54 research outputs found
Electrical properties tomography: a methodological review
Electrical properties tomography (EPT) is an imaging method that uses a magnetic resonance (MR) system to non-invasively determine the spatial distribution of the conductivity and permittivity of the imaged object. This manuscript starts by providing clear definitions about the data required for, and acquired in, EPT, followed by comprehensively formulating the physical equations underlying a large number of analytical EPT techniques. This thorough mathematical overview of EPT harmonizes several EPT techniques in a single type of formulation and gives insight into how they act on the data and what their data requirements are. Furthermore, the review describes machine learning-based algorithms. Matlab code of several differential and iterative integral methods is available upon request.Imaging- and therapeutic targets in neoplastic and musculoskeletal inflammatory diseas
Effects of simulated error-sources on different 3-D CSI-EPT strategies
Three-dimensional contrast source inversion-electrical properties tomography (3-D CSI-EPT) is an iterative reconstruction method that estimates the electrical properties of tissue from transmit field magnetic resonance data. However, in order to bring 3-D CSI-EPT into practice for complex tissue structures and to understand the origin and effect of errors, insight in the sensitivities of reconstruction accuracy to the major error-sources is necessary. In this paper, different strategies for implementing 3-D CSI-EPT, including their iterative structure, are presented, of which the regularized implementation shows the most potential to be used in practice. Moreover, the influence of initialization, noise, stopping criteria, incident fields, B1-maps, transceive phase and domain truncation are discussed. We show that of all these different error-sources, initialization, accurate coil models and domain truncation have the most dramatic effect on electrical properties reconstructions in practice.Imaging- and therapeutic targets in neoplastic and musculoskeletal inflammatory diseas
Anisotropic EEG/MEG volume conductor modeling based on Diffusion Tensor Imaging
Die vorliegende Arbeit befasst sich mit der Volumenleitermodellierung auf
Basis der Finiten Elemente für EEG/MEG Untersuchungen unter Einbeziehung
von Anistropieinformation, die mit Hilfe der
Magnetresonanzdiffusionstensorbildgebung (MR-DTI) gewonnen wurde. Im ersten
Teil der Arbeit wurde der Einfluss unvollständig bestimmter
Wichtungsparamter (b-Matrix) auf die zu rekonstruierenden
Diffusionstensoren untersucht. Die Unvollständigkeit bezieht sich dabei auf
die Tatsache, dass im Allgemeinen nur die starken Diffusionsgradienten zur
Berechnung der b-Matrix herangezogen werden. Es wurde gezeigt, dass
besonders bei Aufnahmen mit hoher räumlicher Auflösung der Anteil der
Bildgradienten an der b-Matrix nicht mehr vernachlässigbar ist. Weiterhin
wurde gezeigt, wie man die b-Matrizen korrekt analytisch bestimmt und damit
einen systematischen Fehler vermeidet. Für den Fall, dass nicht ausreichend
Informationen zur Verfügung stehen um die analytische Bestimmung
durchzuführen, wurde eine Lösung vorgeschlagen, die es mit Hilfe von
Phantommessungen ermöglicht eine parametrisierte b-Matrix zu bestimmen. Der
zweite Teil widmet sich der Erstellung hochaufgelöster realistischer
Volumenleitermodelle detailliert beschrieben. Besonders die Transformation
der Diffusionstensordaten in Leitfähigkeitstensoren. Zudem wurde eine
Vorgehensweise beschrieben, die es erlaubt, einen T1-gewichteten
MR-Datensatz vollautomatisch in fünf verschiedene Gewebesegmente (weiches
Gewebe, graue und weiße Substanz, CSF und Schädelknochen) zu unterteilen.
Der dritte Teil der Arbeit befasst sich mit dem Einfluss der anisotropen
Leitfähigkeit in der weißen Hirnsubstanz auf EEG und MEG unter Verwendung
eines Tier- sowie eines Humanmodells. Um den Einfluss der verschiedenen
Methoden der Transformation von DTI Daten in Leitfähigkeitsdaten zu
untersuchen, wurden verschiedenen Modelle sowohl mit gemessener als auch
mit künstlicher Anisotropie erstellt. In der Tiermodellstudie wurden EEG
und in der Humanmodellstudie EEG und MEG Simulationen sowohl mit den
anisotropen Modellen als auch mit einem isotropen Modell durchgeführt und
miteinander verglichen. Dabei wurde gefunden, dass sowohl der
topographische Fehler (RDM) als auch der Magnitudenfehler stark durch das
Einbeziehen von Anisotropieinformationen beeinflusst wird. Es wurde auch
gezeigt, dass sowohl die Position als auch die Orientierung einer
dipolaren Quelle in Bezug auf das anisotrope Segment einen großen Effekt
auf die untersuchten Fehlermaße hat.In this work anisotropic electric tissue properties determined by
means of
diffusion tensor imaging were modeled into high resolution finite element
volume conductors. In first part of the work the influence of not
considering imaging gradient in the calculation of the b-matrices on the
correct determination of diffusion tensor data is shown and it was found
that especially with high resolution imaging protocols the contributions of
the imaging gradients is not negligible. It was also shown how correct
b-matrices considering all applied gradients can be calculated correctly.
For the case that information about the sequence are missing an
experimental approach of determining a parameterized b-matrix using phantom
measurements is proposed. In the second part the procedure of generating
anisotropic volume conductor models is regarded. The main focus of this
part was to facilitate the derivation of anisotropy information from DTI
measurements and the inclusion of this information into an anisotropic
volume conductor. It was shown, that it is possible to generate a
sophisticated high resolution anisotropic model without any manual steps
into five different tissue layers. The third part studied the influence of
anisotropic white matter employing an animal as well as a human model. To
compare the different ways of converting the anisotropy information from
DTI into conductivity information, different models were investigated,
having artificial as well as measured anisotropy. In the animal study the
EEG and in the human study the EEG and MEG forward solution was studies
using the anisotropic models and compared to the solution derived using an
isotropic model. It was found that both, the topography error (RDM) as well
as the magnitude error (MAG), are significantly affected if anisotropy is
considered in the volume conductor. It was also shown, that the position as
well as the orientation of the dipole with respect to white matter has a
large effect on the amount of the error quantities. Finally, it is claimed
that if one uses high resolution volume conductor models for EEG/MEG
studies, the anisotropy has to be considered, since the average error of
neglecting anisotropy is larger than the accuracy which can be achieved
using such models
The use of magnetic fields to create high-speed plasma jets for spacecraft propulsion
The aim of the work reported in this thesis has been to explore arrangements of magnetic fields and electric currents to create novel plasma thrusters that are more efficient and less complex than existing designs. Two original devices are discussed, firstly a thruster that uses a magnetic nozzle in combination with a High Power Impulse Magnetron Sputtering source (HiPIMS) to produce a jet of copper plasma and secondly, a thruster that uses the phenomena of magnetic reconnection that occurs between opposing magnetic fields in a plasma to produce a plasma jet. While HiPIMS has been normally employed to create thin films, the use of a solenoidal magnetic field to accelerate and focus the ions produced by that source has not been previously investigated as a means of creating a thruster. The HiPIMS thruster has a specific impulse (Isp) of 1543s. Magnetic reconnection has been studied for decades by geophysicists and by astronomers. Despite that effort, so far there has been little interest in exploiting the phenomena as a means of producing high-speed plasmas in a thruster despite the evidence of jets in those environments. A thruster consisting of two slit coaxial tubes of copper was constructed. Evidence for the occurrence of magnetic connection was fourfold. (1) A significant electron current that coincided with the rise of the magnetic field that was followed by a large ion current. (2) Ion currents were found to increase as the plasma became less collisional. (3) Ions with energies greater than 130 eV corresponding to a speed of 2.50 x 104 m/s and an Isp of 2550s were detected. (4) The ratio between the estimated speed of ions flowing into the diffusion layer (350m/s) and the measured speed of the out-flowing ions (2.50 x 104 m/s) was approximately 68. The physics indicates that such a thruster could have a basic energy efficiency of 50%
Construction and testing of the scintillating fibre trackers for MICE
The discovery of neutrino mass through experimental evidence of neutrino oscillations
at the end of the last century has provided the first proof that the Standard
Model of particle physics is incomplete. To be able to extend the Standard
Model to incorporate massive neutrinos first requires many theoretical uncertainties
surrounding the particle and its interactions to be understood. Therefore, a
dedicated experimental programme is needed over the coming decades to provide
precision measurements of the neutrino oscillation parameters and also a possible
measurement of CP violation in the lepton sector, which could have astrophysical
consequences.
An intense source of neutrinos is required to achieve these precision measurements
and the leading contender proposed to provide this neutrino beam, is the Neutrino
Factory. Before a Neutrino Factory facility can be realised, a number of technological
challenges need to be evaluated and understood first. One of which, is reduce the
large phase space volume (emittance) of the initial muon beam, which is eventually
stored and through decay provides the neutrino beam. Ionisation cooling is the
chosen method to achieve this and the Muon Ionisation Cooling Experiment (MICE)
at Rutherford Laboratory in the UK, is required to demonstrate ionisation cooling
and its feasibility for a Neutrino Factory.
To demonstrate ionisation cooling, a section of a cooling channel will be constructed
and single-particle measurements of emittance of a muon beam before and after the
cooling channel from particle spectrometers will be compared. To measure emittance
accurately requires precision measurements of the momenta and spatial coordinates
at the spectrometers by tracking devices in a uniform magnetic field. The focus
of this thesis is based around the construction and testing of the MICE tracker(s),
including a study of its simulated performance and also construction and testing of a prototype
Dynamical systems : mechatronics and life sciences
Proceedings of the 13th Conference „Dynamical Systems - Theory and Applications"
summarize 164 and the Springer Proceedings summarize 60 best papers of university
teachers and students, researchers and engineers from whole the world. The papers were
chosen by the International Scientific Committee from 315 papers submitted to the
conference. The reader thus obtains an overview of the recent developments of dynamical
systems and can study the most progressive tendencies in this field of science
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