Diffusion Tensor Imaging as a Diagnostic and Research Tool: A Study on Preterm Infants

Diffusion tensor imaging (DTI) is an advanced magnetic resonance imaging (MRI) technique. DTI is based on free thermal motion (diffusion) of water molecules. The properties of diffusion can be represented using parameters such as fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity, which are calculated from DTI data. These parameters can be used to study the microstructure in fibrous structure such as brain white matter. The aim of this study was to investigate the reproducibility of region-of-interest (ROI) analysis and determine associations between white matter integrity and antenatal and early postnatal growth at term age using DTI. Antenatal growth was studied using both the ROI and tract-based spatial statistics (TBSS) method and postnatal growth using only the TBSS method. The infants included to this study were born below 32 gestational weeks or birth weight less than 1,501 g and imaged with a 1.5 T MRI system at term age. Total number of 132 infants met the inclusion criteria between June 2004 and December 2006. Due to exclusion criteria, a total of 76 preterm infants (ROI) and 36 preterm infants (TBSS) were accepted to this study. The ROI analysis was quite reproducible at term age. Reproducibility varied between white matter structures and diffusion parameters. Normal antenatal growth was positively associated with white matter maturation at term age. The ROI analysis showed associations only in the corpus callosum. Whereas, TBSS revealed associations in several brain white matter areas. Infants with normal antenatal growth showed more mature white matter compared to small for gestational age infants. The gestational age at birth had no significant association with white matter maturation at term age. It was observed that good early postnatal growth associated negatively with white matter maturation at term age. Growth-restricted infants seemed to have delayed brain maturation that was not fully compensated at term, despite catchup growth.

The northern provinces of Sweden and Norway with part I, of Russia / published under the superintendence of the Society for the Diffusion of Useful Knowledge.

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Prospects of Bitcoin diffusion

Currency is something people deal with every day in their lives. The contemporary society is very much revolving around currencies. Even though technological development has been rapid, the principle of currency has stayed relatively unchanged for a long time. Bitcoin is a digital currency that introduced an alternative to other digital currencies, and to the traditional physical currencies. Bitcoin is peer-to-peer, open source, and it erases the need of a third party in transactions. Bitcoin has since inception gained certain fame, but it has not established itself as a common currency in the world. The purpose of this study was to analyse what kind of potential does Bitcoin have to become a widely accepted currency in day-to-day transactions. The main research question was divided into three sub questions: • What kind of a process is the diffusion of new innovations? • What kinds of factors speak for the wider adoption of Bitcoin? • What kinds of factors speak against the wider adoption of Bitcoin? The purpose of the study was approached by having diffusion of innovations as the theoretical framework. The four elements in diffusion of innovations are, innovation, communication, time, and social system. The theoretical framework is applied to Bitcoin, and the research questions answered by analysing Bitcoin’s potential diffusion prospects. The body of research data consisted of media texts and statistics. In this study, content analysis was the research method. The main findings of the study are that Bitcoin has clear strengths, but it faces a large amount of uncertainty. Bitcoin’s strong areas are the transactions. They are fast, easy, and cheap. From the innovation diffusion perspective Bitcoin is still relatively unknown, and the general public’s attitudes towards it are sceptical. The research findings purport that Bitcoin has potential demand especially when the financial system of a region is dysfunctional, or when there is a financial crisis. Bitcoin is not very trusted, and the majority of people do not see a reason to start using Bitcoin in the future. A large number of people associate it with illegal activities. In general people are largely unaware of what Bitcoin is or what are the strengths and weaknesses. Bitcoin is an innovative alternative currency. However, unless people see a major need for Bitcoin due to a financial crisis, or dysfunctionality in the financial system, Bitcoin will not become much more widespread as it is today. Bitcoin’s underlying technology can be harnessed to multiple uses. Developments in that field in the future are something that future researchers could look into.

Two-dimensional drift-diffusion simulation of organic solar cells

The aim of this master's thesis is to develop a two-dimensional drift-di usion model, which describes charge transport in organic solar cells. The main bene t of a two-dimensional model compared to a one-dimensional one is the inclusion of the nanoscale morphology of the active layer of a bulk heterojunction solar cell. The developed model was used to study recombination dynamics at the donor-acceptor interface. In some cases, it was possible to determine e ective parameters, which reproduce the results of the two-dimensional model in the one-dimensional case. A summary of the theory of charge transport in semiconductors was presented and discussed in the context of organic materials. Additionally, the normalization and discretization procedures required to nd a numerical solution to the charge transport problem were outlined. The charge transport problem was solved by implementing an iterative scheme called successive over-relaxation. The obtained solution is given as position-dependent electric potential, free charge carrier concentrations and current densities in the active layer. An interfacial layer, separating the pure phases, was introduced in order to describe charge dynamics occurring at the interface between the donor and acceptor. For simplicity, an e ective generation of free charge carriers in the interfacial layer was implemented. The pure phases simply act as transport layers for the photogenerated charges. Langevin recombination was assumed in the two-dimensional model and an analysis of the apparent recombination rate in the one-dimensional case is presented. The recombination rate in a two-dimensional model is seen to e ectively look like reduced Langevin recombination at open circuit. Replicating the J-U curves obtained in the two-dimensional model is, however, not possible by introducing a constant reduction factor in the Langevin recombination rate. The impact of an acceptor domain in the pure donor phase was investigated. Two cases were considered, one where the acceptor domain is isolated and another where it is connected to the bulk of the acceptor. A comparison to the case where no isolated domains exist was done in order to quantify the observed reduction in the photocurrent. The results show that all charges generated at the isolated domain are lost to recombination, but the domain does not have a major impact on charge transport. Trap-assisted recombination at interfacial trap states was investigated, as well as the surface dipole caused by the trapped charges. A theoretical expression for the ideality factor n_id as a function of generation was derived and shown to agree with simulation data. When the theoretical expression was fitted to simulation data, no interface dipole was observed.