1,125 research outputs found
Improving MRI Surface Coil Decoupling to Reduce B1 Distortion
As clinical MRI systems continue to advance, larger focus is being given to image
uniformity. Good image uniformity begins with generating uniform magnetic fields, which
are easily distorted by induced currents on receive-only surface coils. It has become an
industry standard to combat these induced currents by placing RF blocking networks on
surface coils. This paper explores the effect of blocking network impedance of phased array surface coils on B1 distortion. It has been found and verified, that traditional approaches for blocking network design in complex phased arrays can leave undesirable B1 distortions at 3 Tesla. The traditional approach of LC tank blocking is explored, but shifts from the idea that higher impedance equals better B1 distortion at 3T. The result is a new design principle for a tank with a finite inductive reactance at the Larmor Frequency. The solution is demonstrated via simulation using a simple, single, large tuning loop. The same loop, along with a smaller loop, is used to derive the new design principle, which is then applied to a complex phased array structure
Poise and Power
According to the author, Whatever we wish to attain or accomplish, a continuous increase of power is required. After this, Larson explains that the newest science of life discovered that the best method by which to achieve power, is poise. However, ...not poise in the usual or external sense, but POISE in a deep interior sense; that is, where we feel the soul-serenity of the limitless power of the great within. In the pages that follow, Larson explains how to achieve poise, and thus more power. He describes the path the one should follow in order to attain power, and accomplish whatever one wishes to accomplish.https://openworks.wooster.edu/motherhomeheaven/1012/thumbnail.jp
An N Server Cutoff Priority Queue Where Customers Request a Random Number of Servers
Consider a multi-priority, nonpreemptive, N-server Poisson arrival queueing system. The number of servers requested by an arrival has a known probability distribution. Service times are negative exponential. In order to save available servers for higher priority customers, arriving customers of each lower priority are deliberately queued whenever the number of servers busy equals or exceeds a given priority-dependent cutoff number. A queued priority i customer enters service the instant the number of servers busy is at most the respective cutoff number of servers minus the number of servers requested (by the customer) and all higher priority queues are empty. In other words the queueing discipline is in a sense HOL by priorities, FCFS within a priority. All servers requested by a customer start service simultaneously; service completion instants are independent. We derive the priority i waiting time distribution (in transform domain) and other system statistics
An N Server Cutoff Multi-Priority Queue
Consider a multi-priority, nonpreemptive, N-server Poisson arrival queueing system. Service times are negative exponential. In order to save available servers for higher priority customers, arriving customers of each lower priority are deliberately queued whenever the number of servers busy equals or exceeds a given priority-dependent cutoff number. A queued priority i customer enters service the instant there are fewer than the respective cutoff number of servers busy and all higher priority queues are empty. The principal result is the priority i waiting time mean, second moment, and distribution (in transforms). The analysis is extended to systems in which any subset of priority levels may overflow to some other system, rather than join infinite capacity queues. The paper concludes with illustrative computational results
Bridging length and time scales in sheared demixing systems: from the Cahn-Hilliard to the Doi-Ohta model
We develop a systematic coarse-graining procedure which establishes the
connection between models of mixtures of immiscible fluids at different length
and time scales. We start from the Cahn-Hilliard model of spinodal
decomposition in a binary fluid mixture under flow from which we derive the
coarse-grained description. The crucial step in this procedure is to identify
the relevant coarse-grained variables and find the appropriate mapping which
expresses them in terms of the more microscopic variables. In order to capture
the physics of the Doi-Ohta level, we introduce the interfacial width as an
additional variable at that level. In this way, we account for the stretching
of the interface under flow and derive analytically the convective behavior of
the relevant coarse-grained variables, which in the long wavelength limit
recovers the familiar phenomenological Doi-Ohta model. In addition, we obtain
the expression for the interfacial tension in terms of the Cahn-Hilliard
parameters as a direct result of the developed coarse-graining procedure.
Finally, by analyzing the numerical results obtained from the simulations on
the Cahn-Hilliard level, we discuss that dissipative processes at the Doi-Ohta
level are of the same origin as in the Cahn-Hilliard model. The way to estimate
the interface relaxation times of the Doi-Ohta model from the underlying
morphology dynamics simulated at the Cahn-Hilliard level is established.Comment: 29 pages, 2 figures, accepted for publication in Phys. Rev.
Intraprostatic Injection of Alcohol Gel for the Treatment of Benign Prostatic Hyperplasia: Preliminary Clinical Results
Benign prostatic hyperplasia (BPH) is one of the most common diseases ailing older men. Office-based procedures offer the advantage of being more effective than medications, while limiting the adverse effects, cost, and recovery of surgery. This study presents preliminary data on a new procedure that utilizes intraprostatic alcohol gel injection to ablate prostatic tissue. The purpose of this study is to evaluate the feasibility of using this gel as a treatment for BPH
The Role of Solid Friction in the Sedimentation of Strongly Attractive Colloidal Gels
We study experimentally and theoretically the sedimentation of gels made of
strongly aggregated colloidal particles, focussing on the long time behavior,
when mechanical equilibrium is asymptotically reached. The asymptotic gel
height is found to vary linearly with the initial height, a finding in stark
contrast with a recent study on similar gels [Manley \textit{et al.} 2005
\textit{Phys. Rev. Lett.} \textbf{94} 218302]. We show that the asymptotic
compaction results from the balance between gravity pull, network elasticity,
and solid friction between the gel and the container walls. Based on these
ingredients, we propose a simple model to account for the dependence of the
height loss on the initial height and volume fraction. As a result of our
analysis, we show that the static friction coefficient between the gel and the
container walls strongly depends on volume fraction: the higher the volume
fraction, the weaker the solid friction. This nonintuitive behavior is
explained using simple scaling arguments.Comment: 13 pages, 5 figures. Submitted to JSTA
Consistency between ARPES and STM measurements on SmB
Strongly correlated topological surface states are promising platforms for
next-generation quantum applications, but they remain elusive in real
materials. The correlated Kondo insulator SmB is one of the most promising
candidates, with theoretically predicted heavy Dirac surface states supported
by transport and scanning tunneling microscopy (STM) experiments. However, a
puzzling discrepancy appears between STM and angle-resolved photoemission
(ARPES) experiments on SmB. Although ARPES detects spin-textured surface
states, their velocity is an order of magnitude higher than expected, while the
Dirac point -- the hallmark of any topological system -- can only be inferred
deep within the bulk valence band. A significant challenge is that SmB
lacks a natural cleavage plane, resulting in ordered surface domains limited to
10s of nanometers. Here we use STM to show that surface band bending can shift
energy features by 10s of meV between domains. Starting from our STM spectra,
we simulate the full spectral function as an average over multiple domains with
different surface potentials. Our simulation shows excellent agreement with
ARPES data, and thus resolves the apparent discrepancy between large-area
measurements that average over multiple band-shifted domains and
atomically-resolved measurements within a single domain
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MEG and EEG data analysis with MNE-Python
Magnetoencephalography and electroencephalography (M/EEG) measure the weak electromagnetic signals generated by neuronal activity in the brain. Using these signals to characterize and locate neural activation in the brain is a challenge that requires expertise in physics, signal processing, statistics, and numerical methods. As part of the MNE software suite, MNE-Python is an open-source software package that addresses this challenge by providing state-of-the-art algorithms implemented in Python that cover multiple methods of data preprocessing, source localization, statistical analysis, and estimation of functional connectivity between distributed brain regions. All algorithms and utility functions are implemented in a consistent manner with well-documented interfaces, enabling users to create M/EEG data analysis pipelines by writing Python scripts. Moreover, MNE-Python is tightly integrated with the core Python libraries for scientific comptutation (NumPy, SciPy) and visualization (matplotlib and Mayavi), as well as the greater neuroimaging ecosystem in Python via the Nibabel package. The code is provided under the new BSD license allowing code reuse, even in commercial products. Although MNE-Python has only been under heavy development for a couple of years, it has rapidly evolved with expanded analysis capabilities and pedagogical tutorials because multiple labs have collaborated during code development to help share best practices. MNE-Python also gives easy access to preprocessed datasets, helping users to get started quickly and facilitating reproducibility of methods by other researchers. Full documentation, including dozens of examples, is available at http://martinos.org/mne
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