Selective photothermolysis

The aforementioned laser parameters—power density, fluence, and wavelength—are the fundamental principles in the operation of medical lasers in the concept known as SP. Anderson and Parrish described SP in 1983, when they outlined the essential factors necessary for discrete laser-induced tissue damage to occur. SP is a method for localizing tissue damage to specific chromophore targets at the cellular level; therefore, it can be used to minimize undesired thermal damage to the surrounding tissue caused by thermal diffusion.

The rate of thermal diffusion of a given tissue is known as the thermal relaxation time (T_R) and is defined as the time required for a given heated tissue to lose 50% of its heat through diffusion. It is measured in terms of the area affected and the thermal diffusivity (D) of the target tissue, as follows:

T_R = r²/4 D, where r is the radius of target tissue.

Therefore, significant thermal diffusion (and hence thermal damage) is minimized if the duration of the laser pulse is shorter than the T_R of the target tissue.

For example, water (the primary constituent by weight of living cells) has a high absorption coefficient of 230 cm^-1 at 10,600 nm, the wavelength emission of a CO₂ laser, and a T_R of 326 µs. With these properties, if a CO₂ laser contacts the skin for less than 326 µs, most of the radiation is absorbed by the water in the targeted skin, with almost no thermal diffusion. However, if the duration of the laser impingement on the tissue is longer than 326 µs, heat is transmitted to the surrounding nontargeted tissue and results in undesirable thermal injury.

Therefore, for proper SP to occur, the target tissue (through its chromophores) must possess greater optical absorption than the nontargeted surrounding tissue does, and the laser of choice must have a pulse duration shorter than the T_R of the target tissue. Because soft tissues in humans generally have a T_R of less than 1 ms, the laser pulse must be extremely short and high-powered to be medically beneficial and minimally destructive. Because many types of lasers exist, selection is crucial and must be tailored to the specific procedure.

Дата добавления: 2015-07-25