Optical Coherence Tomography Flashcards
What is an example where OCT is used in day to day life?
Opticians
What does optical refer to in OCT?
Visable light (400-700nm)
What does coherence refer to in OCT?
Waves that move together and the specific properties that some light sensors have
What does tomography refer to in OCT?
Takes 3D images that we an slice at any dimension to give a full overview of the body
Generally, how does OCT work?
Similar to ultrasound, there is relfection from boundaris in tissues where the refractive index changes
What is refractive index?
Is a material property that describes how the material affects the speed of light travelling through it
What is the refractive index dependent on?
- Speed of light
- Phase velocity of light
What is a temporally coherent source?
Over a time period, points on the wave move together
The source is monochromatic (a single wavelength)
What is a temporally incoherent source?
Changing wavelength/combination of wavelengths over time
What is a spatially coherent source?
In a set time, points on the wavelength move together
They are ‘in phase’, related to size of emitter, distance from source, wavelength
What is a spatially incoherent source?
Points on the wavefront do not move together
Which coherence is important for OCT?
Temporal coherence
What is the resolution range of OCT?
1-10micrometers
What is the penetration range of OCT and why?
- Relatively narrow (between 1 mm and 1cm)
- Light goes through and gets absorbed by chromophores such as Hb and meltatonin
- Anything in the yellow or green range gets absorbed and scattered- can therefore only penetrate red
- Fujimoto J.G, Schmitt J et al 2020
What are the three properties of light that are relevant for OCT?
- Coherence
- Wavelength (relates to penetration depth in the tissue and how to maximise this)
- Power (energy/unit of time) (want to stay within the damage threshold of the tissue)
What is the temporal coherence for monochromatic light and what is an example of a light source?
- Temporal coherence is strong
- A laser
- Singular wavelength
What is the temporal coherence for ‘white’ light?
- Contains all wavelengths
- Has low/no coherence
- Huge spectral bandwidth
What is the spectral brandwidth?
- The width of frequency distribution
What is the perfect temporal coherence for OCT?
- Need a medium amount of wavelengths
- Medium spectral bandwidth
- Complicated envelope of waves
What is the wave envelope?
The line over the top of the waves
What does coherence length (cl) tell us?
Tells us how coherent the light source is
This is proportional to the wavelength spread and the bandwidth of the light source
Also relates to the resolution
What are the basic components within an OCT?
- 2 ‘arms’
- Light source
- Beam splitter (semi silvered mirror)- allows light to pass through in both directions
- Mirror that can move backwards and forwards
- Detector
- Huang D, Wand J et al 1991
What does the michaelson interferometer do?
Splits light into reference and sample paths, then recombines reflected beams to create interference patterns.
These patterns are used to measure tissue depths and generate high-resolution cross-sectional images.
How can you work out the depth probed in the sample using OCT michaelson interferometer?
Can determine the depth to a high accuracy if you know the mirror scan distance
What are some inefficiencies with the classic michaelson interferometer OCT?
- Most tissue is turbid (scatters the light)
- Reflection is therefore poor- signal to noise ratio is not good
- Ideally the signals to the detector would be balanced
How can you get around the inefficiencies of the classic Michaelson interferometer OCT?
Use a fibre based system with a circulator
What does a circulator do in OCT?
A circulator directs light in an OCT system, sending light from the source to the sample and routing reflected signals to the detector.
It ensures efficient signal separation, minimises losses, enhances signal-to-noise ratio and simplifies system design.
What is axial resolution in terms of imaging resolution?
What is it dependent on & What is it inversely proportional to?
- Resolution in the depth of the tissue
- Axial resolution is dependent on the bandwidth (and hence coherence length) of the light source
- Axial resolution is inversely proportional to the bandwidth of the light source
What is lateral resolution in terms of imaging resolution?
- Resolution across the surface
- Lateral resolution is dependent on the optics of the system (lens properties) and the wavelength (as per optical microscopy-related to the diffraction limit)
- Lateral resolution is proportional to wavelength and focal distance of the lens and inversely proportional to the siameter of the light souce incident on the lens
What determines the choice of wavelength when considering imaging a tissue?
- Predetermined due to available wavelength of sources deliverable by fibre, designed for telecoms
- Absorbancy of the molecule
- Optical window (to get deeper into the tissues and avoid damage)
- Scattering in the tissue (want to avoid this)
What are some different absorbing molecule wavelengths?
- Melanin, water and Hb are strong absorbers
- Hb has two peaks 500-600nm
- They drop down as we go into the infrared side
What is the most common wavelength/light source used in OCT and what are the values associated with it?
- Superluminescent diodes (or other broadband light sources)
- GaAs: 800nm
- Bandwidths: 30nm
- Axial resolution: 10 micrometers in tissue
- Drexler w, Fujimoto JG, Springer 2013
What does a larger bandwidth mean in terms of resolution in OCT?
A larger bandwidth results in higher axial resolution in OCT by shortening the coherence length, allowing finer structural detail to be resolved.
Which light source produces the largest bandwidth?
- Femtosecond (10^-15) lasers
- Near IR bandwidths (800nm-1micrometer)
- Drexler w, Fujimoto JG, Springer 2013
What is a 1D image in terms of OCT?
- Is an A-mode scan (amplitude) and is a depth profile of the sample
- Single point measure into depth of tissue, showing boundaries as peaks in reflected signal
What is a 2D image in terms of OCT?
- B-mode/2D is made up of a series of z-axis scans into depth resolution of microns into tissues.
- Single plane, cross-section through sample
- ‘Optical biopsy’
What is a 3D image in terms of OCT?
- En-Face/3D builds up 2D scans by raster scanning over the surface
- X-Y scan of the surface of the tissue, surface reflection, surface topography
What is a spectral/fourier domain set up of OCT?
- Can be higher speed (100x)
- Higher resolution and have better signal to noise than michaelson interferometer
- Reflectance interference between the reference arm and the tissue is calculated simultaneously at all points along the depth of the A-scan
What does spectral/fourier domain OCT rely on?
Obtaining the spectrum of reflected light
What is the most common commercial OCT set up?
Spectral/fourier domain
What is the michaelson interferometer OCT set up?
- ‘Time domain’
- Most basic
See page 31 of OCT lecture to see time vs spectral domain depiction
Outline how the spectral/fourier domain OCT works?
- No moving mirror
- The signal from different depths in the tissue has a different time delay
- Leads to different interference signal depending on the depth in the tissue
- Possible to tell from the different ‘echo’ delays (measures by spectrometer in frequency (not wavelength) the magnitude of the axial measurements).
What is polarisation used for in functional OCT imaging?
- Birefringence in tissues- e.g. scarring
What is spectroscopy used for in functional OCT imaging?
- Differences in tissue bonding (identification of different material in the tissue)
- Uses absorbance curves without dyes- different properties in terms of the spectrum
What is doppler/speckle used for in functional OCT imaging?
- Blood flow
- Look at the speed of tissues
Are functional or structural changes better for noticing disease?
Functional changes are often a precursor to structural changes in disease
What is bireringence property?
The refractive index of the material differs depending on the pathway and polarisation of light
How does polarisation sensitive OCT work?
- Polarised light can be used to detect changes in the birefringence of tissues caused by disease
- Birefringence is a property of tissue because it is often composed of long chain molecules e.g. collagen
- Chain become disrupted due to damage
- CHanges in structure and orientation of the chains lead to different optical properties in the tissue including birefringence
- Need polarised light source and optics
How does spectroscopic OCT work?
- Utilises the absorbtion spectrum by comparing what we expect the seen absorbtion
- Can be used to contrast enhancement and to measure the concentration of tissue chromophores such as Hb
What is the doppler principle?
- When light backscattered from a moving particle interferes with the reference beam, a doppler frequency shift occurs in the interference fringe
- Shift in frequency is proportional to the velocity of the moving molecule
Where anatomically is OCT normally used? (4)
- Eye
- Skin- easy to get at and test
- Heart-easy if already imaging inside vessels, can get an optical fibre into the heart
- Gut- is easy to get a camera into
What is the fundus of the eye?
Composed of the retina, macula, optic disc, fovea, vasculature
What is the fovea of the eye?
Small depression in the retina, located in the macula, aids vision
Where is the choroid thin located?
Between the retina and the sclera
Why is the eye perfect for OCT?
The front section is transparent so easy to get light into the retina for optical imaging
What are the two main structures visable in a OCT of the eye?
- Macula
- Optic nerve
What is the macula?
Controls central vision, colour vision and fine detail
What is the optic nerve?
Is a fibre bundle which carries visual information from the retina to the brain
What is the health of eye vessels indicitive of?
- Eye disease
- Systemic diseases
Outline how light is transmitted into the eye and the different absorption properties of the tissues
- Light travels through the cornea and the lens to get to the retina (350-700nm)
- UVB light (<315) is absorbed by the cornea
- UVA light (315-400) is absorbed mostly by the lens
What alters the absorption spectrum of the eye lens?
Ageing, causing a greater increase in blue light absorption
What is OCT angiography (OCTA)?
- Looks into small vessels (no dye needed)
- Analogous to dopper- uses speckle
Name 4 conditions that cause microvascular changes in the angiography of an eye.
- Diabetes
- Hypertension
- Cardiovascular diseases
- Neurodegenerative diseases
What is healthy perfusion density of the eye?
45%
What are some examples that OCTA can be used in/for?
- En face small field images (on the face of the tissue)
- Superficial and deeper vessels resolved into layers
- Choriocapillaris
- Cross section of structure and function (overlay blood vessels on top)
- Widefield image
- Widefield image with superficial and deeper vessels resolved into layers
What is the choriocapillaris?
Network of capillaries that supply oxygen and nutrients to the retina (the innermost layer of the choroid)
What are three common conditions of the eye diagnosed by OCT?
- Diabetic retinopathy
- Glaucoma
- Age related macular degeneration (wet/dry)
What is the cause of diabetic retinopathy?
High blood glucose levels damage the retinal vessels
What are the consequences and observed outcomes of diabetic retinopathy?
- The vessel walls weaken causing protrusions in vessel walls.
- This increases the permeability causing leaking and oedema in retinal tissues
- Causes blurred vision and floater
What are two treatment options for diabetic retinopathy?
- Anti VEGF
- Laser surgery
What is the cause of glaucoma?
Increased pressure in the eye due to an excess of aqueous humor (drainage blocked)
What are the observed outcomes and consequences of glaucoma?
- Pressure leads to compression (thinning of layers) and loss of nerve fibres
- Initially peripheral vision loss, then central
What are the 3 treatment options for glaucoma?
- Eye drops: Lower IOP by reducing fluid production or increasing drainage.
- Trabeculoplasty: Laser creates holes in the trabecular meshwork to unblock drainage.
- Iridotomy: Laser makes holes in the iris to relieve pressure in angle-closure glaucoma.
What causes dry age-related macular degeneration?
- Macular thinning due to aging.
- Protein accumulation (drusen) under the retina, disrupting retinal function.
- Responsible for ~80% of AMD cases.
What causes wet age-related macula degeneration?
Angiogenesis (formation of new leaky vessels) of permeable vessels leading to scarring of the macula (oedema)
What is the observed outcome and consequences for both age-related macula degeneration?
- Dry- macula holes
- Wet- similar to diabetic retinopathy
- Central vision loss, blurred vision
What is the treatment for dry age-related macula degeneration?
Eye injections to delay loss of vision
What is the treatment for wet age-related macula degeneration?
- Anti-VEGF therapy: Reduces abnormal blood vessel growth.
- Laser surgery: Seals leaky vessels or destroys abnormal vessels.
What two locations is eye OCT taken in?
- Looking straight ahead to see for retinopathy/macula degeneration
- To the side to see for glaucoma and the optic nerve cup
What does a retinal photograph of diabetic retinopathy show?
- Microaneurysms
- Harmorrhages
- Hard exudates (protein build ups)
What does an en face OCT mage of diabetic retinopathy show?
- Microaneurysms
- Areas of capillary nonperfusion
What does an OCT of a glaucoma show?
Cup of the optic nerve increases relative to the optic disc
What aids the diagnosis of a glaucoma?
Ratio of cup/disc (C/D)
What is seen on a dry age-related macular degeneration OCT?
- Ischaemic areas (no perfusion)
- Protein disrupting the underlying layers
- No signs of fluid
- Foveal contour is intact
What is seen on a wet age-related macular degeneration OCT?
- Haemorrhages
- Subretinal fluid and intraretinal fluid
What are 3 types of functional OCT and what can they show about the eye?
- Doppler OCT- Can image the direction of blood flow
- Polarisation- can see glaucoma (loss of birefringence is indicative of damage)
- Spectral OCT- measurement of O2 in vessels based on oxyHb and deoxyHb absorption spectra
What are the main absorbers in the skin called and what are they?
Chromophores
- Haemoglobin
- Melanin
- Water
What are the 4 layers of the skin?
- Epidermis
- Dermal epidermal Junction
- Dermis
- Hypodermis
What is a limitation of the different layers of the skin for OCT?
Limitation on depth of penetration due to scattering and light
Outline layer by layer OCT structural images of the skin
- Bright initial surface reflection (change in refractive index)- stratum corneum
- Dark band- epidermis
- bright band at boundary epudermis to dermis
- Dermis gradually becomes darker as light is absorbed and signal is lost
- Can often visualise hair roots, sweat glands
What is an important thing to consider about different skin tissues?
They are all different
What are 4 examples of skin conditions studies in OCT?
- Skin cancers
- Inflammatory skin conditions such as psoriasis (hyperkeratosis- extra layers of tissue) and dermatitis
- Vascular structural abnormalities e.g. hemangioma
- Diabetes
HOWEVER, tends to more research than clinical
When used clinically, how is OCT used in skin conditions?
Image before and after treatment to see id there are differences
What is high resolution OCT imaging similar to?
Microscopy and the corresponding histology of spectometry
Explain a diabetic mice polarisation OCT skin experiment
- Mice given injections to induce diabetes
- Skin imaged over 4 weeks
- See different interpretations of birefringence of the polarisation signal
- Assocaited changes in skin collagen with time
What are 3 key points for skin and eye OCT?
- Interpretation is complex
- AI technology may play a role in identifying changes/diseases
- Multimodal imaging aids diagnosis
Which OCT- eye or skin is further along the clinical line?
- Eye
- Skin is at a much more immature stage (mostly research)
