Ancillary diagnostic imaging interpretation
Optical coherence tomography
A normal optical coherence tomography demonstrates the presence of multiple neural retinal layers including the nerve fiber layer, ganglion cell layer, inner plexiform layer, outer plexiform layer, outer nuclear layer, external limiting membrane, photoreceptor layers, and retinal pigment epithelium. The nerve fiber layer represents the axons of the ganglion cell nuclei. The ganglion cell layer contains nuclei of the ganglion cells, the axons of which become the optic nerve fibers. The inner plexiform layer is the region of synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells. The inner nuclear layer contains the nuclei of the bipolar cells. The outer plexiform layer is the region of synapse between the rod and cone projections and the bipolar cells. The outer nuclear layer contains the cell bodies of the rods and cones. The external limiting membrane separates the inner segment portion of the photoreceptors from the cell nucleus. The photoreceptor layer contains both rods and cones. However, the foveola has mostly cones and as a result, the outer retina in this area appears lightly bowed (Figure 1.1). The retinal pigment epithelium is a single layer of cuboidal epithelial cells, underneath but in contact with the photoreceptors.
Fluorescein angiography (FA) examines the circulation of the retina and choroid. FA does not involve the use of ionizing radiation. Fluorescein is injected intravenously and then photographs of the retina are taken using a blue light of 490 nm that causes fluorescence of the dye. There is an exciter filter that allows only blue light of 490 nm to travel to the retina, and a barrier filter, which only allows yellow-green light of 525 nm light to enter the camera (Figure 1.2).
In normal patients approximately 10 seconds after the dye is injected, initial choroidal filling (choroidal flush) should be apparent. By 10–12 seconds following injection, the retinal arteries should begin to fill and should completely fill within 1–2 seconds. The early venous stage (laminar venous filling) occurs at 14–15 seconds, with the late venous stage at 18–20 seconds. Normal arteriovenous transit should be no more than 11 seconds. By 5 minutes, late staining may be apparent, which can be used to demonstrate the presence of abnormal vasculature and leakage.
Indocyanine green angiography
Indocyanine green (ICG) angiography is performed by injecting a cyanine dye intravenously, and allows the visualization of the retinal and choroidal circulation (Figure 1.3). ICG has a peak spectral absorption of 800 nm, which is near the infrared range. It penetrates retinal layers allowing for visualization of deeper structures such as the choroidal circulation. ICG can be helpful in evaluation of choroidal masses and diseases such as polypoidal choroidal vasculopathy, central serous chorioretinopathy, and retinal angiomatous proliferation.
Fundus autofluorescence (FAF) is a noninvasive tool to examine the fluorophores of the ocular fundus. A2E within lipofuscin is a naturally occurring fluorophore and appears bright (hyperautofluorescent) (Figure 1.4).
Other fluorophores include advanced glycation end products and the redox pair FAD–FADH2, which provides information on retinal energy metabolism. Areas of sick or atrophied retinal pigment epithelium will appear dark (hypoautofluorescent). FAF may be helpful in evaluation of retinal diseases such as age-related macular degeneration, retinitis pigmentosa, central serous retinopathy, pseudoxanthoma elasticum, and macular dystrophies.
In red-free photography, the imaging light is filtered to remove red colors, improving contrast of vessels and other structures (Figure 1.5).
Figure 1.6: Color fundus photographs. (a) Right eye. (b) Left eye. Note the clear media, normal caliber of vessels and sharp foveal light reflex. There is some mild cup to disc asymmetry between the two eyes, which may be physiologic.
It can be helpful for visualization of microaneurysms and hemorrhages, which might be missed clinically.
Normal color photographs should demonstrate the presence of clear media, sharp optic nerve margins without cupping, normal caliber of retinal vessels, and a sharp foveal reflex (Figure 1.6).
A B-scan ultrasonography (B scan) uses ultrasound to create a two-dimensional image of the eye (Figure 1.7). This is accomplished using high frequency sound waves of 10 mHz that are transmitted from a probe in direct contact with the eye. B scan can be useful when it is not possible to see ocular structures such as the posterior chamber that can occur in the presence of a dense cataract, dense vitreous hemorrhage, or anterior segment opacification. B scan can evaluate for the presence of retinal detachment, choroidal detachment, posterior scleritis, optic nerve head drusen, and choroidal or scleral masses.