Dr Rajvin Samuel PonrajDr Rajvin Samuel PonrajMS ophthalmologyMS ophthalmologyOptical CoherenceTomography (OCT)
Optical coherence tomographyOptical coherence tomography Optical Coherence Tomography, or OCT, is aOptical Coherence Tomography, or OCT, is anoncontact, noninvasive imaging technique used tononcontact, noninvasive imaging technique used toobtain high resolution 10 um cross-sectional imagesobtain high resolution 10 um cross-sectional imagesof the retina and anterior segment.of the retina and anterior segment. Reflected light is used to create image instead ofReflected light is used to create image instead ofsound wavessound waves Infrared ray of 830 nm with 78 D internal lens.Infrared ray of 830 nm with 78 D internal lens. Based on interferometryBased on interferometry– involves interference between reflected light andinvolves interference between reflected light anda reference beam.a reference beam.
Physics: Cross sectional images are generated byscanning the optical beam in a transversedirection thus yielding a two dimensionaldata set that can be displayed as a falsecolour or gray scale image.
The "line" scan simply scans in a single,straight line. The length of the line can bechanged as well as the scan angle.
The "circle" scans in a circle instead of a line.
The "radial lines" scans 6 consecutive linescans in a star pattern
INTRODUCTIONINTRODUCTION OCT of the retina -> vertical biopsy section ofOCT of the retina -> vertical biopsy section ofthe retina.the retina. Instead of a knife, light is used.Instead of a knife, light is used. Instead of viewing a stained section under aInstead of viewing a stained section under amicroscope, we are presented with a "false-microscope, we are presented with a "false-color" view with micron level resolution.color" view with micron level resolution.
Optical coherence tomography-Optical coherence tomography-The processThe processis similar to that of ultrasonography, except that light is used insteadis similar to that of ultrasonography, except that light is used insteadof sound waves.of sound waves.Analog toultrasound
Transverse ScanningBackscatter IntensityAxialScan(Depth)Construction of tomographicConstruction of tomographicimageimage
•When all of the A-scans are combined into one image, the image has aresolving power of about 10 microns vertically and 20 micronshorizontally.•Compare that to the resolution of a good ophthalmic ultrasound at 100microns, or 1/10th of a millimeter.
Difficulties and limitationsDifficulties and limitationsLimited by intraocular mediaLimited by intraocular mediaopacities , which attenuateopacities , which attenuatemeasurement beam andmeasurement beam andreflected lightreflected light
OCT Image of Normal FoveaOCT Image of Normal Fovea
Qualitative and Quantitative AnalysisQualitative and Quantitative Analysis The OCT allows both qualitative and quantitativeThe OCT allows both qualitative and quantitativeanalysis of the retina.analysis of the retina. Qualitative analysis involves describing or identifyingQualitative analysis involves describing or identifyingmorphological changes and anomalous structures inmorphological changes and anomalous structures inthe retina. Morphology is the study of forms andthe retina. Morphology is the study of forms andstructures of organisms.structures of organisms. Quantitative analysis is possible because the OCTQuantitative analysis is possible because the OCTsoftware is able to identify and "trace" two key layerssoftware is able to identify and "trace" two key layersof the retina, the NFL and RPE. The software canof the retina, the NFL and RPE. The software canthen measure the distance between these twothen measure the distance between these twolayers, which represents retinal thickness.layers, which represents retinal thickness.
RegionsRegionsFor purposes of analysis, the OCT image of the retina can beFor purposes of analysis, the OCT image of the retina can besubdivided vertically into four regions:subdivided vertically into four regions: the pre-retinathe pre-retina the epi-retinathe epi-retina the intra-retinathe intra-retina and the sub-retinaand the sub-retina
ProfilesProfilesOCT retinal morphology (form and structure)OCT retinal morphology (form and structure)can be subdivided into four "profiles": Eachcan be subdivided into four "profiles": Eachprofile has its own set of deformations andprofile has its own set of deformations andanomalous structures.anomalous structures. 1. pre-retinal profile1. pre-retinal profile 2. overall retinal profile2. overall retinal profile 3. foveal profile3. foveal profile 4. macular profile4. macular profile
The pre-retinal profileThe pre-retinal profile A normal pre-retinal profile is black space, as pictured below,A normal pre-retinal profile is black space, as pictured below,because the normal vitreous space is translucent, meaning it hasbecause the normal vitreous space is translucent, meaning it hasminimal reflective properties. The small, faint, bluish dots in the pre-minimal reflective properties. The small, faint, bluish dots in the pre- retinal space is "noise". This is an electronic aberration created byretinal space is "noise". This is an electronic aberration created byincreasing the sensitivity of the instrument to better visualize lowincreasing the sensitivity of the instrument to better visualize lowreflective structures.reflective structures.
Anomalous structures that have been observed in the pre-retinal profileinclude the following:1. pre-retinal membrane2. epi-retinal membrane3. vitreo-retinal strands4. vitreo-retinal traction5. pre-retinal neovascular membrane6. pre-papillary neovascular membrane
The over-all retinal profileThe over-all retinal profileA slightly concave curvature that you would expect from observing thesurface of a globe. Abnormal profiles would include exaggeratedconcavity and convexity. Retinal folds would also result in an abnormalover-all profile.
Pigment epithelial detachment is causing the convexity.
Abnormal concavity -the retinal detachment suggesting thelong eye of a significant myope.
The foveal profileThe foveal profileThe normal foveal profile is a slight depression in retina surface
The macular profileThe macular profile The macular profile can, and often does, include the fovea asits center. Therefore, a common OCT scan length of 6 mmwould include 3 mm of the macula on each side of the fovea.
Deformations that have been observed in the macular profile includethe following:1. serous retinal detachment (RD)2. serous retinal pigment epithelial detachment (PED)3. hemorrhagic pigment epithelial detachmentA serous PED is pictured below. We know that it is a PED becausethe fluid (black space around the arrow) is pushing up underneaththe retinal pigment epithelium, identified by the relatively highlyreflective (red and orange) line (arrow).
Intra-retinal anomalies that have beenidentified in the macular profileinclude:1. choroidal neovascular membrane2. diffuse intra-retinal edema3. cystoid macular edema4. drusen5. hard exudates6. scar tissue7. atrophic degeneration8. sub-retinal fibrosis9. RPE tear
Cystoid Macular EdemaOCT is capable of detecting small, fluid-filled,cystic spaces within the macula.
Central Serous ChorioretinopathyCentral serous chorioretinopathy is characterized by thepresence of fluid between the RPE and neurosensoryretina.
Diabetic RetinopathyExudates appear as accumulation of dense materialwithin the neurosensory retina.
ArtifactsArtifacts Artifacts in the OCT scan are anomalies in the scan that are not accurateArtifacts in the OCT scan are anomalies in the scan that are not accurateimages of actual physical structures, but are rather the result of an externalimages of actual physical structures, but are rather the result of an externalagent or action.agent or action. Notice the large gap in the middle of the scan below. This is an artifactNotice the large gap in the middle of the scan below. This is an artifactcaused by a blink during scan acquisition. The was a high resolution scan,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 towhich takes about a second for the scan pass, which is plenty of time torecord a blink.record a blink.
The scan below has waves in the retinal contour. These arenot retinal folds, but rather movement of the eye during thescan pass.
Anterior segment optical coherencetomography (OCT)••High-speed anterior segment optical coherencetomography (OCT) offers a non-contact method forhigh resolution cross-sectional and three-dimensional imaging of the cornea and the anteriorsegment of the eye.•Anterior Segment Optical Coherence Tomographyenhances surgical planning and postoperative care fora variety of anterior segment applications by expertlyexplaining how abnormalities in the anterior chamberangle, cornea, iris, and lens can be identified andevaluated
On the leading edge of anteriorOn the leading edge of anteriorsegment imaging:segment imaging: Mapping of corneal thickness and keratoconusMapping of corneal thickness and keratoconusevaluationevaluation Measurement of LASIK flap and stromal bedMeasurement of LASIK flap and stromal bedthicknessthickness Visualization and measurement of anterior chamberVisualization and measurement of anterior chamberangle and diagnosis of narrow angle glaucomaangle and diagnosis of narrow angle glaucoma Measuring the dimensions of the anterior chamberMeasuring the dimensions of the anterior chamberand assessing the fit of intraocular lens implantsand assessing the fit of intraocular lens implants Visualizing and measuring the results of cornealVisualizing and measuring the results of cornealimplants and lamellar proceduresimplants and lamellar procedures Imaging through corneal opacity to see internal eyeImaging through corneal opacity to see internal eyestructuresstructures
Image shows an anterior-chamber angle as viewedImage shows an anterior-chamber angle as viewedwith gonioscopy and the OCTwith gonioscopy and the OCTThe latter replaces subjective evaluation with objectivemeasurement.
Anterior Segment Optical CoherenceTomography is a must-have for anteriorsegment, refractive, cornea, and glaucomasurgeons.