Macular segmentation with optical coherence tomography

Abstract

Purpose: To develop a software algorithm to perform automated segmentation of retinal layer structures on linear macular optical coherence tomography (StratusOCT; Carl Zeiss Meditec, Inc., Dublin, CA) scan images and to test its performance in discriminating normal from glaucomatous eyes in comparison with conventional circumpapillary nerve fiber layer (cpNFL) thickness measurement.

Methods: Four layer structures within the retina were defined: the macular nerve fiber layer (mNFL), the inner retinal complex (IRC; retinal ganglion cell [RGC] layer + inner plexiform and nuclear layers), outer plexiform layer (OPL), and outer retinal complex (ORC; outer nuclear layer + photoreceptor layer). Normal and glaucomatous eyes underwent fast macular map and fast NFL OCT scans. Linear macular images were analyzed using the developed algorithm, and the results were compared with the cpNFL thickness measurement.

Results: Forty-seven subjects (23 normal and 24 with glaucoma) were analyzed. mNFL, cpNFL, IRC, and the total retinal thicknesses were significantly greater in normal than in glaucomatous eyes (P < or = 0.0002; Wilcoxon), whereas OPL thickness did not show a significant difference (P = 0.46). ORC thickness was significantly greater in glaucomatous than normal eyes (P = 0.035). Areas under the receiver operator characteristic curve (AROCs) for discriminating normal from glaucomatous eyes were highest with mNFL + IRC (0.97) and lowest with OPL (0.56). AROCs for OPL and ORC were significantly smaller than those for mNFL, IRC, mNFL+IRC, and cpNFL (P < or = 0.01). AROCs for IRC, mNFL + IRC, and cpNFL were significantly larger than for retinal thickness (P < or = 0.049). Among the best-performing parameters (mNFL, IRC, mNFL + IRC, and cpNFL) there was no significant difference in AROCs (P > or = 0.15).

Conclusions: The newly developed macular segmentation algorithm described herein demonstrated its ability to quantify objectively the glaucomatous damage to RGCs and NFL and to discriminate between glaucomatous and normal eyes. Further algorithm refinement and improvements in resolution and image quality may yield a more powerful methodology for clinical glaucoma evaluation.

Figure 1

Preprocessing of the image. (a) A raw OCT macular scan image was aligned by cross-correlation. (b) A modified mean filter was applied to the aligned image above.

Figure 2

Segmentation of a sampling line shows a typical sampling line (analogous to an ultrasound A-scan line) plot of tissue reflectivity. The first increase from the noise level was registered as the ILM. The second major peak is RPE complex. The first increase within the RPE complex was the interface between inner and outer segments of the photoreceptors. The RPE was a gap or notch posterior to the interface. Retinal layer structures (NFL, IRC, OPL, and ORC) were defined between the ILM and the interface. The locations of the borders were determined with an adaptive thresholding technique, where cutoff threshold values were calculated based on reflectivity characteristics of each sampling line.

Figure 3

Macular segmentation analysis sample. (a) Detected borders superimposed on the aligned image shown in Figure 1a. (b) Detected borders superimposed on the filtered aligned image shown in Figure 1b.

Figure 4

Comparison of the mean colored maps of each detected layer. Six pairs of mean colored maps, calculated from all 24 normal and 24 glaucomatous eyes, are shown, visually illustrating the quantitative findings. All layers except for the OPL showed statistically significant differences between normal and glaucoma groups. Note that the scale of the color scheme is adjusted for the range of each measured layer.

Figure 5

Color macular segmentation mapping. (a) NFL+IRC thickness mapping on a normal eye (OD). A C pattern thickening was clearly seen, where the nasal region was thicker in the papillomacular bundle than the temporal region. Mean NFL+IRC thickness was 123.9 μm. (b) NFL+IRC thickness map on a glaucomatous eye (OD) and the corresponding SITA visual field (MD −10.8 dB, PSD 11.3 dB). Superior NFL+IRC thinning was depicted clearly, which agreed with the inferior arcuate visual field defect pattern. Mean NFL+IRC thickness was 99.1 μm.