Thermal interactions of pulsed laser radiation and cryogen spray cooling with skin
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Laser applications in dermatology typically involve heating of a subsurface target, such as a blood vessel, hair follicle, or tumor. The goal of such procedures is to heat and destroy the target while minimizing collateral thermal injury. Unfortunately, it is difficult to achieve both goals due to competitive absorption of incident laser light by nontargeted regions of tissue. Previous studies have shown that use of a surface cooling agent in conjunction with pulsed laser radiation can significantly reduce the amount of thermal damage to overlying tissue. Experiments were conducted to 1) study dynamics of laser heating of skin, 2) evaluate infrared temperature measurements as a tool for monitoring tissue surface temperatures, 3) study effects of thermal damage on tissue properties, and 4) characterize cryogen spray cooling (CSC) dynamics. Radiometric temperatures were measured during pulsed CO2 laser ablation of in vivo rat skin. The time required for viii these temperatures to return to near baseline levels, which is critical for determining laser pulse repetition rate, was considerably longer than the theoretically-defined thermal relaxation time. An analytical model was derived to estimate the effective cooling time required for tissue temperatures to reduce to a specified fraction of the peak value. A numerical model was developed to study the potential discrepancy between radiometric and actual surface temperatures during laser heating and cryogen spray cooling of tissue. Superficial temperature gradients due to strong absorption of incident laser light and cryogen film formation led to potentially significant differences between radiometric and actual surface temperatures. Radiometric temperatures were subsequently measured during pulsed CO2 laser irradiation of gelatin tissue phantoms and used as experimental verification of the model. Knowledge of dynamic tissue properties is crucial for identifying the appropriate dosimetry in treatment plans. Infrared- and acoustic-based measurements were performed to identify changes in tissue optical and electrical properties. The significance of acoustic relaxation during optoacoustic-based measurements of optical properties was studied and a numerical deconvolution algorithm developed to overcome the effects of acoustic relaxation on optical property measurements. Pulsed laser ablation of soft tissue is commonly perceived as being mediated by water vaporization. Recent studies have suggested that by simultaneous targeting of protein and water absorption bands, ablation efficiency can be enhanced. ix Comparative skin ablation studies were conducted using a free electron laser, and it was determined that ablation rates are higher when protein absorption is targeted only at high radiant exposures. A dynamic numerical ablation model was developed to explain the underlying reasons for comparable ablation rates at lower radiant exposures. Imaging of the sprayed region during CSC was performed to identify dynamics of CSC as a function of relative humidity (RH). The results suggested that cryogen film/frost formation was affected by RH. Since the presence of the cryogen film impedes radiometric measurements of the skin surface, backside infrared imaging was used to ascertain lateral cooling gradients induced during CSC of a thin aluminum sheet. A gaussian-like temperature distribution was identified from the infrared images. The region over which cooling is uniform was considerably smaller than recommended laser spot sizes used in clinical treatment of port wine stains.