Technical approach the laser beam-tissue interaction

The main parameters, which can be chosen or adjusted by the doctor, are:
• Wavelength of laser radiation,
• time of impulse duration (cw or pulse),
• radiant power,
• Size of working area (spot size).
The criteria which determinate these parameters are as follows, for:

Wavelength
This selection has direct influence on the quantity of absorbed power in tissue, i.e. on the power used. We observe the power P in interaction with tissue in the following mathematical way:
Where, x - depth of the penetration in tissue,mi - attenuation coefficient of tissue, P -refracted power

Impulse duration
Power emission mode may be continuous or pulsed. Impulse duration (temporal pulse width) gives the energy of interaction with tissue during single pulse. Important parameter in this reaction is the tissue reaction on radiant power, which is called thermal relaxation time - Tr . This is the time required for tissue to cool to 50% of the temperature achieved immediately after the laser exposure without conducting heat to the surrounding tissue. Otherwise, the laser pulse duration must be smaller than Tr . Thermal relaxation time depends on:
• the tissue molecular structure,
• wavelength,
• Spot size of interaction.
Applied radiant power
Applied radiant power is one of the parameters, which determinate the power density for interaction and this predicts the effect which will be achieved on the skin (vaporization or coagulation). The power distribution across the spot is common Gausian. Therefore, if greater power is applied, we might expect injury in the spot center caused by this inhomogeneity in power distribution.
Spot size
This parameter is second and it determines the energy density of irradiation on the tissue. It is recommended to apply as little spots as possible ( 1 mm), because we then have less problems with inhomogeneous power distribution and heat conducting to the surrounding tissue.
In ideal case the tissue should be exposed as short as possible, otherwise we have negative effects in tissue. Those effects are presented in the Figure 1.

Negative laser influence on tissue
. The main parameters, which can be chosen or adjusted by the doctor, are:
• Wavelength of laser radiation,
• time of impulse duration (cw or pulse),
• radiant power,
• Size of working area (spot size).
The criteria which determinate these parameters are as follows, for:

Wavelength
This selection has direct influence on the quantity of absorbed power in tissue, i.e. on the power used. We observe the power P in interaction with tissue in the following mathematical way:
Where, x - depth of the penetration in tissue,  - attenuation coefficient of tissue, P -refracted power
 is the sum of two factors:
Where,  - absorption coefficient and  - scattering coefficient. Absorption coefficient, , is the sum of each type of molecules in tissue structure absorption coefficients. Scattering coefficient, , is the sum of Mie and Rayliegh scattering coefficients, which depends on molecule size. Both,  and , depend on the involved wavelength for given medium type.
Impulse duration
Power emission mode may be continuous or pulsed. Impulse duration (temporal pulse width) gives the energy of interaction with tissue during single pulse. Important parameter in this reaction is the tissue reaction on radiant power, which is called thermal relaxation time - Tr . This is the time required for tissue to cool to 50% of the temperature achieved immediately after the laser exposure without conducting heat to the surrounding tissue. Otherwise, the laser pulse duration must be smaller than Tr .
Thermal relaxation time depends on:
• the tissue molecular structure,
• wavelength,
• Spot size of interaction.
Applied radiant power
Applied radiant power is one of the parameters, which determinate the power density for interaction and this predicts the effect which will be achieved on the skin (vaporization or coagulation). The power distribution across the spot is common Gausian. Therefore, if greater power is applied, we might expect injury in the spot center caused by this inhomogeneity in power distribution.
Spot size
This parameter is second and it determines the energy density of irradiation on the tissue. It is recommended to apply as little spots as possible ( 1 mm), because we then have less problems with inhomogeneous power distribution and heat conducting to the surrounding tissue.
In ideal case the tissue should be exposed as short as possible, otherwise we have negative effects in tissue. Those effects are presented in the Figure 1.

Figure 1 Negative laser influence on tissue
THERMAL
Secondary emission
Absorption
Chemical process activation
MECHANICAL
Light pressure
Electrostriction
Pressure of volume thermal increase
Internal steam generation
Dielectric breakdown