Selective photothermolysis (part 1)
SP relies on fundamental choices being made correctly – wavelength, pulse duration, fluence, exposure spot size, and use of skin cooling. First, a wavelength (or, with IPLs, a range of wavelengths) must be used that is preferentially absorbed by the intended ‘target’ structures such as hair follicles, microvessels, tattoo inks, or melanocytes. Thus far, all lasers utilizing SP operate in the visible and near-infrared (NIR) spectrum. Generally, in the visible light spectrum, a target chromophore is treated using wave-lengths of light of a complimentary color. For example, red tattoo ink absorbs green light and can be effectively treated with a frequency doubled Q-switched Nd : YAG laser operating at the green wavelength of 532 nm. Similarly, green tattoo ink is best removed with a red Q-switched laser, such as the ruby laser at 694 nm. Preferential absorption implies the avoidance of competing chromophores, not simply strong absorption in the intended target. For example, when treating dermal targets such as blood vessels it is important to minimize unwanted damage to the epidermis. Since every photon that reaches a blood vessel must first travel through the overlying epidermis, the best wavelengths for port-wine stain treatment are not simply those with strong absorption by blood. The proper wavelength(s) must also penetrate deeply enough to reach the intended targets. Across the visible and near-infrared spectrum from 400 to 1200 nm, longer wavelengths penetrate deeper into tissue. These reasons account for the use of yellow light pulsed dye lasers rather than the very strongly absorbed blue wavelengths for treating superficial vascular lesions. Long-pulsed dye lasers are the first example of a laser designed specifically for a medical application: treatment of port-wine stains in children . On the microscopic scale, microvessels are selectively heated and damaged, with minimal injury to the rest of the skin structures. However, for a hypertrophic or deep vascular lesion, such as many adult port-wine stains and venous malformations, much better efficacy is often obtained using the deeply penetrating 755 nm near-infrared alexandrite laser, and it is easy to observe that hemoglobin absorbs yellow light much more strongly than at 755 nm, a wave-length that is also well absorbed by melanin.) When alexandrite lasers are used for vascular lesion treatment, it is therefore imperative to use excellent skin cooling for epidermal protection.
