oct for everyone
TRANSCRIPT
OCT Basic For ALL
Gyanendra Lamichhane, MD
Phaco surgeon and Retina fellow,Japan
Lumbini Eye Institute
Bhairahawa,Nepal
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• Non-invasive imaging test.
• OCT is a unique test as it allows to look at cross-sectional layers of the retina.
( HISTOLOGY like)
• Each of the ten layers in the retina can be detected.
• OCT allows to measure the thickness of each layer to aid in the early detection and diagnosis of retinal diseases and conditions.
• Reflected light is used instead of sound waves.
• Infrared ray of 830 nm with 78D internal lens.2
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. Stratus) • 10 µm axial resolution
• scan velocity of 400 A-scans/sec
• 2004 – Concept of spectral domain OCT introduced
• 2007 – Spectral domain OCT• 1-15 µm axial resolution • up to 52,000 A-scans/sec
Theories and principle
• OCT 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 laser
Principle
Light from Reference arm & Sample arm combined
Division of the signal by wavelength
Analysis 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
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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
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Hyper reflective scans
• RNFL • ILM, RPE • RPE-
choriocapillaries complex
• PED • Drusen , ARMD
• CNVM lesions • Anterior face of
hemorrhage • Disciform scars • Hard Exudates • Epiretinal
membrane
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• Optical coherence tomography (OCT) is a non-invasive imaging test that uses light
waves to take cross-section pictures of your retina,
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• 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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• 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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The human eye
OCT in ophthalmology: Fercher and Fujimoto groups(early 1990’s)
High resolution OCT: Fujimoto, Drexler (late 1990’s)
• 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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• 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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• 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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DIABETIC MACULAR EDEMA
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Macular hole
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Mild macular edema good vision
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Macular edema improved after lucentis
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Sub retina fluid
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• 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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• 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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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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Please give your valuable feedback about this presentation
Contact
Dr Gyanendra Lamichhae
Vitreo retina Department
Lumbini Eye Instititue
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