JP2008264048A - Fundus examination apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fundus examination apparatus easily and highly precisely measuring the thickness of the optic nerve fiber layer. <P>SOLUTION: A Stokes vector computing section 53 finds Stokes vector based on a signal acquired by a fundus examination section 60. A phase difference distribution computing section 54 finds a phase difference distribution of a fundus reflection light by a specific area (Henle's outer fiber layer) of the fundus based on the Stokes vector. An anterior segment slow phase axis computing section 55 and an anterior segment phase difference computing section 56 find anterior segment components (a slow phase axis of the anterior segment and a phase difference caused by the anterior segment) based on the phase difference distribution. An NFL (Nerve Fiber Layer) component extraction section 57 extracts the NFL component from the polarization state of the fundus reflection light based on the anterior segment component. The NFL thickness computing section 58 computes the thickness of the stratum opticum based on the NFL component. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fundus inspection apparatus for inspecting the fundus of a subject's eye, and more particularly to a technique for measuring the thickness of the layer tissue of the fundus.

  Layered tissues such as the retina, choroid and sclera are present on the fundus. Furthermore, the retina is formed including layer tissues such as an optic nerve fiber layer, a photoreceptor cell layer, and a retinal pigment epithelium layer. The optic nerve fiber layer is also called a retinal nerve fiber layer, a nerve fiber layer, an optic nerve fiber bundle, or the like.

  Information on the thickness of these layer tissues is an important diagnostic material in the ophthalmic field. For example, in glaucoma eyes, it is known that the optic nerve head is compressed as the intraocular pressure increases, and the surrounding optic nerve fiber layer becomes thinner. Using this knowledge, thickness information of the optic nerve fiber layer around the optic disc is used as a diagnostic material for glaucoma.

  Methods for measuring the thickness of the layer tissue of the fundus (particularly the optic nerve fiber layer) include a method using polarized light and a method using OCT (Optical Coherence Tomography). The former method is disclosed in Patent Document 1, for example. The latter technique is disclosed in, for example, Patent Document 2.

  Although measurement of the thickness of the layer tissue of the fundus is useful as described above, the devices described in Patent Documents 1 and 2 are generally expensive, so it seems that there are many medical institutions that cannot introduce this. In response to such circumstances, Patent Document 3 proposes an apparatus that can be provided at a lower cost.

  The fundus examination apparatus of Patent Document 3 irradiates the fundus with circularly polarized illumination light, analyzes the polarization characteristics of the fundus reflection light, and obtains the Stokes vector (Stokes parameter), thereby reducing the thickness of the optic nerve fiber layer. It is what you want. This fundus inspection apparatus has a configuration similar to that of a normal fundus camera, and has an advantage at least in price compared with the apparatuses of Patent Documents 1 and 2.

  This fundus examination apparatus includes a fundus camera and a computer. 6 and 7 show the configuration of the optical system of the fundus camera. The fundus camera has an illumination optical system 120 and a light receiving optical system 121. The illumination optical system 120 is provided with a xenon lamp 122 and a halogen lamp 123. The xenon lamp 122 and the halogen lamp 123 are each arranged at a conjugate position with respect to the half mirror 124.

  The illumination light output from the xenon lamp 122 is relayed to the vicinity of the ring-shaped aperture X by the condenser lens 125. Similarly, the illumination light output from the halogen lamp 123 is relayed to the vicinity of the ring-shaped aperture X by the condenser lens 126.

  The illumination light that has passed through the ring-shaped stop X enters the eye E via the relay lens 127, the total reflection mirror 128A, the relay lens 128, the perforated mirror 129, and the objective lens 130, and illuminates the fundus oculi Ef.

  A polarization unit 137 is inserted in the optical path between the relay lens 128 and the perforated mirror 129. The polarization unit 137 converts the polarization characteristics of the illumination light. The polarization unit 137 includes a green filter (or interference filter) 138, a polarization filter 139 having linear polarization characteristics, and a quarter wavelength plate 140. With such a configuration, the polarization unit 137 acts to convert the illumination light into circularly polarized light. The polarization unit 137 is retracted from the optical path when observing the fundus oculi Ef.

  The light receiving optical system 121 guides the fundus reflection light P of the illumination light to the CCD camera 135 and the photographic film 136. The light receiving optical system 121 includes an objective lens 130, a perforated mirror 129, a focusing lens 132, a relay lens 133, a quick return mirror 134, a CCD camera 135, and a photographic film 136. The CCD camera 135 takes an image projected on the imaging surface 135a. Note that a similar CMOS camera can be used instead of the CCD camera 135. The photographic film 136 and the imaging surface 135a are respectively arranged at conjugate positions with respect to the quick return mirror 134.

  The quick return mirror 134 is inserted / retracted with respect to the optical path. Thereby, the light receiving medium of the fundus reflection light P is switched between the photographic film 136 and the CCD camera 135.

  On the imaging surface 135a of the CCD camera 135, as shown in FIG. 7, a number of light receiving elements Gij (i = 1, 2,..., M; j = 1, 2,..., N) are two-dimensional. Are arranged. The light receiving elements Gij are arranged in a matrix along two directions (horizontal direction and vertical direction) orthogonal to each other.

  One surface of the polarizing plate 143 is disposed in contact with the imaging surface 135a. Further, one surface of the phase plate array 142 is disposed in contact with the other surface of the polarizing plate 143.

  The phase plate array 142 changes the phase state of the fundus reflection light P. As shown in FIG. 7, the phase plate array 142 has a plurality of minute phase plates 144 arranged two-dimensionally (in a matrix) vertically and horizontally. The plurality of phase plates 144 function in units of a total of four groups (for example, phase plates 144a to 144d) that are arranged two each in the vertical and horizontal directions. It can be considered that the phase plate array 142 is such that the phase plate group U1 including the four phase plates 144 is arranged vertically and horizontally.

  The phase plates 144a to 144d of the phase plate group U1 have different fast axis angles. Note that the counterclockwise direction is the positive direction.

  The polarizing plate 143 has a polarization axis AR1. The polarizing plate 143 transmits only the polarization component that vibrates in the direction of the polarization axis AR1.

  The light receiving elements Gij of the CCD camera 135 function in units of a total of four groups (for example, the light receiving elements Ga to Gd) arranged two by two in the vertical and horizontal directions. It can be considered that the light receiving element group U2 including the four light receiving elements Gij is arranged vertically and horizontally on the imaging surface 135a.

  As shown in FIG. 7, the light receiving elements Ga to Gd of the light receiving element group U2 respectively receive the light that has passed through the phase plates 144a to 144d of the phase plate group U1 at the corresponding positions. That is, the fundus reflection light P is phase-changed by the phase plates 144a to 144d, the polarization direction is aligned by the polarizing plate 143, and received by the light receiving elements Gij.

  The CCD camera 135 inputs a signal based on the light reception result by each light receiving element Gij to the computer. The computer analyzes this signal, obtains a Stokes vector based on the fundus reflection light P, and calculates the thickness of the optic nerve fiber layer of the fundus oculi Ef.

Japanese National Patent Publication No. 8-508903 JP-T-2004-502484 JP 2006-204517 A

  By the way, in order to measure the thickness of the layer tissue of the fundus of the living subject eye, it is necessary to project illumination light from the cornea side toward the fundus. That is, the illumination light passes through the cornea and the crystalline lens, is reflected by the fundus, and again passes through the crystalline lens and the cornea and is emitted from the eye to be examined. Therefore, the fundus reflection light includes information necessary for measuring the thickness of the layered tissue and the influence of optical characteristics such as the cornea. The influence of the cornea or the like is one of the causes for degrading the accuracy of the thickness measurement of the layer structure.

  With the techniques of Patent Document 2 and Patent Document 3, it is difficult to remove the influence of the cornea and the like. On the other hand, the apparatus of Patent Document 1 is provided with a configuration for offsetting the influence of the cornea and the like. However, an optical system that performs detection for that purpose is separately required, and further, feedback control based on the detection result needs to be executed. Therefore, in order to realize the device of Patent Document 1, a complicated configuration is required. Moreover, in the apparatus of patent document 1, it is necessary to perform the preparation (setting of an apparatus) for performing the said detection effectively precisely.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a fundus examination apparatus that can easily and highly accurately measure the thickness of the optic nerve fiber layer.

  In order to achieve the above object, an invention according to claim 1 is directed to an illumination optical system that projects circularly polarized illumination light onto an eye to be examined, and a light receiving optical system that receives fundus reflected light of the illumination light from the eye to be examined. And a fundus examination device that calculates the thickness of the optic nerve fiber layer in the optic nerve head region of the fundus based on the polarization state of the received fundus reflected light, the computing unit comprising the fundus The polarization characteristics of the anterior segment of the eye to be examined are calculated based on the polarization state of the fundus reflection light of the illumination light from the specific region, and the calculated polarization properties of the anterior segment and the fundus of the illumination light from the optic disc region The thickness of the optic nerve fiber layer is calculated based on the polarization state of the reflected light.

  The invention according to claim 2 is the fundus examination apparatus according to claim 1, wherein the calculation means is configured to calculate illumination light from the optic nerve head region based on the calculated polarization characteristic of the anterior segment. Identifying a component caused by the anterior ocular segment included in the polarization state of fundus reflected light, extracting a component caused by the optic nerve fiber layer from the polarization state of the fundus reflected light based on the identified component, The thickness of the optic nerve fiber layer is calculated based on the extracted component.

  The invention according to claim 3 is the fundus examination apparatus according to claim 1, wherein the light receiving optical system includes two phase plate groups each including four phase plates having different fast axis directions. Corresponding to four phase plates of each phase plate group of the phase plate array, phase plate array arranged in dimension, polarizing plate that transmits the polarization component of the fundus reflection light transmitted through the phase plate array in a specific direction A light-receiving element group including four light-receiving elements at a position where the light-receiving element is detected, and a light-receiving unit that detects the intensity of the polarization component transmitted through the polarizing plate. A Stokes vector of the fundus reflected light transmitted through the phase plate group is calculated based on the intensity of the fundus reflected light from the specific region transmitted through each phase plate, and the fundus according to the specific region is calculated based on the calculated Stokes vector Reflected light Calculating a phase difference distribution, it calculates the polarization characteristics of the anterior segment based on the computed phase difference distribution, characterized in that.

  Further, the invention according to claim 4 is the fundus examination apparatus according to claim 3, wherein the calculation means is configured to calculate the polarization characteristics of the anterior eye part based on the calculated phase difference distribution. The slow axis direction of the eye part and the phase difference caused by the anterior eye part are calculated.

  The invention according to claim 5 is the fundus examination apparatus according to claim 4, wherein the calculation means calculates the maximum value of the phase difference of the fundus reflected light by the specific region based on the phase difference distribution, and A minimum value is obtained, and a phase difference caused by the anterior segment is calculated based on the maximum value and the minimum value.

  The invention according to claim 6 is the fundus examination apparatus according to any one of claims 3 to 5, wherein the specific region includes a fovea of the fundus and the phase difference distribution. Is a distribution of phase differences on a circumference centered about the fovea of the fundus.

  The invention according to claim 7 is the fundus examination apparatus according to any one of claims 1 to 6, wherein the specific region is located in a peripheral region of the central fovea of the fundus. It is a fibrous layer.

  According to the fundus examination apparatus of the present invention, the polarization characteristic of the anterior segment is calculated based on the polarization state of the reflected light from the specific region of the fundus, and the polarization property of the anterior segment and the polarization state of the reflected light by the optic disc region Thus, the thickness of the optic nerve fiber layer can be measured with high accuracy by removing the influence of the anterior segment.

  Further, since the fundus examination apparatus of the present invention is designed to remove the influence of the anterior segment by means of arithmetic processing, it does not require a complicated configuration as in the prior art and does not require precise settings. Therefore, it is possible to easily measure the thickness of the optic nerve fiber layer with high accuracy.

  An example of an embodiment of a fundus examination apparatus according to the present invention will be described in detail with reference to the drawings.

  The fundus examination apparatus of this embodiment is an apparatus that measures the thickness of the optic nerve fiber layer of the fundus in the same manner as in Patent Document 3. Further, this fundus examination apparatus obtains the influence of the polarization characteristics of the anterior segment such as the cornea based on the fundus reflected light from the specific region of the fundus, and obtains the thickness of the optic nerve fiber layer in consideration of this influence. .

  Here, the anterior ocular segment means a region of the eye to be examined that exists in front of the fundus (corneal side). Therefore, a part existing on the optical path of illumination light or its fundus reflection light, such as the cornea and the crystalline lens, is included in the anterior eye part. In this embodiment, the influence of the cornea will be described in particular.

  The polarization characteristic is a characteristic of an object (substance) and means a characteristic that affects the polarization state of light transmitted through the object. Objects such as the cornea and the fundus layer are birefringent. Birefringence is a property of separating light incident on the object into two light beams having vibration directions orthogonal to each other. The two separated lights have a phase difference. The direction in which the phase advances relatively is called the fast axis, and the direction in which the phase is delayed is called the slow axis.

  Hereinafter, the measurement principle of the thickness of the optic nerve fiber device in this embodiment will be described, and then the fundus examination apparatus using this measurement principle will be described.

[Measurement principle]
In this embodiment, circularly polarized illumination light is projected onto the eye to be examined. As described above, the illumination light passes through the anterior segment, reaches the fundus and is reflected. This fundus reflection light is emitted from the eye to be examined again via the anterior eye portion. The fundus reflection light has elliptically polarized light. This is due to birefringence of the cornea, optic nerve fiber layer, and the like. In addition, the fundus reflection light includes a scattering component. This is due to scattering by layers deeper than the optic nerve fiber layer. In addition, since a scattering component is generally very small, it is ignored in this embodiment.

The polarization state of light is represented by a Stokes vector. The Stokes vector expresses the polarization state of light with four components. If the Stokes vector is S = (s ₀ , s ₁ , s ₂ , s ₃ ) ^T (T is the transpose of the vector), s ₀ is the average intensity, s ₁ is the linearly polarized component in the x-axis and y-axis directions, s ₂ represents a 45-degree and 135-degree linear polarization component, and s ₃ represents a circular polarization component. When a scattering component is present, s ₀ > (s ₁ ² + s ₂ ² + s ₃ ² ) ^1/2 . The x-axis and the y-axis are coordinate axes orthogonal to the light traveling direction (z-axis). The xyz coordinate system defines a three-dimensional orthogonal coordinate system.

  Consider the situation shown in FIG. The xyz coordinate system is the orthogonal coordinate system described above. The birefringent material O has a thickness d (in the z direction) and has a fast axis F and a slow axis S. The refractive index in the fast axis F direction is n1, and the refractive index in the slow axis S direction is n2. In addition, the angle of the fast axis F with respect to the x axis is θ. Since the x axis and the y axis are orthogonal to each other, and the fast axis and the slow axis are orthogonal to each other, the angle θ is also an angle formed by the slow axis and the y axis of the birefringent material. Note that the angle is positive in the counterclockwise direction toward the z direction.

The Stokes vector of the light incident on the birefringent material O changes according to the Mueller matrix T _θ C _α T _−θ , as shown in Equation (1).

(S ₀ , s ₁ , s ₂ , s ₃ ) ^T on the right side is a Stokes vector of light incident on the birefringent material O, and (s ₀ ′, s ₁ ′, s ₂ ′, s ₃ ′) on the left side. ^T is a Stokes vector of light that has passed through the birefringent material O.

In addition, the matrices T _θ and C _α are matrices shown in Expression (2), respectively. The angle θ is an angle formed by the fast axis F of the birefringent material O and the x axis.

Here, Γ is the difference between the thickness d of the birefringent material O and the refractive index n ₁ of the fast axis F and the refractive index n ₂ of the slow axis S (n ₁ − n ₂ ) and the phase difference determined by the wavelength of light λ.

Mueller matrix shown in Equation (1) with putting the _{T θ C α T -θ = M} , the Mueller matrix by corneal _{M c,} the Mueller matrix of the nerve fiber layer is denoted by _{M r.} Further, consider a model in which a mirror that reflects light while maintaining phase is arranged on the retina. At this time, assuming that right circularly polarized light is incident, the equation (1) can be transformed into the following equation (4) (Knightton RW, Huang RN, Greenfield DS: Analytical Model of Scanning Laser Polarization for Reetry Fiber Layer Assessment: Investigative Ophthalmology & Visual Science, February Vol. 43, No. 2 383-392 (2002).

  Here, it is assumed that light is reflected in the retina while maintaining the polarization state. The product of the matrix on the right side of Equation (4) is expressed as in Equation (5) below.

When the Stokes vector on the left side of Expression (4) is acquired, the phase difference Γ corresponding to the thickness d when the cornea and the optic nerve fiber layer are regarded as one birefringent substance can be calculated from the value of s ₃ ′. . This phase difference Γ is expressed as the following equation (6).

  If it can be assumed that the influence of the cornea is small, the phase difference Γ in the equation (6) is sufficient, but in this embodiment, the accuracy is improved by removing the influence of the cornea.

  Further, the angle θ can be calculated by the following equation (7).

Next, removal of the influence of the cornea will be described. When the phase difference Γ in equation (6) is calculated from the matrix M _{t in} equation (5) and imaged, a characteristic pattern is obtained in the macular region of the fundus. This is due to birefringence of the Henle's outer fiber layer and the birefringence of the cornea. The Henle fiber layer is a tissue in which fibers run radially around the fovea (located in the macula).

  FIG. 5 shows the state of the cornea C and the Helen fiber layer H. The cornea C has a fast axis Fc and a slow axis Sc. The Helen fiber layer S has a plurality of slow axes Sh1, Sh2, Sh3,... Radially arranged around the fovea. On the circle Q having an arbitrary radius centered on the fovea, the thickness of the Helen fiber layer H is substantially constant. That is, the Helen fiber layer H is a tissue whose thickness changes substantially concentrically according to the distance from the fovea.

  As can be seen from FIG. 5, in the direction in which the slow axis Sc of the cornea C and the slow axis Shi (one of i = 1, 2,...) Of the Henle fiber layer coincide, Are added to increase the phase delay. On the other hand, in the direction orthogonal to this, the phase lag and the phase advance cancel each other, and the phase difference becomes small.

  Therefore, the slow axis Sc of the cornea C can be obtained by specifying the direction in which the phase delay is large based on the image of the phase difference Γ, for example.

Further, for example, by obtaining the maximum value Γ _max and the minimum value Γ _min of the phase difference Γ on the circle Q and substituting them into the following equation (8), the phase difference Γ _c due to the cornea C can be obtained.

In the expression (8), “±” is selected when the phase change due to the Henle fiber layer is larger than the phase change due to the cornea, and “+” is selected in the opposite case. The By assuming either case, it is uniquely determine the Mueller matrix M _c of the cornea C.

By multiplying an inverse matrix of the Mueller matrix M _c of the determined cornea C from the horizontal direction to both sides of the equation (5), the following equation (9) is obtained.

Matrix M _t can be obtained from the detection result of the fundus reflection light, and since the Mueller matrix M _c of the cornea C is determined by the calculation by the equation (9), can be calculated the Mueller matrix M _r of the optic nerve fiber layer, optic nerve The phase difference Γ _{r due} to the fiber layer and the angle of the fast axis and slow axis of the optic nerve fiber layer (angle with respect to the x axis and y axis) θ _r can be obtained.

The calculation may be performed analytically by the method described above, or may be performed using a simulation method (optimization process). Examples of the latter substitutes the value of the Mueller matrix M _c of the cornea in the formula (5), by changing the values of the angle theta _r of the phase difference gamma _r of the Mueller matrix M _r of the optic nerve fiber layer, wherein There is a method for searching for optimum values of Γ _r and θ _r that satisfy (5).

[Constitution]
The fundus examination apparatus of this embodiment obtains the thickness of the optic nerve fiber layer using the measurement principle described above. 1 to 3 show an example of the configuration of the fundus examination apparatus according to this embodiment.

  The fundus examination apparatus 1 includes a fundus camera and a computer, for example, as in the fundus examination apparatus of Patent Document 3. This computer is a general-purpose computer, for example, and includes a microprocessor, RAM, ROM, hard disk drive, display device, operation device, communication interface, and the like.

  As shown in FIG. 1, the fundus examination apparatus 1 includes a control unit 10, a storage unit 20, a display unit 30, an operation unit 40, an information processing unit 50, and a fundus examination unit 60.

[Fundamental examination part]
The fundus examination unit 60 includes the fundus camera. The fundus examination unit 60 includes an optical system similar to the conventional one (see FIGS. 6 and 7). This optical system includes an illumination optical system that projects circularly polarized illumination light onto the eye to be examined and a light receiving optical system that receives reflected light of the illumination light from the fundus of the eye to be examined. As described in [Measurement Principle], the illumination light is right circularly polarized light, but a configuration using left circularly polarized illumination light is also applicable.

  The fundus camera constituting the fundus examination unit 60 may be a non-mydriatic type or a mydriatic type. This fundus camera includes a CCD camera as a light receiving means for receiving fundus reflected light of illumination light. The CCD camera inputs a signal based on the light reception result by each light receiving element to the computer. The light receiving means may be a CMOS camera or the like. As the light receiving means, it is desirable to use a two-dimensional photosensor array in which light receiving elements are two-dimensionally arranged.

  In addition, a phase plate array and a polarizing plate are provided on the front side of the CCD camera as in the conventional case. In the phase plate array, a phase plate group composed of four phase plates having different fast axis angles is arranged vertically and horizontally. The polarizing plate is a polarizing filter that transmits only the polarization component that vibrates in the direction of the polarization axis.

FIG. 3 shows one phase plate group 70 of the phase plate array, a polarizing plate (a part thereof) 80, and one light receiving element group 90 of the CCD camera. The phase plate group 70 includes four phase plates 71, 72, 73 and 74. The light receiving element group 90 includes four light receiving elements 91, 92, 93 and 94. Reference sign P is fundus reflection light of illumination light. The light receiving elements 91, 92, 93, 94 receive the fundus reflection light P that has passed through the phase plates 71, 72, 73, 74 at the corresponding positions, respectively. The light receiving elements 91, 92, 93, and 94 output signals representing the intensities I ₁ , I ₂ , I ₃ , and I ₄ of the received fundus reflected light P, respectively.

(Control part)
The control unit 10 controls each unit of the fundus examination apparatus 1. In addition, the control unit 10 receives a signal input from the fundus examination unit 60. Further, the control unit 10 causes the display unit 30 to display various images and information. In addition, when an operation is performed using the operation unit 40, the control unit 10 causes the fundus examination apparatus 1 to execute a process corresponding to the operation content. In addition, the control unit 10 performs a process of reading information stored in the storage unit 20 and a process of storing information in the storage unit 20. The control unit 10 includes a microprocessor of the computer 10 and a communication interface.

[Storage section]
The storage unit 20 stores a computer program for controlling the fundus examination apparatus 1 and various types of information. The storage unit 20 includes a hard disk drive. Note that a RAM or a ROM may be included.

[Display section]
The display unit 30 is a display device that displays information under the control of the control unit 10. The display unit 30 is configured by, for example, a flat panel display such as a liquid crystal display, a CRT display, or the like.

[Operation section]
The operation unit 40 is operated by an operator when operating the fundus examination apparatus 1 or inputting information into the fundus examination apparatus 1. The operation unit 40 inputs a signal corresponding to the operation by the operator to the control unit 10. The control unit 10 operates the fundus examination apparatus 1 based on this signal. The operation unit 40 includes, for example, a keyboard, a mouse, a trackball, a joystick, a dedicated control panel, and the like.

  The display unit 30 and the operation unit 40 do not need to be configured from individual devices. For example, an apparatus in which a display device and an operation device are integrated, such as a touch panel type liquid crystal display, can be used as appropriate.

[Information Processing Department]
The information processing unit 50 performs various processes on the information acquired by the fundus examination unit 60. The information processing unit 50 includes a microprocessor. The information processing unit 50 is an example of the “calculation unit” in the present invention. The information processing unit 50 is provided with an anterior segment information calculation unit 51 and a layer thickness calculation unit 52.

(Anterior segment information calculation unit)
The anterior segment information calculation unit 51 calculates the polarization characteristics of the anterior segment based on the polarization state of the reflected light of the illumination light from the specific region of the fundus. In order to perform this calculation process, the anterior segment information calculation unit 51 includes a Stokes vector calculation unit 53, a phase difference distribution calculation unit 54, an anterior segment slow axis calculation unit 55, and an anterior segment phase difference calculation unit 56. Consists of.

  Note that the specific region of the fundus preferably includes a tissue having birefringence and a known fast axis direction and slow axis direction. In this embodiment, the fundus region including the Henle fiber layer is applied as the specific region.

(Stokes vector calculation unit)
The Stokes vector calculation unit 53 obtains a Stokes vector based on a signal output from the CCD camera of the fundus examination unit 60. A specific example of this process will be described below.

A phase difference between the phase plates 71, 72, 73, 74 of the phase plate group 70 is represented by Δ. The angles of the fast axes of the phase plates 71, 72, 73, and 74 with respect to the x axis are φ1, φ2, φ3, and φ4, respectively. The Stokes vector of the fundus reflection light P is S = (s ₀ , s ₁ , s ₂ , s ₃ ) ^T. The intensity of the fundus reflection light P detected by the four light receiving elements 91, 92, 93, 94 of the light receiving element group 90 is I = (I ₁ , I ₂ , I ₃ , I ₄ ) ^T. At this time, the following equation (10) holds.

The Stokes vector calculation unit 53 substitutes the intensities I ₁ , I ₂ , I ₃ , and I ₄ of the fundus reflected light P obtained by the CCD camera of the fundus examination unit 60 into the left side of the equation (10), and calculates the right side matrix By applying the inverse matrix from the left side, the Stokes vector S = (s ₀ , s ₁ , s ₂ , s ₃ ) ^T of the fundus reflected light P is obtained.

  The Stokes vector is calculated for each light receiving element group. The Stokes vector corresponding to each light receiving element group is given positional information (coordinate information) of the light receiving element group on the imaging surface of the CCD camera.

  Note that it is desirable to obtain the matrix on the right side of Equation (10) in advance in consideration of the positional deviation between the phase plate group 70 and the light receiving element group 90. This matrix is obtained, for example, by making light of a known polarization state incident and calculating each matrix component based on the light reception result.

(Phase difference distribution calculation unit)
The phase difference distribution calculation unit 54 is based on the Stokes vector S = (s ₀ , s ₁ , s ₂ , s ₃ ) ^T calculated by the Stokes vector calculation unit 53, and the phase difference of the fundus reflection light by the specific region of the fundus Find the distribution.

  For this purpose, the phase difference distribution calculation unit 54 first forms an image of the above-described phase difference Γ based on a plurality of (the number of light receiving element groups) Stokes vectors obtained by the Stokes vector calculation unit 53.

  Next, the phase difference distribution calculation unit 54 specifies an image region corresponding to the specific region of the fundus in the image of the phase difference Γ. The identification method may be performed in response to designation by the operator, or may be automatically performed by the phase difference distribution calculation unit 54.

  An example of the former specific method will be described. The control unit 10 causes the display unit 30 to display an image of the phase difference Γ. The operator looks at this image and searches for the aforementioned characteristic pattern. This characteristic pattern is attributed to the Henle fiber layer and cornea in the macular region. The operator operates the operation unit 40 to designate the image area of this characteristic pattern. This designation operation can be performed by, for example, a mouse drag operation. The phase difference distribution calculation unit 54 employs an image region designated by the operator as an image region corresponding to a specific region of the fundus.

  The latter specific method will be described. The storage unit 20 stores a template image of the above characteristic pattern in advance. The phase difference distribution calculation unit 54 executes matching processing using the template image, and specifies an image region having a large correlation with the template image from the image of the phase difference Γ. The specified image area is adopted as an image area corresponding to the specific area of the fundus. In the latter specifying method, not only a method using a template image but also any image processing technique for specifying an image area of a predetermined pattern can be applied.

  In this embodiment, a Henle fiber layer is employed as a specific region of the fundus. As described above, the henle fiber layer has the slow axis radially arranged around the fovea, and the thickness on the circle centered on the fovea is substantially constant. Therefore, in this case, it is sufficient to obtain a phase difference distribution on a circle centering on the fovea as an image region corresponding to a specific region of the fundus.

  For this purpose, the phase difference distribution calculation unit 54 specifies, for example, the position of the fovea in the image of the phase difference Γ, and acquires the phase difference distribution on a circle centered on this specific position. Also in this case, the position of the fovea may be designated by the operator or may be automatically detected by image processing.

  Alternatively, the position of the fovea may be omitted and the position of the circle may be directly specified. Also in this case, the operator may specify the position of the circle on the image of the phase difference Γ, or the position of the circle may be automatically specified by image processing.

  Thus, the image area obtained from the image of the phase difference Γ is an image representing the phase difference distribution of the fundus reflection light by the specific area of the fundus. As described above, the image area obtained from the image of the phase difference Γ may be a two-dimensional image area (two-dimensional distribution of phase differences), or a one-dimensional image area (one-dimensional phase difference). Distribution, such as a circle).

(Anterior segment slow axis calculation unit)
The anterior ocular segment slow axis calculator 55 obtains the slow axis (direction) of the anterior ocular segment based on the phase difference distribution in the specific region of the fundus acquired by the phase difference distribution calculator 54.

  When the Henry fiber layer is applied as a specific region of the fundus, as described above, the phase delay is added in the direction in which the slow axis Sc of the cornea C and the slow axis Shi of the Henry fiber layer coincide with each other, and the phase delay is added. Becomes larger.

  By utilizing this, the anterior segment slow axis calculation unit 55 is based on the image region corresponding to the Henry's fiber layer obtained by the phase difference distribution calculation unit 54 in a direction in which the phase delay is large (for example, the phase delay is The maximum direction) is specified, and this specific direction is adopted as the direction of the slow axis Sc of the cornea C.

  At this time, with respect to the phase difference distribution on the circle centering on the fovea, the direction in which the phase difference becomes maximum can be obtained and used as the slow axis of the cornea. It is also possible to analyze a two-dimensional phase difference distribution, find a direction having the maximum phase difference on a circle with a predetermined radius centered on the fovea, and use this as the slow axis of the cornea.

(Anterior phase difference calculation unit)
The anterior segment phase difference calculation unit 56 obtains a phase difference caused by the anterior segment based on the phase difference distribution in the specific region of the fundus acquired by the phase difference distribution calculation unit 54.

The anterior segment phase difference calculation unit 56 obtains, for example, a maximum value Γ _max and a minimum value Γ _min in the phase difference distribution. Then, the phase difference Γ _c due to the cornea is calculated by substituting the maximum value Γ _max and the minimum value Γ _min into the equation (8).

  In Expression (8), either “+” or “−” is applied depending on the magnitude relationship between the phase change caused by the Henle fiber layer and the phase change caused by the cornea. The anterior segment phase difference calculation unit 56 selectively applies “+” and “−” based on, for example, general clinical data or a test result on the eye to be examined. This selection may be made by the operator or automatically.

  Further, the phase difference due to the cornea may be calculated in both cases of “+” and “−”. In that case, an appropriate value is selected by calculating the layer thickness in both cases by the layer thickness calculator 52.

As described above, the anterior segment information calculation unit 51 obtains the slow axis of the cornea and the phase difference due to the cornea as the polarization characteristics of the anterior segment. As a result, a corneal Mueller matrix _Mc is obtained as shown in Equation (2).

As can be seen from the equation (4), the corneal Mueller matrix M _c is derived from the cornea included in the Mueller matrix M _c M _r M _r M _c = M _t corresponding to the polarization state of the fundus reflected light by the optic nerve fiber layer. It is an ingredient. In this way, the component (M _c ) due to the cornea included in the polarization state (M _t ) of the fundus reflected light by the optic nerve head region is specified. The component specified in this way will be referred to as an anterior segment component. The optic disc region is a region including at least a part of the optic disc, and includes, for example, a disc region and a cup region of the optic disc and a region including them.

(Layer thickness calculator)
The layer thickness calculation unit 52 determines the thickness of the optic nerve fiber layer based on the polarization characteristics of the anterior segment acquired by the anterior segment information calculation unit 51 and the polarization state of the fundus reflected light from the optic disc region of the fundus. Calculate. For this purpose, the layer thickness calculation unit 52 is provided with an NFL component extraction unit 57 and an NFL thickness calculation unit 58. NFL is an abbreviation for (Optic) Nerve Fiber Layer, that is, an optic nerve fiber layer.

  The layer thickness calculator 52 may calculate the thickness of the optic nerve fiber layer only for the optic nerve head region, or may calculate the thickness of the optic nerve fiber layer for a wider region. The area for obtaining the thickness may be designated by the operator or may be designated automatically. In the latter case, the optic disc area is detected using a known image processing technique.

(NFL component extraction unit)
The NFL component extraction unit 57 extracts a component (NFL component) caused by the optic nerve fiber layer from the polarization state of the fundus reflection light based on the anterior segment component specified by the anterior segment information calculation unit 51.

For this purpose, first, the NFL component extraction unit 57 substitutes the Stokes vector calculated by the Stokes vector calculation unit 53 into the right side of the equation (4) to express the Mueller matrix M _c M representing the influence of both the cornea and the optic nerve fiber layer. determine the _{r M r M c = M t} .

The corneal Mueller matrix _Mc is acquired by the anterior segment information calculation unit 51 as described above. The NFL component extraction unit 57 calculates an inverse matrix M _c ⁻¹ of the corneal Mueller matrix M _c , and substitutes the inverse matrix M _c ⁻¹ and the Mueller matrix M _t into Equation (9) to obtain the matrix M _r M _r. Further, the Mueller matrix M _r of the optic nerve fiber layer is obtained.

Thereby, the phase difference Γ _{r due} to the optic nerve fiber layer and the angle θ _{r of the} fast axis and slow axis of the optic nerve fiber layer are obtained. The NFL component extraction unit 57 may obtain the phase difference Γ _r and the angle θ _r by the above-described simulation method.

The Mueller matrix M _r (phase difference Γ _r , angle θ _r ) corresponds to the NFL component. The NFL component is obtained by removing the anterior ocular segment component resulting from the corneal birefringence as described above, and is a component resulting from the birefringence of the optic nerve fiber layer.

(NFL thickness calculator)
The NFL thickness calculator 58 calculates the thickness of the optic nerve fiber layer based on the NFL component extracted by the NFL component extractor 57.

  The NFL thickness calculator 58 obtains the flatness of elliptically polarized light based on the NFL component, and calculates the thickness of the optic nerve fiber layer based on the flatness, as in the conventional case. In addition, as an index of the degree of flatness, for example, a flatness ratio or an eccentricity ratio can be used.

  The control unit 10 causes the display unit 30 to display the thickness of the optic nerve fiber layer acquired by the NFL thickness calculation unit 58. As the display mode, for example, the thickness at the position designated by the operator using the operation unit 40 may be displayed as a numerical value, or an image representing the thickness distribution may be displayed. As the latter image, for example, an image obtained by color-coding the thickness of the optic nerve fiber layer or an image obtained by connecting portions having the same thickness as lines such as contour lines can be used.

[Action / Effect]
The operation and effect of the fundus examination apparatus 1 configured as described above will be described.

  According to the fundus examination apparatus 1, when determining the thickness of the optic nerve fiber layer, it is possible to remove the influence of the polarization characteristics of the anterior segment, so that the thickness of the optic nerve fiber layer can be measured with high accuracy. It is.

  In addition, the fundus examination apparatus 1 has substantially the same configuration as a conventional fundus camera, and further, the influence of the anterior segment can be removed by arithmetic processing, so that a conventional complicated configuration is not necessary. There is no need to make precise settings. Therefore, according to the fundus examination apparatus 1, it is possible to easily measure the thickness of the optic nerve fiber layer.

[Modification]
The embodiment described above is merely an example of the fundus examination apparatus of the present invention. Therefore, arbitrary modifications within the scope of the present invention can be made as appropriate.

  For example, the examination for the specific area of the fundus and the examination for the optic disc area may be performed simultaneously or separately. When performing simultaneously, the illumination light which can illuminate both a specific area | region and an optic nerve head area | region simultaneously is applied.

It is a schematic block diagram showing an example of composition of an embodiment of a fundus examination device concerning this invention. It is a schematic block diagram showing an example of composition of an embodiment of a fundus examination device concerning this invention. It is the schematic showing an example of the structure of the optical system of embodiment of the fundus examination apparatus concerning this invention. It is a schematic explanatory drawing for demonstrating the measurement principle of embodiment of the fundus inspection apparatus concerning this invention. It is a schematic explanatory drawing for demonstrating the measurement principle of embodiment of the fundus inspection apparatus concerning this invention. It is the schematic showing an example of a structure of the optical system of the fundus camera in the conventional fundus inspection apparatus. It is the schematic showing an example of a structure of the optical system of the fundus camera in the conventional fundus inspection apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fundus examination apparatus 10 Control part 50 Information processing part 51 Anterior eye part information calculation part 52 Layer thickness calculation part 53 Stokes vector calculation part 54 Phase difference distribution calculation part 55 Anterior eye part slow axis calculation part 56 Anterior eye part phase difference calculation Unit 57 NFL component extraction unit 58 NFL thickness calculation unit 60 fundus examination unit 70 phase plate group 71, 72, 73, 74 phase plate 80 polarizing plate 90 light receiving element group 91, 92, 93, 94 light receiving element C cornea H henile fiber layer

Claims (7)

An illumination optical system that projects circularly polarized illumination light onto the eye to be examined, a light receiving optical system that receives fundus reflected light of the illumination light from the eye to be examined, and the fundus based on the polarization state of the received fundus reflected light A fundus examination apparatus having a computing means for computing the thickness of the optic nerve fiber layer in the optic nerve head region of
The calculation means calculates a polarization characteristic of the anterior ocular segment of the eye to be examined based on a polarization state of fundus reflected light of illumination light by a specific region of the fundus, and calculates the polarization characteristic of the anterior ocular segment and the optic nerve. Calculating the thickness of the optic nerve fiber layer based on the polarization state of the fundus reflection light of the illumination light by the nipple region,
A fundus examination apparatus characterized by the above. The computing means identifies a component caused by the anterior eye part included in a polarization state of fundus reflected light of illumination light by the optic nerve head region based on the computed polarization characteristic of the anterior eye part, and is identified. Extracting the component resulting from the optic nerve fiber layer based on the obtained component from the polarization state of the fundus reflection light, and calculating the thickness of the optic nerve fiber layer based on the extracted component.
The fundus examination apparatus according to claim 1. The light receiving optical system includes a phase plate array in which a plurality of phase plate groups each including four phase plates having different fast axis directions are arranged two-dimensionally, and a specific direction of fundus reflection light transmitted through the phase plate array A polarizing component that transmits the polarizing component, and a light receiving element group including four light receiving elements at positions corresponding to the four phase plates of each phase plate group of the phase plate array. Light receiving means for detecting the intensity of
The computing means computes a Stokes vector of the fundus reflected light that has passed through the phase plate group based on the intensity of the fundus reflected light from the specific region that has passed through the phase plate of each phase plate group of the phase plate array. Calculating the phase difference distribution of the fundus reflected light by the specific region based on the calculated Stokes vector, and calculating the polarization characteristics of the anterior segment based on the calculated phase difference distribution,
The fundus examination apparatus according to claim 1. The calculation means calculates, based on the calculated phase difference distribution, as a polarization characteristic of the anterior segment, a slow axis direction of the anterior segment and a phase difference caused by the anterior segment,
The fundus examination apparatus according to claim 3. The calculation means obtains a maximum value and a minimum value of a phase difference of fundus reflected light by the specific region based on the phase difference distribution, and a phase difference caused by the anterior eye portion based on the maximum value and the minimum value. ,
The fundus examination apparatus according to claim 4. The specific region includes a fovea of the fundus,
The phase difference distribution is a distribution of phase differences on a circumference that is substantially centered on the fundus fovea.
The fundus examination apparatus according to any one of claims 3 to 5, wherein the fundus examination apparatus is characterized. The specific region is a henle fiber layer located in a peripheral region of the fundus fovea.
The fundus examination apparatus according to any one of claims 1 to 6, wherein the fundus examination apparatus is characterized.

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