Optical Coherence Tomography


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OCT is a non contact, non invasive, micron resolution cross-sectional study of retina which correlates very well with the retinal histology.

It was unbelievable, histopathology without biopsy of a structure which was literally untouchable (Retina).

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Optical Coherence Tomography

  1. 1. Optical Coherence Tomography Dr Vijay Joshi
  2. 2. INTRODUCTION  OCT is a non contact, non invasive, micron resolution cross-sectional study of retina which correlates very well with the retinal histology.  It was unbelievable, histopathology without biopsy of a structure which was literally untouchable (Retina).
  4. 4. OPTICAL COHERENCE TOMOGRAPHY  The Principle:  2 or 3 dimensional cross sectional imaging of retina by measuring echo delay and intensity of back reflected infra-red light from internal tissue structures.  Combination of low-coherence interferometry with a special broadband width light.
  5. 5. OCT  Based on principle of Michelson interferometry  Low-coherence infra-red light coupled to a fiber-optic 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 & reference mirror, the reflected light and the reference mirror interacts to produce an interference pattern.
  7. 7. THE OCT SETUP Broadband source Detector Fiber-optic beamsplitter Tissue Scanning reference mirror Computer Amplifier Bandpass filter
  8. 8. OCT  The Interference is measured by a photodetector and processed into a signal. A 2 D image is built as the light source moves along the retina, which resembles a histology section.  Digital processing aligns the A-scan to correct for eye motion. Digital smoothing techniques further improve 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.  Intraretinal cross sectional anatomy is displayed with an axial resolution < 10 microns and transverse resolution of 20 microns
  9. 9. OCT  The Interferometer integrates several data points over 2mm depth to construct a tomogram of retinal structures.  It is a real time tomogram with false colours.  Diff colours rep deg of light scattering from diff depths of retina.  Highly reflective structures are shown in bright colours (white & red) & those with low reflectivity are rep by dark colours (black & blue). Intermediate reflectivity is shown as green.
  10. 10. 1 mm 1 cm 10 cm Penetration depth (log) 1 µm 10 µm 100 µm 1 mm Resolution (log) OCT Confocal microscopy Ultrasound Standard clinical High frequency OCT vs. standard imaging
  11. 11. OCT VS USG  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 OCT USG
  12. 12. TYPES OF OCT  Time Domain OCT  In TD-OCT, a mirror in the reference arm of the interferometer is moved to match the delay in various layers of the sample.  The resulting interference signal is processed to produce the axial scan waveform.  The reference mirror must move one cycle for each axial scan. The need for mechanical movement limits the speed of image acquisition.  Furthermore, at each moment the detection system only collects signal from a narrow range of depth in the sample. This serial axial scanning is inefficient.
  13. 13.  Fourier Domain  In FD-OCT , the reference mirror is kept stationary. The spectral pattern of the interference between the sample and reference reflections is measured.  The spectral interferogram is Fourier transformed to provide an axial scan. The absence of moving parts allows the image to be acquired very rapidly.  Furthermore, reflections from all layers in the sample are detected simultaneously. This parallel axial scan is much more efficient, resulting in both greater speed and higher signal-to-noise ratio. Fourier transform (DFT) converts a finite list of equally spaced samples of a function into the list of coefficients of a finite combination of complex sinusoids, ordered by their frequencies, that has those same sample values. It can be said to convert the sampled function from its original domain (often time or position along a line) to the frequency domain.
  14. 14.  FD-OCT can be implemented in two ways:  In the swept-source implementation, a tunable laser is used to sequentially sweep through the spectrum, while the signal is collected by a single-element photodetector. This is called swept-source OCT (SS-OCT) or optical Fourier domain imaging (OFDI).  In the spectrometer-based implementation , a broad-spectrum light source, such as a superluminescent diode (SLD), is used, and a spectrometer is utilized in the detector arm of the interferometer. The spectrometer uses a grating or a prism to spread the light into a spectrum. The spectrum is typically detected by a line camera. This technique has been called FD-OCT, spectral OCT spectral domain (SD) OCT, spectral RADAR, and frequency domain (FD) SPECTRAL DOMAIN OCT
  16. 16. FD OCT VS TD OCT  FD OCT can capture 2000 pixels simultaneously, while TD OCT captures one pixel at a time. So in the time it take TD OCT to form one single axial scan, FD OCT can capture an entire image.  The higher speed and resolution of FD-OCT allows higher definition, or more pixels per image. As anyone familiar with high definition TV know, this makes the picture much sharper.  Details such as small blood vessels and the photoreceptor inner and outer segment boundary become clearly visible.  Because the FD OCT picture is captured in a small fraction of a second, there is no motion artifact that is commonly seen in conventional OCT images.  Finally, because of the efficiency of simultaneous signal acquisition, FD OCT actually has higher signal, or appear brighter and cleaner, than TD OCT. Even deep choroidal vessels can be visible in normal eyes.
  17. 17. A GENERATIONAL LEAP  RTVue has 65x speed & 2x resolution of Stratus Zeiss OCT1/2 1996 Zeiss Stratus 2002 OptoVue RTVue 2006 26,000 400 100 16 10 5 Speed (A-scans /sec) Resolution (µm) Fourier domain Time domain
  18. 18. FD OCT Simultaneous 2048 pixels at a time TD OCT Sequential 1 pixel at a time Higher speed, higher definition and higher signal. 1024 A-scans in 0.04 sec 512 A-scans in 1.28 sec Motion artifactSmall blood vessels IS/OS Choroidal vessels
  19. 19. THE OCT MACHINE  The OCT system comprises :  Fundus viewing unit  Interferometric unit  Computer display  Control panel  Colour inkjet printer
  20. 20. GENERATIONS OF OCT  OCT 1, i.e first generation of OCT machine has a transverse and axial resolution of 20 and 10 µ, respectively.  OCT 2, i.e second generation of OCT machine has a resolution similar to OCT 1 but with an improved user interface.  Both OCT 1 and OCT 2 acquire 100 vertical scans in a standard OCT scan in an acquisition time of approximately 1.2 seconds.  OCT 3, i.e third- generation OCT unit has improved axial resolution of 7-8 µ and acquires 512 vertical scans.
  21. 21. PROCEDURE  Switch on the system: This activates all the components & takes 45 secs to start window.  The menu and toolbar in the main window has several options including Select patient, Acquisition protocol, Analysis protocol.  Appropriate category can be selected. Data entry made for a new patient. The appropriate protocol is selected
  22. 22. PROCEDURE  A 3 mm pupil is necessary for adequate visualization  Patient is seated with his chin on the chin-rest and eye at the level of the mark on the side of the frame.  Once the patient is seated comfortably, the OCT machine is moved slowly towards the eye to within 1 cm, with the joystick till an image appears on the screen  Then the z offset of the image is optimised to bring the image to the centre. The polarisation is optimised next to create a clear image.  A signal strength of 5 and above gives a clear image
  24. 24. PROCEDURE  Patient is asked to look inside the ocular lens – internal fixation- onto the green target light inside the red rectangular field or external fixation- onto the external target by the other eye in pts with poor vision. The pt is encouraged to blink in between scan acquisition.  There is no discomfort to the pt & an experienced operator can acquire the required scans within 1-3 mts in each eye. The actual time taken by the machine is one sec- the addl time is for patient positioning and optimizing scan quality
  25. 25. PROCEDURE  Normally the patient can look at this field for several minutes at a time without discomfort  During scan alignment, the patient sees the scan pattern in motion on the red field  During scan acquisition, the patient sees a bright greenish-white flash, when the scan image is stored into the camera.  It is possible to acquire the scans without the flash, which is more comfortable to the patient.
  26. 26. PRODUCTION AND DISPLAY OF IMAGE  On Z axis, 1024 points are captured over a 2 mm depth to create a tissue density profile, with resolution of 10 µ.  On X-Y axis, tissue density profile is repeated up to 512 times every 5-60 µ to generate a cross sectional image. Several data points over 2 mm of depth are integrated by the interferometer to construct a tomogram of retinal structures.  Image thus produced has an axial resolution of 10 µ and a transverse resolution of 20 µ.  The tomogram is displayed in either gray scale or false scale on a high resolution computer screen.
  27. 27. NORMAL OCT SCAN OF RETINA  The OCT scan of retina allows cross sectional study of the macular, peripapillary region including RNFL, and ONH region.
  28. 28. COLOUR CODING IN OCT SCAN  Red-yellow colours represent areas of maximal optical reflection and backscattering.  Blue black colours represent areas of minimal signals.
  29. 29. SCAN AND ANALYSIS PRINTOUT  This yields a lot of data 1- Cross-sectional images of each of 6 scans 2- Mean and SD of the data. 3- Retinal thickness measurements in 9 regions of macula 4- Surface map reconstruction display 5- Retinal volume contained under the previous areas
  30. 30. OCT INTERPRETATION  2 modes of interpretation  Objective & Subjective  For accurate interpretation both have to be combined  OCT reading must be done in 2 stages: 1- Qualitative & Quantitative analysis 2- Deduction and synthesis
  31. 31. OCT INTERPRETATION  Qualitative Analysis- this includes  Morphologic studies-  (a)overall ret struc changes, changes in ret outline, ret struc changes & morpho changes in the post layers  (b)Anomalous strucs- pre/epi/ intra/sub retinal  Reflectivity study- hyper/ hypo/ shadow areas  Quantitative analysis- Thickness, volumetry & shadow areas
  32. 32. INTERPRETATION OF RETINAL SCAN  Viterous anterior to the retina is non reflective and is seen as a dark space.  Viteroretinal interface is well defined due to contrast between the non reflective viterous and the backscattering retina.  Retinal layers are represented as below:  Anterior boundary of retina formed by highly reflective RNFL is seen as a red layer due to bright backscattering.  Posterior boundary of retina is also seen as a red layer representing highly refractive retinal pigment epithelium(RPE) and choriocapillaries.  Outer segment of retinal photoreceptors, being minimally reflactive are represented by a dark layer just anterior to RPE – choriocapillaries complex.  Different intermediate layers of neurosensory retina b/w the dark layer of photoreceptors and red layer of RNFL are seen as an alternating layers of moderate and low reflectivity.
  33. 33. 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
  34. 34. 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.
  35. 35. ANOMALOUS STRUCTURES IN PRE RETINAL AREA  Pre-retinal membrane  Epi-retinal membrane  Vitreo-retinal strands  Vitreo-retinal traction  Pre-retinal neovascular membrane  Pre-papillary neovascular membrane
  37. 37. A pre-retinal membrane with traction on the fovea
  38. 38. A pigment epithelial detachment is causing the convexity
  39. 39. Aside from the retinal detachment, notice the underlying concave curvature of the retina, suggesting the long eye of a significant Myope
  40. 40. THE FOVEAL PROFILE The normal foveal profile is a slight depression in the surface of the retina
  41. 41. MACULAR SCAN PROTOCOLS  Radial lines- 6-24 equally spaced line scans pass through a central common axis. 6 lines of 6 mm pass through an aiming circle which can be varied in size. For ret thickness/vol analysis  Macular thickness map- Similar to radial lines except that the aiming circle has a fixed diameter of 6 mm. For macular thickness  Fast macular thickness map- Quick protocol which takes only 2 secs to acquire 6 scans of 6 mm size. Useful in patients with poor co-operation
  42. 42. SCAN PROTOCOLS FOR MACULA  Raster lines- Multiple line scans that are parallel, equidistant 6-24 in no, are placed over a rectangular region of pathology giving scans at multiple levels. 3 mm scan with 6 lines usually  Repeat- Allows to repeat any of the previously saved protocols using the same set of parameters. To monitor retinal changes- thus is reproducible  The direction of the arrow indicates the direction of the scan- the base of the arrow indicates the left side of the scan image and the head of the arrow the right side
  43. 43. 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)
  44. 44. MACULAR CYST  Macular hole, stage 1 (no depression, cyst present)
  45. 45. MACULAR HOLE, STAGE 2  Macular hole, stage 2 (partial rupture of retina, increased thickness)
  46. 46. MACULAR HOLE, STAGE 3  Macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid)
  47. 47. MACULAR HOLE, STAGE 4, OPERCULUM SUSPENDED BY THE HYALOID MEMBRANE  Macular hole, stage 4 (complete hole, edema at margins, complete PVD)
  48. 48. THE MACULAR PROFILE The macular profile can, and often does,  include the fovea as it's center
  49. 49. DEFORMATIONS IN THE MACULAR PROFILE  Serous retinal detachment (RD)  Serous retinal pigment epithelial detachment (PED)  Hemorrhagic pigment epithelial detachment
  51. 51. INTRA-RETINAL ANOMALIES IN THE MACULAR PROFILE  Choroidal neovascular membrane  Diffuse intra-retinal edema  Cystoid macular edema  Drusen  Hard exudates  Scar tissue  Atrophic degeneration  Sub-retinal fibrosis  RPE tear
  55. 55. DME CLASSIFICATION BASED ON OCT  Based on OCT DME can be classified into different pattern such as:  Spongy swelling of the retina  Cystoid macular edema,  Serous detachment, macular traction, with hard exudates  Taut posterior hyaloid membrane.
  57. 57. 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.
  58. 58. PED
  59. 59. OPTIC DISC SCAN  Optic disc scan consists of equally placed line scans 4 mm in length, at 30° intervals and centered on the optic disc.  Characteristic description of an optic disc scan:  Optic disc boundaries and diameter: the point at which choriocapillaries terminates at lamina cribrosa determines the disc boundaries. Extrapolation of these points to retinal surface defines a line segment which measures optic disc diameter.  Optic cup is determined by the points at which nerve fibre layer terminates.  The high resolution imaging of the optic disc with OCT allows an accurate assessment of the size of the optic cup, disc area, C:D ratio, volume of the cup and thickness of RNFL. Serial measurement records are very useful to monitor glaucoma changes.
  60. 60. RNFL ASSESSMENT WITH OCT  RFNL is highly reflective and its thickness increases from macula to the optic disc margin.  OCT 3 offers a variety of RNFL thickness measurement and analysis protocols like RNFL thickness circle scan, fast circle scan, concentric three ring protocol, RNFL map and proportional circles.  Circular scan of 1.34 mm radius centered on the ONH has been shown to exhibit maximum reproducibility for RFNL measurement.  The mean RNFL thickness is calculated using age adjusted RNFL thickness average analysis protocol.
  61. 61. CLINICAL APPLICATIONS OF POSTERIOR OCT SCAN I. Macular disorders: OCT is very useful in confirming macular pathologies which are not apparent clinically. 1. Macular Hole: OCT allows confirmation of diagnosis of macular hole and differentiates it from the clinically simulating condition such as lamellar hole, foveal pseudocyst. It is also useful in monitoring the course of the disease and the response to surgical intervention. 2. Macular Edema: In an OCT scan the macular oedema is characterized by the intraretinal areas of decreased reflectivity and retinal thickening. Round, optically clear regions within the neurosensory retina are noted in cystoid macular edema. Measurement of retinal thickness is performed between two well defined highly reflective red layers of the nerve fibre layer and the RPE/ choriocapillaris layer.
  62. 62.  Quantitative measurement of retinal thickness can be used to monitor the course of macular oedema secondary to diabetes, vascular occlusions, uveitis and post – cataract surgery. 4. Age Related Macular Degeneration: OCT because of its high resolution capacity is able to image:  Morphological changes in the non exudative ARMD.  Subretinal fluid, intraretinal thickening and sometimes, choroidal neovascularization in exudative ARMD.  This is especially useful when visualization of choroidal neovascularization is obscured on fluorescein angiography by a thin layer of fluid or haemorrhage. 5. Central serous chorioretinopathy: In an OCT scan the CSR is characterized by an area of decreased reflectivity(black area) between two highly reflective layers- the neurosensory retina and RPE. An associated PED may be present.
  63. 63. 5. EpiRetinal membrane: is diagnosed on OCT by the presence of highly reflective diaphanous membrane over the surface of retina. The OCT provides information about membrane thickness, cystic changes and its adherence to retinal surface. 6. Solar retinopathy: On OCT scan is characterized by formation of an outer retinal hole.
  64. 64. II. OCT in Glaucoma Optic disc scan is very useful in diagnosing and monitoring the glaucomatous change. It is also useful in evaluating the RNFL for early (pre- perimetric) glaucoma detection. Detection, study and follow up of the macular changes in hypotony induced maculopathy after glaucoma. Evaluation of cystoid macular edema after combined cataract and glaucoma surgery.
  65. 65. THE FAST OPTIC DISC SCAN   The optic cup profile can be evaluated by capturing a "Fast Optic Disc" scan  The patient fixes on the target, which is automatically placed at the edge of the scan window so that the optic nerve is viewed toward the center of the video window.   The operator then moves the scan so that the star pattern is centered on the optic nerve head.   Centering can be aided by clicking on the scan window to view the white centering lines.
  66. 66. The optic nerve scan can be analyzed with the "optic nerve head analysis" protocol
  67. 67. THE FAST RNFL THICKNESS SCAN Nerve fiber layer thickness can be evaluated with the "Fast RNFL Thickness" scan.  This is a circular scan that requires the operator to place the circle so that the center of the circle is centered on the optic nerve head.
  68. 68. The analysis software places lines on the top and bottom of the nerve fiber layer and the distance between the two lines is interpreted to be the thickness of the nerve fiber layer
  69. 69.  Care must be take to make sure that the image is captured with the circle centered on the optic nerve  The placement of the circle can make a big difference in the analysis of the nerve fiber layer thickness
  70. 70.  These two scans (OD) are of a normal eye.  The scan in the first analysis is well centered and the RNFL thickness falls within the normal range.   The scan in the second analysis is of the same eye (OD), but the scan is not well centered.  The analysis is abnormal (black arrows).
  71. 71. WHAT’S NEW  OCT with SLO  OCT with HRA (FA and ICG)  Increase in resolution to 5 microns  Overlays, 3D imaging
  73. 73. CONVENTIONAL OCT VS OCT-SLO  Conventional OCT provides very good cross-sectional images of the posterior segment and has become a vital investigative modality in the management of macular diseases.  However, the exact site of pathology cannot be localized in terms of anteroposterior relationship. OCT-SLO is a new imaging modality, made commercially available , combines the abilities of SLO and OCT, & provides coronal images along various depths.  Hence images with this combo machine seem to have better resolution and localization and an ability to demonstrate subtle lesions.
  74. 74.  OCT-SLO has a longitudinal resolution of appox. 8µ and transverse resolution of 20 µ with a maximum scanning field size of 25°. The scanning depth can be varied from 0.5 to 6mm for both transverse and longitudinal scanning. It can also be used to detect ONH and RNFL characteristics.  OCT-SLO produces confocal SLO and OCT images simultaneously and displays them in cross- sectional (B Scan) and coronal (C-Scan) sections. With pixel to pixel correlation of the two images in a true 3-D characteristic of the pathology can be obtained. This allows more precise localization of the lesion.
  75. 75. 3D OCT A three-dimensional rendering shows the cystic spaces and their relationship with the posterior hyaloid. The tractional forces exerted by the vitreous on the inner retina become apparent in this inferior-oblique view (blue arrows). An isosurface volumetric three-dimensional rendering reveals the cystic spaces (yellow) and the posterior hyaloid (blue) with the inner limiting membrane sandwiched in between (green). The green surface below the cysts represents the retinal pigment epithelium (RPE). Isosurface reconstruction portrayed the retinal surface corrugations caused by an epiretinal membrane. A ridge at the edge of the epiretinal membrane was not as obvious on the 2D OCT images
  76. 76. OCT 3D TOPOCAN
  77. 77. 3-D Reconstruction: In vivo images of human eye using spectral-domain OCT RPE NFL I T N S I S TN
  78. 78. LIMITATIONS OF THE OCT 1. Being purely dependent on optical principles, it requires a minimal pupillary dia of 4 mm to obtain a high quality. 2. OCT has limited applications in patients with poor media clarity due to corneal oedema, dense cataract, viterous haemorrhage and asteroid hyalosis. 3. High Astigmatism, decentered IOL can compromise quality of OCT scan. 4. Limited transverse sampling.
  79. 79. ARTIFACTS  Artifacts in the OCT scan are anomalies in the scan that are not accurate images of actual physical structures, but are rather the result of an external agent or action.    Notice the large gap in the middle of the scan below.  This is an artifact caused by a blink during scan acquisition.  The was a high resolution scan, which takes about a second for the scan pass, which is plenty of time to record a blink.
  80. 80. • The scan below has waves in the retinal contour. These are not retinal folds, but rather movement of the eye during the scan pass.
  81. 81. OCT FOR ANTERIOR SEGMENT IMAGING AND BIOMETRY  Carl Zeiss Meditec Inc., USA, has introduced Vistane OCT system for following applications: 1. Anterior Segment Imaging: The anterior segment can be evaluated and measured pre and postoperatively after image acquisition, using the analysis mode of the system’s software.  Practical tools enable planning and measurement of anterior segment ocular structures, including AC depth, anterior chamber angles and anterior chamber diameter (angle to angle distance
  82. 82. 2. Corneal Imaging and Pachymetry: The OCT provides high resolution corneal. Rapid acquisition during the pachymetry scan ensures an accurate and repeatable pachymetry map result for application in refractive and glaucoma care. 3. New Lasik information: In addition to providing a full thickness pachymetry map prior to laser surgery. OCT is the first non contact device to image, measure and document both corneal flap and thickness and residual stromal thickness immediately following LASIK surgery.
  83. 83.  A unique flap tool in the analysis mode enables quick measurement of the flap and residual stromal thickness at any location. 4. IOL and implant imaging: OCT may also aid post operative evaluation by allowing imaging and visualization of IOL’S and implants in the eye.
  84. 84. OCT IS USED TO FOR ANTERIOR EYE DISEASES AND SURGERY AS WELL LASIK Lens implantation Seeing through opaque cornea Narrow angle glaucoma
  85. 85. POST-LASIK INTERFACE FLUID & EPITHELIAL INGROWTH 056-CP Fibrosis Epithelial ingrowth Fluid
  87. 87. CLASSIC CNVM
  88. 88. OCCULT CNVM
  89. 89. PED WITH SRF
  90. 90. SCAR WITH CME
  91. 91. Post PDT 6 mths Post Avastin 2 mths
  93. 93. 6/9 Pretreatment Post Lucentis 4 mths Post Lucentis 1 mth 6/18
  94. 94. Pretreatment Post Lucentis 4 mths 4/60 6/9
  95. 95. Pretreatment Post Avastin 2 mths Post Avastin 6 mths Post Avastin 4 mths 5/60 6/12
  96. 96. THANKS