• #3-2. Laser in Medicine: Basic Laser Terminology


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    Thermal Relaxation Time(TRT)


    The thermal relaxation time (TRT) is defined as the time taken for the target tissue to cool down to 50% or less ofthe tissue temperature increased due to the photothermal reaction after laser irradiation while the temperature isdiffused.


    The TRT varies depending on the size and type of chromophores. For instance, the TRT is 1µs for the melanosome of 0.5 to 1µm size; and 300µs for the melanocyte of 10µm size. The TRT is 1ms for the blood vessel of 50µm size; 5ms for the blood vessel of 100µm size; and 20ms for the blood vessel of 200µm size. The concept of TRT is often used mainly when the selective photothermolysis is explained.


    Thermal Damage Time(TDT)


    The thermal damage time (TDT) is the time taken for the entire target tissue including the primary chromophore and surrounding target to cool down by 63% of the temperature increased after laser irradiation. The TDT is a concept used, more often than TRT, mainly in the field of hair removal. TDT of hair follicle is the time required for the laser energy absorbed into the hair to diffuse up to the follicle-associated hair stem cell.


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    Photothermal Effect


    Once a tissue is irradiated by the laser, the photon energy is converted into the heat. This heat results in the reversible/irreversible denaturation, coagulation, and vaporization of the tissue, which is called photothermal effect. The lasers using the photothermal effect include CO₂, Diode, Er:Glass, and Er:YAG Laser with water as the primary chromophore.


    These lasers are used during the removal of benign tumors such as spots, moles, liver spots, syringomas, milia, and soft fibromas; the face resurfacing and the photo-rejuvenation; and the treatment of telangiectasia, by bringing change to spot size, pulse duration, fluence, fractional mode, etc.


    Photochemical Effect


    A tissue irradiated by the laser with a high photon energy generates high heat, exhibiting a photothermal effect. However, a tissue irradiated by the laser with a low photon energy stimulates and then activates intra-tissue cells, denatures the molecular structure, or creates active oxygen and then destroys cells, instead of generating heat.


    This is mainly used for the low-level laser therapy and the photodynamic therapy. Low-level laser functions differently per wavelength. For instance, the wavelength of 430nm exerts a strong bactericidal action; and the wavelengths ranging from 600 to 900nm increase blood circulation, raise the speed of wound healing, decrease the inflammatory response, promote tissue regeneration, and decreases oedema and pain.


    The photodynamic therapy is a treatment that involves administeringor applying a photosensitizer to the human body and exposing the involved tissue or cell to the laser, in order to selectively destroy them. This is used to treat some forms of cancer and acne.


    - To be continued

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