ABSTRACT
Decreased scattering coefficient μ¢s and anisotropic constant g = ·cos qÒ relate to the scattering coefficient μs as m m
¢ = -( )1 g (22.12)where g can be −1 (totally backward scattering), 0 (isotropic scattering), or 1 (totally forward scattering). In general, biological tissues have this value in the range of 0.65 to 0.95. In practice, optical absorption and scattering always appear at the same time. They rely on the wavelength asm lS¢ = A b
(22.13)
From Eq. 22.13, both scattering and absorption are mostly suppressed, whereas absorption is dominant if the wavelength is increased from UV to IR in biological tissues. Light-tissue interactions can be modeled in many ways. When m m a
>> ¢ , we use the Lambert-Beer law (l £ 300; l ≥ 2000 nm). In the case of m m S
¢ >> , we use the diffusion approximation (600 £ l £ 1000 nm). If m m S
¢ ª , we can use the Monte Carlo method (300 £ l £ 600 nm; 1000 £ l £ 2000 nm). Light transport in tissues is expected to offer some functional data about tissue, such as O2 consumption and hemodynamics. Due to the therapeutic window (600 £ l £ 1000 nm), the diffusion theory through the time-resolved method is suggested to carry more information about the path length of light. This theory is, therefore, very useful to the quantification of drug delivery. The time-dependent optical diffusion equation can be written as1
c t D r r r t S r t
∂ ∂
-—◊ —+È ÎÍ
˘ ˚˙
=( ) ( ) ( , ) ( , )m a
F (22.14)for the analysiswhere D r r r
( ) ( ) ( )
= +ÈÎ ˘˚ 1
3 m m a S
and c is the light velocity, D is the diffusion coefficient, F is the light flux, S is the source term, r is the measuring position, and t is the measuring time. The image reconstructions of the scattering and absorption coefficients are called diffuse optical tomography (DOT). Usually, solving this equation is a difficult task. Simple fluorescence diffuse optical tomography (FDOT) algorithm, which is thus reduced to a DOT problem, successfully reconstructs the image of the fluorescence target. The concept of virtual total light is very useful, and it will be discussed in Chapter 24. 22.3 Problems 1. What is tissue? 2. Discuss the absorption and scattering processes and their parameters. 3. Give some examples of absorbers and scatterers in tissues. 4. Explain the types of optical signal. 5. Explain the metrics of absorption. 6. Discuss the Lambert-Beer law in absorption fundamental. 7. Discuss the difference between elastic and inelastic scattering. 8. Explain the concepts of Rayleigh and Mie scatterings. 9. Write the diagram of hierarchy of ultrastructure in tissue. 10. Compare the modeling method to understand light propagation in tissue.
