Detailed anatomical data were assembled from past publications into a small database of values. This information was used to make a scale drawing. Using analysis of the available data, it appears likely that reassignment of the anatomical correlates of the bands in the outer retina will be needed. From this preliminary analysis, the 4 outer bands seen in a typical spectral-domain OCT scan in order are the ELM, the ellipsoid section of the photoreceptors, the contact cylinder of the cones, and finally the RPE. While a drawing is not likely to be the final arbiter of band assignments, the findings of the present study suggest that current assignments of Bands 2 and 3 are not correct and additional work needs to be done. This includes methodology to extract actual band thicknesses from the raw OCT data before any transformation is used to display the data and detailed light and electron microscopic analysis of layer thicknesses in the retina. We have begun these projects based on the findings of this preliminary study. Correct band assignment will aide in proper evaluation of both normal and pathologic states in the retina. Early adoption of naming systems for the outer bands provided a way to communicate anatomical findings, but incorrect assignment may pose problems. After adoption of naming assumptions, it is common to base conclusions on these corresponding anatomical correlates in assessing retinal anatomy and pathophysiology. These secondary conclusions and assumptions would need to be revised as well.
In addition to anatomical correlations visible by comparing the OCT images with the drawing, there are other factors that can be used to help validate some of the conclusions. The second band seen in OCT images has commonly been attributed to the IS/OS boundary. A boundary imaged by OCT should have a thickness as influenced by the PSF and the variation in how the various cellular features align with each other. However, the anatomy of the transition between the ISs and OSs raises concerns. First for cones there actually is no sharp boundary. Cone OS disks are actually comblike projections of the plasma membrane that start in steps near the IS. Rod OS are plasma membrane-bounded stacks of disks clearly separate from the IS and connected to it by the cilium. Second, extensions of the ellipsoid cytoplasm over the OSs (the calycal processes) would have the optical characteristic of blurring the boundary of refractive index change. This strategy is used to reduce reflection in optical design.67
Third, an increasing deviation from normal occurs for light rays striking photoreceptors with increasing eccentricity; the nodal point of the eye is anteriorly displaced to the radius of the retinal surface.34
This would decrease the reflection with increasing displacement from the optical axis. The reflective potential of the “boundary” would be enhanced if it actually were a sharp transition. Even making this assumption, the proportion of incident light reflected (LR
) for light normal to the boundary can be approximated by the Fresnel equation as LR
, where nI
is the refractive index of the ISs and nO
is the refractive index of the OS. Given the known refractive indexes (as shown in the ), the proportion of light reflected by a boundary would be approximately 0.00021% for cones and 0.00012% for rods. This result is not entirely unexpected because it would not be efficient for a light receptor system to reflect light. Thus, ascribing one of the brightest OCT bands in the posterior fundus to a region with an extremely low reflectance is not likely to be correct. In addition, a boundary should have an observed thickness close to the PSF of the OCT instrument. The second band is usually nearly as thick as the fourth band, which represents at least the thickness of an entire RPE cell (10–14 μ
m). Finally, because of its very small size, one other candidate for the second band, the connecting cilium,68
would not be visible using OCT. The cilium is outnumbered by a factor greater than 100:1 by the IS mitochondria, which also makes it difficult to ascribe any significant observable reflection arising from the cilium.
Band 3 in the OCT appears to correspond to the contact cylinder between the RPE apical processes and the external portion of the cone OS. This layer is clearly not Verhoeff membrane. It also, strictly speaking is not the photoreceptor tips, which would imply the reflection only comes from the ends of the OS. The third band has a definite thickness and so the reflection must come from a region, not a boundary. Some investigators have claimed that outside of the central macular region, the cone OS tips and rod OS tips (what they called ROST) formed different bands in OCT images.69,70
Curiously the layer termed COST in these reports was not a discrete band in higher-magnification illustrations.69,70
However, the rod OS terminate at the RPE at a distance well below the resolution of OCT, and thus could not be detected as a band distinct from the fourth band by OCT. In addition, if the rod OS tips were visible, one would expect some degree of radial symmetry around the fovea, and the extra band should be seen in many eyes, but was not demonstrated in published reports.69,70
The band reported to be the rod OS tips has been seen only in retinal regions around the optic nerve and in only some eyes. A separation of the rod OS tips from the RPE in this area has not been described in histologic reports. An alternate explanation may be advanced based on known histology.23,58
Cones become wider and shorter with increasing distance from the fovea. However, there are curious patches of cones near the optic nerve that are much shorter and wider (8 μ
m) than elsewhere.23,58
It is possible that these localized subpopulations of cones provide a different reflective level for Band 3 in this region leading to the ambiguity causing some to think the band was arising from the rods ().
Fig. 8 Near the nerve, there are isolated patches of short, wide cones seen in otherwise normal donor eyes.26 Because of the displacement of the contact cylinder anteriorly, it is possible for more than one level to be observed. The rods are not shown in this (more ...)
While the correspondence between the ellipsoid portion of the photoreceptors to Band 2 and the contact cylinder to Band 3 appears to be robust, the widths of the bands corresponding to the ELM and the RPE are somewhat less so. In the case of the ELM, the observed band width is broadened by the PSF of the OCT and is likely to originate from a thin structure. In addition, the log transform broadens any detected band. Thus, the LRP peak projected from the OCT to the drawing ( and ) is misleading and actually should be much narrower. The RPE width is harder to explain. In the perifoveal location, the width of the RPE band matches up fairly closely with the size of the RPE in the drawing. In the foveal location, the fourth band is much thicker than the RPE cell in the drawing. There are many possible explanations for this discrepancy, but one significant problem is that it is not known at present what, in addition to the RPE, is contained in the fourth band. There are regional differences in Bruch membrane and the choroid, and to the extent these are also contained in the fourth band, topographic thickness variations could occur.
This article represents a preliminary analysis comparing OCT-derived anatomical information with previously published data on outer retinal histology. The present study is a novel cross-check of established knowledge of retinal anatomy and compared that specific reflectance information obtained by OCT. Additional analysis of the imaging characteristics of OCT instruments and the reflective properties of boundaries was performed to provide supportive evidence. The results of this initial study suggest that names and anatomical structures commonly attributed to some of the hyperreflective bands visualized by OCT in the outer retina are not correct. The present study has the potential for numerous weaknesses. Interest in one structure or another in the outer retina varied over the years and no one paper or book described or measured all of the values needed for this article. Many studies were based on a limited number of eyes, and values reported may have been inaccurate. Another limitation is that our conclusions were drawn on the basis of images generated by and subject to the constraints of commercially available instruments. Other instruments may display the bands differently as OCT technology continues to evolve. This study was a hypothesis generation effort looking at broad principles and was not designed to evaluate specific OCT instruments or potential in subject variation related to age, gender, or race. Additional studies could include comparison between OCT in vivo with histologic correlative evaluations; these are currently underway. The impetus for these additional studies would not have occurred without first analyzing the possibility that the outer lines may have not been correct in the first place. Our goal is to develop nomenclature that is internally consistent and based on anatomical ground truth. Comprehensive and accurate measures of the width, spacing, and regional differences in OCT bands and candidate correlates for them in outer retina will be useful for reaching that goal. Given the clinical utility of OCT and the numerous assumptions concerning health and disease made based on OCT imaging, rapid and definitive resolution of these basic band assignments is important.