Patent Application
20230000341
The invention relates to a method for measuring at least one parameter indicative of an optical transmission quality of the eye, such as information on absorptive or scattering structures that affect the propagation of light between the cornea and the retina and/or information on the imaging quality, e.g. the point-spread-function (PSF) of the eye.
EP 2710950 describes a method for measuring intraocular scattering and, in particular, the point-spread-function (PSF) of the eye by shining an annular or circular light beam into the eye and by measuring its reflection from the retina.
In such a method, complex measures are required for eliminating reflection effects from the anterior part of the eye and for deriving the PSF of the eye.
The problem to be solved by the present invention is to provide a method of this type that is able to reliably measure at least one parameter indicative of the optical transmission quality of the eye.
This problem is solved by the method and device of the independent claims.
Accordingly, the method comprises at least the following steps:
Hence, optical coherence tomography data can be recorded for each A-scan. This allows to easily distinguish reflection values originating from the anterior part of the eye and from the retina, i.e. to isolate the reflection values ri from the retina. The reflection value ri depends on the eye's transmission properties along the respective A-scan at location xi, yi, which allows to obtain a spatially resolved indication how well the eye transmits light along the probe beam of A-scan i.
This information can be used for determining a large number of different parameters. Some examples include:
Advantageously, the plurality of A-scans includes a first plurality of A-scans, advantageously at least 10 A-scans, in particular at least 100 A-scans, having parallel directions of incidence. In other words, the A-scans differ by their locations xi, yi but not by the direction of the light beams outside the eye as it impinges on the cornea. This allows to record the eye's transmission properties for light coming from a given direction. Further, for an eye accommodated to infinity, all such A-scans will substantially be incident on a common location of the retina, thereby providing better robustness of the measurement against spatially varying retina reflection.
In particular, the “parallel direction of incidence” may be parallel to the eye's visual axis, which allows to record the transmission properties along the patient's natural viewing direction.
In this context, “parallel” advantageously is understood to encompass a parallelism within 5°, in particular within 1°.
Advantageously, the A-scans include a plurality of A-scans, advantageously at least 10 A-scans, in particular at least 100 A-scans, that do not overlap at a cornea of the eye. In other words, these A-scans enter the eye at different locations xi, yi, allowing to record information with good spatial resolution.
In this context, advantageously, two A-scans do not overlap if their center distances on the cornea are larger than their half-width-diameter. The “half-width-diameter” is the diameter, in the x-y-plane perpendicular to the direction of the A-scan outside the eye, over which the intensity of the light used for the A-scan drops by 50%.
In another important embodiment, the probe beams of at least part of the A-scans are focused onto an anterior part of the eye, i.e. the probe beams have their minimum diameter in this anterior part. This allows to spatially resolve scattering or structures in that part of the eye.
In this context, an A-scan is advantageously considered to be focused at an anterior part of the eye if the minimum diameter of its probe beam is located somewhere within 1 mm before the cornea and 5 mm behind the eye's lens.
Alternatively or in addition thereto, the focus may, for at least part of the probe beams, also be located between the posterior surface of the eye's lens and the retina. This may be useful to e.g. detect vitreous floaters.
In one embodiment, the invention comprises the step of displaying the reflection values ri as a function of the locations xi, yi. Hence, an image is displayed representing the reflection value ri as a function of xi and yi. For example, the image can comprise pixels, where the pixel coordinates are mapped to the coordinates xi, yi and the pixel color and/or brightness is a function of the reflection value ri. Such an image allows to locate the regions where light rays are poorly transmitted through the eye, e.g. due to scattering and/or absorption.
This e.g. allows to locate absorbing structures in the anterior section of the eye. Again, the anterior section of the eye may e.g. be a section between the cornea and a location 5 mm behind the eye's lens.
To achieve good lateral resolution for absorbing or scattering structures in the vitreous, it is advantageous to place the focus between the posterior lens surface and the retina.
In another embodiment, the invention comprises a Fourier analysis of the dataset ri(xi, yi). This Fourier analysis comprises at least the following steps:
Advantageously, a two-dimensional Fourier transform is used, which allows to assess the PSF (or a parameter thereof) in two dimensions.
Alternatively or in addition to using the Fourier transform, ray tracing may be used for calculating the PSF, which allows to take into account the eye's refractive structures and in particular their aberrations, as they can e.g. be determined by means of the OCT measurements.
The method may further comprise at least one of the following steps:
Further the data from the A-scans may be used to extract the topology of at least one structure of the eye. This structure may e.g. be at least one of the following:
In that case, the method can further include the step of determining the at least one parameter using the reflection values ri and the topology of the structure, e.g. using ray tracing calculus.
The invention also relates to an ophthalmologic device comprising
The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:
(Note: All grayscale images in the figures have been half-toned for better reproducibility. Half-toning is typically not used when representing the images on an electronic display.)
Device Overview
The ophthalmologic device of
It comprises an optical coherence tomography interferometer 10-26.
The interferometer has a light source 10, which, in the present embodiment, is a swept-source light source, i.e. it generates narrowband light that can be adjusted in wavelength.
The light from light source 10 passes a beam splitter 12, in particular a fiber beam splitter, and is sent into two interferometer arms 14, 16.
The first arm is the reference arm 14, which comprises a collimating lens 17 and mirror 18 at one end. Light impinging on mirror 18 is sent back into beam splitter 12 and from there, at least in part, to a light detector 20.
The second arm is the sample arm 16. It comprises collimation optics 22 for collimating the probe light coming from beam splitter 12. The light is then fed through two scanner mirrors 24a, 24b and an objective lens 26 for generating a probe beam 28. Depending on the position of the scanner mirrors 24a, 24b, probe beam 28 can be laterally offset in an x-y-plane perpendicular to the optical axis z of the device.
In the present embodiment, an interferometer generating telecentric probe beams 28 is used, i.e. the probe beams 28 for various x- and y-coordinates (such as beam 28 and beam 28′ in
In the shown embodiment, the probe beams are shown to be focused on the anterior surface of the cornea, but they may also be focused on any other part of the eye 30 that is of particular interest. For the reasons mentioned above, the probe beams are advantageously focused on the anterior segment of the eye.
The focusing optics, e.g. the position and/or the power of lenses 22 and/or 26, may be adjustable to vary the position of the focus along the z-direction.
Probe beam 28 enters eye 30, where it is reflected or scattered by the structures of the eye. Light cast back from such structures is returned to beam splitter 12, where it can interfere with the light from reference arm 14, and from there, at least in part, to light detector 20.
The device of
Spectral analysis, in particular a Fourier transform, of the signal from detector 20 can then be used for generating the reflection values of eye 30 along axis z for the given A-scan. Reflection values are meant to relate to reflected and scattered light as described above. As customary in OCT imaging, reflection values might be represented by values proportional to the reflected intensity or by values proportional to a logarithm of the reflected intensity or e.g. by other range-compressed values. In more general terms, a “reflection value” is indicative of the amount of light returned from a certain position along an A-scan. Advantageously, it may be linear to the amount of light or a logarithm thereof or any other function thereof.
This type of OCT measurement is known to the skilled person, and it is e.g. described in EP 3572765 and the references cited therein.
The device further comprises a control unit 32, which may e.g. be provided with a microprocessor 34a and with a memory 34b as well as with a display 34c. Memory 34b may hold the data as well as the program instructions required for carrying out the steps of the present method. Display 34c may e.g. be used for showing the data determined thereby and in particular for displaying plots or images as described below.
Advantageously, the measuring range (for a single A-scan) of the OCT interferometer 10-26 extends at least from the cornea to the retina of a typical eye. In other words, with a single A-scan (i.e. for an SS-OCT with a single sweep of the light source), depth-resolved information over at least 40 mm (in air) can be obtained. This allows to apply the techniques described in the following to be used over the whole axial eye length without the need to e.g. apply stitching for combining different measurements.
A-Scan Analysis
As known to the skilled person, the various structures of the eye generate different peaks in the reflection values corresponding to different depths z1, z2, z3 . . . A first major peak at a depth z1 may e.g. correspond to the (anterior surface of) the cornea 36, a second peak at z2 to the anterior surface 40 of the lens 38, a next peak at z3 to the posterior surface 42 of the lens 38, and a last peak at z4 to the retina 44.
The A-scans recorded in this manner can optionally be corrected for eye motion, e.g. by using at least the following steps:
The parameters obtained in the fitting step 2 can then be used for translating the OCT measurements, and in particular the incident coordinates xi, yi as well as the z-coordinates obtained from the A-scan, into a coordinate system that is fixed with the frame of the eye.
Suitable motion correction techniques are e.g. described in WO 2013/107649 or U.S. Pat. No. 7,452,077.
These steps allow to determine the location of various structures in the eye, such as cornea 36, the anterior and/or posterior surfaces 40, 42 of lens 38, and/or the anterior surface of the iris 46 and to identify their reflection values.
Transmission Analysis
As mentioned above, a reflection value of particular interest is the reflection value ri corresponding to the reflection of the probe beam of A-scan i at the retina 44.
This reflection value ri can e.g. be obtained by one of the following methods:
A more robust reflection value r′i can be obtained by combining values ri1, ri2, . . . , rin, of n A-scans i at points xi1/yi1, xi2/yi2, . . . xin/yin, having mutual distances smaller than a threshold d, e.g. with d<1 mm, <0.5 mm or <0.25 mm, by means of e.g. calculating an average, median, or weighted average of ri, ri2, . . . , rin.
The reflection value ri obtained in this manner is not only a function of the reflectivity of the retina but also a function of the transmission of the eye along the path of probe beam 28.
Hence, if the eye comprises scattering and/or absorbing structures along the path of probe beam 28, the reflection value ri decreases.
In a typical measurement, a plurality of A-scans i with i=1 . . . N (with N being at least 10, in particular at least 100, advantageously at least 1000) is performed.
Advantageously, the directions of incidence D of the probe beams outside the eye are parallel to each other and, advantageously, parallel to the eye's visual axis A.
For parallel probe beams 28, 28′ and an eye accommodated to infinity, the probe beams will all hit the retina 44 at a common location 48 (corresponding to the fovea if the direction of incidence of the A-scans outside the eye correspond to the eye's visual axis A).
Hence, the difference between reflection values ri for the retina for these two A-scans will primarily be due to the eye's different transmission for the two probe beams 28, 28′.
In other words, the reflection values ri of the retina describe how the transmission of the eye varies as a function of A-scan location xi, yi.
If, for example, there are local, scattering or absorbing structures 50a-50f in the anterior section of the eye, they can be detected and spatially resolved (at least in the directions x and y if not necessarily along z) by reviewing the reflection values ri as a function of scan location xi, yi.
For example, these structures may include scattering or absorbing structures 50a-50c at the posterior surface of the lens or scattering and/or absorbing structures 50d-50f in the anterior half of the eye behind the lens.
This is illustrated in
In each image, the pupil can be recognized easily. Positions where the A-scan hits the iris have low reflection values ri from the retina and are, therefore, white.
Eye C of
Eyes A, B, and D show eyes where the transmission is impaired for some locations xi, yi, which is indicative of defects in the eye's anterior region.
It must be noted that the present techniques allows to detect not only scattering but also absorbing structures. The latter are notoriously hard to detect by other methods.
PSF Analysis
An analysis of the reflection values ri as a function of xi, yi allows to obtain an estimate of the eye's PSF.
The relevant techniques are e.g. described in Goodman J W, “Introduction to Fourier optics”, 2nd edition (1996).
In particular, and assuming that the eye's lens and cornea provide perfect imaging only impaired by defects 50a-50f in the eye's anterior section, the PSF can be calculated by the Fourier transform FT of the modulation transfer function MTF of the anterior eye, i.e.
PSF=FT(MTF) (1)
The modulation transfer function can be estimated from the reflection values ri(xi, yi) as obtained by the measurements described in the section “PSF analysis” above. Advantageously the MTF is interpolated to a regular 2D grid since this allows to use the efficient FFT algorithm to perform the FT.
In particular, and in good approximation
PSF(u,v)=FT(ri(θxi, θyi)) (2)
with θxi, θyi being the angles of propagation on the posterior side of the lens, of probe beam for A-scan i and u, v being retina coordinates. The angles θxi, θyi are measured in to the axis A of the eye.
For a quantitative analysis, the values θxi, θyi can be calculated from xi, yi using the axial length L of the eye. In this context, this axial length L may be defined as the distance, along axis A, between the center of lens 38 and retina 44. Alternatively, it may e.g. be defined as the distance, along axis A, between any other part of lens 38 and retina 44 or the distance between the apex of cornea 36 and retina 44.
In particular, the values θxi, θyi can be calculated using ray tracing techniques.
This axial length L of the eye can easily be determined from the OCT A-scans by determining the positions of the respective peaks in the A-scan spectra. In the example of
Hence, the present method advantageously comprises the step of using the axial length L in order to estimate a parameter descriptive of the absolute size of the PSF, such as a half-width of the PSF in horizontal and/or vertical direction.
In addition, for a quantitative analysis, the absolute values of xi, yi need to be known, e.g. from one or more of the following sources:
Alternatively to calculating the Fourier transform of a dataset derived from ri(xi, yi), ray tracing can be used for determining at least one parameter of the eye, such as one or more parameters describing the PSF of the eye.
Such ray tracing can be based e.g. on the following steps:
For each simulated beam, a transmission value is determined based on one or more of the reflection values ri, assuming that the reflection values ri are e.g. proportional to the transmission at the points xi, yi, and xi, yi are in vicinity of the coordinates of the simulated beam (e.g. having a distance of less than 10 spot sizes). This transmission value ri (or a composite value r′i) can be used as weighting factor for that particular simulated beam. The PSF resulting from such a simulation represents the optical imaging quality of the eye including effects of aberrations and obstructions (scattering and/or absorption).
The simulation can be further improved by taking into account the angle of incidence of each beam with respect to the retina and weighting each beam according to the Stiles-Crawford effect (Stiles and Crawford 1933), i.e. the angle-dependence of the retinal sensitivity.
Techniques for carrying out such ray tracing calculus are e.g. described in:
The eye's PSF can e.g. be directly displayed to the operator using graphs as shown in
Notes
Advantageously, the A-scans used for measuring the parameter includes plurality of A-scans, advantageously at least 10 A-scans, in particular at least 100 A-scans, that have mutual distances, at the cornea, of at least 1 mm, i.e. a macroscopic region of the eye is examined.
In particular, the plurality of A-scans is distributed over the whole pupil of the eye, which allows to measure the transmission over the whole pupil. The distribution can be even or irregular. Advantageously, it has a resolution of at least ten points horizontally (i.e. along x) as well as vertically (along y).
In the embodiments above, the A-scans i all have the same direction of incidence, i.e. they are, before entering the cornea, parallel to direction D, which is advantageously parallel to the optical or visual axis A of the eye.
In another embodiment, probe beams having different directions of incidence can be used.
For example, a first plurality of A-scans for probe beams having mutually parallel directions of incidence, along a first direction (e.g. D), can be recorded. In addition, a second plurality of A-scans for probe beams having mutually parallel directions of incidence along a second direction (e.g. D′ in
In yet another embodiment, the focal position of the probe beams can be varied while recording the A-scans. For example, for a given location xi, yi, at least two A-scans with different focal positions may be recorded. Since the spatial resolution for defects 50a-50f is best at the focal plane of the probe beams, this allows to e.g. focus the measurement on a specific region of the eye and/or to gain more information about the z-position of given defects.
The present techniques can be used with any kind of OCT, in particular for time-domain OCT as well as frequency-domain OCT. Frequency-domain OCT, and in particular swept-source OCT, is, however, advantageous for its ability to obtain an A-scan quickly.
While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/051189 | 1/17/2020 | WO |
