Biomedical Engineering Reference
In-Depth Information
produces broad fringes when it is normalized to the same max value, since it has a simple
cos
2
(
/2) dependence on the phase (dashed lines in the figure). An example of a fiber optic-
based Fabry-Perot interferometer is the commercially available intracranial fluid pressure
monitoring system for patients with severe head trauma or hydrocephalus (increased
amount of cerebral spinal fluid in the ventricles and/or subarachnoid spaces of the brain).
d
17.4 BIOC
HEMICAL MEASUREMENT TECHNIQUES USIN
G LIGHT
In the last few years, there has been much enthusiasm and considerable effort from med-
ical device companies and universities to perform diagnostic procedures such as cancer
detection and quantifiably monitor blood chemicals, such as glucose, lactic acid, albumin,
and cholesterol using various optical approaches. The most well-known optical monitoring
approach used clinically in vivo is the pulse oximeter, which indirectly measures oxygen
saturation and changes in blood volume by detecting changes in the optical absorption
peaks of oxygenated and deoxygenated hemoglobin as it is pulsed through the arteries.
It is often used to also measure heart rate.
The most common optical approaches for diagnostic and sensing applications include
absorption, scattering, luminescence, and polarimetry. The primary variable for each of
these approaches is a change in the light intensity as it passes through the medium, which
can change as a function of the wavelength or polarization of the light. The lifetime and/or
phase can also be used as a variable. For absorption and luminescence, the intensity will
change nearly linearly for moderate analyte concentrations and nonlinearly for high
concentrations.
17.4.1 Spectroscopic Measurements Using Light Absorption
Many investigators have suggested infrared absorption as a potential route to blood
chemical monitoring and diagnostic sensing, in particular for glucose monitoring and can-
cer detection. The governing Eq. (17.17) for purely absorbing media as well as fluorescence
was described previously using the Beer-Lambert law. Expressed logarithmically, the equa-
tion becomes
A
¼
ln
ð
T
Þ¼
ln
ð
I
o
=
I
Þ¼m
z
¼
z
Se
i
C
i
ð
17
:
64
Þ
in which
A
is absorbance,
T
is transmittance,
I
o
is the incident light,
I
is the transmitted
light, z is the path length, and
is the absorption coefficient, or rather the sum of the mul-
tiplication of the molar absorptivity times the concentration of all the different components
in the analyte. For tissue and blood this equation is valid in the midinfrared wavelength
region (wavelengths of 2.5-12 micrometers), in which the absorption peaks due to various
chemicals are distinguishably sharp and the scatter is weak. Unfortunately, the absorption
of the light due to water within tissue in this region is orders of magnitude stronger than
any of the blood chemicals, which results in the possibility of only short path length sample
investigations (on the order of micrometers). This brings about the possibility for surface
or superficial investigations on the skin or, with the use of fiber optics, on internal body
m
