Posterior segment evaluation with optical coherence tomography (OCT) allows visualization of the vitreous, retinal layers, retinal pigment epithelium (RPE) and the choroidal layers.
Figure 1: SD-OCT scan of a normal right eye. The vitreous, retinal layers and choroidal layers are visible. This scan was acquired with the Cirrus SD-OCT with 5-line raster set at 9mm with enhanced depth imaging (EDI).
Vitreous
The posterior hyaloid of the vitreous can be identified on macula scans especially with spectral-domain (SD-OCT). This is especially useful in the evaluation of posterior vitreous detachments, vitreomacular traction or vitreopapillary traction. The instrument uses ~840nm wavelength of light and is not able to penetrate denser vitreous opacities like Weiss rings or asteroid hyalosis.
Figure 2: Asteroid hyalosis blocking OCT scans and causing a shadow effect
Figure 3: Posterior hyaloid membrane is visible. Vitreous detachment from the macula with still vitreopapillay adhesion. Stage 2 posterior vitreous detachment.
Figure 4: Vitreomacular traction with significant macular edema. The posterior hyaloid membrane is visible and attached to the macula and optic nerve.
Retina
Each distinct retinal layer is visible on the OCT and corresponds well to histological studies. Certain types of retinal conditions can affect different retinal layers. The retinal nerve fiber layer and retinal ganglion cell layer are affected in conditions like glaucoma. The inner retinal layers can thin secondary to retinal vascular occlusions like retinal artery occlusions. The outer retinal layers can become atrophy secondary to conditions that affect the RPE, for example in age-related macular degeneration or central serous chorioretinopathy.
Figure 5: SD-OCT scan of a normal right eye. ILM- inner limiting membrane; RNFL- retinal nerve fiber layer; GCL- ganglion cell layer; IPL- inner plexiform layer; INL- inner nuclear layer; OPL- outer plexiform layer; ONL- outer nuclear layer; ELM- external limiting membrane; EPIS or EZ or IS/OS junction- ellipsoid portion of the inner segments or ellipsoid zone, previously referred to as inner segment outer segment junction line; COST or IZ- cones outer segment tips line or interdigitation zone; RPE- retinal pigmented epithelium.
Four distinct hyper-reflective lines are seen in the outer retinal layers of a SD-OCT scan on a healthy eye. These four hyper-reflective lines should always be identified when evaluating a macula scan.
Figure 6: From left to right
RPE- retinal pigment epithelium
COST- cones outer segment tips line or IZ- interdigitation zone
EPIS - ellipsoid portion of the inner segments or EZ- ellipsoid zone or IS/OS- inner segment outer segment junction line
ELM- external limiting membrane
Choroid
The choroid can be visualize on SD-OCT scans. Utilizing enhanced-depth imaging (EDI) or imaging with longer wave-length OCT the details and thickness can be intensified. Certain conditions that affect the choroid have been found to have an abnormally thicker choroid. In central serous chorioretinopathy and polypoidal choroidal vasculopathy the choroidal thickness is thicker with visible large dilated choroidal vessels compared to age-matched normals.
Figure 4: SD-OCT utilizing enhanced-depth imaging (EDI) of a patient with a branch retinal artery occlusion. The choroid and choroidal vessels are easily visible secondary to overlying retinal atrophy. The borders of the choroid-scleral junction can be seen.