Digital imaging and microtexture analysis of the nerve fiber layer

To describe the design of a digital retinal nerve fiber layer (RNFL) imaging techniques and present a new approach to measure the differences in RNFL patterns.

A digital camera body is connected to a wide-angle camera to obtain images of the RNFL, which are displayed in workstations throughout the clinic. In the on-line archive, images in Joint Photographics Experts Group (JPEG) format (100 KB per frame) are used. The hypothesis that changes in RNFL structure can be seen as changes in the microtexture of digital images was tested using an information theoretical approach (Kullback Information Distance, KID). A large KID value indicates a large difference, and a small KID value indicates a small difference in microtexture between the two regions. The material of this pilot study consists of 9 patients with glaucoma, 8 patients with ocular hypertension, and 7 normal subjects.

The median KID value in the glaucoma group was 3.5, compared with the median KID values of 0.6 in the control groups. Although a trend could be seen in the measured values, because of a small sample size, the differences were not statistically significant. Five of 24 (21%) KID values overlapped between the glaucomatous group and the other two groups.

Although digital imaging produces good quality RNFL images, further research is needed to establish minimum accepted specifications for digital imaging. In this pilot study, only the microtexture of the RNFL was measured in digital images. In the future, the approach can be expanded to include also properties of macrotexture and full color palette.