Apparatus for determining the length of an object, in particular the length of an eye

Abstract

The invention relates to an apparatus (1) for performing optical coherence tomography (OCT), comprising an interferometer (1a) for directing a light beam (4) onto a dispersive object to be examined, which influences the propagation speed of light depending on the frequency of the light (4a, 4b, 4c), and an evaluation unit (5) for determining the length (3) of said object. With regard to the task of determining the length of an object viewed by the apparatus for performing optical coherence tomography as reliably as possible, said apparatus is characterized in that the evaluation unit (5) analyzes interference data obtained from the OCT signal or interference spectrum, determines dispersion-related data from the interference data, and determines the length (3) using the dispersion-related data.

Description

[Technical Field]

[0001] The present invention relates to a device according to the preamble of claim 1. [Background technology]

[0002] The term "optical coherence tomography" (usually abbreviated as OCT) is understood to mean an imaging method by which two- and three-dimensional images of light-scattering structures can be obtained.

[0003] In this method, light of a given band is typically split into two partial beams in a beam splitter: the first partial beam is incident on the sample or object to be examined, and the second partial beam passes through a reference section.

[0004] The light reflected by the sample or object interferes with the reference beam, and the interference signal can be used to examine the sample with depth resolution, i.e., at the depth of the optical axis of the first partial beam, by means of a so-called A-scan.

[0005] Additionally, the first partial beam can be used to scan the sample horizontally or laterally to obtain a three-dimensional OCT image.

[0006] In so-called TD-OCT (Time Domain OCT), the signal is detected continuously in the time domain when the optical path length of the reference arm is changed, i.e. the intensity is detected as a function of time.

[0007] In so-called FD-OCT (Frequency Domain OCT), the interference of individual spectral components is detected, i.e. the intensity is detected as a function of the frequency or wavelength of the light.

[0008] In OCT imaging of the posterior segment of the eye, the lateral scale of the image is often known only as the field angle. Converting the field angle to absolute length or distance requires information about the ocular length and the optical properties of the individual eye examined.

[0009] Additionally, if one wishes to correct the OCT images to display them at the correct scale with the correct curvature, the eye length needs to be known.

[0010] Eye size and visual impairment vary widely within the population. A-scanned cSLO images or OCT sections will vary in size unless corrected.

[0011] Against this background, there are currently two main known methods for determining eye length.

[0012] Using an eye model, the length can be estimated with a set refractive value and a manually entered corneal radius.

[0013] It is also possible to measure the length directly using a separate device. Against this background, biometric devices are known which simultaneously take a rough image of the retina and can use the device-determined length to determine the scale.

[0014] However, this image is not suitable for diagnostic purposes due to its poor quality.

[0015] Separately measuring the ocular length or corneal curvature (corneal curvature) would require additional effort from the user, and therefore these are not always measured, nor are the results transferred to software for evaluating posterior segment OCT images.

[0016] If these values ​​are entered manually, there is also the possibility of transmission errors. The addition of biometric technology to a diagnostic retinal OCT device significantly increases the complexity of the system. Summary of the Invention [Problem to be solved by the invention]

[0017] The invention is therefore based on the object of determining as reliably as possible the length of an object viewed by a device for performing optical coherence tomography. [Means for solving the problem]

[0018] The present invention achieves the above object by the features of claim 1.

[0019] The present invention recognizes for the first time that light passing through a dispersive object to be examined is dispersed, i.e., that the object affects the propagation speed of light depending on the frequency of the light. Furthermore, it is recognized that the apparatus must be equipped with an evaluation unit that analyzes interference data obtained from the OCT signal or interference spectrum, determines dispersion-related data from the interference data, and determines the length of the object based on the dispersion-related data. According to the present invention, the axial length of an object, particularly a human eye, is measured by detecting dispersion. The present invention recognizes that dispersion is typically a disturbing factor that deteriorates image quality, and therefore the dispersion effect in the OCT apparatus is compensated for by hardware and / or software. However, according to the present invention, the effect of dispersion is used to measure length.

[0020] Such a device allows for a relatively trouble-free evaluation of the OCT signal to estimate ocular length, and furthermore, allows for measurement of ocular length while taking OCT images of the posterior segment.

[0021] Existing devices use corneal curvature and refraction to estimate the axial length of the eye, which in turn determines the scale. However, deviations from the used model eye due to unaccounted ocular parameters, such as the corneal curvature of the second surface, the anterior chamber depth, and lens parameters, can invalidate the determination of the axial length of the eye. This is especially true when the patient's refractive error has been corrected with an intraocular lens (IOL) or refractive surgery. Therefore, a direct measurement of the axial length of the eye is a much more robust parameter for determining the scale. A better correspondence between the scale of the test data and the reference data can increase the power of the classification method.

[0022] Additionally, methods that do not capture high-density volumes and use fixed-scale scan patterns have the advantage of a better correspondence between the captured OCT cross-sectional image and the target position, e.g., a circular scan with an absolute radius selected, especially in the millimeter range, reduces the variability of the actual radius of the eye.

[0023] The evaluation unit can determine the length of the object by fitting or based on a model that describes the effect of dispersion on light as a function of the path length traveled by the light in the dispersive medium. In a transparent medium, the refractive index depends on the frequency of the light incident on the medium. This effect of dispersion is used to determine the length of the object.

[0024] The evaluation unit can use predefined models that theoretically describe the dispersion, i.e., the influence of a medium on the propagation speed of light in the medium. Such models can be stored in the storage medium of the device. Depending on the object viewed through it, the best-fit model can be used to approximately theoretically describe the dispersion behavior of each examined object and to determine the object's length based on experimental values.

[0025] The model can represent the dispersion behavior of one or more media in the human eye. In this way, ocular length can be reliably measured. When FD-OCT is performed without additional hardware, the naturally occurring dispersion of ocular media can be used to determine ocular length.

[0026] The interference data can include A-scans or OCT images generated from subspectra of the interference spectrum, where A-scans are generated at depth along the beam, allowing the effect of dispersion over the optical path length to be determined.

[0027] Against this background, the evaluation unit can determine the axial distance between every two A-scans or every two OCT images along the beam direction to determine dispersion-related data.

[0028] The evaluation unit can generate a fit or fitted curve through values ​​obtained from the dispersion-related data to determine the length of the inspected object, whereby the length dimension can be determined using experimentally determined data and comparing theoretical and experimental values.

[0029] The device used herein may be designed as FD-OCT, i.e. as a device suitable for performing frequency-domain optical coherence tomography (FD-OCT), in particular spectral-domain optical coherence tomography (SD-OCT) or swept-source optical coherence tomography (SS-OCT).

[0030] The apparatus as described herein provides a method for determining the length of a human eye, the method comprising: recording an interference spectrum using the device; dividing the interference spectrum into two or more partial spectra; calculating an A-scan or OCT image for each subspectrum; determining the relative axial offset in optical path length of every two A-scans or every two OCT images relative to each other to determine the dispersion behavior of the eye; and Determining the eye length based on a model, in particular a curve fit or fitting, that represents the effect of dispersion on light as a function of the path length traveled by the light in a dispersive medium. The method can be used in a method having the following structure:

[0031] Such a method allows for estimation of the eye length using frequency domain optical coherence tomography (FD-OCT), comprising at least the steps of recording FD-OCT data for the eye, determining the dispersion behavior, preferably via a suitable method, and calculating the axial length of the eye by fitting, in particular a fitted or curve-fitted curve, based on a model that describes the dispersion behavior as a function of the length of the dispersive ocular medium traversed.

[0032] Various methods can be considered for determining dispersion from OCT signals. One suitable method is to determine dispersion via spectral axial shift, or the so-called "walk-off shift" method, which consists of recording an interference spectrum using spectral-domain OCT; dividing this spectrum into two or more partial spectra; calculating an A-scan or OCT image for each of these partial spectra; determining the relative axial offset in optical path length of the A-scan or OCT image for each partial spectrum with respect to each other; and determining a dispersion curve by model fitting.

[0033] Multiple A-scans or OCT images, especially adjacent A-scans or OCT image segments, can be used together to determine the relative axial offset of the OCT image from the partial spectrum, thereby allowing the relative axial position of the OCT image to be inferred from the partial spectrum with greater accuracy.

[0034] Distributed A-scans or A-scans from various positions on the fundus can be calculated to determine the shape of the fundus. For OCT devices that scan the retina, the thickness of the traversed medium, and in particular the length of the eye, can also be determined separately for each scan coordinate by decentralized A-scans, which can provide more detailed information about the shape of the fundus.

[0035] The retinal OCT signal can be recorded to determine ocular length, and / or the posterior segment OCT signal can be recorded to determine ocular length. This allows for ocular length determination when examining the fundus, particularly the retina. Advantageously, a separate measurement of ocular length is not required, eliminating sources of error due to data transfer or missing data entry. This improves the average accuracy of the absolute scale representation of retinal OCT images and simultaneously acquired cSLO images, i.e., confocal scanning laser ophthalmoscopy (cSLO) images.

[0036] The ocular length and the axial position of the OCT image can be used to measure and / or set the distance between the device's camera and the eye. Therefore, fewer adjustment steps must be performed by the user to examine the eye. The camera-to-eye distance can be calculated from the calculated ocular length and the absolute axial position of the OCT image. If the software can additionally use information about the location of the fovea, such as from an anatomical positioning system, the central length of the eye can be determined more reliably.

[0037] After determining the eye length, the OCT signal of the posterior segment, specifically the retina, can be used as a direct control variable for automatically setting the distance of the device's camera from the eye. Advantageously, the distance measurement calculated from the eye length and the OCT position on the retina allows the correct camera-to-eye distance to be confirmed during imaging. This information can be used to adjust the camera manually or automatically. Once the eye's optical distance is known, the retinal OCT signal can be used as a direct control variable for automatic distance setting (retinal signal at the sweet spot of the reference arm length optimally set for the eye length).

[0038] The curvature of the posterior segment of the eye, particularly the retina, can be determined. This method has the advantage that no additional hardware is required beyond the FD-OCT system. Measurement of retinal curvature is relevant to various conditions, such as myopia. The curvature of the retinal signal within the OCT image field is highly dependent on the working distance between the apex of the objective lens and the corneal apex.

[0039] Once the axial optical distance is known, the working distance can be reliably determined from the known parameters of the reference arm length and the sample arm length to the objective apex. This allows the true curvature of the retina to be determined much more accurately using a corresponding eye model. In principle, this has the advantage that the measurements are performed simultaneously compared to methods that measure the distance to the retina and the distance to the cornea sequentially, for example by varying the reference arm length. This largely or completely eliminates errors in the length measurement due to axial movements of the eye.

[0040] The teachings described herein allow for more accurate scale-dependent classification methods, facilitate manual adjustment of specified ocular distance or derived metrics, which on average results in higher image quality, assist the device's auto-adjustment capabilities, and allow the retina to be displayed at the correct scale with its actual curvature. [Brief explanation of the drawings]

[0041] [Figure 1] A schematic diagram showing imaging of the posterior segment of the eye using OCT or cSLO is shown. The angle of view is known as the device parameter φ, but the size of the imaging area on the fundus d cannot be directly obtained because the optical properties and size of the eye are unknown. [Figure 2] Schematic diagrams of various eyes, each with a different length. [Figure 3] A graph showing the refractive index n on the y-axis versus the wavelength of light (μm) on the x-axis is shown. [Figure 4] A schematic diagram of the eye in which three light rays of different frequencies have different wavelengths within the eye due to the dispersive nature of the ocular medium. [Figure 5] 1 shows a schematic diagram of an apparatus for performing optical coherence tomography (OCT), which directs a light beam into an eye to be examined, and an evaluation unit which analyzes interference data obtained from the OCT signal or interference spectrum, determines dispersion-related data from the interference data, and uses the dispersion-related data to determine the eye length. [Figure 6] 1 shows a schematic diagram illustrating the calculation of two OCT images from partial spectra of a spectrum. [Figure 7] FIG. 7 shows a schematic diagram illustrating the axial offset of the two registered OCT images of FIG. 6. [Figure 8] 1 depicts a fitted curve that allows for determining ocular length based on axial offset, the fitted curve comprising a graph plotting ocular length (mm) on the x-axis and axial offset on the y-axis. DETAILED DESCRIPTION OF THE INVENTION

[0042] 1 shows a schematic diagram illustrating imaging of the posterior segment of the eye using an apparatus 1' for performing optical coherence tomography. The angle of view is known as the apparatus parameter φ, but the size of the imaging area at the fundus d cannot be directly obtained because the optical properties and size of the eye 2 are unknown. FIG. 2 shows various eyes 2 with different lengths 3.

[0043] Figure 3 mathematically illustrates the frequency dependence of the propagation speed of light in a dispersive medium. The refractive index of a medium, n, is calculated from the ratio of the wavelength of light in a vacuum to the wavelength of light in the material.

[0044] Figure 4 shows a schematic representation of three light rays with different frequencies having different wavelengths within the eye 2, and therefore different propagation velocities. Due to dispersion, different wavelengths have different optical path lengths over the same physical distance. The more dispersive the medium through which the light travels, the greater the difference in optical path length for each wavelength. In other words, the longer the eye 2, the stronger the effect of dispersion.

[0045] The eye length can be calculated from the strength of the dispersive effect using a dispersive model of the eye, n(λ,z). In one example of a uniformly constructed eye model, the following applies:

number

[0046] where L is the optical path length for light of a given wavelength λ, n is the refractive index, and d is the physical distance traveled by light in a dispersive medium, i.e., the length of the model eye. Then, for the difference in optical path length L, the following applies:

number

[0047] Therefore, the length d of the model eye is:

[0048]

number

[0049] Using an OCT device, ΔL, the difference in optical path length, can be measured.

[0050] FIG. 5 shows an apparatus 1 for performing such optical coherence tomography (OCT), which comprises an interferometer 1 a for directing a light beam 4 onto a dispersive object to be examined, i.e., an eye 2, which affects the propagation speed of light depending on the frequency of the light, and an evaluation unit 5 for detecting the length 3 of the object, i.e., the eye 2.

[0051] The interferometer 1a splits light of a given band into two partial beams in a beam splitter. The first partial beam is incident on the object to be examined, i.e. the eye 2, while the second partial beam passes through a reference section. The light reflected by the object interferes with the reference beam. The signal resulting from the interference can be used to examine the object with depth resolution, i.e. at the depth of the optical axis of the first partial beam, by so-called A-scan or OCT imaging.

[0052] The evaluation unit 5 analyzes the interference data obtained from the OCT signal or the interference spectrum, determines dispersion-related data from the interference data, and determines the length 3 of the eye 2 using the dispersion-related data.

[0053] 6 shows that, based on the so-called "split spectrum method," the interference data includes A-scans 6a, 6b or OCT images 6'a, 6'b generated or calculated from partial spectra of the interference spectrum 7. The OCT images 6'a, 6'b from each partial spectrum are registered.

[0054] 7 illustrates that the evaluation unit 5 determines the axial distance 8 of two A-scans 6a, 6b or OCT images 6'a, 6'b along the beam direction to determine the dispersion-related data. Thus, in particular, the dispersion-related data includes at least one axial offset 8 of the OCT images 6'a, 6'b.

[0055] The evaluation unit 5 uses multiple A-scans 6a, 6b or OCT images 6'a, 6'b together, i.e. in particular fragments of adjacent A-scans 6a, 6b or OCT images 6'a, 6'b, to determine the relative axial offset 8 of the OCT images 6'a, 6'b from the partial spectra.

[0056] FIG. 8 shows that the evaluation unit 5 determines the length 3 of the eye 2 by fitting or based on a model that describes the effect of dispersion on light as a function of the path length traveled by light in a dispersive medium.

[0057] In particular, the evaluation unit 5 uses a predetermined model that theoretically describes dispersion, i.e. the influence of a medium on the propagation speed of light in said medium. The model according to Fig. 8 describes the dispersion behavior of one or more media in the human eye 2. The length 3 of the eye 2, i.e. the path length traversed, can be read off from the x-axis of the fitting curve according to Fig. 8 if the axial offset 8 is known.

[0058] The evaluation unit 5 determines the length 3 by creating a curve fit or curve fitting through values ​​obtained from the dispersion-related data.

[0059] The device is designed as FD-OCT, i.e. as a device suitable for performing frequency-domain optical coherence tomography (FD-OCT), in particular spectral-domain optical coherence tomography (SD-OCT) or swept-source optical coherence tomography (SS-OCT).

[0060] 1. A method for determining the length 3 of a human eye 2 using a device as described herein, comprising: recording an interference spectrum 7 using the device 1; dividing the interference spectrum 7 into two or more partial spectra; calculating an A-scan 6a, 6b or an OCT image 6'a, 6'b for each partial spectrum; determining the relative axial offset 8 in the optical path length of every two A-scans 6 a, 6 b or every two OCT images 6 ′ a, 6 ′ b relative to each other to determine the dispersion behavior of the eye 2; and Determining the length 3 of the eye 2 based on a model, i.e., a fitted or fitted curve, that represents the influence of dispersion on the light as a function of the path length travelled by the light 4 in the dispersive medium. The method comprises:

[0061] To determine the relative axial offset 8 of the images 6'a, 6'b from the partial spectrum, multiple A-scans 6a, 6b or OCT images 6'a, 6'b, particularly adjacent A-scans 6a, 6b or OCT images 6'a, 6'b, are used together. Using this method, the OCT signal of the retina is recorded to determine the length 3 of the eye 2. Specifically, the OCT signal is recorded at the posterior segment to determine the length 3 of the eye 2.

[0062] Using the length 3 of the eye 2 and the axial position of the OCT images 6'a, 6'b, the distance between the camera 1b of the device 1 and the eye 2 can be measured and set as needed. The distance between the camera 1b and the eye 2 can be calculated from the length 3 of the eye 2 and the axial OCT position. The length 3 of the eye 2 can be measured at any number of points in the B-scan.

[0063] After determining the length 3 of the eye 2, the OCT signal of the posterior segment, in particular the retina, is used as a direct control variable for automatically setting the distance of the camera 1b of the device 1 from the eye 2. The determined distance to the eye 2 is used to adjust the camera 1b during imaging. In particular, a fully automatic adjustment of the camera 1b is supported.

[0064] Determine the curvature of the posterior segment of the eye, specifically the retina. Using the length 3 of the eye 2, the retina can be displayed to the correct scale with its natural curvature. [Explanation of symbols]

[0065] 1 device 1a 1 interferometer 1b 1 camera 2 eyes 3. Length of 2 4 luminous flux 4a-c Light, light rays of a specified wavelength 5 Evaluation Units 6a,6b A-scan 6'a, 6'b OCT images 7. Spectrum 8 Axial Offset

Claims

1. An apparatus (1) for performing optical coherence tomography (OCT), comprising: an interferometer (1 a) for directing a light beam (4) onto a dispersive object to be examined, which influences the propagation speed of light (4 a, 4 b, 4 c) depending on the frequency of the light, and an evaluation unit (5) for determining the length (3) of said object, 10. The apparatus according to claim 9, wherein the evaluation unit (5) analyzes interference data obtained from an OCT signal or an interference spectrum, determines dispersion-related data from the interference data, and determines the length (3) using the dispersion-related data.

2. 2. The device according to claim 1, wherein the evaluation unit (5) determines the length of the object (3) by fitting to or on the basis of a model that represents the influence of dispersion on the light (4a, 4b, 4c) as a function of the path length traveled by the light (4a, 4b, 4c) in a dispersive medium.

3. 3. The device according to claim 2, characterized in that the evaluation unit (5) uses a predetermined model that theoretically describes the dispersion, i.e. the influence of a medium on the propagation speed of light (4a, 4b, 4c) in said medium.

4. 4. The device according to claim 3, wherein said model represents the dispersion behavior of one or more media in the human eye (2).

5. 2. The apparatus according to claim 1, wherein the interference data comprises A-scans (6a, 6b) or OCT images (6'a, 6'b) generated from subspectra of the interference spectrum (7).

6. 6. The apparatus according to claim 5, wherein the evaluation unit (5) determines the axial distance (8) between every two A-scans (6a, 6b) or every two OCT images (6'a, 6'b) along the beam direction to determine the dispersion-related data.

7. 7. The device according to claim 6, wherein the evaluation unit (5) determines the length (3) by creating a curve fit or a curve fit through values ​​obtained from the dispersion-related data.

8. 6. The device (1) according to claim 5, characterized in that it is designed as FD-OCT, i.e. as a device suitable for performing frequency domain optical coherence tomography (FD-OCT), in particular spectral domain optical coherence tomography (SD-OCT) or swept-source optical coherence tomography (SS-OCT).

9. A method for determining the length (3) of a human eye (2) using a device (1) according to any one of claims 1 to 8, comprising: recording an interference spectrum (7) using the device (1); Dividing the interference spectrum (7) into two or more partial spectra; calculating an A-scan (6a, 6b) or an OCT image (6'a, 6'b) for each partial spectrum; determining the relative axial offset (8) of every two A-scans (6a, 6b) or every two OCT images (6'a, 6'b) in the optical path length relative to each other to determine the dispersion behavior of the eye (2); and Determining the length (3) of the eye (2) on the basis of a model, in particular a curve fit or fitting, that represents the influence of dispersion on the light (4a, 4b, 4c) as a function of the path length travelled by said light (4a, 4b, 4c) in a dispersive medium. A method having the following.

10. 10. The method according to claim 9, characterized in that a plurality of A-scans (6a, 6b) or OCT images (6'a, 6'b), in particular segments of adjacent A-scans (6a, 6b) or OCT images (6'a, 6'b), are used together to determine the relative axial offset (8) of the images (6'a, 6'b) from the partial spectra.

11. 10. The method of claim 9, wherein distributed A-scans or A-scans from various positions on the fundus are calculated to determine the shape of the fundus.

12. 11. The method according to claim 10, characterized in that the length (3) of the eye (2) is determined by recording an OCT signal of the retina and / or by recording an OCT signal of the posterior segment of the eye (2).

13. 10. The method according to claim 9, characterized in that the length (3) of the eye (2) and the axial position of the OCT images (6'a, 6'b) are used to measure and / or set the distance between the camera (1b) of the device (1) and the eye (2).

14. 10. The method according to claim 9, characterized in that, after determining the length (3) of the eye (2), the OCT signal of the posterior segment of the eye, in particular the retina, is used as a direct control variable for automatically setting the distance of the camera (1b) of the device (1) from the eye (2).

15. 10. The method of claim 9, wherein the curvature of the posterior segment of the eye, in particular the retina, is determined.

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