Energy deposition profile in human skin upon irradiation with a 1,342 nm Nd:YAP laser

Objective monitoring of laser tattoo removal in human volunteers using an innovative optical technique: A proof of principle

OBJECTIVES

Assess the suitability of the technique for objective monitoring of laser tattoo removal by an extended treatment protocol.

MATERIALS AND METHODS

One half of the tattoo in the first volunteer was treated with nanosecond and the other half with picosecond laser pulses at 1064 nm. In the second subject, four test areas were treated repeatedly using different radiant exposures from 1.5 to 6 J/cm2 . Measurements of diffuse reflectance spectra and photothermal radiometric transients were performed 4-20 weeks after each treatment session. Inverse Monte Carlo analysis based on a three-layer model of tattooed skin was applied to assess the tattoo characteristics and analyze their changes.

RESULTS

The results clearly indicate a gradual reduction of the ink content and an increase of the subsurface depth of the tattoo layer with all treatments at a radiant exposure of 3 J/cm2 or higher. The observed dependences on laser pulse duration, radiant exposure, and a number of treatments are in excellent agreement with visual fading of the tattoo.

CONCLUSIONS

The presented methodology enables noninvasive characterization of tattoos in human skin and objective monitoring of the laser removal treatment.

Pulsed Photothermal Radiometric Depth Profiling of Bruises by 532 nm and 1064 nm Lasers

Optical techniques are often inadequate in estimating bruise age since they are not sensitive to the depth of chromophores at the location of the bruise. To address this shortcoming, we used pulsed photothermal radiometry (PPTR) for depth profiling of bruises with two wavelengths, 532 nm (KTP laser) and 1064 nm (Nd:YAG laser). Six volunteers with eight bruises of exactly known and documented times of injury were enrolled in the study. A homogeneous part of the bruise was irradiated first with a 5 ms pulse at 532 nm and then with a 5 ms pulse at 1064 nm. The resulting transient surface temperature change was collected with a fast IR camera. The initial temperature-depth profiles were reconstructed by solving the ill-posed inverse problem using a custom reconstruction algorithm. The PPTR signals and reconstructed initial temperature profiles showed that the 532 nm wavelength probed the shallow skin layers revealing moderate changes during bruise development, while the 1064 nm wavelength provided additional information for severe bruises, in which swelling was present. Our two-wavelength approach has the potential for an improved estimation of the bruise age, especially if combined with modeling of bruise dynamics.

Laser stimulation of the skin for quantitative study of decision-making and motivation

Neuroeconomics studies how decision-making is guided by the value of rewards and punishments. But to date, little is known about how noxious experiences impact decisions. A challenge is the lack of an aversive stimulus that is dynamically adjustable in intensity and location, readily usable over many trials in a single experimental session, and compatible with multiple ways to measure neuronal activity. We show that skin laser stimulation used in human studies of aversion can be used for this purpose in several key animal models. We then use laser stimulation to study how neurons in the orbitofrontal cortex (OFC), an area whose many roles include guiding decisions among different rewards, encode the value of rewards and punishments. We show that some OFC neurons integrated the positive value of rewards with the negative value of aversive laser stimulation, suggesting that the OFC can play a role in more complex choices than previously appreciated.

High-performance diode-end-pumped Nd:YLF laser operating at 1314 nm

A stable, efficient, and powerful 1314 nm Nd:YLF laser inband-pumped by a wavelength-locked narrowband 880 nm laser diode is demonstrated. The influence of mode-to-pump ratio on the performance of the diode-end-pumped Nd:YLF laser has been systematically investigated by taking into account the thermal effect and the energy transfer upconversion effect. For the optimum mode-to-pump ratio of 0.84, the maximum continuous wave output power of 21.9 W was extracted under the pump power of 70 W, which corresponded to the optical power efficiency of 31.3% and the beam quality of M² ≈ 1.6. The resultant output power stability was determined to be 0.059% (RMS) within 1 h. In addition, by increasing the mode-to-pump ratio to 1.0, the near-diffraction-limited beam (M² ≈ 1.3) was achieved with the output power of 17.0 W and the optical power efficiency of 24.3%.

11-watt single-frequency 1342-nm laser based on multi-segmented Nd:YVO4 crystal

High power continuous-wave (CW) single-frequency 1342 nm lasers are of interest for fundamental research, particularly, for laser cooling of lithium atoms. Using the popular Nd:YVO₄ laser crystal requires careful heat management, because strong thermal effects in the gain medium are the most severe limitations of output power. Here, we present a multi-segmented Nd:YVO₄ crystal design that consists of three segments with successive doping concentrations, optimized using a theoretical model. In order to quantify the optimization, we measured the thermal lens power of conventional crystal designs and compare them to our multi-segmented design. The optimized design displays a two times lower thermal lens dioptric power for the same amount of absorbed pump power in the non-lasing case. Using the optimized design, we demonstrate a high power all-solid-state laser emitting 10.0 W single-frequency radiation at 1342 nm when operating the laser crystal at room temperature. Further integration of the laser allows us to operate the laser crystal below room temperature for improving output power up to 11.4 W at 8°C. This is explained by the reduction of energy-transfer upconversion and excited-state absorption effects. Stable free-running operation at the low temperature of 8 °C is achieved with the power stability of ± 0.42 % by peak-to-peak fluctuation and frequency peak-to-peak fluctuation of ± 72 MHz in three hours.

Temperature Depth Profiles Induced in Human Skin In Vivo Using Pulsed 975 nm Irradiation

BACKGROUND AND OBJECTIVES

The aim of this study was to determine the temperature depth profiles induced in human skin in vivo by using a pulsed 975 nm diode laser (with 5 ms pulse duration) and compare them with those induced by the more common 532 nm (KTP) and 1,064 nm (Nd:YAG) lasers. Quantitative assessment of the energy deposition characteristics in human skin at 975 nm should help design of safe and effective treatment protocols when using such lasers.

STUDY DESIGN/MATERIALS AND METHODS

Temperature depth profiles induced in the human skin by the three lasers were determined using pulsed photothermal radiometry (PPTR). This technique involves time-resolved measurement of mid-infrared emission from the irradiated test site and reconstruction of the laser-induced temperature profiles using an earlier developed optimization algorithm. Measurements were performed on volar sides of the forearms in seven volunteers with healthy skin. At irradiation spot diameters of 3-4 mm, the radiant exposures were 0.24, 0.36, and 5.7 J/cm² for the 975, 532, and 1,064 nm lasers, respectively.

RESULTS

Upon normalization to the same radiant exposure of 1 J/cm ² , the assessed maximum temperature rise in the epidermis averaged 0.8 °C for the 975 nm laser, 7.4 °C for the 532 nm, and 0.6 °C for the 1,064 nm laser. The characteristic subsurface depth to which 50% of the absorbed laser energy was deposited was on average 0.31 mm at 975 nm irradiation, and slightly deeper at 1,064 nm, and 0.15 mm at 532 nm. The experimentally obtained relations were reproduced in a dedicated numerical simulation.

CONCLUSIONS

The assessed energy deposition characteristics show that the pulsed 975 nm diode laser is very suitable for controlled heating of the upper dermis as required, for example, for nonablative skin rejuvenation. The risks of nonselective overheating of the epidermis and subcutis are significantly reduced in comparison with irradiation at 532 and 1,064 nm, respectively. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Retinal damage thresholds from 100-millisecond laser radiation exposure at 1319 nm: a comparative study for rabbits with different ocular axial lengths

With the widespread use of high-power laser systems in the wavelength spectrum between 1300 and 1400 nm, the risk of ocular damage becomes more serious and concerning. Existing ocular bio-effects studies have revealed unique damage characteristics, the damage mechanisms involved, and the trends of damage thresholds in this wavelength range. However, the influence of ocular axial length on retinal damage thresholds has not been investigated quantitatively. In this paper, using a 1319 nm continuous-wave laser, the in-vivo retinal damage thresholds were determined for two groups of chinchilla grey rabbits with the ocular axial lengths of 15.97 and 17.25 mm, respectively. The incident corneal irradiance diameter was fixed at 5 mm and the exposure duration was 0.1 s. The determined ED₅₀ values at 24-h post-exposure for the axial lengths of 15.97 and 17.25 mm were 1.06 and 1.79 J, respectively. Detailed analysis revealed that a sufficient margin existed between the damage threshold and MPE for adult humans, but for the newborn eyes, the safety factor may be less than 2.3.

Physiological and structural characterization of human skin in vivo using combined photothermal radiometry and diffuse reflectance spectroscopy

In this proof-of-concept study we combine two optical techniques to enable assessment of structure and composition of human skin in vivo: Pulsed photothermal radiometry (PPTR), which involves measurements of transient dynamics in mid-infrared emission from sample surface after exposure to a light pulse, and diffuse reflectance spectroscopy (DRS) in visible part of the spectrum. The analysis involves simultaneous fitting of measured PPTR signals and DRS with corresponding predictions of a Monte Carlo model of light-tissue interaction. By using a four-layer optical model of skin we obtain a good match between the experimental and model data when scattering properties of the epidermis and dermis are also optimized on an individual basis. The assessed parameter values correlate well with literature data and demonstrate the expected trends in controlled tests involving temporary obstruction of peripheral blood circulation using a pressure cuff, and acute as well as seasonal sun tanning.

Model development and experimental validation for analyzing initial transients of irradiation of tissues during thermal therapy using short pulse lasers

BACKGROUND AND OBJECTIVES

Short pulse lasers with pulse durations in the range of nanoseconds and shorter are effective in the targeted delivery of heat energy for precise tissue heating and ablation. This photothermal therapy is useful where the removal of cancerous tissue sections is required. The objective of this paper is to use finite element modeling to demonstrate the differences in the thermal response of skin tissue to short-pulse and continuous wave laser irradiation in the initial stages of the irradiation. Models have been developed to validate the temperature distribution and heat affected zone during laser irradiation of excised rat skin samples and live anesthetized mouse tissue.

STUDY DESIGN/MATERIALS AND METHODS

Excised rat skin samples and live anesthetized mice were subjected to Nd:YAG pulsed laser (1,064 nm, 500 ns) irradiation of varying powers. A thermal camera was used to measure the rise in surface temperature as a result of the laser irradiation. Histological analyses of the heat affected zone created in the tissue samples due to the temperature rise were performed. The thermal interaction of the laser with the tissue was quantified by measuring the thermal dose delivered by the laser. Finite element geometries of three-dimensional tissue sections for continuum and vascular models were developed using COMSOL Multiphysics. Blood flow was incorporated into the vascular model to mimic the presence of discrete blood vessels and contrasted with the continuum model without blood perfusion.

RESULTS

The temperature rises predicted by the continuum and the vascular models agreed with the temperature rises observed at the surface of the excised rat tissue samples and live anesthetized mice due to laser irradiation respectively. The vascular model developed was able to predict the cooling produced by the blood vessels in the region where the vessels were present. The temperature rise in the continuum model due to pulsed laser irradiation was higher than that due to continuous wave (CW) laser irradiation in the initial stages of the irradiation. The temperature rise due to pulsed and CW laser irradiation converged as the time of irradiation increased. A similar trend was observed when comparing the thermal dose for pulsed and CW laser irradiation in the vascular model.

CONCLUSION

Finite element models (continuum and vascular) were developed that can be used to predict temperature rise and quantify the thermal dose resulting from laser irradiation of excised rat skin samples and live anesthetized mouse tissue. The vascular model incorporating blood perfusion effects predicted temperature rise better in the live animal tissue. The models developed demonstrated that pulsed lasers caused greater temperature rise and delivered a greater thermal dose than CW lasers of equal average power, especially during the initial transients of irradiation. This analysis will be beneficial for thermal therapy applications where maximum delivery of thermal dose over a short period of time is important.

Optical-thermal light-tissue interactions during photoacoustic breast imaging

Light-tissue interactions during photoacoustic imaging, including dynamic heat transfer processes in and around vascular structures, are not well established. A three-dimensional, transient, optical-thermal computational model was used to simulate energy deposition, temperature distributions and thermal damage in breast tissue during exposure to pulsed laser trains at 800 and 1064 nm. Rapid and repetitive temperature increases and thermal relaxation led to superpositioning effects that were highly dependent on vessel diameter and depth. For a ten second exposure at established safety limits, the maximum single-pulse and total temperature rise levels were 0.2°C and 5.8°C, respectively. No significant thermal damage was predicted. The impact of tissue optical properties, surface boundary condition and irradiation wavelength on peak temperature location and temperature evolution with time are discussed.