Optical Coherence Tomography - OCT (Full)

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INTRODUCTION:
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• OCT is an optical instrument that can perform cross-sectional image of biological tissue within less than 10 micron axial resolution using light waves
• Retina is easily accessible to the external light, hence it is specially suited for retinal disorder
• The information provided by OCT is similar to in vivo histopathology of the retina

EXAMPLE:
 Zeiss stratus OCT
 Topcon 3D OCT-1000

PRINCIPLE OF OCT:
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• It is a imaging technology projected light beam (820nm) near infrared light
• The beam is then split into two beam (Probe beam & Reference beam) by Beam splitter
• Probe beam reach to the target tissue (retina) & reference beam reach to the reference mirror
at a known distance
• The echo time delay of light reflected various layer of target tissue (retina) is compared with
the echo time delay of light reflected from the reference mirror
• A positive interference is produced when light reflected from target tissue & reference mirror
arrives simultaneously
• This interference is measured by a photodetector which finally produce a range of time delays
for comparison
• The interferometer integrates several data points over 2mm depth to construct a tomogram
of retinal structures
• It is real time tomogram using false color scale & different colors represent light
backscattering from the different layers of retina

OCT SYSTEM CONSIST OF:
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• Fundus viewing unit
• Interferometer unit
• Computer display
• Control panel
• Color inkjet printer

GENERATION OF
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• OCT- 1:
o 1st generation
o Transverse resolution 20 micron
o Axial resolution 10 micron
• OCT-2:
o 2nd generation
o Transverse resolution 20 micron
o Axial resolution 10 micron
o Better user interference
• BOTH OCT-1 & OCT-2:
o acquire 100 vertical scan in approximately 1.2 sec
• OCT-3:
o 3rd generation
o Axial resolution 7-8 micron
o Acquire 512 vertical scan

COLOR CODING OF OCT SCAN:
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• RED-YELLOW COLORS: represents areas of maximal optical reflection & backscattering
• BLUE & BLACK: Represents areas of minimal optical refelection & backscattering

VARIOUS PATTERN OF B-SCAN:
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• CIRCULAR SCAN FOR THE ONH RNFL:
This generates a plot of the peripapillary RNFL thickness which is important in
glaucoma diagnosis & monitoring
• RADIAL LINE THROUGH ONH:
Consist of 6-24 slices through a common central point on the ONH
• MACULAR RADIAL LINES: Used to measure retinal thickness

PROCEDURE OF OCT:
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• STEP-1: PATIENTS DATA
 Activation of instrument & entering patients data
• STEP-2: PATIENTS PREPARATION:
 Pupil dilate with mydriatics (tropicamide)
 Asked to look into the internal fixation target light in the
ocular lens
• STEP-3: PROTOCOL FOR SCAN ACQUISITION:
 Selected as per the case requirements
 The scanning beam is placed on the target area and scans
are obtained
• STEP-4: PRODUCTION & DISPLAY IMAGE:
 Several data points are integrated by the interferometer to
construct a tomogram of the target area
 The tomogram is displayed in either grey scale or false color
on a high-resolution computer screen