Diagnosis of glaucoma using telemedicine – the effect of compression on the evaluation of optic nerve head cup–disc ratio

A photograph of the optic nerve head requires a lot of disk space (over 1 MByte) for storage and may require substantial bandwidth and time for transmission to a remote practitioner for a second opinion. To test whether compression degrades the image quality of the images, 302 slides were digitized at an optical resolution of 2400 pixels/inch (945 pixels/cm) and 30 bit/pixel. The images were saved both in non-compressed TIFF format and in compressed JPEG (compression ratio of 60) format. A blinded observer measured the optic nerve head cup-disc ratio for all three groups: the original slides, uncompressed TIFF and compressed JPEG images. The results showed that digital images were less accurate than slides. However, compression, even up to a ratio of 40, did not make matters worse.