Thermal kinetic selectivity and lasers

The concept of selective photothermolysis, introduced by Anderson and Parrish, revolutionized the use of lasers in dermatology by enabling targeted thermal damage to tissues based on wavelength, pulse width, and thermal relaxation time (TRT). However, the principle of thermal kinetic selectivity, which involves heat diffusion and selective destruction of non-homogeneous tissues, has received limited attention in literature. This paper explores thermal kinetic selectivity as a key factor in laser-mediated tissue destruction, especially in cases where the target tissue and chromophore are spatially separated or have varying sizes and properties. Selective photothermolysis functions through confined heat generation, where the pulse duration is shorter than the TRT of the target tissue, preventing damage to surrounding structures. However, in non-homogeneous targets, heat diffusion becomes a critical factor in tissue destruction. The extended theory of selective photothermolysis emphasized this heat diffusion through the concept of thermal damage time (TDT). TDT varies with the geometry and dimensionality of the target, with planar structures dissipating heat more efficiently than cylindrical or spherical ones. The theory of thermal kinetic selectivity is particularly useful when treating tissues with different TRTs, such as larger blood vessels and smaller capillaries, by adjusting the pulse duration to selectively damage the desired target without affecting the surrounding tissues. This concept also applies to non-ablative treatments, such as skin rejuvenation and body contouring, where homogenous heating of the dermis is achieved through extended pulse durations. In conclusion, the integration of thermal kinetic selectivity enhances our understanding of laser-tissue interactions, in a range of dermatological and esthetic applications. In addition to the concept of TRT, light-absorption profiles, tissue geometry, heat conduction from the absorber to target tissues, and the light device output temporal profiles all have an effect on the thermolysis.