Optical Absorption in Biological Tissues

Optical Absorption

• Biological tissue is made of molecules (water, proteins, lipids, DNA, etc.)
• Light absorbed by tissue is absorbed by specific molecules within the tissue, called chromophores.
• Different chromophores absorb different wavelengths of light.
• Absorption spectra vary due to differences in molecular or atomic structure of chromophores.

Molecular Energy Levels

• Energy exists in many forms.

• Two broad classes are kinetic energy (energy of motion) and potential energy (energy due to position.)

• In tissue optics, light absorption results in a transfer of energy to internal energy of a molecule.

• This can cause thermal or chemical changes.

• Molecules can only hold specific amounts of energy, called energy levels.

Internal Energy

• Molecules hold energy in several different forms:
  • Rotational kinetic energy
  • Vibrational energy (potential & kinetic)
  • Translational kinetic energy (if in a fluid)
  • Electronic potential energy.
• The structure of these energy levels determines a molecule's absorption spectrum.
• Internal energy is the total energy of a molecule due to its various energy forms.
• The kinetic part of internal energy is related to thermal motion and temperature.

Rotational Kinetic Energy

• Molecules that aren't connected to others can rotate as a solid body, storing energy in the rotational motion.
• The two parts of a molecule connected by a bond can also rotate around each other, storing rotational energy.
• Rotational modes are easily excited with less energy.
  • This usually corresponds to absorption in the microwave region of the electromagnetic (EM) spectrum (longer wavelengths than visible light).

Vibrational Energy

• Parts of a molecule can vibrate relative to one another.

• The bonds holding atoms together are not rigid; they are elastic.

• Vibrational modes need significantly more energy than rotational modes, so absorption occurs in the near-infrared or red end of the visible spectrum.

Electronic Energy

• Electronic energy levels refer to how electrons relate to the nucleus.
• Moving an electron to a higher energy orbit requires more energy.
• Moving electrons to higher levels requires ultraviolet or high-energy visible light (UV) photons.
• A molecule excited by a photon is more likely to react with neighboring molecules; this begins photochemistry.

Light Absorption/Resonance Absorption

• Absorption is the process of a vibrating system (molecule) absorbing energy from another system (light) when their natural frequencies match. This is a resonance effect.
• Light absorption is easier when frequencies of the light and the molecule match.
• The likelihood of a transition from a low energy state to a higher energy state is related to the absorption cross-section, the ratio of power absorbed to incident power per a unit area.

Absorption Spectra

• Absorption spectra show how much different wavelengths of light are absorbed.
• In complex biological tissue, spectra are smooth and continuously varying because of many types of molecules.
• Endogenous absorbers occur naturally in the body and exogenous ones are introduced (e.g., contrast agents, tattoos).
• Important endogenous absorbers are hemoglobin, water, lipids, melanin, and collagen.

Near Infrared Window

• The wavelength range of 650-1300 nm is known as the near-infrared window.
• These wavelengths, due to less absorption by endogenous absorbers and less scattering, can penetrate deepest into tissue.

Absorption Coefficient

• Absorption coefficients relate to the likelihood of absorption at specific frequencies.
• Molar absorption coefficient is the absorption of a mole of a substance.
• Total absorption coefficient of a material is the sum of absorptions of all the absorbers' weighted by their concentrations.

Nonlinear Effects

• Multiphoton absorption, depletion, saturation and bleaching, photostability are processes that occur outside the standard linear absorption properties of materials and molecules.

This quiz explores the principles of optical absorption in biological tissues, focusing on the role of chromophores in light absorption. Additionally, it covers concepts related to molecular energy levels and the various forms of internal energy molecules can hold. Test your understanding of how these processes influence thermal and chemical changes within tissues.