There is no question that optical coherence tomography (OCT) has revolutionized how optometrists and ophthalmologists examine the eye.
OCT has become indispensable for the medical eye care practitioner. Whether it is helping to localize a retinal lesion to a certain retinal layer, or finding retinal nerve fiber layer defects, OCT is as important to medical eyecare as the phoropter is to refraction. Though OCT cannot replace a dilated fundus exam and a keen eye and mind, it has significantly eased the clinician’s burden in differentiating ocular pathology.
Optical coherence tomography is a noninvasive technology for imaging various ocular tissues using light. Modern day OCT allows indirect visualization of the posterior vitreous, retina, retinal pigment epithelium (RPE), and choroid. Unlike fundus photography or funduscopy, OCT images are false views of the retinal anatomy, generating a map of changes in refractive indices. After significant research and clinical work, these optical maps have been correlated to known retinal anatomy.
Large changes in the index of refraction, such as between the vitreous and the retina, will lead to a very distinct demarcation on OCT. Optically homogeneous structures (which reflect little light) will be relatively dark (such as the vitreous) whereas optically heterogeneous structures (which reflect a lot of light) will be relatively bright (such as the retinal nerve fiber layer).1
When looking at OCT B-scan images with the false color scale, highly reflective structures are represented by warmer colors such as red and orange whereas less reflective structures will be illustrated as cooler colors like green and blue.
When looking at an OCT image of the macula, we often forget that we are not looking at a true image due to the near histologic resolution that modern day OCTs are able to achieve.
As we examine the following OCT images, keep in mind that we are not viewing retinal tissue as it truly is, but as the machine interprets it. It is always helpful to keep a copy of a normal OCT image when examining pathology or to explain pathology to patients.
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For this article, all images will be presented in grayscale as I have found it easier to discern the various layers and to spot pathology in grayscale than it is with color images.
The Zeiss Cirrus HD-OCT and the Zeiss Cirrus Photo are both straightforward to use but produce high quality and reproducible images. For these reasons, we chose to use images rendered with a Cirrus 5000 and Cirrus Photo in this article.
The Zeiss Cirrus is a full capability OCT which can image the cornea, iridocorneal angle, retina and optic nerve. Images of the retina can be performed with different types of scans varying in orientation and resolution.
The 512 x 128 Macular Cube Scan is the workhorse of retinal imaging. It allows for a broad scan of the whole macula. Numerous post-scan analyses such as Macular Thickness Map, Guided Progression Analysis, and RPE Segmentation Map can be performed to aid in the diagnosis and management of retinal disease. The numerous HD and Raster scans provide higher resolution scans which can pick up minute changes in retinal architecture.
The Zeiss Cirrus Photo combines all these imaging modalities with a high-quality fundus camera. The correlated images allow for quick and easy distinction of retinal lesions. All the following images except for the Normal Retinal OCT were completed with the Zeiss Cirrus HD-OCT during routine clinic evaluation using the 512 x 128 Macular Cube Scan; the Normal Retinal OCT image was captured with a 5 Line Raster Scan.
Want more content dedicated to ocular disease? Check out our medical optometry resource page for tons of clinical cases and images and information on using OCT and ultra-widefield imaging.
Table of Contents
- Normal Retinal Optical Coherence Tomography
- Vitreomacular Traction on Optical Coherence Tomography
- Macular Hole on Optical Coherence Tomography
- Epiretinal Membrane on Optical Coherence Tomography
- Macular Edema on Optical Coherence Tomography
- Central Serous Chorioretinopathy on Optical Coherence Tomography
- Hard Exudate on Optical Coherence Tomography
- Cotton Wool Spot on Optical Coherence Tomography
- Drusen on Optical Coherence Tomography
- Congenital Hypertrophy of the Retinal Pigment Epithelium on Optical Coherence Tomography
- Choroidal Nevus on Optical Coherence Tomography
Normal Retinal Optical Coherence Tomography
This OCT image is the right eye of a healthy 27-year-old male’s right eye.
Eighteen anatomic landmarks can be visualized here. These landmarks have recently been standardized by the International Nomenclature for Optical Coherence Tomography Panel which is staffed by several of the world’s leading OCT experts.2
There is a stark change in the index of refraction between the optically empty vitreous and the highly
reflective internal limiting membrane (ILM). The retinal nerve fiber layer (RNFL) is thin on the temporal side of the fovea and thick on the nasal side, corresponding to the papillomacular bundle.
Often the posterior cortical vitreous can be seen just barely attached to the macula or floating above it in the case of a complete posterior vitreous detachment.
The ganglion cell layer is significantly less reflective than the RNFL and appears as the darker layer below it. The inner and outer plexiform layers are made up of cell layer connections which are optically irregular leading to higher reflectivity and lighter layers in the image. The inner and outer nuclear layers are composed of densely organized nuclei which are minimally reflective and thus show up as relatively dark layers. The external limiting membrane (ELM) shows up as the first of several thin bands in the outer retina.
The ellipsoid zone (EZ), which has had a contentious naming history and has previously been called the connecting cilia, inner segment-outer segment junction, photoreceptor integrity line, and inner ellipsoid zone, is significantly reflective and separates the myoid zone and the photoreceptor outer segments.3
The integrity of the EZ often correlates to potential retinal acuity. What was once referred to as the RPE or RPE/Bruch’s Complex has now been renamed the interdigitation zone and the RPE/Bruch’s Complex. Under high-resolution conditions, these two layers will show up as two distinct highly reflective bands but under lower resolution conditions can meld into one thicker highly reflective band.
Going forward, I will refer to the interdigitation zone and RPE/Bruch’s Complex simply as the RPE (as there is little clinical value for basic clinical interpretation).
The choroid is visualized posteriorly as three layers, the choriocapillaris, Sattler’s Layer, and Haller’s Layer. These layers may be hard to distinguish with a poorer resolution OCT image. The choroidal-scleral junction is the outer limit of the decipherable OCT layers. The sclera is imaged as the large, homogeneous, outermost structure where little detail is noted.
Vitreomacular Traction on Optical Coherence Tomography
Vitreomacular traction (VMT) is a relatively common vitreoretinal disorder that is often the product of normal vitreal liquefaction and vitreous cortex separation.4
In the majority of cases, this normal age-related process will culminate in subjective floaters and the presence of a Weiss ring upon dilated fundus exam.
In a minority of cases, the separation of the posterior cortical vitreous from the internal limiting membrane will be incomplete leading to VMT. Ophthalmoscopically, VMT might be difficult to discern but can seen as loss or distortion of the foveal reflex.
The classification of VMT, as visualized by OCT, is broken down by size into focal (≤1500µm) or broad (≥1500µm) and whether it is isolated or if concurrent macular pathology is present.4 VMT causes disruption of the macular architecture in the form of foveal distortion, elevation, schisis, edema, or pseudocyst formation.4