Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2000-05-26
2002-04-30
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06379006
ABSTRACT:
The present invention relates to a method and apparatus for the stereoscopic examination of the fundus of the eye. This invention has applications in the investigation and diagnosis of diseases that affect the posterior chamber of the eyeball. The invention will be described in reference to the above application, however, it is possible and envisioned that the apparatus and technique of the present invention may be used for stereoscopic imaging in other medical processes.
Visualisation of the ocular fundus can provide important information about the state of the eye and of the body. Knowledge regarding ocular and systemic diseases, such as glaucoma, macula degeneration, diabetes or hypertension can be gained from examination of the posterior pole of the eye. In the past, imagining of the ocular fundus has been performed through the use of an ophthalmoscope, in which a direct view of the retina may be obtained. Other methods include the use of fundus cameras to obtain photographic images. However, these techniques usually require the use of mydriatic dilating drugs. The amount of light required to illuminate the fundus may also be uncomfortable for the patient.
Recent developments have resulted in the emergence of a new imaging tool for the ophthalmologist, in which an image of the eye may be observed in real-time and captured on a television monitor or screen, during procedures such as fluoreacein angiography. This instrument, known as a scanning laser ophthalmoscope (SLO), first described in U.S. Pat. No. 4,213,678, is currently used to produce representations of the ocular fundus in two dimensions. U.S. Pat. Nos. 4,765,730, 5,268,711 and 5,430,509 describe different embodiments of the scanning laser ophthalmoscope. All utilise a laser beam or light source that is directed through the pupil and onto the retina by way of two, directional, scanning mechanisms. The light from the laser is reflected off the retinal wall towards a photosensitive detection device. Electro-optical circuitry is employed to convert the light into synchronized signals, so that it is possible to display an image of the fundus on a television screen or monitor.
U.S. Pat. No. 5,430,509 describes a different embodiment of the scanning laser ophthalmoscope in which three or more scanning mechanisms are employed to develop a video image of the fundus. Scanning occurs in three directions, horizontal, vertical and in the direction of the optical axis. The inventors suggest that the use of three scanning devices, each with a different scanning frequency, will reduce the demands and wear on the scanner's bearings. However, despite the three scanning mechanisms, only a two dimensional image can be reconstructed on screen in real time.
The prior SLO technology described herein above does not permit stereoscopic viewing of the ocular fundus. Nevertheless, the optic disc region and retinal layers have three dimensional structure. Stereoscopic images of the ocular fundus can impart valuable information that cannot otherwise be derived from a two dimensional representation, especially in relation to the diagnosis of glaucoma. Efforts have therefore been made to create a device capable of producing three dimensional fundus images, while improving on the contrast and resolution of conventional SLO images.
Frambach, Dacey & Sadun (1992, 1993) describe a method of producing a three dimensional fundus picture during fluorescein angiography, using a modified SLO. To obtain stereoscopic data the SLO was manually moved from side to side during angiogram proceedings, much like a fundus camera is moved to enable viewing from two different positions. Individual frames from the video tape were chosen from left and right perspectives to provide a three dimensional image. An alternative approach employed by the authors involved the use of a modified Allen separator. A piece of flat glass was attached to an extended rod, coupled to the Allen separator, so that the glass was interposed between the eye and the SLO. The glass was then rapidly rotated to provide the left and right perspectives. The resulting frames were digitized by computer and viewed directly on a video screen. Superimposed images were formed by breaking a stereo pair down into corresponding fields and recombining them to form a single frame. LCD glasses were then used to view the left and right fields with the corresponding eye.
Frambach et al (1993) illustrate that achieving a stereoscopic image from a conventional scanning laser ophthalmoscope is possible. However, the methods involved exhibit a number of disadvantages. Frambach et al's first method of shifting the SLO involved awkward and confusing adjustments, resulting in poor stereoscopic image quality. In the second method, interference due to unwanted back reflections from the Allen separator would hinder in the transmission of stereoscopic information. Unwanted scattered light would impinge on the photodetecting element, causing a decrease in the contrast and resolution of the images.
Improvements in SLO image resolution and contrast are possible if the detector receives light only from the plane of interest and not scattered light from the media of the eye. A scanning laser ophthalmoscope that could provide high resolution, high contrast images of an ocular fundus was realised with the invention of the confocal scanning laser ophthalmoscope (cSLO), such as that described in U.S. Pat. Nos. 5,170,276 and 5,071,246. The confocal SLO utilises a pinhole or slit aperture to focus the light reflected from the fundus onto a photodetecting element. The aperture is located at a plane in which the opening is conjugate with the plane of the fundus of the eye. In this way, only the light reflected from the plane of interest impinges on the photodetecting element and any light scattered or reflected from out-of-focus planes is prevented from degrading the image.
A further technique to produce three dimensional images of the ocular fundus, known as scanning laser triangulation, is described by Milbocker & Reznichenko (1991). Triangulation is a method commonly used for measuring distances. Combined with a confocal aperture, this method involves synchronized scanning of a pixel of light across the fundus by way of two mirrors. The illumination and detection paths are arranged symmetrically and are defined by the two mirrors. The axial distance is measured by the displacement of the illuminated spot in the confocal plane, enabling calculation of the depth. (That is, the points above and below the ‘average’ position of the retinal wall). Disadvantages of this method include impractical computational speeds for clinical practice and an inflexible stereo base; large pupil and numerical apertures cannot be used with this technique.
Confocal scanning laser ophthalmoscopes are also currently used to provide three dimensional information concerning the ocular fundus. The confocal aperture of the cSLO allows the user to precisely focus on specific layers of the retina. By adjusting the focal plane of the aperture, images can be captured at different levels in the fundus, to reproduce desired depth characteristics. In this way a number of “optical sections” can be produced. A computer can then be used to extract depth information, through the process of “stacking” a selection of the optical sections taken at different levels of the retina. Information regarding the third dimension can therefore be interpolated.
Nevertheless, stereoscopic imaging via the method described is a time consuming process. A large number of images must be acquired at different focal planes. Interference from patient head and eye movement is likely to influence the resultant three dimensional image. Computer processing times must also be taken into account. These factors make confocal sectioning impractical in a clinical setting, where real time images are required for fast diagnosis or treatment.
U.S. Pat. No. 4,900,144 (also see Optics Communications: 87(1,2):
9-14
) describes a scanning laser ophthalmoscope that utilises an alternative confocal
Eikelboom Robert Henry
Reinholz Fred Norbert
Van Saarloos Paul Phillip
Baker & Botts LLP
Manuel George
The Lions Eye Institute of Western Australia Incorporated
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