Evgueni Parilov

Real-time rendering of textures with feature curves

The standard bilinear interpolation on normal maps results in visual artifacts along sharp features, which are common for surfaces with creases, wrinkles, and dents. In many cases, spatially varying features, like the normals near discontinuity curves, are best represented as functions of the distance to the curve and the position along the curve. For high-quality interactive rendering at arbitrary magnifications, one needs to interpolate the distance field preserving discontinuity curves exactly. We present a real-time, GPU-based method for distance function and distance gradient interpolation which preserves discontinuity feature curves. The feature curves are represented by a set of quadratic Bezier curves, with minimal restrictions on their intersections. We demonstrate how this technique can be used for real-time rendering of complex feature patterns and blending normal maps with procedurally defined profiles near normal discontinuities.

Generalized theoretical treatment and numerical method of time-resolved radially dependent laser pulses interacting with multiphoton absorbers

We introduce a generalized numerical method to calculate short-pulsed laser propagation in a wide class of multiphoton absorbing materials. The method has no restrictions on the input pulse widths varying from nanosecond to femtosecond, and its numerical solution is both radially and temporarily dependent, enabling us to check numerically the validity of assuming radially constant solutions, which ensures that the true peak intensity falls below the damage causing level. A new feature of our technique enables us to determine quantitatively the contributions to the total absorption due to every electronic energy level. We found excellent agreement between our calculations and experiments using sample materials ranging from reverse saturable absorbers, two-photon absorbers with excited-state absorption to three-photon absorbers. We applied our technique to a two-photon absorber with excited-state absorption and found approximately 1 order of magnitude increase in the absorption when femtosecond pulses were used in place of nanosecond pulses.

Interstitial Photodynamic Therapy-A Focused Review.

Multiple clinical studies have shown that interstitial photodynamic therapy (I-PDT) is a promising modality in the treatment of locally-advanced cancerous tumors. However, the utilization of I-PDT has been limited to several centers. The objective of this focused review is to highlight the different approaches employed to administer I-PDT with photosensitizers that are either approved or in clinical studies for the treatment of prostate cancer, pancreatic cancer, head and neck cancer, and brain cancer. Our review suggests that I-PDT is a promising treatment in patients with large-volume or thick tumors. Image-based treatment planning and real-time dosimetry are required to optimize and further advance the utilization of I-PDT. In addition, pre- and post-imaging using computed tomography (CT) with contrast may be utilized to assess the response.

Numerical simulation of laser pulse propagation in rare-earth-doped materials

We describe a general numerical method for calculating short-pulse laser propagation in rare-earth-doped materials, which are very important as gain materials for solid-state lasers, fiber lasers and optical amplifiers. The split-step, finite difference method simultaneously calculates changes in the laser pulse as it propagates through the material and calculates the dynamic populations of the rare-earth energy levels at any position within the material and for times during and after the laser pulse has passed through the material. Many traditional theoretical and numerical analyses of laser pulse propagation involve approximations and assumptions that limit their applicability to a narrow range of problems. Our numerical method, however, is more comprehensive and includes the processes of single- and multi-photon absorption, excited state absorption (ESA), energy transfer, upconversion, stimulated emission, cross relaxation, radiative relaxation and non-radiative relaxation. In the models, the rare-earth dopants can have an arbitrary number of energy levels. We are able to calculate the electron population density of every electronic level as a function of, for example, pulse energy, dopant concentration and sample thickness. We compare our theoretical results to published experimental results for rare-earth ions such as Er3+, Yb3+, Tm3+ and Ho3+.

Light fluence dosimetry in lung-simulating cavities

Accurate light dosimery is critical to ensure consistent outcome for pleural photodynamic therapy (pPDT). Ellipsoid shaped cavities with different sizes surrounded by turbid medium are used to simulate the intracavity lung geometry. An isotropic light source is introduced and surrounded by turbid media. Direct measurements of light fluence rate were compared to Monte Carlo simulated values on the surface of the cavities for various optical properties. The primary component of the light was determined by measurements performed in air in the same geometry. The scattered component was found by submerging the air-filled cavity in scattering media (Intralipid) and absorbent media (ink). The light source was located centrally with the azimuthal angle, but placed in two locations (vertically centered and 2 cm below the center) for measurements. Light fluence rate was measured using isotropic detectors placed at various angles on the ellipsoid surface. The measurements and simulations show that the scattered dose is uniform along the surface of the intracavity ellipsoid geometries in turbid media. One can express the light fluence rate empirically as φ =4S/As*Rd/(1- Rd), where Rd is the diffuse reflectance, As is the surface area, and S is the source power. The measurements agree with this empirical formula to within an uncertainty of 10% for the range of optical properties studied. GPU voxel-based Monte-Carlo simulation is performed to compare with measured results. This empirical formula can be applied to arbitrary geometries, such as the pleural or intraperitoneal cavity.

Overview of computational simulations for PDT treatments based on optimal choice of singlet oxygen

Effective photodynamic therapy (PDT) treatment planning and treatment monitoring requires computer simulations of both light transport in tissue and photosensitizer (PS) photophysics to accurately estimate singlet oxygen. Simply using fixed prescribed values of light dose (fluence) or PDT dose (the time integral of ‘PS concentration’ times the ‘fluence rate’) – one value for all patients – does not account for differences in the amount of singlet oxygen formed when fluence rates change or patient tissue parameters change. We will focus on singlet oxygen dose which is calculated by solving the photokinetics rate equations and which determines the effectiveness of the subsequent reactions of singlet oxygen with the cancer target and the negative effect of PS photobleaching.

Optimizing experimental conditions for stimulated emission depletion microscopy in biophotonics

Using a novel numerical method we show how to optimize the resolution enhancement of stimulated emission depletion (STED) by simulating the entire process including the absorption, overlapping multiple beams and stimulated emission. We provide calculations showing that for fixed donut pulse energy, a longer donut pulse length can result in greater resolution enhancement than a shorter donut pulse length. These results show how it is possible to use our simulations to design the best experimental conditions for STED resolution enhancement and illustrate the importance of having a software program that includes both multiple beams and stimulated emission.

Optimizing laser and probe molecules for multi-photon microscopy

Multiple fluorescent probes (multi-dyes) and single or multi-laser configurations can significantly extend the applications and accuracy of microscopy. Multiple fluorescent probes enable the user to identify more than one target, but difficulties can arise due to overlapping spectral emissions of the different probes. In particular, spectral overlapping of fluorescent and/or phosphorescent emission signals can lead to incorrect analysis. We present a method to numerically calculate overlapping spectra. An accurate modeling tool would be valuable to predict the best laser-probes combinations for selection and screening stages. We use a numerical method that simulates both time and space so that we can calculate on a near-instantaneous basis the absorption of laser light and electron populations. We can then calculate the intensity of the emitted signal and determine the overlap of the spectra.

Biomedical applications involving multiphoton probes

Many techniques in biological and clinical science use multiphoton absorbers for fluorescence. The applications include medical imaging for living cells, diagnostic techniques for disease and spectroscopy. The intrinsic value of the multiphoton absorber coefficients is therefore of the utmost importance. Additionally, the laser intensity at which the absorber saturates can determine which absorber, dye or protein is useful for a particular application. Yet, experimental methods for determining the optical coefficients often yield different results. We describe several common methods of 2PA measurements and describe their features. As an example of the importance of applying the correct analysis to measurements, we fit experimental data and obtain values for multiphoton absorbers and accurately obtain their intrinsic values. Finally, we present the optical properties of several multiphoton materials used in biology.

Graphical computational method for active materials in simulation of optical electromagnetics

Traditional numerical analyses of laser beam transmission through “active” nonlinear materials have involved many assumptions that narrow their general applicability. As more complex optical phenomena are widely employed in research and industry, it is necessary to expand the use of numerical simulation methods. Historically, laser-matter interactions have involved calculations of “classical” wave propagation by Maxwell’s equations and photon absorption through rate equations using numerous approximations. We describe a novel numerical modeling framework that adapts itself for simulation of different types of active materials provided by a simple graphical input. Our framework combines classical electric field propagation with “active” photon absorption kinetics using computational active photonic building blocks (APBB). It allows investigating a plane electromagnetic wave propagating through generic organic or inorganic photoactive materials; while, “active” photo-transitions are implemented using the APBB algorithm on the user interface. To date we have used the method in multiphoton absorbers, upconversion, semiconductor quantum dots, rare earth ions, organic chromophores, singlet oxygen formation, energy transfer, and optically-induced chemical reactions. We will demonstrate the method with applications of amplification in rare-earth ions and multiple two-photon absorbers materials in tandem.