Andreas Pitzschke

Optical properties of rabbit brain in the red and near-infrared: changes observed under in vivo, postmortem, frozen, and formalin-fixated conditions

Abstract. The outcome of light-based therapeutic approaches depends on light propagation in biological tissues, which is governed by their optical properties. The objective of this study was to quantify optical properties of brain tissue in vivo and postmortem and assess changes due to tissue handling postmortem. The study was carried out on eight female New Zealand white rabbits. The local fluence rate was measured in the VIS/NIR range in the brain in vivo, just postmortem, and after six weeks’ storage of the head at −20°C or in 10% formaldehyde solution. Only minimal changes in the effective attenuation coefficient μeff were observed for two methods of sacrifice, exsanguination or injection of KCl. Under all tissue conditions, μeff decreased with increasing wavelengths. After long-term storage for six weeks at −20°C, μeff decreased, on average, by 15 to 25% at all wavelengths, while it increased by 5 to 15% at all wavelengths after storage in formaldehyde. We demonstrated that μeff was not very sensitive to the method of animal sacrifice, that tissue freezing significantly altered tissue optical properties, and that formalin fixation might affect the tissue’s optical properties.

Understanding the core density profile in TCV H-mode plasmas

Results from a database analysis of H-mode electron density profiles on the Tokamak a Configuration Variable (TCV) under stationary conditions show that the logarithmic electron density gradient increases with collisionality. By contrast, usual observations of H-modes showed that the electron density profiles tend to flatten with increasing collisionality. In this work it is reinforced that the role of collisionality alone, depending on the parameter regime, can be rather weak, and in these dominantly electron heated TCV cases, the electron density gradient is tailored by the underlying turbulence regime, which is mostly determined by the ratio of the electron to ion temperature and that of their gradients. Additionally, mostly in ohmic plasmas, the Ware-pinch can significantly contribute to the density peaking. Qualitative agreement between the predicted density peaking by quasi-linear gyrokinetic simulations and the experimental results is found. Quantitative comparison would necessitate ion temperature measurements, which are lacking in the considered experimental dataset. However, the simulation results show that it is the combination of several effects that influences the density peaking in TCV H-mode plasmas.

Determination of the radiance of cylindrical light diffusers: design of a one-axis charge-coupled device camera-based goniometer setup

Abstract. A one-axis charge-coupled device camera-based goniometer setup was developed to measure the three-dimensional radiance profile (longitudinal, azimuthal, and polar) of cylindrical light diffusers in air and water. An algorithm was programmed to project the two-dimensional camera data onto the diffuser coordinates. The optical system was designed to achieve a spatial resolution on the diffuser surface in the submillimeter range. The detection threshold of the detector was well below the values of measured radiance. The radiance profiles of an exemplary cylindrical diffuser measured in air showed local deviations in radiance below 10% for wavelengths at 635 and 671 nm. At 808 nm, deviations in radiance became larger, up to 45%, most probable due to the manufacturing process of the diffuser. Radiance profiles measured in water were less Lambertian than in air due to the refractive index matching privileging the radial decoupling of photons from the optical fiber.

Optical properties of the deep brain in the red and NIR: changes observed under in-vivo, post-mortem, frozen and formalin-fixated conditions

Photobiomodulation (PBM) is a promising approach to treat Parkinson’s disease (PD) symptoms in cellular or animal models. Unfortunately, little information is available on the optical parameters playing a role in the light dosimetry during PBM. We conducted a study to determine the effective attenuation coefficient μeff of PD-relevant human deep brain tissues at 671 and 808 nm, using a multichannel fluence rate-meter comprising sub-millimeter isotropic detectors. The first step involved measurements of tissue modifications induced by postmortem situation and tissue storage on rabbit brains. The parameter μeff was measured using various tissue conditions (in vivo, immediately after sacrifice, after six weeks’ storage at −20°C or in 10 % formaldehyde solution) on eight female New Zealand white rabbits. In the second step, fluence rate was measured at various locations of a frozen human deep brain when the deep brain was illuminated from the sphenoidal sinus. The results were processed by an iterative Monte-Carlo algorithm to generate sets of optical parameters, and results collected on rabbit brains were used to extrapolate the μeff value that would be observed in human deep brain tissues in vivo. Under all tissue conditions, the value of μeff at 808 nm was smaller than that at 671 nm. After long-term storage for six weeks at −20°C, μeff decreased, on average by 15 to 25 % at all wavelengths, while it increased by 5 to 15 % at all wavelengths after storage in formaldehyde. Therefore, a reasonable estimate of in vivo human deep brain μeff values at 671 and 808 nm can be obtained by multiplying the data we report by 120 %.

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

The dynamics of electron temperature and density profiles during quasi-stationary H-mode phases in the TCV tokamak has been investigated for type III and type I edge-localized modes (ELMs) using electron cyclotron heating (ECH) to vary the collisionality. At heating power levels close to the threshold for the L–H transition, ELMs of type III were observed, with an energy loss per ELM below 10% of the total plasma energy. Electron temperature and pressure showed no significant increase during the phase before the ELM crash. ELMs of type I were characterized by a lower repetition rate, but caused fractional energy losses reaching 20%. For this ELM type, a clear increase in pedestal pressure and pressure gradient was observed before the collapse associated with the ELM event. The rapid drop in electron temperature also affected the plasma core explaining the rather large energy losses per ELM. Ideal MHD stability calculations of the edge pedestal with the KINX code showed that high-n ballooning modes restricted the achievable pressure gradient at high collisionality and ELMs of type III. With additional heating and type I ELMs, stability limits were set by medium-n kink-ballooning modes. Comparing the values of the pressure gradients and current densities obtained from experimental data with those of ideal MHD stability calculations showed good agreement. The model, however, needs to account for the radial displacement of the location of maximum pressure gradient during the ELM cycle.