energy JðÞ¼power WðÞtime sðÞð 6 : 23 ÞThus, if the power is doubled and the time is halved, the same energy is delivered to
the tissue but a different biological effect may be observed. A more precise dosage
parameter definition therefore is to prescribe a given irradiance during a specific
illumination time.
Example 6.10Consider two tissue samples that have areas of 0.1 and 1 cm^2.
If the samples each are illuminated for 1 s with an irradiance of 10 mW/cm^2 ,
what is the energy deposited in each of these tissue samples?
Solution: The energy is given by
Energy = Irradiance (W/cm^2 )×pulse time(s)×area (cm^2 )
(a) Energy 1 = (10 mJ/s/cm^2 )×1s×0.1 cm^2 =1mJ
(b) Energy 2 = (10 mJ/s/cm^2 )×1s×1cm^2 =10mJThe fundamental light-tissue interaction modes for therapeutic and surgical uses
can be broadly categorized into the following six classes:
- Photobiomodulation: This interaction class [also calledbiostimulationorlow-
level light therapy(LLLT)] deals with the stimulation of biological systems by
means of low irradiance levels. In contrast to other interaction modes, here the
light levels do not result in tissue ablation, heating effects, or cell destruction.
Instead the irradiances induce a photochemical effect to stimulate tissue to heal
and recover from inflammation and other effects. Its clinical applications include
alleviating acute and chronic pain, treating sprains and strains from sports
injuries, speeding up wound healing, treating nerve and brain injuries, and
promoting tissue and bone healing. - Photochemical reaction: An increase in the irradiance beyond that used for
photobiomodulation can cause an electron in a molecule to absorb a high-energy
photon. As a result, molecules with an electron in a higher energy state can
undergo a chemical reaction by exchanging or sharing electrons with other
molecules. Such a photochemical light-tissue interaction can destroy unwanted,
cancerous, or diseased cells. This mode can be used to treat certain skin diseases
and to kill cancer cells, for example, in processes such as photodynamic therapy. - Thermal effects: Coagulation and vaporization are thermal effects that are based
on localized heating of a selective region using a wavelength at which the tissue
exhibits high absorption. The conversion of photon energy into heat energy
occurs via increased molecular vibrations due to photon absorption and from
collisions of photons with molecules. The localized coagulation and vaporiza-
tion effects are used in disciplines such as tissue cutting in laser surgery, tissue
ablation for scar removal and facial contouring, and tissue bonding in
ophthalmology.
6.5 Light-Tissue Interaction Mechanisms 173