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Lasers use beams of highly focused or collimated light which are now used in several surgical applications to cut and coagulate various types of tissues. This article presents a very brief overview of how they achieve this goal.
Temperature-Related Laser Effects
Lasers increase the temperature of cells in several ways, leading to coagulation of proteins. This occurs in many ways.
The earliest change begins with hyperthermia or heating of the tissue to 40-50 °C, which breaks some molecular bonds to change the membrane characteristics and alter enzyme activity. At this point these effects may be reversed.
As the temperature goes beyond 60 °C proteins undergo denaturation, including collagen, causing tissue coagulation. At supratolerable durations, this leads to cell necrosis and the tissue becomes pale.
At still higher temperatures of over 80 °C, chemical disequilibrium occurs as the cells become hyperpermeable and many cell mediators leak out.
At the boiling point of water, 100 °C, water in the tissue vaporizes to form a thermomechanic effect, in which the tissue remains at the same temperature due to the absorption of latent heat by the water. Gas bubbles form within the tissue, and expand, destroying it both mechanically and by heat.
Once the water is completely vaporized, at about 150 °C or more, the tissue becomes carbonized, leading to char formation with blackening of the surrounding tissue and smoking from the skin cells.
Beyond 300 °C, the tissue may melt.
Laser Impact Upon Tissue
The laser beam incident upon a tissue may undergo reflection or refraction, scattering, absorption or transmission, depending on the optical properties of the tissue and the laser characteristics. Absorption is the most important with respect to tissue effects, as the light energy is partly transformed into heat or vibration energy. The intensity of the light is reduced in proportion to factors such as atomic and molecular arrangement, laser wavelength, tissue thickness and temperature. The Beer-Lambert law expresses the relationship between thickness and concentration of the tissue and absorption of the laser energy.
Laser ablation is determined chiefly by how far it penetrates the tissue, and is inversely proportional to the absorption coefficient. In the infrared region of the spectrum water molecules are responsible for most of the absorption, but in the visible and ultraviolet regions proteins and pigments are chiefly involved. Absorption in turn leads to photochemical and photothermal effects on the tissue, either singly or in combination.
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This is the most commonly used mechanism and depends upon heating of the tissue, leading to coagulation, vaporization or cutting. The absorption of photons by the tissue causes biological effects by increasing molecular movement in rotational, vibrational and transitional modes, which basically shakes the biomolecules (proteins, nucleic acid, membranes and membrane structures) until they unwind and lose their functional structure, becoming denatured at about 40-100 °C. The maximum movement occurs when the photon frequency of the incident laser is in proximity to the characteristic frequencies of these kinetic modes, called resonance absorption. This heats up the tissue, and heat is diffused through the surrounding tissue as well depending on its thermal coefficient, the difference in temperature between the irradiated and surrounding tissue, and the tissue characteristics. The heating can lead to any of the thermal effects. To avoid peripheral damage, the duration and peak temperature achieved must be carefully adjusted. The thermal relaxation time is a critical criterion in this. Only above this duration is it possible for adjacent tissue to be damaged.
This group of effects is due to direct electronic bond excitation by absorption of laser energy. Most of these molecules are bonded with energy corresponding to ultraviolet radiation. With laser of similar wavelengths, the tissue components are excited causing molecular bonds to rupture. The temperature is largely unchanged. Instead, tissue effects are due to biochemical interactions with the electromagnetic field or molecular bond rupture by photons of high energy (photodecomposition) occurring with long low-power laser exposure.
Photodynamic Therapy (PDT)
This is a type of photochemical reaction in which chromospheres are introduced from outside, to absorb low energy lasers and become photosensitizers. They transfer this energy downstream to another molecule. This may become a toxic molecule such as an oxygen free radical, that can destroy the DNA of the tissue and cause cell death. PDT is used to destroy some tumor types. Another mechanism is activating some cell components which then induce specific reactions or even derail the entire cell metabolism.
First propounded by Srinivasan and Mayne-Banton (1982), photoablation is a technique of ablative photodecomposition, where high energy laser decomposes tissue by breaking down the molecular bonds, causing volume stress (a mechanical force) and producing controlled tissue removal without coagulation or necrosis, with clean edges. It requires an intensity of 104-1010 W cm-2 for 10-3-10-10 seconds. Ultraviolet excimer laser is most commonly used for this application.
Over a certain power density, ultrashort pulses of laser at picosecond range may release multiple photons that lead to ionization of atoms and molecules causing optical breakdown, which results in shock waves and transformation of matter to plasma. Jet formation and cavitation may also occur within soft tissue or tissues with a large fluid content. The final effect is controlled and clear-cut tissue ablation sparing adjacent tissue, provided the laser parameters are well adjusted. The most critical of these is the local electric field, which must exceed a threshold to cause optical breakdown even in media which lack pigmented laser-absorbing cells. If the laser is used for nanoseconds even, shock waves are always formed which typically lead to inevitable damage to adjacent tissue. The duration is therefore kept to pico- or femtosecond pulses. This reduces plasma energy and unwanted tissue disruption.
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