1,387 research outputs found
Generalized Signal Models and Direct FID-Based Dielectric Parameter Retrieval in MRI
No full textIn this article, we present full-wave signal models for magnetic and electric field measurements in magnetic resonance imaging (MRI). Our analysis is based on a scattering formalism in which the presence of an object or body is taken into account via an electric scattering source. We show that these signal models can be evaluated, provided that Green's tensors of the background field are known along with the dielectric parameters of the object and the magnetization within the excited part of the object. Furthermore, explicit signal expressions are derived in the case of a small homogeneous ball that is embedded in free space and for which the quasi-static Born approximation can be applied. The conductivity and permittivity of the ball appear as explicit parameters in the resulting signal models and allow us to study the sensitivity of the measured signals with respect to these dielectric parameters. Moreover, for free induction decay signals, we show through simulations that, under certain conditions, it is possible to retrieve the dielectric parameters of the ball from noise-contaminated induction decay signals that are based on electric or magnetic field measurements. Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Signal Processing System
Mt. Borah
No full textA mountain rises above some wooded foothills. Description reads: ""Telephoto view of Mt. Borah (12,655 ft. elevation) highest mountain in Idaho, taken from Grazing Service CCC Camp Chilly #111. Forest: Challis, State: Idaho, Date: 7/1940, Author: P.S. Bieler""
Hyndman Peak
No full textA mountain is visible across a valley and between two hills. Description reads: ""Hyndman Peak (12,078 ft. elevation) as seen from upper Big Lost River near Kane Creek on Forest Road to Ketchum. Forest: Challis, State: Idaho, Date: 7/1940, Author: P.S. Bieler""
MRI based Electrical Properties Tomography: Electromagnetic inversion
No full textIn magnetic resonance imaging (MRI) the interest in electric properties tomography (EPT) is growing. In current EPT applications the reconstruction is performed based on the Helmholtz equation which relies on the assumption of a homogeneous contrast. The goal of this thesis is to present new approaches to reconstruct the electrical properties that require less assumptions on the contrast. Two fundamentally new approaches are presented, one based on first order differentiation and one on the global integral field equations using a contrast-source variable. In this thesis these methods are described alongside the existing Helmholz based approach, the contrast source inversion (CSI) - EPT approach, and a deconvolution approach. Reconstruction of both two- and three-dimensional simulations as well as the reconstruction of an in vivo measurement are performed to compare the five different methods. It can be concluded from this comparison that methods that are not based on the homogeneous contrast assumption perform much more accurate (overal) than the Helmholtz equation based method. Both CSI and the direct inversion method based on the global integral equations perform comparable, but the latter is significantly faster and offers almost the same range of flexibility regarding regularisation and preconditioning. The direct inversion method is an improvement on the deconvolution method, performing equally well regarding noise robustness, but offering better reconstructions in all cases due to the lack of an apodisation step. The first order differential method provides a surprisingly robust, accurate and extremely fast way to get insight into the data, and shows that the inversion problem in MRI is actually very well behaved as far as inversion problems go. These new methods provide new insight into the inversion problem in MRI, specifically for EPT and get us one step closer to accurate electric properties reconstruction from an MRI scan.Electrical Engineering, Mathematics and Computer ScienceMicroelectronicsElectrical Engineering — Signals and System
Electromagnetic Fields in MRI: Analytical Methods and Applications
No full textElectrical properties, the conductivity and permittivity of tissue, are quantities that describe the interaction of an object and electromagnetic fields. These properties influence electromagnetic fields and are influenced themselves by physiological phe- nomena such as lesions or a stroke. Therefore, they are important in identifying or diagnosing the severity of pathologies, and they are essential in magnetic resonance imaging (MRI) safety and efficiency by determining tissue heating or sensitivity to excitation pulses and antenna designs. In two-dimensional electromagnetic fields, which occur in specific measurement geometries, it is possible to simplify the relationship between electromagnetic fields and electrical properties, and reconstruct these properties using essentially a forward operation, foregoing a full inversion scheme. These insights also help to find, and ex- plain, the cause of specific artefacts, such as those caused by mismatches in incident field used in the computation of the full electromagnetic fields. The two-dimensional field assumption necessary for the simplified relationship described above is subsequently tested, and it is shown that this assumption does not hold when the object is sufficiently translation variant in the longitudinal direction. That is, even if the fields for a translation invariant object would be two-dimensional, they become three-dimensional through the interaction of the tissue parameters with the fields, which cause out of plane current and field contributions. Another interesting application of closed form expressions between currents and fields is the target field method, which solves the inverse source problem between electric currents and static magnetic fields in a regularised manner by constraining their relationship to a cylindrical geometry. This method is adapted for transverse oriented magnetic fields to be used with Halbach type magnet arrays, and an open source tool is developed to make the method easy to apply for various design con- siderations. Moving away from constraints on the field or current structure, we show the intri- cate relationship between electrical properties and the measured signal in an MRI scanner. This is done by deriving the electro- (and magneto-) motive force for a typ- ical MRI scenario without any assumptions on the object or electro-magnetic fields. This model can then even be used to reconstruct electrical properties from the sim- plest MRI signal, namely the free induced decay (FID) signal. To round off our investigation of tissue properties we take a small detour to the magnetic tissue property, the permeability or magnetic susceptibility. For reconstruct- ing this tissue property a dipole deconvolution is required, where the dipole convolu- tion loses information of the original object through the zeros of the dipole kernel. A new machine learning based approach to reconstruct the lost information is investi- gated in the final chapter of this thesis.Signal Processing System
Author Correction: New perspectives on Neanderthal dispersal and turnover from Stajnia Cave (Poland)
No full textThe Author contributions section now reads:“W.N., A.N. and S.T. designed research; A.P., M.H., W.N., S.B., M.U., A.M., H.F., M.D.B., P.S., K.S., M.Ż., A.W., A.N. and S.T. performed research; A.P., M.H., W.N., S.B., M.U., A.M., H.F., M.D.B., P.S., K.S., M.Ż., A.W., A.N. and S.T. analysed data; A.P., M.H., S.T., W.N. and S.B. wrote the paper with the collaboration of all the co-authors.
P.S.: Further Thoughts from a Lifetime of Listening
No full textThe pieces in P.S. reflect Studs’s wide-ranging interests and travels, as well as his abiding connection to his hometown, Chicago. Here we have a fascinating conversation with James Baldwin, possibly Studs’s finest interview with an author; pieces on the colorful history and culture of Chicago; vivid portraits of Studs’s heroes and cohorts (including an insightful and still timely interview with songwriter Yip Harburg, known for his “Brother, Can You Spare a Dime”); and the transcript of Studs’s famous broadcast on the Depression, the very moving essence of what was to become Hard Times.https://corescholar.libraries.wright.edu/dlpp_all/1204/thumbnail.jp
Author Correction:A 41,500 year-old decorated ivory pendant from Stajnia Cave (Poland)
No full textCorrection to: Scientific Reports https://doi.org/10.1038/s41598-021-01221-6, published online 25 November 2021The original version of this Article contained errors in the author list where Marjolein D. Bosch was omitted from the author list, and Mikołaj Urbanowski was incorrectly listed as an author of the original Article, and has subsequently been removed.The Author contributions section now reads:“S.T. W.N. and A.N. conceived the project; S.T., W.N., A.P., M.B., S.C., M.D., H.F., A.M., M.D. B., D.P., M.P.R., C.M.R., V.S-M., G.M.S., P.S., M.S., K.S., A.V., F.W., H.W., A.W., M.Z., S.B., A.N., J-J. H., performed research; S.T., A.P., W.N., M.B., M.D.B., S.C., M.D., H.F., A.M., D.P., M.P.R., C.M.R., V.S-M., G.M.S., P.S., M.S., K.S., A.V., F.W., H.W., A.W., M.Z., S.B., A.N., J-J. H. analysed all archaeological data; S.T. and A.P. wrote the paper with the collaboration of all the co-authors.”The original Article and its accompanying Supplementary Information file have been corrected
Limitations of 2-D Field Structure Assumptions in Electrical Properties Tomography and its 3-D CSI-EPT Solution
No full textCSI-EPT is an Electrical Properties Tomography (EPT) reconstruction method that uses a Contrast Source Inversion (CSI) optimization approach to retrieve the conductivity and permittivity profiles of tissue based on -data. The method can handle variations in tissue profiles and was originally implemented for profile reconstructions in the midplane of a birdcage coil, where the RF field exhibits an E-polarized field structure [1]. Recently, CSI-EPT has been extended to a fully 3-D volumetric reconstruction method that is generally applicable (in- or outside the midplane) and no particular field structure or smoothness is assumed [2]. This is a major step towards turning CSI-EPT into a practical reconstruction method. Unfortunately, the computation times significantly increase (hours or even days, depending on the reconstruction domain of interest) and from this point of view a 2-D approach may be preferable. We show, however, that a 2-D approach is only warranted under very specific circumstances and having an E-polarized field structure is a necessary but not sufficient condition. In particular, we show that to obtain accurate tissue reconstructions based on 3-D -data, it is in general necessary to take all electromagnetic field components into account and a 2-D reconstruction approach will lead to reconstruction artefacts.Signal Processing System
Wanderers' Attitude to Soviet Power: Old Believer P.S. Morozov's Unknown Work
No full textРассматривается неизвестное ранее историческое сочинение старовера-странника П. С. Морозова, написанное в 1920 г. для председателя ВЦИК М. И. Калинина с целью защиты старообрядцев города Данилова Ярославской губернии от преследований местных чекистов. Главное внимание уделено особенностям осмысления старовером проблемы отношения своего согласия к советской власти.The author studies the previously unknown historic work of P.S. Morozov, a strannik (wanderer) Old Believer, written in 1920 for M.I. Kalinin, the All-Russian Central Executive Committee (VTsIK) Chairman, and intended to defend the Old Believers in the town of Danilov, Yaroslavl Province, from being pursued by Cheka officers. The author's main attention is focused on Old Believers' understanding of the attitude problem to Soviet power
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