METHOD AND ARRANGEMENT FOR CORRECTING AN IMAGE

Description

[0001] The present invention relates to a method for the direct correction of an image based on an examination of a target with an examination beam of an optical coherence tomography device, wherein the optical path length of the examination beam to and / or from the target changes discretely or continuously during the generation of the image. The invention further relates to an ophthalmological examination system.

[0002] Both ophthalmic examination systems and therapy systems are becoming increasingly complex. In ophthalmology, the goal is also to integrate examination systems into therapy systems, enabling a therapeutic procedure, such as laser surgery, to be performed while simultaneously monitoring and ultimately controlling the patient, possibly even actively controlling the patient using data acquired through the use of the examination systems.

[0003] At the same time, however, there is also a desire for smaller systems that require less valuable space in an operating room. Such systems should also be mobile so that they can be "put away" after use. This is advantageous when performing complex treatments, especially complex operations involving multiple steps and requiring the use of different devices within a complex examination and therapy system.

[0004] One example of this is cataract surgery, where various incisions must be made using a short-pulse therapy laser beam, but there are also examination and treatment steps for which the therapy laser device is not required. The therapy laser device—if it were not movable—would significantly limit the available operating space during these steps, and thus the surgeon's freedom of movement.

[0005] A compact system, particularly but not exclusively for use in cataract surgery, is described in WO 2016 / 058931 A2: This is a movable system with a housing and articulated arms mounted on the housing, via which both examination radiation and therapeutic laser radiation are transmitted from their source in the housing via the articulated arms to a patient's eye. The articulated arms allow for a high degree of working freedom: They can be "turned away" and parked when not needed. They also allow for very flexible adjustment of the working position to the patient's position and the wishes of the treating physician.

[0006] Since a therapy laser beam or an examination beam exits the respective articulated arm and thus the system at the movable end of the articulated arm, a change in the position of the articulated arm may influence the beam guidance, as this can, in particular, change the relative position of individual components and / or cause torsion (e.g., when using optical fibers). This can lead to changes in the optical path to be traversed and / or the polarization of the respective beam.

[0007] Secondly, such a compact and movable system requires the use of compact optics. Otherwise, due to the relatively large field of view required by the objective lens – especially for cataract treatment, but also for the correction of refractive errors – the corresponding articulated arms would have to be very large and would also represent a significantly greater weight at the free end of an articulated arm. Therefore, to keep such a system, and especially its arms, small, a laterally movable objective lens is preferably used, whose individual field of view is smaller than the field of view typically required for laser treatment. However, this field of view of the objective lens can be shifted laterally by a corresponding movement of the objective lens, thereby considerably enlarging the overall field of view.

[0008] This movable objective lens can be used to guide both the examination beam during the examination of ocular tissue and the therapy laser beam during surgical intervention on the eye. If necessary, both beams can be directed simultaneously onto ocular tissue via this laterally movable objective lens. However, such a movable objective lens constantly changes the optical path length that a corresponding therapy laser beam or an examination beam must travel from the radiation source on its way to the eye (and, for the examination beam, possibly also from the ocular tissue back to an analysis device). The polarization of a therapy laser beam or an examination beam can also change along this path – for example, by rotating the articulated arms.

[0009] Such a change in the optical path length to be traversed and the polarization is irrelevant for a therapy laser beam—for example, a short-pulse laser, especially a femtosecond laser—used to treat organic tissue—preferably eye tissue—by photodisruption, ablation, or coagulation. For the therapy laser beam, the only decisive factor is the reliability of the focus positioning or focus shift. The movement of the articulated arms can, of course, also influence this positioning. For this purpose, WO 2016 / 058931 A2 proposes a system for stabilizing the beam passage through the articulated arm.

[0010] However, if the optical path and / or the polarization of a radiation influences the outcome of an examination or therapy, changes in these paths due to movement of the system may distort these results. This is the case in an examination of organic tissue, in particular eye tissue, using optical coherence tomography (OCT). Systems in which the optical path and / or the polarization of the radiation change due to simple movements in the system during the examination for the purpose of measurement are described in the documents WO 2016 / 058931 A2, DE 10 2014 115 153 A1, WO 2010 / 098204 A1, and DE 10 2010 019657 A1, respectively.

[0011] An examination system that works according to the principles of optical coherence tomography, but which contains movable subsystems (such as movable articulated arms and / or a movable objective lens), as described in particular in the document WO 2016 / 058931 A2, therefore generates different images of one and the same target in different positions of its movable components, which images are additionally modified, for example distorted or otherwise falsified, by changes in the optical path traversed and / or the polarization of the examination beam due to the corresponding additional movements (e.g. the articulated arms of WO 2016 / 058931 A2).

[0012] DE 10 2004 028 204 B3 presents a method for signal evaluation in a time-domain OCT, which determines a rate of change of the time-varying optical path length of a phase modulator in the reference arm in one measuring cycle and uses it in subsequent measuring cycles to demodulate the OCT measuring signal.

[0013] The task is therefore to demonstrate a method and corresponding systems with which an unadulterated image (of a structure) of a target can be generated in an examination system which comprises a device for optical coherence tomography and which contains a beam guiding device in which an optical path length and / or a polarization of an examination beam is variable, i.e. an image (of a structure) which is not modified by the change in the optical path length and / or the polarization.

[0014] This object is achieved by a method for correcting an image according to claim 1 and an ophthalmological examination system according to claim 5.

[0015] In a method for correcting an image based on the examination of a target with an examination beam from an optical coherence tomography (OCT) device, an image of a target structure or image data of an image of a target structure is used. To generate this image, the optical path length traveled from a source of the examination beam to the target structure and / or from the target structure to an analysis device and / or a polarization of the examination beam is evaluated. The evaluation, in turn, is based on a comparison of the examination beam with a reference beam: While the examination beam travels the path from the source to the target and from the target to the analysis device, the reference beam is "split" from the examination beam on its path to the target—usually using a beam splitter—and instead guided into a reference arm.The examination beam returning from the target to the analysis device is then compared with the reference beam returning from the reference arm to the analysis device. The examination beam and reference beam are brought into interference before the analysis device using a beam combiner. This "comparison" (by generating an interference signal) of the examination beam and reference beam can vary greatly depending on the OCT method: In TD ("time domain") OCT, the length of the reference arm is varied, and the intensity of the interference is continuously measured, with the spectrum playing no role. In FD ("frequency domain") OCT, however, the interference of individual spectral components is analyzed. The length of the reference arm remains constant, but a spectrometer is required for detection. In SS ("swept source") OCT, on the other hand, the wavelength of the radiation source is tuned over time.This means that neither a variation of the reference arm length nor spectral detection is necessary.

[0016] A target can be an organic material, in particular a tissue of the eye, for example, the lens, the lens sac, the vitreous body, the cornea, or the sclera. In general, however, the target can be any material that can be penetrated by an examination beam from an optical coherence tomography device, i.e., that is transparent to this radiation. A target to be examined has structures – on its surface and / or within its volume – that are to be determined during the examination.

[0017] If an OCT examination is performed using an absolutely static examination system, then for one and the same position within a target volume, the examination beam always travels the same optical path from the radiation source to that target position (and from the target to an analysis device). When examining the structure of a target using an A-scan or B-scan of optical coherence tomography in this otherwise immobile system, this structure is therefore imaged correctly and does not need to be corrected, in particular, "distorted," shifted, or contrast enhanced.

[0018] However, for various reasons, the optical path length of the examination beam from the radiation source to and / or from the target to the analysis device and / or its polarization may change discretely or continuously during the generation of the image of the target structure. The discrete or continuous change in the optical path length that the examination beam must travel to and / or from the target refers to the optical path length to a specific position on or within the volume of a target: thus, the relative position between the target itself and an optical coherence tomography device or its components changes.Such a change in the optical path length that the examination beam has to travel from the radiation source to and / or from a specific position of the target to an analysis device and / or a change in the polarization of the examination beam is given, for example, by a movable examination system, in particular an examination system that contains movable components in its beam guidance device.

[0019] For the imaging based on the examination of a target with an examination beam from an optical coherence tomography (OCT) device, the change in the optical path length and / or the polarization of the examination beam in the area where it actually runs separately from the reference beam is important - and for OCT methods, this concerns the beam guidance of the examination beam from a beam splitting element behind the radiation source (by means of which a common beam emerging from the radiation source is first split into examination beam and reference beam) to the target and from the target to a beam combining element (by means of which the examination beam and reference beam are combined in order to interfere) in front of an analysis device.In the following, this region, in which the examination beam actually exists separately from the reference beam, is referred to as the "optical path length of the examination beam (to and / or from the target)." The change in the optical path length for an OCT procedure thus corresponds to the change in the difference between the length of the optical path of the examination beam and the length of the optical path of the reference beam (each to a specific position on or within the volume of the target).

[0020] If, for example, an A-scan or a B-scan of optical coherence tomography is performed in a moving examination system, a change in the optical path length overlays the "normal" OCT scan movement. Typically, such a change in the optical path length only occurs in the beam path of the examination beam: The beam path of the reference beam is usually arranged in such a way that it is exempt from such changes, or potential changes are monitored and corrected. It can remain in a stable area of ​​the examination system because it is not directed at the target. However, if there are changes in the optical path length in the beam path of the reference beam that are attributable to influences other than the function of the (TD)-OCT, these influences must also be taken into account. For example, path length changes, e.g.due to temperature changes in the area of ​​the beam path of the reference beam, are detected and corrected accordingly.

[0021] The effect of a change in the optical path length could, of course, also be "circumvented" if the separation into examination beam and reference beam using a beam splitter, or their recombination in the beam path, occurred only after the moving sections (as seen from the source of the examination beam toward the target) that cause a change in the optical path length. However, there is generally no room for this, since the moving sections of the ophthalmic examination system extend almost to the front of the eye.

[0022] By changing the optical path length of the examination beam and / or its polarization, the corresponding image is altered or modified. The change in the optical path length leads to a shift and distortion of the image, while the change in polarization leads to a change or deterioration in contrast, primarily due to a deterioration in the signal-to-noise ratio. The term "modification of the image," as used here, therefore encompasses a change in the position of the structures in the image compared to their true position, as well as a change in contrast when both factors—the change in the optical path length and the polarization—play a role.

[0023] A change in the optical path length during the generation of the image or the image data of the image can occur continuously—for example, due to a continuous movement of the examination system. Or it can occur discretely, for example, through a discrete change in the settings between two imaging intervals. The same applies to a change in polarization.

[0024] According to the invention, the change in the optical path length of the examination beam to and / or from the target and / or the change in its polarization is now known or measured: When such a (continuous or discrete) change in the optical path length of the examination beam to and / or from the target and / or a change in its polarization occurs during a corresponding examination, it now depends on the type and manner of movement in the examination system whether the respective change in the optical path length and / or the polarization of the examination beam is calculable or predeterminable and can thus be described as "known": For example, in a movable examination system, the changes in the optical path length and / or the polarization can be predetermined for all possible movement positions of the examination system and stored in a table ("look-up table").Then, during the actual OCT examination of the structure of a target, only the respective movement position or the respective change in position (e.g. a displacement or rotation of the arms) needs to be registered in comparison to the previous movement position of the examination system in order to determine the respective change in the optical path length and / or the polarization using the data from the table.

[0025] Or, due to a physical describability of the movement in the examination system, it is possible to calculate, in particular, the change in the optical path length as a function of the respective movement position, so that here too, during the actual OCT examination of the structure of a target, only the respective movement position or a shift in comparison to the previous movement position needs to be registered in order to determine the change in the optical path length.

[0026] If neither predetermination nor calculation is possible or advisable, the change in the optical path length is determined using an additional reference measurement. For this purpose, a known surface or structure from the overall system can be used as a reference surface or reference structure. This should preferably be positioned in the beam path close to the position of the eye in a position where easy imaging is possible, but in any case behind the area whose movement changes the optical path length. If the correct position of the reference surface or reference structure is known, the OCT image can be shifted or the image data of the image can be corrected so that the reference surface or reference structure is in the expected location.

[0027] The change in the polarization of the examination beam can be measured by measuring it in the examination beam (i.e., in the "signal arm") using an additional beam splitter. A new "measurement beam" is generated in the examination beam using a beam splitter downstream of the elements that change the polarization of the examination beam. The change in polarization alone can, for example, be easily measured using a reference arrangement that includes a beam splitter in the beam path of the examination beam and a photodiode behind a polarizer. The beam splitter directs part of the radiation onto the photodiode.

[0028] Furthermore, according to the invention, the known or measured change in the optical path length and / or the polarization of the examination beam is now used to adjust an adjustable correction element or to directly correct the image or the image data of the image, such that the influence of the change in the optical path length of the examination beam to and / or from the target and / or its polarization on the image is compensated.

[0029] By means of an adjustable correction element, the aim is to cancel out the change in the optical path length that the examination beam has to traverse, i.e. to counteract the change in the optical path length in the order of magnitude of the change in the optical path length, but in the opposite direction.

[0030] Such an adjustable correction element can be used in the beam path of the examination beam or alternatively in the beam path of the reference beam: When used in the beam path of the reference beam, however, a correction element must act in such a way that it causes the same change in the optical path length or polarization that is also effective in the examination beam.

[0031] The known or measured change in the optical path length and / or polarization of the examination beam is used to directly correct the image or to directly correct the image data of the image. The known or measured change in the optical path length of the examination beam is computationally accounted for in order to correct and thus eliminate the modification of the image caused by the change in the optical path length of the examination beam, particularly its shift and distortion. For polarization, however, correction using an adjustable correction element is far preferable to computational post-processing.

[0032] The method according to the invention is therefore a method for the direct correction of an image which is applied to a method for generating an image or the image data of the image which are obtained under non-static conditions.

[0033] A method equivalent to the method according to the invention can furthermore be applied to any examination of a target with an examination beam according to an examination method in which the optical path traversed and / or the polarization of the beam influences the result of the examination.

[0034] In a particular embodiment of the method for direct image correction, the examination beam is guided by means of an optical fiber and / or freely through a system whose internal positional relationship can be varied. A change in the polarization of the examination beam occurring during a change in the positional relationship of the system is corrected by two movable paddles or by a phase-delay plate ("wave plate"). The paddles are used to correct the polarization in an optical fiber, and the phase-delay plate is used to correct the polarization in a free beam path. This movement of the paddles, i.e., optical "paddles," thus occurs depending on the movement of the system in such a way that the change in polarization, usually a polarization rotation, is canceled and (approximately) returned to its original state.

[0035] A free guidance of the examination beam is in particular a guidance or guidance of the examination beam by means of lenses, mirrors, etc., i.e. not inside an optical fiber, but via a free optical path.

[0036] The measurement of the change in polarization of the beam under investigation is carried out, as already mentioned above, for example by means of a reference arrangement which contains a beam splitter in the beam path behind a collimator arranged at the output of an optical fiber, and a photodiode behind a polarizer: the beam splitter thus directs part of the radiation onto the photodiode, with which the polarization can be analyzed.

[0037] The change in the internal positional relationship of system components can be triggered by an external movement, such as the system's movable arms. On the other hand, there are also purely internal changes in the positional relationship of components, such as the previously described lateral movement of the lens to expand the field of view. System components here include any optical components, such as lenses and lens systems, adjustable and fixed mirrors, optical freeform surfaces, optical fibers, or other beam guidance components.

[0038] A change in the internal positional relationship can also occur for a completely different reason, namely when replacing system components that cannot be used in exactly the same size and position. Even then, a correction – usually a one-time one – is necessary if direct comparability with tests conducted before the replacement of the corresponding components is to be ensured after the replacement of components.

[0039] In another particular embodiment of the method for direct image correction, the examination beam is freely guided through a system whose internal positional relationship of its components is variable, and a change in the optical path length that occurs during a change in the system's positional relationship is corrected by a variable additional fiber or a delay line. Using such a variable additional fiber or delay line, the resulting change in the optical path length of the examination beam is compensated for, so that an original state (as without variation in the positional relationship of the system's components) can be restored with regard to the optical path length of the examination beam.

[0040] In a preferred embodiment of the method for correcting an image, the examination beam is freely guided through a system whose internal positional relationship can be varied. During this change in the system's positional relationship, the optical path length of the examination beam changes, thus influencing the image or the image data of the image. In correlation with the change in the optical path length of the examination beam and thus as a "zero-order effect," a shearing of the optical coherence tomography image occurs, which is perceived as a distortion of the image. To counteract this effect, the image or the image data of the image are corrected directly, i.e., computationally, in this embodiment depending on the change in the optical path length.

[0041] It is also possible to use an adjustable correction element to partially correct a change in the optical path length of the examination beam and to correct another part of the change in the optical path length directly, i.e., mathematically. Thus, physical correction through the use of adjustable correction elements and direct, mathematical correction can be combined.

[0042] If the optical path length of the examination beam changes continuously, uniformly, or non-uniformly, the rate of change of the optical path length (dOPD / dt) of the examination beam should also be taken into account. In another preferred embodiment of the method for correcting the image, in particular an image for which a "time-domain" (TD) OCT or "swept source" (SS) OCT method is used, a shift in the image is directly corrected as a function of the rate of change of the optical path length (dOPD / dt) of the examination beam. This additional shift in the image as a function of the rate of change of the optical path length is a "first-order effect," which, although generally much less pronounced in its impact on the image than the "zero-order effect" described above, is nevertheless significant.For optimal image correction, both the "0th order effect" and the "1st order effect" should be taken into account.

[0043] Furthermore, it is advantageous if, in a method for direct image correction based on an SS-OCT examination, a sweep range is defined where optimum image resolution is achieved. In contrast to static OCT measurements, the peak width broadens due to the change in the optical path length, even if the sweep speed is assumed to be constant, which in turn leads to a reduction in resolution.

[0044] An ophthalmological examination system comprises a device for optical coherence tomography (OCT). It comprises a device for generating an examination beam, i.e., a radiation source, in particular a laser source. Furthermore, it comprises an analysis device for analyzing the examination beam returned by a target structure and for generating image data of an image of the target structure.

[0045] The ophthalmological examination system may comprise further examination devices or may also be part of an ophthalmological therapy system, in particular part of a system for eye surgery.

[0046] The basic design of an optical coherence tomography (OCT) device includes a so-called "signal arm," through which the examination beam passes, and a "reference arm," through which the reference beam passes. The beam generated by the radiation source is split into an examination beam and a reference beam by a beam splitter. Depending on their paths—the examination beam to the target and back again, the reference beam in the "reference arm"—the examination beam is compared with the reference beam in the analysis device. For this purpose, the examination beam and the reference beam are caused to interfere before the analysis device, and these are analyzed, for example, with a conventional detector in the analysis device. The analysis device then generates image data of a representation of the structure of a target, which can then be used to generate an image on a display device or screen.

[0047] The ophthalmological examination system further includes a beam guidance device for guiding the examination beam from the device for generating the examination beam to the target and from the target to the analysis device. Such a beam guidance device can be used solely to guide the examination beam of an optical coherence tomography device or can also be used simultaneously or sequentially to guide other radiation, i.e., to guide the radiation of other systems of the ophthalmological examination system, at least over a partial path—sequentially or simultaneously. In particular, the beam guidance device can also be used to guide the therapy beam.

[0048] The beam guidance device can contain scanners, movable or fixed mirrors, movable or fixed lenses, or other optical elements that change the direction and / or focus of an examination beam to guide the radiation. The beam guidance device extends from the device for generating an examination beam through the signal arm to the analysis device.

[0049] The target to be examined is located in an examination volume, which can encompass a region of the eye and has a defined spatial extent in all three spatial directions. The target contains structures that are to be precisely determined by the examination.

[0050] In the present ophthalmological examination system, an optical path length and / or a polarization of the examination beam in the beam guidance device can be discretely or continuously varied. The reason for this is, for example, an examination system that can be varied in the internal positional relationship of its components. In particular, a movable objective lens and / or movable articulated arms through which the examination beam is guided and / or an exchange of components of the ophthalmological examination system are the cause of the change in the optical path length and / or the polarization of the examination beam. Such a change is discrete if it occurs once (e.g. in the case of the exchange of components) or repeatedly through discrete adjustments (e.g. position adjustments of the arms or in particular of the movable objective lens), or continuously if the optical path length changes constantly (e.g.by continuously moving the movable objective lens, whereby the latter changes the optical path length not only between two A-scans but also during the performance of an A-scan of the OCT).

[0051] The ophthalmological examination system further comprises a control device for controlling it, wherein the control device can be designed in one piece or can comprise several control units that are also spatially separated from one another.

[0052] According to the invention, the discrete or continuous change in the optical path length and / or the polarization of the examination beam as a function of time is now known or measurable by means of a reference arrangement.

[0053] The ophthalmological examination system according to the invention further comprises an adjustable correction element for compensating for the change in the optical path length and / or the polarization. At the same time or alternatively, the control device has a correction unit which is configured for the direct correction of the image or the image data of the image as a function of the change in the optical path length of the examination beam and / or the polarization in the beam guiding device.

[0054] This correction unit is therefore coded to perform a direct correction of the image or the image data of the image depending on the change in the optical path length and / or the polarization of the examination beam. The correction is performed in such a way that the influence of the change in the path length traveled by the examination beam to and / or from the target and / or the polarization of the examination beam on the image or the image data of the image is compensated, thus directly correcting the modification of the image or the image data of the image. For polarization, however, correction using an adjustable correction element is far preferable to computational post-processing.

[0055] The ophthalmological examination system preferably further includes a display device for visualizing the image using the image data of the image of a structure of the target. Such a display device is generally a screen on which the structure of the target being examined is depicted. However, a mere display of the image data is also possible. Such a display device can, but does not have to, be physically located on or within the ophthalmological examination system. A display device that communicates with the ophthalmological examination system via a data connection at a spatial distance from the system is also possible. Alternatively, an ophthalmological examination system can operate entirely without a display device if the image data of the image is stored and visualized on an independent system.

[0056] In one embodiment of the ophthalmological examination system, the beam guidance device comprises a system in which the internal positional relationship of its components can be varied and contains an optical fiber. At least part of the beam guidance device thus runs, for example, through a movable articulated arm. Guidance through an optical fiber results in an essentially constant optical path length (for the length of the optical path that runs through the optical fiber; temperature-dependent changes in the optical path length in the optical fiber can, of course, occur during temperature fluctuations but are taken into account accordingly). However, polarization rotation is possible due to movement of the optical fiber, for example, due to movement of an articulated arm through which the optical fiber is guided.

[0057] The term optical fiber refers to any type of optical fiber through which an examination beam can be passed without major losses.

[0058] In an alternative embodiment or in continuation of the above-described embodiment of the ophthalmological examination system, the beam guiding device comprises a system whose internal positional relationship of its components is variable, and at least a part of the beam guiding device is configured to guide the examination beam freely through the system whose internal positional relationship of its components is variable.

[0059] Freely guiding an examination beam here means that the beam is not guided via an optical fiber, but rather by means of optical components such as fixed or movable mirrors, fixed or movable lenses, etc., thus ensuring that the optical path is free. This may lead to changes in the optical path (e.g., if the relative position of two optical components changes) and / or the polarization.

[0060] In a preferred embodiment of the ophthalmological examination system, the adjustable correction element used for direct compensation comprises two movable paddles that move depending on the movement of the system, or a phase delay plate to compensate for changes in the polarization of the examination beam, and / or a variable additional fiber or a delay line to compensate for changes in the optical path length of the examination beam. The adjustable correction element is preferably contained in one of the aforementioned embodiments or in a combination of these embodiments in the beam guidance device of the examination beam and / or in an alternative beam guidance device for a reference beam.

[0061] The reference beam is the beam that runs in an alternative beam guidance device of the reference arm and is used in the analysis device for comparison by interference with the examination beam in order to generate the image of the structures of the target to which the examination beam has run or the image data of this image.

[0062] Preferably, such a correction element is therefore only contained in the beam guidance device of the examination beam, so that the change in the optical path length and / or polarization can be directly counteracted there. However, if the correction element is contained in the alternative beam guidance device for the reference beam and not in the beam guidance device of the examination beam, then the correction element must bring about a change with the same effect as the existing discrete or continuous change in the optical path length and / or polarization of the examination beam. The latter variant of positioning the correction element is more error-prone than positioning a correction element in the beam guidance device of the examination beam, since the spatially different course of this alternative beam guidance device means that the correction is made at a location other than the location of origin.

[0063] In another preferred embodiment of the ophthalmological examination system, a correction unit of the control device is configured to correct a shearing of the optical coherence tomography image that occurs as a function of the change in the optical path length, i.e., the "0th order effect," which directly correlates with the change in the optical path length of the examination beam. This can be done, for example, by directly applying a correction value to the image data of the image that depends on the change in the optical path length at the respective time.

[0064] In an embodiment of the ophthalmological examination system in which a correction unit of the control device performs direct corrections to the image or the image data of the image, and an optical path length of the examination beam changes continuously, the rate of change of the optical path length (dOPD / dt) of the examination beam is preferably also taken into account: If a device for optical coherence tomography of an ophthalmological examination system thus comprises, in particular, a device for "time-domain" (TD) OCT or "swept source" (SS) OCT, its correction unit is also configured to correct a shift in the optical coherence tomography image as a function of the rate of change of the optical path length (dOPD / dt), i.e., the "first-order effect."

[0065] Preferably, therefore, especially in OCT versions or other examination methods for which the optical path length and / or polarization plays a role, and in which the change in the optical path length occurs, the correction of the image or the image data of the image is carried out with regard to both the "0th order" effect and the "1st order effect".

[0066] In a preferred embodiment, the beam guidance device of the examination beam of the ophthalmological examination system contains a movable optical system, in particular a laterally movable objective lens. A laterally movable objective lens is therefore movable perpendicular to an optical axis z in an xy plane. Beam guidance can then be achieved using similarly movable deflection elements, such as movable and tiltable mirrors, which follow the movement of the objective lens. This movement of the objective lens is achieved with the aim of achieving a total field of view whose extent corresponds to a multiple of the field of view of the objective lens.

[0067] The movable optics can also be realized by several movable lenses, which have lateral mobility, but can also have mobility parallel to the optical axis.

[0068] Both the movable optics and the deflection elements can be designed to be movable in more than one lateral direction.

[0069] Furthermore, it is advantageous if the ophthalmological examination system comprises a device for optical coherence tomography (OCT) which is divided into closed, independently exchangeable and individually correctable subsystems with defined transitions, but preferably into a first subsystem which contains an interferometer and a second subsystem which comprises an optical fiber with a collimator.

[0070] Alternatively, the system can be divided into a first subsystem containing an interferometer, a second subsystem containing a long optical fiber passing through a system whose internal positional relationship can be varied, and a third subsystem comprising a short optical fiber and a collimator. Such a subdivision of the optical coherence tomography device significantly simplifies the replacement of individual parts of this device: a correction element, as well as a correction using the correction unit of the control device, can then be used very easily and in a known manner.

[0071] For production and subsequent service, it is highly advantageous to divide the ophthalmic examination system into various correctable subsystems whose relationships to one another are easily determined. A division of an examination system, especially an ophthalmic examination system that includes movable articulated arms and operates according to the principles of optical coherence tomography, is therefore carried out into at least two or three subsystems as described above. However, other divisions are also conceivable – provided they create defined interfaces.

[0072] Such a division of the ophthalmic examination system into subsystems, which can be easily corrected according to the principles outlined here, allows for the replacement of individual components. To ensure the optical path length in the movable articulated arm, over which the examination beam is guided, remains constant even when a subcomponent is replaced, an additional option for correcting the change in optical path length is included when using an examination system that operates according to the principles of optical coherence tomography, either in the corresponding subsystem or in a reference arm. This can be, for example, a variable additional fiber available in various lengths or a delay line – i.e., an adjustable correction element that, in this case, is adjusted once after servicing the examination system in which a subcomponent has been replaced.

[0073] If the variable additional fiber or the delay line is installed in the reference arm, the signal-to-noise ratio is less affected than if it is installed in the corresponding subsystem of the signal arm.

[0074] The collimator can also be mounted in a small tube. This allows for the collimator to be variably glued into the tube to achieve the same optical path length from the fiber inlet to the end of the metal tube for all optical fibers with collimators. This ensures that variations in fiber lengths can be compensated for by variable gluing. Due to manufacturing tolerances, the fiber length from the fiber inlet to the collimator is not exactly the same. The additional tube can be used to artificially equalize the optical path length by adding more air path to the end of the tube for some fibers and less for others. This principle is an alternative to an additional delay line, but can also be used in combination with a delay line.

[0075] It is also advantageous if an ophthalmological examination system additionally comprises a therapy laser device, wherein either a device for generating the examination beam is also configured to generate the therapy laser beam, or an independent device for generating the therapy laser beam is included alongside the device for generating the examination beam. In the first case, the energy of the laser radiation, which is used both for the examination with a first energy and for laser therapy with a second, significantly higher energy, can be controlled accordingly over a wider energy range. If an independent device is used for generating the examination beam and for generating the therapy laser beam, the respective generating devices can be more easily adapted to the respective requirements.A preferred device for generating the therapy laser beam comprises a short-pulse laser source, in particular a femtosecond or picosecond laser source.

[0076] A correction unit for a control device of an ophthalmological examination system comprising an optical coherence tomography (OCT) device is configured to generate an image of a structure of a target based on an examination of the target with an examination beam of an optical coherence tomography (OCT) device, and during the generation of which image a traveled optical path length from a source of the examination beam to the structure of the target and / or from the structure of the target to an analysis device and / or a polarization of the examination beam is evaluated, wherein the optical path length of the examination beam to and / or from the target and / or its polarization changes discretely or continuously during the generation of the image of the structure of the target,and the change in the optical path length of the examination beam to and / or from the target and / or the change in its polarization is known or measured, in such a way that the modification of the image is corrected by the known or measured change in the optical path length and / or the polarization of the examination beam.

[0077] The above applies to the target, the material in the target area, and changes in the optical path length of the examination beam to the target and / or changes in the polarization of the examination beam. Regarding how a change in the optical path length and / or polarization of the examination beam can be known or measured, reference is also made to the above statements.

[0078] As already mentioned above, there are numerous causes for a change in the optical path length and / or polarization of the examination beam, the effects of which on the image of the structure can be corrected with such a correction unit. These causes include movable components of a corresponding examination system through which the examination beam passes, such as movable lenses, in particular a movable objective lens with the necessary deflection elements such as mirrors; an externally movable system, in particular an examination system that has movable articulated arms via which an examination beam is guided to a target; but also modifications or replacement of parts of the examination system that are responsible for beam guidance.

[0079] For most causes, the change in optical path length and / or polarization can be described as a function of time and / or the position of components of the system.

[0080] In one variant of the correction unit, a shearing of the image caused by a change in the optical path length is corrected as a function of the change in the optical path length. This is the correction of the "0th order effect," i.e., the correction of the tilt or shearing due to the change in the optical path length as a function of the current optical path length.

[0081] In one variant of the correction unit, an additional image shift is corrected depending on the rate of change of the optical path length (dOPD / dt). This additional correction of the "first-order effect" is thus performed depending on the (possibly variable) rate of change of the optical path length.

[0082] If an SS-OCT is used, this correction also depends on the frequency of the OCT radiation.

[0083] A correction unit for a control device of a device for swept-source OCT is configured to define a sweep range in which an optimum resolution of the image is achieved.

[0084] A sweep range is defined depending on the sweep speed because, compared to a classic, static OCT measurement in which there is no change in the optical path length of the examination beam, the resolution is reduced due to a broadening of the peak width, especially when the speed of change of the optical path length varies.

[0085] The principles of the method according to the invention for correcting an image based on an examination by means of optical coherence tomography (OCT) and of the examination system according to the invention for examinations during which an optical path length and / or the polarization of the examination beam changes are now specified at this point for the specific application of the swept source (SS)-OCT examination: If an ophthalmological examination system is used which contains moving articulated arms via which the examination beam is guided to the eye of a patient, and if the examination beam of an SS-OCT module is guided through an articulated arm by means of an optical fiber, the output polarization in an applicator at the end of the articulated arm changes due to the fiber laying in the movable articulated arm when the articulated arm is moved.To compensate for the polarization rotation, two motorized paddles are installed in an OCT module, which can reset any desired output polarization.

[0086] As described above, an analyzer is installed at the end of the articulated arm in the applicator, which, in combination with a photodiode, can determine the polarization. After each arm movement, the polarization is readjusted using the paddles, ensuring an optimal signal-to-noise ratio. Without the correction, the signal-to-noise ratio would be worse, or a complex algorithm would have to be applied to optimize the signal-to-noise ratio differently.

[0087] If an SS-OCT is used in combination with a moving optics system, the optics move while the individual OCT scans are acquired. This changes the optical path length to the target.

[0088] First, this results in an offset that directly correlates with this change. This change can be directly incorporated into the image, thereby correcting the distortion.

[0089] Since the movement also occurs during an A-scan, a shift in the image also results depending on the current speed of the moving optics, or more precisely, the current speed of the optical path length change, including its sign. For this purpose, the speed of the optical path length change is calculated for each A-scan, and the appropriate additional correction is calculated using this and the scan speed of the swept source. This makes it possible to determine a correct B-scan even though the optical path length actively changes during the individual A-scans.

[0090] Alternatively, an examination beam from an SS-OCT in an ophthalmic examination system that includes movable articulated arms, via which the examination beam is guided to a patient's eye, can also be guided freely through the articulated arm. In particular, if the ophthalmic examination system also includes a therapy laser device, and thus a therapy laser beam is also guided to the patient's eye via a movable articulated arm, the therapy laser beam and the examination beam of the SS-OCT can be guided together through the movable articulated arm.

[0091] Freely guiding the examination beam through a movable articulated arm requires good anti-reflective coatings on the optics to prevent excessive reflections, as well as, if necessary, special coatings for the wavelength of the examination beam to maintain polarization. However, if the polarization changes, this can also be corrected using movable paddles. If the phase drift is very significant across the wavelength, the optimal signal-to-noise ratio can only be adjusted for one wavelength.

[0092] Solutions for correcting an image, especially when using optical coherence tomography in an ophthalmic examination system, are also required when the optical path length and / or polarization change during service measures. This is done as described above.

[0093] The present invention will now be explained using exemplary embodiments. It shows: the Fig. 1 an embodiment of an ophthalmological examination system; the Fig. 2 an optical system with devices for generating an examination beam or a therapy beam and beam guidance device, which is used in an ophthalmological examination system according to Fig. 1 can be used; the Fig. 3a to 3d : the influence of a movable objective lens on the optical path length of an examination beam in a beam guidance device of an ophthalmological examination system; the Fig. 4 : The change in the optical path length of an examination beam, in particular an OCT examination beam, when using a movable objective lens;

[0094] Fig. 5a to 5c: a schematic image of a cornea and a lens without modification of the image, i.e., not distorted and in the correct position; with shearing due to the influence of the change in the optical path length; and additionally shifted due to the influence of the speed of change in the optical path length when using an OCT line scan (B-scan as a sequence of many A-scans);

[0095] Fig. 6a and 6b : a schematic image of a cornea and a lens, undistorted and in the correct position; with shear due to the influence of the change in the optical path length; and additionally shifted due to the influence of the rate of change in the optical path length for a chirped OCT with circular scan;

[0096] Fig. 7 : the influence of z-velocity on a swept-source OCT signal;

[0097] Fig. 8: The dependence of the resolution of the optical path length, in particular the resolution optimum, on the tuning range;

[0098] Fig. 9 : An OCT module of an ophthalmological examination system.

[0099] In the Fig. 1An embodiment of an ophthalmological examination system 100 according to the invention, which comprises a therapy laser device—in this case, strictly speaking, an ophthalmological examination and therapy system 100—is shown in a side view. This ophthalmological examination system 100 contains a base body 110, which is mounted on a movement device 180 having rollers. Two support structures 160, 170, which act as axes, protrude from this base body 110, each of which has an articulated arm 120, 130 arranged thereon.The first articulated arm 120 contains a surgical microscope head 320, while a laser applicator head 220 is arranged on the second articulated arm 130. During operation, a focused, pulsed laser radiation is generated from a laser source (not shown here) located in the base body 110—in this case, a femtosecond laser—and is guided to the laser applicator head 220 via a beam guidance device in the support structure 170 and articulated arm 130, and a beam focusing optics as part of the beam guidance device. Both articulated arms 120, 130 are movable in almost any direction in space via their joints 140. All components of this ophthalmological examination system 100 are controlled by a control device 500.

[0100] The two articulated arms 120, 130 can be connected to each other by connecting two parts of a coupling structure 150, the first part of which is arranged on the surgical microscope head 320 of the first articulated arm 120 and the second part of which is arranged on the laser applicator head 220 of the second articulated arm 130. To ensure that the ophthalmic examination and therapy system 100 maintains its stability in any position of the articulated arms 120, 130 and prevents them from tipping over, the articulated arms 120, 130 have weight-balancing structures 145.

[0101] An examination beam from an optical coherence tomography (OCT) device is also guided through these articulated arms. This is possible in both articulated arms 120, 130 using an optical fiber. In articulated arm 130, through which the pulsed laser radiation from the therapy laser device is guided to the laser applicator head 220, this is alternatively possible using the beam guidance device with its beam focusing optics, which is used by the pulsed laser radiation. In this case, the examination beam is guided freely through the articulated arm 130.

[0102] The Fig. 2 shows an optical system with devices for generating an examination beam or a therapy laser beam and beam guiding devices, as used in an ophthalmological examination system according to Fig. 1An OCT module 400, here a swept source (SS) OCT module, which contains an OCT radiation source 405 for generating an examination beam, a reference beam path, an interferometer, a detector and one or more switching points 420, is controlled by a control unit 500 - not shown in Fig. 2- controlled. Furthermore, the system for generating the therapy laser beam includes a femtosecond (fs) laser source 210. A therapy laser beam emanating from this fs laser source 210 passes through a first optical system, essentially composed of a divergence-varying system 211, a relay optics 213, and a focus-adjusting lens system 212. Subsequently, the therapy laser beam is laterally deflected by an xy scanning system 240 at its movable focal point in the target—i.e., a patient's eye 900. Various mirrors 230, 224 guide the therapy laser beam through the articulated arm 130 and finally onto a movable optics system, in particular a laterally movable objective lens 225, which is arranged in a laser applicator head 220.There it enters the eye 900 via a patient interface 600, which serves to create a connection between the examination and therapy system 100 and the eye 900 of a patient.

[0103] The ophthalmological examination and therapy system 100 contains further examination devices in addition to the OCT module 400: A surgical microscope head 320 is located at the end of a first movable articulated arm 120, which is only schematically indicated. An objective lens 330 and, if necessary, a further lens 335 for adjusting the optics of the surgical microscope head 320 when the therapy laser optics are located in the further beam path, form the radiation output of the surgical microscope head 320. The examination and therapy system 100 also contains a confocal detector 260.

[0104] In the Fig. 2Various variants of the integration of an ophthalmological examination system 100, which operates according to the principles of optical coherence tomography, and a laser therapy device are outlined, which can alternatively be used simultaneously. When used simultaneously, switching between the different variants is obviously necessary. All variants of the examination system 100, which operates according to the principles of optical coherence tomography, pass through movable articulated arms 120, 130 in some way, as well as a movable optic, in particular a laterally displaceable objective lens 225, whereby the optical path length and / or the polarization can change during the examination.

[0105] The switching points 420 guide the radiation emitted by the OCT radiation source 405 and the radiation of the OCT radiation source 405 returning from the eye 900 in a first state only via the second articulated arm 130 and the applicator head 220, and in a second state only via the first articulated arm 120 and the microscope head 320. This enables, for example, the use of the OCT module together with the surgical microscope head 320 for the surgical part of inserting the intraocular lens (IOL), in which the applicator head is not needed and remains decoupled from the surgical microscope head 320 in a parked position, and on the other hand ensures that the illumination and detection beam path of the OCT module 400 for making the incisions using the focus of the fs therapy laser beam corresponds to the beam path of the fs therapy laser beam, thereby avoiding alignment errors. Through the switching point orThis is possible with switching points 420 without the need to integrate an additional OCT module.

[0106] In the various variants of the setup, the same OCT module 400 is used, which contains a short-coherence light source as the OCT radiation source 405 and an interferometer. The examination beam generated in the OCT radiation source 405 is coupled either into a surgical microscope head 320 or into the laser applicator head 220. Accordingly, the light from the OCT radiation source 405, reflected back from the eye 900 via the same path, is superimposed on the reference beam of the reference arm in an interferometer 402 and detected by a detector. The interferometer with the detector, which is contained in the OCT module 400, does not have to be arranged in close proximity to the OCT radiation source.Rather, it is advantageous to split the beam from the OCT radiation source 405 by means of a beam splitter into a reference beam in a "reference arm" and an examination beam in a "signal arm", via which the beam is guided to the target, only relatively close to the target, if otherwise the optical path length and / or the polarization in the "signal arm" could change compared to the "reference arm" due to corresponding movement of the components used, thus generating an offset that distorts the evaluation of the measurement.

[0107] In a first variant, the therapy laser beam of the fs laser source 210 and the examination beam of the OCT short-coherence radiation source 405 are fed via the same mirror-containing second articulated arm 130 to the laser applicator head 220 and, via the latter, to the target in the eye 900, which is connected to the ophthalmic examination system 100 via a patient interface 600. After combining the radiation from both radiation sources, both are deflected laterally via the same x / y mirror scanner 240.

[0108] The change in polarization is measured using a reference arrangement comprising a unit 450-V1 consisting of a beam splitter in the beam path of the examination beam and a photodiode behind a polarizer. The beam splitter directs part of the radiation onto the photodiode. This unit 450-V1 is located in the beam path behind a collimator 440-V1, which collimates the examination beam before it enters the part of the beam guidance device running in the articulated arm 130.

[0109] This solution has the advantage that only a single beam guiding device 230, here in the form of a supply optics formed with the aid of mirrors, is necessary for the fs-therapy laser beam and the examination beam of the OCT radiation source 405 to the laser applicator head 220.

[0110] As an alternative to the second articulated arm 130 containing a mirror, in another variant, a photonic crystal fiber, i.e., an optical fiber 410, can be used as the beam guiding device 230 for supplying the examination beam of the OCT short-coherence radiation source 405. This fiber runs either via the first articulated arm 120 into the surgical microscope head 320 or via the second articulated arm 130 into the laser applicator head 220. Even if the examination beam is to be guided into the laser applicator head via the second articulated arm 130, the beam guidance does not necessarily have to run freely over the x / y mirror scanner 240.

[0111] Finally, in the applicator head 220, the fs-therapy laser beam and the examination beam of the OCT radiation source are combined and guided into the eye 900 via a laterally movable lens 225. If an optical fiber 410 is used, a unit consisting of a beam splitter and photodiode 450-V2, 450-V3 is arranged directly behind a collimator 440-V2, 440-V3 at the end of the respective articulated arm 120, 130 and in front of the exit from the surgical microscope head 320 or the laser applicator head 220 to determine the change in polarization.

[0112] In all variants, however, a change in the optical path length is determined by means of a reference measurement, using a known surface or structure from the overall system as the reference surface 470 or reference structure. This is arranged in the beam path near the position of the eye 900 in a position behind the collimator 440-V1, 440-V2, 440-V3 where simple imaging is possible, but in any case behind the area whose movement leads to a change in the optical path length. If the correct position of the reference surface 470 or the reference structure is known, the OCT image can be shifted or the image data of the image can be corrected so that the reference surface 470 or the reference structure is in the expected location.

[0113] The laser applicator head 220 contains a laterally scanning, movable lens 225. In one embodiment, the coherence length of the OCT radiation source 405 in air is greater than 45 mm, particularly preferably greater than 60 mm.

[0114] If a change in the optical path length or a change in polarization is corrected using correction elements, polarization correction elements such as paddles are preferably used between the OCT radiation source 405 and the analyzer in an optical fiber 410, while a phase delay plate is arranged in the free beam path. The correction elements for correcting the change in the optical path length, such as variable additional fibers or delay lines, can also be used in principle across the entire range. (The correction elements are used in Fig. 2 not shown.)

[0115] The Fig. 3a to 3dshow the influence of a movable objective lens 225 on the optical path length of an examination beam in a beam guiding device of an ophthalmological examination system 100. The objective, or in particular the objective lens 225 as the main lens of the objective, is laterally movable in the x- and y-direction. Upstream deflection mirrors 224, 240 can be used, the distance between which is adjusted and which move synchronously with the objective lens 225. This is shown in the Fig. 3a to 3d schematically shown, which also show, purely by way of example, the use of the ring mirror (or pinhole mirror) 430. This ring mirror 430 is one possibility of superimposing the examination beam 406 with an fs therapy laser beam.

[0116] The Fig. 3a and 3bthe two end points of the displacement of the examination beam by the movable lens 225, which causes a change in the optical path length OPD of the examination beam, in the x-direction. Fig. 3c and 3d , on the other hand, show the two endpoints of the displacement in the y-direction. The change in the position of the examination beam in an image field of an eye 900 is also shown in each case.

[0117] The Fig. 3a to 3d show that the lens body of the objective 225 is firmly coupled to the mirror 224, thus moving with it. This has advantages in terms of image quality, since the lens body of the objective 225 is always correctly irradiated to its optical axis. The three-dimensional representation of the Fig. 3a to 3dshows clearly the adjustment effect of the mirrors 240 and 224 while moving the respective downstream elements (lens body of the objective 225 together with the mirror 224 or mirror 224 and lens body of the objective 225 when the mirror 240 is moved).

[0118] In order to compensate for the influence of the movement of the lens 225 on the OCT signal, the differences in the optical path lengths - of up to 25 mm, but usually up to 13 mm for different lens positions - are preferably taken into account when calculating the A-scans from the OCT signals.

[0119] The Fig. 4 schematically shows the change in the optical path length of an examination beam, in particular an OCT examination beam 406, when using a movable objective lens. The change in the optical path length OPD is composed of a component in the x-direction and a component in the y-direction.

[0120] The Fig. 5ashows a schematic representation of the structures 950 of a target 900, here a cornea and a lens of an eye, initially as seen in static images—that is, without movement of the articulated arms 120, 130 or individual optical components such as the objective lens 225, and thus without changes in the optical path length and / or polarization during the examination using optical coherence tomography (OCT). This image is therefore the unmodified image, i.e., the undistorted image in the correct position at this point. It shows the real geometry.

[0121] The Fig. 5bshows a schematic (modified) image of the structures 950' of the target 900' in a movable examination system 100. A shear of the image in the z-direction occurs due to the influence of the change in the optical path length OPD - the "0th order effect" - as occurs, for example, when the objective lens 225 moves during an examination using optical coherence tomography.

[0122] Finally, the Fig. 5cA schematic (modified) image of structure 950"-1 or 950"-2 of target 900"-1 or 900"-2, which, in addition to the shearing caused by the change in the optical path length OPD, also exhibits a shift of the image in the z-direction, i.e., along the optical axis. This shift of the image is caused by the influence of the rate of change in the optical path length dOPD / dt—the "first-order effect"—when using an OCT line scan as a B-scan in a series of multiple A-scans. The direction of the shift depends on the direction of the rate of change in the optical path length.

[0123] As explained above, a swept-source OCT is preferably used in combination with a moving optic, in particular a transversely movable objective lens 225. While the individual OCT scans are acquired, this optic moves, and the optical path length OPD to the target 900 changes (also during an A-scan). This leads, on the one hand, to a shearing of the image of the structure 950 of the target 900, i.e., the OCT image, which directly correlates with the change in the optical path length OPD. This change can be directly incorporated into the image, thus correcting the distortion.

[0124] The movement also results in a non-intuitive shift in the image depending on the current speed of the moving optics, or more precisely, the current speed of the optical path length change, including its sign. For this purpose, the speed of the optical path length change is calculated for each A-scan, and the appropriate additional correction is calculated using this and the sweep speed of the SweptSource OCT's radiation source. For line scans with a constant speed, the offset is constant, whereas for circular scans it is location-dependent, as explained below.

[0125] In this way, it is possible to determine a correct B-scan even though the optical path length actively changes during the individual A-scans.

[0126] In concrete terms, the application of the SweptSource OCT principle in a moving, and therefore non-static, examination system requires the correction of several influences in order to generate correct OCT images from the raw signal: 1. The shearing of the OCT image (also called "distortion") between the different A-scans due to the change in the optical path length (OPD) in the signal arm, i.e., the active arm of the interferometer, which ultimately leads to a distortion of the image. 2. Due to the simultaneous change in the OCT frequency k at a rate v K = dk / dt and the optical path length (OPD) at a speed v OPD = dOPD / dt due to scanning in the x and y directions, additional correction terms arise that must be included to correct the "shift" or correct the OCT signal.

[0127] Only when both corrections are applied is there no significant deviation between the real geometry of the scanned object and the OCT images or the image data of the OCT image. Only when scanning is very slow is the second term, the "first-order effect," negligible.

[0128] Specifically, the detector signal D, i.e. the intensity of the interference of the examination beam and the reference beam, or of an examination wave and a reference wave, is: D = I 0 sin k * OPD for a single backscatter reflection (δ-reflex) of the backscatter intensity I=I 0 *δ(OPD); or D = ∫ I OPD ∗ sin k ∗ OPD dOPD for a complex / multi-layered scattering object, with k: wave vector of the measuring radiation k=2π / λ; (e.g. 2π / 1060nm=5.93E6m -1< , 2π / 1020nm=6.16E6m -1< ) ​​OPD: change in the optical path length between signal arm and reference arm, also called optical path difference.

[0129] The Fourier transform of D yields the desired representation I(OPD) of the depth layering of the scattering object.

[0130] In the SweptSource method, k is tuned linearly, i.e.: k = k M + V k * t with k M : mean wave vector at the start time of tuning (t=0), (e.g. 6.04E6m -1< ) V k : Tuning speed (sweep speed) dk / dt, (e.g. 0.233E6m -1< * 2000Hz = 0.466E9m -1< s -1< ) t: Time (t = -T / 2..T / 2), T: Sweep duration, Δk=vk *T: Tuning range ("sweep range")

[0131] The tuning range Δk determines the OPD resolution, which is explained further below.

[0132] This means that D=sin((k M +vk *t)*OPD) or D=sin(k M *OPD+vk *OPD *t)

[0133] D contains a constant phase term and a time-varying signal; a δ-reflex at the optical path length appears at a frequency ω ω = v k * OPD z . B . v K = 466 kHz / mm .

[0134] From the backscattering intensity D(ω) the desired signal I(OPD) is reconstructed with: I OPD = D ω / v k .

[0135] If the optical path length is changed between different A-scans by the scanning system, a shift in the frequency domain between the A-scans occurs, which, after inverse transformation in the OCT signal processing, naturally leads to a shift in the recorded signal. This is Fig. 5b compared to Fig. 5a (without such a change in the optical path length) for the representation of a cross-section of structures of the eye, here the lens and the cornea, is clearly shown.

[0136] In the Fig. 5b An OCT image can be seen in which the optical path length of the examination beam in the signal arm was changed (in this case extended) from A-scan to A-scan during the B-scans, without the target to be measured, i.e. the eye with its structures, changing.

[0137] In the real system, both the laser frequency or wave vector k and the optical path length change simultaneously. If, during the tuning of the examination beam, the optical path length that the examination beam has to travel to reach the target, i.e., the backscattering object, changes linearly with time, i.e. OPD = OPD M + v OPD * t , the detector signal becomes: D = sin k M + v k * t * OPD M + v OPD * t or D = sin k M * OPD M + v k * OPD M * t + v OPD * k M * t + v k * v OPD * t 2

[0138] The Fourier analysis then produces a signal at: ω = v k * OPD M + v OPD * k M and appears like a signal at: OPD = ω / v k = OPD M + v OPD * k M / v k and thus has a shift of v OPD *k M / vk to the real position of the target, i.e. the backscattering object.

[0139] To stay with the example data: The shift is 13 ms* v OPD ; i.e., 13 µm at a rate of change of the optical path length v OPD of 1 mm s -1< , or 130 µm at a rate of change of the optical path length v OPD of 10 mm s -1< . This is not negligible given the required measurement accuracy of the OCT of a few 10 µm.

[0140] For illustration purposes, the Fig. 5c to Fig. 5b The comparison between statically scanned geometry and scanning with change of the optical path length at a speed v OPD is shown. Fig. 5b The shearing of the image shown as a result of the change in the optical path length is additionally shifted with or against the direction of the tuning velocity vk, depending on the direction of the velocity v OPD.

[0141] To compensate for this, the following procedure is used: If the change in the optical path length OPD to the target, i.e. to the backscattered object, per time as a result of the xy movement of the scanning system is known, the signal I(OPD) can simply be shifted by -v OPD *k M / vk in the optical path length in order to obtain the real scattered signal. It should be noted that the velocity addition is non-Euclidean, i.e. v OPD = vx + vy , since the optical path length OPD is added to the scan movement in x and thus, for example, v OPD can also be 0 if vx = - vy even though the examination beam moves laterally over the target.

[0142] This shift can be easily achieved with known sweep parameters k M , vk . Experimentally, the shift with a known scan speed v OPD in one direction and -v OPD in the opposite direction can be reduced to a shift 2*v OPD *Δk / vk, and from this, the speed-proportional correction factor Δk / vk can be determined experimentally.

[0143] The example of Fig. 6a and 6b shows a schematic image of the structures 950 of an eye 900 - here a cornea and a lens - not modified, ie here undistorted and in the correct position ( Fig. 6a ); with shearing due to the influence of the change in the optical path length in the continuous lines of the Fig. 6b (structures 950'); and additionally shifted by the influence of the speed of change of the optical path length in the dotted lines of the Fig. 6b (Structures 950") for a chirped OCT with circular scan.

[0144] For example: x=Rsin(Ωt) and y=Rcos(Ωt).

[0145] The change that the optical path length experiences is then: ΔOPD = x + y

[0146] The rate of change of the optical path length is described by v OPD = RΩ 2 sin π 4 − Ωt

[0147] The image must therefore be provided with a sinusoidal correction to compensate for the shear and the shift caused by the change in the optical path length and the speed of this change.

[0148] In a circular scan, such as that required for a capsulotomy, the optical path length does not change linearly. This nonlinear change also results in a varying rate of change in the optical path length, which also changes sign during the circular scan. This is shown in the Figures 6a and 6b In the Fig. 6aThe structures 950, i.e. the interfaces of the cornea and the lens of the eye, are shown as they would be expected with a perfectly centered cornea and lens when measured in a static examination system. Fig. 6b The structures 950' with solid lines show the image that would result solely from the change in the optical path length during a circular scan without correction. The structures 950", shown with dotted lines, contain the additional effect of displacement due to the variable rate of change in the optical path length during a circular scan.

[0149] In the Fig. 6bIt is clearly evident that displacement and shear are possible in both directions. At points where the optical path length is constant, the resulting shear and displacement are, as expected, zero. It becomes clear that in the case of a circular scan, a variable correction of the shear and displacement is necessary throughout the scan.

[0150] The Fig. 7 illustrates the influence of velocity in the z-direction, i.e., along the optical axis, on a swept-source OCT signal; the shear in the z-direction is proportional to the change in the optical path length during an A-scan, and the displacement in the z-direction is proportional to the rate of change in the optical path length.

[0151] In a swept source OCT procedure, a frequency range (or wavelength range) is repeatedly scanned during the examination.

[0152] The dependence of the resolution of the optical path length, especially the resolution optimum, on the tuning range is shown in the Fig. 8 Compared to a static OCT measurement, an OCT measurement in a moving system, in which the optical path length changes during the examination, exhibits an optimum resolution: The peak width broadens due to the change in speed, resulting in a reduction in resolution. It is therefore advantageous to define a sweep range where an optimum resolution of the image is achieved.

[0153] In addition, the ω varies during tuning by + / - vk *v OPD *T / 2, see quadratic t-term. This corresponds to a linearly varying ω, which causes a peak broadening in the Fourier-transformed signal and thus a resolution reduction of the optical coherence tomography by d OPD = v OPD *Δk / vk = v OPD *T. This corresponds to the intuitively expected sweeping of the static signal peak over the change in the optical path length during the time of the A-scan T, e.g. 0.5 µm at 1 mm / s or 5 µm at 10 mm / s. The reduction in OCT resolution of up to 5 µm is just tolerable given the required OCT measurement accuracy of a few 10 µm.

[0154] The Fig. 8now shows the dependence of the resolution of the optical path length on the sweep range Δk. Curve K3 shows a "static" curve – no change in the optical path length during the A-scan. Curve K2 is a curve showing the change in the optical path length during an A-scan – with a typical value of the rate of change of the optical path length of v OPD = 10 mm / s with an operating point at 2.3E5m -1< (see large triangle) and a minimum (see cross). Curve K1 is a curve showing the change in the optical path length during the A-scan, with a high value of the rate of change of the optical path length of v OPD = 60 mm / s. The minimum of the peak width lies before the 2.3E5m -1< operating point; a reduction of the tuning width to 2.2E4m -1< is advantageous.

[0155] While in static OCT measurements the resolution (peak width w of a δ-reflex) is determined by the tuning range Δk: ws =2π / Δk and becomes finer with larger tuning ranges, the broadening due to the change in the optical path length is larger with larger tuning ranges (assuming a constant tuning speed vk ). Therefore, a resolution optimum is obtained at Δk = 2 π v k v OPD .

[0156] Using the above example data for vk and v OPD, the optimum is at Δk=0.541E6m -1< , twice the sweep range offered by the laser source. Only at scan speeds > 55mm / s and thus v OPD > 55mm / s does the optimum lie before the used 0.233E6m -1< sweep range, and restricting the sweep range would be advantageous: In this case, such a restriction leads to better resolution, a shorter sweep time, and thus a higher A-scan repetition rate.

[0157] While in a static measurement one would use a swept source with the widest possible frequency range or k-range in order to achieve the highest possible resolution, in a swept source OCT examination, in which the optical path length changes because, for example, the lens is moved laterally during the examination, this can lead to a tuning range that is too wide not increasing the resolution but actually deteriorating it.

[0158] The tuning speed should always be kept at the highest technically manageable value to minimize resolution losses in v OPD and achieve a higher A-scan repetition rate. Limiting factors are the maximum tuning rate of the laser source and the cutoff frequency of the detectors, including data acquisition.

[0159] The Fig. 9shows an embodiment of an OCT module of an ophthalmological examination system according to the invention. The OCT module contains an OCT radiation source 405 as a device for generating an examination beam, an interferometer 402 for analysis by superimposing the examination beam returned from the target 900 with a reference beam, and a signal processing unit 510 consisting of a data acquisition card in combination with photodiodes and electrical amplifiers, which digitizes the acquired data and transmits it to the control unit 500. The interferometer 402 is connected to the OCT radiation source 405, the signal processing unit 510, and finally the path to the eye via optical fibers 410. A collimator 450 is arranged at the exit of the optical fiber. This collimator sends the radiation through a beam splitter 442 onto the target 900, i.e., the eye.Second, a portion of the radiation is reflected onto photodiode 445, which serves as a reference array. If a change in the polarization of the examination beam is detected, this change is corrected using two movable paddles 460-1, 460-2 to restore the original polarization.

[0160] The features of the invention mentioned above and explained in various embodiments can be used not only in the combinations given as examples, but also in other combinations or alone.

[0161] A description of a device related to process features applies analogously to the corresponding process with regard to these features, while process features represent functional features of the described device.

Claims

1. Method for directly correcting an image that is based on an examination of a target using an examination beam (406) of an optical coherence tomography (OCT) device, - wherein an image of a structure (950) of the target (900) is used or image data of an image of the structure of the target (900) are used, for the creation of which an optical path length travelled from a source of the examination beam to the structure of the target (900) and / or from the structure of the target (900) to an analysis device is evaluated, - wherein the optical path length of the examination beam (406) to and / or from the target (900) changes discretely or continuously in the course of the creation of the image of the structure (950) of the target (900), - and the change in optical path length (OPD) of the examination beam (406) to and / or from the target (900) is known or is measured, and - the known or measured change is used to set an adjustable correction element or else to directly correct the image or the image data of the image, in such a way that the influence of the change in optical path length (OPD) of the examination beam (406) to and / or from the target (900) on the image is compensated for, - wherein the examination beam (406) is freely guided through a system that can be varied in terms of its internal positional relationship of its components, the optical path length of the examination beam (406) changes during the change in positional relationship of the system, and a resulting shearing of the image is corrected for on the basis of the change in optical path length (OPD), and - in particular an image created using a "Time-Domain" (TD)-OCT or "Swept Source" (SS) -OCT method is used or image data of such an image are used, in which a displacement of the image is directly corrected on the basis of the rate of change of the optical path length (dOPD / dt).

2. Method according to Claim 1, - wherein the image of the structure (950) of the target (900) is used or image data of the image of the structure of the target (900) are used, for the creation of which a polarization of the examination beam (406) is evaluated, - wherein the polarization of the examination beam (406) changes discretely or continuously in the course of the creation of the image of the structure (950) of the target (900), - and the change in polarization is known or is measured, and - the known or measured change is used to set an adjustable correction element or else to directly correct the image or the image data of the image, in such a way that the influence of the change in polarization of the examination beam (406) on the image is compensated for.

3. Method according to Claim 2, wherein the examination beam is guided by means of an optical fibre (410) and / or freely through a system that can be varied in terms of its internal positional relationship of its components, and a change in polarization of the examination beam resulting during a change in positional relationship of the system is corrected by two movable paddles (460) or by a phase retardation plate.

4. Method according to any of claims 1 to 3, wherein the examination beam (406) is freely guided through a system that can be varied in terms of its internal positional relationship of its components, and a change in optical path length (OPD) resulting during a change in positional relationship of the system is corrected by a variable additional fibre or a delay line.

5. Ophthalmological examination system (100) comprising an optical coherence tomography (OCT) device, and containing - a device for creating an examination beam (405), - an analysis device for analysing the examination beam (406) returned by a structure of a target and for creating image data of an image of the structure of the target, - a beam guiding device for guiding the examination beam (406) from the device for creating (405) the examination beam to the target and from the target to the analysis device (402), wherein an optical path length of the examination beam (406) in the beam guiding device is discretely or continuously changeable, - a control device (500) for controlling the ophthalmological examination system, wherein - the discrete or continuous change in optical path length as a function of time is known or measurable by means of a reference arrangement, and - the control device comprises a correction unit configured to directly correct the image or the image data of the image on the basis of the change in optical path length (OPD) of the examination beam (406) (406) in the beam guiding device, wherein the correction unit is configured to directly correct shearing of the optical coherence tomography image that occurs dependent on the change in optical path length (OPD), and its optical coherence tomography device in particular comprises a device for "Time-Domain" (TD)-OCT or "Swept Source" (SS)-OCT, and the correction unit is configured to directly correct a displacement of the optical coherence tomography image on the basis of the rate of change of the optical path length (dOPD / dt).

6. Ophthalmological examination system (100) according to Claim 5, further comprising an adjustable correction element for compensating for the change in optical path length (OPD).

7. Ophthalmological examination system (100) according to Claim 5 or 6, - wherein a polarization of the examination beam (406) in the beam guiding device is discretely or continuously changeable, - wherein the discrete or continuous change in polarization as a function of time is known or measurable by means of a reference arrangement, and - the ophthalmological examination system (100) comprises an adjustable correction element for compensating for the change in polarization, and / or the control device comprises a correction unit configured for directly correcting the image or the image data of the image on the basis of the change in polarization of the examination beam (406) in the beam guiding device.

8. Ophthalmological examination system (100) according to Claim 6, wherein the adjustable correction element comprises a delay line for compensating for the change in optical path length (OPD) of the examination beam (406), and the adjustable correction element is contained in the beam guiding device and / or in an alternative beam guiding device for a reference beam.

9. Ophthalmological examination system (100) according to Claim 7, wherein the adjustable correction element comprises two movable paddles (460) or a phase retardation plate for compensating for the change in polarization, and the adjustable correction element is contained in the beam guiding device and / or in an alternative beam guiding device for a reference beam.

10. Ophthalmological examination system (100) according to any of Claims 5 to 7, furthermore containing a display device for rendering the image visible with the aid of the image data of the image of a structure (950) of the target (900).

11. Ophthalmological examination system (100) according to any of Claims 5 to 10, wherein the beam guiding device both comprises a system that can be varied in terms of its internal positional relationship of its components and contains an optical fibre (410).

12. Ophthalmological examination system (100) according to any of Claims 5 to 11, wherein the beam guiding device comprises a system that can be varied in terms of its internal positional relationship of its components, and at least a part of the beam guiding device is configured to guide the examination beam freely through the system that can be varied in terms of its internal positional relationship of its components.

13. Ophthalmological examination system (100) according to any of Claims 5 to 12, the beam guiding device of which contains a movable optical unit, in particular a laterally movable objective lens (225).

14. Ophthalmological examination system (100) according to any of Claims 5 to 13, whose optical coherence tomography (OCT) device is subdivided into closed, independently interchangeable and individually correctable subsystems with defined transitions, preferably into a first subsystem containing an interferometer (402) and a second subsystem comprising an optical fibre (410) with a collimator (450) or, in an alternative to that, preferably into a first subsystem containing an interferometer (402), a second subsystem containing a long optical fibre (410), which passes through a system that can be varied in terms of its internal positional relationship of its components, and into a third subsystem comprising a short optical fibre (410) and a collimator (450).

15. Ophthalmological examination system (100) according to any of Claims 5 to 14, comprising a therapy laser device (200), wherein a device for creating the examination beam (406) is also configured to create the therapy laser beam, or an independent device for creating (210) the therapy laser beam is included beside the device for creating (405) the examination beam.