Assessment of digital stereoscopic optic disc images using a Z screen

ASSESSMENT OF DIGITAL STEREOSCOPIC OPTIC DISC IMAGES USING A Z SCREEN

J.E. MORGAN,1 N.J.L. SHEEN2, R. GOYAL1, J.M. WILD2 and R.V. NORTH2

1School of Ophthalmology, University of Wales College of Medicine; 2Department of Optometry and Vision Sciences, Cardiff University; Cardiff, UK

Abstract

Purpose: To determine the ability of digital stereoscopic optic disc analysis to identify glaucomatous optic nerve damage in eyes with early visual field loss. Methods: Simultaneous (Nidek 3Dx) and sequential (Nikon F505) optic disc images of 56 glaucoma and 60 normal subjects of similar age were obtained. The diagnosis of glaucoma was established by grading of two consecutive Humphrey Program 24-2 SITA Standard fields. Optic disc appearance and intraocular pressure were not used to make the diagnosis. Both sequential and simultaneous optic disc images were digitized at high resolution, and viewed stereoscopically using a Monitor Z screen (Stereographics Corp.). Images were viewed stereoscopically, using an interlaced display method. Three observers (residents in training) who were masked to patient diagnosis subjectively graded disc images as normal or glaucomatous. They then outlined the optic disc and cup borders using a cursor whose depth, in viewing space, could be adjusted to match that of the scleral rim. The scaling factors for each image were determined for the calculation of neuroretinal rim area (at 30° intervals) and total disc areas using established algorithms. Results: The group mean MD for the normal group was -0.38dB (PSD 1.88 dB) and -4.46 dB (PSD 5.28) respectively, for the glaucoma group. Best subjective grading gave a sensitivity of 80.3% (93.3% specificity) for the discrimination of glaucomatous optic discs. When account was then taken of optic disc area, by performing linear regression analysis of optic disc area and the log transformed neuroretinal rim area (in 30° segments), glaucomatous optic discs could be detected with a sensitivity of 82.3% at a specificity of 93.3%. Conclusions: Digital stereoscopic viewing systems, in conjunction with simultaneous stereoscopic optic disc photography, enable the discrimination of glaucomatous optic neuropathy with clinical useful sensitivity and specificity. Similar levels of diagnostic precision could be achieved when the images were analyzed subjectively and quantitatively.

Introduction

Glaucomatous optic neuropathy is characterized by loss of retinal ganglion cells with associated thinning of the neuroretinal rim and excavation of the optic disc cup. Changes in the appearance of the optic nerve head and retinal nerve fiber layer, in general, precede those in the visual field. In some cases, retinal nerve fiber layer defects can be seen up to six years before the onset of visual field loss as detected using currently available clinical methods.1-3

Address for correspondence: J.E. Morgan, MD, Department of Ophthalmology, University of Wales, College of Medicine, Heath Park, Cardiff CF4 4XW, UK

Perimetry Update 2002/2003, pp. 309–316

Proceedings of the XVth International Perimetric Society Meeting, Stratford-upon-Avon, England, June 26–29, 2002

edited by David B. Henson and Michael Wall

© 2004 Kugler Publications, The Hague, The Netherlands

These findings have driven the development of imaging techniques to quantify optic nerve head4,5 and retinal nerve fiber layer damage6,7 for the earliest detection of glaucomatous damage. Notable success has been achieved, particularly in the use of scanning laser tomographic devices for the quantification of topographical changes in the optic nerve head and peripapillary retinal nerve fiber layer. With the Heidelberg Retina Tomogram (HRT), for example, when account is made of optic disc size, normal and glaucomatous optic nerve heads can be discriminated with high sensitivity and specificity.5 Other devices have been developed for the more direct quantification of retinal nerve fiber layer thickness based on its birefringence8 or refractive difference9 by scanning laser polarimetry (SLP) and optical coherence tomography (OCT), respectively. These devices have now entered clinical practice and are likely to play an important role in the detection and management of glaucomatous optic neuropathy.

Despite these advances, the techniques have their limitations in that they do not provide a full illustration of the clinical appearance of the disc and may, therefore, fail to highlight some of the pathological features which are useful markers for disease.10 For example, peripapillary atrophy can cause problems with the topographical determination of retinal surface height and retinal nerve fiber layer hemorrhages can be difficult to detect in tomographic scans.11 With devices such as the OCT and SLP, there is some debate concerning the accuracy with which retinal structures are quan- tified.12-14

The gold standard by which these devices are tested usually includes the assessment of the optic nerve head by stereoscopic optic disc photography (SODA).15 In expert hands, this imaging method can detect disease with high sensitivity and specificity, which can match or even exceed that obtained with state-of-the-art digital imaging devices.16 Despite this, stereoscopic optic disc imaging is not in widespread clinical use, even though fundus cameras for the acquisition of the relevant images are widely available. Several factors have limited the application of this technology. Viewing of stereoscopic images in the clinical setting is time-consuming and has relied on the precise alignment of slide projection systems. A further limitation is that quantitative analysis can be difficult to integrate within the viewing system, and the image can only be viewed by one observer at any one sitting.

Recently, digital technology has been developed for the display and analysis of stereoscopic images using digital imaging systems.17,18 These systems are compact and ideally suited for the analysis of digital stereoscopic images in the clinical setting. In this study, we describe our experience with the use of such a system, and evaluate its use in the detection of glaucomatous optic neuropathy.

Methods

The cohort contained 56 patients with glaucoma and 60 normal individuals. The patients with glaucoma were recruited in the course of their normal clinical care at the University Hospital of Wales. The normal individuals were recruited from the Eye Clinic at the Department of Optometry and Vision Sciences, Cardiff University. The study was conducted with institutional ethical approval, and all patients provided informed and written consent.

Glaucoma patients were defined on the basis of a reproducible visual field defect,

as defined on the basis of at least two consecutive visual fields with two or more contiguous locations corresponding to the retinal nerve fiber distribution exhibiting abnormality on Pattern Deviation Probability analysis at p ≤ 0.02,19 or a single location at p ≤ 0.01. The group mean MD for the normal group was -0.38 dB (PSD 1.88 dB) and -4.46 dB and 5.28, respectively, for the glaucoma group.

Optic disc appearance and intraocular pressure were not used to make the diagnosis.20 Images of the optic disc were acquired using a Nidek 3Dx stereocamera with 35mm film. Both images of the stereo-pair were captured within the 35-mm frame using Kodak Ektackrome film (ISO 100) and were then digitized at high resolution (1394 by 1200, 24 bits per pixel) for display on a 19-inch computer monitor.

For stereoscopic imaging, images were processed for display using the above and below (interlaced) display method.21 This display format requires compression of images that comprise a stereo-pair to 50% of their full height (image width is held constant). The vertical synchronization signal for the video monitor is adjusted using proprietary hardware (Stereographics Corporation, San Rafael, CA) so that the raster lines corresponding to the right and left stereo-image pairs alternate in the vertical axis of the monitor. The monitor operates at greater than 50 Hz vertical refresh rate, and ensures that either image can be presented at a refresh rate that exceeds the critical fusion frequency for the human eye.

The images were perceived stereoscopically by ensuring that the correct image of the stereo-pair was perceived by the appropriate eye. In the majority of digital stereoscopic viewing systems, this is achieved using liquid crystal (LC) plates that rapidly alter their plane of polarization so that, with the observer wearing polarized glasses with the appropriate polarization axis, either eye is rapidly and alternately occluded with respect to the image on the video-monitor. With the Z Screen (Stereographics Corporation, San Rafael, CA), the LC plate is placed over the computer monitor to achieve this image-eye dissociation and the observer wears Polaroid (unpowered) glasses (Fig. 1). The change in the polarization state of the Z screen is synchronized to the refresh rate of the monitor in order to ensure that the LC plate is effectively opaque for the right eye when the left stereo-image is displayed and opaque for the left eye when the right stereo-image is displayed.

Custom software was developed for the display of the stereo-images using the above and below (interlaced) method for personal computers running Microsoft Windows (Microsoft Corporation) operating system. For the analysis of stereo-images, the Windows (Microsoft Corporation) mouse cursor was disabled and a cursor developed for each stereo-image so that the cursor could be moved in the X,Y plane of the monitor but also in the Z plane (depth). By varying the horizontal separation of the cursors, the user was able to adjust the depth of the cursor to match the depth of the structure(s) of interest within the stereoscopic image.

For each optic disc, a single observer demarcated the boundary of the optic nerve head as the inner border of Elschnig’s rim, with the cursor set to the depth of the scleral rim. The cursor was then maintained at this depth for demarcation of the inner border of the neuroretinal rim. In those optic discs in which Elschnig’s rim was tilted, the depth of the cursor could be adjusted to follow the contour of the scleral rim around the optic disc. Fine adjustment of the demarcation of either the neuroretinal rim or optic disc boundary could be made by adjusting the length of the cursor line to represent the width of the neuroretinal rim at 30° intervals around the disc (Fig. 2).

Fig. 3. Plot of total neuroretinal rim against optic disc area (mm2) for all eyes in the study. Filled symbols: normal eyes; open symbols: glaucomatous eyes. 95% prediction intervals are shown, above and below the regression line.

Software then computed total disc area (mm2) neuroretinal rim area and 30° segment neuroretinal rim area. Images were scaled for each eye for absolute values from keratometry and refraction data according to published methods.22

For discrimination of normal from glaucomatous discs, the neuroretinal rim area was regressed against total optic disc area to correct for the variation of neuroretinal rim area with disc size.23 If the neuroretinal rim area, either for the whole disc or in any 30° segment, fell below either the 95 or 98% confidence intervals, the optic disc was classified as abnormal (Fig. 3). This analysis was performed both with and without logarithmic transformation of the neuroretinal rim areas in order to take account of the greater dispersion in neuroretinal rim area with increasing optic disc size.23 Results are reported for a single observer who was experienced in the analysis of optic discs, had good stereoscopic vision (greater than 45 minutes of arc) and was familiar with the software. Statistical analysis was performed using SPSS 10.0 (SPSS Inc).

Results

We found that the quality of the stereoscopic image was consistently better with simultaneously acquired (Nidek 3Dx) stereoscopic images compared with sequentially acquired (Nikon) images, and for this reason the analysis is based on images acquired with the Nidek fundus camera. Data are presented for a single observer who was masked to patient diagnosis.

314 J.E. Morgan et al.

In the first analysis of the images, the observer was asked to make a clinical judgment of the diagnosis based on the clinical impression of the optic disc. This subjective grading (which did not rely on information with respect to size of the optic disc) gave a sensitivity of 80.3% and a specificity of 93.3% for the detection of glaucomatous optic discs.

In the second analysis of the images, we achieved an optimal discrimination of normal from glaucomatous discs with a sensitivity of 82.3% and a specificity of 93.3% following regression of the log of the neuroretinal rim against optic disc area and using the 98% confidence interval as the cut-off measure.

Discussion

We have demonstrated that stereoscopic optic disc imaging using digital display methods is an effective method for the discrimination of normal from glaucomatous optic discs in eyes with early to moderate glaucomatous damage. The system provided excellent views of the optic disc and was well tolerated by the observer. It is important to stress that we report results for eyes with early to moderate disease as defined on the basis of visual field loss. Since substantial optic disc damage will have occurred prior to the onset of reproducible visual field loss, it is clear that many of these discs would have been identifiable as abnormal on the basis of subjective assessment. This is apparent by the similarity of the diagnostic precision for detection of both the subjective and quantitative methods. It is also possible that recruitment on the basis of visual field loss could have introduced a bias for the presence of focal rather than diffuse visual field damage. Focal damage would tend to be associated with the presence of notches in the neuroretinal rim which do not require quantitative evaluation for their detection. Such a bias might have inflated the sensitivity of the technique for the detection of disease.

Several technical aspects of the system require comment. The display format was designed to run on a computer with relatively low specification that is suitable for the clinical environment. Specialized graphics cards are not required for full-screen stereoscopic displays to provide a flicker-free stereoscopic image. A key feature of the Z screen system was that the stereoscopic images could be observed with polarized glasses that were low cost, unpowered and robust. Several pairs of glasses can be distributed to enable multiple observers to view the same optic disc.

An important novel feature of the software developed for disc analysis was that the mouse driven cursor could be moved in the depth (Z) axis, as well as in the transverse (XY) axes. This allowed the determination of the size of the optic disc at the level (in depth) of Elschnig’s rim, thereby minimizing parallax errors. It also facilitated a precise definition of the inner border of the neuroretinal rim as the intersection of the surface of the optic disc cup with the plane of Elschnig’s rim. In eyes with shallow cups, this would place the retinal surface above this plane, thereby recording a disc with an absent cup. To our knowledge, this is the first system that uses cursors that are adjustable in depth (Z space). We anticipate that this will improve agreement between observers in the quantification of optic disc parameters.

One of the limitations of the current display method is that it required compression of the image pairs by 50% in the vertical axis for display in the above and below

method. Since large images were used in this study with a vertical resolution of over 1200 pixels, this effectively reduces the vertical resolution to 600 pixels and it is apparent on detailed inspection of the stereo-image that the image pairs were offset by one (raster) line on the video-monitor. While the storage of large images to obtain a high final resolution is not a major problem, it is clear that the interlaced method will limit the possible resolution with which the stereoscopic image can be viewed. Another significant limitation is that desynchronization of the monitor refresh rate to achieve the interlaced display also affects the graphical interface controls. Although it is possible to overcome this by disabling the Windows interface over the image, it is not possible to apply this limitation to the use of Windows controls. The observer therefore has to deal with two Windows cursors, and has to determine which of these is active with respect to Windows-based controls, such as dialogue boxes.

Notwithstanding these problems, digital stereoscopic imaging is a very effective solution to the analysis of stereoscopic images. It does not require the use of mirrorbased viewing systems and allows the analysis of high resolution true color images using cursor controls that define regions of interest in both the transverse and depth planes of the image. Given the limitations of the interlaced display, we are currently developing tools for use in the analysis of stereoscopic images in an alternative (quad buffered) format, in which the images occupy the same space on the monitor but are refreshed alternately at over 50 Hz to achieve a flicker-free stereoscopic view using the Z screen. This viewing method has the advantage that it runs within a given graphical interface and allows continued use of the mouse cursors for control of the analysis software. It also permits high resolution displays of stereo-images (exceeding 1024 in each axis at 24 bits per pixel) and avoids the artefacts that are evident with the interlaced method. However, the high refresh rates that are needed (exceeding 100 Hz) are only possible on selected monitors and require graphics cards with sufficient ‘memory’ to hold the component images. Our preliminary experience with these displays is that they are easier to use compared to the interlaced format and allow additional functionality including the ability to enlarge parts of the image while viewing stereoscopically, to pan images that have been magnified, and to have multiple stereoscopic images displayed within graphical windows at any one time (thereby facilitating comparison of optic disc images in a time series).

Acknowledgment

Funding: WORD, NERC (UK).

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