scholarly journals Characterization and correction of time‐varying eddy currents for diffusion MRI

2021 ◽  
Author(s):  
Jake J. Valsamis ◽  
Paul I. Dubovan ◽  
Corey A. Baron
Keyword(s):  
Diffusion Mri ◽  
Eddy Currents ◽  
Time Varying

scholarly journals New insights about time-varying diffusivity and its estimation from diffusion MRI

2016 ◽  
Vol 78 (2) ◽  
pp. 763-774 ◽  
Author(s):  
Lipeng Ning ◽  
Kawin Setsompop ◽  
Carl-Fredrik Westin ◽  
Yogesh Rathi
Keyword(s):  
Diffusion Mri ◽  
Time Varying

scholarly journals Evaluating corrections for Eddy‐currents and other EPI distortions in diffusion MRI: methodology and a dataset for benchmarking

2018 ◽  
Vol 81 (4) ◽  
pp. 2774-2787 ◽  
Author(s):  
M. Okan Irfanoglu ◽  
Joelle Sarlls ◽  
Amritha Nayak ◽  
Carlo Pierpaoli
Keyword(s):  
Diffusion Mri ◽  
Eddy Currents

Micromagnetic calculations of eddy currents with time-varying fields

2006 ◽  
Vol 372 (1-2) ◽  
pp. 290-293
Author(s):  
Levent Yanik ◽  
Edward Della Torre ◽  
Michael J. Donahue
Keyword(s):  
Eddy Currents ◽  
Time Varying

scholarly journals High-fidelity, high-spatial-resolution diffusion MRI of the ex-vivo whole human brain on the 3T Connectom scanner using structured low-rank EPI ghost correction

2021 ◽  
Author(s):  
Gabriel Ramos-Llordén ◽  
Rodrigo Lobos ◽  
Tae Hyung Kim ◽  
Qiyuan Tian ◽  
Thomas Witzel ◽  
...  
Keyword(s):  
Human Brain ◽  
Spatial Resolution ◽  
Diffusion Mri ◽  
Eddy Currents ◽  
High Spatial Resolution ◽  
Ex Vivo ◽  
Low Rank ◽  
Mesoscopic Scale ◽  
Background Magnetic Field ◽  
Strong Diffusion

Diffusion MRI (dMRI) of whole, intact, fixed postmortem human brain at high spatial resolution serves as key bridging technology for 3D mapping of structural connectivity and tissue microstructure at the mesoscopic scale. Ex vivo dMRI offers superior spatial resolution compared to in vivo dMRI but comes with its own technical challenges due to the significantly reduced T2 relaxation times and decreased diffusivity incurred by tissue fixation. The altered physical properties of fixed tissue necessitate the use of alternative acquisition strategies to preserve SNR and achieve sufficient diffusion weighting. Multi-shotor segmented 3D echo planar imaging (EPI) sequences have been used to shorten echo times (TEs) with reduced distortions from field inhomogeneity and eddy currents on small-bore MR scanners and have been adopted for high b-value dMRI of ex vivo whole human brain specimens. The advent of stronger gradients on human MRI scanners has led to improved image quality and a wider range of diffusion-encoding parameters for dMRI but at the cost of more severe eddy currents that result in spatial and temporal variations in the background magnetic field, which cannot be corrected for using standard vendor-provided ghost correction solutions. In this work, we show that conventional ghost correction techniques based on navigators and linear phase correction may be insufficient for EPI sequences using strong diffusion-sensitizing gradients in ex vivo dMRI experiments, resulting in orientationally biased dMRI estimates. This previously unreported problem is a critical roadblock in any effort to leverage scanners with ultra-high gradients for high-precision mapping of human neuroanatomy at the mesoscopic scale. We propose an advanced reconstruction method based on structured low-rank matrix modeling that reduces the ghosting substantially. We show that this method leads to more accurate and reliable dMRI metrics, as exemplified by diffusion tensor imaging and high angular diffusion imaging analyses in distributed neuroanatomical areas of fixed whole human brain specimens. Our findings advocate for the use of advanced reconstruction techniques for recovering unbiased metrics from ex vivo dMRI acquisitions and represent a crucial step toward making full use of strong diffusion-encoding gradients for neuroscientific studies seeking to study brain structure at multiple spatial scales.

Improved Concept and Model of Eddy Current Damper

2005 ◽  
Vol 128 (3) ◽  
pp. 294-302 ◽  
Author(s):  
Henry A. Sodano ◽  
Jae-Sung Bae ◽  
Daniel J. Inman ◽  
W. Keith Belvin
Keyword(s):  
Magnetic Field ◽  
Theoretical Model ◽  
Eddy Current ◽  
Eddy Currents ◽  
Opposite Polarity ◽  
Image Method ◽  
Time Varying ◽  
Transverse Beam ◽  
Eddy Current Damper ◽  
Improved Model

When a conductive material experiences a time-varying magnetic field, eddy currents are generated in the conductor. These eddy currents circulate such that they generate a magnetic field of their own, however the field generated is of opposite polarity, causing a repulsive force. The time-varying magnetic field needed to produce such currents can be induced either by movement of the conductor in the field or by changing the strength or position of the source of the magnetic field. In the case of a dynamic system the conductor is moving relative to the magnetic source, thus generating eddy currents that will dissipate into heat due to the resistivity of the conductor. This process of the generation and dissipation of eddy current causes the system to function as a viscous damper. In a previous study, the concept and theoretical model was developed for one eddy current damping system that was shown to be effective in the suppression of transverse beam vibrations. The mathematical model developed to predict the amount of damping induced on the structure was shown to be accurate when the magnet was far from the beam but was less accurate for the case that the gap between the magnet and beam was small. In the present study, an improved theoretical model of the previously developed system will be formulated using the image method, thus allowing the eddy current density to be more accurately computed. In addition to the development of an improved model, an improved concept of the eddy current damper configuration is developed, modeled, and tested. The new damper configuration adds significantly more damping to the structure than the previously implemented design and has the capability to critically damp the beam’s first bending mode. The eddy current damper is a noncontacting system, thus allowing it to be easily applied and able to add significant damping to the structure without changing dynamic response. Furthermore, the previous model and the improved model will be applied to the new damper design and the enhanced accuracy of this new theoretical model will be proven.

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