Andrej Duh

Magnetic Resonance Imaging System Based on Earth's Magnetic Field

Abstract This article describes both the setup and the use of a system for magnetic resonance imaging (MRI) in the Earths magnetic field. Phase instability caused by temporal fluctuations of Earths field can be successfully improved by using a reference signal from a separate Earths field nuclear magnetic resonance (NMR) spectrometer/magnetometer. In imaging, it is important to correctly determine the phase of the NMR signal. A reference signal of a fixed‐frequency oscillator cannot be used since the Larmor frequency changes with time, following temporal fluctuations of Earths magnetic field. The reference signal frequency and phase, provided by a separate NMR spectrometer, change in the same way as Earths field, creating thereby, a stable rotating frame of reference for the measured signal. In principle, excellent homogeneity of the magnetic field enables scanning of very large volume samples. Reduction in S/N ratio due to the weak magnetic field can be partly compensated by the receiving coil design and shielding of electromagnetic pick‐up in audio frequency (AF) range. The smallest voxel examined so far is on the order of 50 mm3. Unlike in the case of strong magnetic fields, detection and processing of low frequency signal are less demanding for the electronics. The techniques used and the results of measurements are briefly presented.

Diffusion and flow in a porous structure by the gradient spin echo spectral analysis

The frequency analysis of relation between the NMR gradient spin echo method and the correlation of molecular motion throws a new light upon the measurement of molecular transport in porous media by magnetic resonance. The spectral analysis provides, in some other way, a known ffiffiffiffiffiffi Dt p early time dependence of attenuation or the pulse gradient spin echo sequence, and at intermediate times, it gives a not-known Dpt þ dð1 � expð� t=trÞÞ: When the displacements are getting larger than the size of compartments, the spin echo is levelling into a time-independent asymptote. In the system of packed poly-dispersed beds, the spin echo measurement of flow dispersion perpendicular to flows confirms the predicted spin echo decay. It demonstrates a clear distinction between different time regimes of signal decay, from which different properties of the porous structure can be revealed. The results gives almost identical long-time dispersion coefficient, D 0 ¼ Dp; for different flows, but the shortening of the dispersion correlation time tr with the increase of interstitial velocity. In combination with the modulated gradient sequence, the method extends the measuring range of spin echo over multi-pore length scale, and opens a new way to provide information about important properties of porous media like average pore size, the interconnectivity and the tortuosity. r 2001 Elsevier Science B.V. All rights reserved.

Enhancing Identification of Opinion Spammer Groups

Consumer-generated media buzz has significant influence on brands and is also the reason that has forced companies to communicate with consumers through consumer-generated channels. Recent investigations about brand recognition however show that many prominent brands are using fake followers or fake opinions for their promotion or to discredit their opponents. It is of paramount importance to detect such fake activities to ensure that the Web remains a trusted source of valuable information. So far, it is still quite easy to detect fake followers. On the other hand fake opinions are very hard to recognize as fake ones by just reading them. Most of the research in the field of fake opinions has been targeting the detection of fake opinions or reviews originating from single opinion spammers. Since groups of opinion spammers can simply mislead the most sophisticated algorithms that focus on content of the fake opinion we propose a novel idea to enhance the detection of fake profiles based on content originated by such profiles using additional features from quantitative psycho-linguistic text analytics tools.

The elimination of magnetic susceptibility artifacts in the micro-image of liquid-solid interfaces: internal gradient modulation by the CPMG RF train

Distortions of magnetic resonance images near solid-liquid interface appear as the result of the restriction to spin self-diffusion in the proximity of impermeable boundary as well as of a susceptibility difference. The spectral analysis of spin echo enables to resolve, in a simple way, how various RF-gradient pulse sequences reduce the effect of the internal magnetic field induced by the susceptibility difference at interfaces. The 1D diffusion-weighted imaging of water in the narrow notch tested efficiency of some sequence. The notch was milled in a piece of Plexiglas. The method can be used to distinguish the susceptibility effect from the effects of applied gradients when investigating the transport of fluid through a porous structure.

Guerrilla Media: Interactive Social Media

In the convergence culture we are witnessing the rise of social or consumer-generated media—social networks, social blogs, weblogs, podcasts, pictures and videos—that increase communication intensity between people. Key properties of social media, such as frequency, immediacy, permanence and interactivity, should also be the key properties of traditional media—television and newspapers. Unfortunately this is rarely the case.

Collective behaviour of social bots is encoded in their temporal Twitter activity.

Computational propaganda deploys social or political bots to try to shape, steer, and manipulate online public discussions and influence decisions. Collective behavior of populations of social bots has not been yet widely studied, although understanding of collective patterns arising from interactions between bots would aid social bot detection. In this study, we show that there are significant differences in collective behavior between population of bots and population of humans as detected from their Twitter activity. Using a large dataset of tweets we have collected during the UK-EU referendum campaign, we separated users into population of bots and population of humans based on the length of sequences of their high-frequency tweeting activity. We show that, while pairwise correlations between users are weak, they co-exist with collective correlated states; however the statistics of correlations and co-spiking probability differ in both populations. Our results demonstrate that populations of social bots and human users in social media exhibit collective properties similar to the ones found in social and biological systems placed near a critical point.

Pulse Gradient Spin Echo Measurement of Flow Dynamics in a Porous Structure: NMR Spectral Analysis of Motional Correlations

NMR measurement of spin migrations by the spin-echo [1] and magnetic resonance microscopy [2] have important implication on understanding of the molecular transport in porous media. Among various approaches, the analysis of self-diffusion and flow through a porous structure by Modulated Gradient Spin Echo method (MGSE) demonstrated that a properly shaped gradient sequence can be is a powerful non-invasive probe for study of molecular dynamics by measuring the low frequency features of the velocity self-correlation function (VCF) [3, 4]. The method provides new information that might be relevant to a wide range of scientific, technological and medical inquiries such as oil reservoir appraisal and management, aquifers behaviour, distillation and filtration processes, heterogeneous catalyst bed design and performance, ion channelling through membranes, cell migration in biological processes etc.