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High-quality Images (high-quality + image)

Distribution by Scientific Domains
Medical Sciences40%
Earth and Environmental Science40%

Selected Abstracts

Calculation of the wave propagation angle in complex media: application to turning wave simulations

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2009
Xiaofeng Jia
SUMMARY The wave propagation angle is one of the key factors in seismic processing methods. For the dual-domain propagators, it is sometimes necessary to acquire the wave propagation angle in the space-frequency domain instead of the wavenumber domain or the angle domain. We propose a method dealing with this problem, in which the wavefield gradient is used for the calculation of the wave propagation angle. The wavefield gradient can be directly obtained by either the finite difference approximation or the marching expression of the propagator. This method is not applicable in the case of extremely low frequency due to the comparability between the wavelength and the grid interval. Combined with the superwide-angle one-way propagator, this approach is instrumental in simulating the turning wave, which is hard to be handled by the traditional one-way propagator. Numerical examples show the good performance of the superwide-angle one-way propagator with our approach involved. The turning wave is modelled accurately; as a result, a high-quality image of the overhanging salt flank can be obtained. [source]

DTI in Context: Illustrating Brain Fiber Tracts In Situ

COMPUTER GRAPHICS FORUM, Issue 3 2010
Pjotr Svetachov
Abstract We present an interactive illustrative visualization method inspired by traditional pen-and-ink illustration styles. Specifically, we explore how to provide context around DTI fiber tracts in the form of surfaces of the brain, the skull, or other objects such as tumors. These contextual surfaces are derived from either segmentation data or generated using interactive iso-surface extraction and are rendered with a flexible, slice-based hatching technique, controlled with ambient occlusion. This technique allows us to produce a consistent and frame-coherent appearance with precise control over the lines. In addition, we provide context through cutting planes onto which we render gray matter with stippling. Together, our methods not only facilitate the interactive exploration and illustration of brain fibers within their anatomical context but also allow us to produce high-quality images for print reproduction. We provide evidence for the success of our approach with an informal evaluation with domain experts. [source]

Implications of bulk motion for diffusion-weighted imaging experiments: Effects, mechanisms, and solutions

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2001
David G. Norris PhD
Abstract This review article describes the effect of bulk motion on diffusion-weighted imaging experiments, and examines methods for correcting the resulting artifacts. The emphasis throughout the article is on two-dimensional imaging of the brain. The effects of translational and rotational motion on the MR signal are described, and the literature concerning pulsatile brain motion is examined. Methods for ameliorating motion effects are divided into three generic categories. The first is methods that should be intrinsically insensitive to macroscopic motion. These include motion-compensated diffusion-weighting schemes, single-shot EPI, projection reconstruction, and line scanning. Of these, only single-shot EPI and projection reconstruction methods can obtain high-quality images without compromising on sensitivity. The second category of methods is those that can be made insensitive to bulk motion. The methods examined here are FLASH and RARE. It is shown that for both sequences motion insensitivity is in general attained only at the cost of a 50% reduction in sensitivity. The final set of methods examined are those that correct for motion, primarily navigator echoes. The properties and limitations of the navigator echo approach are presented, as are those of methods which attempt to correct the acquired data by minimizing image artifacts. The review concludes with a short summary in which the current status of diffusion imaging in the presence of bulk motion is examined. J. Magn. Reson. Imaging 2001;13:486,495. © 2001 Wiley-Liss, Inc. [source]

Diffusion imaging in humans at 7T using readout-segmented EPI and GRAPPA

MAGNETIC RESONANCE IN MEDICINE, Issue 1 2010
Robin M. Heidemann
Abstract Anatomical MRI studies at 7T have demonstrated the ability to provide high-quality images of human tissue in vivo. However, diffusion-weighted imaging at 7T is limited by the increased level of artifact associated with standard, single-shot, echo-planar imaging, even when parallel imaging techniques such as generalized autocalibrating partially parallel acquisitions (GRAPPA) are used to reduce the effective echo spacing. Readout-segmented echo-planar imaging in conjunction with parallel imaging has the potential to reduce these artifacts by allowing a further reduction in effective echo spacing during the echo-planar imaging readout. This study demonstrates that this approach does indeed provide a substantial improvement in image quality by reducing image blurring and susceptibility-based distortions, as well as by allowing the acquisition of diffusion-weighted images with a high spatial resolution. A preliminary application of the technique to high-resolution diffusion tensor imaging provided a high level of neuroanatomical detail, which should prove valuable in a wide range of applications. Magn Reson Med 64:9,14, 2010. © 2010 Wiley-Liss, Inc. [source]

Imaging geophysical data,taking the viewer into account

ARCHAEOLOGICAL PROSPECTION, Issue 1 2004
T. J. Dennis
Abstract A common way of presenting geophysical data from two-dimensional sources is as a grey-scale image. Some theoretical background to discrete image representation is described, and the deleterious effects of inappropriate (too sparse) sampling and display of such images discussed in an archaeological context. In high-quality images, such as magazine illustrations or digital television, the sampling densities can be sufficiently high to avoid the appearance of artefacts. Geophysical images in contrast are often sampled at very low densities; if the effective area of each sample is significantly less than the sample spacing, then the classic effect called ,aliasing' in communication engineering, caused by the violation of Nyquist's criterion, will be seen. Knowledge of the sensor's footprint can be used to select an appropriate sample density, and so minimize this source of distortion. To maximize the visibility of what may be low-contrast structures immersed in a high level of background noise, it is helpful also to consider the bandpass nature of the spatial frequency response of the human visual system. The non-linear phenomenon of visual masking is shown to influence the choice on presentation methods. Copyright © 2004 John Wiley & Sons, Ltd. [source]