Optical Absorption in Biological Tissue

Optical Absorption

• Biological tissue is composed of molecules such as water, proteins, lipids, DNA, etc.
• When light interacts with tissue, it's absorbed by specific molecules called chromophores.
• Different chromophores have different absorption spectra, meaning they absorb different wavelengths of light with varying strengths.
• These differences in absorption spectra are due to differences in the molecular or atomic structure of the chromophores.
• Optical absorption in tissue is important. In therapeutic applications of tissue optics, light absorption leads to changes in the internal energy of the molecule, which can cause thermal or chemical effects.

Molecular Energy Levels

• Energy exists in many forms.
• Two primary forms are kinetic energy (energy of motion) and potential energy (energy of position).
• Molecules can only hold specific amounts of energy, called energy levels.
• Internal energy is the sum of all types of molecular energies (rotational, vibrational, translational, and electronic).
• The kinetic part of internal energy is related to thermal motion and temperature.

Rotational Kinetic Energy

• Molecules in a system that are not bound to each other can rotate and store energy in the form of rotational motion.
• Two parts of a molecule, held together by a bond that allows them to rotate, can store rotational energy
• Rotational modes are easily excited with low energy, leading to absorption in the microwave region of the EM spectrum.

Vibrational Energy

• Different parts of molecules can also vibrate relative to each other.
• The bonds holding the atoms together are not perfectly rigid, allowing for vibrations.
• Vibrational modes require more energy to excite than rotational modes, which results in absorption in the near-infrared or red part of the visible spectrum.

Electronic Energy Levels

• Electronic energy levels relate to the potential energy of electrons relative to the nucleus.
• These energy transitions typically require higher energies (Ultraviolet or X-ray) than vibrational or rotational ones, and are associated with the highest energy changes.
• Excitation of electrons can lead to chemical reactions.

Internal Energy and Absorption Spectra

• The structure of the energy levels of a molecule determines its absorption spectrum.
• The total absorption spectra is often a combination of many spectra of the different molecules present in biological tissue.

Light Absorption / Resonance Absorption

• Light absorption is typical in vibrating systems when the driving frequency (of the light) matches the natural frequency.
• This condition is called resonance.
• The likelihood of absorption is influenced by the absorption cross section. The absorption cross-section is the ratio of the power absorbed by the molecule to the incident power per unit area.

Absorption Spectra

• Absorption spectra in complex biological matter, like tissue, exhibit smooth variations rather than distinct peaks.
• This is due to combinations of multiple vibrational levels transitions between the various chromophores, and broadening across multiple vibrational states.
• The measured absorption spectrum of a complex material, like tissue, is a blend of the constituent molecules present. Substances like water, hemoglobin, melanin, and lipids all have distinct absorption ranges.

Near Infrared Window

• The wavelength range between 650-1300 nm is known as the near-infrared window.
• Light in this range penetrates tissue more deeply than light at shorter wavelengths due to lower absorption of endogenous chromophores.

Absorption Coefficient

• The absorption coefficient describes the likelihood of a transition from one energy state to another due to photon absorption.
• It is often expressed as a molar absorption coefficient which represents the absorption by a mole of molecules.
• The total absorption coefficient of a material is the sum of the molar absorption coefficients for each of its components weighted by the concentrations of those components.
• Nonlinear effects like multiphoton absorption, ground state depletion and saturation and bleaching are further factors in energy absorption and transfer in biological substances.