Laser-tissue interactions are crucial in biophotonics. They involve absorption, scattering, and reflection of light in tissues, leading to thermal and photochemical effects. Understanding these processes is key to developing safe and effective laser-based medical treatments.

Laser safety is paramount when working with high-powered beams. Maximum permissible exposure limits and laser safety classes guide safe usage. Protective measures, including eyewear and engineering controls, are essential to prevent accidents and ensure responsible laser application in medical settings.

Light-Tissue Interactions

Absorption and Scattering Processes

• Absorption occurs when light energy transfers to tissue molecules
  • Depends on wavelength and tissue composition
  • Chromophores (light-absorbing molecules) determine absorption spectrum
    • Water, hemoglobin, and melanin are primary chromophores in biological tissues
• Scattering redirects light within tissue without energy transfer
  • Caused by refractive index mismatches between cellular components
  • Mie scattering dominates in biological tissues (particle size similar to wavelength)
  • Rayleigh scattering occurs for smaller particles (proportional to 1/λ⁴)
• Reflection happens at tissue surface or internal boundaries
  • Specular reflection occurs on smooth surfaces (mirror-like)
  • Diffuse reflection occurs on rough surfaces (scatters in multiple directions)

Thermal and Photochemical Effects

• Thermal effects result from light energy conversion to heat
  • Localized temperature increase can cause protein denaturation (coagulation)
  • Thermal relaxation time determines heat dissipation rate
  • Continuous wave lasers typically produce more thermal effects than pulsed lasers
• Photochemical effects involve light-induced chemical reactions
  • Photodynamic therapy uses light-activated drugs to produce reactive oxygen species
  • UV light can induce DNA damage through photochemical processes
  • Photobiomodulation stimulates cellular processes without significant heating

Laser Ablation Mechanisms

Photoablation and Plasma-Induced Ablation

• Photoablation removes tissue through direct bond breaking
  • Typically uses UV lasers (excimer lasers)
  • Precise removal of thin layers of tissue (micrometers)
  • Minimal thermal damage to surrounding tissue
  • Applications include corneal reshaping in refractive surgery (LASIK)
• Plasma-induced ablation creates a highly ionized gas (plasma)
  • Occurs at very high laser intensities (>10¹⁰ W/cm²)
  • Plasma absorbs incoming laser energy, shielding deeper tissue
  • Shock waves from plasma expansion can cause mechanical tissue damage
  • Used in cataract surgery and dental procedures

Photodisruption and Cavitation

• Photodisruption breaks tissue through mechanical effects
  • Utilizes extremely short laser pulses (picoseconds to femtoseconds)
  • Creates localized plasma that rapidly expands, causing tissue separation
  • Minimal thermal damage due to ultra-short interaction time
  • Applications include posterior capsulotomy after cataract surgery
• Cavitation bubbles form during photodisruption
  • Rapid plasma expansion creates a void filled with water vapor and gases
  • Bubble collapse can cause additional mechanical damage
  • Cavitation effects extend the zone of tissue disruption beyond the focal point

Laser Safety

Maximum Permissible Exposure and Safety Classes

• Maximum permissible exposure (MPE) defines safe exposure limits
  • Measured in J/cm² or W/cm² depending on exposure duration
  • Varies with wavelength, pulse duration, and exposure time
  • Based on ANSI Z136.1 standard in the United States
• Laser safety classes categorize lasers based on potential hazards
  • Class 1: Safe under all conditions of normal use
  • Class 2: Safe for accidental viewing (visible lasers only, <1 mW)
  • Class 3R: Low risk, but potentially hazardous (1-5 mW)
  • Class 3B: Hazardous for direct beam viewing (5-500 mW)
  • Class 4: High power lasers, hazardous to eyes and skin (>500 mW)

Protective Measures and Equipment

• Protective eyewear blocks specific laser wavelengths
  • Optical density (OD) indicates attenuation level
  • Must match laser wavelength and power
  • Different materials used for different wavelengths (polycarbonate, glass)
• Engineering controls reduce laser hazards
  • Interlocks prevent unauthorized access to laser areas
  • Beam enclosures contain stray radiation
  • Key switches prevent unauthorized laser activation
• Administrative controls include training and standard operating procedures
  • Laser Safety Officer (LSO) oversees laser safety program
  • Warning signs and labels indicate laser hazards
  • Regular maintenance and alignment procedures minimize risks