Light in Medicine: Properties and Applications

Light in Medicine

Properties of Light

• Light changes speed when it goes from one material to another
• The ratio of the speed of light in a vacuum to its speed in a given material is called the index of refraction
• Light behaves both as a wave and as a particle
• As a wave, it produces interference and diffraction, which are of minor importance in medicine
• As a particle, it can be absorbed by a single molecule, and when absorbed, its energy is released in various ways

Heating Effect of Light

• When light is absorbed, its energy generally appears as heat
• This property is the basis for the use of IR light to heat tissues in medicine
• This property is known as fluorescence

Reflection of Light

• Light is reflected to some extent from all surfaces (e.g., endoscope)
• Reflection of light is used in medicine for imaging and diagnosis

Light Amplification by Stimulated Emission of Radiation (Laser)

Einstein's Theory

• In 1917, Einstein postulated that incident photons of energies exactly equal to the energy that an excited atom must eject if it falls to its lower energy state
• These incident photons stimulate the excited atom to fall to its lower state and release a photon in phase with the incident photon

Types of Lasers

• Pulse laser (e.g., ruby laser, semiconductor laser, glass laser)
• Continuous wave laser (e.g., neon-helium laser, argon laser)

Ruby Laser

• In 1960, T.H. Maiman produced a laser beam from a ruby crystal
• The active material in the ruby is the chromium ions
• Ruby laser is used in medicine for various applications, including eye surgery and skin treatments

Laser in Medicine

• Lasers are used in medicine primarily to deliver energy to tissue
• The laser wavelength used should be strongly absorbed by tissue
• Short wavelengths (400-600 nm) are always absorbed better than long wavelengths (700 nm)
• Laser energy directed to human tissue causes a rapid rise in temperature and can destroy the tissue
• The amount of damage to living tissue depends on the time the tissue is exposed to increased temperature

Applications of Laser in Medicine

• Used by surgeons for the painless removal of eye tumors
• Used as a "bloodless knife" in surgery
• Repairing retinal tears or holes that develop prior to retinal detachment (photocoagulation)
• Treatment of diabetic retinography (photocoagulation)
• Used in medical research for special three-dimensional imaging called holography

Application of Visible Light in Medicine

Curved Surfaces

• Curved lenses (convex, concave, and cylindrical) are used in medicine
• Curved mirrors are used in ophthalmoscope, otoscope, and other medical instruments

Endoscopes

• Endoscopes are used for viewing internal body cavities
• Based on the principle of total internal reflection
• Endoscopes are supplied with IR-absorbing filters to minimize tissue heating

Transillumination

• It is the transmission of light through the tissues of the body
• Used clinically in the detection of:
  • Hydrocephalus (water-head) in infants
  • Pneumothorax (collapsed lung) in infants
  • Sinuses
  • Breasts
  • Gums
  • Testes

Phototherapy

• Premature infants recover from jaundice when they are exposed to visible light

Microscopy

• The smallest objects that can be resolved are about 1 μm in diameter
• Visible light microscopes are adequate for resolving cells (diameters 5-50 μm)

Application of Ultra-Violet (UV) and Infra-Red (IR) Light in Medicine

UV Light

• Photon has energy greater than visible photon
• Wave length: 100-400 nm
• Used in medicine to:
  • Kill germs
  • Produce more reactions in the skin than visible light
  • Cause tanning
  • Treat skin cancer

IR Light

• Photon has energy less than visible photon
• Wave length: 700-10000 nm
• Used in medicine to:
  • Heat deep tissues
  • Provide warmth
  • Not usually hazardous even when focused by the cornea and lens of the eye onto the retina