Device for determining length of object, in particular of eye

Abstract

The invention relates to a device (1) for performing optical coherence tomography (OCT), comprising: an interferometer (1a) for guiding a light beam (4) into a dispersive object to be examined, which influences the propagation rate of the light (4a, 4b, 4c) depending on the frequency thereof; and an evaluation unit (5) for detecting the length (3) of the object. With regard to the problem of detecting the length of a transmitted illumination object as reliably as possible by means of a device for carrying out optical coherence tomography, the device is characterized in that an evaluation unit (5) analyzes interference data obtained from an OCT signal or an interference spectrum and determines dispersion-dependent data of the interference data and uses the dispersion-dependent data to determine the length (3).

Description

Technical Field

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

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

[0003] In this method, light with a certain bandwidth is usually split into two partial beams in a beam splitter. A first partial beam is incident on the sample or object to be examined, and a second partial beam passes through a reference section.

[0004] The light reflected from the sample or object interferes with the reference beam. Through the signal from the interference, the sample can be examined with depth resolution by a so-called A-scan, thereby examining in the depth of the optical axis of the first partial beam.

[0005] Furthermore, the first partial beam can be used to scan the sample flatly 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, ie the intensity is detected as a function of time, while the optical path length in the reference arm changes.

[0007] In so-called FD-OCT (Frequency Domain OCT), the interference of the individual spectral components is detected, ie 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 referred to simply as the field angle. Information about the length of the eye and about the optical properties of the individual examined eye is necessary to convert the field angle into an absolute length or distance.

[0009] Furthermore, if one wants to perform a correction of the OCT image, it is necessary to know the length of the eye in order to display the OCT image in the correct proportions with the correct curvature.

[0010] Eye size and visual impairments vary widely among the population. Uncorrected A-scan cSLO images or OCT segment sizes vary.

[0011] Against this background, two possibilities are currently known to determine the length of the eye.

[0012] The estimation of the length can be performed by means of a set refraction and a manually entered radius of the cornea with the aid of an eye model.

[0013] Direct measurement of the length can also be performed using a separate device. In this context, biometric devices are known which simultaneously record a rough image of the retina, for which the length determined by the device can be used to determine the scale.

[0014] However, these images are not suitable for diagnostic purposes due to their poor quality.

[0015] Separate measurements of the length of the eye or the curvature of the cornea (keratometry) are additional efforts for the user. Therefore, these measurements are not always performed, or the results are not transferred to the software used to evaluate OCT images of the posterior segment of the eye.

[0016] If these values ​​are entered and they are entered manually, transmission errors may also occur.The additional installation of biometric technology in a diagnostic retinal OCT device will significantly increase the complexity of the system. Summary of the invention

[0017] The invention is therefore based on the object of detecting the length of a transmitted-illuminated object as reliably as possible by means of a device for performing optical coherence tomography.

[0018] The invention achieves this object by means of the features of claim 1 .

[0019] According to the invention, it has first been recognized that light penetrating into a dispersive object to be examined experiences dispersion, i.e. the influence of the object on the propagation speed of the light depends on the frequency of the light. Furthermore, it has been recognized that an evaluation unit must be provided in such a device, which analyzes the interference data obtained from the OCT signal or the interference spectrum, determines dispersion-related data of the interference data, and determines the length of the object via the dispersion-related data. According to the invention, the axial length of an object, in particular a human eye, is measured by detecting the dispersion. According to the invention, it has been recognized here that dispersion is generally a disturbing factor leading to poorer image quality, since the dispersion effects in the OCT device are compensated via hardware and / or software. However, according to the invention, the dispersion effect is used to measure the length.

[0020] Using such a device, the length of the eye can be estimated by evaluating the OCT signal in a relatively unproblematic manner. In addition, the length of the eye can be measured during the capture of OCT images on the posterior segment of the eye.

[0021] In existing devices, corneal curvature and refraction are used to estimate the axial length of the eye and thus determine the scale. However, if the eye parameters not taken into account, such as corneal curvature of the second surface, anterior chamber depth and lens parameters, deviate from the model eye used, the determination of the axial length of the eye is flawed. This is particularly the case if the patient's refractive defects are corrected by using an intraocular lens (IOL) or by refractive surgery. Therefore, a direct measurement of the axial length of the eye is a significantly more robust parameter for the determination of the scale. Due to the better correspondence of the scale between the test data and the reference data, the classification method can achieve a higher test power.

[0022] Methods that do not record dense volumes, but use a scanning pattern with a fixed scale, additionally benefit from a better correspondence of the recorded positions of the recorded OCT cross-sectional images with the target positions. For example, in the case of circular scans with a chosen absolute radius (especially in the millimeter range), the actual radius of the eye differs less.

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

[0024] The evaluation unit can utilize a predetermined model which theoretically describes the dispersion, i.e. the influence of a medium on the propagation speed of light in the medium. Such a model can be stored in a storage medium of the device. The best-fit model can be used depending on the object being transmitted illuminated in order to approximately theoretically describe the dispersion behavior of the respective object being examined and to infer the length of the object based on experimental values.

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

[0026] The interferometric data may include A-scans or OCT images generated as partial spectra of the interference spectrum. The A-scans are generated over the depth in the beam direction so that the longitudinal dispersion effect on the optical path length can be derived therefrom.

[0027] In this context, the evaluation unit can determine the axial distance along the beam direction of every two A-scans or every two OCT images in order to determine the dispersion-related data.

[0028] The evaluation unit can plot a matching curve or a fitting curve via the values ​​obtained from the dispersion-related data in order to determine the length of the object under examination. Thus, a length dimension can be derived from the experimentally determined data, wherein the theoretical value is compared with the experimental value.

[0029] The device used in this article can be designed as FD-OCT, that is, a device suitable for performing frequency domain optical coherence tomography (FD-OCT), in particular for performing spectral domain optical coherence tomography (SD-OCT) or for performing swept source optical coherence tomography (SS-OCT).

[0030] An apparatus of the type described herein may be used in a method for determining the length of a human eye, wherein the method comprises the following steps:

[0031] - Use a device to record the interference spectrum;

[0032] - splitting the interference spectrum into two or more partial spectra;

[0033] - Calculate an A-scan or OCT image for each partial spectrum;

[0034] - Determine the relative axial offset of each two A-scans or each two OCT images with respect to each other in the optical path length to infer the dispersion behavior of the eye;

[0035] - Determining the length of the eye based on a model (in particular a matching curve or a fitting curve) that represents the effect of dispersion on light as a function of the path length traveled by the light in a dispersive medium.

[0036] Using this method, an estimation of the length of the eye is achievable by using frequency domain optical coherence tomography (FD-OCT). The method comprises at least the following steps: The method comprises a step of recording FD-OCT data on the eye, a step of determining the dispersion behavior, preferably via a suitable method, and a step of calculating the axial length of the eye by means of a fitting (in particular a curve fitting or matching curve) based on a model of the dispersion behavior depending on the length of the medium of the dispersive eye through which it passes.

[0037] Different methods can be used to determine the dispersion from the OCT signal: One suitable method is the determination of the dispersion via spectral axial shift (i.e. the so-called "ex vivo shift"), which method consists of the following method steps: The interference spectrum is recorded using spectral domain OCT. The spectrum is split into two or more partial spectra. For each individual of these partial spectra, a calculation of an A-scan or OCT image is performed. The relative axial shift of the A-scan or OCT images of the different partial spectra relative to each other on the optical path length is determined. The dispersion curve is determined by model fitting.

[0038] Multiple A-scans or OCT images can be used together, in particular adjacent strips of A-scans or OCT images, to determine the relative axial offset of the OCT images from the partial spectra. The relative axial positions of the OCT images can thus be estimated with greater accuracy from the partial spectra.

[0039] It is also possible to calculate scattered A-scans or A-scans from different positions of the fundus in order to determine the shape of the fundus. In conjunction with an OCT device that scans the retina, it is also possible to use non-central A-scans and perform a determination of the thickness of the medium passed (in particular the length of the eye) for each scan coordinate individually. Thus, more detailed information about the shape of the fundus can be obtained.

[0040] The OCT signals of the retina can be recorded, and thus the length of the eye can be determined, and / or the OCT signals of the posterior segment of the eye can be recorded, and thus the length of the eye can be determined. The length of the eye can thus be determined when examining the fundus, in particular the retina. Advantageously, a separate measurement of the length of the eye is not necessary, and thus no sources of error occur in the data transmission or due to non-input of data. The average accuracy of the absolute scale specification of the retinal OCT recordings and of the simultaneously recorded cSLO images, i.e. images from a confocal scanning laser ophthalmoscope (cSLO), is thus increased.

[0041] The length of the eye and the axial position of the OCT image can be used to measure and / or set the distance between the camera of the device and the eye. Thus, the user has to perform fewer adjustment steps in order to examine the eye. The distance between the camera and the eye can be calculated from the calculated length of the eye together with the axial absolute position of the OCT image. If information about the position of the fovea is additionally available in the software, for example from an anatomical positioning system, the central length of the eye can be determined with greater reliability.

[0042] The OCT signal of the posterior segment of the eye, in particular the retina, can be used as a direct control variable for the automatic setting of the distance from the camera of the device to the eye (after determining its length). It can be advantageously checked during recording whether the distance from the camera to the eye is correct by means of a distance measurement, which is calculated from the length of the eye and the OCT position on the retina. This information can be used for manual or automatic adjustment of the camera. If the optical length of the eye is known, the OCT signal of the retina can be used as a direct control variable for the automatic distance setting (retinal signal in the optimal light spot for the reference arm length set optimally for the eye length).

[0043] The curvature of the posterior segment of the eye, in particular the retina, can be determined. The method advantageously does not require any additional hardware outside the FD-OCT system. The measurement of retinal curvature is relevant to various pathologies, for example, in myopic patients. The curvature of the retinal signal within the OCT image field essentially depends on the working distance vertex-objective lens / vertex-cornea.

[0044] If the optical axis length is known, the working distance can be reliably determined from the known parameters reference arm length and sample arm length to the objective vertex. The true curvature of the retina can thus be determined significantly more accurately using a corresponding eye model. In principle, this has advantages over methods in which the distances to the retina and cornea are measured sequentially, for example, because the reference arm length is varied, the measurements are carried out simultaneously. Errors in the length measurement due to axial movements of the eye are therefore largely or completely excluded.

[0045] The teaching described herein allows a more precise classification method that depends on the scale. The teaching aids manual adjustment by specifying the interocular distance or derived indications, thus achieving a higher image quality on average. The automatic adjustment function of the device is assisted. It is possible to display the retina in the correct proportion with the actual curvature. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A schematic representation of the recording of the posterior segment of the eye by OCT or cSLO is shown, where the field of view angle is referred to as the device parameter φ, while the size of the recording area d at the fundus is not directly accessible because the optics and dimensions of the eye are unknown,

[0047] Figure 2 Different eyes are shown, each eye showing a different length,

[0048] Figure 3 A graph showing the relationship between the refractive index n on the y-axis and the wavelength of light in μm on the x-axis is shown,

[0049] Figure 4 shows a schematic representation of the eye, where three light beams with different frequencies have different wavelengths inside the eye due to the dispersive properties of the eye medium,

[0050] Figure 5 A schematic representation of an apparatus for performing optical coherence tomography (OCT) is shown, the apparatus directing a light beam into an examined eye, and an evaluation unit analyzing interference data obtained from the OCT signal or interference spectrum and determining dispersion-related data of the interference data and determining the length of the eye via the dispersion-related data,

[0051] Figure 6 A schematic representation of the calculation of two OCT images from partial spectra of the spectrum is shown,

[0052] Figure 7 Shown from Figure 6 Schematic diagram of the axial offset of two registered OCT images, and

[0053] Figure 8 A fitting curve is shown from which eye length can be determined based on the axial offset, wherein the fitting curve comprises a graph in which eye length in millimeters is plotted on the x-axis and the axial offset is plotted on the y-axis. DETAILED DESCRIPTION

[0054] Figure 1 A schematic representation of recording the posterior segment of an eye by means of a device 1 ' for performing optical coherence tomography is shown, Figure 1 In , the field of view angle is referred to as the device parameter φ. However, the size of the recording area d at the fundus is not directly accessible, since the optics and dimensions of the eye 2 are unknown. Figure 2 Different eyes 2 with different lengths 3 are shown.

[0055] Figure 3 It is shown mathematically that the speed of light propagation in a dispersive medium depends on its frequency. The refractive index n of the medium is calculated from the quotient of the wavelength of light in a vacuum and the wavelength of light in the material.

[0056] Figure 4 It is schematically shown that three light beams with different frequencies have different wavelengths and therefore different propagation speeds inside the eye 2. Due to dispersion with equal physical distance, the optical path lengths are different for different wavelengths. The more dispersed the medium through which the light passes, the greater the difference in optical path lengths for different wavelengths. In other words, the longer the eye 2, the stronger the effect of dispersion.

[0057] The length of the eye can be calculated from the strength of the dispersion effect using a model for the dispersion of the eye, n(λ, z). In an exemplary model for a uniformly constructed eye, the following applies:

[0058] L(λ)=n(λ)·d

[0059] In this formula, L is the optical path length of light of a particular wavelength, n is the refractive index, and d is the physical distance that light travels in the dispersive medium, and is therefore the length of the modeled eye. The following then applies to differences in the optical path length L:

[0060] ΔL=L(λ1)-L(λ2)=(n(λ1)-n(λ2))·d

[0061] Therefore, the length d of the modeled eye results in the following:

[0062]

[0063] An OCT device can be used to measure the difference in optical path length.

[0064] Figure 5 Such a device 1 for performing optical coherence tomography (OCT) is shown, comprising an interferometer 1a for guiding a light beam 4 into a dispersive object to be examined (i.e. the eye 2), whose influence on the propagation speed of the light depends on the frequency of the light, and an evaluation unit 5 for detecting a length 3 of the object (i.e. the eye 2).

[0065] The interferometer 1a splits the light with a certain bandwidth into two partial beams in a beam splitter. The first partial beam is incident on the object to be inspected (i.e. the eye 2), while the second partial beam passes through a reference section. The light reflected from the object interferes with the reference beam. By means of the signal from the interference, the object can be inspected with depth resolution by a so-called A-scan or OCT image, thus inspecting the object in the depth of the optical axis of the first partial beam.

[0066] The evaluation unit 5 analyses the interference data obtained from the OCT signal or the interference spectrum and determines dispersion-dependent data of the interference data and determines the length 3 of the eye 2 via the dispersion-dependent data.

[0067] Figure 6 It is shown that the interference data comprises A-scans 6a, 6b or OCT images 6'a, 6'b generated or calculated from partial spectra of the interference spectrum 7 based on the so-called "segmented spectroscopy method". The OCT images 6'a, 6'b from the respective partial spectra are registered.

[0068] Figure 7 It is shown that the evaluation unit 5 determines an axial distance 8 of two A-scans 6a, 6b or OCT images 6'a, 6'b along the beam direction to determine dispersion-related data. In particular, the dispersion-related data thus comprises at least one axial offset 8 of the OCT images 6'a, 6'b.

[0069] The evaluation unit 5 uses a plurality of A-scans 6a, 6b or OCT images 6'a, 6'b together (ie certain strips of adjacent A-scans 6a, 6b or OCT images 6'a, 6'b) in order to determine a relative axial shift 8 of the OCT images 6'a, 6'b from the partial spectra.

[0070] Figure 8 It is shown that the evaluation unit 5 determines the length 3 of the eye 2 by fitting a model or based on a model representing the influence of dispersion on light as a function of the path distance travelled by the light in the dispersive medium.

[0071] Specifically, the evaluation unit 5 uses a predetermined model which theoretically describes the dispersion, ie the influence of a medium on the propagation speed of light in the medium. Figure 8 The model describes the dispersion behavior of one or more media of the human eye 2. If the axial offset 8 is known, it can be obtained from Figure 8 The fitting curve of reads the length 3 of the eye 2 on the x-axis (ie, the distance of the path traversed).

[0072] The evaluation unit 5 plots a matching curve or a fitting curve by means of the values ​​obtained from the dispersion-related data in order to determine the length 3 .

[0073] The device is designed as FD-OCT, i.e. a device suitable for performing frequency domain optical coherence tomography (FD-OCT), in particular for performing spectral domain optical coherence tomography (SD-OCT) or for performing swept source optical coherence tomography (SS-OCT).

[0074] Using the device described here, a method for determining the length 3 of a human eye 2 is performed, the method comprising the following steps:

[0075] - recording an interference spectrum 7 using the device 1;

[0076] - splitting the interference spectrum 7 into two or more partial spectra;

[0077] - calculation of an A-scan 6a, 6b or an OCT image 6'a, 6'b for each partial spectrum;

[0078] - determining the relative axial offset 8 of each two A-scans 6a, 6b or each two OCT images 6'a, 6'b relative to one another in the optical path length in order to infer the dispersion behavior of the eye 2;

[0079] - Determining the length 3 of the eye 2 based on a model (ie matching curve or fitting curve) which represents the effect of dispersion on light as a function of the path distance travelled by the light 4 in the dispersive medium.

[0080] A plurality of A-scans 6a, 6b or OCT images 6'a, 6'b, in particular strips of adjacent A-scans 6a, 6b or OCT images 6'a, 6'b, are used together in order to determine from the partial spectrum the relative axial offset 8 of the images 6'a, 6'b. Using this method, the OCT signals of the retina are recorded and the length 3 of the eye 2 is determined. In particular, the OCT signals are recorded in the posterior segment of the eye and the length 3 of the eye 2 is determined.

[0081] The length 3 of the eye 2 and the axial position of the OCT images 6'a, 6'b can be used to measure and possibly set the distance between the camera 1b of the device 1 and the eye 2. The distance between the camera 1b and the eye 2 can be calculated from the length 3 and the axial OCT position of the eye 2. The length 3 of the eye 2 can be measured at a plurality of arbitrary points of the B-scan.

[0082] The OCT signal of the posterior segment of the eye (particularly 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 after the length 3 thereof has been determined. The determined distance to the eye 2 is used for adjusting the camera 1b during recording. In particular, a fully automatic adjustment of the camera 1b is facilitated.

[0083] Determine the curvature of the posterior segment of the eye, in particular the retina. The length 3 of the eye 2 can be used to display the retina in the correct scale with its natural curvature.

[0084] List of reference numerals:

[0085] 1 Device

[0086] 1a 1 interferometer

[0087] 1b 1 camera

[0088] 2 Eyes

[0089] 3 2 length

[0090] 4 Beam

[0091] 4a-c Light, beam of light with a specific wavelength

[0092] 5 Evaluation Units

[0093] 6a, 6b A-scan

[0094] 6'a, 6'b OCT images

[0095] 7 Spectrum

[0096] 8 Axial offset

Claims

1. A device (1) for performing optical coherence tomography (OCT), the device (1) comprising an interferometer (1a) for guiding a light beam (4) into a dispersive object to be examined, the interferometer (1a) influencing the propagation speed of the light (4a, 4b, 4c) as a function of the frequency of the light (4a, 4b, 4c), and an evaluation unit (5) for detecting the length (3) of the object, It is characterized in that The evaluation unit (5) analyses interference data obtained from the OCT signal or the interference spectrum and determines dispersion-dependent data of the interference data and determines the length (3) from the dispersion-dependent data.

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

3. The device according to claim 1 or 2, characterized in that: The evaluation unit (5) uses a predetermined model which theoretically describes the dispersion, ie the influence of a medium on the propagation speed of light (4a, 4b, 4c) in the medium.

4. The device according to claim 2 or 3, characterized in that The model describes the dispersion behavior of the medium or media of the human eye (2).

5. A device according to any one of the preceding claims, characterised in that The interference data comprises an A-scan (6a, 6b) or an OCT image (6'a, 6'b) generated from a partial spectrum of the interference spectrum (7).

6. The device according to claim 5, characterized in that The evaluation unit (5) determines the axial distance (8) of every two A-scans (6a, 6b) or every two OCT images (6'a, 6'b) along the beam direction in order to determine the dispersion-related data.

7. The device according to claim 6, characterized in that The evaluation unit (5) determines the length (3) by plotting a matching curve or a fitting curve by using the values ​​obtained from the dispersion-related data.

8. Device (1) according to any one of the preceding claims, characterized in that The device (1) is designed as FD-OCT, that is, the device (1) is designed as a device suitable for performing frequency domain optical coherence tomography (FD-OCT), in particular, performing 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 the preceding claims, comprising the following steps: - recording the interference spectrum (7) using the device (1); - splitting the interference spectrum (7) into two or more partial spectra; - calculating an A-scan (6a, 6b) or OCT image (6'a, 6'b) for each partial spectrum; - determining the relative axial offset (8) of each two A-scans (6a, 6b) or each two OCT images (6'a, 6'b) with respect to each other in the optical path length in order to infer the dispersion behavior of the eye (2); - determining the length (3) of the eye (2) based on a model, in particular a matching curve or a fitting curve, which represents the influence of dispersion on light (4a, 4b, 4c) as a function of the path distance travelled by the light (4a, 4b, 4c) in a dispersive medium.

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

11. The method according to claim 9 or 10, characterized in that: Scattered A-scans, or A-scans from different locations on the fundus, are calculated to determine the shape of the fundus.

12. The method according to any one of claims 9 to 11, characterized in that The OCT signals of the retina are recorded and the length (3) of the eye (2) is determined and / or the OCT signals of the posterior segment of the eye are recorded and the length (3) of the eye (2) is determined.

13. The method according to any one of claims 9 to 12, 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. The method according to any one of claims 9 to 13, characterized in that After determining the distance between the camera (1b) of the device (1) and the eye (2), the OCT signal of the posterior segment of the eye, in particular the retina, is determined as a direct control variable for automatically setting the distance between the camera (1b) of the device (1) and the eye (2).

15. The method according to any one of claims 9 to 14, characterized in that The curvature of the posterior segment of the eye, in particular the retina, is determined.

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