Device for measuring the length of an object
Patent Information
- Application Number
- EP2023735260
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-09-14
- Filing Date
- 2023-06-22
- Publication Date
- 2025-07-02
AI Technical Summary
Current optical coherence tomography (OCT) devices face challenges in measuring the length of the eye with high precision while managing high data rates and maintaining image quality, as they require large measuring depths and high resolutions, which are costly and difficult to handle.
A device with a path length switching unit that alternates between two optical path lengths to focus on the cornea and retina areas, allowing for precise measurement of the eye length with reduced data rates and optimized signal-to-noise ratio, using adjustable telescopes and numerical phase corrections to improve image quality.
Enables precise measurement of the eye length with high precision (approximately 10 μm) while reducing data rates and maintaining high image quality, allowing for efficient processing and accurate biometry.
Smart Images
Figure 1.1
Abstract
Description
[0001] Patent application
[0002] Applicant: Heidelberg Engineering GmbH
[0003] Max-Jarecki-Str. 8 69115 Heidelberg
[0004] Device for measuring the length of an object
[0005] The invention relates to a device according to the preamble of claim 1.
[0006] The term optical coherence tomography (OCT) refers to an imaging procedure.
[0007] This technique can be used to obtain two- and three-dimensional images from light-scattering structures. 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 a reference path.
[0008] The light reflected from the sample or object interferes with the reference beam. Signals from the 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 OCT images.
[0009] Against this background, WO 2012 / 104 097 A1 discloses a method for acquiring cross-sectional images using a path-length switching unit. The path-length switching unit changes the path length of a sample beam and / or a reference beam of an interferometer, allowing depth-sectional images to be generated at different depths within a sample. The path length is changed by redirecting beam paths along different geometric paths.
[0010] To measure the eye length, i.e., the length from the cornea to the retina, with full OCT resolution, i.e., with a resolution of less than 10 pm, a correspondingly large measurement depth of approximately 40–45 mm is currently required. This involves measuring the full depth at full resolution. Another approach could be to reduce the resolution, i.e., the OCT bandwidth.
[0011] Using the full OCT bandwidth or resolution results in very high data rates that are difficult or impossible to process. The technical complexity and associated costs are correspondingly high. However, reducing the resolution would have the disadvantage of compromising the quality of the measurement results and noticeably degrading the image quality.
[0012] The invention is therefore based on the object of reconciling, as far as possible, the competing goals of high resolution or image quality and the generation of the lowest possible data rates when measuring the length of objects. The present invention solves the aforementioned problem by the features of claim 1.
[0013] According to the invention, an optical path length is alternately shifted from a first value to a second value by at least one path length switching unit in order to detect essentially only two structures which are necessary for measuring the length of an object, for example a human eye.
[0014] First of all, it has been recognized that the approaches mentioned above are disadvantageous in that when measuring a human eye, large parts of the acquired data sets would cover the relatively uninteresting area of the vitreous body, which usually does not provide any information relevant for measuring eye length.
[0015] Against this background, it has been further recognized that the teaching of WO 2012 / 104 097 A1 requires an extension to virtually mask out the relatively uninteresting region of the vitreous body. According to the invention, the path length switching unit described in the cited document is used to alternately switch between imaging the corneal region and the retinal region. The intrinsic image depth of the OCT system can be significantly smaller than the length of the eye.
[0016] This invention makes it possible for the first time to measure eye length with a very high precision of approximately 10 pm, while simultaneously optimizing the measurement speed, signal-to-noise ratio (SNR), and image quality, while keeping data rates as low as those required for an OCT with a comparatively shallow image depth (approximately 10 mm). The path length switching unit could change the first value of the optical path length to the second value after a defined time interval and then, after the expiration of the time interval or a subsequent time interval, change the second value back to the first value. This enables automated path length variation, allowing reproducible measurements.
[0017] The path length switching unit could repeat the alternating change of the values over a specified period of time, preferably periodically at a defined frequency. By specifying the period as the measurement period and / or specifying the frequency, the signal-to-noise ratio can be variably adjusted and optimized.
[0018] Given this, the frequency could be in the range of 1 to 1000 Hz. Data rates within this frequency range are easily processable.
[0019] The difference between the optical path lengths at the first and second values could be variably adjustable depending on the length of the object being measured. The expected length can be input as an external parameter into the evaluation unit or a control unit. This allows calibration of the device.
[0020] The evaluation unit could determine structures of the object from the recorded and processed signals, which can be detected during the setting of the first value and the second value, whereby the spatial distance between these structures can be determined and output as the length of the object by the evaluation unit. This makes it possible, for example, to record the cornea and the retina as structures in a human eye and to use their distance to determine the length of the eye. The path length offset between the two paths or path lengths is preferably selected so that all relevant eye lengths are covered. The offset is precisely calibrated when the device is put into operation. By quickly alternating between the two positions and the subsequent detection of the cornea and retina position in the OCT signals, the total eye length can be determined if the offset is known.
[0021] An adjustable telescope could be provided to keep a structure to be captured in focus. This can improve the signal-to-noise ratio (SNR) in the area of the structure. Specifically, the image in two imaging paths could be adjusted using individual optics so that the corneal image has a focus in the cornea. The retinal image is designed to have a focus in the retina. This ensures an optimal signal-to-noise ratio (SNR) in both areas. To further optimize the SNR, an adjustable focus telescope could be used to compensate for any possible refractive error of the eye and to always keep the retina in focus.
[0022] The evaluation unit could differentiate the complex conjugate plane of a signal from the real plane of the signal. This could reduce data rates. To reduce data rates, particularly but not exclusively in the area of retinal scanning – similar to full-range OCT – the complex conjugate (CC) plane of the signal could be differentiated from the real plane of the signal.
[0023] Against this background, the evaluation unit could be capable of performing a numerical phase correction to determine the plane in which a signal from a structure of the object to be detected lies. A numerical phase correction could be used to determine in which of these planes, complex or real, the retinal signal lies, and thus the true distance from the reference arm. This is possible by applying a real and a complex conjugate phase correction to each measurement and comparing both signals. This can break the Hermitian symmetry for FD-OCT, which makes it difficult to determine unambiguous optical lengths between two image areas.
[0024] A dispersive element, preferably an optical fiber, could be arranged in the reference arm or sample arm. If sufficiently strong dispersion is introduced into the OCT interferometer or setup, a significant amplitude difference occurs after Fourier transformation of the signals, since signals that do not originate from the plane matching a phase vector are severely distorted. If the interferometer itself exhibits hardly any dispersive properties, optical elements with dispersive properties can be inserted to sufficiently increase this effect. This can be achieved, for example, by inserting a piece of fiber with dispersive properties into the reference arm or sample arm.
[0025] In a method for determining the length of an eye, a device of the type described here could be used, wherein at least one path length switching unit alternates an optical path length from a first value to a second value in order to direct or focus light into the region of the cornea at the first value and to direct or focus light into the region of the retina at the second value. This allows the length of an eye to be reliably determined. Against this background, the local position of the cornea could be detected at the first value and the local position of the retina could be detected at the second value, the length of the eye being determined by the distance between the positions.
[0026] By evaluating multiple or multiple measurements of the corneal and retinal positions, eye movement during the measurements could be captured. The results of this evaluation could be used to correct errors in corneal and retinal detection. This allows eye movements to be compensated for, increasing the accuracy of the measurement when determining eye length.
[0027] The retina or its associated signal could be located either in a real plane or a real image part of an OCT image or in a complex conjugate plane or a complex conjugate image part of an OCT image, whereby to determine the length of the eye either the movement trajectories of the cornea and retina are evaluated or a phase analysis of the signals from the cornea and retina is performed.
[0028] Images of the cornea and retina could be displayed and / or presented on a monitor in real time, allowing a person to evaluate and evaluate the images.
[0029] The device described here can carry out all the process steps described here individually or in combination.
[0030] The device described here can be used for eye length measurement, axial length measurement, biometry, and fundus length measurement. The drawing shows
[0031] Fig. 1 is a schematic representation of a device with a path length switching unit,
[0032] Fig. 2 is a schematic representation of the imaging in the corneal area and the retinal area, and
[0033] Fig. 3 various lens configurations for focusing.
[0034] Fig. 1 shows a device 1 with an interferometer. The device 1 comprises a light source 2 and a beam splitter 3, which splits the light from the light source 2 into a sample beam on a sample arm 4 and a reference beam on a reference arm 5.
[0035] A returning sample beam is reflected from a sample 6, namely an eye, as returning light 4a and interferes with a returning reference beam reflected by a mirror 7 as returning light 5a.
[0036] An evaluation unit 8 evaluates the signals of the interfering beams or lights 4a, 5a and creates depth-section images from the signals.
[0037] To generate the depth-section images, the sample beam is directed by a deflection unit 9 to various lateral positions on the sample 6. These positions define the measuring area 6a.
[0038] An optical unit 10 can focus the sample beam to a specific depth of the sample 6 if necessary. The depth-section image is recorded at the depth 11 of the sample 6. The depth 11 can be determined independently of the position or movement of the mirror 7 and the distance 12 of the device 1 from the sample 6 by a path length switching unit 13a and / or 13b arranged in the beam path of the sample arm 4 and / or the reference arm 5.
[0039] In this respect, Fig. 1 shows a device 1 for determining the length of the object 6 by carrying out optical coherence tomography, comprising an interferometer with a light source 2, a sample arm 4 and a reference arm 5, wherein the light emitted by the light source 2 can be split by a beam splitter 3 so that first light 4a can be guided on the sample arm 4 in the outgoing and returning directions and second light 5a can be guided on the reference arm 5 in the outgoing and returning directions, wherein the first and second returning lights 4a, 5a can be brought into interference.
[0040] The evaluation unit 8 is arranged to detect and process signals of the interfering first and second light 4a, 5a, wherein a path length switching unit 13a, 13b is arranged in the beam path of the sample arm 4 and / or in the beam path of the reference arm 5, which changes the optical path length of the light 4a, 5a passing through the path length switching unit 13a, 13b.
[0041] At least one of the path length switching units 13a, 13b alternates an optical path length from a first value to a second value.
[0042] At least one of the path length switching units 13a, 13b spends the first
[0043] Value after a defined time interval to the second value and then after the time interval or a further time interval has elapsed the second value back to the first value.
[0044] The path length switching unit 13a, 13b performs the alternating change of the values over a predefined period of time, preferably periodically at a defined frequency. The frequency is in the range of 1 to 1000 Hz.
[0045] The difference between the optical path lengths at the first value and the second value can be variably adjusted depending on the length of the object 6 being examined to be measured, whereby the expected length can be entered as an external parameter into the evaluation unit 8 or into a control unit 8a.
[0046] The evaluation unit 8 can determine structures of the object from the recorded and processed signals, which can be detected during the setting of the first value and the second value.
[0047] The spatial distance between these structures can be determined and output as the length of the object 6 by the evaluation unit 8.
[0048] An adjustable telescope is provided as the optical unit 10 in order to keep a structure to be detected in focus.
[0049] The evaluation unit 8 differentiates the complex conjugate plane of a signal from the real plane of the signal. A numerical phase correction can be performed with the evaluation unit 8 to determine the plane in which a signal from a structure of the object 6 to be detected lies. A dispersive element, preferably a light-conducting fiber, could be arranged in the reference arm 5 or sample arm 4. However, this is not shown here.
[0050] Fig. 2 schematically shows a method for determining the length of an eye, in which a device 1 of the type described above is used, wherein at least one path length switching unit 13a, 13b alternates an optical path length from a first value to a second value in time in order to direct or focus light into the region of the cornea (cornea) 14 at the first value, and to direct or focus light into the region of the retina (retina) 15 at the second value.
[0051] In Fig. 2, the left column schematically shows the setting of first values for detecting the area of the cornea 14 and the right column shows the setting of second values for detecting the area of the retina 15. The arrow represents the difference in the optical path lengths at the respective values, i.e. the path length offset.
[0052] The first value detects the local position of the cornea 14 and the second value detects the local position of the retina 15, whereby the length of the eye is determined by the distance between the positions.
[0053] By evaluating a plurality or series of measurements of the positions of the cornea 14 and the retina 15, the movement of the eye during the measurements can be detected and the results of this evaluation can be used to correct errors in the detection of the cornea 14 and the retina 15.
[0054] The retina 15 or its associated signal could be located either in a real plane or a real image part of an OCT image or in a complex conjugate plane or a complex conjugate image part of an OCT image, wherein to determine the length of the eye either the movement trajectories of the cornea 14 and retina 15 are evaluated or a phase analysis of the signals from the cornea 14 and retina 15 is carried out.
[0055] Images of the cornea and retina are displayed on a monitor 16 in real time.
[0056] Specifically, the eye length measurement is carried out with device 1 as follows:
[0057] Two optical paths are realized by at least one path length switching unit 13a, 13b.
[0058] The first optical path, represented by the first value, has an optical path length that is equal to the reference arm 5 just before the patient's cornea 14 ("DC position"). It is realized by suitable lenses such that the focus is in the area of the cornea 14.
[0059] The second optical path, represented by the second value, has a path length such that the DC position is in the retina area 15 of a typically long eye.
[0060] In Fig. 2, the OCT imaging area is marked by the boxes. The middle dividing line, partially dashed, corresponds to the DC position, the minus sign indicates the complex conjugate plane, and the plus sign the real plane. The upper boxes show the conditions for a standard eye, the middle boxes for a short eye, and the lower boxes for a long eye.
[0061] The offset represented by the double arrow is the difference in the path lengths of the two beam paths, which is predetermined, in particular mechanically, by the structure of the path length switching unit 13a, 13b.
[0062] To minimize the influence of patient movements on the measurement result, rapid switching and alternating measurement of the positions of cornea 14 and retina 15 is necessary.
[0063] Using the path length switching unit 13a, 13b, this is possible in the millisecond range. Ideally, a whole series of alternating positions is measured. From this series, the movement trajectory of the eye / measuring instrument can be deduced in order to correct any residual error.
[0064] When using the device 1 as described here, it is possible that the retina 15 is located either in the conjugate complex or in the real image part.
[0065] An assignment, and thus the correct determination of eye length, can be done in two ways.
[0066] This can be done either by checking the movement trajectories between cornea 14 and retina 15, where these are in the same direction when retina 15 is also in the real image part, and in the opposite direction when it is in the complex conjugate, or based on a phase analysis of the signals. To further optimize the signal yield at retina 15, an optimal focus on retina 15 for different eye lengths can be achieved using a focus telescope.
[0067] For this purpose, a lens configuration can be implemented that achieves a constant pupil filling level, ensuring maximum numerical aperture for different eye lengths to further optimize the SNR. Naturally, this approach achieves higher SNRs for shorter eyes.
[0068] Fig. 3 shows examples of such lens configurations.
[0069] List of reference symbols:
[0070] 1 device
[0071] 2 light source
[0072] 3 beam splitters
[0073] 4 sample arm
[0074] 4a outgoing and returning light
[0075] 5 Reference arm
[0076] 5a outgoing and returning light
[0077] 6 Sample (eye)
[0078] 6a Measuring range
[0079] 7 mirrors
[0080] 8 Evaluation unit
[0081] 8a Control unit
[0082] 9 Deflection unit
[0083] 10 optical unit
[0084] 11 Depth
[0085] 12 Distance of device 1 to sample 6
[0086] 13a, 13b Path length switching unit
[0087] 14 Cornea
[0088] 15 Retina
[0089] 16 monitors
Claims
Patent claims Device (1) for determining the length of an object (6) by carrying out optical coherence tomography, comprising an interferometer with a light source (2), a sample arm (4) and a reference arm (5), wherein the light emitted by the light source (2) can be split by a beam splitter (3) or circulator, so that first light (4a) can be guided on the sample arm (4) in the outgoing and returning directions and second light (5a) can be guided on the reference arm (5) in the outgoing and returning directions, wherein the first and the second returning light (4a, 5a) can be brought into interference, wherein an evaluation unit (8) for detecting and processing signals of the interfering first and second light (4a, 5a) is arranged and wherein a path length switching unit (13a, 13b) is arranged in the beam path of the sample arm (4) and / or in the beam path of the reference arm (5), which switch unit determines the optical path length of the the path length switching unit (13a,13b) of the light (4a, 5a) passing through, characterized in that at least one path length switching unit (13a, 13b) changes an optical path length from a first value to a second value in time. Device according to claim 1, characterized in that the path length switching unit (13a, 13b) changes the first value to the second value after a defined time interval and then, after the expiration of the time interval or a further time interval, changes the second value back to the first value. Device according to claim 1 or 2, characterized in that the path length switching unit (13a, 13b) changes the alternating change of the, values repeatedly over a specified period of time, preferably periodically with a defined frequency.
4. Device according to claim 3, characterized in that the frequency is in the range 1 to 1000 Hz.
5. Device according to one of the preceding claims, characterized in that the difference between the optical path lengths at the first value and at the second value is variably adjustable depending on the length of the object being examined to be measured, wherein the expected length can be input as an external parameter into the evaluation unit (8) or into a control unit (8a).
6. Device according to one of the preceding claims, characterized in that the evaluation unit (8) can determine from the recorded and processed signals structures of the object which are detectable during the setting of the first value and the second value, and that the spatial distance of these structures from one another can be determined and output as the length of the object (6) by the evaluation unit (8).
7. Device according to one of the preceding claims, characterized in that an adjustable telescope is provided in order to keep a structure to be detected in focus.
8. Device according to one of the preceding claims, characterized in that the evaluation unit (8) differentiates the conjugate complex plane of a signal from the real plane of the signal.
9. Device according to claim 8, characterized in that with the evaluation unit (8) a numerical phase correction can be carried out is to determine the plane in which a signal of a structure of the object (6) to be detected lies. Device according to one of the preceding claims, characterized in that a dispersive element, preferably a light-conducting fiber, is arranged in the reference arm (5) or sample arm (4). Method for determining the length of an eye, in which a device (1) according to one of the preceding claims is used, wherein at least one path length switching unit (13a, 13b) alternates an optical path length from a first value to a second value in order to direct or focus light into the region of the cornea (14) at the first value, and to direct or focus light into the region of the retina (15) at the second value.Method according to claim 11, characterized in that the local position of the cornea (14) is detected for the first value and that the local position of the retina (15) is detected for the second value, the length of the eye being determined by the distance between the positions. Method according to claim 12, characterized in that the movement of the eye during the measurements is detected by evaluating a plurality or a series of measurements of the positions of the cornea (14) and retina (15), and that the results of this evaluation are used to correct errors in the detection of the cornea (14) and the retina (15). Method according to one of claims 11 to 13, characterized in that the retina (15) or its associated... The signal is located either in a real plane or a real image part of an OCT image or in a complex conjugate plane or a complex conjugate image part of an OCT image, wherein, to determine the length of the eye, either the movement trajectories of the cornea (14) and retina (15) are evaluated or a phase analysis of the signals from the cornea (14) and retina (15) is performed. Method according to one of claims 11 to 14, characterized in that images of the cornea (14) and the retina (15) are displayed and / or displayed on a monitor (16) in real time.