Methods and limits of digital image compression of retinal images for telemedicine

PURPOSE. TO investigate image compression of digital retinal images and the effect of various levels of compression on the quality of the images. METHODS. JPEG (Joint Photographic Experts Group) and Wavelet image compress ion techniques were applied in five different levels to 11 eyes with subtle retinal abnormalities and to 4 normal eyes. Image quality was assessed by four different methods: calculation of the root mean square (RMS) error bet ween the original and compressed image, determining the level of arteriole branching, identification of retinal abnormalities by experienced observers , and a subjective assessment of overall image quality. To verify the techn iques used and findings, a second set of retinal images was assessed by cal culation of RMS error and overall image quality. RESULTS. Plots and tabulations of the data as a function of the final image size showed that when the original image size of 1.5 MB was reduced to 29 KB using JPEG compression, there was no serious degradation in quality. The smallest Wavelet compressed images in this study (15 KB) were generally st ill of acceptable quality. CONCLUSIONS. For situations where digital image transmission time and costs should be minimized, Wavelet,image compression to 15 KB is recommended, al though there is a slight cost of computational time. Where computational ti me should be minimized, and to remain compatible with other imaging systems , the use of JPEG compression to 29 KB is an excellent alternative.