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OCT

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OCT FOR BEGINNERS a simplified presentation on the technique and its uses in ophthalmology.

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OCT

  1. 1. OPTICAL COHERENCE TOMOGRAPHY BY: Dr Vaibhav Khanna Dept Of ophthalmology KIMS, Hubli
  2. 2. PHYSICS • WAVELENGTH – The distance over which the wave’s shape repeats
  3. 3. PHYSICS • FREQUENCY – It is the number of occurrences of a repeating event per unit time. • Wavelength is inversely proportional to frequency
  4. 4. non contact non invasive micron resolution cross-sectional study of retina correlates very well with the retinal histology Principle – Low coherence interferometry (Michelson interferometer)
  5. 5. INTERFERENCE In physics , interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude
  6. 6. • In physics two waves are coherent if they have a constant phase difference and same frequency and are non coherent if there is a constant changing phase difference COHERENCE
  7. 7. TOMOGRAPHY • Tomogram – It’s a two – dimensional image representing a slice or section through a three – dimensional object i.e. cross-sectional image • Combining these tomograms we get a three- dimensional structure of the object which is being analyzed
  8. 8. PRINCIPLE • Effectively ‘optical ultrasound’ with the following differences : a. Uses near red infrared (830-850nm) light coupled to a fibre optic system b. Does not require any contact • OCT images obtained by measuring – echo time delay – intensity of reflected light • Optical properties of ocular tissues, not a true histological section
  9. 9. The process is similar to that of ultrasonography, except that invisible light is used instead of sound waves. Analog to ultrasound
  10. 10. COHERENCE • Coherence length • Coherence time • Coherence length is dependent on : a)Wavelength b)Bandwidth / spectrum (broad or narrow )of light source • Any light which remains in the tissue and bounces back is no more coherent. That is why depth of reach of OCT is few mm’s Whereas the confocal microscopy it is 50 microns
  11. 11. • Low coherence near infra-red(monochromatic) light coupled to a fibreoptic travels through a beam splitter and is directed through the ocular media to the retina and a reference mirror • The distance between the beam splitter and reference mirror is continuously varied • When the distance between light source & retinal tissue = distance between light source and reference mirror , the reflected light and the reference mirror interacts to produce an interference pattern
  12. 12. • The interference is measured by a photo detector and processes in to a signal. A 2D image is built as the light source moves along the retina , which resembles a histology section. The small faint bluish dots in the pre-retinal space is noise NOTE -This is an electronic aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures and is corrected by Digital smoothening technique
  13. 13. • The interferometer integrates several data points over 2mm depth to construct a real time tomogram of retinal structures with false colours. • Highly reflective structures are shown in bright colours (white and red) . • Those with low reflectivity are represented by dark colours (black and blue). • Intermediate reflectivity is shown Green.
  14. 14. CONFOCAL MICROSCOPY –Spatial rejection only whereas OCT has spatial + coherent rejection Confocal pinhole ANY LIGHT ABOVE AND BELOW THE FOCAL PLANE IS GONNA BE REJECTED BY THE CONFOCAL PINHOLE –SPATIAL REJECTION
  15. 15. ADVANTAGES • Its noncontact unlike USG, and noninvasive, unlike FFA,ICG. • Children easily tolerate it. • Very helpful for quantitative information about macular thickness. • Valuable teaching tool for the ophthalmologist as well as patient. DISADVANTAGES • Media opacity. • Scan quality depends on the skill of OCT operator. • Not possible with uncooperative patients. • Measurement of Fovea Thickness not accurate if scan not through the center of fovea.
  16. 16. •When all of the A-scans are combined into one image, the image has a resolving power of about 10 microns vertically and 20 microns horizontally. •Compare that to the resolution of a good ophthalmic ultrasound at 100 microns, or 1/10th of a millimeter. •The image on the right has 1/10th of the pixels per inch that the image on the left does. The image on the right would represent the resolution of the ultrasound as compared to the resolution of the OCT on the left.
  17. 17. • Time domain-OCT
  18. 18. NO MOVING PARTS MOVING PARTS PRESENT
  19. 19. Spectral Domain OCT
  20. 20. SPECTERAL DOMAIN OCT TIME DOMAIN OCT BENEFIT OF SPECTERAL DOMAIN LIGHT SOURCE 840 nm Broader Bandwidth 820 nm Provides higher resolution DETECTOR Spectrometer Single detector No moving parts – faster acquisition less motion artifacts AXIAL RESOLUTION 6-7 microns 10 microns Better visualization of retinal layers and pathology TRANSVERSE RESOLUTION 10 microns 20 microns SCAN DEPTH 2mm 2mm Slightly better penetration of light SCAN SPEED About 28,000 A- scans per second 400 A-scans per second Better registration , 3- D scanning and analysis
  21. 21. Resolution of an OCT • Resolution – Is the capability of the sensor to observe or measure the smallest object clearly with distinct boundaries. • Image resolution is an important parameter that determines the size of the smallest feature that can be visualized • Axial resolution (is governed by coherence length ) -Wavelength and -Bandwidth of the light source Long wavelength - visualisation of choroid, laminar pores, etc
  22. 22.  Transverse resolution - Based on spacing of A- scans i.e. spot size Since there is a trade-off between spot size and depth of focus , most commercial OCT systems use a 20 micron transverse resolution in order to have a sufficient depth of focus.
  23. 23. Procedure • Machine is activated • Patients pupils are dilated (however 4mm is the limit for pupil size during the scan ) • Patient seated comfortably • Asked to look into the target light in the ocular lens • Discouraged to blink • Protocol selected as per case requirement -A protocol is simply a pre-determined procedure or method
  24. 24. • Obtaining a scan 1)Scan acquisition protocol 2)Scan analysis protocol a)Image processing b)Quantitative analysis
  25. 25. SCAN ANALYSIS PROTOCOL IMAGE PROCESSING PROTOCOL • Normalize: this eliminates background noise and improves signal strength • Align: this protocols corrects the errors that have resulted from the patient movements in the axial direction • Normal and align : performs both above functions • Gaussian smoothing: this protocol balances the background noise and improves colour of the scan image • Median smoothing: similar to gaussian without removing minor details. • Proportional : gives an image that is true in its horizontal and vertical proportions so that images are not stretched/compressed to fit in the
  26. 26. The Align Process This tool "straightens" motions artifacts
  27. 27. Proportional analysis Proportional analysis produces an image with its true horizontal and vertical proportions
  28. 28. SPECIFIC SCANNING PROTOCOLS • Macular scans • Retinal nerve fibre layer analysis • Optic nerve head analysis
  29. 29. Macular scan
  30. 30. Line scan • acquire multiple line scans • Default angle is 0 degree • Scan line length is usually 5mm (can be varied) • But on increasing the length the resolution decreases
  31. 31. Radial lines- • Default number is 6 lines separated by 30degrees angle- can be altered up to 24 lines • Default length of each line is 6mm which can be altered but only after saving the first study. • Used to determine entire macular scan and macular thickness/ volume scan
  32. 32. Macular thickness map- • same as radial lines except that aiming circle has a fixed number of lines and diameter of 6mm. • Fast macular thickness map: similar to macular thickness map, but is quick takes only 1.92 sec to acquire and can be used for comparative retinal thickness
  33. 33. The Fast Macular Thickness Scan • The Fast Macular Thickness Scan consists of 6 radial line scans in a spoke pattern. It is a low resolution scan that was designed for quantitative analysis (thickness and volume) • When scanning the macula, the patient simply looks at the fixation target. The center of the FMT scan lines up with the fixation target by default
  34. 34. Each of the 6 scans can be viewed individually by clicking on the thumbnails on the left of the scan selection screen
  35. 35. Normal macular thickness map appears green (210-270microns) foveal depression appears blue. normal thickness ranges from 190 –210 microns
  36. 36. • The thickness of the macula is depicted by colour codes • Blue150-210 microns • Green210-270 microns • Yellow270-320microns • Orange320-350microns • Red350-470 microns • White>470 microns
  37. 37. Raster line • This protocol provides an option of acquiring series of lines scans that are parallel, equally spaced and are in 6-24 in numbers • Multiple line scans are placed over a rectangular region , the area of which can be adjusted so as to cover the entire area of pathology • This is useful where one wishes to obtain scans at multiple levels • Default setting has 3mm square with 6 lines
  38. 38. • 5 Line Raster – 5 six mm lines closely arranged 1 mm vertically • Each line consists of 4096 -A scans. • This scan gives the greatest resolution • Each Raster lines setis done superior to inferior and each line moves from nasal to temporal • Raster line scan is important in CNVM lesion where one wishes to obtain scans at multiple levels
  39. 39. • Macular cube 200x200 combo • 6mm square grid ,acquire 200 horizontal scans lines each composed of 200 A-scans • Macular cube 512 x 128 combo 6mm square grid acquire 128 horizontal scans each composed of 512 A scans Greater resolution in each line from left to right(512 A scans) but lines are separated further apart giving less resolution from top to bottom.
  40. 40. En Face scans • Scans adapted to the natural concavity of the posterior pole for example : a) CSR – They allow the study of dimensions and shape of detachment also giving details of the shape , thickness , smoothness of the walls b) Diabetic retinopathy and CME –It gives the details about the type and extension of retinal edema with their evolution pattern and stage.
  41. 41. REPEAT • Enables one to repeat any of the previously saved protocols using the same set of parameters that include scan size , angle , placement of fixation LED and landmark. • Landmark – Is a pulsating point of light that can act as a reference point for telling the location of the lesion.
  42. 42. The Cross Hair Scan Cross Hair Scan performs a high resolution horizontal line scan and then automatically flips to a vertical line scan without having to exit the protocol This is a common technique used in B-scan ultrasonography
  43. 43. PRINT OUT
  44. 44. • Section 1: Patient related data, examination date, list and signal strength • Section 2: Indicates whether the scan is related to macula with its pixel strength (as in this • picture) or optic disc cube (It also displays the laterality of the eye: OD • (right eye), OS (left eye). • Section 3: Fundus image with scan cube overlay. 3A: Color code for thickness overlays. • Section 4: OCT fundus image in grey shade. • Section 5: The circular map shows overall average thickness in nine sectors. It has three • concentric circles representing diameters of 1 mm, 3 mm and 6 mm, and except for the • central circle, is divided into superior, nasal, inferior and temporal quadrants. The central circle has a radius of 500 micrometers. • Section 6: Slice through cube front. Temporal – nasal (left to right). • Section 7: Slice through cube side. Inferior – superior (left to right). • Section 8: Thickness between Internal limiting membrane (ILM) to retinal pigment epithelium (RPE) thickness map. 8A: Anterior layer (ILM). 8B: Posterior layer (RPE). All these are 3-D surface maps. • Section 9: Normative database uses color code to indicate normal distribution percentiles. • Section 10: Numerical average thickness and volume measurements.
  45. 45. Scan Quantitative analysis protocol for retina 1) Retinal thickness (single eye) Obtained by measuring the distance between the first highly reflective band corresponds to vitreoretinal surface & second to the RPE • Displacement between anterior surfaces of these interfaces gives the retinal/macular thickness
  46. 46. 2) Retinal Map: 2 maps of Retinal thickness a) color code b) numerical values in 9 maps sectors.(table form) Map diameter default adjusted to 1 ,3 and 6 mm centered on the macula.
  47. 47. Retinal thickness/volume tabular output • Gives data table that displays thickness ,volume quadrant average , ratios and differences among various quadrants
  48. 48. ONLY OCT..???? PATIENT HISTORY CLINICAL EXAMINATION (including MOBiRET) FFA / ICG STUDY (Assuming it’s a retina case) OCT DIAGNOSIS
  49. 49. Regions For purposes of analysis, the OCT image of the retina can be subdivided vertically into four regions • the pre-retina • the epi-retina • the intra-retina • the sub-retina
  50. 50. • THICKNESS – 2D • VOLUME -3D • Always move from VR interface to Choroid to study the OCT SCAN • Comment about the contour of the Fovea if included in the scan • Grey and White scans ?? Allows me to assess slight variation in the intensities of grey and white and make out details in the scan which is rather difficult by pseudo colors (high contrast ) QUALITATIVE ANALYSIS
  51. 51. • The interferometer integrates several data points over 2mm depth to construct a real time tomogram of retinal structures with false colours. • Highly reflective structures are shown in bright colours (white and red) . • Those with low reflectivity are represented by dark colours (black and blue). • Intermediate reflectivity is shown Green.
  52. 52. The over-all retinal profile NOTE : The NFL increases in thickness towards the optic disc
  53. 53. RPE BRUCH’S AND PHOTORECEPTOR • RPE -Is defined as in a good scans as three parallel strips of which 2 are hyper reflective sandwiching a thin hypo reflective layer • RPE-Bruch’s complex - Normally it is impossible to distinguish Bruch’s membrane from RPE but becomes evident in detachments , drusens etc. • BRUCH’s Membrane – Studied under these – Integrity , Discontinuity , Reflectivity , Anomalous Structures • Above RPE – Strong Reflective line that represents junction b/w inner and outer segment of photoreceptors , these follow RPE hyper reflective lines and takes the shape of circumflex accent which is the great vertical dimension of the CONES
  54. 54. 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 aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures.
  55. 55. Anomalous structures • pre-retinal membrane • epi-retinal membrane • vitreo-retinal strands • vitreo-retinal traction • pre-retinal neovascular membrane • pre-papillary neovascular membrane
  56. 56. EPIRETINAL MEMBRANE Highly reflective diaphanous membrane over the surface of retina According to the degree of distortion classified as: a.Cellophane Maculopathy b.Crinkled cellophane Maculopathy Globally Adherent c.Macular Pucker:
  57. 57. • A epi-retinal membrane with traction on the fovea • These membranes may be associated with true or pseudo macular holes Clearly seperable
  58. 58. SHADOWING • It produces a screen like effect that masks the deeper outer structures. • Blood in the cavity produces shadow effect , blood cells form a moderate hyper reflective band beneath the elevated space/ cavity. • Photoreceptor lesion – IS/OS is blurred / fuzzy / interrupted. • Sup Hemorrhages & cotton wool spots produce shadow cone only if very dense or very thick.
  59. 59. Shadowing vs photoreceptor lesions
  60. 60. HARD EXUDATE Dense hyper reflective nodular structure deep in the retina casting a shadow cone on the posterior layers of the retina.
  61. 61. ARMD ( Age related Macular Degeneration) • Degenerative disorder affecting Macula • Presence of specific clinical findings including Drusen and RPE changes , choriocapillaris and Bruch’s membrane changes as early feature with no evidence the signs are secondary to some another disorder. • Conventionally divided in to two : a. Dry/non-exudative/Non-neovascular – Drusen ,Geographical Atrophy b. Wet/exudative/Neovascular – CNV ,PED
  62. 62. Dry/non-exudative/Non-neovascular DRUSEN • Seen as areas of focal elevation of RPE • Low modulations in the RPE associated with shallow borders with no shadowing underneath • No shadowing underneath differentiates it from PED GEOGRAPHICAL ATROPHY • Well demarcated pigment epithelial / choriocapillaris atrophy • OCT shows Increased optical reflectivity from the choroid due to increased penetration of the light through the overlying atrophic retina
  63. 63. SOFT DRUSEN Replacement of three hyper reflective bands with a single irregular hyper reflective band with decreased photoreceptor thickness
  64. 64. In a case of Retinal atrophy OCT shows a highly reflective choroid signal , which allows greater beam penetration in to the choroid and greater reflectivity. The retinal map allows the quantification of the decreased retinal thickness
  65. 65. Disease progression will show replacement of the three hyper reflective bands with a single irregular hyper reflective band with decreased photoreceptor layer thickness.
  66. 66. CNVM • Integrity of RPE – Bruch complex is lost , with abnormal growth of blood vessels complex from the choriocapillaris. OCCURS IN OLDER AGE GROUP (ABOVE 60 YRS ) • (FFA CLASSIFICATION) • Two types : a) classic • b)occult • Major use of OCT in the management of CNVM is in monitoring the response to treatment for which it provides an accurate quantitative assessment
  67. 67. Classic cnvm -The normal RPE-Choriocapillaris complex forms a Highly Reflective continuous band .Thickening of this band with thickened edges demarcating the boundaries of CNVM
  68. 68. The normal RPE-Choriocapillaris complex forms a Highly Reflective continuous band Thickening of this band with thickened edges demarcating the boundaries of CNVM
  69. 69. Occult CNVM
  70. 70. Boundaries are poorly defined
  71. 71. How OCT is helpful in CNVM…? • Diagnosing – Even soft confluent drusen occult CNVM can often be missed on FFA. • Response to Treatment :Monitor response to thermal laser , photocoagulation , Photodynamic Therapy (PDT) and VEGF inhibitors. Following PDT a staging has been described to check for the progression or regression of the disease by using the REPEAT protocol.
  72. 72. DETACHMENTS • Serous detachment appears as a hypo reflective , shallow separation of the neurosensory retina from the RPE • Retinal pigment detachments appears as well defined , dome shaped hypo reflective elevation of the RPE • Subretinal fibrinoid deposits appear as moderate high reflectivity collection within neurosensory serous detachments
  73. 73. PED (Pigmentary epithelial detachment ) • Basically due to the reduction of the hydraulic conductivity of a thickened and dysfunctional Bruch’s membrane , thus impending the movement of fluid from the RPE to choroid. • Types : a. Serous b. Fibro vascular c. Drusenoid d. Haemorrhagic
  74. 74. SEROUS PED Elevation of the RPE with an optically clear space underneath. The underlying choroid shows minor optical shadowing.
  75. 75. Fibrovascular PED Elevation of the RPE with a demarcation b/w RPE and underlying structures . Optical shadowing
  76. 76. FIBROVASCULAR PED Elevation of the RPE with a demarcation b/w RPE and underlying structures . Optical shadowing from the underlying choroid is absent
  77. 77. HAEMORRHAGIC PED Same as serous PED except backscattering from the RPE attenuates towards the outer retina with absent choroidal reflections.
  78. 78. CSR PATHOLOGY : Localized serous detachment of the sensory retina at the macula secondary to leakage from choriocapillaris through focal , or less commonly diffuse hyper permeable RPE defects. OCCURS IN YOUNG PATIENTS 25-45 YRS
  79. 79. CSR with PED
  80. 80. RPE TEAR PATHOLOGY : Occurs at the junction of attached and detached RPE if Tangential stress becomes sufficient. Tears may occur spontaneously following laser photocoagulation , or after intravitreal injection etc
  81. 81. CRVO • Macula in venous occlusions shows 1. intraretinal fluid accumulation 2. serous retinal detachment 3. Cystoid macular edema • Also an excellent modality to study the response of the macula to any intervention
  82. 82. Diabetic retinopathy • Macular edema: 5 distinct pattern of macular edema are defined on Oct. Type 1: focal macular thickening Type 2: Diffuse thickening without cysts Type 3: diffuse cystoid macular edema Type 4: Tractional macular edema 4A: posterior hyaloid traction 4B: epiretinal membrane 4C: both posterior hyaloid and epiretinal membrane Type 5: DME from one of the previous types associated to a macular serous retinal detachment • Useful in monitoring response to any intervention • Helps in quantifying retinal thickness
  83. 83. • How has it helped ..???? • Foveal TRD and TPHM are indications of Pars plana Vitrectomy. • Both the above lead to Recalcitrant Macular Edema • CSME secondary to foveal TRD and TPHM are non-responsive to laser photocoagulation and thus an indication for pars plana vitrectomy.
  84. 84. Sponge like retinal thickening
  85. 85. CSME with cystoid macular edema
  86. 86. CSME with tractional foveal detachment
  87. 87. CSME with taut posterior hyaloid membrane
  88. 88. Deformations in the foveal profile • macular pucker • macular pseudo-hole • macular lamellar hole • macular cyst • macular hole, stage 1 (no depression, cyst present) • macular hole, stage 2 (partial rupture of retina, increased thickness) • macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) • macular hole, stage 4 (complete hole, edema at margins, complete PVD)
  89. 89. MACULAR Pseudo Hole
  90. 90. Macular hole • Differentiating from other vitreo-retinal lesions • Diagnosing vitreofoveal traction • Distinguishing true from lamellar macular hole • Staging and diameter of macular hole • Evaluation for surgical intervention • To study progression • Visual outcome prediction • GASS CLASSIFICATION
  91. 91. Macular cyst stage 1a (no depression, cyst present) Pathology : Inner retinal layers (Muller cell cone) detach from the underlying photoreceptor layer , with the formation of schisis cavity
  92. 92. satge1B: Occult macular hole Pathology : Loss of the structural support causes the photoreceptor layer to undergo centrifugal displacement.
  93. 93. Macular hole, stage 2 (partial rupture of retina, increased thickness(edema) Pathology : Dehiscence develops in the roof of schitic cavity , often with persistent vitreofoveolar adhesion
  94. 94. Macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) Pathology : Avulsion of the roof of the cyst with an operculum and persistent parafoveal attachment of the vitreous cortex
  95. 95. Macular hole, stage 4 (complete hole, edema at margins, complete PVD)
  96. 96. OCT and Fluorescein Angiography in retinal diagnosis FAs provide excellent characterization of retinal blood flow over time, as well as size and extent information on the x and y axis (north- south, east-west) The OCT gives us information in the z (depth) axis, telling us what layers of the retina are affected
  97. 97. OCT for glaucoma • Analysis of optic nerve head • Analysis of RNFL
  98. 98. • Glaucoma is characterized by irreversible damage to the retinal ganglion cells, resulting in retinal nerve fibre layer loss • Structural damage precedes functional loss: RNFL loss precedes visual field defects. • 40% axonal loss occur prior to any detectable change in visual function • The recognition of RNFL loss inpatients with normal visual fields has led to the concept of pre- perimetric glaucoma
  99. 99. Optic nerve head evaluation – Optical disc scan – Fast optical disc scan protocol
  100. 100. 1. Optical disc scan consists of equally placed 6 (minimum)Axial scans 4mm(fixed) in length at 30 degree intervals, centred on the optic disc. • The number of lines can be adjusted between 6-24 lines until the first scan in the series is saved with each radial spoke composed of 128 sampling points. 2. Fast optical disc scan compresses 6 optical disc scan into one scan in short time of 1.92sec
  101. 101. The various optic nerve head parameters 1.Disc area , 2.Cup area 3.Rim area 4.rim volume 5.cup volume 6.avg cup-disc ratio and 7.horizontal and vertical cup-disc ratio
  102. 102. RNFL SCANNING PROTOCOL Scan ACQUISATION protocol • RNFL thickness(3.4): this protocol consists of three circle scan of 3.4 mm diameter around the optic disc.Average of three scans taken. • RNFL thickness (2.27xdisc):this protocol enables one to acquire a single circular scans around the optic disc which is 2.27 times the radius of the aiming circle (size of the patient optic disc- 1.5 mm approx)
  103. 103. • Fast RNFL thickness (3.4) this protocol compresses the three RNFL (3.4) scans into one scan in a time period of 1.92 seconds • RNFL map: this consists of a set of six concentric circle scans of predetermined radius increasing from the centre. It is designed to thoroughly assess RNFL thickness at increasing distances from the disc margin
  104. 104. RNFL analysis protocol 1. RNFL thickness: is used to obtain graphs of RNFL thickness along circle scans made around the optic disc • Normal RNFL graph appears as a double hump because to the increased RNFL thickness at the superior and inferior poles of disc
  105. 105. 2.RFNL thickness average: • Creates two maps of retinal nerve fibre thickness around the optic disc • One map shows RNFL Thickness using a colour code (A) the other shows average RNFL thickness in microns(B) • Summary parameters (C) – Gives maxm avg thickness in each quadrant and their ratios. 3.RFNL thickness serial analysis : • Comparison of RNFL thickness over time. • It utilizes the circle scans around the optic disc to analyse the changes in RNFL thickness from one visits to the next. • The RNFL thickness graphs of up to 4 visits can be superimposed on the same chart and each visit is colour coded.
  106. 106. 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
  107. 107. 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
  108. 108. 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.
  109. 109. • Best discriminating parameter for diagnosis of glaucoma was the average inferior RNFL thickness and the C/d area ratio of the ONH analysis
  110. 110. 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 • These artifacts can be operator induced (focussing , depolarizing & out of range images) ; can be patient induced (off center fixation leading to incorrectly centered retinal thickness maps , blink and motion artifacts) ; may be due to limitations of imaging techniques( Td-OCT has lower imaging speed and thus frequent blink and motion artifacts)
  111. 111. • 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. • Misidentification of inner retinal layer: Occurs due to software breakdown, mostly in eyes with epiretinal membrane vitreomacular traction or macular hole.
  112. 112. • Misidentification of outer retinal layers: Commonly occurs in outer retinal diseases such as central serous retinopathy ,ARMD, CME and geographic atrophy.
  113. 113. • 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
  114. 114. • Degraded image: – Degraded images are due to poor quality image acquisition. • Cut edge artifact: – This is an artifact where the edge of the scan is truncated. – Result in abnormality in peripheral part of the scan and do not affect the central retinal thickness measurements
  115. 115. • 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
  116. 116. • 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.
  117. 117. 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
  118. 118. 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
  119. 119. THANK YOU

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