Arrangement for improved imaging of eye structures

Abstract

Arrangement for improved imaging of eye structures, particularly in the anterior eye segment, comprising an interferometric measuring arrangement (2) with a measuring arm (3) and a reference arm (4), a light source (1) for illuminating the measuring arm (3) and the reference arm (4), an optical element arranged in the measuring arm (3) or reference arm (4) for influencing the polarization state of the light in the measuring arm (3) or reference arm (4) prior to its interferometric superposition, a scanning unit (7) arranged in the measuring arm (3) for realizing OCT scans, a detector (9) for recording the generated interference patterns, and an evaluation and documentation unit (11), wherein the optical element arranged in the measuring arm (3) or reference arm (4) of the interferometric measuring arrangement (2) is capable of generating at least two different polarization states of the light in the measuring arm (3) or reference arm (4), the detector (9) is designed such thatthat it is suitable for recording the interference patterns generated in the interferometric measuring arrangement (2) when illuminated with light having at least two different polarization states, an existing control module (10) has connections to the scanning unit (7), the optical element for influencing the polarization state of the light in the measuring arm (3) or reference arm (4), the detector (9) and the evaluation and documentation unit (11), and the evaluation and documentation unit (11) is suitable for reconstructing OCT scans from the interference patterns transmitted by the detector (9) (11.2) and combining them (11.4), as well as displaying and / or storing (11.5) the resulting OCT signals, characterized in that the light source (1) has a stable degree of polarization between 1:3 and 1:1000, in particular between 1:10 and 1:200.

Description

The present invention relates to an arrangement for improved imaging of eye structures, in particular in the anterior eye segment, based on optical coherence tomography.Various methods and measuring devices are known for tomographic imaging of eye structures, which are based on confocal scanning systems or optical coherence tomography (OCT=optical coherence tomography).Therein, ophthalmoscopes based on confocal scanners, in particular confocal scanning laser ophthalmoscopes (confocal laser scanning ophthalmoscopes, for short cSLO) represent an important tool for diagnosis and therapy in eye healing (U.S. Pat. No. 6,769,769 B2). Confocal scanners can provide three-dimensional spatial resolution by restricting the depth of a spatially variable focus using spatial filtering and do not rely on the utilization of interference effects, unlike OCT.In contrast, in the OCT methods, coherent light is used with the aid of an interferometer for distance measurement and imaging on reflective and scattering samples. On the human eye, the OCT methods deliver measurable signals during the depth scan, due to the changes in the refractive index occurring at optical boundary surfaces and due to volume scattering. Optical coherence tomography is a very sensitive and rapid method for interferometric imaging, which has been widely used in the medical field and in fundamental research in particular (Wolfgang Drexler, James G. Fujimoto; "Optical Coherence Tomography Technology and Applications", Springer Verlag 2008). OCT images (OCT scans) of eye structures are frequently used in eye healing for diagnosis and therapy guidance, as well as for planning interventions and for selecting implants. An example of OCT-based diagnosis is the use of OCT scans of the retina for the determination of retinal nerve fiber layer thicknesses (RNFLs) for the diagnosis of glaucoma and for tracking the course of the disease.The basic principle of the OCT method described, for example, in U.S. Pat. No. 5,321,501 A is based on white light interferometry and compares the transit time of a signal with the aid of an interferometer (usually a Michelson interferometer). In this case, the arm with a known optical path length (=reference arm) is used as a reference to the measurement arm in which the sample is located. The interference of the signals from both arms yields a pattern from which the scattering amplitudes can be determined as a function of the optical delay between the arms and thus a depth-dependent scattering profile, which is referred to as an A-scan in analogy to ultrasonic technology. Fast variations of the optical delay between the measuring arm and the reference arm can be realized, for example, by means of fiber paths (EP 1 337 803 A1) or so-called rapid-scanning optical delays (RSOD) (U.S. Pat. No. 6,654,127 B2). In the multi-dimensional raster methods, the beam is then transversely guided in one or two directions, with the result that a two-dimensional B scan or a three-dimensional volume tomogram can be recorded. If the reference arm length is kept constant, a two-dimensional C scan can be obtained during lateral scanning of the measurement beam in two directions.An important example of the use of optical coherence tomography is biometrics in the anterior eye section by means of an anterior chamber OCT (AC-OCT for short) in order to be able to subsequently make a selection of implants, such as intraocular lens implants (IOLs), in particular also phalk IOLs, for refraction correction. However, the most common application of IOLs is in the exchange of the natural eye lens in the presence of a lens turbidity (cataract), wherein, however, refraction and imaging errors are increasingly also corrected.Besides OCT, optical coherence domain reflectometry is also used for interferometric biometrics, with the aid of which intraocular distances can be determined, which are required as biometric parameters for selecting I-OLs (US 2005 / 018137 A1, US 7 400 410 B2).The most important biometric parameters are the axial length (distance from the cornea to the retina), the corneal curvature and refractive power, and the depth of the anterior chamber (distance to the eye lens). In order to ensure the most optimal vision possible after the operation, it is necessary to determine these biometric parameters with correspondingly high accuracy. The selection of a suitable replacement lens on the basis of the determined measured values is carried out on the basis of established formulae and calculation methods.The basic principle of the OCT method is based on white light interferometry or short coherence interferometry and compares interferometrically the delay time of a backscattered sample signal (or else measurement signal) with a reference signal with the aid of an interferometer (usually a Michelson interferometer). In this case, not only reflections on surfaces can be evaluated, but also small, varying scattering signals from different sample depths.In this case, the arm with a known optical path length (=reference arm) is used as a reference to the measurement arm (also referred to as sample arm). The interference of the signals from reference and sample arm yields an interference pattern from which the relative optical path length of scattering signals within an A scan (depth signal) can be read out. In the one-dimensional raster methods, the beam is then guided transversely in one or two directions, analogously to the ultrasonic technique, with the result that a two-dimensional B scan, a C scan or a three-dimensional tomogram can be recorded. A C-scan is usually understood to mean a planar tomogram which has been obtained by two-dimensional scanning at a constant reference arm length into a time-domain OCT. In the following, however, this term is to be used as synonym for all scans which are based on two-dimensional scanning, that is to say also for volume scans. The amplitude values of the individual A scans are represented in linear or logarithmized grayscale or false color values.It is also known that volume scans can be corrected by comparison with B scans with respect to disturbances due to sample movements (U.S. Pat. No. 7,365,856 B2). It is also known that additional information about dynamic processes can be obtained and displayed by phase-resolved measurement, in particular by Doppler signal evaluations (Adrian H. Bachmann, Martin L. Villiger, Cedic Blatter, Theo Lasser and Trainer A. Leitgeb "Resonant Doppler flow imaging and optical visualization of retinal blood vessels", Vol. 15, No. 2 / OPTICS EXPRESS 408).The acquisition of A scans is usually carried out at 400 Hz to 400 kHz, in exceptional cases even in the MHz range. Ophthalmological OCT systems have typical sensitivities of 80 dB to 110 dB. The wavelength used depends on the desired scanning range and the absorption and scattering behavior of the tissue. Retinal OCTs usually operate in the range from 700 nm to 1100 nm, while anterior chamber OCT preferably use longer-wave radiation, for example 1300 nm, which is absorbed in the vitreous body. Anterior chamber OCTs can, however, also be realized by switching retinal OCTs.The axial measurement resolution of the OCT method is determined by the so-called coherence length of the light source used, which is inversely proportional to the bandwidth of the radiation used, and is typically between 3 μm and 30 μm (Kurzkohärenzinterferometrie). The lateral measurement resolution is determined by the cross section of the measurement beam in the scanning region and is between 5 μm and 100 μm, preferably below 25 μm. Because of its particular suitability for examining optically transparent media, the method is widely used in ophthalmology.The OCT methods used in ophthalmology have adopted two different basic types. In order to determine the measured values, the length of the reference arm is changed in the first type and the intensity of the interference is continuously measured without the spectrum being taken into account in this case. This method is referred to as a "time domain" method (U.S. Pat. No. 5,321,501 A). In the other method referred to as "frequency domain", by contrast, the spectrum is taken into account for determining the measured values and the interference of the individual spectral components is detected. One therefore speaks, on the one hand, of the signal in the time domain (time domain) and, on the other hand, of the signal in the frequency domain (frequency domain).The advantage of the frequency domain method lies in the simple and fast simultaneous measurement, wherein complete information about the depth can be determined without requiring moving parts. This increases the stability and the speed (U.S. Pat. No. 7,330,270 B2).In the case of frequency-domain OCT, a further distinction is made whether the spectral information is obtained by means of a spectrometer ("spectral domain OCT", SD-OCT) or by means of the spectral tuning of the light source ("swept source OCT", SS-OCT).The great technological advantage of OCT is the decoupling of the depth resolution from the transverse resolution. In contrast to microscopy, the three-dimensional structure of the object to be examined can thereby be detected. The purely reflective and thus contactless measurement enables the generation of microscopic images of living tissue (in vivo).DE 196 24 167 A1 describes a method for coherence biometrics and tomography with increased transverse resolution. The measurement of the position of light-emitting points along a measurement path on the surface and in the interior of objects is effected here by means of a measurement light beam of a short coherence interferometer. Here, short coherence interferometry fundamentally means that light of short coherence length is used and the length to be measured in the measurement beam is determined by changing the length in the reference beam until interference occurs, which is only the case if the length of the two beam paths is identical within the coherence length of the light used. The known length of the reference beam is then equal to the sought length in the measurement beam.Whereas in the method for coherence biometrics the entire depth of the measurement object in the z-direction is detected with the aid of a measurement beam, in coherence tomography a whole series of such interferometric distance measurement is carried out at adjacent locations (for example in the x-direction) and combined to form an image.For this purpose, the light beam illuminating the object is displaced with respect to the object after each A scan, for example in the x direction, so that the object structure is scanned in the z direction line by line. These lines are later assembled into a sectional image (tomogram).With the solution described here, an equally good and high optical transverse resolution over the entire interferometric measurement path is achieved in that, by suitable optical imaging of the (dynamic) focus generated by the moved optical element into the object, the alignment of the optical lengths of reference beam path and measurement beam path up to the (thereby coherent) measurement focus is simultaneously ensured.A reduction of reflection losses at the surfaces of the components and for optimizing the beam splitters of the interferometer arrangement described here can additionally be used polarization-effective optical components. No variation of the polarization-effective optical components during the measurements is disclosed.An efficient optical coherence tomography system for fast three-dimensional imaging is described in U.S. Pat. No. 7,145,661 B2. In this case, polarized light is radiated into the OCT system via a polarization beam splitter, so that the OCT detector operates in a low-noise regime.Another system for tomographic imaging based on a partially coherent frequency-swept laser source is described in U.S. Pat. No. 7,359,062 B2. The radiation in the interferometer is phase modulated to eliminate DC and autocorrelation noise as well as the mirror image.When scanning an eye, a pixel image with a low frequency component and one with a high frequency component can thus be generated simultaneously from each point by the system detector. While the low frequency component pixel image is similar to the image realized with a scanning laser ophthalmoscope (SLO), the high frequency component image corresponds to a two-dimensional OCT image. Due to the pixel-to-pixel correspondence between the simultaneously captured SLO and OCT images, the OCT image for the scanned region may be converted pixel-by-pixel "on-the-fly" corresponding to the SLO image into a 3D image.The solution described here provides a system with which precise three-dimensional OCT images of the eye tissue can be realized very quickly. A variation of the polarization-effective optical components during the measurements is also not disclosed here.DE 10 2009 041 996.9, which has not yet been published, relates to an ophthalmological biometrics or image generation system and a method for the acquisition and evaluation of measurement data, for the determination of sizes, distances and / or geometric relationships of eye structures. To optimize the measurement value acquisition, the measuring arrangement has, among other things, a control circuit which is formed by a drive unit, an optical scanning unit and a position sensor. In an advantageous embodiment, polarization adaptation is effected between the measuring arm and the reference arm of the OCT interferometer in order to ensure sufficient signal strengths as far as possible in all regions of the scan. The polarization adaptation can be effected, for example, by rotated birefringent waveplates, motorizedally moved fibre paddle, birefringent or polarization-rotating liquid crystal modulators or fast electro-optical polarization modulators.US 2007 / 291277 A1 describes a further optical coherence tomography system which is preferably based on a Mach-Zehnder interferometer. Here too, polarization adaptation between the measuring arm and the reference arm of the interferometer is carried out in order to ensure sufficient signal strengths in all regions of the scan, wherein fiber paddle devices are used for this purpose. With the setting of these fibrous paddle, however, often not all the central and peripheral corneal areas can be recorded equally well. In particular, the problem also arises that depth-dependent changes in the polarization state of the backscattered light can also be observed in the cornea.Further optical OCT systems which operate on the basis of "frequency domains" and additionally use optical elements in order to generate different polarization states of the light in the measurement or reference arm and thus to optimize the OCT signals are also described, for example, in the documents U.S. Pat. No. 6,927,860 B2 and US 2007 / 0038040 A1.The strength of OCT signals is known to depend on the adaptation of the polarization states of the superimposed light from the sample arm and reference arm of the OCT interferometer. Thus, maximum interference is achieved with identical polarization states, while no interference signals can be detected with mutually orthogonally oriented (for example linear or even circular) polarization states.Deviations between the polarization states can be caused, for example, by different beam guides in the sample arm and reference arm, for example, by mirrors in periscope arrangement, or polarization-effective optical components. Secondly, birefringent samples such as corneas, crystal lenses or retinal nerve fiber layers of the human eye are responsible for this purpose.According to the solutions of the known prior art, in OCT systems a partial or predominant polarization adaptation takes place by means of adjustable optical elements influencing the polarization effect, wherein fiber paddle devices, which represent manually or motorized rotatable fiber loops, are most frequently used for this purpose. By birefringence, fibre paddle displays have a similar effect to known waveplates.However, the solutions known from the prior art are not capable, despite the use of optical elements for polarization adaptation, of obtaining optimized signals from samples with a depth-dependent, polarization-changing effect.Another example is measurements on the birefringent retinal nerve fiber layer (RNFL) whose thickness rating is very important for glaucoma diagnosis and progression analysis. Complex polarization-resolved measurements with the aid of scanning laser ophthalmoscopes (SLO for short) are known. By means of so-called polarization-sensitive OCT systems (PS-OCT), OCT signals can be recorded with polarization resolution, wherein separate detection paths for orthogonally polarized light components are realized for this purpose. Yasuno et al. (Optics Express Vol. 17, Iss. 5, pp. 3980-3996 (2009)), a PS-OCT system for the anterior chamber region, which is based on the swept source frequency domain optical coherence tomography. E. Gotzinger et al. ("High speed spectral domain polarization sensitive optical coherence tomography of the human retina"; 13, 2005, 25, 10217-10229. OPTICS EXPRESS) describe PS-OCT systems which are based on spectral domain OCT and are used for fundus imaging.In this connection, it is examined by B. Cense et al. ("Thickness and Birefringence of Hearing Retinal Nerve Fiber Layer Tissue Measured with Polarization-Sensitive Optical Coherence Tomography"; 45, 2004, 2606-2612; Inverse Opthalmology & Visual Science) whether polarization-sensitive optical coherence tomography can also be used to detect thinnings of the retina and the retinal nerve fiber layer (RNFL), in order to be able to diagnose glaucoma at an early stage.For example, in an SD-OCT system, the resulting interference spectra of two orthogonally polarized reference light components, which are each superimposed with the backscattered sample light, are recorded separately by means of two spectrometers.Separate detection channels can also be realized by time-fast multiplexing, by means of very fast polarization modulators, such as acousto-optical modulators (AOM), electro-optical modulators (EOM) or else piezo fiber extenders.It is disadvantageous that all these systems are very complicated with regard to their beam guidance, polarization separation, separate detection or realization of high-frequency and high-precision modulation signals and do not achieve an optimum signal-to-noise ratio in certain situations.An example: sample light is arbitrarily already polarization-adjusted in an optimum manner with respect to a detection channel. However, due to the necessary division of the sample light into two detection channels (e.g. 50:50), only one signal can be detected with one channel that contains 50% of the sample light (SNR reduced by 3 dB compared to optimum), while nothing is detected in the other channel.When OCT methods are used to image the anterior eye segments, it must be taken into account that the image quality and the visibility of the cornea and of other intraocular structures and implants depend on the lighting conditions and settings, such as contrast, brightness, noise behavior, saturation and polarization, and may be limited. Although these settings can be optimized by the user, some structures, in particular intraocular lenses (IOL), are very difficult to image.The Stratus OCT™ developed a device for high-resolution optical coherence tomography in the non-contact method for examining the anterior eye sections, in particular for the diagnosis and follow-up of retinal diseases, by Carl Zeiss Meditec AG. With the introduction of the Visante™ OCT system, high-resolution, contactless optical coherence tomography is also made usable for the anterior eye sections. The Visante™ OCT system for the first time offers clear and high-detail sectional images of the anterior chamber, including the chamber angle, without a typical anaesthesia or a time-consuming water bath being necessary for this purpose. For this purpose, the image quality is improved by an extended scan mode in which four successive scans are carried out with the same settings and the measured values are averaged as a result. The averaged image has a higher resolution and an improved noise behavior than the individual scan images. Despite these substantial improvements, some structures, particularly implants, are still difficult to image. Furthermore, this OCT system also offers the possibility of recording star-shaped meridian scans of the anterior chamber, for example in order to determine the thickness distribution of the cornea.The signal processing in the OCT is based on the assumption that the polarization state of the reference light coincides with that of the sample light. However, practice has shown that structures and / or implants can entirely alter the polarization state of the sample light. In extreme cases, the polarization states of sample light and reference light could be perpendicular to one another, so that no interference patterns can occur, since the OCT signal completely disappears.A polarization controller may change the polarization state of the light source to minimize the effect described above. However, even if a polarization controller is provided in the OCT system, the optimization of settings is very time consuming. In addition, only one polarization state is set for a longer period of seconds to minutes. In the case of imaging structures of different polarization-changing nature, changes in the state of polarization are required each time. Thus, different structures are not simultaneously optimally visible.The object of the present invention is to develop a solution for improved imaging of eye structures, in particular in the anterior eye segment, with which clear and detailed sectional images of the anterior chamber, including the chamber angle, can be realized. In particular, the solution should be suitable for obtaining optimized OCT signals at low cost also from samples with a location- and / or depth-dependent polarization-changing effect. In this case, it should serve to support the operation planning and postoperative follow-up and work contactless as far as possible.According to the invention, the object is achieved by the features of the independent claims. Preferred refinements and refinements are the subject matter of the dependent claims.This object is achieved with the present arrangement for improved imaging of eye structures, in particular in the front eye segment, consisting of an interferometric measuring arrangement, having a measuring arm and a reference arm, a light source for illuminating the measuring arm and reference arm, an optical element arranged in the measuring arm or reference arm for influencing the polarization state of the light in the measuring arm or reference arm before its interferometric superposition, a scanning unit arranged in the measuring arm for realizing OCT scans, a detector for recording the generated interference patterns, and an evaluation and documentation unit, wherein the optical element arranged in the measuring arm or reference arm of the interferometric measuring arrangement is capable of generating at least two different polarization states of the light in the measuring arm or reference arm, the detector being designed in such a way, According to the invention, an interference pattern generated in the interferometric measuring arrangement when illuminated with light having at least two different polarization states is suitable for receiving the interference patterns generated in the interferometric measuring arrangement, an existing control module has connections to the scanning unit, the optical element for influencing the polarization state of the light in the measuring or reference arm, the detector and the evaluation and documentation unit, and the evaluation and documentation unit is suitable for reconstructing OCT scans from the interference patterns transmitted by the detector and combining them, and for displaying and / or storing the resulting OCT signals, characterized in that the light source (1) has a stable degree of polarization between 1:3 to 1:1000, in particular between 1:10 to 1:200.The arrangement according to the invention based on optical coherence tomography for improved imaging of anterior eye segments can be used for pre- and post-operative image recording for analysis and measurement. In addition to the accurate measurement of the eye structures in the front section including the depth of the front chamber, the angle of the front chamber and the diameter of the front chamber, the solution is also suitable for imaging, measuring and documentation of the flap thickness and residual stroma thickness directly following the LASIK treatment.In addition to these mentioned applications, the solution for improved imaging of anterior eye segments can also be used for pachymetry, since in the case of rapid image recording it supplies high-resolution corneal scans, from which a precise and reproducible corneal thickness map can be determined for use in refractive surgery and glaucoma diagnosis.The invention is described in more detail below with reference to exemplary embodiments. For this purpose, the following show: FIG. 1 shows an arrangement for recording three scans at predetermined polarization states, FIG. 2 shows an arrangement for adaptively determining suitable polarization states, and FIG. 3 shows an arrangement for recording zone-by-zone scans at different polarization states.The arrangement according to the invention for improved imaging of eye structures, in particular in the front eye segment, comprises an interferometric measuring arrangement, having a measuring arm and a reference arm, a light source for illuminating the measuring arm and reference arm, an optical element arranged in the measuring arm or reference arm for influencing the polarization state of the light in the measuring arm or reference arm before it is interferometrically superimposed, a scanning unit arranged in the measuring arm for realizing OCT scans, a detector for recording the interference patterns generated, and an evaluation and documentation unit. In this case, the optical element arranged in the measurement arm or reference arm of the interferometric measurement arrangement is capable of generating at least two different states of polarization of the light in the measurement arm or reference arm. For this purpose, the detector is designed such that it is suitable for recording the interference pattern generated in the interferometric measuring arrangement when illuminated with light having at least two different polarization states. An existing control module has connections to the scanning unit, the optical element for influencing the polarization state of the light in the measuring or reference arm, the detector and the evaluation and documentation unit. The evaluation and documentation unit reconstructs OCT scans from the interference patterns transmitted by the detector and combines them, and displays and / or stores the resulting OCT signals.According to the invention, it is advantageous if the light source supplies light of a sufficiently stable, defined polarization state, i.e. if the light has a degree of polarization (DOP degree of polarization) of between 1:3 and 1:1000, in particular between 1:10 and 1:200, and causes signal fluctuations dependent on the polarization state of less than 25%, measured by an analyzer which is oriented with respect to maximum transmission. This ensures a sufficient signal-to-noise ratio and ensures a sufficient effect of the optical elements for influencing the polarization state.The interference of the signals (optical cross-correlation) from the measurement arm and reference arm of the interferometric measurement arrangement yields an interference pattern from which the relative optical path length within a depth profile (A-scan) can be read out. In the one-dimensional raster methods, the light beam is guided transversely by the scanning unit in one or two directions, whereby a planar tomogram (B-scan) or a three-dimensional volume (C-scan) can be recorded.The prominent feature of the OCT methods is the decoupling of the transverse resolution from the longitudinal resolution. In conventional light microscopy, both axial (depth) and transverse (lateral) resolution depend on the focusing of the light beam. The parameter for focusability is the numerical aperture. In OCT, the axial resolution is limited only by the bandwidth of the light used, which means that a high resolution is achieved with a wide bandwidth (with wide spectra) and thus small details can be resolved. By using short-coherent light, a high axial spatial resolution can be ensured.The polarization state of the illumination light can be influenced either in the measurement arm or in the reference arm of the interferometric measurement arrangement. This ensures that the polarization adaptation of the light between the reference arm and the measuring arm is varied, namely before they are superimposed interferometrically.While orthogonal polarization states can be avoided by using at least one optical element, the use of a set of 3 or more optical elements allows complete matching of the polarization state. Preferably, at least one polarization-optical element is used which can convert a linear state into a circular state, such as a quarter wave plate.Here, λ plates (wave plates), polarization rotators, in-line fiber polarization controllers or fiber paddlels are used as optical element for influencing the polarization state of the illumination light. In principle, polarization-variable modulators such as electro-optical, acousto-optical or magneto-optical modulators are also suitable, but the expenditure is considerably higher in this case.Fibre paddle arrangements have proven to be particularly advantageous since orthogonal states can be avoided. The polarization-changing effect is defined here by the bending radius and angle of the birefringent fiber. For the realization of different polarization states, the fiber paddle has an adjustment mechanism which is connected to the control module in order to change the polarization states of the illumination light in predetermined steps or steps adapted as a function of the signal conditions.In order to be able to achieve improved images of the front eye segments with the aid of the interferometric measuring arrangement, it is necessary for the optical element to implement at least two, but preferably three, defined polarization states of the illumination light for the measurements.The reconstructed OCT scans are combined by the evaluation and documentation unit pointwise or zonewise or completely in the form of A, B or also C scans. In this connection, it has been found to be expedient to evaluate the reconstructed OCT scans with respect to mutual displacements or deformations and to correct these, if appropriate (registration). The evaluation and documentation unit is also suitable for combining the reconstructed OCT scans by forming weighted or unweighted mean, maximum or else median values. The weighting factors required for this are generated from the local contrast, the reaching of minimum or maximum values or the exceeding of threshold values for signal intensities or the signal-to-noise ratio.The control module with its functions can also be integrated into the evaluation and documentation unit.In a first advantageous embodiment, three OCT scans are recorded at predetermined, defined polarization states with the aid of the arrangement according to FIG. 1.Starting from the light source 1, the object 5 located in the measuring arm 3 of an interferometric measuring arrangement 2 and the mirror element 6 arranged in its reference arm 4 and displaceable in the direction of the light beam for compensating the path length between the measuring and reference arms are illuminated with short-coherent light. In addition, the interferometric measuring arrangement 2 has in its measuring arm 3 a scanning unit 7 for realizing OCT scans in the form of A, B and C scans, and also an optical element arranged in its reference arm 4 for influencing the polarization state of the illumination light in the form of a fibre paddle 8 with an adjusting mechanism. At the output of the interferometric measuring arrangement 2 there is a detector 9 for recording the short-coherent light reflected by the object 5 and by the mirror element 6. The present control module 10 is connected to the scanning unit 7, the adjusting mechanism of the fibre paddle 8 and the detector 9 and serves for controlling the latter. The recorded interference patterns transmitted by the detector 9 are reconstructed and combined by an evaluation and documentation unit 11 to form OCT scans, and the resulting OCT signals are displayed and stored.To accommodate three interference patterns, the positions for realizing the three different, predefined polarization states of the illumination light are transmitted to the adjustment mechanism of the fiber paddle 8 by the present control module 10 for this purpose. After previous reconstruction 11.2 of these three interference patterns to form OCT scans, a combination 11.4 to form resulting OCT signals and their display and / or storage 11.5 takes place in the evaluation and documentation unit 11. To clarify the differences of the interference patterns recorded in different polarization states, the OCT scans as a result of their reconstruction 11.2 and their display and / or storage 11.5 are schematically shown.The combination 11.4 can be effected here by weighted or unweighted mean, maximum or else median value formation. In this case, it is expedient for the evaluation and documentation unit 11 to subject the three interference patterns to a registration 11.3 before the combination 11.4 to form resulting OCT signals, in which the interference patterns are evaluated with respect to mutual displacements and these are corrected if appropriate.A particular challenge is the registration of OCT scans, which can show partial or complete signal dropouts depending on the polarization. According to the invention, this problem is solved by using either a very strong signal component (e.g. iris or cornea surface), which still delivers signals suitable for registrations even in unfavourable polarization conditions. Or, methods of signal registration known to the person skilled in the art can be used, in which incomplete signal components are supplemented according to specific plausibility criteria and weights. For example, weighted fits or curve adaptations of the front or rear surface of the cornea or of its center of gravity are suitable in this case. Suitable weighting parameters are, for example, the strength and sharpness of an intensity jump of the signal-to-noise ratio, which is a measure of the probability of the presence of a corneal surface at this position.In a second advantageous embodiment, three scans are recorded with adaptively determined polarization states with the aid of the arrangement according to FIG. 2.The overall arrangement corresponds to that of the first advantageous embodiment according to FIG. 1, with the exception that the evaluation and documentation unit 11 contains an additional signal evaluation 11.1. In this signal evaluation 11.1, the interference pattern recorded and transmitted by the detector in a first polarization state of the illumination light is evaluated with respect to specific criteria before its reconstruction 11.2 into OCT scans in order to determine adapted polarization states.In contrast to FIG. 1, the OCT scans as a result of their reconstruction 11.2 and their display and / or storage 11.5 are not schematically shown, but rather show real images of an eye.Criteria used for this purpose are, for example, the contrast, the reaching of minimum or maximum values, the exceeding of threshold values for signal intensities or the signal-to-noise ratio.The signal evaluation can also be carried out without complete reconstruction of OCT scans. For example, the contrast of spectral interference modulations can be evaluated without the need for complex complete signal reconstruction by means of Fourier transformations.After a series of at least two, but preferably three, adapted polarization states of the illumination light have been determined, the three interference patterns are recorded. For this purpose, the positions for realizing the three adapted polarization states are transmitted from the present control module 10 to the adjustment mechanism of the fiber paddle 8.In this second advantageous embodiment, the three interference patterns are recorded by the detector 9 and transmitted to the evaluation and documentation unit 11, which, after previous reconstruction 11.2 of these three interference patterns to form OCT scans, combines 11.4 to form resulting OCT signals and displays and / or stores 11.5 thereof. Here too, the combination 11.4 can be carried out by weighted or unweighted mean, maximum or even median value formation and it can be expedient for the evaluation and documentation unit 11 to subject the OCT scans reconstructed from the three interference patterns to a registration 11.3 before their combination 11.4 to give resultant OCT signals, in which the interference patterns are evaluated with respect to mutual displacements and these are optionally corrected.In a third advantageous embodiment, three zone-by-zone scans at different polarization states are recorded with the aid of the arrangement according to FIG. 3.Since the polarization states can be both predetermined and adapted, the overall arrangement can correspond either to the first advantageous embodiment according to FIG. 1 or to the second advantageous embodiment according to FIG. 2. The exception is only to be seen in the fact that no complete interference patterns are recorded by the detector 9, but instead different zones are recorded and transmitted to the evaluation and documentation unit 11.Here too, the three zone interference patterns from the evaluation and documentation unit 11 undergo a reconstruction 11.2 to form OCT scans. The OCT scans are, as a result of their reconstruction 11.2 and their display and / or storage 11.5, again not schematically, but rather are shown as real images of an eye.In contrast to the solutions described so far, the combination 11.4 to give resulting OCT signals is not effected by weighted or unweighted mean, maximum or else median value formation, but rather by simple joining (adding) of the partial or zone scans. They are then likewise displayed and / or stored 11.5.In particular, it is expedient here that the three zone interference patterns are subjected to a registration 11.3 by the evaluation and documentation unit 11 before the combination 11.4 to form resulting OCT signals, in which registration the zone interference patterns are evaluated with respect to mutual displacements and these are corrected if appropriate. This has the advantage that the zones fit exactly with one another and no defects are produced. In this context, it is advantageous if the individual zones overlap at least minimally, wherein favorable overlap widths are 100 μm to 1 cm. Depth-dependent polarization effects can thereby be compensated and registration can be simplified.The most important advantage of this solution compared with the previously described advantageous embodiments lies in the time saving. An optimum polarization state can be determined and used for each zone to be scanned. A combination by weighted or unweighted mean, maximum or even median formation can thus be dispensed with.In this third advantageous embodiment, the scans to be combined do not necessarily have to be zone scans, but rather it is also possible to combine complete scans with zone scans in order to save measuring time. For example, it is possible to repeatedly scan and combine zone scans only the problematic, peripheral corneal regions in different polarization matching states.In the described arrangements according to the invention for improved imaging of the anterior eye segments, the polarization adaptation between measurement light and reference light is varied before they are superimposed interferometrically. After a spatial registration of the OCT scans, in which mutual displacements are evaluated and these are optionally corrected, a combination of the at least two, but preferably three scans takes place in order to obtain an optimum signal.The use of 3 and more scans offers in particular the possibility of recording intermediate states as well, if parts of the OCT scans appear completely complementary in two polarization positions, because for example a depth-dependent sample scattering causes polarization states that are orthogonal to one another in places. By means of the signal coupling that can be realized via these intermediate states, registration can then be simplified, for example, or OCT evaluations of dynamic processes can also be carried out, which are based on the comparison of local states at different times, such as the evaluation of phase or speckle changes, for example. Such evaluations serve, for example, for the determination of blood flows in vessels or also other movements.Furthermore, there is the possibility that, in addition to the variation of the pure polarization adaptations, spatial position changes between the scans are also carried out. For example, these scans could form the meridian scans already mentioned. As a result, the inventive improvement of the OCT signals can be realized by combining adjacent meridians in the same way as the known evaluation with regard to the corneal thickness distributions.The proposed invention provides a solution for improved imaging of the anterior eye segments, with which clear and detailed sectional images of the anterior chamber, including the chamber angle, can be realized. In particular, the solution is suitable for obtaining optimized OCT signals at low cost also from samples with a location- and / or depth-dependent polarization-changing effect. The solution serves to support surgical planning and postoperative follow-up for applications at the anterior eye section and also works contactless.The proposed solution relates to the field of high-resolution, contactless optical coherence tomography for imaging eye structures, in particular in the front eye segment, and can also be used for supporting surgical planning, diagnosing and monitoring retinal diseases. Extremely clear and detailed sectional images of the anterior chamber including the chamber angle are produced.

Claims

Arrangement for improved imaging of eye structures, in particular in the front eye segment, consisting of an interferometric measuring arrangement (2), having a measuring arm (3) and a reference arm (4), a light source (1) for illuminating the measuring arm (3) and reference arm (4), an optical element arranged in the measuring arm (3) or reference arm (4) for influencing the polarization state of the light in the measuring arm (3) or reference arm (4) before its interferometric superposition, a scanning unit (7) arranged in the measuring arm (3) for realizing OCT scans, a detector (9) for recording the interference patterns generated, and an evaluation and documentation unit (11), wherein the optical element arranged in the measuring arm (3) or reference arm (4) of the interferometric measuring arrangement (2) is capable of, at least two different states of polarization of the light in the measuring arm (3) or reference arm (4), the detector (9) is designed such that it is suitable for recording the interference patterns generated in the interferometric measuring arrangement (2) when illuminated with light having at least two different states of polarization, an existing control module (10) has connections to the scanning unit (7), the optical element for influencing the state of polarization of the light in the measuring arm (3) or reference arm (4), the detector (9) and the evaluation and documentation unit (11), and the evaluation and documentation unit (11) is suitable for reconstructing (11.2) OCT scans from the interference patterns transmitted by the detector (9) and combining them (11.4) and for displaying and / or storing (11.5) the resulting OCT signals, characterized in that, the light source (1) has a stable degree of polarization of between 1:3 and 1:1000, in particular between 1:10 and 1:200.Arrangement according to Claim 1, characterized in that the optical element realizes at least two, but preferably three, different states of polarization of the light in the measurement arm (3) or reference arm (4).Arrangement according to one of Claims 1 and 2, characterized in that wave plates, polarization rotators, in-line fibre polarization controllers or fibre pads (8) are used as optical element for influencing the state of polarization of the light in the measurement arm (3) or reference arm (4).Arrangement according to one of the preceding claims, characterized in that a set of wave plates, polarization rotators, in-line fibre polarization controllers or fibre pads (8) is used as optical element for influencing the polarization state of the light in the measuring arm (3) or reference arm (4).Arrangement according to one of the preceding claims, characterized in that the optical element has an adjustment mechanism for realizing different states of polarization of the light in the measuring arm (3) or reference arm (4), which adjustment mechanism is connected to the control module (10) in order to change the states of polarization of the illumination light in predetermined or adapted steps.Arrangement according to one of the preceding claims, characterized in that the evaluation and documentation unit (11) is suitable for partially spatially overlapping OCT scans (11.2) to be reconstructed from the interference patterns transmitted by the detector (9) and for these to be combined (11.4) and for the resulting OCT signals to be displayed and / or stored (11.5).Arrangement according to one of the preceding claims, characterized in that the interference pattern recorded and transmitted by the detector (9) in the case of a first polarization state of the light in the measuring arm (3) or reference arm (4) is evaluated (11.1) by the evaluation and documentation unit (11) in order to determine adapted polarization states with regard to specific criteria, such as the contrast, the reaching of minimum or maximum values, the exceeding of threshold values for signal intensities or the signal-to-noise ratio.Arrangement according to one of the preceding claims, characterized in that the reconstructed (11.2) OCT scans are combined (11.4) point by point or zone by zone or completely in the form of A, B or also C scans by the evaluation and documentation unit (11).Arrangement according to one of the preceding claims, characterized in that the reconstructed (11.2) OCT scans are evaluated (11.3) with respect to mutual displacements by the evaluation and documentation unit (11) and these are optionally corrected.Arrangement according to one of the preceding claims, characterized in that the evaluation and documentation unit (11) is suitable for combining (11.4) the reconstructed (11.2) OCT scans, in which weighted or unweighted mean, maximum or even median values are formed.Arrangement according to one of the preceding claims, characterized in that the evaluation and documentation unit (11) is suitable for generating weighting factors from the local contrast, the reaching of minimum or maximum values or the exceeding of threshold values for signal intensities or the signal-to-noise ratio.Arrangement according to one of the preceding claims, characterized in that the control module (10) is integrated with its functions into the evaluation and documentation unit (11).Arrangement according to one of the preceding claims, characterized in that the OCT scans realized and recorded in at least 3 different polarization states are evaluated by the evaluation and documentation unit (11) with regard to dynamic changes, in particular by means of Doppler signal evaluations.

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