Neuroretinal rim measurement error using PC-based stereo software

Automated assessment of the optic nerve head on stereo disc photographs

PURPOSE

To develop automated software for optic nerve head (ONH) quantitative assessment from stereoscopic disc photographs and to evaluate its performance in comparison with human expert assessment.

METHODS

A fully automated system, including three-dimensional ONH modeling, disc margin detection, cup margin detection, and calculation of stereometric ONH parameters, was developed and tested. One eye each from 54 subjects (23 healthy, 17 suspected glaucoma, and 14 glaucoma) was enrolled. The majority opinion of three experts defined disc and cup margins on the disc photographs was used for comparison. Seven ONH parameters, disc area, rim area, rim volume, cup area, cup volume, cup-to-disc (C/D) area ratio, and vertical C/D ratio, were computed based on both machine- and expert-defined margins and compared between the methods.

RESULTS

All automated ONH measurements showed good correlation with the expert defined margins (Pearson r = 0.90, disc area; 0.56, rim area; 0.78, rim volume; 0.88, cup area; 0.93, cup volume; 0.69, C/D area ratio; and 0.67, vertical C/D ratio; all P <or= 0.0001). No statistically significant difference was found in the glaucoma-discriminating ability of all seven ONH parameters (P >or= 0.21). The mean or median of automatically defined disc and cup areas was significantly higher than the subjective assessment (disc area P = 0.0001, t-test; cup area P = 0.036, Wilcoxon signed ranks test), although they had high correlation coefficients. The software failed to detect the disc margin for all the disc photographs with peripapillary atrophy.

CONCLUSIONS

The automated ONH analysis method provides an objective and quantitative ONH evaluation using widely available stereo disc photographs.

Digital imaging of the optic nerve head: monoscopic and stereoscopic analysis

AIMS

To compare monoscopic and stereoscopic assessment of the optic disc using novel software for the digital stereoscopic analysis of optic disc stereopairs.

METHODS

Software was developed for the stereoscopic display of digital optic disc images using an interlaced display method. Neuroretinal rim width was determined at 10 degree intervals around the optic disc using a custom (stereoscopic) cursor whose depth was adjusted to that of Elschnig's rim. Measurements were taken, first viewing the disc monoscopically and at a separate sitting, stereoscopically.

RESULTS

Measurements were made in 35 eyes from 35 patients (1260 estimates for each observer) using three observers. The mean cup to disc ratio (CDR) ranged from 0.57 to 0.66 (SD 0.13-0.14) for monoscopic viewing compared with 0.64 to 0.69 (SD 0.12-0.14) for stereoscopic viewing. Stereoscopic assessments gave higher CDRs in temporal, superior, nasal, and inferior aspects of the optic disc (p<0.001, Mann-Whitney U test). Agreement between observers in estimating CDR was high for monoscopic assessment (intraclass correlation coefficient 0.74 (CI 0.72 to 0.76) increasing to 0.80 (0.78 to 0.82) for stereoscopic assessment.

CONCLUSION

Digital stereoscopic optic disc assessment provides lower estimates of neuroretinal rim width and higher levels of interobserver agreement compared with monoscopic assessments.

The reengineering of a software system for glaucoma analysis

Glaucoma is a destructive eye disease that causes blindness in individuals displaying little or no symptoms. There is no cure as yet though there are treatments that can arrest its effects or slow its development. The earlier the disease is detected, the more likely the treatment will be successful; however early detection of the disease can be difficult. This highlights the importance of ophthalmologists having access to tools that can assist in accurately diagnosing glaucoma and other retinal diseases as early as possible. The stereo optic disc analyser (SODA) software package is a tool intended to be used by ophthalmologists, to aid in the accurate detection of retinal diseases. SODA will use stereoscopy and three-dimensional image analysis to assist in accurately detecting changes in the retina, caused by diseases such as glaucoma. This paper will focus on the reengineering and redesign of the SODA software package to overcome the shortcomings inherent in its prototype implementation and develop a package that can be commercialised. Software Engineering principles and the software development lifecycle, along with principles of object-orientation and usability, have been used to establish a framework for SODA, improve its accuracy, enhance its usability and to redevelop the product into an implementation that can later be commercialised.

Computer-assisted planimetry associated with Sturge-Weber syndrome

Photographs of the optic nerve head (optic disc) in the eye are used for the clinical assessment of the disease glaucoma. These images are usually subjectively assessed by a clinician. A case of Sturge-Weber Syndrome which includes glaucoma as a symptom, is presented here. Narrowing of the rim of the optic disc was measured using custom-made measurement software confirming glaucomatous progression. To maximize the information obtained from optic disc images, low cost software can assist with quantifying disc parameters aiding clinical interpretation.