Thermal and Non-Thermal Interactions

Question 1

What does reflection from the surface of tissue depend on?

Answer 1

• Wavelength.
• Pigment.
• Texture.

Question 2

What does absorption in tissues depend on?

Answer 2

• Wavelength.
• Chromophores.
• Tissue type.

Question 3

At what temperature are lasting results left in tissues?

Answer 3

40 degC.

Question 4

At what temperature may tissues be welded together?

Answer 4

45-70 degC.

Question 5

What condition must be met for tissue to heat past 100 degC?

Answer 5

All water in tissue must be evaporated.

Question 6

What happens when all the water has evaporated?

Answer 6

Tissue temperature rises to ablation temperature.

Question 7

What are the three types of non-thermal photo effects?

Answer 7

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.

Question 8

What are the main aspects of LASER assisted surgery and at what temperature to they occur?

Answer 8

- 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 surgery

Question 9

What are the four processes that can occur to incident light in a medium?

Answer 9

• Absorption
• Transmission
• Scattering
• Specular Reflection

Question 10

What equation governs the amount of reflected light from a beam of light in air incident on a medium with refractive index n?

Answer 10

I_r/I_0=[(n-1)/(n+1)]^2

Question 11

What is the equation that governs the absorbed light through a medium?

Answer 11

I_a=I_0exp(-alpha.x)

Question 12

What is the extinction length?

Answer 12

The length at which 99% of the incident light has been absorbed.
L_E=4.6/alpha

Question 13

What causes surface scatter?

Answer 13

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.

Question 14

What causes bulk scattering?

Answer 14

Reflection from scattering centres in the medium (e.g. tissue structure).

Question 15

What is the equation that governs bulk scattering?

Answer 15

I_sb=I_0.exp(-beta.x)

Question 16

What consideration of backscatter must be taken when using a near-infrared LASER?

Answer 16

Backscatter occurs from the body of tissue as well as the skin surface. This may be important when using the microscope or slit lamp.

Question 17

What is the equation for the energy supplied in non-ablative heating in thermally isolate tissue?

Answer 17

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

Question 18

What is the equation for the energy supplied when heating and vaporising thermally isolated tissue?

Answer 18

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

Question 19

What is the corneal ablation threshold?

Answer 19

30mJ/cm^2

Question 20

What is thermal relaxation time?

Answer 20

The time taken for the temperature within the core of the region to drop to a half.

Question 21

What is the equation for thermal relaxation?

Answer 21

T_t=(L^2)/4K
L is the diffusion length
K is the thermal diffusivity

Question 22

How can knowing the thermal relaxation time help us choose better pulse lengths and frequencies?

Answer 22

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.

Question 23

What sort of damage can occur to tissue due to irradiation with LASER?

Answer 23

Denaturation of proteins and/or loss of function.

Question 24

What does tissue damage depend on?

Answer 24

• Cell capacity for protein loss
• Temperature threshold for damage
• Time spent at that temperature.

Question 25

What is the idea beam profile?

Answer 25

A top hat function delivers maximum energy to the required area and none outside (similar to excimer laser profile).

Question 26

What is needed for optimal surgical LASER parameters?

Answer 26

• 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.