Biomedical Optics 3 MCQ

Absorption and Scattering

• The third session of the course covers absorption and scattering topics.
• Today's session content includes: absorption coefficient, absorption cross-section, molar absorption coefficient, absorption coefficient as a probability, spectrophotometry including measuring absorption spectra, transmittance and absorbance, and spectroscopic inversion.
• Optical scattering topics discussed are: optical scattering, the physical basis of scattering, molecular scattering refractive index, and scattering coefficient.
• Absorption coefficient is often given in terms of absorption cross-section.
• Absorption cross-section is the ratio of the power absorbed by the molecule and the incident Power per unit of area.

Absorption Coefficient

• Absorption coefficient is a property for describing the amount of a single molecule absorbing light.
• In tissue optics, it is necessary to describe the absorption of bulk matter consisting of many molecules.
• Molar absorption coefficient is obtained by summing up the absorption cross-sections of the molecules in one mole of substance.
• The mathematical representation of the molar absorption coefficient is α(ν) = ΝΑσ(ν), where ΝΑ = Avogadro's constant, and σ(ν) = absorption cross section.

Absorption Coefficient as a Probability

• The probability of a photon being absorbed while travelling a distance δz is μaδz where μa is the absorption coefficient.
• A plane wave of continuous light irradiating a purely absorbing, non-scattering medium also contributes to this exponential decay.
• Beer's Law describes the relationship: Φ = Φ0 exp(-μaZ) where Φ0 is the initial intensity of light
• Spectrophotometry is used to measure optical absorption spectra
• The intensity will decrease with propagation direction as light is absorbed.

Measuring Absorption Spectra

• The measurement of optical absorption spectra is spectrophotometry.
• Spectrophotometry equation is: I_L(λ) = I₀(λ) exp(-µ_a(λ)d) which describes the detected intensity and gives us the coefficient μ_a(λ) and α(λ).

Spectroscopic Inversion

• Given a sample with known chromophores, concentrations of each constituent can be estimated via measurements at different wavelengths.
• A sample containing multiple chromophores a, b, and c with their respective extinction coefficients ε_a(λ), ε_b(λ), ε_c(λ) can be analyzed.
• Measurements of absorbance are taken with a spectrophotometer at wavelengths λ₁, λ₂, λ₃.

Optical Scattering

• Scattering is often the dominating characteristic in biological tissue.
• Scattering can be categorized as elastic, where energy is conserved and the scattered light has the same frequency and inelastic, where energy is lost or gained.
• Elastic scattering is significant in biological tissues with visible and near-infrared wavelengths.
• The scattering mechanism works because the oscillating dipole (a moving charge) radiates an electromagnetic wave.
• A photon of equal energy to the incoming photon will be reradiated in a random direction.

Refractive Index

• Light travels at a specific velocity in vacuum, and the speed in the glass is c/v.
• The refractive index is related to why light travels at slower speeds in a glass medium.

Scattering Coefficient

• The scattering coefficient (μ_s) describes the mean free path between scattering events and relates to tissue scattering properties.
• The reduced scattering coefficient µ_s' incorporates scattering coefficient μ_s and anisotropy g. μ_s' = μ_s(1−g).
• The mean free path refers to the average distance travelled by a moving particle (atom, molecule, or photon) between impacts.

Other Important Concepts

• Tissue optical properties can be analyzed with reduced scattering coefficient.
• A g-factor measures forward-peaked light distribution.