Biophotonics_Concepts_to_Applications

interactions and can be generated by either CW or pulsed laser light. The energy
that produces thermal reactions is significantly larger than the optical energy
associated with irradiance levels that produce photochemical reactions. The irra-
diance associated with thermal interactions ranges from approximately 10 W/cm^2
in application times of seconds to 10^6 W/cm^2 over time periods of microseconds.
Depending on the duration and the peak value of the temperature increase of the
tissue, the resulting effects in the tissue can include coagulation, vaporization,
carbonization, and melting. Table6.4lists the effects on tissue at different tem-
perature levels.
When a light beam impinges on a tissue area, most of the light that is absorbed is
converted to heat. The rate of heat production per volume of tissue at a specific
point r is the product of the totalfluence rate (given in J/cm^2 ) at r and the absorption
coefficient at r. For a short pulse of light of durationτand irradiance E 0 impinging
on a tissue with an absorption coefficientμa, the resultant maximum temperature
riseΔTis

DT¼

laE 0 s

qC

ð 6 : 24 Þ

Table 6.4 Effects on tissue
at different temperature levels

Temperature (°C) Biological effect

37 Normal body temperature

45 Hyperthermia

50 Enzyme activity reduction, cell

immobility

60 Coagulation

80 Membranes become permeable

100 Vaporization and ablation

>100 Carbonization

>300 Melting

Vaporization

Carbonization

Coagulation

Hyperthermia

Irradiating

laser beam

Tissue surface

Tissue

volume

Fig. 6.23 Thermal effect on
tissues at different
temperature levels include
hyperthermia, coagulation,
carbonization, and
vaporization

180 6 Light-Tissue Interactions