Optical coherence tomography (OCT) provides in vivo, three-dimensional (3D), high-speed and high-resolution cross-sectional imaging of anterior and posterior eye structure.
1 It has significantly improved our understanding of eye physiology and the pathogenesis of various ocular diseases and helped in disease diagnosis and management.
2 Optical coherence tomography not only provides high-definition imaging of the morphology of normal or diseased eye tissue, but also makes quantitative measurement available. Currently, there are two automatic measurements provided by most commercial OCT machines: peripapillary retinal nerve fiber layer (RNFL) thickness and total retinal thickness. Advanced image analysis further provides measurement of optic nerve head,
3 macular ganglion cell complex,
4 choroidal thickness,
5 and so on. The caliper function is also provided by commercially available OCT software to manually measure the size of macular hole and make other distance-based measurements.
Currently, most of the available OCT quantitative measurements provide spatially dimensional parameters. Optical coherence tomography images, however, deliver two basic types of information: dimension and signal/reflection intensity. Measurement of optical intensity on OCT may provide additional information augmenting morphology-based quantitative characteristics. It has been qualitatively observed that OCT optical intensity changes in several ocular diseases. For example, optical intensity of inner retina increases in retinal artery occlusion.
6 In age-related macular degeneration, the optical intensity increases with development and regression of choroidal neovascularization.
7 However, quantitative assessment of OCT intensity remains much less reported compared to morphologic analysis. In 2000, Pons et al.
8 reported that the internal reflectivity of RNFL was lower in patients with glaucoma compared to controls. This observation was recently confirmed using spectral-domain OCT.
9,10 The optical intensity was also quantitatively investigated in subretinal space,
11,12 photoreceptor,
13 and intraretinal space
14 and in choroidal neovascularization.
15 Outside the retina, it has also been investigated in filtering blebs
16 and posterior capsule.
17 However, to the best of our knowledge, no study has been devoted to optical intensities of retinal layers in normal subjects. Our work is an attempt to overcome the lack of such information about the normal ranges and physiological variations of optical intensities in individual retinal layers.
In this study, we developed an automatic, 3D measurement of the optical intensities in all retinal layers segmented on spectral-domain OCT using an extended research version of the Iowa Reference Algorithm (Retinal Image Analysis Lab, Iowa Institute for Biomedical Imaging, Iowa City, IA).
18–22 Using this technique, we investigated the variations of and relationships among retinal layer optical intensities in normal subjects.