A comparison of retinal nerve fiber layer (RNFL) thickness obtained with frequency and time domain optical coherence tomography (OCT)

Abstract

To diagnose glaucoma and other diseases of the retinal ganglion cell/ optic nerve, the thickness of the retinal nerve fiber layer (RNFL) is routinely measured with optical coherence tomography. Until recently, these OCT measurements were made almost exclusively with a time domain OCT (tdOCT) machine from a single manufacturer. Recently, a number of OCT machines, based upon an improved frequency domain OCT technology (fdOCT), have appeared. We compared measurements made using a new fdOCT machine to those from the older tdOCT machine. The results were comparable. More importantly, we learned that the key factor determining whether results from different machines will be comparable is the algorithm used to segment RNFL thickness, not the type of OCT.

Fig. 1

Sample data from a glaucoma suspect. A. Location of tdOCT circle scan. B. tdOCT single circle scan. Distance between white lines is RNFL thickness. C. RNFL thickness as a function of the distance around the optic disc for the scan in panel B. D. Fundus view showing location of the fdOCT line scans. E. 3-D representation of RNFL thickness. F. A reconstructed fdOCT scan for circle (dashed curve in panel E). Distance between white lines is RNFL thickness. G. RNFL thickness (thin red line) as a function of the distance around the optic disc for the scan in panel F. Bold red line is RNFL thickness for profile in panel F; thin dotted line is the same data after spatial smoothing. See View 1 for fdOCT data and View 2 for tdOCT data; additional datasets are also available for fdOCT (Case003) and tdOCT (Case004).

Fig. 2

A. Scatter plot of fdOCT overall average RNFL thickness versus tdOCT overall average RNFL thickness. Each point represents the results for a single eye, controls (green), suspects (blue) and patients with glaucoma (red). Dashed line has a slope of 1.0 and is locus of equal fdOCT and tdOCT values. B. Bland-Altman plot showing the difference between overall RNFL thickness on fdOCT and tdOCT versus the average of the fdOCT and tdOCT values. Dashed line has a slope of 0 and is locus of equal fdOCT and tdOCT values. Bold solid line is the mean of the average fdOCT and tdOCT values.

Fig. 3

Bland-Altman plots showing the difference between RNFL thickness on fdOCT and tdOCT versus the average of the fdOCT and tdOCT values for the superior (panel A), nasal (panel B), inferior (panel C) and temporal (panel D) quadrant. Each point represents the results for a single eye, controls (green), suspects (blue) and patients with glaucoma (red). Dashed line has a slope of 0 and is locus of equal fdOCT and tdOCT values. Bold solid line is the mean of the average fdOCT and tdOCT values.

Fig. 4

A. tdOCT (blue) and fdOCT (red) RNFL profiles for a glaucoma suspect. The green curve is the difference between the tdOCT and fdOCT profiles. B. tdOCT scan for the blue curve in panel A. C. fdOCT scan for the red curve in panel A. Dashed red lines indicate the two places where the two scans differ the most.

Fig. 5

A. tdOCT (blue) and fdOCT (red) RNFL profiles for a patient with glaucoma. The green curve is the difference between the tdOCT and fdOCT profiles. B. tdOCT scan for the blue curve in panel A. C. fdOCT scan for the red curve in panel A. Dashed red lines indicate the two places where the two scans differ the most.