Method for filtering reflexes in full-field setups for ophthalmologic imaging by separated illumination and detection apertures

A parallel detecting optical coherence tomography (OCT) setup and method, in which the light paths of the illumination of the sample and of the detection of the backscattered light do not use the same apertures. The separation of illumination and detection apertures filters these disturbing reflexes from the backscattered light of the sample and significantly increases image quality.

FIELD OF THE INVENTION

The present invention relates to the field of ophthalmologic imaging, and more particularly to systems and methods for filtering reflexes in full-field setups for ophthalmologic imaging by separated illumination and detection apertures.

BACKGROUND

In ophthalmology tomographic imaging of scattering structures of the eye is of high interest. The standard technique is optical coherence tomography (OCT), employing an interferometric setup and a spatially coherent light source with a short temporal coherence length. Conventional OCT systems acquire volumetric data by scanning a focused beam over the sample and consequently, measurement speed is limited by the scanning speed. To further increase imaging speed methods with parallel detection of scattered light have been developed. Since in those tomographic ophthalmic imaging techniques that employ parallel detection no confocal imaging is used, disturbing reflexes reduce the image quality significantly.

Below are some existing ophthalmology tomographic imaging techniques:

In Fourier-Domain optical coherence tomography (FD-OCT) an interference signal over a broad spectral width is recorded in an interferometric setup. This is achieved by either detecting the signal of a broadband light source spectrally resolved (Spectral-Domain OCT) or by recording an interference signal over time, while a laser source is spectrally tuned (Swept-Source OCT). Considering the entire spectral range, the short temporal coherence length of the light source allows for an optical path length measurement of backscattered and/or reflected light from a sample in one of the interferometer arms. The path length is encoded in the interference signal, which is generated by superimposing light from sample and reference arm. Measurements of interference signals at several wavelengths allow a depth encoded profile of the sample (A-scan). The main area of application for OCT is ophthalmologic imaging, especially tomographic images of the retina (posterior eye segment) and structures in the anterior eye segment (cornea, ocular lens, iridocorneal angle).

Swept-Source OCT (SS-OCT) uses a setup with a tunable light source. While tuning the light source a wavelength-dependent interference spectrum is measured. The tuning range of the laser, which corresponds to its entire spectrum defines the coherence length of the system. The coherence length of a single wavelength is defined by the instantaneous line width. A tomographic data volume is measured conventionally by sequential data acquisition at different points, i.e., lateral scanning of the sample. Generally the sample is screened by two lateral scanners and the backscattered light is detected with a point detector. Measurement speed for sequential data acquisition might be limited by the speed of the scanners in this scenario. A phase stable detection over the whole volume is not possible in most cases. Scanning fiber based OCT systems have a confocal gating, i.e. only sample structures in the focus are illuminated and detected, while out-of-focus photons are rejected. Therefore multiple scattered photons occurring in strongly scattering media are suppressed.

A setup with a partly parallelized data acquisition is called Line-Field-OCT or Swept-Source parallel OCT (see reference 1). The backscattered light from the sample is detected in parallel in one lateral dimension, while it is scanned in the other lateral direction. The detector consists of multiple individual elements, which are arranged in a line, e.g., a line scan camera. Measurement speed is generally improved by the partial parallelization. Multiple scattered photons are not suppressed, if their last scattering event is recorded by the detector.

For a completely parallel detection in two dimensions the sample is illuminated homogeneously and spatially coherent (Full-Field Swept-Source OCT, short: FF-SS-OCT). An area scan camera is used as detector. In FF-SS-OCT the sample or rather a part of the sample is imaged onto the area camera. An advantage of this method is the increased measurement speed. While in scanning OCT systems the scanners or the tuning speed limit the measurement speed, in FF-SS-OCT the frame rate of the camera is in general the most limiting factor. All parallel detected A-scans are phase stable to each other. With this method scattered photons from all depths are detected, which are filtered in scanning systems by the confocal gating. Thus the measurement depth is larger in comparison to scanning systems, but the lateral resolution degrades out-of-focus. This degradation limits the useful measurement depth, especially at high lateral resolution. Another disadvantage of FF-SS-OCT is the detection of multiple scattered photons due to the parallel detection. Ophthalmic imaging with FF-SS-OCT has been demonstrated successfully, showing in vivo retina measurements (see reference 2)

Holoscopy is method related to FF-SS-OCT where the sample is not necessarily imaged onto the camera, but wave fields of the backscattered light from the sample are detected (see references 3 and 4). The focusing in all depth is performed in the following reconstructions. This has the advantage, that the lateral resolution does not degrade out-of-focus, but is constant over the whole volume. The reconstruction algorithm for holoscopy is also suitable for increasing the focus depth in FF-SS-OCT data.

By implementing an off-axis reference illumination in FF-SS-OCT or holoscopy it is possible to separate the signal of the interference of the sample with the reference light from its complex-conjugated signal as well as from DC and autocorrelation signals and to suppress the non-relevant signal terms. This increases the sensitivity of the imaging and avoids an overlay of multiple signal parts. (See reference 5).

All OCT techniques and related imaging methods mentioned so far are in particular suitable for scattering samples. If a sample has highly reflecting and weakly scattering parts, the strong reflections induce overexposure artifacts in the images and decrease the sensitivity of the measurements. More significantly, strong reflexes that are not within the measurement range, induce an incoherent background noise on all depth profiles/A-scans.

In other ophthalmologic imaging modalities, which have spatial incoherent illumination, strong reflexes induce image artifacts as well. This is the case for fundus cameras, where photographs of the posterior eye segment are taken, as well as for silt lamps, where all segments of the eye can be visualized enabling a variable slit shaped illumination. The artifacts caused by strong reflexes mainly lead to an overexposure of the detector, or the reflexes overshadow the actual image of the retina. The approaches to reduce those artifacts are based on the separation of illumination and detection apertures.

In conventional fundus camera setups a ring shaped aperture in the illumination light path is imaged into the plane of the pupil (References 6, 7, 8). The ring shaped illumination generates only reflexes in the outer areas, which are reflected at an angle, in a way, that they are not imaged onto the detector. The retina is illuminated divergently. The backscattered light is refracted via the optical elements of the eye and projected onto the detector. Such an aperture is not possible with coherent light, as the light will interfere on the retina and not create a constant illumination.

In conventional slit lamp setups the eye is illuminated at an angle. The angle is adjusted in a way that the backscattered light is detected, while the reflected light does not reach the detection light path (references 9, 10). When using a coherent light source the illumination of the retina is not uniform due to interference effects.

Both fundus cameras and slit lamps use conventional light sources, e.g., filament lamps or halogen bulbs. Those light sources are both spatially and temporally incoherent. Therefore there are no additional interference effects, which prevent a uniform illumination of the sample. In addition, these techniques do not provide depth information for scattering tissue.

So far there is no imaging modality that provides volumetric sample information with a parallel detection and spatially coherent light that implements reflex reduction methods to increase SNR by decreasing stray light.

As described above, existing techniques have drawbacks and disadvantages. Therefore, there is a need for an imaging system and method that allows imaging a larger portion of the eye than possible with any of today's optical imaging systems. Anterior and posterior segment can be imaged in parallel. Using parallel detection, volumetric imaging equivalent to >5 million A-scans per second (comparing to scanned OCT technology) are possible and provide very fast acquisition. The parallel detection scheme has a much higher efficiency, as it misses the confocal gating that is used in the prior art of eye inspection using optical coherence tomography. Hence a larger imaging speed can be applied within the restrictions of eye safety regulations.

SUMMARY

The present invention provides a parallel detecting OCT setup and method where the light paths of the illumination of the sample and of the detection of the backscattered light do not use the same apertures. The separation of illumination and detection apertures filters these disturbing reflexes from the backscattered light of the sample and significantly increases image quality. Similar approaches have been implemented in non-tomographic ophthalmologic imaging modalities with spatially incoherent light sources (mainly fundus cameras, slit lamps). However, different technical constraints apply and special technical arrangement have to be used for tomographic imaging. For example, the light sources that are used in fundus imaging are white light sources (such as a halogen lamp) which are incoherent. For tomographic imaging, spatially (and temporally) coherent radiation (a swept source laser) is required to obtain the depth information. Using the previous approaches (such as ring illumination) to separate the illumination from the detection aperture results in disturbing interference patterns on the retina if coherent radiation is used, whereas everything is fine with incoherent light. This poses a problem when using the ring aperture for tomographic imaging, as the areas with destructive interference cannot be imaged. Therefore, the approaches provided by the present invention have a unique advantage that it does not cause interference signals on the retina.

One embodiment of the present invention provides an ophthalmologic imaging system including: a tunable light source; a first optical system to focus light from the light source onto a mirror, such that the mirror reflects the light onto a sample in a first light path; a second optical system configured to direct scattered light from the sample onto a detector in a second light path, and the second optical system is further configured to direct a reference beam onto the detector, the reference beam being either on-axis or off-axis to the second path; wherein the mirror is configured to reflect the light in the first light path, which is an angle to the second light path.

Another embodiment of the present invention provides an ophthalmologic imaging system including: a tunable light source; a mirror with a hole; a first optical system to focus light from the light source into the hole of the mirror, such that the light passes through the hole is focused onto the cornea by a lens and illuminates the retina divergently; a second optical system configured to direct scattered light from the sample that is reflected by the mirror onto a detector, and the second optical system is further configured to direct a reference beam onto the detector, the reference beam being either on-axis or off-axis to the second path.

Another embodiment of the present invention provides an ophthalmologic imaging system including: a tunable light source; a mirror with a hole; a gradient-index (GRIN) lens placed in the hole; a first optical system to direct light from the light source onto the GRIN lens, such that the light passes through the GRIN lens is focused onto the cornea by a lens and illuminates the retina divergently; a second optical system configured to direct scattered light from the sample that is reflected by the mirror onto a detector, and the second optical system is further configured to direct a reference beam onto the detector, the reference beam being either on-axis or off-axis to the second path.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts.

Ophthalmologic imaging is sensitive to image artifacts that are caused by reflections from the eye itself. Especially the front side of the cornea is strongly reflecting. In swept-source OCT and related tomographic imaging methods these reflections cause a reduced sensitivity due to overexposure, coherent background, and/or incoherent background noise. By implementing separated apertures for the illumination of the eye and the detection of the backscattered light strong reflections at interfaces like the cornea can be filtered before detection. In one embodiment of the invention, an interferometric setup for ophthalmic imaging includes one or more of the following components:A tunable light source; preferably, the total coherence length, defined by the spectral width of the tuning range, does not exceed 50 μm. The instantaneous coherence length, defined by the instantaneous line width of the spectral range during one shot, preferably, is at least 200 μm.A detector consisting of several pixels, preferably, recording data with an acquisition speed of at least 500 images per second.A reference illumination with an incident direction that can differ from the incident direction of the sample light on the detector (off axis).A spatial coherent or partly coherent illumination of the eye.A separated illumination and detection aperture for the suppression of reflected light from plane or curved interface, preferably, of at least 10 dB.

Separated illumination and detection apertures in an interferometric setup could be implemented as follows:

In one embodiment of the invention, the angle under which the sample is illuminated differs from the detection angle of the backscattered light, as illustrated byFIG. 1. A light source110is used to illuminate the sample130. The illumination light from light source110is focused by a collimator120and a lens180onto a mirror140. The mirror reflects the illumination light into the sample130. The backscattered light from the sample130is detected by a camera170. A reference signal150(which can be either on-axis or off-axis) is directed onto the camera170via a beam splitter160. As shown inFIG. 1, the illumination light path192is reflected by the mirror140, which is placed at an angle to the detection light path195. The angle of the mirror is adjusted in a way that the desired area of the sample130is illuminated and the backscattered light detected while the reflexes of the cornea190are not in the detection beam path195.

In another embodiment of the invention, central reflexes of the cornea can be filtered by creating a focus on the cornea or slightly beneath the cornea surface. The illumination of the retina is divergent. Backscattered light from the retina is detected while reflected light from the cornea is filtered, as shown inFIG. 2. A light source210is used to illuminate the eye230. A collimator220and a lens (280) focus the illumination light path through a central hole in an angled mirror240. Further optical elements295create a focus on the cornea, while the retina is divergently illuminated. The backscattered light is converted into the far field by the optics of the eye. The central cornea reflex does not reach the detection camera, since it is imaged back through the hole in the mirror and/or is absorbed by a non-reflecting area around the hole, while most of the light from the retina290is deflected by the mirror240onto the detector260. Only a small part of the sample light is filtered by the mirror. A reference signal250(which can be either on-axis or off-axis) is directed onto the camera260via a beam splitter270.

Another option to create a focus on the cornea is to position a Gradient-index (GRIN) lens with a suitable pitch length in a hole in an angled mirror, in accordance with one embodiment of the invention.FIG. 3shows a GRIN lens380in a hole of the mirror340. The illumination beam is converging when leaving the GRIN lens380and focused by an additional lens397onto the cornea or slightly beneath the cornea surface of the eye330. The backscattered light is converted into the far field by the optics of the eye. The central cornea reflex395does not reach the detection camera, since it is imaged back through the hole in the mirror and/or is absorbed by a non-reflecting area around the hole, while most of the light from the retina is deflected by the mirror340onto the detector360. Only a small part of the sample light is filtered by the mirror. An off-axis reference signal350is directed onto the camera360via a beam splitter370.FIG. 3also shows an arrangement according to an embodiment in which a light source310supplies both the illumination beam and reference beam. A coupler390that splits the light from light source310into two paths, one to the GRIN lens380, another one to a collimator320, which directs the reference beam350to a beam splitter370. The beam splitter370directs the reference beam onto the detector360. It is contemplated that other optical components and combinations thereof, such as lens, prisms, mirrors, fibers, etc., may be used to direct the light from the light source to the illumination path and the reference beam path. Note that although not shown inFIGS. 1 and 2, similar arrangements are contemplated to provide both the illumination beam and reference beam.

The backscattered light from the sample is collected by the detector—either via imaging the scattering volumetric sample or via the detection of wave fields from scatterers from all depths within the sample. In both cases the reconstruction algorithm developed for holoscopy can be used for data reconstruction.

REFERENCES