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