What is Optical Coherence Tomography (OCT)? Basic Interpretation Approach

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What is OCT? 

Optical Coherence Tomography (OCT) is a non-invasive and well-tolerated technology that uses a beam of light to image the layers of the retina resulting in 10-20 times higher resolution than ultrasound images. OCT performs microscale and cross-sectional tomographic imaging which provides a quantitative idea of the macula. This has helped in diagnosing, monitoring and managing various posterior segment diseases. Furthermore, it has helped in the advancement of the research area.

To master any ocular diagnostics tool, a clinician must have command over the three key components which are:

  • Instrumentation,
  • Operational, and
  • Interpretation.

However, this writing is primarily focused on the basic interpretation of the OCT report aiming to ease the clinician but, at the same time, the instrumentation and operational parts are of equal importance too. Before knowing how the diseased retina or macula looks like, it’s very important to learn the normal macula on an OCT at first.

Optical Coherence Tomography Normal Macula Scan

Normal Macular OCT Scan

Normal Eye Macula Optical Coherence Tomography OCT Report
Layers of Retina on OCT Scan Layers of Retina on OCT Scan
Usually, it is characterized by two high reflective structures, the nerve fibre layer (NFL) and retinal pigment epithelium (RPE) choriocapillaris complex (RPE-CC). So, in between the two red (strong) layers, there are the various layers of the sensory retina represented with shades of green corresponding to particular layers of the retina.

A Basic Approach for Interpretation of Optical Coherence Tomography (OCT)

The learning of this part can be achieved only if we are able to ask 4 sets of standard questions to ourselves.

The 4 sets of questions are as:

  1. How does the vitreoretinal interface appear?
  2. What is the foveal contour like?
  3. Is the retinal architecture altered (status of the neurosensory retina)?
  4. Whether the uniformity of retinal epithelial choriocapillaris(RPE-CC) is disrupted?

 

How does the vitreoretinal interface appear?

 A normal vitreoretinal interface is: Above: Optically clear Vitreous Below: A high reflective nerve fibre layer (NFL).
Normal vitreo retinal interface
Sometimes, the vitreoretinal interface abnormalities can be a membrane that may be single or double and they may have no attachment, partial attachment or total attachment.

What is the foveal contour like?

  1. The normal foveal contour is a V-shaped depression
  2. The abnormality in the foveal contour can be in the form of obliteration of the foveal contour which is due to either pulling mechanism (from an overlying membrane) or by pushing mechanism (from underlying intraretinal fluid).
  3. Widening of the foveal contour is another abnormality that indicates thinning of the foveal area.
Foveal contour on OCT Scan

d. Macular Holes: may be full thickness or lamellar (inner lamellar or outer lamellar). Macular hole index (MHI) from various methods can be measured from the OCT.

Meanwhile, we need to rule out the pseudo hole from the inverted mushroom shape of the inner lamellar hole.

Macular hole OCT and Pseudo macular Hole Optical Coherence Tomography
Macular Hole Vs Pseudo Macular Hole

 Is the retinal architecture altered?

Always remember the normal retinal architecture and its orientation and reflectivity.
i) The alteration in the retinal architecture can be due to accumulation of fluid which is either intraretinal or subretinal fluid. Further, the intraretinal fluid accumulation can be diffuse or with cystic spaces.
Retinal architecture on OCT
ii) The hard exudates appear as spots of increased hyperreflectivity with a trail of shadowing behind it.
iii) Schisis, split in the macula, can be easily diagnosed.
How does exudates looks on Optical coherence topography oct scan

Whether the uniformity of retinal epithelial choriocapillaris (RPE-CC) is disrupted?

i) The regular uniformity of the hyperreflectivity of the RPE-CC complex can be disrupted by a bumpy appearance due to drusen or fusiform thickening (CNVM).
ii) There may be elevation noted due to RPE-detachments, commonly called Pigment Epithelial Detachment (PEDs).
  1. Serous PED: Optically clear space with distinct reflectivity from the choroid.
  2. Hemorrhagic PED: Similar to serous PED but the absence of reflectivity from the underlying choroid as because of the shadowing effect of the blood beneath RPE.
  3. Fibrovascular PED: Multiple medium reflectivity echoes and absence of distinct reflectivity of the choroid.
uniformity of retinal epithelial choriocapillaries
This writing has been prepared during the internship period (2012/13) at LVPEI, Visakhapatnam, India.
Acknowledgements:
• Dr Avinash Pathengay, Vitro-Retina Surgeon.
• P. Anand and G. Rekha, Optometry Head.
• LVPEI-Visakhapatnam Team
About the author: Birendra Mahat M.Optom Student Tilganga Institute of Ophthalmology & Consultant Optometrist of Metro Eye Care, Kathmandu Nepal

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