Extended theory of selective photothermolysis.

BACKGROUND AND OBJECTIVE: We present a new theory of selective thermal damage of non-uniformly pigmented structures in biological tissues. Spatial separation of the heavily pigmented areas and the target requires limitation of the pigment temperature and heat diffusion from the pigmented to the targeted areas. STUDY DESIGNS/MATERIALS AND METHODS: A concept of selective target damage by heat diffusion is presented for three target geometries: planar, cylindrical, and spherical. An in vitro experiment is described in which the dependence of thermal damage on pulsewidth at constant fluence was evaluated. RESULTS: The in vitro experiment showed that the size of the damage zone for similar hair follicles was pulsewidth-independent over a very broad range of pulsewidths (30-400 ms). We formulated a new theory (extended theory of photothermolysis) to interpret the experimental results. CONCLUSIONS: Based on this new theory, the treatment pulsewidth for non-uniformly pigmented targets is significantly longer than the target thermal relaxation time (TRT). The theory provides new recommendations for photoepilation and photosclerotherapy parameters.

Evaluation of cooling methods for laser dermatology.

BACKGROUND AND OBJECTIVE: Skin cooling is used to protect the epidermis in a variety of laser dermatology procedures, including leg vein treatment, hair removal, and port wine stain removal. Spray and contact cooling are the two most popular methods, but similarities and differences of these techniques are not well understood. STUDY DESIGN/MATERIALS AND METHODS: A theoretical model of skin cooling is presented for two different regimens: "soft" cooling in which freezing of the skin is not permitted and "hard" cooling in which the skin can be frozen to a given depth. Spray and contact cooling were also compared experimentally using an in vitro model. RESULTS: For a fixed skin surface temperature, spray and contact cooling theoretically produce the same cooling profile in the skin. Anatomic depth of cooling depends on the time for which either the spray or contact is applied. In vitro experiments caused temperature at the simulated basal layer to be between -5 and +5 degrees C for both spray (tetrafluoroethane, boiling point -26 degrees C) and contact (-27 degrees C sapphire plate) cooling. The theoretical precooling analysis shows hard mode to be faster and more selective than soft mode; however, cooling time for hard mode must be carefully controlled to prevent irreversible epidermal damage caused by freezing. CONCLUSIONS: Both spray and contact cooling provide efficient skin cooling. The choice of cooling method depends on other factors such as the target depth, cost, safety, and ergonomic factors.

Histological comparison of corneal ablation with Er:YAG laser, Nd:YAG optical parametric oscillator, and excimer laser.

PURPOSE: To use histological techniques to assess and compare the ablation depth, local damage, and surface quality of corneal ablations by a Q-switched Er:YAG laser, an optical parametric oscillator laser at 2.94 microm, a long pulse Er:YAG laser, and a 193-nm excimer laser. METHODS: Human cadaver eyes and in vivo cat eyes were treated with a 6.0-mm diameter, 30-microm-deep phototherapeutic keratectomy ablation and a 6.0-mm diameter, -5.00-D photorefractive keratectomy ablation. Human cadaver eyes were also treated with a 5.0-mm diameter, -5.00-D laser in situ keratomileusis (LASIK) ablation. Fluences and pulse widths used were 200 mJ/cm2 and 70 ns for the Q-switched Er:YAG, 150 mJ/cm2 and 7 ns for the optical parametric oscillator laser (OPO), 500 mJ/cm2 and 50 microseconds for the long pulse Er:YAG, and 160 mj/cm2 and 20 ns for the excimer laser. In the ablation rate study, 12 porcine eyes were ablated by the OPO laser with a range of layers and at different fluences ranging from 60 to 150 mJ/cm2, all using a 1.5-mm spot on the eye. The ablation depth of these acute ablations was evaluated by light microscopy examination. RESULTS: In the acute damage study, light microscopy showed a thin surface layer in all samples with minimal thermal damage except on the long pulse Er:YAG corneas. Transmission electron microscopy revealed less than 0.3-microm surface damage for all specimens of both the optical parametric oscillator and the excimer laser samples with no evidence of collagen shrinkage. Transmission electron microscopy showed damage layers of 0.5 to 3 microm for Q-switched Er:YAG and 3 to 10 microm for long pulse Er:YAG. Scanning electron microscopy showed smooth surfaces in all eyes, although the excimer was the roughest. In the porcine eye study, ablations were produced in both PTK and PRK modes with the ablation rate per layer increasing with the fluence. At 120 mJ/cm2, the average ablation rate was 1.9 microm per layer. CONCLUSIONS: The histology from the short pulse mid-infrared optical parametric oscillator laser at 2.94 microm was comparable to the 193-nm excimer with a smooth, damage-free, ablation zone when performing PRK and LASIK.

Healing after photorefractive keratectomy in cat eyes with a scanning mid-infrared Nd:YAG pumped optical parametric oscillator laser.

PURPOSE: To evaluate the healing characteristics of cat corneas treated with a new scanning mid-infrared laser system. METHODS: Six adult cats were treated with 6-mm diameter photorefractive keratectomy (PRK) corrections. One eye in each animal was untreated as a control and the other was treated with either a -3.00 or -6.00 diopter ablation. The laser was a new Nd:YAG pumped optical parametric oscillator laser at 2.94 microm with a new scanning delivery system. The pulse width was 7 nanoseconds, the repetition rate was 10 Hz, the size of the laser spot on the eye was 1.0 mm, and the fluence was 150 mJ/cm2. Healing of the cat corneas was followed for 4 months. Slit-lamp and corneal topography evaluations were done at each follow-up examination. Histology was performed at the end of the study. RESULTS: The corneal epithelium healed within 1 week. There was no stromal haze in any eye after the epithelium healed. After the first 2 weeks, slit-lamp examination could not identify which eye was treated. Corneal topography showed corneal flattening. Light microscopy at 4 months revealed normal epithelium and increased keratocyte density in the anterior third of the cornea. Electron microscopy showed discontinuities in the basement membrane and hemidesmosomes. The deep stroma and endothelium were normal. CONCLUSIONS: Cat corneas treated with the new optical parametric oscillator laser healed normally with no adverse effects. Increased keratocyte activity in the anterior stroma was the only noticeable response besides the flattening shown by topography.

Contact cooling of the skin.

Skin precooling can be used to reduce epidermal thermal damage in laser procedures (such as hair removal) where the target structures are located up to several millimetres below the skin surface. We have developed and experimentally verified a computational model that describes contact precooling of a multilayered skin structure prior to laser irradiation. The skin surface is assumed to be brought into thermal contact with a cold plate made of material with a high thermal conductivity. The approximate analytical solution for the skin temperature is obtained by considering the plate as a local heat sink. The time evolution of temperature (in both the skin and the plate) is simulated numerically to yield the optimal cooling parameters. To experimentally verify the numerical results of the model, we performed direct measurements of skin temperature for contact cooling with a sapphire plate held at several different temperatures in the range +10 to -30 degrees C.

High-energy, high-efficiency harmonic generation from a Cr:LiSrAlF(6) laser system.

We have developed a near-diffraction-limited Q-switched Cr:LiSrAlF(6) (Cr:LiSAF) laser system capable of producing 860-nm, 400-mJ pulses at a repetition rate of 2 Hz. Our harmonic-generation experiments with beta-BaB(2)O(4) nonlinear crystals have yielded 200 mJ of energy at 430 nm and 40 mJ at 215 nm. To the best of our knowledge, the 50% second-harmonic conversion efficiency and UV energy demonstrated in this research are the highest yet obtained from a Cr:LiSAF laser.

Tunable optical parametric oscillators pumped by Ti:sapphire lasers.

We report on the application of tunable, pulsed Ti:sapphire lasers as efficient pump sources for optical parametric oscillators. Using a 90-deg phase-matched KTP crystal, we have generated output from 1030 to 1280 nm (signal) and 2180 to 3030 nm (idler) by tuning the pump laser from 700 to 900 nm, with 49 mJ of combined signal and idler output energy at a pump level of 110 mJ. In addition, we have demonstrated degenerate optical parametric oscillator operation with a KNbO(3) crystal over the pump range of 720-818 nm and observed a 44% conversion efficiency.