Biomedical Engineering Reference
In-Depth Information
The diffuse reflectance profile for oblique incidence is centered about the position of the
point sources, so the shift,
x, can be measured by finding the center of diffuse reflectance
relative to the light entry point. The two source model gives the following expression:
D
D
z
ð
1
þ m
eff
r
1
Þ
exp
ðm
eff
r
1
Þ
þ
ðD
z
þ
2
z
b
Þð
1
þ m
eff
r
2
Þ
exp
ðm
eff
r
2
Þ
1
4p
R
ð
x
Þ¼
ð
17
:
45
Þ
r
1
r
2
where
y
t
. Equation (17.45) can be scaled arbitrarily to fit a relative reflectance
profile that is not in absolute units. The effective attenuation coefficient,
D
z
¼
3D cos
m
eff
, is defined pre-
viously, while
r
2
are the distances from the two sources to the point of interest (the
point of light collection; see Figure 17.8). As can be seen in Figure 17.8b, the diffusion coef-
ficient can be calculated from
r
1
and
D
x
:
D
¼ D
x
=ð
3 sin y
Þ
ð
17
:
46
Þ
Optical properties of biological tissues depend on the tissue type and optical wavelength.
For instance, the liver, with its reddish color, would have a much higher absorption coeffi-
cient from a green light source such as an argon laser than a tan piece of tissue such as
chicken breast. Depending on the tissue type and exact wavelength, a typical set of optical
properties for visible or near-infrared light would be 0.1 cm
-1
for the absorption coefficient,
100 cm
-1
for the scattering coefficient, and 0.9 for the anisotropy. In the UV region, light
absorption is dominated by proteins. In the IR region, light absorption is dominated by
water. The near IR (
730 nm) is considered a diagnostic window because of the minimal
absorption and relatively low scattering in this region.
An important application of optical properties is the measurement of hemoglobin oxygen
saturation, which is a critical physiological parameter. Because the oxygenated and deoxy-
genated hemoglobin molecules have different absorption spectra, the relative concentration
ratio between the two forms of hemoglobin can be calculated once the optical properties of
the tissue are measured.
17.3 PHYSI
CAL INTERACTION OF LIGHT AND PHYSICAL
SENSING
After reading Section 17.2, it becomes quite apparent that most of the tissues in our
bodies are not transparent to light—that is, light is typically either absorbed or scattered
in tissues. In this section the physical interaction of the light with the tissue is described
and its use in both sensing and therapeutics. The four fundamental physical interactions
that will be described are the thermal changes induced or measured using light, pressure
changes induced or measured using light, velocity changes manifested as Doppler fre-
quency shifts in the light, and path length changes in the sample, which causes an inter-
ference pattern between two or more light beams.
17.3.1 Temperature Generation and Monitoring
All tissues in our body absorb light at various wavelengths, and the absorption process
can convert the light energy into heat. In addition, our body tissues, as with any object
above absolute zero temperature, also generate light radiation known as blackbody
