Device for determining the length of an object, in particular the length of an 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 object influences the propagation rate of light (4a, 4b, 4c) depending on the frequency thereof; and an evaluation unit (5) for detecting a length (3) of the object. With respect to the problem of detecting the length of a transilluminated object as reliably as possible by means of a device for performing optical coherence tomography, the device is characterised in that the evaluation unit (5) analyses interferometric data obtained from an OCT signal or interference spectrum and determines dispersion-related data of the interferometric data and determines the length (3) using the dispersion-related data.

Description

[0001] Patent application

[0002] Applicant: Heidelberg Engineering GmbH

[0003] Max-Jarecki-Str. 8

[0004] 69115 Heidelberg

[0005] Device for determining the length of an object, in particular the length of an eye

[0006] The invention relates to a device according to the preamble of claim 1.

[0007] Optical coherence tomography (OCT) is an imaging technique that can be used to obtain two- and three-dimensional images of light-scattering structures.

[0008] In this method, light with a certain bandwidth is typically split into two partial beams in a beam splitter. The first partial beam falls on the sample or object under investigation, while the second partial beam travels through a reference path.

[0009] The light reflected from the sample or object interferes with the reference beam. Signals from this interference allow the sample to be examined with depth resolution, i.e., at the depth of the optical axis of the first partial beam, using so-called A-scans. In addition, it is possible to scan the sample flatly or laterally with the first partial beam to obtain three-dimensional OCT images.

[0010] In a so-called TD-OCT (Time Domain OCT), signals are continuously recorded in the time domain when the optical path length in the reference arm changes, i.e. an intensity is recorded as a function of time.

[0011] In the so-called FD-OCT (Frequency Domain OCT), the interference of individual spectral components is recorded, i.e. an intensity is recorded as a function of the frequency or wavelength of the light.

[0012] In OCT imaging of the posterior segment of an eye, the lateral scaling of images is often only known as the field angle. Converting the field angle into absolute lengths or distances requires information about the length of an eye as well as the optical properties of the individual eye being examined.

[0013] In addition, knowledge of the length of an eye is necessary if you want to correct an OCT image in order to display it to scale with a correct curvature.

[0014] Eye sizes and refractive errors vary considerably across the population. A scanned cSLO image or an OCT section varies in size without correction.

[0015] Against this background, there are currently two main methods known for determining the length of an eye. The length can be estimated using a set refraction and a manually entered corneal radius using an eye model.

[0016] One can also measure the length directly using a device. For this purpose, biometric devices are known that simultaneously capture a rough image of the retina, for which a scale can be determined based on the length determined by the device.

[0017] However, due to their poor quality, these images are not suitable for diagnostic purposes.

[0018] A separate measurement of the length of an eye or the curvature of the cornea (corneal curvature) represents an additional effort for a user. Therefore, these measurements are not always performed, or the results are not transferred to software for evaluating OCT images of the posterior segment of the eye.

[0019] Furthermore, if such values ​​are entered manually, transmission errors may occur. The additional integration of biometer technology into a diagnostic retinal OCT device would significantly increase system complexity.

[0020] The invention is therefore based on the object of detecting the length of an X-rayed object as reliably as possible by means of a device for carrying out optical coherence tomography.

[0021] The present invention solves the aforementioned problem by the features of claim 1. According to the invention, it was first recognized that light penetrating a dispersive object under examination undergoes dispersion, namely that the object influences the propagation speed of the light depending on its frequency. It was further recognized that such a device must include an evaluation unit that analyzes interferometric data obtained from an OCT signal or interference spectrum, determines dispersion-related data from the interferometric data, and uses the dispersion-related data to determine the length of the object. According to the invention, the axial length of the object, in particular of the human eye, is measured by detecting the dispersion.The invention recognizes that dispersion is typically a disruptive factor that leads to poorer image quality, which is why dispersive effects in OCT devices are compensated for using hardware and / or software. However, the invention utilizes the effect of dispersion to measure lengths.

[0022] With such a device, it is relatively easy to estimate the length of an eye by evaluating OCT signals. Furthermore, it is possible to measure the length of an eye during OCT image acquisition of the posterior segment of the eye.

[0023] Existing devices use corneal curvature and refraction to estimate the axial length of the eye and thus determine scaling. However, the determination of the axial length of the eye becomes inaccurate if unconsidered eye parameters, such as corneal curvature of the second surface, anterior chamber depth, and lens parameters, deviate from the model eye used. This is particularly the case if a patient's refractive error has been corrected through the use of intraocular lenses (IOLs) or refractive surgery. The direct measurement of the axial length of the eye is therefore a significantly more robust parameter for determining scaling. Due to the better agreement of the scaling between test data and reference data, classification methods can achieve higher test power.

[0024] Procedures that do not acquire a dense volume but use scan patterns or scan patterns with fixed scaling also benefit from better agreement between the acquisition location of the acquired OCT cross-sectional images and the target positions. For example, in circular scans with a selected absolute radius, especially in the millimeter range, the actual radius in the eye will vary less.

[0025] The evaluation unit could determine the object's length by fitting it to a model or using a model that represents the influence of dispersion on light as a function of the distance the light has traveled in a dispersive medium. In transparent media, the refractive index depends on the frequency of the light incident on these media. This effect of dispersion is used to determine the object's length.

[0026] The evaluation unit could access a predefined model that theoretically describes dispersion, namely the influence of a medium on the propagation speed of light in that medium. Such models could be stored in a storage medium of the device. Depending on the object being scanned, the most suitable model can be accessed to approximately theoretically describe the dispersion behavior of the object under investigation and to infer the object's length based on experimental values. The model could describe the dispersion behavior of one or more media of the human eye. This would enable reliable measurements of the length of an eye. The naturally occurring dispersion of eye media can be used to determine the length of the eye when performing FD-OCT, particularly without additional hardware.

[0027] The interferometric data could include A-scans or OCT images generated from partial spectra of an interference spectrum. A-scans are generated at the depth of a beam direction, allowing for inferences about dispersion influences along an optical path length.

[0028] Against this background, the evaluation unit could determine an axial distance between two A-scans or two OCT images along a beam direction to determine the dispersion-related data.

[0029] The evaluation unit could create a fitting curve using values ​​obtained from the dispersion-related data to determine the length of the object under investigation. This allows a length measurement to be derived from experimentally determined data by comparing theoretical values ​​with experimental ones.

[0030] The device used here could be configured as an FD-OCT, namely as 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). A device of the type described here could be used in a method for determining the length of a human eye, the method comprising the following steps:

[0031] Recording an interference spectrum with the device;

[0032] Division of the interference spectrum into two or more sub-spectra; calculation of an A-scan or an OCT image for each sub-spectra;

[0033] Determination of the relative axial offset of two A-scans or OCT images in optical path length to each other in order to draw conclusions about the dispersion behavior of the eye;

[0034] Determination of the length of the eye using a model, in particular a fitting curve or fit curve, which represents the influence of dispersion on light as a function of the distance traveled by the light in a dispersive medium.

[0035] With such a method, an estimation of the length of an eye can be realized using frequency-domain optical coherence tomography (FD-OCT). The method comprises at least the following steps: The method comprises the step of acquiring FD-OCT data from the eye, the step of determining a dispersion behavior, preferably using a suitable method, and the step of calculating the axial length of the eye using a fit, in particular a fit curve or adjustment curve, based on a model of the dispersion behavior as a function of the length of the dispersive eye media passed through.

[0036] Various methods are possible for determining dispersion from an OCT signal. One suitable method is dispersion determination via a spectral axial shift, the so-called "walk-off shift" method, which consists of the following steps: An interference spectrum is recorded using a spectral domain OCT. The spectrum is divided into two or more partial spectra. An A-scan or OCT image is calculated for each of these partial spectra. The relative axial offset in optical path length of the A-scans or OCT images of the different partial spectra is determined. A dispersion curve is determined by a model fit.

[0037] Multiple A-scans or OCT images could be used together, especially strips of adjacent A-scans or OCT images, to determine the relative axial offset of OCT images from the partial spectra. This allows the axial relative position of the OCT images to be estimated with greater accuracy from the partial spectra.

[0038] Decentralized A-scans or A-scans from different fundus locations could also be calculated to determine the fundus shape. In conjunction with an OCT device that scans the retina, the thickness of the traversed medium, particularly the length of the eye, can also be determined using non-central A-scans and separately for each scan coordinate. This allows even more detailed information about the fundus shape to be obtained.

[0039] An OCT signal of the retina could be acquired, and the length of the eye could be determined, and / or an OCT signal could be acquired from the posterior segment of the eye, and the length of the eye could be determined. This allows the length of the eye to be determined during an examination of the fundus, particularly the retina. Advantageously, no separate measurement of the length of the eye is necessary, and there is no source of error in data transmission or due to non-entry of data. This increases the average accuracy of an absolute scaling specification of retinal OCT images, as well as of simultaneously acquired cSLO images, namely confocal scanning laser ophthalmoscopy (cSLO) images.

[0040] The eye length and the axial position of an OCT image could be used to measure and / or adjust the distance between a device's camera and the eye. The user therefore needs fewer adjustment steps to examine an eye. The calculated eye length, together with the absolute axial position of the OCT image, can be used to calculate the distance between the camera and the eye. If additional information about the position of the fovea (fovea) is available in the software, for example, from an anatomical positioning system, the central length of the eye can be determined with greater accuracy.

[0041] The OCT signal from the posterior segment of the eye, particularly the retina, could be used as a direct control variable for automatically adjusting the distance of a device's camera from the eye once the eye's length has been determined. Advantageously, a distance measurement calculated from the eye's length and the OCT position on the retina can be used during the image acquisition to check whether the camera's distance to the eye is correct. This information can be used for manual or automatic camera adjustment. The OCT signal from the retina can be used as a direct control variable (retinal signal in the sweet spot for a reference arm length optimally adjusted to the eye length) for automatic distance adjustment, provided the optical length of the eye is known.

[0042] The curvature of the posterior segment of the eye, particularly the retina, could be determined. Advantageously, the method requires no additional hardware beyond an FD-OCT system. Measuring retinal curvature is relevant for various pathologies, for example, in myopia patients. The curvature of the retinal signal within the OCT image field depends significantly on the working distance between the apex lens and the apex of the cornea.

[0043] Knowing the optical bulb length, the working distance can be reliably determined from the known parameters of the reference arm length and the sample arm length to the lens apex. This allows the true curvature of the retina to be determined much more accurately using appropriate eye models. Compared to methods that measure the distance to the retina and cornea sequentially, for example, by varying the reference arm length, this method has the advantage of simultaneously measuring the distance. Therefore, errors in the length measurement due to axial movements of the eye are largely or completely eliminated.

[0044] The gauge described here allows for more precise classification procedures that depend on scaling. The gauge provides assistance with manual adjustment by specifying the interpupillary distance or derived indicators, thus achieving higher image quality on average. It supports the device's automatic adjustment function. It offers the possibility of a true-to-scale representation of the retina with its actual curvature.

[0045] In the drawing show

[0046] Fig. 1 is a schematic representation of an image of the posterior segment of the eye using OCT or cSLO, in which the field of view angle is known as a device parameter, but the size of the imaged area at the fundus d is not directly accessible because the optics and size of the eye are unknown, Fig. 2 is a schematic representation of different eyes, each showing different lengths,

[0047] Fig. 3 is a diagram showing the refractive index n on the y-axis versus the wavelength of light in pm on the x-axis,

[0048] Fig. 4 is a schematic representation of an eye in which three light beams with different frequencies have different wavelengths inside the eye due to the dispersive properties of the eye media,

[0049] Fig. 5 is a schematic representation of a device for performing optical coherence tomography (OCT), which directs a light beam into an eye to be examined, and an evaluation unit, which analyses interferometric data obtained from an OCT signal or interference spectrum and determines dispersion-related data of the interferometric data and determines the length of the eye using the dispersion-related data,

[0050] Fig. 6 is a schematic representation of a calculation of two OCT images from partial spectra of a spectrum,

[0051] Fig. 7 is a schematic representation of the axial offset of the two registered OCT images from Fig. 6 and

[0052] Fig. 8 shows a fit curve from which the eye length can be determined based on the axial offset. The fit curve comprises a graph in which the length of the eye in mm is plotted on the x-axis and the axial offset on the y-axis. Fig. 1 shows a schematic representation of an image of the posterior segment of the eye using a device 1' for performing optical coherence tomography, in which the visual field angle is known as a device parameter. However, the size of the imaged area on the fundus d is not directly accessible because the optics and size of the eye 2 are unknown. Fig. 2 shows various eyes 2 with different lengths 3.

[0053] Fig. 3 mathematically illustrates that the propagation speed of light in dispersive media depends on its frequency. The refractive index n of a medium is calculated from the quotient of the wavelength of light in a vacuum to the wavelength of light in the material.

[0054] Fig. 4 schematically shows that three light beams with different frequencies have different wavelengths and therefore different propagation velocities inside Eye 2. Due to dispersion, the optical path lengths vary for different wavelengths at the same physical distance. The more dispersive the light travels through, the more the optical path lengths differ for different wavelengths. In other words, the longer Eye 2 is, the stronger the dispersion effect.

[0055] Using a model n (A, z) for the dispersion of an eye, the length of the eye can be calculated from the strength of the dispersion effects. For an example model for a homogeneously constructed eye, the following applies:

[0056] L(A) = n(A) • d

[0057] In this formula, L is the optical path length for light of a specific wavelength A, n is the refractive index, and d is a physical distance that the light must travel in a dispersive medium, i.e., the length of the model eye. Then, the difference between optical path lengths L is:

[0058] AL = L(A X ) — L(A2) = (zi(A1) — n(A2)) • d

[0059] This gives the length d of the model eye as:

[0060] To measure the AL, i.e. the difference in optical path lengths, an OCT device can be used.

[0061] Fig. 5 shows such 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, namely an eye 2, which influences the propagation speed of light as a function of its frequency, and an evaluation unit 5 for detecting a length 3 of the object, namely the eye 2.

[0062] The interferometer 1a splits light with a specific bandwidth into two partial beams in a beam splitter. The first partial beam falls on the object under investigation, namely eye 2, while the second partial beam travels a reference path. The light reflected from the object interferes with the reference beam. Signals from the interference allow the object to be examined with depth resolution, i.e., at the depth of the optical axis of the first partial beam, using so-called A-scans or OCT images.

[0063] The evaluation unit 5 analyzes interferometric data obtained from an OCT signal or interference spectrum and determines dispersion-related data from the interferometric data. Using the dispersion-related data, it determines the length 3 of the eye 2. Using a so-called "split-spectrum approach," Fig. 6 shows that the interferometric data include A-scans 6a, 6b or OCT images 6'a, 6'b, which are generated or calculated from partial spectra of an interference spectrum 7. The OCT images 6'a, 6'b from the various partial spectra are registered.

[0064] Fig. 7 shows that the evaluation unit 5 determines an axial distance 8 between the two A-scans 6a, 6b or the OCT images 6'a, 6'b along a beam direction to determine the dispersion-related data. Specifically, the dispersion-related data therefore includes at least one axial offset 8 between the OCT images 6'a, 6'b.

[0065] In this respect, the evaluation unit 5 uses several A-scans 6a, 6b or OCT images 6'a, 6'b together, namely specifically strips of adjacent A-scans 6a, 6b or OCT images 6'a, 6'b, in order to determine the relative axial offset 8 of OCT images 6'a, 6'b from the partial spectra.

[0066] Fig. 8 shows that the evaluation unit 5 determines the length 3 of the eye 2 by fitting it to a model or using a model that represents the influence of dispersion on light as a function of the distance traveled by the light in a dispersive medium.

[0067] Specifically, the evaluation unit 5 accesses a predefined model that theoretically describes dispersion, namely the influence of a medium on the propagation speed of light in that medium. The model shown in Fig. 8 describes the dispersion behavior of one or more media of the human eye 2. Knowing the axial offset 8, the length 3 of the eye 2, i.e., the distance traveled, can be read from the fit curve shown in Fig. 8 on the x-axis. The evaluation unit 5 creates an adjustment curve or fit curve using values ​​obtained from the dispersion-related data to determine the length 3.

[0068] The device is designed as an FD-OCT, namely as 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).

[0069] The device described here is used to carry out a method for determining the length 3 of a human eye 2, which comprises the following steps:

[0070] Recording an interference spectrum 7 with the device 1;

[0071] Division of the interference spectrum 7 into two or more partial spectra; calculation of one A-scan 6a, 6b or OCT image 6'a, 6'b for each partial spectrum;

[0072] Determination of the relative axial offset 8 of two A-scans 6a, 6b or OCT images 6'a, 6'b in optical path length to each other in order to draw conclusions about the dispersion behavior of the eye 2;

[0073] Determination of the length 3 of the eye 2 using a model, namely a fitting curve or fit curve, which represents the influence of dispersion on light as a function of the path traveled by the light 4 in a dispersive medium.

[0074] Multiple 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, to determine the relative axial offset 8 of images 6'a, 6'b from the partial spectra. Using this method, an OCT signal of the retina is recorded, and the length 3 of eye 2 is determined. Specifically, an OCT signal is recorded at the posterior segment of the eye, and the length 3 of eye 2 is determined.

[0075] The length 3 of eye 2 and the axial position of an OCT image 6'a, 6'b can be used to measure and, if necessary, adjust the distance between a camera 1b of device 1 and eye 2. The distance between camera 1b and eye 2 can be calculated from the length 3 of eye 2 and the axial OCT position. The length 3 of eye 2 can be measured at any number of arbitrary locations within a B-scan.

[0076] The OCT signal of the posterior segment of the eye, particularly the retina, is used as a direct control variable for automatically adjusting the distance of a camera 1b of the device 1 from the eye 2, after the latter's length 3 has been determined. The determined distance to the eye 2 is used to adjust the camera 1b during an image acquisition. In particular, fully automatic adjustment of the camera 1b is supported.

[0077] The curvature of the posterior segment of the eye, especially the retina, is determined. The length 3 of eye 2 can be used to create a true-to-scale representation of the retina with its natural curvature.

[0078] List of reference symbols:

[0079] 1 device

[0080] 1 a Interferometer of 1

[0081] 1 b Camera of 1

[0082] 2 eyes

[0083] 3 length of 2

[0084] 4 light beams

[0085] 4a-c Light, light beam with a specific wavelength

[0086] 5 Evaluation unit

[0087] 6a, 6b A-scan

[0088] 6'a, 6'b OCT image

[0089] 7 Spectrum

[0090] 8 axial offset

Claims

Patent claims Device (1) for carrying out 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 speed of light (4a, 4b, 4c) as a function of its frequency, and an evaluation unit (5) for detecting a length (3) of the object, characterized in that the evaluation unit (5) analyses interferometric data obtained from an OCT signal or interference spectrum and determines dispersion-related data of the interferometric data and determines the length (3) using the dispersion-related data. Device according to claim 1, characterized in that the evaluation unit (5) determines the length (3) of the object by a fit to a model ordetermined using a model that represents the influence of dispersion on light (4a, 4b, 4c) as a function of the path traveled by the light (4a, 4b, 4c) in a dispersive medium. Device according to claim 1 or 2, characterized in that the evaluation unit (5) accesses a predetermined model that theoretically describes the dispersion, namely the influence of a medium on the propagation speed of light (4a, 4b, 4c) in this medium. Device according to claim 2 or 3, characterized in that the model describes the dispersion behavior of one or more media of the human eye (2). Device according to one of the preceding claims, characterized in that the interferometric data comprise A-scans (6a, 6b) or OCT images (6'a, 6'b) generated from partial spectra of an interference spectrum (7). Device according to claim 5, characterized in that the evaluation unit (5) determines an axial distance (8) between each pair of A-scans (6a, 6b) or each pair of OCT images (6'a, 6'b) along a beam direction to determine the dispersion-related data. Device according to claim 6, characterized in that the evaluation unit (5) places an adjustment curve or fit curve through values ​​obtained from the dispersion-related data in order to determine the length (3).Device (1) according to one of the preceding claims, characterized by a design as an FD-OCT, namely 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). A method for determining the length (3) of a human eye (2) using a device (1) according to one of the preceding claims, comprising the following steps: - recording an interference spectrum (7) with the device (1); - division of the interference spectrum (7) into two or more sub-spectra; - Calculation of one A-scan (6a, 6b) or one OCT image (6'a, 6'b) for each partial spectrum; - Determination of the relative axial offset (8) of two A-scans (6a, 6b) or two OCT images (6'a, 6'b) in optical path length to each other in order to infer the dispersion behavior of the eye (2); - Determination of the length (3) of the eye (2) using a model, in particular an adjustment curve or fit curve, which represents the influence of dispersion on light (4a, 4b, 4c) as a function of the path traveled by the light (4a, 4b, 4c) in a dispersive medium.

10. The method according to claim 9, characterized in that several 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), in order to determine the relative axial offset (8) of images (6'a, 6'b) from the partial spectra.

11. Method according to claim 9 or 10, characterized in that decentralized A-scans or A-scans from different locations of the fundus are calculated to determine the shape of the fundus.

12. Method according to one of claims 9 to 11, characterized in that an OCT signal of the retina is recorded and the length (3) of the eye (2) is determined and / or that an OCT signal is recorded at the posterior segment of the eye and the length (3) of the eye (2) is determined.

13. Method according to one of claims 9 to 12, characterized in that the length (3) of the eye (2) and the axial position of an OCT image (6'a, 6'b) are used to measure and / or adjust the distance between a camera (1b) of the device (1) and the eye (2). Method according to one of claims 9 to 13, characterized in that the OCT signal of the posterior segment of the eye, in particular the retina, is used as a direct control variable for automatically adjusting the distance of a camera (1b) of the device (1) from the eye (2) after the length (3) of the eye has been determined. Method according to 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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