Biomedical Engineering Reference
In-Depth Information
Material
phase
Photon
phase
FIGURE 17.5
Two coexisting phases, the photon phase (laser) and the material phase (tissue) govern laser-
tissue interactions. Photons are distributed within the tissue by the processes of absorption and scattering. Energy
balance over a control volume (shown with thicker line) provides the governing equation for light and heat distri-
bution and exchange of energy between the photon and material phase as described later in the chapter.
q
in
Q
L
(rate of energy absorbed by
the "material phase")
U
(r)
q
out
FIGURE 17.6
Control volume of the photon phase with radiative fluxes in and out, radiative energy, and rate
of energy lost to the material phase by absorption.
For a steady-state system in which radiation is collimated and monochromatic, laser light
travels only in the positive
z
direction—that is, assuming only one forward flux—Eq. (17.25)
reduces to
dq
ð
rad
Þ
z
=
dz
¼
Q
L
:
ð
17
:
26
Þ
Considering the speed of light and the dimensions of a biological tissue, the assumption
of a steady-state condition is a reasonable assumption for most applications except when
very fast light sources are used and/or when time-resolved analyses are considered. The
important step at this point is to use the phenomenological relation
Q
L
¼
m
a
I
ð
17
:
27
Þ
