Biomedical Engineering Reference
In-Depth Information
blood vessels, tissue necrosis, or tissue vaporization. Biostimulation has been claimed as
a result of minute heating of the tissue whereby the light-induced heat and/or the elec-
tromagnetic field stimulates nerves and accelerates wound healing. Higher-energy
absorbed laser light can facilitate the joining of tissues—in particular blood vessels (i.e.,
anastomosis)—or can be used to coagulate blood to stop bleeding during a surgical pro-
cedure. If the light-induced heating causes the temperature to rise above 45
o
C, tissue
necrosis and destruction occur as would be desired for the treatment of cancer or
enlarged prostate tissues. At even higher-power densities, ablation or vaporization
ofthetissueoccurs,asisthecaseforthecorrectiveeyesurgeryknownasradial
keratectomy.
The light-induced heating is typically performed with laser light. The lasers used have
different wavelengths (from the ultraviolet to the infrared spectrum), power densities
(i.e., ratio between beam power and irradiated area), and duration times. The amount of
energy imparted to the tissue and therefore temperature rise can be changed by either vary-
ing the power density or the duration of the time pulse of the laser. For high-power densi-
ties, coagulation, necrosis, and vaporization of tissue can occur, while at low-power
densities, minimal heating is observed. For some wavelengths—for instance, those of the
known strong absorption bands of water—the laser beam is highly absorbed, since tissue
is primarily made of water. At these wavelengths the energy is then highly absorbed in a
relatively thin layer near the surface where rapid heating occurs (i.e., radial keratectomy).
The absorption is less for other wavelengths used away from the water absorption bands,
and this results in slower heating of a larger volume of the tissue (i.e., for prostate
coagulation).
Temperature Generation and Rate of Photon Absorption
A thermodynamically irreversible mode of interaction of light with materials is the
process of absorption in which the photon energy is absorbed by the material phase. In
the absence of conduction, the temperatureriseintissueisgovernedbyathermody-
namic equation of state. The equation of state requires that the change in internal energy
of the system be proportional to temperature rise. The change in internal energy of the
system, in the absence of conduction and other heat transfer processes, is equal to the
rate of energy deposition by the laser. Expressing this relation in terms of time deriva-
tives gives
D
U
=D
t
¼ r
C
D
T
=D
t
Q
L
ð
17
:
48
Þ
(W/m
3
) is volumetric rate of photon absorption by the
where
D
T
(K) is temperature rise,
Q
L
(J/m
3
.K) is volumetric heat
capacity, and t is the duration. A very important factor in temperature rise by photons is the
rate of photon absorption
(kg/m
3
) is mass density,
tissue,
r
C
(J/Kg.K) is specific heat,
r
C
, which is also known as the light/laser source term and as the
rate of energy deposition. For ordinary interaction processes, the rate of absorption of
photons by the material is proportional to irradiance, and the constant of proportionality
is the absorption coefficient:
Q
L
Q
L
¼ m
a
f
ð
17
:
49
Þ
