Introduction to laser dermatology 3

Hints laser in dermatology

LASER

• Light• Ampilification• Stimulated • Emission• radiation

 Light:

is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.

Albert Einstein in 1905 in the Swiss patent office in Berne (Lucien Chavan).(Courtesy, The Albert Einstein Archives, The Jewish National and University Library, TheHebrew University of Jerusalem, Israel.)

1919Stimulated emission

Maser (Theodore Schawlow&Charles Townes)

Theodore maiman 1960

Ruby

Leon goldman

Father of laser medicine and surgery

Types of LasersGas Argon , CO2, Excimer laser

Dye Pulsed dye laser

Solid Ruby, Nd YAG and Er Yag

Semiconductor Diode laser

Laser light can be delivered in pulses, continuous wave, or pseudo-continuous wave forms.

Laser components

Optical cavity or the resonating tube: where the amplification of stimulated emission

occurs.

Laser medium: This could be solid, liquid, or gas.

Energy source: which pumps energy to the laser medium to produce

the population inversion.

Wave-guide:as fiber-optic cable.

LASER tissue interactionLASER tissue interaction

When the skin is exposed to LASER, it could be transmitted, or scattered by the skin structures, or absorbed by the chromophores.

Chromophores as oxyhemoglobin, melanin, tattoo ink or water.

Grothus-Draper photobiology law

Light should be absorbed to have a tissue effect if there is no light absorption there will be no effect.

Absorption is controlled by wavelength of laser and the chromophore.

1.Photothermal 2.Photochemical

3.Photomechanical

Theory of selective photothermolysis

In 1983 Drs. R.A. Anderson and J. A. Parrish proposed this theory where a chromophore can be selectively damaged with a light pulse of an appropriate wavelength and suitable pulse duration shorter than the thermal relaxation time of that chromophore.

• Thermal Relaxation Time (TRT)– Time taken for target to dissipate 50% of the energy

absorbed to surrounding tissue– Or time needed for cooling of a given light absorbing

site e’in skin– TRT is roughly equal to the square of the diameter of

target structure– TRT in sec.= ~ d² in mm– Second(S)=

• 10³ millisecons(ms)• 106 microsecond( µs)• 109 nanosecond (ns)• 1012 picosecond(ps)• 1015 femtosecond• 1018 attosecond

TRT

target size TRT

MelanosomeDermal capillary

0.5-1 µm0.1 mm

0.25-1 µs10 ms

1. Exposure time = or < TRT of target lead to selective heating of target without damaging surrounding tissue

2. Exposure time > TRT of target lead to heat diffusion to surrounding tissue

1.2.

Thermal relaxation time

It is time of an object to loose heat energy to the surroundings through thermal diffusion (The time to

disperse the heat absorbed during a laser pulse.)

Pulse duration

It is the time the laser is in contact with the target cell (Chromophore).

Laser parameters

Energy fluence (Energy density) Joules/cm2

= energy delivered to a given unit area for each single pulse.

Irradiance (Power density) (Watt/cm2) = rate of energy delivery = intensity of the energy delivered.

Efficacy of laser depends upon:

The target chromophore Wavelength Pulse duration Energy density