JP2012147976A - Optical coherence tomographic imaging method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem: in an OCT apparatus used in ophthalmology, if a working distance between an eye to be inspected and an objective lens varies, the shape of an obtained tomographic image is changed, and this means that the OCT apparatus cannot be used for examining the variation of eyeball shapes.SOLUTION: This optical coherence tomographic imaging method includes: adjusting and measuring the distance between the eye to be inspected and the objective lens; acquiring a tomographic image of the eye to be inspected including a plurality of linear data; computing the correction quantity in each of the plurality of linear data in the tomographic image; and generating a tomographic image in a reference working distance.

Description

  The present invention relates to an optical tomographic image capturing method and an optical tomographic image capturing apparatus, and more particularly to an optical tomographic image capturing method and an optical tomographic image capturing apparatus used for ophthalmic medical care and the like.

  Currently, various types of ophthalmic equipment using optical equipment are used. For example, an anterior ocular segment photographing machine, a fundus camera, a confocal laser scanning ophthalmoscope (SLO), or the like. Among them, an optical tomographic imaging apparatus based on optical coherence tomography (OCT) using low-coherence light is an apparatus that can obtain a tomographic image of an eye to be examined with high resolution. It is becoming an indispensable device for specialized outpatients. Hereinafter, this is referred to as an OCT apparatus.

  The fundus oculi observation device of Patent Document 1 includes a fundus camera unit, an OCT unit, and an arithmetic control device. The fundus camera acquires a two-dimensional image of the fundus, and the OCT unit acquires a tomographic image of the fundus. Furthermore, the characteristic part of the fundus can be specified, and measurement can be performed by changing the irradiation position of the measurement light. Then, a fundus tomographic image or a three-dimensional image can be formed.

JP 2009-279031 A

  In the OCT apparatus used in ophthalmology, the shape of the obtained tomographic image changes when the working distance that is the distance between the eye to be examined and the objective lens changes. This means that it cannot be used to investigate changes in the shape of the eyeball.

  In order to solve the above problems and obtain a tomographic image that does not depend on a change in working distance, a tomographic imaging method according to the present invention adjusts a first distance between an eye to be examined and an objective lens, and applies measurement light to the eye to be examined. An optical tomographic imaging method for obtaining a tomographic image of an eye to be inspected by a combined light of a return light from a retina of the eye to be obtained and a reference light obtained by irradiating the eye, wherein the first eye and the objective lens A tomographic image based on the first step of measuring the distance, the second step of acquiring information for generating a tomographic image of the eye to be examined comprising a plurality of line-shaped data, and the measured first distance A third step of calculating a correction amount in each of the plurality of line-shaped data, and using the correction amount, a tomographic image obtained at a second distance between the reference eye to be examined and the objective lens from the acquired information A fourth step of generating It is characterized in.

  According to the present invention, a difference in shape due to a difference in working distance can be reduced.

Diagram explaining the OCT apparatus Illustration explaining working distance and scanning radius A diagram showing the relationship between working distance and rotation center-retinal distance Diagram showing changes in tomographic images and two-dimensional images due to changes in working distance Diagram showing changes in anterior segment observation due to changes in working distance The figure which shows the procedure of the signal processing in Example 1. Conceptual diagram of correction amount

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[Example 1]
FIG. 1 is a diagram illustrating a configuration of an optical tomographic imaging apparatus according to the first embodiment.

(Optical system)
The optical tomographic imaging apparatus is configured by a Michelson interference system. The outgoing light 102 of the light source 101 is guided to the single mode fiber 107 and enters the optical coupler 108, and is split into the reference light 103 and the measuring light 104 by the optical coupler 108. Then, the measurement light 104 is reflected or scattered by the measurement portion of the retina 125 to be observed, and returns to the optical coupler 108 as return light 105. Then, the optical coupler 108 combines with the reference light 103 that has passed through the reference optical path to become the combined light 106 and reaches the spectroscope 119.

  The light source 101 is a SLD light source (Super Luminescent Diode) which is a typical low-coherent light source. In consideration of measuring the eye, near-infrared light is suitable for the wavelength. Furthermore, since the wavelength affects the resolution in the lateral direction of the obtained tomographic image, it is desirable that the wavelength be as short as possible. Here, the center wavelength is 840 nm and the wavelength width is 50 nm. Of course, other wavelengths may be selected depending on the measurement site to be observed. Note that although the SLD light source is selected here as the type of light source, it is only necessary to emit low-coherent light, and an ASE light source (Amplified Spontaneous Emission) or the like can also be used.

  Next, the reference light path of the reference light 103 will be described. The reference light 103 divided by the optical coupler 108 is emitted as substantially parallel light by the lens 109-1. Thereafter, the reference light 103 passes through the dispersion compensation glass 110 and changes its direction at the mirror 111. Then, the light is again guided to the spectroscope 119 via the optical coupler 108. The dispersion compensation glass 110 compensates the reference light 103 for dispersion when the measurement light 104 reciprocates between the eye to be examined 124 and the scanning optical system. Here, the length is assumed to be 24 mm assuming a representative value as the average eyeball diameter of Japanese. The optical path length of the reference light can be adjusted in the direction of the arrow by the electric stage 112 to adjust the position of the coherence gate. The coherence gate is a position on the optical path of the measurement light that is equidistant from the length of the optical path of the reference light. The electric stage 112 is controlled by the computer 120.

  Next, the measurement optical path of the measurement light 104 will be described. The measurement light 104 divided by the optical coupler 108 is emitted as substantially parallel light by the lens 109-2, and is incident on the mirror of the XY scanner 113 constituting the scanning optical system. In FIG. 1, the XY scanner 113 is a single mirror, but actually two mirrors, an X scan mirror and a Y scan mirror, are arranged close to each other. The measurement light reaches the eye to be examined 124 via the lens 114, the dichroic mirror 115, and the objective lens 128.

  Here, observation of the anterior segment, that is, the cornea 122 will be described. A ring-shaped light source (not shown) outside the objective lens is used as proof light for observing the anterior segment. The illumination light is reflected by the cornea 122, the reflected light passes through the objective lens 128 again, is reflected by the dichroic mirror 115, and reaches the observation system 118 via the split prism 116 and the optical system 117. The split prism 116 is disposed at a position conjugate with the cornea 122.

  Further, the objective lens 128 is disposed so as to face the eye to be examined 124, and shapes the shape when the measurement light is guided to the eye to be examined 124.

(Working distance)
Next, working distance will be described. Here, the working distance 126 is the length (distance) between the surface of the cornea 122 and the surface of the objective lens 128. This length corresponds to the first distance in the present invention. First, the optical system of a general OCT apparatus is designed so that the pupil 129 of the eye to be examined 124 becomes the rotation center of scanning of the measurement light 104. Therefore, it is desirable to perform OCT measurement by adjusting the distance between the objective lens 128 and the eye 123 so that the working distance becomes a design value. However, since the OCT apparatus has a small NA of the optical system, the depth of focus is deep, and as a result, even if the working distance deviates from the design value, imaging can be performed without any problem. If the design value deviates greatly, the light 127 is blocked or the focal point is blurred.

  Here, the relationship between the working distance, the rotation center 202 of the scanning of the measurement light, and the locus 205 of the coherence gate will be described using the schematic diagram of the eye of FIG. In these figures, the horizontal axis is the fast scan x-axis, and the vertical axis is the depth z-axis. Then, the origin of the rotation center is set as the pupil position which is a design value. In this figure, the center of rotation 202 is a point where light rays incident on the retina 201 are extended as they are, and is not a point where light refracted by the cornea or the crystalline lens 123 is connected.

  The rotation center 202 of the measurement light scan moves as the working distance 126 changes. Further, the scanning radius 204 and the scanning angle 206 when the measurement light 104 is scanned also change as the working distance 126 changes. The xy scanner 113 is often composed of two mirrors. For example, it is assumed that the rotation center of the y axis is on the 1 mm objective lens side, for example. In such a case, movement in the y-axis direction by 3D measurement needs to be separately corrected. Here, the discussion is made assuming that the rotation centers of the x-axis and y-axis are the same.

  FIG. 2A shows a case where the center of rotation 202 is closer to the retina 201 than the pupil 129 because the working distance 126 is shorter than the design value. Naturally, the distance between the rotation center 202 and the retina 201 is shorter than the design value. In the OCT measurement, the coherence gate is arranged on the vitreous body side so that the retina 201 can be observed. The position of the locus 205 of the coherence gate can be changed by the reference mirror 111. FIG. 2B shows a time when the working distance 126 substantially matches the design value. The distance between the rotation center 202 and the retina 201 is a design value. FIG. 2C shows a case where the working distance 126 is longer than the design value and the center of rotation 202 is on the objective lens 128 side with respect to the pupil 129.

  As shown in FIGS. 2A to 2C, the locus becomes flat as the scanning radius 204 becomes longer. That is, in the OCT apparatus, the difference between the retina 201 and the locus 205 of the coherence gate is displayed as an image. Therefore, the apparent curvature becomes larger as the scanning radius 204 becomes longer. However, even if the working distance 126 changes, the light that is incident at the same angle with respect to the optical axis is focused on the same part of the eyeball, so the scanning range 207 does not change significantly.

Here, the relationship between the distance change of the working distance 126 and the position of the rotation center 202 will be further described. The difference 210 from the design value of the working distance 126 is expressed as a variable g by a spatial distance. Since the origin is the pupil 129, the value of the variable g is the value of the z axis. The standard working distance corresponds to the second distance of the present invention, and is, for example, a design value (g = 0). Here, the spatial distance from the rotation center 202 to the retina 201 is expressed as f (g) using a variable g. The coherence gate is based on the standard eye retinal position, and is specifically 24 mm from the pupil. A difference 203 on the z-axis from the reference is expressed as a variable s by a spatial distance. Using these, the scanning radius 204L (g) is expressed as Equation 1.

Movement of the coherence gate can be expressed by Equation 2 using the refractive index n _h of the eye in terms of movement amount ΔM of the reference mirror 111.

  Incidentally, the change in the spatial distance f (g) from the rotation center 202 to the retina 201 is not proportional to the change in the working distance 126. This is because the light is refracted by the cornea 122 or the crystalline lens 123. FIG. 3 shows a simulation result of the spatial distance f (g) from the rotation center 202 to the retina 201. The horizontal axis is a change g from the design value of the working distance 126. The vertical axis represents the spatial distance from the rotation center 202 to the retina 201. When the working distance 126 is on the minus side, it can be seen that the movement of the rotation center is smaller than the amount of change of the working distance 126. On the contrary, when it is on the plus side, it can be seen that the movement of the rotation center is larger than the change amount of the working distance 126. In this simulation, a model with an axial length of 24 mm was used, but f (g) naturally changes when the axial length changes. In this case, a simulation is required for each eye axis length model. Further, when the rotation center is different between the x axis and the y axis, further simulation is required.

  Here, the example which imaged the model eye with the OCT apparatus is demonstrated using FIG. In the model eye, radial and circular patterns are arranged in a portion corresponding to the retina. The portion corresponding to the retina is the surface of the glass. These images were measured by changing the working distance 126 and adjusting the tomographic image so that the vertex of the retina position of the model eye was equidistant from the coherence gate. 4A is a tomographic image when the working distance 126 is 4 mm shorter than the design value, FIG. 4B is a two-dimensional projection image thereof, and FIG. 4C is a tomographic image when the working distance 126 is the design value. FIG. 4D shows the two-dimensional projection image, FIG. 4E shows the tomographic image when the working distance 126 is 4 mm longer than the design value, and FIG. 4F shows the two-dimensional projection image. In each tomographic image, the retina of the model eye is captured as an arc of different curvature. Moreover, the concentric circle and the radiation of the projected image are picked up and projected from the concentric circle and the radiation recorded at the retina position of the model eye. Focusing on the intersection 404 of the circle and the straight line, if the auxiliary line 405 is drawn on the corresponding tomographic image, the image of the retina moves to the lower side of the auxiliary line in FIG. 4A and to the upper side of FIG. You can see that However, as shown in FIGS. 4B, 4D, and 4F, the measurement range has almost no change.

  Next, working distance measurement will be described with reference to FIG. FIG. 5 is an image of the cornea by the anterior ocular segment observation system 118, in which the pupil 501 and the iris 502 are observed. The image of the pupil 501 is designed such that a positive region and a negative region in the y direction are separated from each other by the beam split prism and formed on the observation system 118 with the x axis as a boundary. 5A shows a case where the working distance 126 is shorter than the design value, FIG. 5B shows a case where the working distance 126 substantially matches the design value, and FIG. 5C shows a case where the working distance 126 is longer than the design value. is there. When the working distance 126 substantially matches the design value, the pupil 501 is an unseparated image. On the other hand, when the working distance 126 is shorter than the design value, the image above the pupil moves to the right side, and when it is longer, the image above the pupil moves to the left side. By measuring the difference 503 between the images of the upper and lower pupils 501, the length of the working distance 126 can be known.

  The working distance 126 corresponds to the first distance in the present invention described above, and the above-described configuration for knowing the length of the working distance 126 includes the configuration for adjusting the position of the objective lens 128 (not shown). Corresponds to means for adjusting and measuring the first distance in Or the structure which knows the length of this working distance 126 respond | corresponds to the acquisition means which acquires the 1st distance between the eye to be examined and the objective lens corresponding to the tomographic image in this invention.

  Incidentally, the distance from the center of rotation to the retina can be determined by measuring the moving distance of the working distance 126 and the mirror 111. That is, when the coherence gate is arranged at the design value and coincides with the retina, it can be seen that the axial length is 24 mm as designed. If they do not match, move the coherence gate and look for a location that matches the retina. The true eye axis length can be known from the amount of movement. Although the retina is thick, for example, the boundary between the vitreous body and the nerve fiber layer is considered as a design value.

(Signal processing)
Here, the signal processing of the OCT measurement will be described with reference to FIG.

  In step A1, measurement is started. In this state, the OCT apparatus is activated and the eye to be examined is placed at the measurement position.

  In a step A2 corresponding to the first step in the present invention, the working distance (WD) 126 is adjusted and measured. Here, the position of the pupil is the origin of the coordinate system. The alignment is performed while observing the cornea 122 by the anterior ocular segment observation system 118. The working distance 126 is adjusted with a goal of entering about ± 5 mm from the design value. In particular, when the curvature is large, specifically, when the value of the curvature is larger than a predetermined value, the objective lens may be further brought closer to the eye to be examined in a range where the measurement light is not blocked by the glow. This is an operation of providing a means for measuring curvature and bringing it closer to the eye to be examined according to the measured value. By applying this operation, it is possible to suitably obtain a tomographic image and execute the later-described A3 step and subsequent steps even for an eye to be examined having a large retina curvature. In addition, minus is a direction in which the objective lens approaches the cornea. Naturally, the coherence gate and focus position are adjusted along with the adjustment of the working distance.

  OCT measurement is performed in step A3 corresponding to the second step in the present invention. The scanning range 207 is, for example, 6 mm for imaging the macula, 10 mm for imaging the macula and the nipple. Here, assuming that a range of 6 mm is imaged, 512 lines of data are acquired in the x direction and 512 lines are acquired in the y direction. One-dimensional array of line data (1024 pixels) is acquired for each line from the spectroscope 119 and sequentially sent to the computer 120. Then, 512 lines in the x direction are stored in units of two-dimensional array data. The data size is 1024 × 512 × 12 bits. This can be 512 sheets in the y direction.

  A tomographic image (B-Scan image) can be obtained from the measured two-dimensional array data by performing removal of fixed noise, wavelength wave number conversion, Fourier transform, and the like. If this tomographic image is confirmed and it is determined that a desired measurement has been made, the eye to be examined is removed from the measurement position. The above-described configuration for performing the OCT measurement corresponds to the means for acquiring information for generating a tomographic image of the eye to be inspected comprising a plurality of line data in the present invention.

Next, the correction amount is calculated in step A4 corresponding to the third step in the present invention. First, a general tomographic image is displayed relative to the position of the coherence gate. However, as shown in FIG. 7A, the locus 205 of the coherence gate draws a fan-shaped arc. Assuming that point B is the j-th line scan, a point C projected on the z-axis is defined as point C. The amount of change d (g) of how much the coherence gate changes with respect to the point A on the z-axis is expressed by Equation 4. Here, it is assumed that the scanner rotates by θ (g) / (N−1) using a scanning angle (θ (g)) 206 when the working distance is different from the design value g. Further, j is an integer satisfying 0 to N−1, and N is the number of lines 512 in the x direction.

Note that the distance f (g) from the center of rotation 202 viewed from the retina 201 to the retina 201 and the scanning angle 206 change due to the change in the working distance 126, but the scanning range 207 (W) does not substantially change. Further, the scanning angle θ (g) has a relationship as shown in Equation 5. Of course, θ (g) may be obtained by simulation.

Since f (g) may be simulated in advance as shown in FIG. 3, d _j (g) can be obtained. By using the mathematical formula for obtaining the change amount d _j (g), it is possible to correct each of the line-shaped data described above. Further, the A4 step is performed by the computer 120, and the computer 120 corresponds to a means for calculating a correction amount in each of the plurality of line data in the present invention. As described above, in the step A3, the first distance (g) between the eye to be examined and the objective lens, the distance from the rotation center of the scan viewed from the retina when scanning the measurement light on the retina (from the retina ( f (g)), the scanning angle (θ (g)) when scanning the measurement light viewed from the retina, and the coherence gate position (s) determined by the optical path length of the reference light, and correction of each of the line data A quantity is required.

Furthermore, a tomographic image is generated in step A5 corresponding to the fourth step of the present invention. Since Equation 3 is a spatial distance, after multiplying the refractive index n _h , division by pixel resolution is performed to calculate the number of pixels to be shifted. FIG. 7B shows a conceptual diagram in which each line 701 is corrected with the obtained correction amount. This example shows a short case with respect to the design value of the working distance. The entire imaging range indicated by the dotted line indicates that the image is cut out with reference to the line of the central portion. In the portion 702 where there is no data, noise level data is added. If extra data is calculated in the depth direction, fill it with it. Naturally, the standard working distance is not a design value but may be another value.

  The process ends in step A6. After confirming that the desired data has been obtained, the process ends. Note that the above tomographic image generation step corrects the tomographic image acquired in the tomographic image corresponding to the second distance, which is the reference working distance between the eye to be examined and the objective lens, which is different from the first distance described above. Implemented by the computer 120. The said structure respond | corresponds to the correction | amendment means in this invention.

  As a result, even if the working distance is different from the design value, an image of the reference working distance can be created. That is, even for a person with a large curvature, the measurement is performed in consideration of the curvature, and images with relatively the same working distance are obtained.

  As described above, the measurement light is operated on the retina by two x scanners and y scanners each having different rotation axes, that is, rotation centers. Therefore, it is necessary to correct the tomographic image in consideration of the difference in the position of the rotation center of these scanners. In the present invention, in the fourth step, a tomographic image in which such a difference in the rotation center is reflected in the working distance may be generated. As a result, a more appropriate tomographic image can be obtained in an actual OCT apparatus.

  As described above, according to the present embodiment, by adjusting and measuring the working distance of the eye to be examined and the objective lens and correcting the image, the difference in shape due to the difference in working distance can be reduced. Even in the case of an eye to be examined having a large curvature, the obtained tomographic image can be used for shape analysis. Further, it is not necessary to match the working distance to the design value at the time of imaging, so that the measurement time can be shortened.

(Other examples)
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (9)

The first distance between the eye to be examined and the objective lens is adjusted, and the combined light of the return light from the retina of the eye to be examined and the reference light obtained by irradiating the eye to be examined with measurement light, An optical tomographic imaging method for obtaining a tomographic image,
A first step of measuring the first distance between the eye to be examined and the objective lens;
A second step of acquiring information for generating a tomographic image of the eye to be examined comprising a plurality of line-shaped data;
A third step of calculating a correction amount in each of the plurality of line-shaped data of the tomographic image based on the measured first distance;
A fourth step of generating a tomographic image obtained at a second distance between the eye to be examined and the objective lens from the acquired information using the correction amount;
An optical tomographic imaging method characterized by comprising:   The correction amount is the first distance between the eye to be examined and the objective lens, the distance from the rotation center of the scan viewed from the retina when scanning the measurement light on the retina, and the retina, 2. The optical tomographic imaging method according to claim 1, wherein the optical tomographic image imaging method is performed using a scanning angle when scanning the measurement light viewed from the retina and a coherence gate position determined by an optical path length of the reference light. Two scanners each having a different center of rotation for scanning the measuring light,
The optical tomographic image according to claim 1, wherein the fourth step further includes a step of correcting a difference in position of the rotation center of each of the scanners when generating the tomographic image. Imaging method.   In the second step, when the curvature of the retina in the eye to be examined is measured and the curvature is larger than a predetermined value, the first distance between the eye to be examined and the objective lens is shortened, The optical tomographic imaging method according to claim 1, wherein the tomographic image is acquired. An optical tomographic imaging apparatus that acquires a tomographic image of the eye to be inspected based on a combined light obtained by combining the return light from the eye to be examined irradiated with the measurement light via the objective lens and the reference light corresponding to the measurement light Because
Obtaining means for obtaining a first distance between the eye to be examined and the objective lens corresponding to the tomographic image;
Correction means for correcting the tomographic image to a tomographic image corresponding to a second distance between the eye to be examined and the objective lens different from the first distance;
An optical tomographic imaging apparatus characterized by comprising:   The optical tomographic imaging apparatus according to claim 5, further comprising a spectroscope that acquires a plurality of line-shaped data for generating the tomographic image and sends the line-shaped data to the correction unit.   7. The optical tomographic imaging apparatus according to claim 5, further comprising means for adjusting the first distance by moving the objective lens. Two scanners each having a different center of rotation for scanning the measuring light,
The optical tomographic imaging according to claim 5, wherein the correction unit corrects a difference in position of the rotation center of each of the scanners when generating the tomographic image. apparatus.   The correction means measures the curvature of the retina in the eye to be examined and, when the curvature is larger than a predetermined value, shortens the first distance between the eye to be examined and the objective lens, and The optical tomographic imaging apparatus according to claim 5, wherein an image is acquired.

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