Biomedical Engineering Reference
In-Depth Information
wavelength range has been proposed to overcome the autofluorescence problem, since the
fluorescence component falls off with increasing wavelength.
Excitation in the NIR region offers the added advantage of longer wavelengths that pass
through larger tissue samples with lower absorption and scatter than other spectral regions,
such as visible or ultraviolet. However, in addition to fluorescence falling off with wave-
length, the Raman signal also falls off to the fourth power as wavelength increases. Thus,
there is a tradeoff between minimizing fluorescence and maintaining the Raman signal.
The eye has been suggested as a site for analyte concentration measurements using Raman
spectroscopy to minimize autofluorescence, but the disadvantage to using the eye for Raman
spectroscopy is that the laser excitation powers must be kept low to prevent injury and this
significantly reduces the signal-to-noise ratio. Finally, like infrared and NIR absorption, to
quantifiably determine the inherently low concentrations of analytes in vivo, the presence
of different chemicals must be accounted for that yield overlapping Raman spectra.
17.4.3 Use of the Luminescence Property of Light for Measurement
As described previously, luminescence is the absorption of photons of electromagnetic
radiation (light) at one wavelength and reemission of photons at another wavelength. The
photons are absorbed by the molecules in the tissue or medium, raising them to some
excited energy state, and then, upon returning to a lower energy state, the molecules emit
radiation or, rather, luminesce at a different wavelength. The luminescent effect can be
referred to as fluorescence or phosphorescence. Fluorescence is luminescence that has
energy transitions that do not involve a change in electron spin, and therefore the reemis-
sion occurs much faster. Consequently, fluorescence occurs only during excitation, while
phosphorescence can continue after excitation (i.e., after the light source is turned off). As
an example, a standard television while turned on is producing fluorescence, but for a very
short time after it is turned off the screen will phosphoresce.
The measurement of fluorescence has been used for diagnostic, monitoring, and research
purposes. Fluorescent microscopes are now being used for research that produce outstand-
ing images of cells and tissues that provide a variety of information about them. Obtaining
diagnostic information, in particular with respect to cancer diagnosis or the total plaque in
arteries, has also been attempted using both intrinsic fluorescence of tissue and extrinsic
dyes. The intrinsic fluorescence is due to the naturally occurring proteins, nucleic acids,
and nucleotide coenzymes within the tissue, while extrinsic fluorescence is induced by
the uptake of certain dyes in the tissue. Extrinsic fluorescence has also been investigated,
for instance, to monitor such analytes as glucose, intracellular calcium, proteins, and
nucleotide coenzymes. Unlike the use of fluorescence in chemistry on dilute solutions, the
intrinsic or autofluorescence of tissue, as well as the scattering and absorption of the tissue,
acts as a noise source for the extrinsic approach.
The response of a fluorescence sensor can be described in terms of the output intensity as
I
f
¼ F
f
ð
I
o
I
Þ
ð
17
:
65
Þ
in which
I
f
is the radiant intensity of fluorescence,
F
f
is the fluorescence efficiency,
I
o
is the
radiant intensity incident on the sample, and
I
is the radiant intensity emerging from the
