A method for analyzing a stereoscopic image of a fundus, and an apparatus for executing that method

Abstract: A method for analyzing an optic disk part on the stereoscopic image of the fundus, which can provide highly reliable results, even if the fundus is inclined. The method includes: a first step of designating a disk line in the stereoscopic image of the fundus; a second step of determining a lowest point in a disk region which is determined by the designated disk line; a third step of determining a cross section of the disk region at each predetermined angle centering on the lowest point; a fourth step of determining highest points at both ends of a line of longitude on each cross section of the disk region; a fifth step of setting as a cup point a level which is lower from the highest points by a predetermined depth; and a sixth step of forming a cup rim by consecutively connecting the cup points in the cross sections of the disk region determined in the fifth step.

Description: Background of the invention The present invention relates to a method for analyzing a stereoscopic image of a fundus to detect and analyze the optic disk on the basis of the stereoscopic image of the fundus, and an apparatus for executing that method. Glaucoma is known as a disease of the visual function. In the examination of glaucoma, it is said that the observation of patient's optic disk on the basis of an image of the fundus is important. In recent years, it has become possible to quantitatively measure the optic disk due to progress in the technology of image analysis. As the results of analysis, the following parameters are computed: the C/D ratio (a distance ratio between a rim of an optic disk cupping (the optic disk cupping being referred to as a cup) and a rim of the optic disk (the optic disk being referred to as a disk)), the area of the plane within the cup rim, the depth of the cup, the volume of the cup, and the like. The rim of the cup which serves as a basis for the computation of these parameters is conventionally detected such that, by using the disk as a reference, positions which are lowered by an arbitrary depth (e.g., 150 µm) from the disk are estimated to plot the rim of the cup. However, since it is not uncommon that overall disk regions of the patent's eyes are inclined, there arise cases where the rim of the cup cannot be detected by the above-described detection method. It is possible to obviate the above-described problem if an operator designates three points on the disk region and effects analysis by using as a reference plane an inclined plane thus determined. In accordance with this method, however, there is a problem in that the data of the cups detected vary for each operator, and the data can be different on each occasion of analysis even for the same operator, with the result that the reliability and reproducibility of the results of analysis are poor. T. TAKAMOTO ET AL.: 'Stereo measurement of the ocular fundus in ophthalmology' PROCEEDINGS OF SPIE, vol. 361, 24-27 August 1982, SAN DIEGO (US), pages 324-329, discloses a method for assessing ocular fundus topography. The method comprises the steps of taking three to five independent stereophotographs of an optic disc with a fundus camera. Discrete points on each stereophotograph of the disc are digitized in the three-dimensional corrdinates and stored. Then, the digital model is transformed into a polar coordinate system in which the horizontal plane is parallel to the disk margin with the pole at its center of gravity. From the transformed digital model radial sections from the center of the optic disc, the pole, are produced for each discrete radial angle using nearby spot readings. In view of the above-described problems of the conventional art, a technical task of the present invention is to provide a method and an apparatus for analyzing a stereoscopic image of the fundus which make it possible to obtain highly reliable results of analysis even if the fundus is inclined and which give excellent reproducibility. According to the invention, the object is solved by the features of the independent claims. The respective sub claims contain further preferred developments of the invention. Brief description of the drawings In the accompanying drawings: Fig. 1 is a schematic block diagram of the apparatus in accordance with an embodiment; Fig. 2 is a diagram illustrating a flowchart of the overall analysis of an image of the fundus of the eye; Fig. 3 is a diagram illustrating a flowchart for detecting the cup; Fig. 4 is a diagram explaining the steps for determining the cup level in a cross section of the disk; Fig. 5 is a diagram explaining a method for determining cup points when there are a plurality of points which reached the cup level; and Fig. 6 is a diagram explaining a method for obtaining a cup plane S. Detailed description of the preferred embodiment Referring now to the drawings, a description will be given of an embodiment of the present invention. Fig. 1 is a schematic block diagram of an apparatus in accordance with the embodiment. An analyzing apparatus 1 is comprised of a computing and analyzing unit 2, a keyboard 3 and a mouse 4 which constitute a designation input section, a display 5 for displaying an image of a fundus, the results of analysis, and the like, and a printer 6. A commercially available personal computer can be used as the analyzing unit 2. Also, commercially available ones can be used as the other component parts. Reference numeral 7 denotes a stereoscopic retinal (fundus) camera, wherein a bundle of illumination light rays reflected from the fundus is separated into two bundle of rays by a bifurcating (two-hole) diaphragm to obtain a pair of right and left stereoscopic images. The stereoscopic image of the fundus photographed by the stereoscopic retinal (fundus) camera 7 is converted into image data by an image reading apparatus 8, and is inputted to the computing and analyzing unit 2. As the stereoscopic retinal (fundus) camera 7, it is possible to use a type in which image of the fundus is photographed on a transparency film, or a type in which an image on the fundus is photographed by a CCD camera. In the case of the latter type, after the image of the fundus is recorded in an image recording means as a still picture, the image of the fundus is inputted into the computing and analyzing unit 2 directly or via a recording medium such as a floppy disk. Next, a description will be given of the analysis of an image of the fundus by means of the computing and analyzing unit 2 (see Figs. 2 to 5). First, a pair of stereoscopic image data of the fundus photographed by the stereoscopic retinal (fundus) camera 7 is inputted into the computing and analyzing unit 2. The computing and analyzing unit 2 incorporates correction based on a parallax, a magnification of image formation, a distortion due to aberrations and the like into the inputted stereoscopic image data, subjects the corrected stereoscopic image data to stereoscopic matching, and obtains three-dimensional data of the fundus on the basis of the same. As the stereoscopic matching, it is possible to use an apparatus disclosed in Japanese Patent Application Laid-Open No. 8-567 (Title of the Invention: Fundus Oculi Measuring Instrument). Subsequently, an outer peripheral line of the disk is designated. On the basis of the three-dimensional data of the fundus, a bird's-eye view (or a triangular net) showing a three-dimensional image (configuration) of the fundus and a front view showing a planar image (frontal, planar configuration) of the fundus are displayed on the display 5, and the operator designates a disk region in the displayed three-dimensional image or planar image of the fundus. The designation of the disk region is effected as follows. Since a pointer for designating an outer peripheral line of the disk is displayed in the three-dimensional image and the planar image on the display 5, the pointer is moved and then the mouse 4 is clicked to designate a multiplicity of (preferably, 10 or more) points on the outer peripheral line of the disk. After the designation of the outer peripheral line of the disk, the operator instructs the execution of cup analysis processing to the computing and analyzing unit 2. The computing and analyzing unit 2 first effects the detection of the cup. The detection of the cup is effected in accordance with the flowchart shown in Fig. 3. The designated points are connected by a quartic spline curve in such a manner as to form a smooth line, and that line is depicted in such a manner as to be superposed on the planar image on the display 5. By depicting the designated points by the spline curve, it is possible to determine the outer peripheral line of the disk in a shape which approximates a real one very closely. In addition, it is possible to cope with even the outer periphery of the disk which has a distorted shape. The computing and analyzing unit 2 sets its inner side as the disk region, and determines the position of its center of gravity (Step 1). The position of the center of gravity serves as a tentative central position, on the basis of which the position of a lowest point within the disk region is determined. The computing and analyzing unit 2 determines a cross section of the disk for each predetermined detection angle (e.g., for each 1 degree) by using as the center the position of the center of gravity determined in Step 1 (Step 2), and determines a lowest point (a deepest point) for each cross section of the disk (Step 3). A minimum lowest point as well as its position in the disk region are determined among the lowest points for the respective cross sections of the disk determined in Step 3 (Step 4). By using as the center the position of the minimum lowest point determined in Step 4, the cross section of the disk is determined again for each predetermined detection angle (e.g., for each 1 degree) (Step 5). Further, lowest points in the respective cross sections of the disk determined in Step 5 are determined (Step 6), and a minimum lowest point is determined among the lowest points determined in Step 6 (Step 7). This minimum lowest point determined in Step 7 approximates a true lowest point within the disk region. Two highest points in both directions between the lowest point in each cross section of the disk determined in Step 6 and each of the disk point on either side (either end of the cross section of the disk) are respectively determined (Step 8), and the level which is lower from each of the determined highest points by a predetermined distance (e.g., 150 µm) is set as each cup level (Step 9, see Fig. 4). Points on the cup level are searched in each cross section of the disk left and right directions from the lowest point determined in Step 6, the number of points which reached the cup level are found (Step 10), and if the number of points which reached the cup level is one for each of the left and right directions from the lowest point determined in Step 6, that point is set as the cup point (Step 11). When there are a plurality of points which reached the cup level, a point which is closest to the lowest point determined in Step 6 is set as the cup point (Step 12, see Fig. 5). The cup points determined for the respective cross sections are connected to obtain the cup rim (Step 13). Through the above-described method, even if the plane of the disk is inclined, it is possible to detect the cup rim accurately and improve reproducibility as well. Subsequently, the computing and analyzing unit 2 computes various parameters of the optic disk on the basis of the computed data of the cup. In this embodiment, in the computation of the parameters of the optic disk, as shown in Fig. 6, a cup plane S is obtained by taking a highest point of the cup rim as a reference position and by projecting a curved surface S' of the cup defined by the cup rim onto a horizontal plane (reference plane) including the reference position. The outer peripheral line of this cup plane S is used in computation as the cup rim data. As the parameters of the optic disk, the following are computed among others: the distance ratio (C/D ratio) between the cup rim and the disk rim, the area (cup area) of the plane within the cup rim, the area ratio (C/D area ratio) between the cup and the disk, the depth of the cup (maximum cup depth), and the cup volume. It should be noted that the parameters of the maximum cup depth and the cup volume are computed by incorporating correction processing in which the values are shifted in correspondence with the cup plane S. The computed parameters of the optic disk are displayed on the display 5, and are stored and recorded. Through the parameters of the optic disk obtained in the above-described manner, it is possible to quantitatively measure the optic disk, which can be made useful in the diagnosis of glaucoma. As described above, in accordance with the present invention, it is possible to reduce variations due to errors among operators and repeated analysis, and obtain highly reliable results of analysis. In addition, it is possible to improve the reproducibility of the results of analysis.

Claims: A method for analyzing a stereoscopic image of a fundus to analyze an optic disk on the basis of the stereoscopic image of the fundus, the method comprising: a first step of obtaining a three-dimensional data of the fundus on the basis of the stereoscopic image of the fundus; a second step of designating an outer peripheral line of the optic disk on the basis of the three-dimensional data of the fundus; a third step (STEP 1-4) of determining a deepest point in an optic disk region which is determined by the designated outer peripheral line of the optic disk; a fourth step (STEP 5) of determining a plurality of cross sections of the optic disk region which pass through the determined deepest point in the optic disk region; charaterised in that the method further comprises a fifth step (STEP 6) of determining a deepest point in each cross section; a sixth step (STEP 8) of determining two highest points in each cross section between the determined deepest point in each cross section and both end points of each cross section; a seventh step (STEP 9-12) of setting, as cup points in each cross section, two points at levels which are respectively deeper from the two highest points in each cross section by a predetermined depth; and a eighth step (STEP 13) of determining a rim of an optic disk cupping by consecutively connecting the set cup points in all cross sections. A method for analyzing a stereoscopic image of a fundus according to claim 1, wherein said third step includes: a step (STEP 1) of determining a tentative central point in the optic disk region determined by the designated outer peripheral line of the optic disk; a step (STEP 2) of determining a plurality of first cross sections of the optic disk region which pass through the determined tentative central point; a step (STEP 3) of determining a deepest point in each of the first cross sections; and a step (STEP 4) of determining, as the deepest point in the optic disk region, a deepest point among the deepest points determined in the respective first cross sections. A method for analyzing a stereoscopic image of a fundus according to claim 1 or 2, further comprising: a step (STEP 7) of determining a deepest point among the deepest points in each cross section. A method for analyzing a stereoscopic image of a fundus according to one of claims 1-3, wherein in said seventh step, if a plurality of points exists at the level deeper from one of the highest points by the predetermined depth, then one of the points which is closest to the determined deepest point in each cross section, is set as the cup point. A method for analyzing a stereoscopic image of a fundus according to one of claims 1-4, further comprising a step of obtaining a cup plane of the optic disk cupping by taking as a reference position a highest point of the rim of the optic disk cupping as a result of comparison of the determined highest points and by projecting a curved surface defined by the rim of the optic disk cupping onto a horizontal plane including the reference position. A method for analyzing a stereoscopic image of a fundus according to one of claims 1-5, further comprising a step of computing optic disk parameters. An apparatus for analyzing a stereoscopic image of a fundus to analyze an optic disk on the basis of the stereoscopic image of the fundus, the apparatus comprising: data input means (7, 8) for inputting data on the stereoscopic image of the fundus; designating means (4, 5) for designating an outer peripheral line of the optic disk on the basis of a three-dimensional data of the fundus obtained on the basis of the input stereoscopic image of the fundus; storage means (2) for storing a program for detecting a rim of an optic disk cupping on the basis of the designated outer peripheral line of the optic disk and the three-dimensional data of the fundus, said program being adapted to perform the following steps when executed on a program executing means: a first step (STEP 1-4) of determining a deepest point in an optic disk region which is determined by the designated outer peripheral line of the optic disk; a second step (STEP 5) of determining a plurality of cross sections of the optic disk region which pass through the determined deepest point in the optic disk region; a third step (STEP 6) of determining a deepest point in each cross section; a forth step (STEP 8) of determining two highest points in each cross section between the determined deepest point in each cross section and both end points of each cross section; a fifth step (STEP 9-12) of setting, as cup points in each cross section, two points existing at levels which are respectively deeper from the two highest points in each cross section by a determined depth; and a sixth step (STEP 13) of forming a rim of an optic disk cupping by consecutively connecting the set cup points in all cross sections; program executing means (2) for executing said program; parameter computing means (2) for computing predetermined parameters for analysis of the optic disk on the basis of the detected rim of the optic disk cupping; and display means (5) for displaying a result of the computation. An apparatus for analyzing a stereoscopic image of a fundus according to claim 7, wherein said first step includes: a step (STEP 1) of determining a tentative central point in the optic disk region determined by the designated outer peripheral line of the optic disk; a step (STEP 2) of determining a plurality of first cross sections of the optic disk region which pass through the determined tentative central point; a step (STEP 3) of determining a deepest point in each of the first cross sections; and a step (STEP 4) of determining, as the deepest point in the optic disk region, a deepest point among the deepest points determined in the respective first cross sections.

Inventors: KAWAMURA MASUNORI, NANJO TSUGUO, NODA MANABU, KAWAMURA, MASUNORI, NANJO, TSUGUO, NODA, MANABU
Applicants: NIDEK KK, NIDEK CO., LTD.
Application year: 1997
Main classification: A61B3
IPC codes: G06T1, A61B3