Biomedical Engineering Reference
In-Depth Information
cavities. Unfortunately, the optical fibers available in this region tend to be toxic, hydro-
scopic, and/or rigid. Consequently, the combination of the water absorption and lack of
good fibers makes in vivo diagnostics and sensing very difficult in the mid-IR region.
The light sources used in this region include the broad-based tungsten bulb, nernst glower,
nichrome wire, and globar rod, and narrowly tunable, typically liquid nitrogen cooled, laser
diodes. The detectors used include the cooled mercury cadmium telluride (MCT), thermo-
pile, and thermistor. The optics include sodium chloride and potassium bromide, mirrors
(typically gold coated), and gratings.
In the near-infrared (NIR) wavelength region, the spectrum is not affected by water to
the same degree as the midinfrared region, allowing for path lengths within the tissue of
1 mm to 1 cm to be used. In addition, low OH silica fibers are quite transparent across this
range and are nontoxic and nonhydroscopic. The NIR region (700-2,500 nm) exhibits
absorptions due to low-energy electronic vibrations (700-1,000 nm), as well as overtones
of molecular bond stretching and combination bands (1,000-2,500 nm). These bands result
from interactions between different bonds (
NH) to the same atom. Typically,
only the first, second, and third overtones of a molecular vibration are detectable and are
broad in nature. Thus, only at high concentrations of the chemical species are these over-
tones qualitatively detectable with the intensity dropping off rapidly as overtone order
increases. The NIR absorption bands are also influenced by temperature, pressure, and
hydrogen bonding effects and can overlap significantly. In the 700 to 1,200 nm region, they
are also influenced by scattering effects. Thus, unlike mid-IR spectroscopy, NIR spectros-
copy is primarily empirical and not well suited for qualitative work. However, using tech-
niques such as multivariate statistics, quantitative analysis is possible with the NIR
spectrum. The light sources used in this region include the broad-based tungsten bulb, light
emitting diodes (LEDs), laser pumped solid-state lasers such as the tunable titanium sap-
phire laser, and laser diodes. The detectors used include silicon (good to 1 micrometer), ger-
manium (good to 1.7 micrometers), and indium antiminide (good to 5.5 micrometers). The
optics include low OH glasses, quartz, glass, gratings, and mirrors.
The means for separating the wavelength of the light of a broadband source include dis-
persive and nondispersive methods. The dispersive approach uses either a ruled reflective
grating or transmissive prism to separate the wavelengths of light. The nondispersive
systems include a series of wavelength selection filters or a Fourier transform infrared
(FTIR)-based instrument. The FTIR method uses an interferometer similar to the Michelson
interferometer shown in Figure 17.9 to collect the entire spectrum and then deconvolutes it,
using Fourier transform techniques. Both approaches can be configured to cover the NIR
and mid-IR ranges. The advantages of the dispersive systems include higher resolution
and separation of closely spaced wavelength bands, while the nondispersive systems
generally have better throughput, since all the light passes through the sample.
CH,
OH,
17.4.2 Monitoring Approaches Using Scattered Light
There are fundamentally two types of optical scattering for diagnostics and monitoring:
elastic and inelastic. The elastic scattering can be described using Mie theory (or Rayleigh
scattering for particles that are small compared to the wavelength), in which the intensity
of the scattered radiation can be related to the concentration, size, and shape of the
