Home About us Contact
|
Home About us Contact | ||
|
|
||
Constructive Interference (constructive + interference)
Selected AbstractsCentral brightening due to constructive interference with, without, and despite dielectric resonance,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2005Christopher M. Collins PhD Abstract Purpose To aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head. Materials and Methods We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere. Results With two strategically placed surface coils it is possible to create central brightening even when one coil alone excites an image intensity pattern either bright on one side only or bright on both sides with central darkening. This central brightening can be created with strategic coil placement even when the resonant pattern would favor central darkening. Results in a conductive sample show that central brightening can similarly be achieved in weakly conductive dielectric materials where any true resonances would be heavily damped, such as in human tissues. Conclusion Constructive interference and wavelength effects are likely bigger contributors to central brightening in MR images of weakly conductive biological samples than is true dielectric resonance. J. Magn. Reson. Imaging 2005;21:192,196. © 2005 Wiley-Liss, Inc. [source] Australian drought: the interference of multi-spectral global standing modes and travelling wavesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2003Warren B. White Abstract Extreme drought has devastated the flora, fauna, and regional economy in rangeland grazing districts over Australia for 3,5 years duration every 20 to 30 years throughout the 20th century. We investigate the source of drought occurring in five example grazing districts in eastern and central Australia. We find year-to-year variability in grazing district rainfall (GDR) during the summer rainy season (November to March) composed of quasi-biennial, interannual, quasi-decadal, and interdecadal signals from 1900 to 1999. However, the longer period signals dominate, accounting for the interdecadal quasi-periodicity of the drought/flood cycle. We find these GDR signals associated with corresponding global standing modes and travelling waves in covarying sea-surface temperature (SST) and sea-level pressure (SLP) anomalies. These global SST/SLP modes/waves influence the GDR signals by altering the troposphere moisture flux converging onto the grazing districts from regional tropical and extra-tropical oceanic source regions. We construct statistical models to determine whether the evolution of these global SST/SLP modes/waves over the oceanic source regions can hindcast corresponding GDR signals from one year to the next. When these models allow for modulation of the modes/waves, they hindcast ,1/3 of the variance in the GDR indices at 1 year lead, including the drought episodes. We find drought resulting from the constructive interference of the dry phases of the quasi-decadal and interdecadal global SST/SLP modes/waves, accompanied by a weakening of year-to-year variability associated with either weak quasi-biennial and interannual modes/waves or their destructive interference. Copyright © 2003 Royal Meteorological Society [source] Central brightening due to constructive interference with, without, and despite dielectric resonance,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2005Christopher M. Collins PhD Abstract Purpose To aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head. Materials and Methods We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere. Results With two strategically placed surface coils it is possible to create central brightening even when one coil alone excites an image intensity pattern either bright on one side only or bright on both sides with central darkening. This central brightening can be created with strategic coil placement even when the resonant pattern would favor central darkening. Results in a conductive sample show that central brightening can similarly be achieved in weakly conductive dielectric materials where any true resonances would be heavily damped, such as in human tissues. Conclusion Constructive interference and wavelength effects are likely bigger contributors to central brightening in MR images of weakly conductive biological samples than is true dielectric resonance. J. Magn. Reson. Imaging 2005;21:192,196. © 2005 Wiley-Liss, Inc. [source] Magnetic Resonance Imaging in Patients Diagnosed With Papilledema: A Comparison of 6 Different High-Resolution T1- and T2(*)-Weighted 3-Dimensional and 2-Dimensional SequencesJOURNAL OF NEUROIMAGING, Issue 2 2002Johannes Seitz MD ABSTRACT Purpose. To evaluate visualization and signal characteristics of macroscopic changes in patients with ophthalmologically stated papilledema and to find a suitable high-resolution magnetic resonance imaging (MRI) protocol. Method. Nine consecutive patients with 12 ophthalmologically stated papilledemas underwent MRI of the head and orbits, which consisted of the following high-resolution sequences: 3-dimensional (3D), T2*-weighted (T2*w) constructive interference in steady-state sequence (CISS); 3D, T1-weighted (T1w) magnetization prepared-rapid gradient echo sequence (MP-RAGE) (with and without intravenous contrast medium); transverse 3D and 2-dimensional (2D) (2mm), T2-weighted (T2w) turbo spin echo (TSE); transverse 2D (2mm), contrast-enhanced T1w TSE with fat-suppression technique; and transverse 2D (5mm), T2w TSE. A quantitative and qualitative evaluation of the papilla, optic nerve, optic nerve sheath, optic chiasm, and the brain was performed. The 6 high-resolution sequences were compared. Results. The elevation of the optic disc into the optic globe in ophthalmologically stated papilledema was best visualized in T2w, 3D CISS sequence. The pathological contrast enhancement was best seen in T1w contrast-enhanced 2D TSE sequence with fat-suppression technique. The mean width of the optic nerve sheath directly behind the globe was 7.54 mm (± 1.05 mm) in the pathological eyes, compared to 5.52 mm (± 1.11 mm) in the normal eyes. In all patients, the cerebral indices calculated showed no signs of increased intracranial pressure or other abnormalities changing the volume of the brain or ventricles. The contrast of the orbital fat versus the optic nerve sheath, the optic nerve sheath versus the surrounding cerebrospinal fluid (CSF), the surrounding CSF versus the optic nerve, the optic chiasm versus the CSF, and the optic papilla versus the optic globe were best visualized in the 3D, T2*w CISS sequence. An enhancement of the swollen optic nerve head was best seen in all 12 cases in the T1w contrast-enhanced 2D TSE sequence with fat-suppression technique. Conclusion. An MRI protocol consisting of a 5-mm transverse T2w TSE sequence; a T2*w, 3D CISS sequence; a T1w, 3D MP-RAGE sequence with and without contrast medium; and a transverse T1w, (2-mm) 2D TSE sequence with fat-suppression technique with intravenous contrast medium is suitable to visualize the macroscopic changes in papilledema. In addition, this combination is an excellent technique for the examination of the orbits and the brain. [source] Resonant gravity-wave drag enhancement in linear stratified flow over mountainsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 609 2005M. A. C. Teixeira Abstract High-drag states produced in stratified flow over a 2D ridge and an axisymmetric mountain are investigated using a linear, hydrostatic, analytical model. A wind profile is assumed where the background velocity is constant up to a height z1 and then decreases linearly, and the internal gravity-wave solutions are calculated exactly. In flow over a 2D ridge, the normalized surface drag is given by a closed-form analytical expression, while in flow over an axisymmetric mountain it is given by an expression involving a simple 1D integral. The drag is found to depend on two dimensionless parameters: a dimensionless height formed with z1, and the Richardson number, Ri, in the shear layer. The drag oscillates as z1 increases, with a period of half the hydrostatic vertical wavelength of the gravity waves. The amplitude of this modulation increases as Ri decreases. This behaviour is due to wave reflection at z1. Drag maxima correspond to constructive interference of the upward- and downward-propagating waves in the region z
| | ||||||||||||