As indicated in Fig.6.11, the sizes of scattering elements in biological tissue
range from about 0.01 to 20μm. This corresponds to the sizes of biological tissue
elements ranging from cell membranes to whole cells. As shown in Fig.6.11,
eukaryotic cell sizes generally range from 10μm and higher. For example, red
blood cells have the shape of a biconcave disk with a 7.8-μm diameter and 2.6-μm
thickness, white blood cells are spheroids ranging in size from 7 to 20μm, and
platelets are oblate spheroids that are approximately 3.6μm in diameter and 0.9μm
in thickness.
Photons are scattered most strongly by particles that have a size approximately
equal to a wavelength used in diagnostic processes (500–1600 nm) and with a
refractive index similar to the surrounding medium in which the particles are
embedded. Thus two primary strong scattering elements in a cell are cell nuclei and
mitochondria. The average refractive index of biological tissue ranges from 1.34 to
1.62, which is higher than the 1.33 value of water. Table6.2lists the refractive
indices of some common tissue components.
Scattering can be either an elastic or inelastic process. Inelastic scatteringthe
incident and scattered photons have the same energy. This type of scattering
describes most light-tissue interactions. The uses of this phenomenon in biopho-
tonics techniques include elastic scattering spectroscopy and diffuse correlation
spectroscopy (see Chap. 9 ).Inelastic scatteringinvolves the exchange of energy
between a photon and the molecule responsible for the scattering. As Sect.6.3.2
describes, during inelastic scattering, energy can be transferred either from the
photon to the molecule or the photon can gain energy from the molecule. This
phenomenon is used in biophotonics techniques such as fluorescent
scattering-based microscopy, multiphotonflorescence scattering, Raman scattering
microscopy, and coherent anti-Stokes Raman scattering.
6.3.1 Elastic Scattering
For a spherical particle of any size, light scattering can be analyzed exactly by the
Mie scattering theory [ 16 , 27 ]. Note that even though the shapes of some scattering
elements in biological tissue are not necessarily spherical, their scattering behavior
Table 6.2 Refractive indices
of some common tissue
components
Tissue component Refractive index
Cytoplasm 1.350–1.375
Epidermis 1.34–1.43
Extracellularfluids 1.35–1.36
Human liver 1.367–1.380
Melanin 1.60–1.70
Mitochondria 1.38–1.41
Tooth enamel 1.62–1.73
Whole blood 1.355–1.398162 6 Light-Tissue Interactions