Therapeutic Uses of NIR Flashcards
1
Q
What are the four thermal interactions between NIR and tissue? At what temperatures do they occur?
A
- Coagulation: >60degC
- Vaporisation: 100degC
- Carbonisation: >100degC
- Melting: >200degC
2
Q
Where is the therapeutic window?
A
600-1200nm
3
Q
At what range does Rayleigh scattering become important in tissue?
A
UV
4
Q
What is the equation for the thermal penetration depth?
A
z(therm)=sqrt(4kt)
5
Q
What are the four mechanisms used in LASER ophthalmology?
A
- Photo-ablation: Resurfacing of the cornea (Excimer)
- Photo-disruption: Iridotomy (Nd-YAG)
- Plasma induced ablation: Resurfacing the cornea (fs LASER)
- Thermal: Rental reattachment (Ar, diode)
6
Q
Describe the Photo-ablation process.
A
- Works only on larger molecules (polymers, or organic matter)
- Covalent bonding between fragments have relatively weak attractive forces and strong repulsive forces.
- Photon energy is absorbed by the individual fragments creating a thermal increase locally, as the repulsive forces means that this does not spread easily.
- High photon energy density within a short time is enough to break the covalent bonds.
7
Q
Why are ArF (EXCIMER) LASERs good for reshaping the cornea?
A
- Very small amount of residual energy ends up as heat.
8
Q
Describe how LASIK Surgery is performed.
A
- Prior to Surgery: Patient stops wearing contacts for 1 week, to minimise small blood vessels within cornea.
- Pre-Surgery: Corneal thickness checked - corneal topography measured with low-powered LASER.
- Surgery: Eye partially immobilised with suction cup. Iris tracking and multi-dimensional eye tracking stop LASER if pupil is outside pre-defined zone.
9
Q
Describe how ablation occurs due to plasma formation.
A
- Interaction between photons and unbound electrons
- Free electrons gain energy from incoming photons but within the confines of the atom (Inverse bremstrahlung)
- Further electrons liberated producing plasma
- Further interaction with incoming photons: plasma shielding.
- For visible and IR alpha_plasma»alpha
- Pulse length of around 100fs for lowest threshold energy density to produce useful plasma ablation.
10
Q
What is a mode locked LASER?
A
- Manipulation of the multi-ode beam within the LASER cavity to select those points in time when all modes are acting together.
- This gives foreshortened pulses.
- Active mode locked by electro-optical device with absorbs according to a triggering signal.
11
Q
What is a Q-switched LASER?
A
- Manipulation of the beam within the cavity to allow population inversion to complete.
- This gives the train a foreshortened pulse of higher intensity.
- Active Q-switching by electro-optical device which absorbs according to a triggering signal.
12
Q
How is the absorber made for Q-swithced or Mode-locked diode LASERS?
A
- Absorber is reverse biased section on the same substrate.
- For mode-locked it attenuates based on the return time of flight for a single pulse.
- For Q-switched it attenuates based on the time taken to re-saturate a population inversion.
- Pulse width is determined by a trade off between natural broadening and switching efficiency.
13
Q
Describe the photo-disruptive effects from a LASER.
A
- High intensity LASER has a high associated electric field.
- High electric field causes plasma formation creating free electron diffusion.
- This creates a supersonic shockwave and adiabatic heating.
- Finally cavitation bubbles occur causing fluid motion and jet formation at boundaries.
14
Q
What is a jet?
A
- When cavitation bubble collapse near a sold boundary constraints on fluid flow at the boundary cause high speed liquid jets normal to the surface.
15
Q
Explain how photo disruption is used in Iridotomy?
A
- Pressure build up behind the lens due to blockage of natural drains resulting from glaucoma.
- Focussed Nd-YAG used to create artificial drainage.
- Nd-YAG preferred as less chance of holes resealing (cold LASER)
- In tissue plasma shielding means that alpha increases sharply with depth in plasma.
- Shockwave directed mainly proximally sparing deeper parts of the eye.
