DE102022123426A1 - Device for measuring the length of an object - Google Patents

Abstract

A 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 source (2) The light emitted can be split by a beam splitter (3), so that the first light (4a) can be guided on the sample arm (4) in the outflow and return directions and the second light (5a) can be guided on the reference arm (5) in the outflow and return directions, whereby the first and the second returning light (4a, 5a) can be brought into interference, an evaluation unit (8) being arranged for detecting and processing signals of the interfering first and second light (4a, 5a) and 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), is in view of the task, to reconcile the actually competing goals of high resolution or image quality and the generation of the lowest possible data rates when measuring the lengths of objects, characterized in that at least one path length switching unit (13a, 13b) alternates an optical path length from a first value to a second value spent.

Description

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

The term optical coherence tomography (in English “Optical Coherence Tomography”, usually abbreviated to OCT) refers to an imaging procedure.

With this process, two- and three-dimensional images can be obtained from light-scattering structures. In this process, light with a certain bandwidth is usually split into two partial beams in a beam splitter. The first partial beam falls on the sample or object to be examined, the second partial beam passes through a reference path.

The light reflected from the sample or object interferes with the reference beam. Using signals from the interference, the sample can 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 in order to obtain OCT images.

Against this background, from the WO 2012/104 097 A1 a method for recording sectional images is known in which a path length switching unit is used. The path length switching unit changes the path length of a sample beam and/or a reference beam of an interferometer so that depth section images can be generated at different depths of a sample. The path length is changed by redirecting beam paths along different geometric paths.

In order to be able to measure the length of the eye, i.e. the length from the cornea to the retina, with full OCT resolution, i.e. with a resolution of less than 10 µm, a correspondingly large measuring depth of approx. 40 is currently required - 45 mm necessary. The full depth is measured with full resolution. Another approach may be to reduce the resolution, i.e. the OCT bandwidth.

When using the full OCT bandwidth or resolution, very high data rates occur that are impossible or difficult to handle or process. The technical effort and the associated costs are correspondingly high. However, reducing the resolution would have the disadvantage that the quality of the measurement results would suffer and the image quality would noticeably decrease.

The invention is therefore based on the object of reconciling as far as possible the actually competing goals of high resolution or image quality and generating the lowest possible data rates when measuring the lengths of objects.

The present invention solves the aforementioned problem through the features of claim 1.

According to the invention, an optical path length is alternately moved from a first value to a second value by at least one path length switching unit in order to essentially detect only two structures that are necessary for measuring the length of an object, for example a human eye.

First of all, it was recognized that the approaches mentioned at the beginning are disadvantageous in that when measuring a human eye, large parts of acquired data sets would cover the relatively uninteresting area of the vitreous body, which generally does not provide any information that is relevant for measuring the length of the eye .

Against this background, it has been further recognized that the teaching of WO 2012/104 097 A1 an extension is required in order to virtually hide the relatively uninteresting area of the vitreous body. The path length switching unit described in the document mentioned is used according to the invention in such a way that switching back and forth alternately between imaging the corneal area and the retinal area. The intrinsic image depth of the OCT system can be significantly smaller than the length of the eye.

With this invention it is possible for the first time to measure the length of the eye with a very high precision of approx. 10 µm, while at the same time the measurement speed, the signal-to-noise ratio (SNR) and the image quality are optimized, and at the same time the data rates are so low as would be required for an OCT with a comparatively small image depth (approx. 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 change the second value back to the first value after the time interval or a further time interval has elapsed. This enables automated path length variation, which allows reproducible measurements.

The path length switching unit could alternately change the values via a predetermined value Repeat for a certain period of time, preferably periodically with a defined frequency. By specifying the time period as the measurement period and/or specifying the frequency, the signal-to-noise ratio can be variably adjusted and optimized.

Given this, the frequency could be in the range 1 to 1000 Hz. In this frequency range, data rates occur that can be processed without any problems.

The difference between the optical path lengths for the first value and the second value could be variably adjustable depending on the length of the examined object to be measured, with the expected length being input as an external parameter into the evaluation unit or into a control unit. This makes it possible to calibrate the device.

The evaluation unit could be able to determine structures of the object from the detected and processed signals, which can each be detected during the setting of the first value and the second value, with the spatial distance of these structures from one another being able to be determined and output by the evaluation unit as the length of the object. 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 chosen 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 then detecting the position of the cornea and retina in the OCT signals, the total eye length can be determined if the offset is known.

An adjustable telescope could be provided to keep a structure to be captured in focus. In this way, the signal-to-noise ratio (SNR) in the area of the structure can be improved. Specifically, the imaging could be adjusted in two imaging paths using individual optics so that the corneal imaging has a focus in the area of the cornea. The retinal image is designed so that there is a focus in the retina area. This ensures an optimal signal-to-noise ratio (SNR) in both areas. To further optimize the SNR, an adjustable focus telescope could be used, which compensates for possible ametropia of the eye in order to always keep the retina in focus.

The evaluation unit could differentiate the conjugate complex level of a signal from the real level of the signal. This allows data rates to be reduced. In order to reduce the data rates, particularly, but not exclusively, in the area of the retinal scan - similar to full-range OCT - the conjugate complex (CC) level of the signal could be differentiated from the real level of the signal.

Against this background, a numerical phase correction could be carried out with the evaluation unit in order to determine the plane in which a signal of 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, and thus at what real distance from the reference arm, the retina signal lies. This is possible by applying a real and a complex conjugate phase correction to each measurement and comparing both signals with each other. This can break the Hermitian symmetry for FD-OCT, which makes it difficult to determine clear optical lengths between two image areas.

A dispersive element, preferably a light-conducting fiber, could be arranged in the reference arm or sample arm. If the dispersion is sufficiently strong in the OCT interferometer or setup, a strong amplitude difference occurs after Fourier transformation of the signals, since signals that do not come from the plane that matches a phase vector are strongly distorted. If the interferometer itself hardly shows any dispersive properties, optical elements with dispersive properties can be inserted so that this effect becomes sufficiently large. This can be done, for example, by using a piece of fiber in the reference arm or sample arm with dispersive properties.

In a method for determining the length of an eye, a device of the type described here could be used, with at least one path length switching unit alternating an optical path length from a first value to a second value in order to direct light into the area of the cornea at the first value ( cornea) to spend or focus, and at the second value to spend or focus light in the area of the retina. This allows the length of an eye to be reliably determined.

Against this background, the local position of the cornea could be recorded with the first value and the local position of the retina could be recorded with the second value, with the length of the eye is determined by the distance between the positions.

By evaluating a plurality or series of measurements of the positions of the cornea and retina, the movement of the eye during the measurements could be detected, with the results of this evaluation being used to correct errors in the detection of the cornea and retina. This allows movements of the eye to be compensated for in order to increase the measurement accuracy when determining the length of the eye.

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, with either the movement trajectories of the cornea being used to determine the length of the eye and retina are evaluated or a phase analysis of the signals from the cornea and retina is carried out.

Images of the cornea and retina could be displayed and/or displayed on a monitor in real time. This allows a person to rate and evaluate the images.

The device described here can carry out all of the method steps described here individually or in combination.

The device described here can be used for eye length measurement, axial length measurement, biometry and fundus length measurement.

• 1 eine schematische Darstellung einer Vorrichtung mit einer Weglängenschalteinheit,
• 2 eine schematische Darstellung der Bildgebung im kornealen Bereich und im retinalen Bereich, und
• 3 diverse Linsenkonfigurationen zur Fokussierung.

Show in the drawing

• 1 a schematic representation of a device with a path length switching unit,
• 2 a schematic representation of the imaging in the corneal area and in the retinal area, and
• 3 various lens configurations for focusing.

1 shows a device 1 with an interferometer. The device 1 has a light source 2 and a beam splitter 3, which splits the light coming from the light source 2 into a sample beam on a sample arm 4 and a reference beam on a reference arm 5.

From a sample 6, namely an eye, a returning sample beam is reflected as returning light 4a and interferes with a returning reference beam reflected by a mirror 7 as returning light 5a.

An evaluation unit 8 evaluates the signals of the interfering beams or lights 4a, 5a and creates depth section images from the signals.

To generate the depth section images, the sample beam is directed to different lateral positions on the sample 6 by a deflection unit 9. These positions define the measuring range 6a.

An optical unit 10 can focus the sample beam into a specific depth of the sample 6 if necessary.

The depth section image is recorded at depth 11 of 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 to the sample 6 by a path length switching unit 13a and / or 13b, which is arranged in the beam path of the sample arm 4 and / or the reference arm 5.

To that extent shows 1 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 outflow and return directions and second light 5a on the reference arm 5 in the outflow and return directions, the first and the second returning light 4a, 5a being able to be brought to interference.

The evaluation unit 8 is arranged for detecting and processing signals of the interfering first and second light 4a, 5a, with a path length switching unit 13a, 13b being arranged in the beam path of the sample arm 4 and/or in the beam path of the reference arm 5, which determines the optical path length of the path length switching unit 13a, 13b each passing light 4a, 5a changed.

At least one of the path length switching units 13a, 13b spends an optical path length alternating in time from a first value to a second value.

At least one of the path length switching units 13a, 13b changes the first value to the second value after a defined time interval and then returns the second value to the first value after the time interval or a further time interval has elapsed.

The path length switching unit 13a, 13b carries out the alternating change of the values over a predeterminable period of time, preferably periodically with a defined frequency. The frequency is in the range 1 to 1000 Hz.

The difference between the optical path lengths for the first value and the second value can be variably adjusted depending on the length of the examined object 6 to be measured, the expected length being able to be entered as an external parameter into the evaluation unit 8 or into a control unit 8a.

The evaluation unit 8 can be used to determine structures of the object from the detected and processed signals, which can be detected during the setting of the first value and the second value.

The spatial distance between these structures can be determined and output as the length of the object 6 by the evaluation unit 8.

An adjustable telescope is provided as the optical unit 10 in order to keep a structure to be captured in focus.

The evaluation unit 8 differentiates the conjugate complex level of a signal from the real level of the signal. With the evaluation unit 8, a numerical phase correction can be carried out in order to determine the plane in which a signal of 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.

2 shows schematically a method for determining the length of an eye, in which a device 1 of the type described above is used, with at least one path length switching unit 13a, 13b spending an optical path length alternating in time from a first value to a second value in order to light at the first value to spend or focus in the area of the cornea 14, and at the second value to spend or focus light in the area of the retina 15.

In 2 The left column shows schematically 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 for the respective values, i.e. the path length offset .

The first value detects the local position of the cornea 14 and the second value detects the local position of the retina 15, the length of the eye being determined by the distance between the positions.

By evaluating a plurality or series of measurements of the positions of the cornea 14 and retina 15, the movement of the eye during the measurements can be recorded and the results of this evaluation can be used to correct errors in the detection of the cornea 14 and the retina 15 .

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, whereby to determine the length of the eye either the movement trajectories of Cornea 14 and retina 15 are evaluated or a phase analysis of the signals from cornea 14 and retina 15 is carried out.

Images of the cornea and retina are displayed on a monitor 16 in real time.

• Es werden durch mindestens eine Weglängenschalteinheit 13a, 13b zwei optische Wege realisiert.

Specifically, the eye length measurement is carried out with the device 1 as follows:

• Two optical paths are implemented by at least one path length switching unit 13a, 13b.

The first optical path, represented by the first value, has an optical path length that is the same length as the reference arm 5 shortly in front of the patient's cornea (cornea) 14 (“DC position”). It is realized using suitable lenses in such a way that the focus is in the area of the cornea 14.

The second optical path, represented by the second value, has a path length such that the DC position is in the area of the retina 15 of a typically long eye.

In 2 the OCT imaging area is marked by the boxes, the middle dividing line, which is partially shown in dashed lines, corresponds to the DC position, the minus sign indicates the conjugate complex plane, the plus sign the real plane.

The top boxes show the conditions for a standard eye, the middle ones for a short eye and the bottom boxes for a long eye.

The offset shown by the double arrow is, in particular mechanical, due to the structure the path length switching unit 13a, 13b predetermined difference in the path lengths of the two beam paths.

In order to minimize the influence of the patient's movements on the measurement result, a quick switchover and an alternating measurement of the positions of cornea 14 and retina 15 is necessary.

With the help of 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 errors.

When using the device 1 described here, it is possible for the retina 15 to be located either in the complex conjugate or in the real part of the image.

An assignment and thus the correct determination of the eye length can be done in two ways.

Either by checking the movement trajectories between cornea 14 and retina 15, whereby these are in the same direction if the retina 15 is also in the real part of the image and in opposite directions if it is in the complex conjugate, or on the basis of a phase analysis of the signals.

To further optimize the signal yield on the Retina 15, an optimal focus on the Retina 15 for different eye lengths can be achieved using a focus telescope.

For this purpose, a lens configuration can be implemented that achieves a constant degree of filling of the pupil, so that the maximum numerical aperture is always used for the different eye lengths in order to further optimize the SNR. Naturally, higher SNRs are achieved in this way with shorter eyes.

3 shows an example of such lens configurations.

List of reference symbols:

1

contraption

2

light source

3

Beam splitter

4

Sample poor

4a

out and back light

5

Reference arm

5a

out and back light

6

sample (eye)

6a

Measuring range

7

Mirror

8th

Evaluation unit

8a

Control unit

9

Deflection unit

10

optical unit

11

depth

12

Distance between device 1 and sample 6

13a, 13b

Path length switching unit

14

Cornea or cornea

15

Retina or retina

16

monitor

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2012104097 A1 [0006, 0013]

Claims (15)

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), the light emitted by the light source (2). Light can be split by a beam splitter (3) or circulator, so that the first light (4a) can be guided on the sample arm (4) in the outflow and return directions and the second light (5a) can be guided on the reference arm (5) in the outflow and return directions, wherein the first and the second returning light (4a, 5a) can be brought into interference, an evaluation unit (8) for detecting and processing signals of the interfering first and second light (4a, 5a) being arranged and 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), characterized in that at least one Path length switching unit (13a, 13b) spends an optical path length alternating in time from a first value to a second value. Device according to Claim 1 , characterized in that the path length switching unit (13a, 13b) sets the first value to the second value after a defined time interval and then returns the second value to the first value after the time interval or a further time interval has elapsed. Device according to Claim 1 or 2 , characterized in that the path length switching unit (13a, 13b) carries out the alternating change in the values repeatedly over a predeterminable period of time, preferably periodically with a defined frequency. Device according to Claim 3 , characterized in that the frequency is in the range 1 to 1000 Hz. 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 can be variably adjusted depending on the length of the examined object to be measured, the expected length being entered as an external parameter in the evaluation unit ( 8) or can be entered into a control unit (8a). Device according to one of the preceding claims, characterized in that the evaluation unit (8) can be used to determine structures of the object from the detected and processed signals, which can each be detected during the setting of the first value and the second value, and that the spatial distance between these Structures from each other as the length of the object (6) can be determined and output by the evaluation unit (8). 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. Device according to one of the preceding claims, characterized in that the evaluation unit (8) differentiates the conjugate complex level of a signal from the real level of the signal. Device according to Claim 8 , characterized in that a numerical phase correction can be carried out with the evaluation unit (8) in order 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) spends an optical path length alternating in time from a first value to a second value in order to achieve the first value to spend or focus light in the area of the cornea (14), and for the second value to spend or focus light in the area of the retina (15). Procedure according to Claim 11 , characterized in that the local position of the cornea (14) is detected at the first value and the local position of the retina (15) is detected at the second value, the length of the eye being determined by the distance between the positions. Procedure according to Claim 12 , characterized in that the movement of the eye during the measurements is recorded by evaluating a large number 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 avoid errors in the recording of the To correct the cornea (14) and the retina (15). Procedure according to one of the Claims 11 until 13 , characterized in that the retina (15) or its associated 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, for determining 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. Procedure according to one of the Claims 11 until 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.

Images