Thermal and Non-Thermal Interactions Flashcards
What does reflection from the surface of tissue depend on?
- Wavelength.
- Pigment.
- Texture.
What does absorption in tissues depend on?
- Wavelength.
- Chromophores.
- Tissue type.
At what temperature are lasting results left in tissues?
40 degC.
At what temperature may tissues be welded together?
45-70 degC.
What condition must be met for tissue to heat past 100 degC?
All water in tissue must be evaporated.
What happens when all the water has evaporated?
Tissue temperature rises to ablation temperature.
What are the three types of non-thermal photo effects?
Photomechanical - Shockwaves and plasma (<10ns) damage tissue. (Some localised heating but effect is largely non-thermal).
Photoablation - Ultra-fast surface heating (~10ns) ejects tissue. (Some localised heating but effect is largely non-thermal).
Photochemical - Responsible for vision and PDT.
What are the main aspects of LASER assisted surgery and at what temperature to they occur?
- Tissue vaporisation (1000C)
\+ e.g. ENT, dentistry, urology
- Coagulation (>600C)
\+ e.g. micro-vascular
- Tissue Welding (45-700C)
\+ e.g. Retina reattachment
- Surface ablation (fast localised action)
\+ e.g. Cornea (sight correction)
\+ e.g. Fractional skin resurfacing
- Photo-disruption
\+ e.g. inter-ocular surgeryWhat are the four processes that can occur to incident light in a medium?
- Absorption
- Transmission
- Scattering
- Specular Reflection
What equation governs the amount of reflected light from a beam of light in air incident on a medium with refractive index n?
I_r/I_0=[(n-1)/(n+1)]^2
What is the equation that governs the absorbed light through a medium?
I_a=I_0exp(-alpha.x)
What is the extinction length?
The length at which 99% of the incident light has been absorbed.
L_E=4.6/alpha
What causes surface scatter?
Surface scatter is caused by reflection from a rough surface, with troughs about equal in size to the incident wavelength.
I_s increases as the area of interaction increases.
What causes bulk scattering?
Reflection from scattering centres in the medium (e.g. tissue structure).
What is the equation that governs bulk scattering?
I_sb=I_0.exp(-beta.x)
What consideration of backscatter must be taken when using a near-infrared LASER?
Backscatter occurs from the body of tissue as well as the skin surface. This may be important when using the microscope or slit lamp.
What is the equation for the energy supplied in non-ablative heating in thermally isolate tissue?
I_0.At=mcdT or I_0.At=pADcdT Where: I_0 is the intensity of the beam A is the area irradiated t is the time of irradiation m is the mass of the irradiated volume c is the specific heat capacity of the tissue dT is the change in temperature p is the density of the tissue D is the depth of the irradiated tissue
What is the equation for the energy supplied when heating and vaporising thermally isolated tissue?
I_0.At=m(cdT+H) or I_0.At=pAD(cdT+H) Where: I_0 is the intensity of the beam A is the area irradiated t is the time of irradiation m is the mass of the irradiated volume c is the specific heat capacity of the tissue dT is the change in temperature p is the density of the tissue D is the depth of the irradiated tissue H is the latent heat of vaporisation
What is the corneal ablation threshold?
30mJ/cm^2
What is thermal relaxation time?
The time taken for the temperature within the core of the region to drop to a half.
What is the equation for thermal relaxation?
T_t=(L^2)/4K
L is the diffusion length
K is the thermal diffusivity
How can knowing the thermal relaxation time help us choose better pulse lengths and frequencies?
Pulse length should be roughly equal to thermal relaxation time, and the frequency should be much less than the reciprocal of the thermal relaxation time.
What sort of damage can occur to tissue due to irradiation with LASER?
Denaturation of proteins and/or loss of function.
What does tissue damage depend on?
- Cell capacity for protein loss
- Temperature threshold for damage
- Time spent at that temperature.
What is the idea beam profile?
A top hat function delivers maximum energy to the required area and none outside (similar to excimer laser profile).
What is needed for optimal surgical LASER parameters?
- Wavelength well absorbed in relevant tissue.
- Pulse length roughly equal to relaxation time.
- Peak irradiance above ablation threshold.
- Top hat profile to minimise collateral damage.
- Low repetition rate to allow for cooling.
