Transcript
OPTICAL COHERENCE TOMOGRAPHY
Group:48Solanki Ujjval
PRESENTATION LAYOUT
IntroductionPrinciplesTypes InterpretationClinical ApplicationsLimitations & AdvantagesLatest Developments
INTRODUCTION
Optical coherence tomography, or OCT is a non-contact, noninvasive imaging technique used to obtain high resolution 10 micro 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.
OPTICAL COHERENCE TOMOGRAPHY
Optical Coherence Tomography, or OCT, is a noncontact, noninvasive imaging technique used to obtain high resolution cross-sectional images of the retina and anterior segment.
Three-dimensional imaging technique with ultrahigh spatial resolution
Measures reflected light from tissue discontinuities
Based on interferometry.
OPTICAL COHERENCE TOMOGRAPHY-THE PROCESS IS SIMILAR TO THAT OF ULTRASONOGRAPHY, EXCEPT THAT LIGHT IS USED INSTEAD OF SOUND WAVES.
Analog to ultrasound
PRINCIPLEOCT images obtained by measuring
echo time intensity of reflected light
Effectively ‘optical ultrasound’
Optical properties of ocular tissues, not a true histological section
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 laser1310 nm : AS-OCT
Light from Reference arm & Sample arm combinedDivision of the signal by wavelengthAnalysis of signal
Interference pattern
A-scan created for each point B-Scan created by combining A-scans
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
COLOR CODING IN OCT Highly reflective structures are shown in bright colures (white and
red) .
Those with low reflectivity are represented by dark colours (black and blue).
Intermediate reflectivity is shown Green.
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
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
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
1. Time domain-OCT
Types of OCT
2. Spectral
Domain OCT
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
SCANNING TIPS1. Communicate with the doctor regarding the size
and location of the pathology of interest.2. Refer to other images of the pathology, e.g. color
photos and FA.3. Review past OCT exams and repeat scan types
used before.4. Dilate the eye well.5. The patient must keep the forehead against the
bar and the chin in the chinrest, with teeth together. Use the marker on the headrest to align the patient vertically. The outer canthus should be even with the line.
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.
INTERPRETATION &CLINICAL
APPLICATIONS
1. SPECTRALIS-Anterior Segment Module
2. OCT – Posterior Segment Module
SPECTRALIS-ANTERIOR SEGMENT MODULE
New dimension to anterior segment imagingCorneaAngle structure Iris details
Consists of Add-on lens and dedicated software
Compatible with all SPECTRALIS SD-OCT models
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
ANTERIOR SEGMENT OPTICAL COHERENCE TOMOGRAPHY (OCT)
•High-speed anterior segment optical coherence tomography (OCT) offers a non-contact method for high resolution cross-sectional and three-dimensional imaging of the cornea and the anterior segment of the eye.
•Anterior Segment Optical Coherence Tomography enhances surgical planning and postoperative care for a variety of anterior segment applications by expertly explaining how abnormalities in the anterior chamber angle, cornea, iris, and lens can be identified and evaluated
ON THE LEADING EDGE OF ANTERIOR SEGMENT
IMAGING: Mapping of corneal thickness and keratoconus evaluation
Measurement of LASIK flap and stromal bed thickness
Visualization and measurement of anterior chamber angle and diagnosis of narrow angle glaucoma
Measuring the dimensions of the anterior chamber and assessing the fit of intraocular lens implants
Visualizing and measuring the results of corneal implants and lamellar procedures
Imaging through corneal opacity to see internal eye structures
IMAGE SHOWS AN ANTERIOR-CHAMBER ANGLE AS VIEWED WITH GONIOSCOPY AND THE OCT
The latter replaces subjective evaluation with objective measurement.
A NARROW ANGLE IS APPARENT WITH OCT IMAGING, IN THIS CASE 9.5°.
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.
OCT – POSTERIOR SEGMENT MODULE
Glaucoma
ONH analysis
Retina
Choroid
GLAUCOMADiagnosis of glaucoma difficult in early stage Infrequency of episodes of rise in the IOP Visual field tests not being sensitive enough
Glaucoma diagnosis traditionally performed by examining optic nerve cupping width of the neuroretinal rim
Limitations of Visual Field Tests:
Visual field loss late clinical findings
Detected only after significant loss of retinal nerve fibers
Difficult to differentiate early glaucoma from normal
POSTERIOR POLE ASYMMETRY ANALYSIS
Combines mapping of the posterior pole retinal thickness with asymmetry analysis
Both eyes
Hemispheres of each eye
RETINAOCT 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
GANGLION CELL COMPLEX
Collective term RNFL Ganglion cell layer and Inner plexiform layer
GCC thought to be affected in early glaucoma
HYPER REFLECTIVE SCANS
RNFL ILM, RPE RPE-choriocapillaries complex
PED Drusen , ARMD
CNVM lesions Anterior face of hemorrhage
Disciform scars Hard Exudates Epiretinal membrane
PED
Drusen of the Retina
DISCIFORM SCAR
HYPO REFLECTIVE SCANS
Retinal atrophyIntraretinal/subretinal fluid
Regions:The Pre-retina
The Epi-retina
The Intra-retina
The Sub-retina
THE PRE-RETINAL PROFILEA 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
Anomalous structures in Pre-retinal area:
Pre-retinal membrane
Epi-retinal membrane
Vitreo-macular traction
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)
LAMELLAR MACULAR HOLE
FULL THICKNESS MACULAR HOLE WITHOUT PVD
DEFORMATIONS IN THE MACULAR PROFILE
Serous retinal detachment
DEFORMATIONS IN THE MACULAR PROFILE
Serous retinal pigment epithelial detachment
MACULAR CYST
INTRA-RETINAL ANOMALIES IN THE MACULAR PROFILE
Choroidal neovascular membraneDrusensHard exudatesScar tissueRPE tear
OCT deformations:
Concavity myopia
Convexity PED Subretinal cysts Subretinal tumors
Disappearance of foveal depression
CSR
Patterns of Diabetic macular edema in OCT: Sponge like thickening of retinal layers:
Mostly confined to the outer retinal layers due to backscattering from intraretinal fluid accumulation
Large cystoid spaces involving variable depth of the retna with intervening septaeInitially confined to outer retina mostly
Serous detachment under fovea
Tractiional detachment of fovea
Taut posterior hyaloid membrane
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.
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.
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 outer retinal layers: Commonly occurs in outer retinal diseases such as central serous retinopathy ,AMD, CME and geographic atrophy.
OCT ARTIFACT AND WHAT TO DO?OCT artifact Remedial measureInner 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 scanOff center artifact Retake the scan/manually plot
the foveaMotion artifact Retake the scanBlink artifact Retake the scan
NEW SPECTRALIS OCT FEATURES
Imaging of deeper tissue structuresDifficult due to :
Pigment from the Retinal Pigment Epithelium (RPE) Light scattering from the dense vascular structure of the
choroid
Enhanced Depth Imaging (EDI) : New imaging modality on the Spectralis OCT Provides an enhanced visualisation of the deeper structures,
like choroid Particularly useful for imaging pigmented lesions in the
choroid such as naevi and melanomas
LIMITATIONS OF OCT Penetration depth of OCT is limited
Limited by media opacities Dense cataracts Vitreous 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
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
Some major limitations in the normative databases
Long term data on monitoring disease progression with SD OCT unknown
Depends on operator skill
ADVANTAGES OF OCTBest axial resolution available so far
Scans various ocular structures
Tissue sections comparable to histopathology sections
Easy to operate
Short scanning time
REF. Internet books >optical coherence tomography- Carmen puliafito
and michael Hee >optical coherence tomography- Gangjun liu Important links: http://www.intechopen.com/books/optical-
coherence-tomography
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