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Bulk Motion (bulk + motion)
Selected AbstractsBulk motion of ultrarelativistic conical blazar jetsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006Gopal-Krishna ABSTRACT Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales, we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux-variability and proper-motion measurements. This is in accord with our earlier finding that such ultrarelativistic (Lorentz factor, , > 30) conical jets can reconcile the relatively slow apparent motions of Very Long Baseline Interferometry (VLBI) knots in TeV blazars with the extremely fast flows implied by their rapid ,-ray variability. This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account. [source] TSE with average-specific phase encoding ordering for motion detection and artifact suppressionJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2007Ling Zhang ME Abstract Purpose To detect motion-corrupted measurements in multiaverage turbo-spin-echo (TSE) acquisitions and reduce motion artifacts in reconstructed images. Materials and Methods An average-specific phase encoding (PE) ordering scheme was developed for multiaverage TSE sequences in which each echo train is assigned a unique PE pattern for each preaveraged image (PAI). A motion detection algorithm is developed based on this new PE ordering to identify which echo trains in which PAIs are motion-corrupted. The detected PE views are discarded and replaced by uncorrupted k-space data of the nearest PAI. Both phantom and human studies were performed to investigate the effectiveness of motion artifact reduction using the proposed method. Results Motion-corrupted echo trains were successfully detected in all phantom and human experiments. Significant motion artifact suppression has been achieved for most studies. The residual artifacts in the reconstructed images are mainly caused by residual inconsistencies that remain after the corrupted k-space data is corrected. Conclusion The proposed method combines a novel data acquisition scheme, a robust motion detection algorithm, and a simple motion correction algorithm. It is effective in reducing motion artifacts for images corrupted by either bulk motion or local motion that occasionally happens during data acquisition. J. Magn. Reson. Imaging 2007;25:1271,1282. © 2007 Wiley-Liss, Inc. [source] Implications of bulk motion for diffusion-weighted imaging experiments: Effects, mechanisms, and solutionsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2001David 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] Automatic quality assessment in structural brain magnetic resonance imaging,MAGNETIC RESONANCE IN MEDICINE, Issue 2 2009Bénédicte Mortamet Abstract MRI has evolved into an important diagnostic technique in medical imaging. However, reliability of the derived diagnosis can be degraded by artifacts, which challenge both radiologists and automatic computer-aided diagnosis. This work proposes a fully-automatic method for measuring image quality of three-dimensional (3D) structural MRI. Quality measures are derived by analyzing the air background of magnitude images and are capable of detecting image degradation from several sources, including bulk motion, residual magnetization from incomplete spoiling, blurring, and ghosting. The method has been validated on 749 3D T1 -weighted 1.5T and 3T head scans acquired at 36 Alzheimer's Disease Neuroimaging Initiative (ADNI) study sites operating with various software and hardware combinations. Results are compared against qualitative grades assigned by the ADNI quality control center (taken as the reference standard). The derived quality indices are independent of the MRI system used and agree with the reference standard quality ratings with high sensitivity and specificity (>85%). The proposed procedures for quality assessment could be of great value for both research and routine clinical imaging. It could greatly improve workflow through its ability to rule out the need for a repeat scan while the patient is still in the magnet bore. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] Fast, three-dimensional free-breathing MR imaging of myocardial infarction: A feasibility studyMAGNETIC RESONANCE IN MEDICINE, Issue 5 2004Manojkumar Saranathan Abstract Imaging delayed hyperenhancement of myocardial infarction is most commonly performed using an inversion recovery (IR) prepared 2D breathhold segmented k -space gradient echo (FGRE) sequence. Since only one slice is acquired per breathhold in this technique, 12,16 successive breathholds are required for complete anatomical coverage of the heart. This prolongs the overall scan time and may be exhausting for patients. A navigator-echo gated, free-breathing, 3D FGRE sequence is proposed that can be used to acquire a single slab covering the entire heart with high spatial resolution. The use of a new variable sampling in time (VAST) acquisition scheme enables the entire 3D volume to be acquired in 1.5,2 min, minimizing artifacts from bulk motion and diaphragmatic drift and contrast variations due to contrast media washout. Magn Reson Med 51:1055,1060, 2004. © 2004 Wiley-Liss, Inc. [source] Forced Alveolar Flows and Mixing in the LungPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009David Borer The air flows deep inside the lung are not only important in gas exchange processes but they also determine the efficiency of particle deposition and retention. The study aims at quantifying the relative influence of different flow components in the transport of small particles in alveolar geometries such as convective breathing patterns, wall movement, gravitational settling and Brownian motion. In addition, the possibility and efficiency of external forcing is studied, relying on the mechanism of internal acoustic streaming. A viscous oscillating boundary layer flow is converted into a steady, viscosity-independent bulk motion which is very efficient at low Reynolds numbers. The streaming can be controlled by external parameters (excitation amplitude, frequency, beam shape) and may thus be of diagnostic and therapeutic relevance. Numerical simulations are performed to analyze the flow patterns in 3D model geometries and to measure deposition rates. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Cluster correlations in redshift spaceMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002N.D. Padilla We test an analytic model for the two-point correlations of galaxy clusters in redshift space using the Hubble volume N -body simulations. The correlation function of clusters shows no enhancement along the line of sight, owing to the lack of any virialized structures in the cluster distribution. However, the distortion of the clustering pattern arising from coherent bulk motions is clearly visible. The distribution of cluster peculiar motions is well described by a Gaussian, except in the extreme high-velocity tails. The simulations produce a small but significant number of clusters with large peculiar motions. The form of the redshift-space power spectrum is strongly influenced by errors in measured cluster redshifts in extant surveys. When these errors are taken into account, the model reproduces the power spectrum recovered from the simulation to an accuracy of 15 per cent or better over a decade in wavenumber. We compare our analytic predictions with the power spectrum measured from the APM cluster redshift survey. The cluster power spectrum constrains the amplitude of density fluctuations, as measured by the linear rms variance in spheres of radius 8 h,1 Mpc, denoted by ,8. When combined with the constraints on ,8 and the density parameter , derived from the local abundance of clusters, we find a best-fitting cold dark matter model with and , for a power spectrum shape that matches that measured for galaxies. However, for the best-fitting value of , and given the value of Hubble's constant from recent measurements, the assumed shape of the power spectrum is incompatible with the most readily motivated predictions from the cold dark matter paradigm. [source] | ||||||||||