Boris Majaron

Numerical optimization of sequential cryogen spray cooling and laser irradiation for improved therapy of port wine stain

Despite application of cryogen spray (CS) precooling, customary treatment of port wine stain (PWS) birthmarks with a single laser pulse does not result in complete lesion blanching for a majority of patients. One obvious reason is nonselective absorption by epidermal melanin, which limits the maximal safe radiant exposure. Another possible reason for treatment failure is screening of laser light within large PWS vessels, which prevents uniform heating of the entire vessel lumen. Our aim is to identify the parameters of sequential CS cooling and laser irradiation that will allow optimal photocoagulation of various PWS blood vessels with minimal risk of epidermal thermal damage.

Thermomechanical laser ablation of hard dental tissues: an overview of effects, regimes, and models

Derivation and predictions of the basic analytical model of thermo-mechanical laser ablation, treating the limit case of constant laser intensity, 1D geometry, negligible heat diffusion and no debris screening, is summarized first as a reference point for further discussion. Each of the above requirements is then omitted in turn, to analyze additional effects observed under various experimental conditions. Scattering and absorption of the laser radiation in ejected debris is treated using a model that allows the debris extinction coefficient to vary dynamically during the ablation process, resulting in influence of pulse duration on the fluence dependence of predicted ablation crater depths. Next, the influence of heat diffusion on ablation efficiency and amount of thermal side effects is analyzed in a semi-quantitative way, leading to rule-of-thumb formulas that predict the ablation regime for a general ablation process from laser pulse fluence and duration as well as optical and thermal properties of the treated tissue. Influence of the laser beam profile on ablation crater depth and shape is demonstrated and discussed for the case of Gaussian beam profile. In the end, fiber-tip contact ablation in the presence of water spray is discussed as a counter-example of experimentally observed effects that - to our best knowledge - are still beyond the reach of quantitative understanding.

Quantitative investigation of thermal damage in Er:YAG laser skin resurfacing

Feasibility of deep coagulation of skin with superficially absorbed Er:YAG lasers is investigated using a numerical model. Unlike most previous models, the skin is treated as a two-component material: water trapped in spherical cavities inside an infinite elastic medium. In describing the interaction of mid-IR laser light with skin, thermodynamic behavior of pressurized hot water and steam is combined with elastic response of the surrounding medium. A one-dimensional treatment of heat diffusion and the Arrhenius model of the protein denaturation process are also included in the model. Temperature evolution, profile and coagulation depth are analyzed as a function of the pulse duration, number of applied pulses and repetition frequency of the sequence. The results indicate that the depth of coagulated layer, which amounts to 15 - 40 micrometer with a single free-generated Er:YAG laser pulse, can be extended up to 200 micrometer with no surface ablation by using a repetitive pulse sequence of suitable single-pulse fluence, repetition frequency and duration.

Influence of water spray on Er:YAG ablation of hard dental tissues

Influence of water-spray cooling on the efficiency of erbium laser ablation of hard dental tissues is investigated experimentally. Consecutive pulses from a dental Er:YAG laser system are applied to dentin and enamel surface in vitro. Depths and diameters of the resulting craters, obtained with and without the use of a pressurized air/water spray, are compared at single-pulse fluences from 10 to 140 J/cm2. Similar measurements are performed also with direct drilling of enamel with a fiber tip. The results show that direct interaction of the cooling water with the laser radiation increases the ablation efficiency for enamel, especially in fiber-tip drilling.

Temperature and gain dynamics in flashlamp-pumped Er:YAG

A novel optical transmission method was used for measuring transient temperature dynamics inside an Er:YAG laser crystal during optical pumping with flashlamp pulses of different duration and spectral composition. Time-resolved measurements of the crystal gain at 2.94 /spl mu/m were also performed for the same set of pumping conditions. The results are compared with predictions of a theoretical model for gain and temperature dynamics during optical pumping of Er:YAG. A good spatial resolution of the implemented temperature measurement method was utilized also to determine the lateral temperature profiles inside the laser rod under various pumping conditions.

Population dynamics in Yb:Er:phosphate glass under neodymium laser pumping

Characteristics of ytterbium to erbium energy transfer in Yb:Er:phosphate glasses have been studied for a case when Nd phosphate glass laser is used as a pumping source. A theoretical model of the energy transfer was developed to reproduce the experimentally observed population dynamics of the Er/sup 3+/ upper and lower laser levels. The results show that saturation and frequency hole burning of the inhomogeneously broadened Yb/sup 3+/ absorption line are of primary importance in determining the dynamics and efficiency of the Yb/sup 3+/-Er/sup 3+/ energy transfer under Nd laser pumping. The influence of other loss channels that have been identified in previous studies is found to be relatively small. The model also yields the lifetime of the frequency hole, as well as the effective rates of energy transfer between groups of Yb/sup 3+/ and Er/sup 3+/ ions in phosphate glasses. >

Fiber-tip drilling of hard dental tissues with Er:YAG laser

Specifics of fiber-tip ablation of hard dental tissues with mid-infrared laser radiation are investigated in vitro. Sequences of free-generated Er:YAG laser pulses are applied to fresh human dentin and enamel slices at a low repetition rate and the resulting craters inspected and measured by optical microscopy. Influence of the laser pulse fluence (up to 55 J/cm2), number of pulses in the sequence (1 to 20), and the gap between the fiber tip and the tissue surface (0 to 1.0 mm), is determined quantitatively. Additionally, the effect of the optical quality of the fiber tip on the ablation efficiency is assessed qualitatively. The results help us identify the optimum working regime for the dental fiber-tip handpieces in both tissues. Additionally, they provide new clues for understanding the process of mid-infrared ablation of hard biological tissues, especially in presence of the water spray, which interacts directly with the laser radiation.

Heat diffusion and ablation front dynamics in Er:YAG laser skin resurfacing

Influence of pulse energy, duration and beam cross-section on the outcome of Er:YAG laser ablation of skin is interpreted on the basis of an analytical model of heat diffusion and ablation front dynamics. Derived expressions enable us to identify different ablation regimes in terms of ablation efficiency and depth of thermally affected tissue layer for any thermally driven laser ablation process. Influence of laser wavelength is also discussed, focusing on a comparison between Er:YAG and carbon-dioxide laser skin resurfacing. Preliminary experimental and clinical evidence in agreement with the model is also presented.

Influence of hole burning on laser pumping dynamics and efficiency in Yb:Er:phosphate glasses

Measured population dynamics of erbium metastable level in a Yb:Er:phosphate glass following the excitation with a Nd:glass laser is reproduced by a theoretical model based on rate equations. To our knowledge, this is the first model to include frequency hole burning of inhomogeneously broadened pumping transition of ytterbium ion. The model explains also the previously observed dependence of pumping efficiency on Nd:glass laser pulse length.

Monte Carlo simulation of radiation transfer in human skin with geometrically correct treatment of boundaries between different tissues

In customary implementation of three-dimensional (3D) Monte Carlo (MC) numerical model of light transport in heterogeneous biological structures, the volume of interest is divided into voxels by a rectangular spatial grid. Each voxel is assumed to have homogeneous optical properties and curved boundaries between neighboring tissues inevitably become serrated. This raises some concerns over realism of the modeling results, especially with regard to reflection and refraction on such boundaries. In order to investigate the above concern, we have implemented an augmented 3D MC code, where tissue boundaries (e.g., blood vessel walls) are defined by analytical functions and thus maintain their shape regardless of grid discretization. Results of the customary and augmented model are compared for a few characteristic test geometries, mimicking a cutaneous blood vessel irradiated with a 532 nm laser beam of finite diameter. Our analysis shows that at specific locations inside the vessel, the amount of deposited laser energy can vary between the two models by up to 10%. Even physically relevant integral quantities, such as linear density of the energy absorbed by the vessel, can differ by as much as 30%. Moreover, the values obtained with the customary model vary strongly with discretization step and don’t disappear with ever finer discretization. Meanwhile, our augmented model shows no such behavior, indicating that the customary approach suffers from inherent inaccuracies arising from physically flawed treatment of tissue boundaries.