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  • Suomela, Jukka (Helsingin yliopisto, 2009)
    This thesis studies optimisation problems related to modern large-scale distributed systems, such as wireless sensor networks and wireless ad-hoc networks. The concrete tasks that we use as motivating examples are the following: (i) maximising the lifetime of a battery-powered wireless sensor network, (ii) maximising the capacity of a wireless communication network, and (iii) minimising the number of sensors in a surveillance application. A sensor node consumes energy both when it is transmitting or forwarding data, and when it is performing measurements. Hence task (i), lifetime maximisation, can be approached from two different perspectives. First, we can seek for optimal data flows that make the most out of the energy resources available in the network; such optimisation problems are examples of so-called max-min linear programs. Second, we can conserve energy by putting redundant sensors into sleep mode; we arrive at the sleep scheduling problem, in which the objective is to find an optimal schedule that determines when each sensor node is asleep and when it is awake. In a wireless network simultaneous radio transmissions may interfere with each other. Task (ii), capacity maximisation, therefore gives rise to another scheduling problem, the activity scheduling problem, in which the objective is to find a minimum-length conflict-free schedule that satisfies the data transmission requirements of all wireless communication links. Task (iii), minimising the number of sensors, is related to the classical graph problem of finding a minimum dominating set. However, if we are not only interested in detecting an intruder but also locating the intruder, it is not sufficient to solve the dominating set problem; formulations such as minimum-size identifying codes and locating dominating codes are more appropriate. This thesis presents approximation algorithms for each of these optimisation problems, i.e., for max-min linear programs, sleep scheduling, activity scheduling, identifying codes, and locating dominating codes. Two complementary approaches are taken. The main focus is on local algorithms, which are constant-time distributed algorithms. The contributions include local approximation algorithms for max-min linear programs, sleep scheduling, and activity scheduling. In the case of max-min linear programs, tight upper and lower bounds are proved for the best possible approximation ratio that can be achieved by any local algorithm. The second approach is the study of centralised polynomial-time algorithms in local graphs these are geometric graphs whose structure exhibits spatial locality. Among other contributions, it is shown that while identifying codes and locating dominating codes are hard to approximate in general graphs, they admit a polynomial-time approximation scheme in local graphs.
  • Lindström, Jan (Helsingin yliopisto, 2003)
  • Pikkarainen, Anna-Liisa (Helsingin yliopisto, 2008)
    The Baltic Sea was studied with respect to selected organic contaminants and their ecotoxicology. The research consisted of analyses of total hydrocarbons, polycyclic aromatic hydrocarbons, bile metabolites, hepatic ethoxyresorufin-O-deethylase (EROD) activity, polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs). The contaminants were measured from various matrices, such as seawater, sediment and biota. The methods of analysis were evaluated and refined to comparability of the results. Polyaromatic hydrocarbons, originating from petroleum, are known to be among the most harmful substances to the marine environment. In Baltic subsurface water, seasonal dependence of the total hydrocarbon concentrations (THCs) was seen. Although concentrations of parent polycyclic aromatic hydrocarbons (PAHs) in sediment surface varied between 64 and 5161 ug kg-1 (dw), concentrations above 860 ug kg-1 (dw) were found in all the studied sub-basins of the Baltic Sea. Concentrations commonly considered to substantially increase the risk of liver disease and reproductive impairment in fish, as well as potential effects on growth (above 1000 ug kg-1 dw), were found in all the studied sub-basins of the Baltic Sea except Kattegat. Thus, considerable pollution in sediments was indicated. In bivalves, the sums of 12 PAHs varied on a wet weight basis between 44 and 298 ug kg-1 (ww). The predominant PAHs were high molecular weight and the PAH profiles of M. balthica differed from those found in sediment from the same area. The PAHs were both pyrolytic and petrogenic in origin, and a contribution from diesel engines was found, which indicates pollution of the Baltic Sea, most likely caused by the steadily increasing shipping in the area. The HPLC methods developed for hepatic EROD activity and bile metabolite measurements proved to be fast and suitable for the study of biological effects. A mixed function oxygenase enzyme system in Baltic Sea perch collected from the Gulf of Finland was induced slightly: EROD activity in perch varied from 0.30 14 pmol min-1 mg-1 protein. This range can be considered to be comparable to background values. Recent PAH exposure was also indicated by enhanced levels (213 and 1149 ug kg-1) of the bile metabolite 1-hydroxypyrene. No correlation was indicated between hepatic EROD activity and concentration of 1-hydroxypyrene in bile. PCBs and OCPs were observed in Baltic Sea sediment, bivalves and herring. Sums of seven CBs in surface sediment (0 5 cm) ranged from 0.04 to 6.2 ug kg-1 (dw) and sums of three DDTs from 0.13 to 5.0 ug kg-1 (dw). The highest levels of contaminants were found in the most eastern area of the Gulf of Finland where the highest total carbon and nitrogen content was found and where the lowest percentage proportion of p,p -DDT was found. The highest concentrations of CBs and the lowest concentration of DDTs were found in M. balthica from the Gulf of Finland. The highest levels of DDTs were found in M. balthica from the Hanö Bight, which is the outer part of the Bornholm Basin close to the Swedish mainland. In bivalves, the sums of seven CBs were 72 108 ug kg-1 (lw) and the sums of three DDTs were 66 139 ug kg-1 (lw). Results from temporal trend monitoring showed, that during the period 1985 2002, the concentrations of seven CBs in two-year-old female Baltic herring were clearly decreased, from 9 16 to 2 6 ug kg-1 (ww) in the northern Baltic Sea. At the same time, concentrations of three DDTs declined from 8 15 to 1 5 ug kg-1 (ww). The total concentration of the fat-soluble CBs and DDTs in Baltic herring muscle was shown to be age-dependent; the average concentrations in ten-year-old Baltic herring were three to five-fold higher than in two-year-old herring. In Baltic herring and bivalves, as well as in surface sediments, CB 138 and CB153 were predominant among CBs, whereas among DDTs p,p'-DDD predominated in sediment and p,p'-DDE in bivalves and Baltic herring muscle. Baltic Sea sediments are potential sources of contaminants that may become available for bioaccumulation. Based on ecotoxicological assessment criteria, cause for concern regarding CBs in sediments was indicated for the Gulf of Finland and the northern Baltic Proper, and for the northern Baltic Sea regarding CBs in Baltic herring more than two years old. Statistical classification of selected organic contaminants indicated high-level contamination for p,p'-DDT, p,p'-DDD, p,p'-DDE, total DDTs, HCB, CB118 and CB153 in muscle of Baltic herring in age groups two to ten years; in contrast, concentrations of a-HCH and g-HCH were found to be moderate. The concentrations of DDTs and CBs in bivalves is sufficient to cause biological effects, and demonstrates that long-term biological effects are still possible in the case of DDTs in the Hanö Bight.
  • Nuopponen, Markus (Helsingin yliopisto, 2008)
    Controlled radical polymerization techniques, such as RAFT polymerization, are modern alternatives for preparing pre-designed polymers. In RAFT polymerization, chain length, molar mass distribution, microstructure (tacticity and sequencing), composition and functionality can be controlled. This allows the synthesis of a variety of novel polymer architectures, such as block and graft copolymers, stars, hybride materials and bioconjugates. The self-organization of synthetic preformed polymers into controllable nanostuctures is one of the most promising topics in the material science. However, the field of block copolymer self-assembly is still relatively young and current polymeric materials are structurally rather simple compared to biological materials. Thus, novel generations of polymer-based materials offer huge opportunities in material science. In this work, amphiphilic di- and triblock copolymers were synthesized by RAFT polymerization, and their organization into specific structures at nanoscale was studied. In all the block copolymer, one of the blocks was thermoresponsive poly(N-isopropylacrylamide). Thus, polymers and studied materials were temperature sensitive. In addition, control over tacticity in N-isopropylacrylamide polymerization was studied. The self-organization in aqueous solutions was strongly affected by the tacticity and the block sequence. Amphiphilic polymers formed various micellar structures in aqueous solutions. These micellar microcontainers have applications in controlled drug delivery. Amphiphiles have also applications as dispersants in coatings and cosmetics. In bulk, all the stable block copolymer morphologies were observed for triblock copolymers. Hydrogels of triblock copolymers can be used as thermoresponsive membrane materials. Polymers synthesized through the RAFT polymerization can be directly used in the synthesis of polymer grafted nanoparticles. Gold nanoparticles have attracted great interest due to the fact that gold is the most stable and inert noble metal possessing unique surface properties and good conductivity. It was shown that the association and optical properties of the gold nanoparticles grafted with smart polymers can be widely varied by pH and temperature. This type of gold nanoparticles have applications in diagnostics, sensors and cell imaging.
  • Mølgaard, Bjarke (Helsingin yliopisto, 2014)
    Epidemiological and toxicological studies have clearly shown that air pollution has adverse effects on human health. Exposure to air pollution mainly occurs in the urban and in the indoor environment. Fine particles form a diverse group of air pollutants, which is responsible for some of the health effects, and they originate from many outdoor and indoor sources. This thesis aims to support efforts for reduction of human exposure to fine particles by development and evaluation of an urban forecast model and by evaluation of an indoor aerosol model and of air cleaners. Measured particle number size distributions were utilised in all studies, and the urban studies additionally utilised meteorological data. The urban aerosol was investigated by using cluster analysis, and the urban particle number concentration forecast model was based on a regression model with an autoregressive structure on its error term. It was evaluated using particle number concentrations from five cities. The utility of a Multi-Compartment Size-resolved Indoor Aerosol Model (MC-SIAM) for estimating airflows in a building was evaluated by comparison with results from a tracer gas technique. The performance of five portable indoor air cleaners was evaluated by applying a simple aerosol model to data from chamber experiments. The urban concentration of particles smaller than 50 nm was found to depend on local sources, while for larger sizes distant sources may dominate the concentration. The forecast model was found to perform best for locations with a strong influence of local sources. The estimates obtained with the MC-SIAM of airflows between the indoors and outdoors were good for periods during which good agreement between the modelled and measured particle number size distributions was obtained. The indoor airflow estimates deviated somewhat due to a commonly used simplification. Portable air cleaners utilising fans and filters performed as expected, but an ion generator was ineffective for most particle sizes. The forecast model can give useful forecasts in its present form under certain conditions but further development is desired to make it more versatile. The MC-SIAM is a valuable tool for the study of indoor aerosols except in case of fluctuating airflows. Portable air cleaners are useful for improving indoor air quality, but not all models perform well.
  • Lahtinen, Petro (Helsingin yliopisto, 2005)
  • Pehrman, Reijo (Helsingin yliopisto, 2012)
    This doctoral thesis is focused on the radiation induced oxidative dissolution of spent nuclear fuel. UO2 is typically used as a model substance for spent nuclear fuel on the dissolution simulation experiments, but transuranium elements and fission products are expected to influence the redox chemistry involved. The dissolution behaviour of NpO2 and PuO2 in H2O2 solution without complexing agent was compared to UO2. Based on the measured rates, the dissolution of the actinides is not expected to be congruent, with Np and Pu release rates lower than the U release rate. The oxidative dissolution of PuO2 was found to be enhanced by the presence of Fe2+ in solution. This enhancement was attributed to hydroxyl radicals produced in the Fenton reaction between Fe2+ and radiolytically produced H2O2. The presence of solid UO2 pellet was found to prolong the lifetime of Fe2+ in solution, leading to further enhancement on the Pu dissolution. Fission product doping of UO2 was found to not have significant effect on the catalytic decomposition of H2O2. Fission product doping was however observed to hinder the reaction of UO2 with oxidants MnO4- and IrCl62-, and the effect of doping to decrease with increasing reduction potential of the oxidants. Uranyl peroxide solid phase formation on UO2 surface was observed to depend strongly on the peroxide concentration on the solution. In high peroxide concentrations oriented UO4∙nH2O crystals formed plate-like formations covering the whole surface, and with decreasing H2O2 concentration the crystals became unoriented and covered the UO2 surface only partially. In situ study showed the phase formation in high H2O2 concentration to take place in hours, and no intermediate phases were detected. Method development was performed on two areas: H2O2 measurement in small solution volumes down to nanomolar concentrations by chemiluminescence was tested and optimal parameters studied, and reference Raman spectra for studtite, schoepite, becquerelite and soddyite was measured.
  • Salminen, Johanna (Helsingin yliopisto, 2009)
    The importance of supercontinents in our understanding of the geological evolution of the planet Earth has been recently emphasized. The role of paleomagnetism in reconstructing lithospheric blocks in their ancient paleopositions is vital. Paleomagnetism is the only quantitative tool for providing ancient latitudes and azimuthal orientations of continents. It also yields information of content of the geomagnetic field in the past. In order to obtain a continuous record on the positions of continents, dated intrusive rocks are required in temporal progression. This is not always possible due to pulse-like occurrences of dykes. In this work we demonstrate that studies of meteorite impact-related rocks may fill some gaps in the paleomagnetic record. This dissertation is based on paleomagnetic and rock magnetic data obtained from samples of the Jänisjärvi impact structure (Russian Karelia, most recent 40Ar-39Ar age of 682 Ma), the Salla diabase dyke (North Finland, U-Pb 1122 Ma), the Valaam monzodioritic sill (Russian Karelia, U-Pb 1458 Ma), and the Vredefort impact structure (South Africa, 2023 Ma). The paleomagnetic study of Jänisjärvi samples was made in order to obtain a pole for Baltica, which lacks paleomagnetic data from 750 to ca. 600 Ma. The position of Baltica at ca. 700 Ma is relevant in order to verify whether the supercontinent Rodinia was already fragmented. The paleomagnetic study of the Salla dyke was conducted to examine the position of Baltica at the onset of supercontinent Rodinia's formation. The virtual geomagnetic pole (VGP) from Salla dyke provides hints that the Mesoproterozoic Baltica - Laurentia unity in the Hudsonland (Columbia, Nuna) supercontinent assembly may have lasted until 1.12 Ga. Moreover, the new VGP of Salla dyke provides new constraint on the timing of the rotation of Baltica relative to Laurentia (e.g. Gower et al., 1990). A paleomagnetic study of the Valaam sill was carried out in order to shed light into the question of existence of Baltica-Laurentia unity in the supercontinent Hudsonland. Combined with results from dyke complex of the Lake Ladoga region (Schehrbakova et al., 2008) a new robust paleomagnetic pole for Baltica is obtained. This pole places Baltica on a latitude of 10°. This low latitude location is supported also by Mesoproterozoic 1.5 1.3 Ga red-bed sedimentation (for example the Satakunta sandstone). The Vredefort impactite samples provide a well dated (2.02 Ga) pole for the Kaapvaal Craton. Rock magnetic data reveal unusually high Koenigsberger ratios (Q values) in all studied lithologies of the Vredefort dome. The high Q values are now first time also seen in samples from the Johannesburg Dome (ca. 120 km away) where there is no impact evidence. Thus, a direct causative link of high Q values to the Vredefort impact event can be ruled out.
  • Leikoski, Tuomo (Helsingin yliopisto, 2014)
    Formation of carbon carbon bonds constitutes the basis of synthetic organic chemistry. The growing demand of safer and environmentally friendlier processes, combined with continuing need for more efficient and selective reactions, has given challenges to industrial and fundamental academic research. The objective of this thesis was to develop novel ways to perform important carbon carbon bond-forming reactions on solid support. Of special focus were palladium- and copper-catalysed reactions of unsaturated amines. Polymer-bound propargylamine and allylamine were arylated successfully by the palladium-catalysed Sonogashira and Heck reaction, respectively. Additionally, allenes were produced in the Crabbé homologation of polymer-bound propargylamine, where copper acetylide is acting as an intermediate. All of these reactions would give rise to biologically interesting molecules: 1,3-arylaminopropanes after hydrogenation of the Sonogashira and Heck products and nitrogen-containing allenes by the Crabbé reaction. By varying the aryl iodide in solution, a series of arylated propargylamines and allylamines were synthesised and isolated as their acetamides. From the polymer-bound propargylamine, various allene amides were obtained after N-acylation followed by the Crabbé reaction. It was also briefly explored if the arylation of propargylamine on solid-phase could be possible without expensive palladium via the Castro-Stephens reaction, using a polymer-bound copper acetylide and the aryl iodide in solution. However, attempts to perform the first Castro Stephens reaction on solid-phase failed. Free amines are problematic in the Sonogashira and Heck reactions, due to coordination with the palladium catalyst and nucleophilicity toward the allene in the Crabbé reaction. These incompatibilities were solved by using the resin linkers simultaneously as protecting groups for the amines: as carbamates in the Sonogashira and Heck reaction, and as N-acyltriazenes in the Crabbé reaction. For the Heck reaction, finding the right reaction conditions turned out to be particularly difficult, the additional challenges being the narrow temperature window and the need to avoid polyarylation. Nevertheless, a regioselective γ-arylation could be performed giving similar yields as in the Sonogashira studies. In summary, alternative methods to perform important carbon carbon bond-forming reactions on solid support were developed.
  • Andersson, Terhi (Helsingin yliopisto, 2007)
    Pressurised hot water extraction (PHWE) exploits the unique temperature-dependent solvent properties of water minimising the use of harmful organic solvents. Water is environmentally friendly, cheap and easily available extraction medium. The effects of temperature, pressure and extraction time in PHWE have often been studied, but here the emphasis was on other parameters important for the extraction, most notably the dimensions of the extraction vessel and the stability and solubility of the analytes to be extracted. Non-linear data analysis and self-organising maps were employed in the data analysis to obtain correlations between the parameters studied, recoveries and relative errors. First, pressurised hot water extraction (PHWE) was combined on-line with liquid chromatography-gas chromatography (LC-GC), and the system was applied to the extraction and analysis of polycyclic aromatic hydrocarbons (PAHs) in sediment. The method is of superior sensitivity compared with the traditional methods, and only a small 10 mg sample was required for analysis. The commercial extraction vessels were replaced by laboratory-made stainless steel vessels because of some problems that arose. The performance of the laboratory-made vessels was comparable to that of the commercial ones. In an investigation of the effect of thermal desorption in PHWE, it was found that at lower temperatures (200ºC and 250ºC) the effect of thermal desorption is smaller than the effect of the solvating property of hot water. At 300ºC, however, thermal desorption is the main mechanism. The effect of the geometry of the extraction vessel on recoveries was studied with five specially constructed extraction vessels. In addition to the extraction vessel geometry, the sediment packing style and the direction of water flow through the vessel were investigated. The geometry of the vessel was found to have only minor effect on the recoveries, and the same was true of the sediment packing style and the direction of water flow through the vessel. These are good results because these parameters do not have to be carefully optimised before the start of extractions. Liquid-liquid extraction (LLE) and solid-phase extraction (SPE) were compared as trapping techniques for PHWE. LLE was more robust than SPE and it provided better recoveries and repeatabilities than did SPE. Problems related to blocking of the Tenax trap and unrepeatable trapping of the analytes were encountered in SPE. Thus, although LLE is more labour intensive, it can be recommended over SPE. The stabilities of the PAHs in aqueous solutions were measured using a batch-type reaction vessel. Degradation was observed at 300ºC even with the shortest heating time. Ketones and quinones and other oxidation products were observed. Although the conditions of the stability studies differed considerably from the extraction conditions in PHWE, the results indicate that the risk of analyte degradation must be taken into account in PHWE. The aqueous solubilities of acenaphthene, anthracene and pyrene were measured, first below and then above the melting point of the analytes. Measurements below the melting point were made to check that the equipment was working, and the results were compared with those obtained earlier. Good agreement was found between the measured and literature values. A new saturation cell was constructed for the solubility measurements above the melting point of the analytes because the flow-through saturation cell could not be used above the melting point. An exponential relationship was found between the solubilities measured for pyrene and anthracene and temperature.
  • Ollinaho, Pirkka (Helsingin yliopisto, 2014)
    Numerical Weather Prediction (NWP) models form the basis of weather forecasting. The accuracy of model forecasts can be enhanced by providing a more accurate initial state for the model, and by improving the model representation of relevant atmospheric processes. Modelling of subgrid-scale physical processes causes additional uncertainty in the forecasts since, for example, the rates at which parts of the physical processes occur are not exactly known. The efficiency of these sub-processes in the models is controlled via so called closure parameters. This thesis is motivated by a practical need to estimate the values of these closure parameters objectively, and to assess the uncertainties related to them. In this thesis the Ensemble Prediction and Parameter Estimation System (EPPES) is utilised to determine the optimal closure parameter values, and to learn about their uncertainties. Closure parameters related to convective processes, formation of convective rain and stratiform clouds are studied in two atmospheric General Circulation Models (GCM): the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the ECMWF model HAMburg version (ECHAM5). The parameter estimation is conducted by launching ensembles of medium range forecasts with initial time parameter variations. The fit of each ensemble member to analyses is then evaluated with respect to a target criterion, and the likelihoods of the forecasts are discerned. The target criterion is first set to be 500 hPa level geopotential height Mean Squared Error (MSE) at forecast days three and ten. After the proof of concept experimentations, the use of total energy norm as the target criterion is explored. EPPES estimation with both likelihoods results in parameter values converging to more optimal values during a three-month sampling period. The improved forecast accuracy of the models with the new parameter values are verified through headline skill scores (Root Mean Square Error (RMSE) and Anomaly Correlation Coefficient (ACC)) of 500 hPa geopotential height and a scorecard consisting of multiple model fields. The sampling process also provides information about parameter uncertainties. Three uses for the uncertainty data are highlighted: (i) parametrization deficiencies can be identified from large parameter uncertainties, (ii) parameter correlations can indicate a need for the coupling of parameters, and (iii) adding parameter variations into an ensemble prediction system (EPS) can be used to increase the ensemble spread. The relationship between medium range forecasts and model climatology is explored, too. Closure parameter modification induced cloud cover changes at forecast day three carry over to the very long range forecasts as well. This link could be used to improve model climatology by enhancing the computationally cheaper medium range forecast skill of the model.
  • Weijo, Ville (Helsingin yliopisto, 2008)
    The Standard Model of particle physics consists of the quantum electrodynamics (QED) and the weak and strong nuclear interactions. The QED is the basis for molecular properties, and thus it defines much of the world we see. The weak nuclear interaction is responsible for decays of nuclei, among other things, and in principle, it should also effects at the molecular scale. The strong nuclear interaction is hidden in interactions inside nuclei. From the high-energy and atomic experiments it is known that the weak interaction does not conserve parity. Consequently, the weak interaction and specifically the exchange of the Z^0 boson between a nucleon and an electron induces small energy shifts of different sign for mirror image molecules. This in turn will make the other enantiomer of a molecule energetically favorable than the other and also shifts the spectral lines of the mirror image pair of molecules into different directions creating a split. Parity violation (PV) in molecules, however, has not been observed. The topic of this thesis is how the weak interaction affects certain molecular magnetic properties, namely certain parameters of nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopies. The thesis consists of numerical estimates of NMR and ESR spectral parameters and investigations of the effects of different aspects of quantum chemical computations to them. PV contributions to the NMR shielding and spin-spin coupling constants are investigated from the computational point of view. All the aspects of quantum chemical electronic structure computations are found to be very important, which makes accurate computations challenging. Effects of molecular geometry are also investigated using a model system of polysilyene chains. PV contribution to the NMR shielding constant is found to saturate after the chain reaches a certain length, but the effects of local geometry can be large. Rigorous vibrational averaging is also performed for a relatively small and rigid molecule. Vibrational corrections to the PV contribution are found to be only a couple of per cents. PV contributions to the ESR g-tensor are also evaluated using a series of molecules. Unfortunately, all the estimates are below the experimental limits, but PV in some of the heavier molecules comes close to the present day experimental resolution.
  • Karvi, Timo (Helsingin yliopisto, 2000)
  • Hakulinen, Ville (Helsingin yliopisto, 2002)
  • Arponen, Heikki (Helsingin yliopisto, 2009)
    This thesis consists of three articles on passive vector fields in turbulence. The vector fields interact with a turbulent velocity field, which is described by the Kraichnan model. The effect of the Kraichnan model on the passive vectors is studied via an equation for the pair correlation function and its solutions. The first paper is concerned with the passive magnetohydrodynamic equations. Emphasis is placed on the so called "dynamo effect", which in the present context is understood as an unbounded growth of the pair correlation function. The exact analytical conditions for such growth are found in the cases of zero and infinite Prandtl numbers. The second paper contains an extensive study of a number of passive vector models. Emphasis is now on the properties of the (assumed) steady state, namely anomalous scaling, anisotropy and small and large scale behavior with different types of forcing or stirring. The third paper is in many ways a completion to the previous one in its study of the steady state existence problem. Conditions for the existence of the steady state are found in terms of the spatial roughness parameter of the turbulent velocity field.
  • Kiljunen, Timo (Helsingin yliopisto, 2008)
    Diagnostic radiology represents the largest man-made contribution to population radiation doses in Europe. To be able to keep the diagnostic benefit versus radiation risk ratio as high as possible, it is important to understand the quantitative relationship between the patient radiation dose and the various factors which affect the dose, such as the scan parameters, scan mode, and patient size. Paediatric patients have a higher probability for late radiation effects, since longer life expectancy is combined with the higher radiation sensitivity of the developing organs. The experience with particular paediatric examinations may be very limited and paediatric acquisition protocols may not be optimised. The purpose of this thesis was to enhance and compare different dosimetric protocols, to promote the establishment of the paediatric diagnostic reference levels (DRLs), and to provide new data on patient doses for optimisation purposes in computed tomography (with new applications for dental imaging) and in paediatric radiography. Large variations in radiation exposure in paediatric skull, sinus, chest, pelvic and abdominal radiography examinations were discovered in patient dose surveys. There were variations between different hospitals and examination rooms, between different sized patients, and between imaging techniques; emphasising the need for harmonisation of the examination protocols. For computed tomography, a correction coefficient, which takes individual patient size into account in patient dosimetry, was created. The presented patient size correction method can be used for both adult and paediatric purposes. Dental cone beam CT scanners provided adequate image quality for dentomaxillofacial examinations while delivering considerably smaller effective doses to patient compared to the multi slice CT. However, large dose differences between cone beam CT scanners were not explained by differences in image quality, which indicated the lack of optimisation. For paediatric radiography, a graphical method was created for setting the diagnostic reference levels in chest examinations, and the DRLs were given as a function of patient projection thickness. Paediatric DRLs were also given for sinus radiography. The detailed information about the patient data, exposure parameters and procedures provided tools for reducing the patient doses in paediatric radiography. The mean tissue doses presented for paediatric radiography enabled future risk assessments to be done. The calculated effective doses can be used for comparing different diagnostic procedures, as well as for comparing the use of similar technologies and procedures in different hospitals and countries.