The interpretation of optical coherence tomography images of the retina

PURPOSE. TO determine the relationship between optical coherence tomography (OCT) images of the retina and retinal substructure in vitro and in vivo. METHODS. in vitro, OCT images of human and bovine retina were acquired afte r sequential excimer laser ablation of the inner retinal layers. Measuremen ts of bands in the OCT images were compared with measurements of retinal la yers on histology of the ablated specimens. In vivo, OCT images were acquir ed of retinal lesions in which there was a displacement of pigmented retina l pigment epithelial (RPE) cells: retinitis pigmentosa and laser photocoagu lation (eight eyes each). RESULTS. The mean thickness of human inner OCT bands (131 mu m; 95% confide nce interval [CI], 122-140 mu m) was 7.3 times that of the retinal nerve fi ber layer (RNFL). This band persisted despite ablation greater than 140 mu m. The inner aspect of the outer OCT band corresponded to the apical RPE, b ut the mean thickness of this band in human tissue (55 mu m; 95% CI, 48-62 mu m) was 2.6 times the thickness of the RPE-choriocapillaris complex. OCT measurement of total retinal thickness was accurate (coefficient of varianc e, 0.05) and precise (coefficient of correlation with light microscopy, 0.9 8). Hyperpigmented lesions gave rise to high signal, attenuating deeper sig nal; hypopigmented lesions had the opposite effect on deeper signal. CONCLUSIONS. The inner band is not RNFL specific, partly consisting of a su rface-related signal. The location, not thickness, of the outer band corres ponds to RPE melanin. Given the additional effect of polarization settings, precise OCT measurement of specific retinal layers is currently precluded.