Biomedical Optics 2 MCQ

Optical Absorption

• Biological tissues are composed of various molecules, including water, proteins, lipids, DNA, etc.
• Light absorption by tissue occurs when light interacts with specific molecules within the tissue.
• These absorbing molecules are called chromophores.
• Different chromophores have distinct absorption spectra, meaning they absorb different wavelengths of light to varying degrees.
• Absorption spectra differences arise from variations in the molecular/atomic structure of the chromophores.

Molecular Energy Levels

• Energy exists in various forms, broadly categorized as kinetic and potential energy.

• Kinetic energy is related to an object's or system's motion.

• Potential energy is connected to the position of an object or system.

• In tissue optics, light absorption leads to an increase in the internal energy of molecules.

• This internal energy can then result in thermal or chemical effects.

• Molecules store energy in fixed amounts, termed energy levels, not arbitrary amounts.

• A molecule stores energy in various forms, including rotational kinetic, vibrational (potential & kinetic), translational kinetic (if the molecules are free to move within a fluid), and electronic potential energy.

• Energy level structures determine a molecule's absorption spectrum.

• Internal energy encompasses all types of molecular energy.

• The kinetic component of the internal energy is associated with thermal motion and correlates with temperature when observed macroscopically.

• Rotational kinetic energy relates to the rotation of a molecule or parts of a molecule held together by a bond that allows them to rotate relative to each other.

• Rotational energy transitions typically absorb low-energy photons in the microwave region of the EM spectrum.

• Vibrational energy arises when different parts of a molecule vibrate with respect to each other due to the elastic nature of the bonds holding constituent atoms together.

• Vibrational energy transitions absorb higher energy photons than rotational transitions, typically in the near-infrared or red end of the visible spectrum.

• Electronic energy levels describe the potential energy of electrons in relation to their distance from the atomic nucleus. Transitions between electronic energy levels require much higher energy than vibrational transitions and involve UV or X-ray photons.

• Excitation of electrons typically leads to photochemical reactions.

Light Absorption and Resonance Absorption

• Light absorption is a typical property of vibrating systems, occurring most effectively when the light's frequency matches the system's natural frequency (resonance).
• In tissue optics, the light's energy transfer into the molecule depends on the correspondence between the light's frequency and the molecule's natural frequency.
• Absorption is highly probable when the light's photon energy matches the difference between the molecule's current and excited states.
• Absorption processes generally occur on a very short timescale (femtoseconds).

Absorption Spectra

• Emission spectra of gases display narrow peaks of strong emission corresponding to specific frequencies (wavelengths or photon energies).

• Strong absorption lines often occur at the same frequencies (wavelengths or photon energies) as those in the emission spectra.

• However, in practice, these lines are not infinitely narrow but exhibit a finite probability of absorbing light at wavelengths slightly different from the resonance frequency – this phenomenon is known as spectral broadening.

• For large and complex biomolecules, the absorption spectra have more smoothly varying characteristics instead of distinct peaks, primarily due to various vibrational energy levels between electronic levels.

• Absorption spectra reflect the presence of various chromophores(different molecules absorbing light), such as water, hemoglobin, melanin, and lipids.

• Endogenous absorbers are molecules naturally present, in contrast to exogeneous absorbers which are externally introduced, like contrast agents or tattoos.

Near Infrared Window

• The near-infrared range, between approximately 650–1300 nm, is often referred to as the "near-infrared window."
• Within this spectral range, light penetration into tissue is more significant because endogenous absorbers, such as water and hemoglobin, display relatively low absorption.
• At these wavelengths, optical scattering is still present but does not hinder light penetration to the same extent as at shorter wavelengths.

Absorption Coefficient

• For a photon of a given frequency, the probability of a molecule undergoing a state transition from 'a' to 'b' is often expressed using the absorption cross-section.
• Absorption cross-section is the ratio of the absorbed power to incident power and is measured in units of area.
• Molar absorption coefficient is a material property.
• It is calculated by combining the absorption cross-sections of all molecules in one mole of a compound.
• The total absorption coefficient of a material composed of multiple chromophores is calculated as the sum of the individual absorption coefficients, weighted by their respective concentrations.

Other

• Nonlinear effects, such as multiphoton absorption, depletion, saturation, and photostability, are included, but not detailed.