Measurement of refractive errors in human eyes using optical coherence tomography

Abstract

Particular embodiments disclosed herein provide an apparatus for performing optometric measurements, the apparatus including an optical coherence tomography (OCT) imaging system configured to output parallel laser beams for scanning on an eye of a patient. A line in the OCT image representing the retina of the eye of the patient is identified. The ametropia calculator then measures ametropia of the eye of the patient using the shape of the wire. The height of the line due to curvature indicates the extent of myopia or hyperopia. The more complex curvature of the line corresponds to higher order aberrations. OCT images of multiple cross-sectional planes may be used to calculate a wavefront elevation map.

Description

Technical Field

[0001] The present disclosure generally relates to methods for measuring refractive error of an eye. Background Art

[0002] An optical coherence tomography (OCT) system directs a coherent beam of light into the patient's eye, some of which is scattered back into the OCT system. The location of the point from which the light was scattered is measured using interference between the scattered light and a portion of the original coherent beam. Ophthalmic OCT is primarily used to measure ocular dimensions and diagnose various ocular pathologies. Ophthalmic OCT can also be used to image the anterior chamber of the eye. Summary of the invention

[0003] The present disclosure generally relates to using OCT to measure refractive error of the eye.

[0004] In the present patent application, an optical coherence tomography (OCT) imaging system is configured to scan a parallel beam of light over an eye of a patient. A computing device is configured to receive OCT signals reflected from the retina. The apparent shape of the retina, in particular the deviation of the apparent shape of the retina from a straight line, is used to calculate the refractive error of the eye. For an eye without any optical error, the apparent image of the retina is a straight line.

[0005] The following description and the associated drawings set forth in detail certain illustrative features of the one or more embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Certain aspects of one or more embodiments are depicted in the accompanying drawings, and therefore should not be considered to limit the scope of the disclosure.

[0007] Figure 1 A perfect aberration-free lens is shown illuminated with multiple parallel light rays so that all rays are focused to a focal point F.

[0008] Figure 2A An OCT image of an aberration-free eye is presented, showing the actual light propagation to a focal point on the retina of the eye.

[0009] Figure 2B An aberration-free eye is shown. An OCT image of an eye showing a stretched representation of the focus according to certain embodiments.

[0010] Figure 3 Demonstrated the application of Fermat's principle to light passing through the lens.

[0011] Figure 4 A surface S is shown, which is a surface where the light travel time from plane A to surface S is equal.

[0012] Figure 5A and Figure 5B An OCT image of plane P is shown, which is offset upward from the focus of the lens in the OCT image, in accordance with certain embodiments.

[0013] Fig. 6A and Figure 6B Shown are apparent images of the retina obtained for a myopic eye and a hyperopic eye according to certain embodiments.

[0014] Figure 7 is a diagram illustrating a method for relating the sagittal height of an apparent image of the retina representing a plane to the distance of the plane from the focal plane of the lens, according to certain embodiments.

[0015] Fig. 8A and Figure 8B is a diagram illustrating a system for testing the relationship between the sagittal height of an apparent image of the retina representing a plane and the distance of that plane from the focal plane of the lens, in accordance with certain embodiments.

[0016] Fig. 9 is an OCT image of the reflective surface P imaged by OCT when the aspheric lens is not in the optical path, where the image of the reflective surface P is a straight line proving that the rays of the scanning OCT beam are parallel to each other and the reflective surface P is perpendicular to the OCT rays.

[0017] Fig.10 is a set of OCT images of the apparent reflective surface P obtained for different vertical positions of the lens according to certain embodiments.

[0018] Fig.11 is an OCT image showing the sagittal height of the apparent image of the reflective surface P, according to some embodiments.

[0019] Fig. 12A and Fig. 12B An apparent image of the reflective surface P is shown, where the planar surface of the aspheric lens faces the incident OCT beam and the lens is tilted to induce higher-order aberrations.

[0020] Fig.13 is a diagram of an eye showing dimensions that may be used to calculate the eye's refractive error based on sagittal height measured in OCT images, in accordance with certain embodiments.

[0021] Fig.14A and Fig. 14B The use of OCT images to measure higher order aberrations according to certain embodiments is demonstrated.

[0022] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0023] The refractive error of the human eye can be measured using a Shack-Hartman wavefront meter, a Talbot-Moire wavefront meter, a Tracey wavefront meter, or a Tscherning wavefront meter. A new device is described below that can measure the refractive error and wavefront error of the human eye. The device is based on the Fermat principle and uses an OCT scanner. Ophthalmic OCT was invented in 1991, however, it has not been used or considered to be able to measure the refractive error and wavefront error of the eye.

[0024] The optical coherence tomography (OCT) system directs one beam from a laser beam (called a signal beam) to the patient's eye and directs another beam from the laser beam (called a reference beam) to the reference light arm. The back-reflected beams from the two light arms are then combined. Due to the combination of these beams, a wavelength-dependent interference spectrum is formed. By analyzing the interference spectrum, two pieces of information can be obtained: (a) the intensity of the back-reflected light from the eye and (b) the Z depth position of the origin of the back-reflected light. The Z position can also be expressed as a Z distance or as a light propagation time T=Z / c, where c is the speed of light. For the purposes of the embodiments described herein, the propagation time T is the most relevant parameter. Measurements taken along one propagation direction are called A scans. By scanning the OCT beam in the X and Y directions, thousands of A scans can be performed. Based on multiple A scans, a 3D image of the eye can be reconstructed. Based on the 3D image, an X / Z or Y / Z side view, or an X / Y frontal image is typically displayed. Ophthalmic OCT is mainly used to image the cornea, anterior chamber, lens, vitreous body, and retina. These images are used to measure eye dimensions and diagnose different kinds of eye pathologies.

[0025] refer to Figure 1 To measure the refractive error of the eye using OCT, the 350-year-old Fermat principle is used. This principle states that the light propagation time from plane A to focus F is the same for any ray. The Fermat principle can be applied to a perfect lens L, which is illuminated with a plane wave propagating parallel to the optical axis of the lens. The geometric path length from plane A to focus F is the shortest for ray 1, but this ray is the one that has traveled the longest path in the lens material. The propagation time in the lens is slower and corresponds to c / n, where c is the speed of light in a vacuum and n is the refractive index of the lens material.

[0026] The following FIG. 2A to FIG. 2B Used to describe how the Fermat principle applies to an OCT image of an eye with zero refractive error. It should be noted that the travel time of light displayed by the OCT device is based on the input position R of the OCT light.

[0027] refer to Figure 2A , parallel OCT light rays propagate to the eye. If the eye has no refractive error, all beams are focused on the retina and the propagation time of all light rays is the same, such as Figure 2A shown.

[0028] When an OCT image is created, the propagation time is plotted against the input position R of the incident beam. When an OCT image of the eye is displayed, refraction and beam deflection at the curved surfaces of the cornea and lens are ignored. Since the propagation time is the same for every ray, the retinal focus appears as a straight line (4), as shown in Figure 2B It should be noted that Figure 2B The line (4) in is not the image of the retina, but Figure 2A The image of the retina in point (4) is stretched horizontally. It is important to remember that the horizontal line is not the image of the retina; it just looks like an image of the retina. In reality, the horizontal line is the image of a small portion of the retina stretched horizontally. The length of the stretch is equal to the scan length of the OCT beam over the cornea. The cornea is usually limited by the size of the pupil.

[0029] Some OCT devices partially account for beam deflection caused by refraction on tilted surfaces, but for the purposes of the embodiments described herein, OCT software is used that does not correct for beam deflection on curved and tilted surfaces.

[0030] Currently, when it comes to OCT imaging, there are two existing methods: retinal OCT and anterior chamber OCT. In anterior chamber OCT, the central axial beam of the OCT above the cornea is always strictly vertical and scanned along the horizontal direction. The beam waist of the OCT incident beam is in the anterior chamber-lens complex, which helps the OCT to efficiently collect backscattered light from the anterior chamber. In retinal OCT, the beam is focused at the level of the retina, and the axial beam of the OCT beam is not strictly parallel. Instead, the incident beam is formed in the shape of a handheld fan, with its axis of rotation located in the pupil of the eye. In this way, a very large angular range of the retina can be scanned.

[0031] The OCT described in this patent application is similar to anterior chamber OCT in that the central axial beam is always strictly vertical and scans horizontally, except that the beam waist is focused to the level of the retina. This focusing ensures efficient collection of backscattered light from the retina. Such a system will be referred to as modified anterior chamber OCT in this patent application. As described below, certain deviations from strict verticality can be corrected so that in some implementations, the axial beams are only substantially vertical and parallel to each other, for example, within 2 degrees of vertical and parallel.

[0032] refer to Figure 3, assuming that the circular surface S is centered at the focus F of the lens L. From Fermat's principle, it can also be concluded that for any light ray, the light travel time from plane A to the spherical surface S is equal. Therefore, S is a surface with equal travel time.

[0033] refer to Figure 4 , assuming that the reflecting flat surface P is located above the focus F. Figure 4 As shown, light rays 1 and 2 are the first rays to arrive at plane P. Light 3 is the last ray to arrive at plane surface P. The arrival time difference Δ is Δ / c, where c is the speed of light. For any light ray, the travel time from plane A to spherical surface S is the same. Figure 5A and Figure 5B , the appearance of surface P on an OCT image measuring refractive error is a curve having a sagittal height Δ measured along the optical axis of the lens L.

[0034] Fig. 6A demonstrated that when a myopic eye is imaged, the lines representing light reflected from the retina will bow upward (towards the OCT scanner). Figure 6B It was demonstrated that when a presbyopic eye is imaged, the lines representing light reflected from the retina will bow downward (away from the OCT scanner).

[0035] Figure 7 The calculation of the sag Δ using an aspheric lens L and an OCT scanner is demonstrated. Δ refers to the sag of the phase front at a certain distance X from the focal plane. The OCT scanner has an OCT scan length of 6 mm (limited by the OCT used) and scans horizontally through an AL108 Edmund Optics aspheric lens with a numerical aperture (NA) of 0.55 and an input diameter of 10 mm. For a horizontal scan length of 6 mm, the NA of the marginal OCT light is NA=0.55*0.6=0.33. The incident angle α of the marginal light is α=arc sin NA=arc sin 0.33=19.27°.

[0036] The sag Δ can be calculated as Δ=X*[(1 / cos19.27°)-1)=X*0.0594. Another way to express Δ is: Δ=59.4 μm per millimeter from the focal plane FP.

[0037] refer to Fig. 8A and Figure 8B, the values ​​of the sag Δ for multiple values ​​of X were measured using an anterior chamber OCT (i.e., Optoview iVue OCT) using an anterior chamber scanning protocol. In the experimental setup, X was changed by moving the lens L relative to the reflective plane surface P. The value of X corresponds to the distance of the lens L from the position when the focus of the lens L is at the reflective plane surface P. Since the distance of the OCT to plane P does not change, the up and down movement of the lens relative to the reflective plane surface P does not move the center point of the arcuate image of the reflective plane surface P up and down. Instead, only the curvature of the arcuate image changes.

[0038] Fig. 9 is an OCT image of plane P captured without the aspheric lens in the OCT beam. The reflective surface P is a flat, polished stainless steel surface. The straight lines prove that the OCT scanner is optically correctly adjusted. That is, the incident OCT light is both (a) parallel and (b) perpendicular to the stainless steel surface.

[0039] Fig.10 The results show that an aspheric lens is more sensitive to light than an occlusal lens in an OCT beam. Fig. 8A OCT images taken at different distances X from the initial position of the lens. When X is a positive value, the focus of the lens L is below the reflective plane surface P. When X is a negative value, the focus of the lens L is above the reflective plane surface P. X=0 corresponds to the focus of the lens at the reflective plane surface P, so when X=0, the image is a straight line.

[0040] Fig.11 The OCT image obtained when X=5 mm is shown. The predicted sagittal height Δ can be calculated as Δ=5 mm*59.4 μm / mm=297 μm. The sagittal height Δ measured from the OCT image is 310 μm, which gives a measurement accuracy of 310 / 297=1.044 or 4.4%. This accuracy is acceptable given the available measurement accuracy of the X shift of aspheric lenses.

[0041] Fig. 12A and Fig. 12B is an OCT image showing an apparent OCT image of a reflective plane surface P when the optical arrangement has high-order aberrations. High-order aberrations are introduced by tilting the lens L and flipping the lens L so that the OCT beam is incident on the flat surface of the lens L (the aspherical lens L is designed so that the curved surface is to be irradiated).

[0042] refer to Fig.13, the concepts outlined above can be used to measure the refractive error of the eye using OCT images. For a typical eye, a 1 mm change in the axial length of the eye will result in a refractive error of 2.82 diopters. This value of 2.82 is used by different IOL (intraocular lens) calculators to select an appropriate IOL after cataract surgery. The following real-life example is used to estimate the resolution of an OCT-based refractive error measurement device. For an 8 mm dilated pupil, the angle of incidence of the marginal ray is α=arcsin(4 / 20)=11.5°. As previously defined, an axial length error of 1 mm will cause a sagittal height Δ: Δ=1mm*[1 / cos11.5°)-1]=20.6μm. In the vitreous, Δ=20.6μm corresponds to a distance of 1.34*20.6μm=27.6μm, where 1.34 is the refractive index of the vitreous. In summary, a 2.82 diopter error will cause a sagittal height Δ = 27.6 μm (in air units) in the OCT image. Assuming the realistic axial resolution of OCT is 4 μm, the estimated refractive error resolution is therefore 2.82 diopters * 4 / 27.6 = 0.41 diopters. For an OCT resolution of 2 μm, which is also technically feasible, the resolution can be about 0.2 diopters. By using appropriate image analysis and segmentation software, the measurement accuracy of Δ can be an order of magnitude less than the depth resolution of OCT. In this way, the measurement result of refractive error can exceed 0.2 diopters.

[0043] Recently so-called phase-resolved OCT is also under development. The software for phase-resolved OCT calculates not only the Fourier amplitude component of the interference spectrum, but also the phase component. Phase-resolved OCT can have a depth resolution much better than 2um, and thus a system based on phase-resolved OCT can have a much better diopter resolution.

[0044] The angle of incidence α and the numerical aperture of the patient's eye are functions of measurements of the diameter of the patient's pupil and the position along the optical axis of the patient's pupil obtained using conventional techniques, including anterior chamber OCT. The angle of incidence α is a function of the distance between the pupil and the patient's retina, which can also be easily measured using an OCT scanner. Alternatively, the diameter of the pupil can be measured using a surgical microscope.

[0045] Fig.14A The sag Δ is shown for an eye with only myopic through-focus error and no astigmatism or higher-order errors, resulting in a parabolic shape that is caused by the distance of the retina from the focal point of the eye. The higher-order aberrations of the eye appear as deviations from the parabolic apparent shape of the retina, such as Fig. 14B shown.

[0046] OCT scanners can be used not only to measure spherical refractive errors, but also to measure the entire wavefront. For this purpose, a large number of multiple A scans arranged in a grating or in a radial arrangement can be used. Accordingly, a set of OCT images is obtained, each image is for a different cross-sectional plane of the patient's eye, so that each pixel of each image represents the eye measurement results at the X, Y and Z coordinates in the patient's eye. Therefore, the pixels representing the retina in multiple images (for example, the pixels representing the lines of the retina as discussed above) can be combined to obtain a surface, i.e., a surface defined by an array of X, Y and Z coordinates. This three-dimensional surface is actually a wavefront elevation map used in wavefront science. For example, the Z value (i.e., elevation) at each (X, Y) coordinate in the three-dimensional surface represents the phase delay value of the wavefront elevation map. In the case where this surface is bowed upward, the patient's eye is myopic, and in the case where this surface is bowed downward, the patient's eye is hyperopic, wherein the amount of the sag of the surface indicates the degree of myopia or hyperopia, as described above.

[0047] The wavefront elevation map may be analyzed using the well-known Zernike, Fourier, or Seidel methods. From the wavefront elevation map (i.e., from the two-dimensional sagittal height of the surface) optical properties of the eye may be derived, such as the modulation transfer function, point spread function, Strehl ratio, Snellen resolution, astigmatism error, coma, spherical aberration, higher order aberrations, lens prescription parameters, etc. The derivation of the optical parameters of the eye using the measured wavefront elevation map is a routine procedure for any technician in the field of human eye optics and need not be described in detail here.

[0048] Refractive error can be measured using any of the methods herein using an OCT scanner by configuring the OCT scanner to generate a strictly parallel incident beam onto the cornea. If there is some error in the parallelism of the OCT beam (e.g. caused by the scanning system of the OCT), a strictly parallel test surface can be measured with OCT. The non-parallelism of the incident OCT beam will appear as a deviation from a straight line on the OCT image of the test surface. This deviation is a calibration error and should be used during the calculation to derive the correct wavefront error of the eye.

[0049] To measure the wavefront aberration map, a reflective plane surface P (see Fig. 8A and Figure 8B ) should ideally be flat. It is also necessary to select a flat anatomical retinal layer to measure the wavefront map of the eye. In the human eye, the ideal flat anatomical layer is the Bruch's membrane-retinal pigment epithelium (RPE) complex. Anatomical studies have shown that the presence of the fovea does not affect the flatness of the RPE / Bruch complex and can be used to measure the sag Δ.

[0050] The following conclusions and applications are drawn from the above methods:

[0051] Using the method described herein, the refractive error of the entire eye can be obtained without knowing the refractive index of the cornea, the refractive index of the lens, or any topographical and geometrical data of the surface. All that is needed is to trust the validity of Fermat's principle, which has been proven to be valid over the past 350 years.

[0052] The method described herein can be performed without performing OCT-based ray tracing. This method is independent of ray tracing.

[0053] • Refractive error measured in diopters is roughly proportional to the sagittal height Δ. The sagittal height Δ is roughly proportional to the square of the pupil diameter.

[0054] - Higher order aberrations appear as deviations from the parabolic sagittal surface of the apparent image of the retina.

[0055] As described below, OCT can be transformed into a multifunctional diagnostic tool by applying certain changes to the OCT beam scanner.

[0056] Anterior chamber OCT can be converted into a wavefront measurement diagnostic tool by (a) making the incident OCT beam strictly parallel and (b) increasing the length of the OCT reference beam arm by the distance between the pupil and the retina (approximately 20 mm) so that the imaging area is shifted downward from the anterior chamber to the retina.

[0057] • An OCT scanner implementing the methods described herein can be built into a surgical microscope used during ophthalmic surgery. In this way, the surgical microscope becomes an intraoperative wavefront aberrometer, like ORA (Alcon) or Holos (Zeiss).

[0058] A computing device may be used to perform a method for measuring a refractive error of an eye of a patient as described herein. The computing device may receive one or more OCT images from an OCT scanner or be incorporated into the OCT scanner itself. The computing device includes one or more processing devices and one or more storage devices operably connected to the one or more processing devices. The one or more storage devices store executable code that, when executed by the one or more processing devices, causes the one or more processing devices to calculate a refractive error of the patient's eye as described above and any other calculations or measurements from the OCT images as described above.

[0059] The foregoing description is provided to enable any person skilled in the art to practice the various embodiments described herein. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the claims are not intended to be limited to the embodiments shown herein, but are to be given the full scope consistent with the language of the claims.

Claims

1. A device for performing optometry measurements, the device comprising: an optical coherence tomography (OCT) imaging system configured to scan a parallel light beam across an eye of a patient; as well as one or more processing devices; One or more storage devices coupled to the one or more processing devices, the one or more storage devices storing executable code that, when executed by the one or more processing devices, causes the one or more processing devices to: receiving one or more OCT images of the patient's eye from the OCT imaging system; identifying an apparent image of a retina of an eye of the patient in the one or more OCT images; and The refractive error of the patient's eye is calculated based on the shape of the apparent image.

2. The device according to claim 1, wherein: The executable code, when executed by the one or more processing devices, further causes the one or more processing devices to: In the event that the apparent image is bowed upward, it is determined that the patient's eye is myopic. In the event that the apparent image is bowed downward it is determined that the patient's eye is hyperopic.

3. The device of claim 1, wherein: The executable code, when executed by the one or more processing devices, further causes the one or more processing devices to calculate higher-order aberrations of the patient's eye based on the shape of the apparent image.

4. The device of claim 1, wherein: The executable code, when executed by the one or more processing devices, further causes the one or more processing devices to measure a refractive error of the patient's eye by calculating a sagittal height of the apparent image.

5. The device of claim 4, wherein: The executable code, when executed by the one or more processing devices, further causes the one or more processing devices to calculate the refractive error of the patient's eye by: obtaining an incident angle α corresponding to a width of a pupil of the patient's eye and a distance between the pupil and the retina; and The refractive error is calculated based on the sagittal height and the angle of incidence.

6. The apparatus of claim 1, further comprising a surgical microscope, the OCT imaging system being incorporated into the surgical microscope.

7. The device of claim 1, wherein: The OCT imaging system is a phase-sensitive OCT imaging system.

8. The device of claim 1, wherein: The OCT imaging system is a modified anterior chamber OCT imaging system.

9. The device of claim 1, wherein: The OCT imaging system is a modified anterior chamber OCT in which a reference optical arm length is selected to image the retina of the patient's eye and is configured to generate a parallel light beam.

10. The device of claim 1, wherein: One of the following: (a) the one or more OCT images comprise a single image, and the apparent image is a line in the single image representing the retina; and (b) the one or more OCT images include multiple images of multiple cross-sectional planes of the patient's eye, and the apparent image is a surface defined by an array of X, Y and Z coordinates corresponding to pixels in the multiple images representing the retina.

11. A device for eye imaging, comprising: an optical coherence tomography (OCT) imaging system configured to scan a parallel light beam across an eye of a patient; one or more processing devices; One or more storage devices coupled to the one or more processing devices, the one or more storage devices storing executable code that, when executed by the one or more processing devices, causes the one or more processing devices to: receiving, from the OCT imaging system, a plurality of OCT images of the patient's eye, each OCT image in the plurality of OCT images corresponding to one of a plurality of cross-sectional planes of the patient's eye such that each pixel in each OCT image in the plurality of OCT images represents an X, Y, and Z coordinate within the patient's eye; Pixels in the plurality of OCT images representing a retina of the patient's eye are identified to obtain a surface corresponding to a wavefront elevation map of the patient's eye.

12. The device of claim 11, wherein: The executable code, when executed by the one or more processing devices, further causes the one or more processing devices to calculate a refractive error of the patient's eye using the wavefront elevation map.

13. A method for performing optometry measurements, the method comprising: scanning a parallel light beam over an eye of a patient by an optical coherence tomography (OCT) imaging system to obtain a plurality of OCT images of a plurality of cross-sectional planes of the eye of the patient; identifying pixels in the plurality of OCT images representing a retina of an eye of the patient to obtain a wavefront elevation map; as well as The wavefront elevation map is used to calculate the refractive error of the patient's eye.

14. The method of claim 13, further comprising: Determining (a) that the wavefront elevation map is upwardly bowed; and In response to (a), it is determined that the patient's eye is myopic.

15. The method of claim 13, further comprising: determining (a) that the wavefront elevation map is downwardly bowed; and In response to (a), it is determined that the patient's eye is hyperopic.

16. The method of claim 13, further comprising calculating higher-order aberrations of the patient's eye based on the shape of the wavefront elevation map.

17. The method of claim 13, further comprising calculating the refractive error by calculating a sagittal height of the wavefront elevation map.

18. The method of claim 17, further comprising: obtaining an angle of incidence α, the angle of incidence being determined by a width of a pupil of the patient's eye and a distance between the pupil and a retina of the patient's eye; and The refractive error is calculated based on the sagittal height and the incident angle α.

19. The method of claim 13, further comprising calculating the refractive error of the patient's eye based on the shape of the wavefront elevation map when ophthalmic surgery is performed using a surgical microscope, and the OCT imaging system is integrated into the surgical microscope.

20. The method of claim 13, wherein: The OCT imaging system is a modified anterior chamber OCT imaging system.

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