3 in the windows. Then the signal processor 182 calculates degrees of polarization uniformity (DOPU) in the windows in accordance with Expression 4.
  • DOPU degrees of polarization uniformity
  • Q m ", "U m “, and “V m” represent values obtained by averaging elements Q, U, and V of the Stokes vectors in the windows.
  • the signal processor 182 performs this process on all the windows included in the B scan images so as to generate a DOPU image (which is also referred to as a "tomographic image representing a degree of polarization uniformity") of the optic disk portion illustrated in Fig. 3C.
  • the DOPU is a numeric value representing a degree of polarization uniformity.
  • the DOPU is a numeric value near 1 in portions in which polarization is maintained and is a numeric value smaller than 1 in portions in which polarization is cancelled.
  • RPE retinal pigment epithelium
  • a value of the DOPU is smaller in a portion corresponding to the RPE in the DOPU image when compared with other regions.
  • light color portions hatchched portions in Fig. 3C
  • the DOPU image is obtained by imaging a layer which cancels polarization, such as the RPE, even when the RPE is deformed due to disease, an image of the RPE is reliably realized instead of use of luminance change.
  • the signal processor 182 generates an orientation image using the phases ⁇ H and ⁇ V of the tomographic signals having the polarization components which are orthogonal to each other.
  • Values ⁇ of pixels in the orientation image represent directions of the optical axis relative to the measurement light in positions of the pixels included in the tomographic images.
  • the values ⁇ are calculated using the phase difference ⁇ of the tomographic signals having the polarization components which are orthogonal to each other in accordance with Expression 5.
  • ⁇ ( ⁇ - ⁇ )/2
  • the direction of the optical axis depends on anisotropy of an internal configuration of the subject.
  • the optical axis is generated along a scanning direction of the nerve fiber. Therefore, by generating an orientation image, a direction of anisotropy of a layer having birefringence may be recognized.
  • the signal processor 182 performs segmentation of the tomographic images using the luminance images described above. First, the signal processor 182 generates images (hereinafter referred to as a "median image” and a “Sobel image”) by applying a median filter and a Sobel filter to the tomographic images to be processed. Next, the signal processor 182 generates a profile for each A-scan using the generated median image and the generated Sobel image. A profile of a luminance value is generated from the median image and a profile of a gradient is generated from the Sobel image. Subsequently, the signal processor 182 detects a peak of the profile generated from the Sobel image.
  • images hereinafter referred to as a "median image” and a "Sobel image”
  • the signal processor 182 extracts boundaries between regions of the retina layers with reference to the profile generated from the median image corresponding to a portion before or after the detected peak or a portion between peaks. Furthermore, the signal processor 182 measures membrane thicknesses in a direction of an A-scan line so as to generate a membrane thickness map of the layers.
  • the signal processor 182 generates a retardation map from a retardation image obtained from a plurality of B-scan images.
  • the signal processor 182 detects the RPE in the B scan images.
  • the RPE is characterized by cancelling polarized light. Therefore, the signal processor 182 checks distribution of retardation in a range from an inner limiting membrane (ILM) to a point which does not include the RPE along a depth direction in the A-scans and sets the maximum values as representative values of the retardation in the A-scans.
  • the signal processor 182 performs the process described above on all retardation images so as to generate a retardation map.
  • ILM inner limiting membrane
  • the retardation map of the optic disk portion is illustrated in Fig. 3D.
  • a deep color portion corresponds to a small phase difference whereas a light color portion (a hatched portion in Fig. 3D) corresponds to a large phase difference.
  • a layer having birefringence is a retina nerve fiber layer (RNFL)
  • the retardation map represents a phase difference caused by the birefringence of the RNFL and a thickness of the RNFL. Therefore, a portion having a large thickness of the RNFL has a large phase difference whereas a portion having a small thickness of the RNFL has a small phase difference. Accordingly, using the retardation map, a thickness of the RNFL of the entire ocular fundus may be recognized and is used for diagnosis of glaucoma.
  • the signal processor 182 linearly approximates a value of a retardation ⁇ in a range from the ILM to the RNFL in each of the-A scan images of the retardation image generated as described above and determines an inclination of the obtained value as birefringence in a position on the retina in each of the A-scan images.
  • the signal processor 182 performs this process on all the obtained retardation images so as to generate a map representing birefringence.
  • a birefringence map of the optic disk portion is illustrated in Fig. 3E.
  • the birefringence map may be drawn as a change of birefringence when a fiber construction is changed even in a case where a thickness of the RNFL is not changed.
  • the tomographic images corresponding to the first polarized light and the second polarized light described above, the retardation image, the DOPU image, and the like are referred to as "tomographic images representing polarization states” where appropriate.
  • the retardation map and the birefringence map described above are referred to as "ocular fundus images representing polarization states”.
  • a change of a polarization state of an optical fiber used as a light guide member will be described.
  • a single mode light fiber when a bending load is applied to the fiber, birefringence is generated in the fiber and a polarization state is changed.
  • the change of the polarization state may be corrected by a polarization controller.
  • the birefringence of the fiber has a temperature dependency, even when correction is performed under a certain temperature, a polarization state is changed with a change of an operating temperature, and accordingly, the polarization state is required to be corrected again.
  • a degree of the change also depends on characteristics of the fiber.
  • an azimuth angle and ellipticity of the polarization of light which is transmitted through the fiber changes by several degrees.
  • the same change occurs in elements including the optical fiber, such as a polarization coupler and a polarization controller.
  • the polarization state is changed, an error occurs in calculated values of the retardation and the orientation. Therefore, a stable image may be obtained by performing the correction.
  • Fig. 4 is a flowchart illustrating a correction process performed by the imaging apparatus. It is assumed that a user selects a correction mode by operating a correction start button (not illustrated) displayed in the display unit 184 or by operating a correction start button physically disposed on the apparatus. Then the control device 180 accepts an instruction for starting the correction, sets a correction mode as an operation mode, and starts the correction process illustrated in Fig. 4.
  • step S100 the driving controller 181 inserts the mirror 210 in the light path of the measurement light (a light path control process) so that the measurement light is irradiated to the model 200 serving as an imaging subject.
  • the measurement light reflected by the model 200 encounters the fiber coupler 123 which divides the measurement light into P-polarized light and S-polarized light which have different polarization directions (a division process), and thereafter, the P-polarized light and the S-polarized light are received by the line cameras 133 and 129, respectively.
  • the line cameras 129 and 133 output signals (tomographic signals) corresponding to amounts of the received light beams.
  • step S101 the control device 180 obtains tomographic signals A H0 and A V0 corresponding to the model 200 from the line cameras 129 and 133.
  • the tomographic signals A H0 and A V0 changes of a polarization characteristic in the light path until the light is supplied from the light source 101 to the line cameras 129 and 133 are superimposed on the polarization characteristic of the model 200.
  • the process in step S101 is an example of a process of detecting the P-polarized light and the S-polarized light which have different interfering light beams and which are obtained by dividing the measurement light by the fiber coupler 123 and outputting signals corresponding to detected values.
  • step S102 the control device 180 calculates correction coefficients by comparing the predetermined retardation ⁇ 0 of the model 200 with the tomographic signals A H0 and A V0 .
  • a ratio between the tomographic signals A H0 and A V0 is changed.
  • ⁇ a correction coefficient for correcting the change of the ratio between the tomographic signals A H0 and A V0
  • the retardation ⁇ 0 and the tomographic signals A H0 and A V0 have the relationship represented by Expression 6.
  • ⁇ 0 arctan[ ⁇ *A V0 /A H0 ] Expression 6
  • control device 180 calculates the correction coefficient ⁇ using Expression 7 in accordance with the relationship represented by Expression 6.
  • the correction coefficient ⁇ is an example of an amount of correction.
  • the correction process is thus terminated.
  • the control device 180 terminates the correction mode.
  • Fig. 5 is a flowchart illustrating a measurement process performed by the imaging apparatus. It is assumed that the user selects a measurement mode by operating a measurement start button (not illustrated) displayed in the display unit 184 or by operating a measurement start button physically disposed on the apparatus. Then the control device 180 accepts an instruction for starting the measurement, sets a measurement mode as the operation mode, and starts the measurement. In step S200, the driving controller 181 retracts the mirror 210 from the light path of the measurement light so that the measurement light is irradiated to the subject. Thereafter, in step S201, the control device 180 obtains tomographic signals A H and A V corresponding to the subject from the line cameras 129 and 133. Note that, in the tomographic signals A H and A V , changes of a polarization characteristic in the light path until the light is supplied from the light source 101 to the line cameras 129 and 133 are superimposed on the polarization characteristic of the subject.
  • step S202 the control device 180 calculates polarization characteristics using the tomographic signals A H and A V so as to generate an image.
  • the control device 180 calculates a retardation ⁇ using the tomographic signals A H and A V and the correction coefficient ⁇ in accordance with Expression 8.
  • ⁇ arctan[ ⁇ *A V /A H ] Expression 8
  • the retardation ⁇ which does not include the change of the polarization characteristic in the light path may be calculated by multiplying the intensity ratio between the tomographic signals A H and A V by the correction coefficient ⁇ .
  • the process in step S202 is an example of a polarization characteristic calculation process of calculating the polarization characteristic of the subject using the correction coefficient ⁇ serving as a correction amount and the tomographic signals A H and A V serving as detected values. The measurement process is thus terminated. When the measurement process is terminated, the control device 180 terminates the measurement mode.
  • the imaging apparatus may perform correction in a condition similar to a condition of the subject by calculating a correction coefficient as a correction amount from a result of measurement of the model and correcting a measurement value using the correction coefficient. That is, the imaging apparatus may perform correction with high accuracy and obtain a stable image.
  • a timing when the correction is performed is not limited to the timing of this embodiment.
  • the control device 180 may automatically start the correction process instead of the operation of a button performed by the user or the like.
  • the control device 180 may execute the correction process when the imaging apparatus is powered or when a predetermined period of time is elapsed.
  • the imaging apparatus may include a thermometer, not illustrated, and the control device 180 may execute the correction process when a temperature is changed by a predetermined amount.
  • the control device 180 may execute the correction process every time a measurement button is pressed.
  • the control device 180 may execute the correction process after tomographic signals of the subject are obtained.
  • the correction coefficient is not limited to that of this embodiment.
  • the control device 180 may calculate the retardation ⁇ of the subject using the tomographic signals A H and A V of the subject and the correction coefficient ⁇ in accordance with Expression 10.
  • ⁇ arctan[A V /A H ] + ⁇ Expression 10
  • control device 180 may selectively use the first correction formula represented by Expression 8 or the second correction formula represented by Expression 9 in accordance with tendency of the change of the polarization characteristic of the imaging apparatus. For example, the control device 180 selects the first correction formula when influence of the change after the division by the fiber coupler 123 is relatively large. As a cause of the change, a change of the polarization state at a time when the light passes the fiber coupler 123 and a change of sensitivity of a spectrometer are taken as examples. On the other hand, the control device 180 selects the second correction formula when influence of the change in a portion of the light path on an upstream side relative to the fiber coupler 123 is large. Note that a degree of the influence of the change is measured by a polarization meter or the like in advance and the user or the like selects one of the correction formulas in accordance with a result of the measurement and sets the selected correction formula in the imaging apparatus.
  • the polarization characteristic is not limited to the retardation.
  • the imaging apparatus may perform correction the same as the correction performed in accordance with the second correction formula represented by Expression 9 even when the orientation is calculated, for example.
  • ⁇ ⁇ 0 -( ⁇ - ⁇ 0 )/2
  • control device 180 calculates the orientation ⁇ of the subject using the tomographic signals A H and A V of the subject and the correction coefficient ⁇ in accordance with Expression 12.
  • ⁇ ( ⁇ - ⁇ 0 )/2 + ⁇ Expression 12
  • control device 180 may correct the orientation.
  • Fig. 6 is a diagram illustrating an imaging apparatus according to a second embodiment.
  • the imaging apparatus includes a first polarization controller 220, a second polarization controller 221, and a third polarization controller 222.
  • the first polarization controller 220, the second polarization controller 221, and the third polarization controller 222 are referred to as "polarization controllers 220 to 222" where appropriate.
  • the first polarization controller 220 is the same as the polarization controller 103 included in the imaging apparatus according to the first embodiment.
  • the three polarization controllers 220 to 222 control polarization under control of a driving controller 181.
  • the first polarization controller 220 is disposed in a light path of light emitted from a light source 101, and performs polarization control so as to convert the emitted light into vertically-polarized light.
  • the second polarization controller 221 is disposed in a light path of reference light, and performs polarization control so as to convert the reference light into vertically-polarized light.
  • the third polarization controller 222 is disposed in a light path of measurement light, and performs polarization control so as to convert the measurement light into vertically-polarized light.
  • a shutter 223 is disposed in the light path of the measurement light so as to block the measurement light under control of the driving controller 181.
  • the polarization controllers 220 to 222 are inline polarization controllers.
  • the present invention is not limited to this. Any polarization controller may be used as the polarization controllers 220 to 222 as long as polarization is controlled under control of the driving controller 181.
  • the polarization controllers 220 to 222 may be paddle polarization controllers or polarization controllers configured by combining a ⁇ /4 wave plate and a ⁇ /2 wave plate with each other.
  • Fig. 7 is a flowchart illustrating a correction process according to the second embodiment. It is assumed that a user selects a correction mode by operating a correction start button (not illustrated) displayed in a display unit 184 or by operating a correction start button physically disposed on the apparatus. Then a control device 180 accepts an instruction for starting the correction, sets a correction mode as an operation mode, and starts the correction process illustrated in Fig. 7.
  • the driving controller 181 inserts the shutter 223 in the light path of the measurement light. By this, the measurement light is blocked and line cameras 129 and 133 only receive the reference light.
  • the control device 180 controls the first polarization controller 220 so that the first polarization controller 220 moves to a predetermined initial position.
  • the predetermined initial position may be a position where preceding correction is performed or a position determined when the imaging apparatus is assembled and adjusted.
  • step S302 the control device 180 obtains reference light signals A HS and A VS representing intensities of the reference light output from the line cameras 129 and 133. Thereafter, in step S303, the control device 180 calculates an intensity of a reference light signal from the reference light signals A HS and A VS , that is, a signal intensity As, in accordance with Expression 13.
  • the control device 180 compares the signal intensity As with a predetermined threshold value.
  • the threshold value for the intensity of the reference light signal is determined in accordance with an intensity of a reference light signal which is measured in advance when the imaging apparatus is assembled and adjusted.
  • the threshold value is a minimum intensity for obtaining at least an image having desired image quality at a time when a subject is measured.
  • step S303 When the signal intensity As is equal to or larger than the threshold value (Yes in step S303), the control device 180 proceeds to step S304.
  • the control device 180 returns to step S301 where the control device 180 controls the first polarization controller 220 again. Specifically, the control device 180 controls the first polarization controller 220 so that the signal intensity exceeds the threshold value.
  • the control device 180 may control the first polarization controller 220 in an entire control range of the first polarization controller 220 or may use a general optimization algorithm, such as a gradient method.
  • step S304 the control device 180 controls the second polarization controller 221 so that the second polarization controller 221 moves to a predetermined initial position.
  • the predetermined initial position may be a position where preceding correction is performed or a position determined when the imaging apparatus is assembled and adjusted.
  • step S305 the control device 180 obtains reference light signals A HS and A VS representing intensities of the reference light beams output from the line cameras 129 and 133.
  • step S306 the control device 180 calculates a ratio of the intensities of the reference light signals. Then the control device 180 determines whether the ratio of the intensities of the reference light signals is included in a first range.
  • the first range is set in advance.
  • step S306 When the ratio of the intensities of the reference light signals is included in the first range (Yes in step S306), the control device 180 proceeds to step S307. When the ratio of the intensities of the reference light signals is not included in the first range (No in step S306), the control device 180 returns to step S304 where the control device 180 controls the second polarization controller 221 again.
  • the reference light when a polarization characteristic in the light path is not changed, the reference light encounters a fiber coupler 123 as a linearly-polarized light polarized by 45° and is divided by an intensity ratio of 50:50. Accordingly, an intensity ratio of the reference light signals is ideally 1. Accordingly, a range in which the intensities of the reference light signals are substantially the same taking accuracy required by the imaging apparatus or control accuracy of the second polarization controller 221 into consideration is set as the first range.
  • the control device 180 may control the second polarization controller 221 in an entire control range of the second polarization controller 221 or may use a general optimization algorithm, such as a gradient method.
  • step S307 the driving controller 181 retracts the shutter 223 from the light path of the measurement light.
  • the driving controller 181 further inserts a mirror 210 so that the measurement light is irradiated to the model 200.
  • step S308 the driving controller 181 moves the third polarization controller 222 to a predetermined initial position.
  • the predetermined initial position may be a position where preceding correction is performed or a position determined when the imaging apparatus is assembled and adjusted.
  • step S309 the control device 180 obtains tomographic signals A H and A V representing intensities of interfering light beams output from the line cameras 129 and 133.
  • step S310 the control device 180 calculates a ratio of intensities of tomographic signals. Then the control device 180 determines whether the ratio of the intensities of the tomographic signals is included in a second range.
  • the second range is a range defined in accordance with a predetermined retardation ⁇ 0 of the model 200.
  • the control device 180 retracts the mirror 210 from the light path of the measurement light so that the measurement light is irradiated to the subject. Thereafter, the control device 180 terminates the correction process.
  • step S310 when the ratio of the intensities of the tomographic signals is not included in the second range (No in step S310), the control device 180 returns to step S308 where the control device 180 controls the third polarization controller 222 again.
  • the process in step S308 is an example of a polarization control process of controlling polarization.
  • the retardation ⁇ 0 of the model 200 is obtained in advance, and when the polarization characteristic of the light path is not changed, the ratio of the intensities of the tomographic signals is represented by Expression 14.
  • a V /A H tan[ ⁇ 0 ]
  • a range in which the intensities of the tomographic signals are substantially the same taking accuracy required by the imaging apparatus or control accuracy of the third polarization controller 222 into consideration is set as the second range.
  • a range of a ratio of intensities of the tomographic signals A H and A V from tan[0.99* ⁇ 0 ] to tan[1.01* ⁇ 0 ] is set as the second range.
  • the control device 180 may control the third polarization controller 222 in an entire control range of the third polarization controller 222 or may use a general optimization algorithm, such as a gradient method.
  • the imaging apparatus according to the second embodiment may perform correction in a condition similar to a condition of the subject using a result of measurement of a model.
  • a timing when the correction process is started is not limited to the timing of the second embodiment as long as the correction process is performed before the measurement of the subject.
  • the imaging apparatus may perform both of the correction process according to the second embodiment described with reference to Fig. 7 and the correction process according to the first embodiment described with reference to Fig. 4.
  • the correction process according to the second embodiment is referred to as a "mechanical correction process” and the correction process according to the first embodiment is referred to as a "numerical correction process” hereinafter.
  • the imaging apparatus executes the numerical correction process after the mechanical correction process is executed.
  • the imaging apparatus executes the numerical correction process after the polarization control by the mechanical correction process is executed.
  • the control device 180 may use the tomographic signals A H and A V obtained by the mechanical correction process in the numerical correction process and may obtain the tomographic signals A H and A V again in the numerical correction process.
  • the imaging apparatus may execute the mechanical correction process once before the measurement of the subject, and thereafter, perform the numerical correction process every time the measurement of the subject is performed immediately before or the immediately after the measurement of the subject is performed.
  • the mechanical correction process requires a long period of time since the polarization controller is required to be controlled.
  • the numerical correction process may obtain a correction coefficient by one measurement process. Therefore, by executing the mechanical correction process and the numerical correction process in combination as described above, the correction may be efficiently executed.
  • the imaging apparatus may correct a change which is not corrected by the mechanical correction process and a change generated after the correction in the numerical correction process by performing the mechanical correction process and the numerical correction process in combination.
  • the imaging apparatus may perform correction with high accuracy when compared with a case where the mechanical correction process or the numerical correction process is solely performed, and obtain a stable image.
  • Fig. 8 is a diagram illustrating an imaging apparatus according to a third embodiment.
  • the imaging apparatus according to the third embodiment changes an irradiation direction of measurement light between a model 200 and a subject by controlling a direction of a Y scanner 110. Note that it is not necessarily the case that a direction of measurement light irradiated from the Y scanner 110 to the model 200 illustrated in Fig. 8 coincides with an actual direction.
  • the model 200 is disposed in a predetermined position facing a predetermined direction.
  • a ⁇ /4 wave plate 230 is disposed between the Y scanner 110 and the model 200.
  • the ⁇ /4 wave plate 230 is inclined by 45° relative to the vertically-polarized measurement light.
  • the measurement light is subjected to polarization control so as to be converted into circularly-polarized light when passing the ⁇ /4 wave plate 230 since a phase of the measurement light is shifted by 90° and is irradiated to the model 200.
  • a lens which collects light or which attains focus may be disposed in front of the model 200.
  • a target of irradiation of the measurement light may be changed by an X scanner 107.
  • the imaging apparatus according to the third embodiment may perform switching between irradiation to the model 200 and irradiation to the subject using the scanner. Specifically, the imaging apparatus according to the third embodiment may perform correction using a smaller number of components. Furthermore, the imaging apparatus may perform correction for each tomographic image, and accordingly, a more stable image may be obtained.
  • Fig. 9 is a diagram illustrating an imaging apparatus according to a fourth embodiment.
  • the imaging apparatus according to the fourth embodiment does not include the mirrors 210 and 211.
  • a model 240 is disposed in a light path of measurement light in a detachable manner.
  • the model 240 includes a retarder 240a which has a predetermined retardation and an optical axis in a predetermined direction and a reflector 240b which reflects the measurement light.
  • a driving controller 181 inserts the model 240 into the light path of the measurement light.
  • the correction process is executed in a state in which the model 240 is disposed in the light path of the measurement light.
  • the model 240 may also serve as a portion of a shutter which blocks the measurement light.
  • the imaging apparatus according to the fourth embodiment may perform correction using a smaller number of components since the imaging apparatus does not require the mirrors 210 and 211.
  • Fig. 10 is a diagram illustrating an imaging apparatus according to a fifth embodiment.
  • a user or the like selects a correction mode when a subject is not positioned in a measurement position and sets a model 250 in a position of the subject.
  • the model 250 includes a retarder 250a which has a predetermined retardation and a reflector 250b which reflects measurement light.
  • the model 250 may be manually installed by the user or may be automatically installed by a control device 180 of the imaging apparatus. In this case, a position and an inclination of the model 250 are defined using a tool or a mechanism (not illustrated) used to fix the model 250.
  • the model 250 is disposed in a position corresponding to the subject.
  • the model 250 is not required to be incorporated in the imaging apparatus according to the fifth embodiment, and therefore, correction may be performed with a simpler configuration.
  • the emission light emitted from the light source 101 is converted into vertically-polarized light by the polarization controller 103 in the foregoing description
  • the emission light may be converted into another linearly-polarized light having a different azimuth direction, such as horizontally-polarized light.
  • an angle of a wave plate and a calculation formula corresponding to the polarized light in the different azimuth direction are employed.
  • the imaging apparatuses according to the foregoing embodiments are applicable to Time-Domain (TD)-OCT and Swept-Source (SS)-OCT in addition to the SD-OCT.
  • a subject of the imaging apparatus is not limited to those of the foregoing embodiments.
  • the imaging apparatus is at least OCT which measures a polarization characteristic of a subject, and may be OCT which measures polarization characteristics of living bodies other than skin, internal organs, blood vessels, teeth, and eyes or samples other than living bodies.
  • Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a 'non-transitory computer-readable storage medium') to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s).
  • computer executable instructions e.g., one or more programs
  • a storage medium which may also be referred to more fully as
  • the computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions.
  • the computer executable instructions may be provided to the computer, for example, from a network or the storage medium.
  • the storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD) TM ), a flash memory device, a memory card, and the like.

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Abstract

L'invention concerne un appareil de traitement d'image qui obtient une image représentant un état de polarisation d'un sujet sur la base de la lumière d'interférence obtenue d'une lumière de référence et d'une lumière de réflexion de la lumière de mesure qui est irradiée sur le sujet. L'appareil de traitement d'image comprend un détecteur configuré pour détecter la lumière d'interférence obtenue de la lumière de référence et la lumière de réflexion de la lumière de mesure irradiée sur le sujet ou sur un modèle ayant une caractéristique de polarisation prédéterminée, une unité de calcul de valeur de correction configurée pour calculer une valeur de correction sur la base d'une valeur de la lumière provenant du modèle détecté par le détecteur et de la caractéristique de polarisation prédéterminée du modèle, et une unité de calcul de caractéristique de polarisation configurée pour calculer une caractéristique de polarisation du sujet sur la base de la valeur de correction et d'une valeur de la lumière provenant du sujet détecté par le détecteur.
PCT/JP2015/003298 2014-07-16 2015-06-30 Appareil de traitement d'image, procédé de traitement d'image et programme WO2016009604A1 (fr)

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