Lecture 6: OCT and glaucoma Flashcards
What is optical coherence tomography?
How is it carried out?
*Non-invasive technique that allows the 2- and 3-dimensional imaging of biological tissue
*Retina can be imaged with a few micrometres in resolution and 1-2 mm in depth
*Subject fixates on a point in the lens of the machine
*You get a sequence of black and white images that slice the retina in its depth.
*OCT is used to target all areas of the eye from the cornea to the retina
What is an A scan?
What is a B scan?
What is a volume scan?
What is a C scan?
*A scan: profiles of signal.
*B scan: 2 dimensional images. Composite of successive A scans
*Volume: when successive b scan are acquired along a direction and placed next to each other they form this 3-dimensional image.
cross-sectional view from above
What principle does OCT use?
*Interferometry is used
*If 2 waves are out of phase, they combine destructively cancelling out both signals
*Coherence length: misalignment of waves is in a certain length
*Infrared light in the OCT is split into 2 arms by the beam splitter
*A sample arm and reference arm with a mirror
*The 2 reflective beams and combined and reflected to a detector
*The combination of the reflected light from sample arm and reference arm gives rise to an interference pattern but only if beams in both arms have travelled the same distance within the coherence length
*If we can control position of the mirror, we can control the depth that we are sampling
What is time domain OCT?
1.The mirror is moved, and the signal is recorded of the time at the detector.
2.The signal at different times corresponds to different depths.
3.since different tissues have different reflectively values, the light that is reflected will change according. This is measured in the signal.
4.The signal represents the a-scan. This profile is represented with Grayscale values.
5.A series of these successive lines are placed side by side
6.To require successive a scan, the mirror is tilted to get b scan then volume
What is frequency domain OCT?
What are the common types?
*Sped up version
*Obtaind a scan with No mirror movement
*Broadband interference is used
*These different frequencies have different penetration in the tissue so replace the movement of the mirror
spectral domain
swept source
How does spectral-domain OCT work?
*A broadband light source is used. This is a beam containing multiple frequencies.
*The reflected light goes through a dispersive detector which separates and processes the different wavelengths individually.
*This allows the reconstruction of the whole a scan In a single acquisition avoiding tilting of mirror.
*More prone to falloff of the signal to noise ratio. -Measures the clarity of the signal.
How does swept-source OCT work?
*Uses different wavelengths in the light source
*Uses same detector as time domain OCT
*Reconstructs the a-scan by decoding the signal overtime
*Similar technology to time domain OCT
*Records a time encoded signal
*Sequential use of light source at different frequencies allows much faster acquisition
What does OCT show?
*Oedema (accumulation of fluid) seen in DR
*OCT shows stratified layers of the human retina.
-thinning of nerve fibre layer in glaucoma
What are the retinal layers?
internal limiting membrane
retinal nerve fiber layer
ganglion cell layer
inner plexiform layer
inner nuclear layer
outer plexiform layer
outer nuclear layer
external limiting membrane
photoreceptor layers
retinal pigment epithelium
Bruch’s membrane
What are the applications of OCT in glaucoma?
*Optic nerve imaging (peripapillary RNFL, neural rim, scleral rim)
*Posterior pole (GCL, IPL, RNFL)
*OCT angiography
*Anterior segment OCT
What is a circumpapillary RNFBL scan?
*Scan around the optic head
*Can segment and quantify different tissues around the optic nerve. measures thickness.
*More interested in RNFL
*Scan is unrolled and displayed as a straight line. B scan performed in a circular way.
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Does green always indicate normal and does red indicate damage on the RNFBL scan ?
*Not always the case. Always look at the image first.
*Errors is segmentation (identification of the boundaries of the RNFL can make measurements inaccurate)
*Vessel position in a healthy person may change the profile so it doesn’t align with normal
*Isolated voids in RNFL only visible on image
*Thinning below the inner limiting membrane may not be picked up by segmentation.
How is Bruch membrane opening-minimum rim width measured?
*Describes cup to disc ratio
-Radial scans that cut across the centre of the nerve head
-Calculates for each scan the distance between the edge of the bruchs membrane opening and the closest point on the inner limiting membrane. Describes how much tissue you have at this area.
-The more tissue you lose, the more this distance is reduced over time
What can you look for on anterior segment OCT?
closed angle
pigment dispersion sysdrome
