Biomedical Engineering Reference
In-Depth Information
the light. Knowing the absorption, scattering, and direction of propagation of the light has
applications for both optical diagnostics and sensing, as well as optical therapeutics. For
instance, many investigators are trying to quantifiably and noninvasively measure certain
body chemicals such as glucose using the absorption of light. In addition, by knowing the
absorption and scattering of light, investigators are modeling the amount of coagulation
or “cooking” of the tissue that is expected for a given wavelength of light, such as in the
case of prostate surgery or cancer therapy.
The optical penetration depth and the volume of tissue affected by light is a function of
both the light absorption and scattering. If it is assumed scattering is negligible or, rather,
absorption is the dominant component, then the change in light intensity is determined
by the absorption properties in the optical path and the illumination wavelength. For
instance, for wavelengths above 1.5 micrometers, the light propagation through all
biological tissues, since they have large water content, is absorption dominant. The intensity
relationship as a function of optical path and concentration for this absorption dominant
case can be described by the Beer-Lambert law as
I
(
z
Þ¼
I
o
exp (
m
a
z
Þ
ð
17
:
17
Þ
in which
I
is the transmitted light intensity,
I
o
is the incident light intensity,
z
is the path
length of the light, and
m
a
(1/cm) is the absorption coefficient. The absorption coefficient
can be further broken down into two terms that include the concentration of the chemically
absorbing species,
(mg/mL), times the molar absorptivity, n (cm
2
/mg), which is a func-
C
tion of wavelength.
Luminescence, also known as fluorescence or phosphorescence, depending on the fluo-
rescent lifetime, is a process that occurs when photons of electromagnetic radiation are
absorbed by molecules, raising them to some excited state, and then, on returning to a
lower energy state, the molecules emit radiation or rather luminesce. Fluorescence does
not involve a change in the electron spin and therefore occurs much faster. The energy
absorbed in the luminescence process can be described in discrete photons by the equation
E
¼
hc
=l
ð
17
:
18
Þ
10
34
J-s),
in which
E
is the energy,
h
is Planck's constant (6.626
c
is the speed of light, and
l
is the wavelength. This absorption and reemission process is not 100 percent efficient,
since some of the originally absorbed energy is dissipated before photon emission. There-
fore, the excitation energy is greater than that emitted
E
excited
>
E
emitted
ð
17
:
19
Þ
and
Þ
The energy level emitted by any fluorochrome is achieved as the molecule emits a pho-
ton, and because of the discrete energy level, individual fluorochromes excited by a given
wavelength typically only fluoresce at a certain emission wavelength. Originally fluores-
cence was used as an extremely sensitive, relatively low-cost technique for sensing dilute
solutions. Only recently have investigators used fluorescence in turbid media such as tissue
for diagnostics and sensing.
l
> l
ð
17
:
20
emitted
excited
