Phase Reordering Research Articles

3D coronary artery imaging with phase reordering for improved scan efficiency

3D coronary artery imaging with phase reordering for improved scan efficiency

MR imaging artifacts that simulate disease: how to recognize and eliminate them.

Occasionally, artifacts may simulate pathologic conditions on magnetic resonance (MR) images. Motion artifacts especially affect images of the chest and abdomen. There are a number of techniques for reducing motion artifacts, including respiratory and cardiac gating, k-space phase reordering, gradient moment nulling, even echo rephasing, and physical restraints. Aliasing occurs when the field of view does not include all of the anatomic structures present in the imaged section. Aliasing artifacts can be eliminated by increasing the field of view, oversampling, and use of saturation pulses or surface coils. Truncation artifacts represent the difference between the original and the reconstructed image and can be reduced with data extrapolation algorithms or image filtering. Chemical shift artifacts and magnetic susceptibility artifacts are due to a local deformity of the magnetic field, resulting in spatial misregistration. Chemical shift artifacts are more severe in images acquired with a narrow-bandwidth technique; magnetic susceptibility artifacts are more severe in images acquired with a long echo time. Pitfalls in the interpretation of MR images can be avoided by becoming familiar with the appearances and causes of common MR imaging artifacts.

Electroencephalogram and visual evoked potential generation in a mathematical model of coupled cortical columns.

This study deals with neurophysiologically based models simulating electrical brain activity (i.e., the electroencephalogram or EEG, and evoked potentials or EPs). A previously developed lumped-parameter model of a single cortical column was implemented using a more accurate computational procedure. Anatomically acceptable values for the various model parameters were determined, and a multi-dimensional exploration of the model parameter-space was conducted. It was found that the model could produce a large variety of EEG-like waveforms and rhythms. Coupling two models, with delays in the interconnections to simulate the synaptic connections within and between cortical areas, made it possible to replicate the spatial distribution of alpha and beta activity. EPs were simulated by presenting pulses to the input of the coupled models. In general, the responses were more realistic than those produced using a single model. Our simulations also suggest that the scalp-recorded EP is at least partially due to a phase reordering of the ongoing activity.

Laser Raman study of the low-temperature phase transformation in [N(CH3)4]2CuCl4 single crystals

Abstract The results of a detailed temperature-dependent Raman study of single crystals of [N(CH 3 ) 4 ] 2 CuCl 4 are reported in different spectral regions. The observed splittings of the symmetric stretching modes of the N(CH 3 ) + 4 ion below 250 K and the CuCl 2− 4 ion below 50 K into doublets have been explained as being due to the presence of ions on two energetically inequivalent positions within the lattice. From a temperature dependence of the half-widths, frequency separation, relative intensities of the two components of the CN stretch and other thermosensitive spectral changes, we have discovered a new first order type phase transition in this system at ∼50 K due to ordering of ions from one configuration to another. The reordering in phase IV up to 50 K takes place via continuous movement of cations from the disordered to ordered configuration, but below 50 K the ions freeze in the two inequivalent configurations. The dynamics of ordering of the ions at different temperatures is discussed in this communication.

Fast limited flip angle MR subtraction angiography.

A fast MR angiography method is introduced that is capable of generating difference images of blood vessels in scan times of 10-20 s. This is an order of magnitude faster than many previous methods. The fundamental concept of this approach is to use cardiac gating and acquire several phase encodings at least twice during each cardiac cycle using limited flip angles (LFAs) and repetition times in the 20 to 50 ms range. The encodings acquired during diastole are subtracted from those acquired during systole to generate the difference image. The contrast in the difference image is due both to the influx of unsaturated spins and to the loss of phase coherence of systolic blood moving at high velocity along a magnetic gradient. The systolic peak of the cardiac cycle is determined during reconstruction by shifting the systolic and diastolic "windows" until the difference signal is maximized. Ghost artifacts due to pulsatile flow are eliminated by a phase reordering technique similar in concept to those developed for suppression of breathing artifacts. Arteries in thick slices are successfully imaged and initial in vivo results are presented.

A fundamental investigation of the composition of auditory evoked potentials.

An investigation, carried out to establish whether auditory evoked potentials (AEP's) are due to phase reordering of the background electroencephalogram or to an additive signal, is described. New phase reordered and additive models of the AEP were introduced and used in conjunction with the techniques of angular statistics. It was established that all of the AEP's studied contained additive energy in at least one harmonic component.