Biomedical Engineering Reference
In-Depth Information
the source, dye if used, sample, and detector. For instance, depending on the tissue probed,
there are strong absorbers, scatterers, and autofluorescence that need to be factored into any
fluorescent system design.
17.4.4 Measurements Made Using Light Polarization Properties
Having discussed some of the fundamental electromagnetic theory of polarized light
generation in Section 17.1.2, some of the applications of polarized light shall now be dis-
cussed. Applications of polarized light include biochemical quantification in vivo such as
noninvasively monitoring glucose for diabetes, measuring birefringence in tissues using
polarized light microscopy, and tissue characterization, in particular to aid in cancer
identification or for use in the measurement of the nerve fiber layer of normal and
glaucomatous eyes.
The rotation of linearly polarized light by optically active substances has been used
for many years to quantify the amount of the substance in solution. A variety of both
polarimeters, adapted to the examination of all optically active substances, and sacchari-
meters, designed solely for polarizing sugars, have been developed. The concept behind
these devices is that the amount of rotation of polarized light by an optically active sub-
stance depends on the thickness of the layer traversed by the light, the wavelength of
the light used for the measurement, the temperature, the pH of the solvent, and the concen-
tration of the optically active material.
Historically, polarimetric measurements have been obtained under a set of standard
conditions. The path length typically employed as a standard in polarimetry is 10 cm
for liquids, the wavelength is usually that of the green mercury line (5,461 Angstroms),
and the temperature is 20
C. If the layer thickness in decimeters is
L
, the concentration
of solute in grams per 100 ml of solution is
C
,
a
is the observed rotation in degrees, and
[
] is the specific rotation or rotation under standard conditions, which is unique for all
chiral molecules, then
a
100a
L
C
¼
ð
17
:
68
Þ
½
a
In the preceding equation, the specific rotation [
a
] of a molecule is dependent on tempera-
ture,
, and the pH of the solvent.
For polarimetry to be used as a noninvasive technique—for instance, in blood glucose
monitoring—the signal must be able to pass from the source, through the body, and to a
detector without total depolarization of the beam. Since the skin possesses high scattering
coefficients, which causes depolarization of the light, maintaining polarization information
in a beam passing through a thick piece of tissue (i.e., 1 cm), which includes skin, would not
be feasible. Tissue thicknesses of less than 4 mm, which include skin, may potentially be
used, but the polarimetric sensing device will encounter greater than 95 percent depolariza-
tion of the light due to scattering from the tissue.
As an alternative to transmitting light through the skin, the aqueous humor of the eye
has been investigated as a site for detection of in vivo glucose concentrations, since this
sensing site is a clear biological optical media. It is also known that glucose concentration
of the aqueous humor of the eye correlates well with blood glucose levels, with a minor
T
, wavelength,
l
