by extension, the ability to use the image for clinical evaluation. Several studies have evaluated the effect of image compression on picture quality [3, 4, 15, 24].

At the University of Alberta, it was found that when digital images were compressed to 16:1 JPEG format, there was excellent reproducibility when compared to ETDRS standard Þlm photography in the diagnosis of diabetic retinopathy [24]. Another study evaluated the identiÞcation of diabetic retinal pathologies with compression at ratios of 55:1 and 113:1. Exact agreement was found in 75Ð100% of images, with k values ranging from 0.46Ð1.00 for 55× compression to 0.60Ð1.00 for 113× compression [3].

Lee et al. [15] at the Scheie Eye Institute investigated high compression of 80:1 and low compression of 30:1 in the identiÞcation of drusen. They found that high compression of 80:1 markedly reduced image quality and that a compression of 30:1 could be suitable for telemedicine and the archiving of images.

3.4Stereoscopic Teleophthalmology Systems

3.4.1University of Alberta

This teleophthalmology system has been in use for many years and has been used to evaluate diabetic retinopathy, glaucoma and age-related macular degeneration [22Ð24, 29, 31]. This system captures non-simultaneous stereoscopic images of the disc and macula (ETDRS Þelds 1 and 2) and nonstereoscopic images of the periphery (Þelds 3Ð7).

Rudnisky et al. [22] compared this system to the accepted standard of contact lens biomicroscopy (CLBM) in the detection of clinically signiÞcant macular oedema (CSME). It was found that exact agreement was greater than 83.6% for all three types of CSME with sensitivities ranging from 50.0% to 90.6% and speciÞcities ranging from 90.0% to 99.0%, validating the use of stereoscopic digital imaging in the detection of CSME. A later study evaluated the beneÞts of stereopsis in the evaluation of CSME. Stereoscopic digital photographs were evaluated for retinal thickening and the presence of hard exudate. These images were

compared to a clinical examination with CLBM for the presence of CSME. It was found that the sensitivities of hard exudate and stereoscopic observation of retinal thickening were similar (93.9% and 90.9%, respectively). However, the speciÞcity of digital stereopsis was 92.9% and signiÞcantly more than hard exudate alone (81.6%, p<0.001) [23]. Screening systems that have lower speciÞcity will refer patients unnecessarily for clinical examination, thus reducing efÞciency and increasing cost. The University fo Alberta has also evaluated the correlation of stereoscopic digital photography to standard slide Þlm in the diagnosis of diabetic retinopathy. The digital gradings for ETDRS level of diabetic retinopathy, CSME and referral thresholds were highly correlated to Þlm gradings (>87%, k>0.71) [24].

Similarly, Somani et al. [29] compared stereoscopic digital imaging to slide Þlm photography for the identiÞcation of age-related macular degeneration (AMD). The correlation between the two was poor for small and intermediate drusen, but was substantial to excellent (k 0.64Ð0.83) for the identiÞcation of moderate to advanced AMD.

This group has also evaluated the practicality of teleophthalmology for optometric referral triage [12]. They found that teleophthalmology triage reduced patient travel distance and time, reduced ofÞce visits to the retina specialist and improved the efÞciency of clinical examination, testing and treatment.

3.4.2Inoveon Diabetic Retinopathy– 3DT System

The Inoveon 3DT system was described in a validation study of seven-Þeld, stereoscopic, digital imaging, using Internet data transfer and a reading centre based on the ETDRS protocol, to detect threshold disease requiring referral. They deÞned threshold disease as a level of diabetic retinopathy greater than ETDRS level 53, macular oedema or an ungradable image in either eye of a patient. Their system demonstrated a sensitivity and speciÞcity for the detection of threshold disease of 98.2% and 89.7%, respectively [10]. As a result of this study, this group replaced Þlm photography with digital, stereoscopic imaging for their commercial diabetic retinopathy screening system.