Biomedical Engineering Reference
In-Depth Information
L A S ER
L i g h t
O
P
T
I
C
S
optical properties
Energy Distribution
& Absorption
(Laser Heat source)
Heat Transfer
(Temperature rise)
T
H
E
R
M
O
D
Y
N
A
M
I
C
S
biochemical props.
Denaturation
(coagulation & necrosis)
Ablation
(Intense Vaporization & Pyrolysis)
FIGURE 17.13
Flowchart of photothermochemical processes in thermal interaction of light with biological
tissues.
Light including laser interaction with biological tissue is composed of a combination of
optical and thermodynamical processes. An overview of these processes can be summa-
rized as shown in Figure 17.13. Once light is irradiated on tissue, the photons penetrate into
the tissue and, depending on the tissue optical properties such as scattering coefficient,
absorption coefficient, and refractive index, the energy is distributed within the tissue.
A portion of this energy is absorbed by the tissue and is converted into thermal energy,
making the light act as a distributed heat source. This light-induced heat source in turn
initiates a nonequilibrium process of heat transfer, manifesting itself by a temperature rise
in tissue. The combined mechanisms of conduction, convection, and emissive radiation dis-
tribute the thermal energy in the tissue, resulting in a time- and space-dependent tempera-
ture distribution in the tissue. The temperature distribution depends on the thermal
properties, conductivity, heat capacity, convective coefficients, and emissivity of the tissue.
As heat deposition and transfer continue, a certain threshold can be reached above which
a process of irreversible thermal injury initiates. This process leads to coagulation of tissue
caused by denaturation of enzymes and proteins and finally leads to necrosis of constituent
cells. As a result of this thermochemical process of injury, properties of the tissue, especially
