OCT is a great technology,Many ophthalmologist find very difficult to understand it ,SO I have tired to simplify it as much as possible .Hope everyone can understand now onwards the basic about OCT .
Every feedback s most welcomed sothat i can improve further in coming days
Please email your feedback to me in the following address
yourgyanu@gmail.com
3. HISTORY- OCT Timeline
• 1991–Concept of
OCT in
ophthalmology
• 1993 - First in vivo
retinal OCT images
• 1994-OCT prototype
• 1994-Anterior
segment/Cornea OCT
• 1995-The First
Clinical Retinal OCT
• 1995-The First
Glaucoma OCT
• 2002 – Time domain OCT (e.g. St r at us)
• 10 µm axial r esolut ion
• scan velocit y of 400 A-scans/ sec
• 2004 – Concept of spect ral domain OCT
int r oduced
• 2007 – Spectral domain OCT
• 1-15 µm axial r esolut ion
• up t o 52,000 A-scans/ sec
4. Theories and principle
• OCT images obt ained by measur ing
– echo time
– intensity of ref lect ed light
• Ef f ect ively ‘opt i cal
ul t ras ound’
• Opt ical proper t ies of ocular t issues, not a
t rue hist ological sect ion
5. • Laser out put f r om OCT is low, using a near-infra-red
br oadband light source
• Measures backscat t er ed or back-r ef lect ed
light
• Source of light : 830nm diode
laser
7. Light f r om Ref erence arm &
Sample arm combined
Division of t he signal by
wavelengt h
Analysis of signal
I nt er f er ence pat t ern
A-scan creat ed f or
each point
B-Scan creat ed
by combining A-scans
8.
9. • Digit al pr ocessing aligns t he A-scan t o cor r ect
f or eye mot ion.
• Digit al smoot hing t echniques f urt her impr oves
t he signal t o noise r at io.
• The small f aint bluish dot s in t he pr e-r et inal
space is noise
This is an elect ronic aberrat ion
creat ed by increasing t he sensit ivit y of
t he inst rument t o bet t er visualize low
17. Ganglion Cell Complex
• Collect ive t er m
– RNFL
– Ganglion cell layer and
– I nner plexif orm layer
• GCC t hought t o be af f ect ed in ear ly
glaucoma
59. • With OCT, each of the retina’s distinctive layers can be seen to map and measure
their thickness. These measurements help with early detection, diagnosis and
treatment guidance for retinal diseases and conditions, including age-related macular
degeneration and, diabetic eye disease, among others.
•
Since OCT relies on light waves, it cannot be used successfully with any condition
that interferes with light passing through the eye, such as dense cataracts or
significant bleeding in the vitreous.
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60. • Optical coherence tomography (OCT) has been developed during the last 20
years. Applications include medical diagnostics.
• Now a days the resolution is approaching that of histology.
• Some of its advantages are that it can be used in situ, no excision of the tissue
investigated is needed and the method favours the use of endoscopes.
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61. The human eye
OCT in ophthalmology: Fercher and Fujimoto groups
(early 1990’s)
High resolution OCT: Fujimoto, Drexler (late 1990’s)
62. • Optical coherence tomography is aninterferometric technique, typically employing
near-infrared light.
• The use of relatively long wavelength light allows it to penetrate into the scattering
medium
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63. • Retina
• OCT showing both macular edema and subretinal fluid in a diabetic patient
• OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in
the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in
diagnosing:
macular hole
• macular pucker
• vitreomacular traction
• macular edema
• detachments of the neurosensory retina and retinal pigment epithelium (e.g. central serous retinopathy or age-
related macular degeneration)
• In some cases, OCT alone may yield the diagnosis (e.g. macular hole). Yet, in other disorders, especially retinal
vascular disorders, it may be helpful to order additional tests (e.g. fluorescein angiogram).
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64. • Optic neuropathies
• OCT is gaining increasing popularity when evaluating optic nerve
disorders such as glaucoma. OCT can accurately and reproducibly
evaluate the nerve fiber layer thickness.
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71. • From its inception, OCT images were acquired in a time domain fashion. Time
domain systems acquire approximately 400 A-scans per second using 6 radial slices
oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be
taken to avoid missing pathology between the slice
• Spectral domain technology, on the other hand, scans approximately 20,000-40,000
scans per second. This increased scan rate and number diminishes the likelihood of
motion artifact, enhances the resolution and decreases the chance of missing
lesions. Whereas most time domain OCTs are accurate to 10-15 microns, newer
spectral domain machines may approach 3 micron resolution. Whereas most time
domain OCTs image 6 radial slices, spectral domain systems continuously image a
6mm area. This diminishes the chance of inadvertently missing pathology.
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72. • Limitations
• Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media
opacities can interfere with optimal imaging. As a result, the OCT will be limited the
setting of vitreous hemorrhage, dense cataract or corneal opacities.
• As with most diagnostic tests, patient cooperation is a necessity. Patient movement
can diminish the quality of the image. With newer machines (i.e. spectral domain ),
acquisition time is shorter which may result in fewer motion related artifacts
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73. Limitations contd
• The quality of the image is also dependant on the operator of the machine. Early
models of OCT relied on the operator to accurately place the image over the desired
pathology. When serial images were acquired over time (e.g. during treatment for
AMD with anti-VEGF therapy), later images could be taken that were off axis
compared to earlier images. Newer technologies, such as spectral domain acquisition
or eye tracking equipment, limit the likelihood of acquisition error.
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74. Please give your valuable feedback about this presentation
Contact
Dr Gyanendra Lamichhae
Vitreo retina Department
Lumbini Eye Instititue
yourgyanu@gmail.com
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Notas do Editor
Optical Coherence Tomography uses low-coherence or white light interferometry to perform high resolution range measurements and imaging.
An optical beam from a laser or light source which emits either short optical pulses or short coherence length light is directed onto a partially reflective mirror (optical beam splitter).
The partially reflecting mirror splits the light into two beams; one beam is reflected and the other is transmitted.
One light beam is directed onto the patient's eye and is reflected from intraocular structures at different distances.
The reflected light beam from the patient's eye consists of multiple echoes which give information about the range or distance and thickness of different intraocular structures.
The second beam is reflected from a reference mirror at a known spatial position. This retro-reflected reference optical beam travels back to the partial mirror (beam splitter) where it combines with the optical beam reflected from the patient's eye
The interference is measured by a photodetector and processed into a signal. A 2D image is built as the light source moves along the retina, which resembles a histology section
Vitreous anterior to retina is non reflective and is seen as a dark space.
Posterior boundary of retina is also seen as a red layer representing highly refractive retinal pigment epithelium and choriocapillaries.
Outer segment of retinal photoreceptors, being minimally reflective are represented by a dark layer just anterior to RPE- choriocapillaries complex.
Vitreo-retinal interface is well defined due to contrast between the non reflective vitreous and the backscattering retina.
Anterior boundary of retina formed by highly refractive RNFL is seen as a red layer due to bright backscattering.
Alterations in the thickness of the retinal nerve fiber layer may be a powerful indicator of the onset of neurodegenerative diseases such as glaucoma.
The NFL appears in the OCT images as a highly backscattering layer in the superficial retina and exhibits increased reflectivity compared to the deeper retinal layers.
The observation of depressions from both the anterior and posterior margins of the NFL is a helpful indicator of actual thinning.