Optical Coherence Tomography

OCT

1. OPTICAL COHERENCE TOMOGRAPHY TYPES, INTERPRETATION AND USES Manoj Aryal B . Optometry Institute Of Medicine, Maharajgunj Medical Campus

2. PRESENTATION LAYOUT Introduction History Theories & Principles Types Interpretation Clinical Applications Limitations & Advantages Latest Developments

3. Introduction Optical coherence tomography, or OCT is a non-contact, noninvasive imaging technique used to obtain high resolution 10 cross sectional images of the retina and anterior segment. Reflected light is used instead of sound waves. Infrared ray of 830 nm with 78D internal lens.

4. HISTORY- OCT Timeline 1991–Concept of OCT in ophthalmology 1993 - First in vivo retinal OCT images 1994-OCT prototype 1994-Anterior segment/Cornea OCT 1995-The First Clinical Retinal OCT 1995-The First Glaucoma OCT 2002 – Time domain OCT (e.g. Stratus) 10 µm axial resolution scan velocity of 400 A-scans/sec 2004 – Concept of spectral domain OCT introduced 2007 – Spectral domain OCT 1 -15 µm axial resolution up to 52,000 A-scans/sec

5. Theories and principle OCT images obtained by measuring echo time intensity of reflected light Effectively ‘ optical ultrasound’ Optical properties of ocular tissues, not a true histological section

6. Laser output from OCT is low, using a near-infra-red broadband light source Measures backscattered or back-reflected light Source of light: 830nm diode laser 1310 nm : AS-OCT

7. Principle

8. L ight from Reference arm & Sample arm combined Division of the signal by wavelength Analysis of signal Interference pattern A-scan created for each point B-Scan created by combining A-scans

9. Digital processing aligns the A-scan to correct for eye motion . Digital smoothing techniques further improves the signal to noise ratio . The small faint bluish dots in the pre-retinal space is noise This is an electronic aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures

10. Color coding in OCT Highly reflective structures are shown in bright colures ( white and red) . T hose with low reflectivity are represented by dark colours (black and blue ). Intermediate reflectivity is shown G reen.

11. OCT Vs USG Advantages Non-invasive Non-contact Minimal cooperation needed Resolution ~ 10 μm Pick up earliest signs of disease Quantitatively monitor disease/staging Disadvantages Best for optically transparent tissues Diminished penetration through Retinal/ subretinal hemorrhage Requires pupil diameter > 4 mm OCT

12. Advantages Resolution of ~ 50 μm Anterior segment of the eye Not limited to optically transparent tissues i.e . opaque corneas Disadvantages Direct contact Penetration of only 4-5 mm Image influenced by Plane of section Distance to anterior chamber Orientation of the probe Room illumination Fixation Accommodative effort USG

13. Resolution of an OCT Axial resolution -Wavelength and -Bandwidth of the light source Long wavelength - visualisation of choroid , laminar pores, etc Transverse resolution Based on spacing of A-scans Limited by optics of eye and media opacity

14. Speed of acquisition Faster acquisition speed in the newer generation OCT Increased signal-noise ratio Reduced motion artifacts Spectral domain OCT :1-15 µm axial resolution & Up to 52,000 A-scans/sec

15. Time domain-OCT Types of OCT

16. Spectral Domain OCT

17. Spectral-domain OCTs: – Spectralis (Heidelberg) Cirrus (Zeiss) RTVue ( Optovue ) Optovue and Cirrus : Anterior eye imaging capabilities in addition to posterior eye Spectralis : Require special lens and anterior segment module for anterior eye imaging

18. SPECTRALIS-Anterior Segment Module New dimension to anterior segment imaging Cornea Angle structure Iris details Consists of Add-on lens and dedicated software Compatible with all SPECTRALIS SD-OCT models INTERPRETATION & CLINICAL APPLICATIONS

19. AS-OCT using light of wavelength 1310 nm Better detail of non-transparent tissues increased penetration & illumination power High-speed Fourier domain optical depth scanning Scan speed of 2000 A scans/second Axial resolution – 18 micron Transverse resolution – 60 micron Reduced motion artifact SD-OCT using light of wavelength 830nm Axial resolution of 5 micron Higher resolution allows better visualization of cornea and angle and it’s structures Provided a scan depth greater than 6.30nm- allowing imaging of entire AC depth Reduced overlap artifacts

20. A study comparing AS-OCT with Goniscopy AS-OCT detected more closed angles than gonioscopy Disparity to attributed Possible distortion of the anterior segment by contact gonioscopy Differences in illumination

21. OCT – Posterior Segment Module Glaucoma ONH analysis Retina Choroid

22. GLAUCOMA D iagnosis of glaucoma difficult in early stage I nfrequency of episodes of rise in the IOP V isual field tests not being sensitive enough Glaucoma diagnosis traditionally performed by examining optic nerve cupping width of the neuroretinal rim

23. Limitations of Visual Field Tests: Visual field loss late clinical findings D etected only after significant loss of retinal nerve fibers D ifficult to differentiate early glaucoma from normal

24. Ganglion cells outside the paramacular region N ot multilayered Early losses more readily detected by VF testing Not central visual field defects However , losses of ganglion cells possibly occur in Paramacular region Outside the paramacular region simultaneously

25. M ultiple layers of ganglion cells in the paramacular & macular region L oss 5 layers of these cells before the visual fields show abnormality in central area

26. 3 -dB sensitivity loss at a single location in the perifoveal area on Humphrey visual field testing associated with loss of approximately 230 ganglion cells compared with loss of 10 ganglion cells in the peripheral posterior pole retinal thickness losses correlated more strongly with the severity of optic nerve cupping than with visual field changes

27. Role of OCT in Glaucoma-Recent Advances Any decrease in the overall retinal thickness an indicator of a loss of the ganglion cell layer and RNFL OCT detect nerve fiber layer thinning before the onset of visual changes Potential of diagnosing glaucoma early examining the retinal thickness in the macular area Nerve fiber layer thickness, as measured by OCT, has been shown to correspond to visual function

28. Circle Scan Differences betweeen average thickness in sectors (along the calculation circle) in each eye OCT Scan with automatic segmentation of RNFL TSNIT RNFL thickness compared to normative database RNFL Thickness in quadrants & sectors compared to normative database

29. Posterior Pole Retinal Thickness Map with Compressed Color Scale in 8x8 Analysis Grid Mean Thickness Hemisphere Analysis with Asymmetry Gray Scale OCT scan of macular region

30. Posterior Pole Asymmetry Analysis Combines mapping of the posterior pole retinal thickness with asymmetry analysis Both eyes Hemispheres of each eye

31. Interpretation of Asymmetry Analysis Posterior Pole Retinal Thickness Map Retinal thickness over the entire posterior pole for each eye Compressed Color Scale H ighlight early retinal loss too small to be detected with standard color scales 8x8 Analysis Grid Positioned along the fovea to disc axis Mean retinal thickness is given for each cell

32. Asymmetry Maps C ompare relative macular thickness between corresponding grid Gray Scale Gray: thickness less than the corresponding cell White : thickness the same or greater than the corresponding cell Hemisphere (S-I and I-S)Asymmetry Compares thickness of cells between hemispheres of the same eye Mean Thickness Mean retinal thickness for the entire grid area and for each hemisphere

33. Case 1: A 53 year old female patient : glaucoma suspect due to borderline IOP of 23 mm Hg Right optic nerve: 0.5 cup with an infero -temporal RNFL loss (arrows ) The visual fields normal in both eyes along with the rest of the eye examination .

34. Case 2: A 55-year- old female diagnosed with primary open angle glaucoma OD

35. NEURO-OPHTHALMIC In the evaluation of ONH Optic disc edema Optic neuritis Optic atrophy

36. RETINA OCT image display , Highest reflectivity - red nerve fiber layer retinal pigment epithelium and choriocapillaris Minimal reflectivity appear blue or black photoreceptor layer choroid vitreous fluid or blood

37. Ganglion Cell Complex C ollective term RNFL G anglion cell layer and I nner plexiform layer GCC thought to be affected in early glaucoma

38. Hyper reflective scans RNFL ILM, RPE RPE- choriocapillaries complex PED Drusen , ARMD CNVM lesions Anterior face of hemorrhage Disciform scars Hard Exudates Epiretinal membrane

39. PED

40. Drusen of the Retina

41. Disciform scar

42. Hypo reflective scans Retinal atrophy Intraretinal/subretinal fluid

43. Yes shadows (cone effect): No shadows . Superficial layers Normal retinal blood vessels Serous collections Dense collection of blood Scanty hemorrhage Cotton wool exudates Deep layers Hard exudates ( lipoproteins ) RPE hyperplasia Intraocular foreign body Dense pigmented scars Choroidal nevi Thick SRNVM

44. Regions : The P re-retina The E pi -retina The I ntra-retina The Sub-retina

45. The pre-retinal profile A normal pre-retinal profile is black space Normal vitreous space is translucent The small, faint bluish dots in the pre retinal space is noise This is an electronic alteration created by increasing the sensitivity of the instrument to better visualize low reflection structures

46. Anomalous structures in P re-retinal area: Pre-retinal membrane Epi -retinal membrane Vitreo -macular traction

47. Deformations in the foveal profile Macular pucker Macular lamellar hole Macular hole, stage 1( no depression, cyst present) Macular hole, stage 2 (partial rupture of retina, incraesed thickness) Macular hole stage 3 (hole extends to RPE, increased thickness, some fluid) Macular hole, stage 4 (complete hole, edema at margins, complete PVD)

48. Lamellar macular hole

49. Full thickness macular hole without PVD

50. Deformations in the macular profile Serous retinal detachment

51. Deformations in the macular profile Serous retinal pigment epithelial detachment

52. Deformations in the macular profile Hemorrhagic pigment epithelial detachment

53. Intra-retinal anomalies in the macular profile Choroidal neovascular membrane Drusens Hard exudates Scar tissue RPE tear

54. OCT deformations: Concavity myopia Convexity PED S ubretinal cysts S ubretinal tumors Disappearance of foveal depression

55. CSR

56. Patterns of Diabetic macular edema in OCT: Sponge like thickening of retinal layers: Mostly c onfined to the outer retinal layers due to backscattering from intraretinal fluid accumulation Large cystoid spaces involving variable depth of the retna with intervening septae Initially confined to outer retina mostly Serous detachment under fovea Tractiional detachment of fovea Taut posterior hyaloid membrane

57. Fovea Loss of foveal photoreceptors can be assessed with OCT, as occurs with full-thickness macular holes central scarring or fibrosis Steepening of the foveal contour epiretinal membranes and macular pseudoholes or lamellar holes . Loss or flattening of the foveal contour impending macular holes foveal edema or foveal neurosensory detachments.

58. OCT: ARTIFACTS Artifacts in the OCT scan are anomalies in the scan that are not accurate the image of actual physical structures, but are rather the result of an external agent or source Misidentification of inner retinal layer : Occurs due to software breakdown, mostly in eyes with epiretinal membrane vitreomacular traction or macular hole .

59. Mirror artifact/inverted artifact : Noted only in spectral domain OCT machines . Subjects with higher myopic spherical equivalent, less visual acuity and a longer axial length had a greater chance of mirror artifacts .

60. Misidentification of outer retinal layers : Commonly occurs in outer retinal diseases such as central serous retinopathy ,AMD, CME and geographic atrophy.

61. Out of register artifact : Out of register artifact is defined as a condition where the scan is shifted superiorly or inferiorly such that some of the retinal layers are not fully imaged.  This is generally an artifact, which is operator dependent and caused due to misalignment of the scan

62. Degraded image: Degraded images are due to poor image acquisition. These images were generally associated with non-retinal diagnosis . Cut edge artifact : This is an artifact where the edge of the scan is truncated. R esult in abnormality in peripheral part of the scan and do not affect the central retinal thickness measurements

63. Off center artifact: Happens due to a fixation error. Happens mostly with subjects with poor vision, eccentric fixation or poor attention. Motion artifact: Noted due to ocular saccades, change of head position or due to respiratory movements

64. Blink artifacts: These are noted when the patient blinks during the process of scan which are noted as areas of blanks in the rendered en-face image and macular thinning on macular map.

65. OCT artifact and what to do? OCT artifact Remedial measure Inner layer misidentification Manual correction Outer layer misidentification Manual correction Mirror artifact Retake the scan in the area of interest Degraded image Repeat scan after proper positioning Out of register scan Repeat the scan after realigning the area of interest Cut edge artifact Ignore the first scan Off center artifact Retake the scan/manually plot the fovea Motion artifact Retake the scan Blink artifact Retake the scan

66. New Spectralis OCT Features I maging of deeper tissue structures Difficult due to : P igment from the Retinal Pigment Epithelium (RPE ) L ight scattering from the dense vascular structure of the choroid Enhanced Depth Imaging (EDI) : N ew imaging modality on the Spectralis OCT P rovides an enhanced visualisation of the deeper structures, like choroid P articularly useful for imaging pigmented lesions in the choroid such as naevi and melanomas

67. Limitations of OCT P enetration depth of OCT is limited L imited by media opacities D ense cataracts V itreous hemorrhage Lead to errors in RNFL and retinal layer segmentation Each scan much be taken in range and in focus must be examined for blinks and motion artifacts Axial motion is corrected with computer correlation software transverse motion cannot be corrected

68. Contd . Unable to visualise neovascular network or analyse if a CNV is active fluorescein angiography still has a significant role OCT images cannot be interpreted in isolation must be correlated with red-free OCT fundus image and photography/ophthalmoscopy Aligning the scanning circle around the optic disc may be difficult in patients with abnormal disc contours

69. Some major limitations in the normative databases Long term data on monitoring disease progression with SD OCT unknown Depends on operator skill

70. ADVANTAGES OF OCT Best axial resolution available so far Scans various ocular structures Tissue sections comparable to histopathology sections Easy to operate Short scanning time

71. references INTERNET