LASCA véráramlásmérő rendszer fejlesztése és a kiértékelést befolyásoló egyes folyamatok fontosságának vizsgálata

When a rough surface is illuminated by coherent light and the scattered light is projected on a screen or onto the image sensor of a camera, a grainy interference pattern is formed, known as laser speckle. This effect is used in a wide range of everyday applications, from optical mouse to 3D imaging. A blood flow measurement method based on laser speckle contrast analysis (LASCA) was first proposed in the early 1980s1 as a simple and cheap alternative to the rather expensive laser Doppler measurement systems, that could monitor blood flow over large areas in near real time. Over the years, the method has evolved considerably, both in terms of technical implementations and theoretical models used in data evaluation, but it still has its limitations. While, for example, evaluation of the measurements performs on the fundus of the eye and the surface of the brain do not pose any particular difficulties, in case of the skin the presence of static scattering centers close to the surface can cause serious measurement and evaluation problems. In the latter case, the obtained perfusion results are strongly influenced by the evaluation model used and the way the measurement system is calibrated. My work focused on the further development of LASCA measurement system available in our laboratory and to study some phenomena, which were mostly omitted in the scientific literature, while these could have an influence on the results of the data evaluation. One of my main tasks was to create small size and lightweight arm-mounted illuminator-camera system. The main challenge was the large mass of the temperature-stabilized laser source, which led me to design an easy to handle optical fiber arrangement. The multimode optical fiber allows for easier coupling of the light with higher power output, but its granular output intensity distribution limited its use in blood flow measurement. As a possible solution to this problem, I investigated the applicability of a beam homogenizing diffuser. I present the steps of the development of the LASCA system used in my measurements, including the tuning of components and the rationale for the choice of instrumentation. I also describe the improvements I made to the control software to make the measurements more efficient. The contrast models also described in the theoretical overview of dissertation are important in correcting for the significant contribution of static scattering, which as a simplification, ignore the importance of so-called mixed scattering, where a given photon scatters on both moving and stationary particles. However, it arises the question of how accurately the models compute the correlation time of intensity fluctuations for different static-dynamic scattering ratios that affect the probability of mixed scattering. Measurements on tissue models showed that when purely dynamic emulsions were covered with a static scattering layer, not only the infinitely long exposure time contrast K(∞) increased due to the static scattering. However, the motion insensitive K(0) contrast, corresponding to infinitely short exposure time, also changed, which indicate an alteration in the optical properties of the sample. Based on this observation, my aim was to perform a systematic study to understand the effect of an induced change in the perfusion state of the tissue on the K(0) and K(∞) values of the same area. The results will help to optimize the measurements and to (partially) calibrate the measurement system. I believe, that the technical development of the LASCA blood flow measurement system and the performed studies will contribute the evolution of the method and increase the reliability of obtained data. In the longer term, this could lead to a broadening of the range of applications of the method in both the research and clinical diagnostics

Nanoparticle generation from nitinol target using pulsed laser ablation

The influence of experimental conditions (wavelength, liquid environment) on the properties of Ni-Ti nanoparticles generated by nanosecond laser ablation of NiTinol (Shape Memory Alloy) target and the elemental distribution of the irradiated surface were investigated. The studied laser wavelengths were 248 (KrF) and 1064 nm (Q-switched Nd:YAG). Nitinol targets were covered with thin liquid layer of distilled water and ethanol, respectively. The samples were irradiated with various numbers of pulses (500 in case of the investigation of the alloy surface and 20000 for the particle generation) and fluences (2 and 5 J/cm2). The morphology of laser treated NiTi surface and the size of the generated nanoparticles were studied as a function of both the laser fluence and the type of the applied liquids using scanning electron microscopy (SEM). Element mappings were realized by energy-dispersive X-ray spectroscopy (EDX). Our results clearly show that when using ethanol both the laser treated surface and the generated particles had a relatively homogeneous elemental distribution. However, under distilled water the irradiated surface and the generated particles showed a separated presence of Ni and Ti elements. For the higher fluence, formation of core-shell structured particles were observed in water environment