Method for realizing ocular fundus photographs that are corrected for scattered light

Abstract

A method by which fundus photographs that are corrected for scattered light can be realised by using a digital fundus camera. The fundus is illuminated and a photograph of the fundus is realized. The photographed area is greater than the illuminated are. The scattered light intensity is determined and is used for correction. Measurement points for determining the scattered light intensity are defined in the non-illuminated area. Values of the scattered light intensity determined at the measurement points are averaged. Average-scattered light intensity thus determined is subtracted from the intensity values of the photograph to correct the photograph.

Claims

1. A method for realizing ocular fundus images that are corrected for scattered light using a digital fundus camera, the method comprising: a) illuminating the ocular fundus by application of an illumination source located within the digital fundus camera and directed through optics of the digital fundus camera into an eve and onto the ocular fundus, and b) capturing a digital image of the ocular fundus through the optics of the digital fundus camera in which an imaged surface area of the ocular fundus is larger than an illuminated surface area of the ocular fundus, wherein in the digital image of the ocular fundus realized in b) a scattered light intensity is determined and is used for a correction, the method further comprising c) defining, by application of the digital fundus camera, measurement points or a measurement area, the measurement points or the measurement area for determining the scattered light intensity being located in a non-illuminated area of the ocular fundus adjacent to or surrounding the illuminated area of the ocular fundus; d) in the case of measurement points in the non-illuminated area of the ocular fundus, eliminating outliers and after the outliers have been eliminated, averaging values of scattered light intensity determined at said measurement points to determine an averaged scattered light intensity by application of the digital fundus camera; and e) using the average scattered light intensity thus determined to correct said image of the ocular fundus realized in method step b) or to define the severity of a cataract by application of the digital fundus camera.

2. The method in accordance with claim 1, further comprising: a) illuminating the ocular fundus at an illumination angle , and b) capturing a digital image of the ocular fundus at a field of view angle , wherein <.

3. The method in accordance with claim 1, further comprising, for determining the scattered light intensity in accordance with method step c), utilizing at least 5 of the measurement points.

4. The method in accordance with claim 3, further comprising, utilizing at least 10 of the measurement points.

5. The method in accordance with claim 3, further comprising utilizing at least 20 of the measurement points and distributing the measurement points as uniformly as possible in the non-illuminated area of the fundus.

6. The method in accordance with claim 1, wherein for defining the measurement points in accordance with method step c), it is necessary for both the non-illuminated and also the imaged area of the fundus either to be known or detected.

7. The method in accordance with claim 6, further comprising, during the determination of the non-illuminated area of the fundus, taking into account the ametropia of the eye to be examined.

8. The method in accordance with claim 6, further comprising detecting an edge of the non-illuminated area of the fundus using image processing methods.

9. The method in accordance with claim 8, wherein the image processing methods used are adapted for detecting geometric shapes of a light screen.

10. The method in accordance with claim 8, wherein the image processing methods used are based on a Hough transformation.

11. The method in accordance with claim 1, further comprising taking into account a safety margin for the measurement points to be defined in method step c), both to the illuminated area of the fundus and also to the outer edge of the imaged area.

12. The method in accordance with claim 1, further comprising, in method step c), wherein the measurement area is used and the measurement area in the non-illuminated area of the fundus has the shape of a circular ring.

13. The method in accordance with claim 12, wherein the measurement area in the shape of a circular ring that is defined for determining the scattered light intensity has a safety margin .sub.1 to an illumination angle and a safety margin .sub.2 to a field of view angle .

14. The method in accordance with claim 12, further comprising, in method step d) using intergration to determine the scattered light intensity across the measurement area.

15. The method in accordance with claim 1, further comprising checking the ocular fundus image realized in method step b) with respect to maintaining a minimum standard of image quality prior to further processing.

16. The method in accordance with claim 15, wherein the minimum standard of image quality is attained with respect to dynamics and brightness if the structures of the fundus are detectable.

17. The method in accordance with claim 15, wherein the minimum standard of image quality has been attained with respect to correct orientation of the fundus camera if the fundus image has homogeneous illumination.

18. The method in accordance with claim 15, wherein, if the minimum standards of image quality are not maintained, the image of the ocular fundus realized in method step b) is discarded.

19. The method in accordance with claim 1, further comprising: e) using the average scattered light intensity thus determined in order to correct said digital image of the fundus realized in method step b) by subtracting the average scattered light intensity thus determined from the intensity values of this fundus image.

20. The method step in accordance with claim 1, further comprising: e) using the average scattered light intensity thus determined in order to define the severity of a cataract, by relating average (absolute) scattered light intensity thus determined to an intensity of a flash lamp of the fundus camera and defining a measure for the severity of a cataract in the eye to be examined as relative scattered light intensity.

21. The method in accordance with claim 20, further comprising storing the relative scattered light intensity and comparing the relative scattered light intensity to prior measured values, later measured values or both prior measured values or later measured values to draw conclusions therefrom regarding the severity of a cataract and any changes.

22. The method in accordance with claim 20, further comprising converting the value of the relative scattered light intensity to diagnostic values for other cataract measurement methods.

23. The method in accordance with claim 22, further comprising converting the value of the relative scattered light intensity to diagnostic values to values of a Scheimpflug camera or a slit lamp.

24. The method in accordance with claim 20, further comprising using a mathematical regression for converting the values of the relative scattered light intensity to diagnostic values for other cataract measurement methods.

25. The method in accordance with claim 12, wherein for defining the measurement points in accordance with method step c), it is necessary for both the non-illuminated and also the imaged area of the fundus either to be known or detected.

26. The method in accordance with claim 25, further comprising, during the determination of the non-illuminated area of the fundus, taking into account the ametropia of the eye to be examined.

27. The method in accordance with claim 25, further comprising detecting an edge of the non-illuminated area of the fundus using image processing methods.

28. The method in accordance with claim 27, wherein the image processing methods used are adapted for detecting geometric shapes of a light screen.

29. The method in accordance with claim 27, wherein the image processing methods used are based on a Hough transformation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a fundus image in which the imaged surface area is larger than the illuminated surface area;

(2) FIG. 2 is the fundus images of two eyes in which the cataracts are of differing severity;

(3) FIG. 3 is a fundus image in which the measurement area for determining the scattered light intensity in the non-illuminated area is marked; and

(4) FIG. 4 is a fundus image in which the minimum standard of image quality is not maintained with respect to dynamics and brightness.

DETAILED DESCRIPTION

(5) In the inventive method for realizing ocular fundus images that are corrected for scattered light using a digital fundus camera, according to the method steps: a) the ocular fundus is illuminated, and b) an image of the ocular fundus is realized in which the imaged surface area is larger than the illuminated surface area.

(6) In the image of the ocular fundus realized in method step b), the scattered light intensity is determined and used for a correction, in that: c) measurement points for determining the scattered light intensity are defined in the non-illuminated area of the fundus, d) after outliers have been eliminated, the values of the scattered light intensity determined at said measurement points are averaged, and e) the average scattered light intensity thus determined is subtracted from the intensity values of the realized fundus image in order to correct said image of the fundus realized in method step b).

(7) One requirement for performing the inventive method is to use a fundus camera that offers the option for the imaged field of view to be larger than the illuminated field of view. This is the case, for instance, with fundus cameras that provide the option of being able to image different angle ranges without optical enlargement.

(8) In accordance with the invention, according to the method steps: a) the ocular fundus is illuminated at an illumination angle , and b) an image of the ocular fundus is realized at a field of view angle , wherein <.

(9) Most fundus cameras offer the option of making images with different illumination angles. Typically images may be made with a 30 aperture angle or with a 45 aperture angle. The suggested method is based on the idea that the fundus is illuminated at an angle of 30 but the larger area of the fundus is imaged by the sensor at 45. However, the inventive method is not limited to these two angles, but instead may even be used for other angles in the same manner.

(10) In this regard, FIG. 1 depicts an image of a fundus in which the imaged surface area is larger than the illuminated surface area. The illuminated surface area is labelled F.sub.B and the non-illuminated surface area is labelled F.sub.U. The imaged surface area F.sub.A thus corresponds to the sum of the two surfaces F.sub.B and F.sub.U. The total rectangular surface area represents the surface area F.sub.BS of the image sensor.

(11) In an ideal case, i.e. without any occurring scattered light, the illuminated surface area F.sub.B would be imaged on the image sensor in front of a black background. Since fewer or more scattering particles are present in the anterior ocular media depending on the severity of the cataract, the illumination light is scattered on these particles, both in the direction of the fundus and in the opposite direction. The scattering of the illumination light towards the retina occurs in two dimensions and at a wide angle so that the entire surface area F.sub.A to be imaged is illuminated.

(12) The scattered light that occurs increases as the severity of the cataract increases, which leads to the actually non-illuminated surface area F.sub.U being illuminated with increasingly higher intensity.

(13) To illustrate this, FIG. 2 provides fundus images of two eyes in which the cataracts are of different severity. While the fundus image on the left shows an eye having a minor or no cataract (that is, little scattered light), the cataract in the eye shown in the fundus image on the right is severe, which may be seen from the greater proportion of scattered light.

(14) The light scattered towards the retina (forward) may be interpreted like additional illumination with a very wide illumination angle. This leads to the illuminated surface area F.sub.B being further illuminated and the actual non-illuminated surface area F.sub.U being illuminated.

(15) In addition, a worsening of the signal-to-noise ratio (SNR) is caused by the direct back-scattering of light on particles of the anterior ocular media in the observation beam path. This is expressed inter alia in a worsening of contrast. As may be seen from the fundus image on the right in FIG. 2, structures are no longer imaged as clearly because of this.

(16) The fundus image is overlaid by interfering scattered light in two dimensions. Thus, due to this scatter effect, the intensities I.sub.B of the fundus image that are present is overlaid by the constant scatter intensity I.sub.S, which is an addition. If the scatter intensity I.sub.S is known, it is possible to calculate the corrected fundus image with the intensities I.sub.B by subtracting the scatter intensity I.sub.S from the intensities of the fundus image affected by scatter.

(17) If it is assumed that the fundus image is overlaid in two dimensions by the interfering scattered light, an equivalent for this may be determined in the outer area of the fundus image and carried over to the entire fundus image. It may be seen from the image on the right in FIG. 2 that the brightness in the outer area is nearly uniform.

(18) In addition, no more structures are detectable in the outer area starting at a certain distance from the transition region from the illuminated to the non-illuminated area. This means that the scattering is very uniform and that the average value in the actually non-illuminated outer area represents a good measure for the average absolute scattered light. In particular the averaging is also robust relative to noise effects over a large range.

(19) For determining the scattered light intensity in accordance with method step c), at least 5 measurement points are used according to an example embodiment, preferably 10 measurement points are used according to another example embodiment , and especially preferably 20 measurement points distributed as uniformly as possible are defined in the non-illuminated area of the fundus are used according to a further example embodiment.

(20) To be able to define the measurement points in accordance with method step c), it is necessary for both the non-illuminated and also the imaged area of the fundus either to be known or detected.

(21) It should be noted that, despite constant illumination parameters, the size of the illuminated area of the fundus may vary. This is highly dependent on the patient's ametropia. Therefore it appears reasonable to maintain a safety margin from the transition region between the illuminated area and the non-illuminated area when the scattered light intensity is determined in the non-illuminated area.

(22) If the eye to be examined suffers from ametropia, the ametropia should be taken into account during the determination of the non-illuminated area of the fundus. Since as a rule the ametropia to be examined must be known in order to be able to focus the fundus camera on the ocular fundus that is to be imaged, the non-illuminated area of the fundus should also be known. For fundus cameras in which focusing on the fundus takes place automatically, i.e. without the knowledge of the specific ametropia, it is necessary to detect the non-illuminated area of the fundus.

(23) The non-illuminated area of the fundus, especially its edge, is detected using known image processing methods that are primarily adapted for detecting geometric shapes of a light screen and are based, for instance, on a Hough transformation.

(24) In an example embodiment, a safety margin for the measurement points to be defined in method step c) is taken into account, both to the illuminated area of the fundus and also to the outer edge of the imaged area. This can minimize the occurrence of so-called outliers. The safety margin to be selected may be smaller if the non-illuminated and the imaged area of the fundus are known precisely or were detected using image processing methods.

(25) In accordance with a first advantageous example embodiment of the inventive method, in method step c), instead of individual measurement points for determining the scattered light intensity, a measurement area in the non-illuminated area of the fundus may be defined that, for example, has the shape of a circular ring. The determination of the scattered light intensity in accordance with method step d) is then accomplished using integration across the defined measurement area.

(26) In accordance with example embodiments of the invention, the safety margin is also provided in the form of an angle, wherein .sub.1 characterizes the safety margin to the illuminated area of the fundus or the illumination angles , and .sub.2 characterizes the safety margin to the outer edge of the imaged area or to the field of view angle . The circular ring is thus defined by the following outer limits: (+.sub.1) and (.sub.2).

(27) The safety margins .sub.1 and .sub.2 may assume the same or different values.

(28) FIG. 3 provides a fundus image in which the measurement area for determining the scattered light intensity in the non-illuminated area is marked. In this case, as well, the imaged surface area F.sub.A equals the sum of the illuminated surface area F.sub.B and the non-illuminated surface area with F.sub.U. For determining the scattered light intensity in the non-illuminated area F.sub.U, a measurement area is defined that is characterized by the two circular rings K.sub.R1 and K.sub.R2.

(29) Here, as well, it is possible to use the method steps described in the foregoing with respect to knowing both the non-illuminated and also the imaged area of the fundus, or detecting them, and to take into account the ametropia of the eye to be examined and maintain a safety margin.

(30) The absolute intensity of the scattered light correlates to the quantity and size of the scattering particles in the anterior media of the human eye and thus, e.g., to the severity of cataract. Moreover, the intensity I.sub.S of the scattered light is approximately a linear function of the intensity I.sub.BB of the flash lamp.

(31) In accordance with one example embodiment of the inventive method, the average (absolute) scattered light intensity determined in method step e) is related to the intensity of the flash lamp of the fundus camera and define a measure for the severity of a cataract in the eye to be examined as relative scattered light intensity.

(32) While the portion relative to the entire light intensity is understood to be a relative portion of the scattered light, the absolute portion is the absolute quantity of the scattered light.

(33) In this context, it is advantageous to store the relative scattered light intensity and compare it to prior and/or later measured values in order to be able to draw conclusions therefrom regarding the severity of a cataract and any changes.

(34) The determined magnitude of the scattered light may also be compared to age-based characteristics in order to be able to detect deviations. Major deviations from an age-based characteristic may provide indications of an eye disorder that is present.

(35) The absolute intensity of the scattered light correlates to the quantity and size of the light scattering particles in the anterior media of the human eye and thus also, e.g., to the severity of a cataract. Moreover, the scattered light has an approximately linear relationship, i.e., if the back-scatter is ignored, to the intensity of the flash lamp and is thus a measure of the severity of the cataract. The inventive method thus permits the severity of the cataract to be measured with a digital fundus camera.

(36) In addition, it is advantageous to convert the value of the relative scattered light intensity to diagnostic values for other cataract measurement methods, such as for instance the values of a Scheimpflug camera or a slit lamp.

(37) A mathematical regression may for example be used for converting the values of the relative scattered light intensity to diagnostic values for other cataract measurement methods.

(38) In accordance with another example embodiment of the inventive method, the ocular fundus image realized in method step b) is checked with respect to maintaining the minimum standard of image quality prior to the further processing, wherein these minimum standards are attained with respect to dynamics and brightness if typical structures such as arteries, veins, pupils, or the area of the macula are detectable in the illuminated area.

(39) The minimum standard of image quality has been attained with respect to correct orientation of the fundus camera if the fundus image has homogeneous illumination.

(40) If the minimum standards of image quality are not maintained, the image of the ocular fundus realized in method step b) is discarded and the scattered light intensity is not determined.

(41) In such a case, the operator of the fundus camera receives, for example, an indication that quality criteria have not be satisfied and reproducibility is reduced for poor image quality.

(42) In this regard, FIG. 4 is a fundus image in which the minimum standard of image quality is not maintained in terms of dynamics and brightness. In contrast to the previous images, this image includes only the illuminated surface F.sub.B, since only this illuminated surface is examined with respect to maintaining the minimum standards. However, the minimum standard for image quality with respect to dynamics and brightness is not maintained here, since the illuminated surface area F.sub.B is over-illuminated at the bottom left.

(43) With the inventive solution, a method is provided with which it is possible to realize ocular fundus images that are corrected for scattered light using a digital fundus camera. With the suggested method, the relative and absolute quantity of scattered light may be measured during the realization of fundus images. Thus the present invention makes it possible to measure and evaluate the severity of the cataract with a digital fundus camera.

(44) The solution is for example based on a digital fundus camera that is adequately known from the prior art and has a simple technical structure and is relatively simple to operate. Using it, it is possible for the determined measurement values to be highly accurate, reliable, and reproducible.

REFERENCE LIST

(45) F.sub.B Illuminated surface area F.sub.U Non-illuminated surface area F.sub.A Imaged surface area F.sub.BS Surface area of the image sensor I.sub.S Scatter intensity I.sub.B Intensity of the fundus image in the illuminated area I.sub.BB Flash intensity of the illumination light K.sub.R1, K.sub.R2 Circular rings for defining the measurement area Field of view angle Illumination angle Angle extension of circular ring .sub.1, .sub.2 Safety margins