We found a good correlation between the log acutance values for the RNFL images and the subjective grading of the image sharpness by two masked and experienced observers. Our results suggest that acutance may form the basis of a practical method for providing an objective assessment of digital RNFL images and may reduce the dependence of the analysis system on expert observation. The provision of acutance values for RNFL image should improve confidence in the assessment of RNFL. For example, if an RNFL image has a high acutance value (that is, good quality) and there is an absence of RNFL striations, one may be more confident in attributing the lack of RNFL striations to true axon loss rather than just secondary to suboptimal image quality.
The use of manual RNFL analysis should be placed in the context of other methods for the quantitative analysis of the optic disc and RNFL. Although devices such as the scanning laser ophthalmoscope (Heidelberg Retina Tomograph, Germany) and the retinal nerve fibre layer analyser (GdX, Laser Diagnostic Technologies, USA) have been used to diagnose glaucoma with high diagnostic precision,19,20
this has not consistently been achieved by all groups.21,22
. It is likely that the highest levels of diagnostic accuracy will be achieved using a combination of imaging techniques (both subjective and objective).23
These analyses do not need to be applied in all cases but in those in which the diagnosis or the presence of progressive disease remains equivocal. Our study employed filters that are currently available in digital fundus camera, which should encourage the use of RNFL image analysis in this clinical approach.
It is important to note that the acutance value calculated and the subjective grading of image quality were not based on the clarity RNFL striations as this is affected by the degree of glaucomatous damage.1–4,12
This is especially true in glaucomatous diffuse loss as the actual reduction in RNFL striations may confound the calculation of the acutance value and subjective grading.
There exists a possibility that the degree of RNFL loss might have an effect on the acutance by increasing the edge effect of pixels representing a vein edge. As there is no reliable objective or subjective method for providing an absolute measure of RNFL loss, we investigated whether there is any correlation between cup:disc ratio (an indirect way of indicating the extent of RNFL loss) and the acutance and the subjective assessment of image clarity as graded by the observers. We have biomicroscopic cup:disc ratio (CDR) data on 53 of the 58 images. There was no significant correlation between CDR and acutance (Pearson correlation efficient, r= 0.002, p>0.05), CDR and log acutance (r=0.05, p>0.05), CDR and observer 1 (r=0.13, p>0.05), and CDR and observer 2 (r=0.05, p>0.05). In addition, we divided the images into two groups, group A (log acutance D 1.00, n=28) and group B (log acutance <1.00, n=25). There was no significant difference between the mean CDR of group A (0.64; SD 0.16; range 0.40–0.95) and group B (0.63; SD 0.23; range 0.20–0.90) (two tailed unpaired t test, p=0.86). Therefore, we can infer that the degree of RNFL loss does not have a major effect on the acutance or observer grading.
Since we sampled a single location in the image, the question arises as to whether this provides a satisfactory estimate of the entire image quality. Our choice of sampling region was made on preliminary observations using other retinal regions. Although these regions provided a reasonable correlation with subjective assessment, we found a higher degree of variability. Furthermore, the areas of interest in the assessment of RNFL were near the optic disc and along the vascular arcades and not at the periphery, which were often not in sharp focus and/or poorly illuminated even in good quality images.
Although this method holds promise as a practical technique, it has several limitations. Firstly, the selection of the region of interest was done subjectively although we took steps to reduce the subjectivity by clearly defining the area (that is, the superotemporal vein, 1 disc diameter from the edge of the optic disc) and standardising it to all the images. However, owing to natural variations of the retinal vasculature, the selection of the region of interest remains just an approximation. Secondly, the images were monochrome and not truly red free. We could expect better results with different filter settings. Thirdly, although the observers were instructed not to consider the clarity of the RNFL striations in their grading, the degree of the RNFL striations might still influence their subjective grading.
Despite the limitations, there are potential clinical applications of this technique. Routine use of acutance could provide a useful measure of quality control of RNFL images. The “cut-off” acutance value of RNFL images could be predetermined so that only images with acutance above this value would be considered of sufficiently good quality for RNFL analysis. Since our software algorithm is semiautomated it could easily be incorporated into digital imaging program allowing efficient automated objective assessment of the quality of RNFL images. These quantitative measures should help in identification of diffuse glaucomatous damage, based on the assessment of digital RNFL images.
Presented in part at the Association for Research in Vision and Ophthalmology, Fort Lauderdale, FL, USA, May 2001.