NL2006726C2

NL2006726C2 - Method and apparatus for scaling of intraocular image.

Info

Publication number: NL2006726C2
Application number: NL2006726A
Authority: NL
Inventors: Aleksey Nikolaevich Simonov; Michiel Christiaan Rombach
Original assignee: Akkolens Int Bv
Priority date: 2010-06-03
Filing date: 2011-05-05
Publication date: 2014-04-07
Also published as: NL2006726A

Description

Method and apparatus for scaling of intraocular image

Introduction. - Scales (reference standards for dimensions), of optical measurements of structures inside the eye (intraocular measurements of sizes of structures), can be 5 distorted by aberrations caused by optical surfaces of the eye, including surfaces of the cornea and the lens. This document discloses a method and apparatus to correct for both longitudinal and transverse scales for the effects of aberrations. The method involves separate biometry, for example, a Scheimpflug procedure, to determine the optical properties of an optical element of the eye prior to measurement of size and, 10 subsequently, adjust scales to the patient eye. Also, this document discloses an optical apparatus comprising a variable-focus lens construction to adjust scales during measurement.

Optical methods for intraocular imaging, providing images of structures inside an eye, 15 include, but are not restricted to, Optical Coherence Tomography, OCT, and the corresponding OCT imaging apparatus, which enables micron-scale cross-sectional imaging of the eye with high resolution. The imaging apparatus is referred to, in this document, as “imaging apparatus and digital processing means” (being existing image apparatus, not disclosed in this document; henceforth: “imaging apparatus”), with 20 methods and apparatus (“apparatus”, being novel additional optics and additional optical apparatus, disclosed in this document). OCT images can be used for a number of applications and diagnostics including measurement of mainly structures in the anterior chamber of the eye, but not restricted thereto.

25 The raw data, for example signal strength map versus optical path length from an imaging apparatus are converted into corresponding images of the eye and corrected, as predicted by the “model eye” (the properties of an average eye) incorporated in that particular imaging apparatus, versus the eye of the patient/person on which diagnostics are performed (“patient eye”). Optical distortions caused by the optical surfaces of the 30 eye are automatically corrected by the integrated software of the imaging apparatus and the resulting image of the intraocular structure is represented in, at least one, in practice generally two, “imaging apparatus scales” (longitudinal and transverse scales, or, alternatively, X-scale and Y-scale) which imaging apparatus scales are derived based on the model eye. However, the scales of the intraocular image are likely inaccurate if the 2 parameters of the patient eye deviate from the parameters of the model eye - for example, if the comeal curvature of the patient eye differs from that of the model eye in combination in combination with a zero total refractive error (in the case when, for example, deviation of cornea curvature is exactly corrected for by particular posterior 5 chamber length).

Measurements of the anterior segment are distorted by the deviations of the corneal curvature mainly (S. Ortiz et al., Opt. Express 18, 2782-2796, 2010). The cornea is responsible for -80%, of the refractive power of the eye and most of the refraction of 10 the eye is provided by the cornea-air interface. The lens provides only -20% of the refracting power of the eye.

Figure captions. - Figure 1 shows the anterior chamber measurement with an optical imaging apparatus, 1. In this example, a scanning ray of light, 2, from the imaging 15 apparatus is refracted by the cornea, 3, (which corresponds to the model eye) and scattered by an anatomical feature, 4, inside the eye. A portion of the scattered light reaches the imaging apparatus and gives rise to a variation in the signal strength. In scanning, 5, the light ray is scattered by another anatomical feature, 6, and the scattered light is detected by the imaging apparatus. Thus, the distance, 7, between the anatomical 20 features of the eye can be determined with the imaging apparatus. The result becomes inaccurate when, for example, the curvature of the cornea, 8, diverts from the comeal curvature of the model eye. Correction of the imaging apparatus scales can be achieved by ray-tracing analysis that takes into account the difference in the comeal curvatures 3 and 8.

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Figure 2 shows the anterior chamber measurement with an optical imaging apparatus, as in Figure 1, including correcting optical element, 9. In this example, the correcting optical element is adjusted to mimic in combination with the cornea of the patient eye the comeal curvature of the model eye.

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Figure 3 shows an example the calibration curve for transverse dimensions in the anterior chamber of the eye. In this example, the comeal power of the model eye is 43 D (the ratio of the comeal radii is 7.7/6.8 according to the Gullstrand eye model), the distance to the transverse plane is 3.5 mm.

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Prior art. - The relevant invention US2004/0068192A1 discloses a method and system for image correction in optical coherence tomography. US2004/0068192A1 differs in all aspects from the methods described in the present document. First, 5 US2004/0068192A1 requires uncorrected/raw OCT data to be corrected by a data processor. Our method does not assume any use of the raw data from the optical imaging apparatus. Moreover, the methods described in the present document are not limited by OCT applications, these methods can be extended to any optical measurements, for example, with a retinal camera. Second, forward and backward 10 mapping approaches specified in US2004/0068192A1 are not applicable since the methods described in the present document do not use uncorrected/raw data. Third, correction of scanning distortions as described in US2004/0068192A1 is not relevant since the methods described in the present document do not imply any knowledge of the light scanning system.

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The method. - This document discloses a method to adjust the imaging apparatus scales of an image of the internal structure of a patient eye provided by an imaging apparatus. The imaging apparatus scales are based on the optical properties of a model eye (“model eye”) incorporated in the imaging apparatus. Adjustment of the transverse, in the X-20 direction and Y-direction, and longitudinal, in the Z-direction, scales is needed if the parameters of a patient eye differ from those of the model eye. The method is adapted to provide adjustment for, at least one, optical property of, at least one, optical element of the patient eye for differences between said optical property and the corresponding optical property of the corresponding optical element of said model eye. So, in 25 summary: the method adjusts, at least one, imaging apparatus scale which is based on a model eye, of an image provided by an imaging apparatus, of, at least one, structure inside a patient eye by adjustment for, at least one, difference between, at least one, optical property of, at least one, optical element of the patient eye and the corresponding optical property of the corresponding optical element of the model eye.

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In practice, scales need adjustment only for defocus aberration, astigmatic aberration and combinations of defocus aberration and astigmatic aberration. With other measurements on the patient eye, for example refractive measurements, cornea topography, ultrasound, all obtained independently, recalculation and adjustment of the 4 scales can be performed, for example, by ray-tracing software or calibration tables calculated in advance (see, for example, the calibration graph provided in Figure 3).

The human eye has two main optical elements, the anterior optical element, the cornea, 5 in combination with the anterior chamber, henceforth: cornea, and the posterior optical element, the crystalline lens, henceforth: lens. The optical properties of the cornea must be taken into account for adjustment of measurement in the anterior chamber, i. e. up to the iris-plane, and the optical properties of the combination of cornea and lens must be taken into account for measurements beyond the iris-plane, for example for 10 measurement in the retinal plane.

The degree of adjustment must correspond to the degree of, at least one, difference in, at least one, optical property between the patient eye and the model eye which difference must be know a priori. Such difference is determined by, for example, calculation of the 15 optical power of the cornea following measurement of its radius/curvature, or by any appropriate ophthalmologic measurements which can also determine the optical properties of the combination of the cornea and lens. The degree of difference is found by subtraction of the optical properties of optical elements of the patient eye from the optical properties of the corresponding optical elements of the model eye or vice versa, 20 depending on how algorithms of the digital processing means are structured.

The method can provide adjustment by placing, at least one, additional optical element in the optical path from the patient eye to the imaging apparatus, for example, a spectacle lens, contact lens, or any lens in between the eye and the imaging apparatus.

25 The additional optical element should have such optical properties that the differences between the patient eye and the model eye can be adjusted, or corrected in a known manner. Said differences can be determined prior to measurements, for example, with a separate refractometer apparatus or cornea topograph. Said adjustment can also be achieved by, at least one, additional digital processing step to correct, at least one, scale 30 for differences in optical properties optical properties between the patient eye and the model eye which differences can be determined prior to measurements.

For example, to measure the iris diameter and adjust the lateral scale of the resulting image the procedure can be as follows: - the radius of curvature of the cornea is 5 measured by, for example, a corneal topographer, - the adjustment factor related to the corneal curvature is determined by, for example, a prepared graph (as in, for example, Figure 3), the diameter of the iris is measured by, for example, an OCT apparatus and the measurement result is corrected by said adjustment factor determined a priori.

5 Alternatively, the adjustment factor can be included in the measurement of the iris diameter directly by adding the appropriate optical correction by additional optical elements, for example, adjusting said variable optical elements to the appropriate values.

10 Note that an astigmatism of the patient eye will result in perception of rotational asymmetry in the image and, for example, result in diagnosis of an ellipsoidal white-to-white rim or ellipsoidal sulcus. Knowledge of the degree of astigmatism, by, for example, cornea topography, prior to the measurement, provides a basis for correction of such perceived rotational asymmetry.

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The apparatus. - The imaging apparatus, for example, an OCT apparatus, comprises imaging optics to project incident light on a sensor, optics and assembly for positioning and targeting, said sensor, digital processing means, display means and appropriate imaging software, for example, software embedded in an OCT apparatus. Apparatus for 20 scaling and/or additional processing means for scaling can be designed according to methods outlined above. Such apparatus provides adjustment of the imaging apparatus scale, based on a model eye, of an image of, at least one, structure inside a patient eye by adjusting said scale for differences in, at least one, optical property of, at least one, optical element of the patient eye and corresponding optical properties of corresponding 25 optical elements of said model eye.

Said apparatus can provide adjustment of the scales by, at least one, additional optical element in the optical path from the patient eye to the imaging apparatus. A construction may include, for example, one or multiple variable, optionally actuator-driven, optical 30 elements for variable defocus in combination with variable astigmatism and in combination with other desired variable aberrations which construction can be part of the imaging apparatus and which optics can, as will be concluded by a man skilled in the arts, be adapted from optics disclosed in W02007037691, which discloses camera modules, in WO2010024668, technical and automotive applications and 6 US2010094413, intraocular lenses, and which documents are considered part of this document by reference.

Clearly, additional processing means can be adapted to calculate alternative scales based 5 on known a priori differences of optical properties of optical elements of the patient eye and the model eye.

Claims

Method for setting at least one scale of an image-forming device on the basis of a model eye of an image provided by an image-forming device of at least one structure in an eye of a patient, characterized in that the method is arranged to enable adjustment for at least one difference between at least one optical characteristic of at least one optical element of the patient's eye and the corresponding optical characteristic of the corresponding optical element of the model eye. 10

Method of adjustment according to claim 1, characterized in that the optical property is defocus aberration.

3. Method of adjustment according to claim 1, characterized in that the optical property is astigmatic aberration.

Method of adjustment according to claim 1, characterized in that the optical property is a combination of defocus aberration and astigmatic aberration. 20

The adjustment method according to claims 1 to 4, characterized in that the optical element is the comea.

6. Method of adjustment according to claims 1 to 4, characterized in that the optical element is a combination of the comea and the lens.

7. Method of adjustment according to claims 1 to 6, characterized in that the degree of adjustment corresponds to at least one degree of difference in at least one optical characteristic between the patient's eye and the model eye, which degree of difference is known a priori.

Method of adjustment according to claims 1-6, characterized in that the method is adapted to enable adjustment by at least one additional optical element with optical properties in the optical path from the eye of the patient to the imaging device to adjust at least one optical characteristic of the patient's eye relative to the corresponding optical characteristic of the model eye.

A method of adjustment according to claims 1-6, characterized in that the method is adapted to enable adjustment by at least one additional digital processing step to adjust at least one optical characteristic of the patient's eye relative to of the corresponding optical property of the model eye. 10

Device for setting the scale of the image-forming device, based on a model eye of an image of at least one structure in the eye of a patient, characterized in that the image-forming device is adapted to set the scale for the difference between at least one optical characteristic of at least one optical element of the patient's eye and the corresponding optical characteristics of the corresponding optical elements of the model eye.

11. Device as claimed in claim 10, characterized in that the device is adapted to provide the adjustment by at least one additional optical element in the optical path.

12. Device with additional optical element according to claim 11, characterized in that the additional optical element is a variable optical element which is adapted to provide combinations of variable defocus and variable astigmatism.

Device as claimed in claim 10, which is adapted to adjust the scale, characterized in that the digital processing means comprise additional digital processing means which are adapted to enable the adjustment. 30