Biomedical Engineering Reference
In-Depth Information
scattering particles. The inelastic scattering, in which the polarization of the particle is not
constant, can be described as Raman scattering. Note that fluorescence, although an absorp-
tion reemission process, is also inelastic.
When using the light scattering phenomenon for sensing, the intensity of the reflected
light is usually considered. The reflection of light, however, can be divided into two forms.
Specular reflection, or a “mirror” type of reflection, occurs at the interface of a medium. The
returned light yields little information about the material other than its roughness, since it
never penetrates the medium. Thus, for applications other than surface roughness, the spec-
ularly reflected light is typically minimized or eliminated with the design of the optical sen-
sor. Diffuse reflection, however, occurs when light penetrates into a medium, becomes
absorbed and multiply scattered, and makes its way back to the surface of the medium.
The model describing the role of diffuse scattering in tissue is based on the radiative trans-
fer theory, as was described in Section 17.2. The same theory applies for sensing as well.
The use of elastically scattered light has been suggested for both diagnostic procedures
such as cancer detection and for monitoring analytes such as glucose noninvasively for dia-
betics. For use as a monitoring application of chemical changes such as glucose, researchers
have used an intensity-modulated frequency domain NIR spectrometer, capable of separat-
ing the reduced scattering and absorption coefficients to detect changes in the reduced scat-
tering coefficient showing correlation with blood glucose in human muscle. This approach
was promising at first as a relative measure over time, since clearly an increase in glucose
concentration in the physiologic range decreases the total amount of tissue scattering. How-
ever, the drawbacks of the light elastic scattering approach for analyte monitoring are still
quite daunting. The specificity of the elastic scattering approach is the biggest concern with
this method, since other physiologic effects unrelated to glucose concentration could pro-
duce similar variations of the reduced scattering coefficient with time, and unlike the
absorption approach, elastic light scattering as a function of the molecule is nearly wave-
length independent.
The measurement precision of the reduced scattering coefficient and separation of scat-
tering and absorption changes is another concern with this approach. It is difficult to mea-
sure such small changes and be insensitive to some of the larger absorption changes in the
tissue, particularly hemoglobin. This approach also needs to take into account the different
refractive indices of tissue. Tissue scattering is caused by a variety of substances and orga-
nelles (membranes, mitochondria, nuclei, etc.), and all of them have different refractive
indices. The effect of blood glucose concentration and its distribution at the cellular level
is a complex issue that needs to be investigated before this approach can be considered via-
ble. An instrument of this type would require in vivo calibration against a gold standard,
since the reduced scattering coefficient is dependent on additional factors such as cell den-
sity. Last, there is a need to account for factors that might change the reduced scattering
coefficient, such as variations in temperature, red blood cell concentration, electrolyte
levels, and movements of extracellular and intracellular water.
As a diagnostic screening tool for cancer detection, measurement of the scatter in thin tis-
sues or cells may hold promise. Many of the changes in tissue due to cancer are morpho-
logic rather than chemical and thus occur with changes in the size and shape of the
cellular and subcellular components (membranes, mitochondria, nuclei, etc.). Thus, the
changes in elastic light scatter should be larger with the morphologic tissue differences.
