Optical coherence tomography (OCT) is a non-invasive imaging technique that uses light to capture high-resolution, cross-sectional images of the retina and anterior segment of the eye. OCT provides depth resolution on the scale of 10 microns, allowing it to visualize and measure individual layers of the retina. OCT can detect various retinal pathologies through qualitative and quantitative analysis of the pre-retinal, overall retinal, foveal, and macular profiles. It is useful for diagnosing conditions like macular edema, retinal detachments, and glaucoma.

1. By DR TAPAN JAKKAL DEPT OF OPHTHALMOLOGY G.M.C.H AURANGABAD Optical Coherence Tomography (OCT)

4. Optical coherence tomography- The process is similar to that of ultrasonography, except that light is used instead of sound waves. Analog to ultrasound

5. OCT (TD – principle) single reflection site

6. Construction of tomographic image Transverse Scanning Backscatter Intensity Axial Scanning (Depth)

9. OCT Image of Normal Fovea The OCT image above can be compared to what we know about retinal anatomy from conventional microscopic sections. The vitreous is the black space on the top of the image. We can identify the fovea by the normal depression. The nerve fiber layer (NFL) and the retinal pigment epithelium (RPE) are easily identifiable. These layers are more highly reflective than the other layers of the retina. This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, white) in the false color representation of the OCT . The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan.

14. Anomalous structures that have been observed in the pre-retinal profile include the following: 1. pre-retinal membrane 2. epi-retinal membrane 3. vitreo-retinal strands 4. vitreo-retinal traction 5. pre-retinal neovascular membrane 6. pre-papillary neovascular membrane A pre-retinal membrane with traction on the fovea is pictured below.

15. The over-all retinal profile The normal over-all retinal profile has a slightly concave curvature that you would expect from observing the surface of a globe. Abnormal profiles would include exaggerated concavity and convexity. Retinal folds would also result in an abnormal over-all profile.

16. The following OCT image demonstrates an abnormal convexity in the over-all retinal profile. In this case, a pigment epithelial detachment is causing the convexity.

17. The image below demonstrates an abnormal concavity to the over-all retinal profile. Aside from the retinal detachment, notice the underlying concave curvature of the retina, suggesting the long eye of a significant myope.

18. The foveal profile The normal foveal profile is a slight depression in the surface of the retina, as pictured below.

19. Deformations that have been observed in the foveal profile include the following: 1. macular pucker 2. macular pseudo-hole 3. macular lamellar hole 4. macular cyst 5. macular hole, stage 1 (no depression, cyst present) 6. macular hole, stage 2 (partial rupture of retina, increased thickness) 7. macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) 8. macular hole, stage 4 (complete hole, edema at margins, complete PVD)

21. macular cyst

25. The macular profile The macular profile can, and often does, include the fovea as it's center. Therefore, a common OCT scan length of 6 mm would include 3 mm of the macula on each side of the fovea.

26. Deformations that have been observed in the macular profile include the following: 1. serous retinal detachment (RD) 2. serous retinal pigment epithelial detachment (PED) 3. hemorrhagic pigment epithelial detachment A serous PED is pictured below. We know that it is a PED because the fluid (black space around the arrow) is pushing up underneath the retinal pigment epithelium, identified by the relatively highly reflective (red and orange) line (arrow).

27. Intra-retinal anomalies that have been identified in the macular profile include: 1. choroidal neovascular membrane 2. diffuse intra-retinal edema 3. cystoid macular edema 4. drusen 5. hard exudates 6. scar tissue 7. atrophic degeneration 8. sub-retinal fibrosis 9. RPE tear

28. Cystoid Macular Edema OCT is capable of detecting small, fluid-filled, cystic spaces within the macula.

29. Central Serous Chorioretinopathy Central serous chorioretinopathy is characterized by the presence of fluid between the RPE and neurosensory retina.

30. Diabetic Retinopathy Exudates appear as accumulation of dense material within the neurosensory retina.

32. The scan below has waves in the retinal contour. These are not retinal folds, but rather movement of the eye during the scan pass .

34. 6.Use the two buttons near the joystick for freezing and saving scans. This saves you from having to juggle the joystick and the mouse. 7.Minimize patient fatigue by keeping scan time to a minimum. Never scan an eye for more than 10 minutes (FDA regulation). 8.Keep the cornea lubricated. Use artificial tears and have the patient blink when you are not saving a scan pass. 9.Move the instrument on the x and y axis (using the joystick) to work around opacities.

37. Image shows an anterior-chamber angle as viewed with gonioscopy and the OCT The latter replaces subjective evaluation with objective measurement.

38. A narrow angle is apparent with OCT imaging, in this case 9.5°.

39. With the increase in popularity of anterior chamber imaging, and anterior segment OCT proving to be the best tool for high resolution biometry, Anterior Segment Optical Coherence Tomography is a must-have for anterior segment, refractive, cornea, and glaucoma surgeons.