The results of this preliminary study show that the performance of an automated algorithm for determination of the cup and rim from close-to-isotropic SD-OCT images correlates highly with that of planimetry by glaucoma experts on the same eye.
To our knowledge, this is the first such algorithm. If these preliminary results can be confirmed in a larger study, such algorithms, to automatically segment cup and rim from SD-OCT, have the potential to replace or substitute for time-consuming expert planimetry, which also suffers from interobserver variability.5
Because evaluation of the optic disc and planimetry are an important part of the evaluation of patients with glaucoma, such algorithms may offer improved management of patients with glaucoma. In this preliminary study, we have not compared the performance of this, SD-OCT-based, algorithm to the performance of our previously published stereo color photograph–based algorithm.6
We are currently studying the performance of an improved version of the stereo color photograph algorithm. The lower accuracy of the algorithm on cup than on rim is to be expected, as glaucoma specialists base their rim planimetry on relatively easy to define image structures, while the cup planimetry is based on more subjective assessment of a boundary on a gradual, relatively featureless, slope. The algorithm is trained on the planimetry from color stereo photos. Because the algorithm is trained for rim voxels on a less variable rim margin human expert reference standard, as well as a more variable cup boundary reference standard, whereas the rim voxels only depend on the more variable cup boundary reference standard, the rim area is less variable than the cup area.
There are several shortcomings in this study. First, the number of subjects is too small to unequivocally determine the performance relative to glaucoma experts. Second, this algorithm currently works on close-to-isotropic SD-OCT volumes. This study was performed on SD-OCT devices from only a single manufacturer, but we obtained subjectively similar results on a second close-to-isotropic SD-OCT device (Topcon, Paramus, NJ). The performance can be expected to be lower on more anisotropic OCT volumes, where the distance between B-scans is much larger. Third, although c/d ratio and expert planimetry are important components of the evaluation of patients with glaucoma, other features of the optic disc, visible through biometry and stereo color photography, such as color changes, are lost. The additional value, if any, of such features remains to be determined, especially for their relevance to the measurement of progression of glaucoma. Fourth, because we expect the main application of this algorithm to be in glaucoma staging, rather than differentiating normal from glaucoma cases, we have not tested the algorithm’s performance on normal eyes. Some of the patients had suspected glaucoma with a small c/d, and the algorithm did not show bias in these cases.
Finally, we essentially compressed the information contained in the ONH OCT volume into a single number, the c/d ratio. The cup and rim are attractive because they are easily understood by clinicians and allow comparisons to other studies. It is possible, and in fact likely, that other parameters of the ONH, determined from SD-OCT volumes, correlate at least as well or better than the cup and rim to glaucoma diagnosis and progression. For example, the work by Burgoyne and Downs13
shows that features of the lamina cribrosa are important parameters of glaucoma progression. Because we detect the rim and cup, we could also have compressed the ONH information into a different number—for example, the rim surface. We have focused in this study on the c/d ratio because it is a measure that is in widespread clinical use, and most clinicians are familiar with its limitations, more so than with those of newer measures. Thus, validation of this algorithm on a larger cross-sectional glaucoma sample as well as the normal control is warranted, to determine unequivocally its correspondence to glaucoma experts. More important, longitudinal evaluation of a sample of persons with glaucoma, will allow its potential, if any, to detect glaucoma progression to be determined.
In summary, in this preliminary study, we developed a novel algorithm to determine the cup and rim in close-to-isotropic SD-OCT images of the ONH and showed that its performance for determination of the cup and rim from these SD-OCT images is essentially equal to that of planimetry performed by glaucoma experts on the same eye.