Hereρis the tissue density (given in gm/cm^3 ) and C is the heat capacity (given in
J/gm/°C). To a good approximation the valueρC≈4.2 J/(cm^3 /°C) holds for tissue.
The thermal effect on tissues at different temperature levels is illustrated in
Fig.6.23. As a tissue increases in temperature from its normal state of 37 °C,
various stages of tissue denaturation are manifested.Denaturationrefers to the
disruption and possible destruction of both the secondary and tertiary structures of
proteins. Cell activity and possibly cell death can result if proteins in a living cell
are denatured. As shown in Table6.4, at temperatures up to about 45 °C and
beyond, a condition known ashyperthermiaresults from weakening or destruction
of molecular bonds and alterations in membrane structures. If such a hyperthermia
condition persists for several minutes, the tissue will undergo necrosis (death of
living cells or tissue). Starting around 50 °C there is a noticeable reduction in
enzyme activity. This effect results in cell immobility, reduced energy transfer in
the cell, and a diminishing of certain cell repair mechanisms.
The desired therapeutic effect of coagulation commences around 55 °C and can
be used up to 90 °C to heal cuts and wounds. Thecoagulationeffect is due to the
denaturation of proteins and collagen. Typically a Nd:YAG laser operating at
1064 nm is a good candidate for the irradiating process, because this wavelength
offers a good penetration depth with sufficient tissue absorption. A common
coagulation procedure is known as thepercutaneous technique, and also is known
as percutaneous laser ablation (PLA), laser-induced interstitial thermotherapy
(LITT), or interstitial laser-induced thermotherapy (ILT). The localized heating is
produced by means of opticalfibers that carry the laser energy into the designated
tissue area.
Increasing the temperature further to 100 °C initiatesvaporizationof the water
molecules contained in most tissues. During the vaporization process, the high
value of the latent heat of vaporization for water (2270 kJ/kg) helps in carrying
away excess heat from the surrounding region, thereby preventing heating damage
to the adjacent tissue. As the water molecules undergo a phase transition, they
create large gas bubbles that induce mechanical rupture and expulsion of tissue
fragments.
Once the water molecules have evaporated, the tissue heats up beyond 100 °C
and the tissue starts to become carbonized. Becausecarbonizationis not a desirable
condition, the tissue can be cooled with water or gas following the vaporization
treatment. When the tissue is heated beyond 300 °C, then melting can occur in
certain materials for sufficient optical power densities and exposure times.
To calculate the desired level of tissue modification during a thermal heating
process is a complex task, which involves knowing both precise tissue character-
istics and laser operational parameters [ 2 ]. First the task requires accurate predic-
tions of temperature changes with time. These temperature predictions necessitate
knowledge of the rate of heat production, which depends on an accurate estimate of
thefluence rate throughout the tissue.
A common application of thermal-based laser-tissue interactions is laser
resurfacingorphotorejuvenationfor the treatment of certain skin conditions and for
wrinkle removal [ 39 ]. The dermatological conditions include skin atrophy, skin
6.5 Light-Tissue Interaction Mechanisms 181