Constant Beam Diameter Research Articles

Development of a Treatment Planning Framework for Laser Interstitial Thermal Therapy (LITT).

Develop a treatment planning framework for neurosurgeons treating high-grade gliomas with LITT to minimize the learning curve and improve tumor thermal dose coverage. Deidentified patient images were segmented using the image segmentation software Materialize MIMICS©. Segmented images were imported into the commercial finite element analysis (FEA) software COMSOL Multiphysics© to perform bioheat transfer simulations. The laser probe was modeled as a cylindrical object with radius 0.7 mm and length 100 mm, with a constant beam diameter. A modeled laser probe was placed in the tumor in accordance with patient specific patient magnetic resonance temperature imaging (MRTi) data. The laser energy was modeled as a deposited beam heat source in the FEA software. Penne's bioheat equation was used to model heat transfer in brain tissue. The cerebrospinal fluid (CSF) was modeled as a solid with convectively enhanced conductivity to capture heat sink effects. In this study, thermal damage-dependent blood perfusion was assessed. Pulsed laser heating was modeled based on patient treatment logs. The stationary heat source and pullback heat source techniques were modeled to compare the calculated tissue damage. The developed bioheat transfer model was compared to MRTi data obtained from a laser log during LITT procedures. The application builder module in COMSOL Multiphysics© was utilized to create a Graphical User Interface (GUI) for the treatment planning framework. Simulations predicted increased thermal damage (10-15%) in the tumor for the pullback heat source approach compared with the stationary heat source. The model-predicted temperature profiles followed trends similar to those of the MRTi data. Simulations predicted partial tissue ablation in tumors proximal to the CSF ventricle. A mobile platform-based GUI for bioheat transfer simulation was developed to aid neurosurgeons in conveniently varying the simulation parameters according to a patient-specific treatment plan. The convective effects of the CSF should be modeled with heat sink effects for accurate LITT treatment planning.

Thermal damage thresholds for multiple-pulse porcine skin laser exposures at 1070 nm

.As solid-state laser technology continues to mature, high-energy lasers operating in the near-infrared (NIR) band have seen increased utilization in manufacturing, medical, and military applications. Formulations of maximum permissible exposure limits establish guidelines for the safe use of these systems for a given set of laser parameters, based on past experimental and analytical studies of exposure thresholds causing injury to the skin and eyes. The purpose of our study is to characterize the skin response to multiple-pulsed laser exposures at the NIR wavelength of 1070 nm, at a constant beam diameter of 1 cm, using anesthetized Yucatan mini-pig subjects. Our study explores three constant total laser-on times of 0.01, 0.1, and 10 s as single- and multiple-pulse sequences. Exposures consisting of 10, 30, and 100 pulses have identical individual pulse durations but different duty cycles in order to include variable degrees of thermal additivity. A plurality of three observers quantifies skin damage with the minimally visible lesion metric, judged at the 1- and 24-h intervals postexposure. Calculation of the median effective dose () provides injury thresholds for all exposure conditions, based on varying laser power across subjects. The results of this study will provide a quantitative basis for the incorporation of multiple-pulsed laser exposure into standards and augment data contained in the existing database.

Study and Validation of Laser Forming Process Numerical Models

This paper presents a statistical analysis of Structural Steel to study the special effects of process constraints (Laser Power, Scan Speed, Spot Diameters and Heat flow diameter). These process parameters are taken as input changeable and twisting angle or deformation considered as the output variable. An Ansys 18.1 licensed software tool is used for modelling and optimization of the laser bending method. The performance of the developed model and results is matched and checked with reference papers. All the process parameters are investigated and set up considerable effect on the bending angle. The effect of the beam diameter, scan speed and heat flow energy on bending angle also compare with research papers. The bending angle enhanced with an increase in beam diameter at stable heat energy but decreased with increase in heat energy at constant beam diameter. The bending angle is also affected by overlapping and gap between beam diameters.

Monte Carlo simulations of nanoscale focused neon ion beam sputtering

A Monte Carlo simulation is developed to model the physical sputtering of aluminum and tungsten emulating nanoscale focused helium and neon ion beam etching from the gas field ion microscope. Neon beams with different beam energies (0.5–30 keV) and a constant beam diameter (Gaussian with full-width-at-half-maximum of 1 nm) were simulated to elucidate the nanostructure evolution during the physical sputtering of nanoscale high aspect ratio features. The aspect ratio and sputter yield vary with the ion species and beam energy for a constant beam diameter and are related to the distribution of the nuclear energy loss. Neon ions have a larger sputter yield than the helium ions due to their larger mass and consequently larger nuclear energy loss relative to helium. Quantitative information such as the sputtering yields, the energy-dependent aspect ratios and resolution-limiting effects are discussed.

Beam diameter dependence of surface damage threshold of fused silica fibers and preforms for nanosecond laser treatment at 1064 nm wavelength

Optical fibers made of fused silica are a common method of transmitting high laser pulse energies. Failure of those fibers is a significant risk. The determination of laser-induced damage thresholds (LIDT) on fiber end facets according to ISO 21254 standard is needed. In the past, single pulse nanosecond laser experiments showed an improvement of LIDT with increasing fiber core diameter for 1064nm wavelength and a constant beam diameter of 50μm.This paper pays particular attention to the influence of the laser beam diameter on damage resistance. All-silica fiber types (LEONI) with different core diameters (100–600μm) were investigated using beam diameters in a range from 30μm to 100μm. For comparison experiments on fused silica preform material (Heraeus F300) were performed. On one hand, surface LIDT of fused silica preform material decreases significantly with increasing beam size. A model considering a random distribution of point defects explains the experimental data qualitatively. On the other hand, LIDT of fiber end facets stays constant. White light microscopy results suggest that the point defect density on fiber end facets is lower compared to the preform surface due to an excellent surface polish quality.

Morphology of laser treated polycrystalline α-Al2O3

Applying metallic coating to ceramics via laser surface processing is an attractive process, due to the ability to reach relatively thick surface coatings, which include reaction products formed between the metal and ceramic. Two main effects must be characterized to implement such processes: damage to the ceramic due to thermal shock, and changes in the ceramic microstructure due to the process. This study focuses on the influence of laser surface processing on the microstructure of α-Al2O3. A CO2 laser with a maximum power of 3 kW was used for the study. The laser beam was scanned across the polycrystalline alumina substrate under argon at a rate of 0.66–2.0 cm/s. Specimens were prepared at laser powers ranging from 200 to 800 W, with a constant beam diameter of 0.2 cm. The substrates were preheated to 400 °C before the laser treatments to minimize thermal shock. The alumina substrates were 96% pure alumina plates, with a thickness of 2 mm. The microstructure within the melt pool varied strongly as a function of distance from the free surface. Different regions were identified by electron microscopy, with grain sizes varying from 2–3 μm up to 60 μm long needle-shaped grains having a {112̄6} preferred orientation with respect to the substrate surface. Due to the presence of Si, Ca, and Mg in the alumina, an amorphous phase was found at the grain boundaries throughout the melt-pool region. The amorphous phase primarily appeared as a relatively thin film at the grain boundaries. However, large spheres (≅ 50 μm) of amorphous regions also appeared in the melt pool, which contained small grains of alumina. The reasons for the strongly varying microstructure will be discussed.

Quasi-optical measurement of complex dielectric constant at 300 GHz

A two beam interferometer in the Martin-Puplett configuration is used to determine the complex dielectric constant at 300 GHz of teflon, TPX-plastics, SPECTRALON and paraffin waxes with melting temperatures of 48° C and 72° C, respectively. The design of the quasi-optical system leads to a constant beam diameter at the power detector independent of path delay and frequency. The power detector signal is recorded not only along one period but over about 50 periods. A spectrum estimation routine allows to determine more exactly amplitude and phase angle of the signal. A basic problem is noticed: imperfect detector and source match cause harmonic distortion of the power detector signal. The effects on processing the loss tangent and the invalidation are shown. Finally loss tangent and dielectric constant are determined indirectly by optimizing an equivalent microwave circuit using a commercial available microwave design system to take multiple reflections and losses in consideration.

Transmission electron microscopy observations of t′-phase after laser sealing of yttria partially stabilized zirconia

The version of the zirconia-rich portion of the zirconia-yttria phase diagram as reported by Scott [1] has until now been considered the most reliable; in this diagram both the equilibrium tetragonal (t) phase and non-equilibrium tetragonal (t') phase, also called non-transformable (sometimes designated as secondary tetragonal) [2] have been reported. The significant differences between these tetragonal phases are that t-phase contains less yttria, corresponding to the phase diagram, while the t'-phase contains approximately the same amount of yttria as the starting material. Moreover, the tetragonality (c/a ratio) of t'-phase is less than that of t-phase and approaches unity; t'-phase is produced by athermal transformation and t-phase is produced by diffusional transformation. Upon decreasing the annealing temperature to below about 1100 °C, the t-phase transforms martensitically to monoclinic (m) phase. Transmission electron microscopy (TEM) investightions have been used to evaluate plasma-sprayed layers and laser-sealed layers of 6 wt % yttria stabilized zirconia (YSZ) and 8 w t % YSZ [3]. Z rO2-6wt% Y203 plasma-sprayed layers were found to have equiaxed grains approximately 500 nm in size, consisting entirely of tetragonal variants in the form of plates of width approximately 0.1 p~m. Thecell size was changed to about 1.75/xm after laser sealing and consisted entirely of tetragonal variants approximately 0.08/xm wide. On the other hand, ZrO; -8 wt% Y203 plasma-sprayed layers [3] consisted of very fine grains of approximately 20 rim, of cubic structure, tetragonal or cubic grains of size 0.1-0.5 tzm and large grains of size 1-5/xm consisting entirely of tetragonal variants of width about 200 nm. After laser sealing, all cells were found to be in the range 1-3/zm and consisted entirely of approximately 150 nm tetragonal variants. In the work reported here, specimens of 8.5 wt % yttria-partially stabilized zirconia (YPSZ) plasmasprayed layers of thickness about 0.5 mm were remelted by CO2 laser radiation at a power of 0.8-1.7 kW at a constant beam diameter of 5 mm and different traverse speeds (5 .3-370mms-i) . Overlapped tracks of approximately 10% overlap were produced. The microstructures of the upper surface and transverse section of the layers were evaluated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) for

Effects of ambient gases on ion emission stability of liquid-metal ion sources

Effects of ambient gases on ion emission stability of Au–Si–Be and Pd–Ni–Si–Be–B liquid‐metal (LM) ion sources have been studied. Water vapor and oxygen gas have significant effects on ion emission stability. These effects are thought to be due to oxidation of the liquid‐metal surface, especially the Be atoms. A higher operating temperature enhances these effects, while it reduces the effects for Ga‐ion sources. Vapor pressures of resultant oxides are discussed to explain these results. We have obtained positionally stable ion beams (fluctuation <0.08 μm) with a constant ion current (fluctuation <0.02%) and a constant beam diameter (fluctuation<0.03 μm at beam diameter of 0.2 μm) by operating the sources under a vacuum pressure of 2×10−7 Torr in a 100 kV FIBI (focused‐ion‐beam implanter).