Selective Photothermolysis the Enabling Principle for Cosmetic Laser Surgery

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The era of cosmetic laser treatment of the skin began in the 1980s with the work of Harvard University dermatologists Rox Anderson and John Parrish, who used laser energy to selectively damage dermal blood vessels. They considered the physical properties of small blood vessels, including their depth, diameter, laser energy absorption of their chromophore (hemoglobin), and thermal relaxation time, a measure of how quickly a structure cools down after being heated to a certain temperature. They theorized that a laser with a high energy level (fluence), a short pulse duration,

Hemoglobin Laser Absorption

Fig. 4.2 The electromagnetic spectrum includes all wavelengths of electromagnetic radiation. Shorter wavelengths have higher energy. Visible light wavelengths range from about 400 nanometers (nm) to 700 nm. Ultraviolet "light" is invisible electromagnetic radiation with higher energy than violet light and wavelengths as short as 10 nm. Infrared radiation has lower energy than red light and wavelengths up to 1 millimeter (1 mm = 1 million nm). Radio waves have wavelengths greater than 1 meter (1m = 1 billion nm).

Fig. 4.2 The electromagnetic spectrum includes all wavelengths of electromagnetic radiation. Shorter wavelengths have higher energy. Visible light wavelengths range from about 400 nanometers (nm) to 700 nm. Ultraviolet "light" is invisible electromagnetic radiation with higher energy than violet light and wavelengths as short as 10 nm. Infrared radiation has lower energy than red light and wavelengths up to 1 millimeter (1 mm = 1 million nm). Radio waves have wavelengths greater than 1 meter (1m = 1 billion nm).

and a wavelength that was highly absorbed by hemoglobin (relative to other skin chromophores) could be used. Pulse duration had to be shorter than thermal relaxation time so that heat would not build up excessively within the blood vessel and then be conducted to surrounding dermal tissue, causing a burn injury (with resultant scarring).

A good analogy of such limited thermal effect is a very brief contact of a finger with a hot pan on top of a kitchen stove. The pan could cause a severe burn if the skin were in contact with it for more than a split second. If the finger is immediately pulled away, insufficient heat energy is absorbed by the skin to cause a burn because the period of contact is so short.

Anderson and Parrish coined the term "selective photothermo-lysis" for their theory. A properly designed laser could cause lysis (damage or destruction) of a selective target through heat (thermal energy) generated by light (photo) from the laser. "Selective" is the key term: only the desired target should be affected. Only a laser could provide sufficient energy at a precise wavelength to enable selective photothermolysis.

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