Echo Effect Research Articles

Optimizing the sequence parameters for double-quantum CRAZED imaging.

The evolution of magnetization during repeated application of the double-quantum-(DQ)-CRAZED sequence is analyzed, with the aim of identifying sequence parameters that maximize sensitivity to signal produced by the distant dipole field (DDF). Phase cycling schemes that allow cancellation of signals following undesired coherence pathways are also described. Simulations and imaging experiments carried out at 3 T on phantoms and the human head were used to verify the analysis. The results indicate that in the absence of phase cycling, the DDF-related signal-to-noise ratio (SNR) per unit time is maximized using TR=2.05 T1, along with values of the RF flip angles (alpha approximately 90 degrees and beta approximately 60 degrees ), and echo time (TE=T2) that have previously been shown to maximize the DDF-related signal at long TR. However, with TR=2.05 T1 there can also be a significant signal contribution due to stimulated echo effects (up to 40% of the signal for water at 3 T and TE=70 ms). Using a two-step phase cycle, the stimulated echo signal is eliminated and the maximum SNR per unit time occurs for TR=2.76 T1. It is also demonstrated that sensitivity to signal changes caused by small variations in T2 is maximized by setting TE=2T2.

Electrical detection of spin coherence in silicon.

Experimental evidence is presented showing that photocurrents in silicon can be used as highly sensitive readout probes for coherent spin states of localized electrons, the prime candidates for quantum bits in various semiconductor based quantum computer concepts. Conduction electrons are subjected to fast Rabi oscillation induced by means of pulsed electron spin resonance. The collective spin motion of the charge carrier ensemble is reflected by a spin-dependent recombination rate and therefore by the sample conductivity. Because of inhomogeneities, the Rabi oscillation dephases rapidly. However, a microwave induced rephasing is possible causing an echo effect whose intensity contains information about the charge carrier spin state and the coherence decay.

Clinical and economic impact of exercise electrocardiography and exercise echocardiography in clinical practice.

Patients with known or suspected coronary disease are often investigated to facilitate risk assessment. We sought to examine the cost-effectiveness of strategies based on exercise echocardiography and exercise electrocardiography. We studied 7656 patients undergoing exercise testing; of whom half underwent exercise echocardiography. Risk was defined with the Duke treadmill score for those undergoing exercise electrocardiography alone, and by the extent of ischaemia by exercise echocardiography. Cox proportional hazards models, risk adjusted for pretest likelihood of coronary artery disease, were used to estimate time to cardiac death or myocardial infarction. Costs (including diagnostic and revascularisation procedures, hospitalisations, and events) were calculated, inflation-corrected to year 2000 using Medicare trust fund rates and discounted at a rate of 5%. A decision model was employed to assess the marginal cost effectiveness (cost/life year saved) of exercise echo compared with exercise electrocardiography. Exercise echocardiography identified more patients as low-risk (51% vs 24%, p<0.001), and fewer as intermediate- (27% vs 51%, p<0.001) and high-risk (22% vs 4%); survival was greater in low- and intermediate-risk and less in high-risk patients. Although initial procedural costs and revascularisation costs (in intermediate-high risk patients) were greater, exercise echocardiography was associated with a greater incremental life expectancy (0.2 years) and a lower use of additional diagnostic procedures when compared with exercise electrocardiography (especially in lower risk patients). Using decision analysis, exercise echocardiography ( in 2615/life year saved) was more cost effective than exercise electrocardiography. Exercise echocardiography may enhance cost-effectiveness for the detection and management of at risk patients with known or suspected coronary disease.

On the Effect of Domed Ceiling in Worship Spaces: A Scale Model Study of a Mosque

Though the importance of domes in worship buildings has been recognised since ancient times, their acoustic effects have not been entirely addressed by acoustics researchers. In addition to the known detrimental effects of late reflections and echoes, more complicated behaviour can be expected as a result of the coupling of the dome volume to the rest of the room. This investigation is focused on the impact of a dome on reverberation time. This is done using a scale model study of a typical mosque. The results show how the parameter is affected and indicate how the phenomenon might be quantified.

Combining of neutron spin echo and reflectivity: a new technique for probing surface and interface order

The recently proposed spin-echo resolved grazing-incidence scattering (SERGIS) uses the well-known neutron spin echo effect for encoding the momentum transfer in reflectometry. By the application of tilted magnetic-field borders, SERGIS measures the scattering angle in grazing incidence experiments in absence of any geometrical beam-defining tool, such as slits. The main difficulty in such set-ups is the realization of geometrically flat field borders. The possibility of the application of neutron resonance spin echo (NRSE) for such a purpose is discussed, where the field borders are defined by current sheets. Prototype SERGIS experiments performed on holographically made optical gratings at a NRSE triple-axis spectrometer are shown.

Relationships among U.S. oil prices and oil industry equity indices

The cointegration analysis suggests that the pure oil industry equity system and the mixed oil price/equity index system offers more opportunities for long-run portfolio diversification and less market integration than the pure oil price systems. On a daily basis, in the oil price systems all oil prices with the exception of the 3-month futures can explain the future movements of each other. In the mixed system, none of the daily oil industry stock indices can explain the daily future movements of the New York Mercantile Exchange (NYMEX) futures prices, whereas these prices can explain the movements of independent companies engaged in exploration, refining, and marketing. The spillover analysis of oil volatility transmission suggests that the oil futures market has a matching or echoing volatility effect on the stocks of some oil sectors and a volatility-dampening effect on the stocks of others. The policy implication is that, during times of high oil volatility, traders should choose the S&P oil sector stocks that match their tolerance for volatility and use the right financial derivative to hedge against or profit from this volatility. The day effect for volatility transmission suggests that Friday has a calming effect on the volatility of oil stocks in general. The effect for Monday is not significant.

Longitudinal collective echoes in coasting particle beams

Longitudinal ballistic and collective beam echoes with diffusion effects are investigated theoretically. In the presence of the space-charge impedance, the collective echo amplitude is obtained as a closed form expression. In contrast to the ballistic case, the collective echo amplitude consists of one maximum at time ${t}_{\mathrm{echo}}$. The echo amplitude grows up and damps down with a rate proportional to the Landau damping rate of space-charge waves. The effect of weak diffusion is found to modify the ballistic and the collective echo amplitudes in the same manner. This effect of diffusion was confirmed using a ``noiseless,'' grid-based simulation code. As a first application the amount of numerical diffusion in our simulation code was determined using the echo effect.

How we perform delayed enhancement imaging.

Recently, numerous studies have demonstrated the effectiveness of a segmented inversion recovery fast gradient echo (seg IR-FGE) sequence for differentiating injured from normal myocardium. This technique for delayed enhancement imaging has been shown to be effective in identifying the presence and extent of myocardial infarction, as well as predicting improvement in contractile function after coronary revascularization. In this article we outline the procedure of delayed enhancement imaging performed at our center, describe the seg IR-FGE sequence in more detail, including our process for choosing sequence settings, review our process of image interpretation, and highlight potential pitfalls (and techniques to overcome them) that we have encountered in our experience with performing the technique in over 1500 patients.

Superfluorescence-stimulated photon echoes.

Two closely spaced 778 nm, 4-ps pulses, two-photon resonant with the 5S-5D transition in Rb vapor generate a ground state grating, which later is excited by a similar third pulse, producing conical yoked superfluorescence echo on the 5S-6P, 420-nm transition. The intensity of this emission as a function of the relative delay between the leading excitation pulses is governed by the dynamics of Doppler dephasing and rephasing. This is the first observation of an echo effect induced by a spontaneous relaxation process.

A computational general equilibrium model with vintage capital

This paper presents a vintage capital model assuming putty–clay investment and perfect foresight. The model is written in discrete time and is simulated by using a second order relaxation algorithm. By computing the eigenvalues of the dynamic system, we have checked the conditions of existence and uniqueness of a solution (Blanchard and Kahn's conditions) and identified the echo effect that characterizes vintage capital models and the related dynamics of creation and destruction. By calibrating the model on French data, it has been proved useful to explain the medium-term movements in the distribution of income in France during the last three decades.

Nonlinear Dynamics of Anisotropic Disturbances in Plane Couette Flow

The dynamics of disturbances to a two-dimensional inviscid Couette flow is examined under the assumption that the disturbances have a cross-stream scale which is asymptotically smaller than their streamwise scale. Such anisotropic disturbances emerge naturally from isotropic perturbations after long times because of the shearing effect of the basic flow. An asymptotic equation describing the nonlinear evolution of anisotropic disturbances is derived using aregular-perturbation technique. The close relationship between this equation and those found in critical-layer problems is discussed. The case of doubly periodic disturbances is examined in detail, since it leads to a remarkable dynamics: formally, the evolution is discontinuous in time and is given by a sequence of jumps which take place when the time is a rational number (multiplied by a fixed geometric factor). The interpretation of such a dynamical system, in a sense intermediate between discrete and continuous systems, is discussed. The asymptotic model is used to study simple interactions between two sheared (Fourier) modes and to investigate the hydrodynamic echo effect. Analytical results are complemented by results of direct numerical simulations.

Inhomogeneity and anisotropy effects on the redistribution term in Reynolds-averaged Navier–Stokes modelling

A channel flow DNS database at Reτ = 590 is used to assess the validity of modelling the redistribution term in the Reynolds stress transport equations by elliptic relaxation. The model assumptions are found to be globally consistent with the data. However, the correlation function between the fluctuating velocity and the Laplacian of the pressure gradient, which enters the integral equation of the redistribution term, is shown to be anisotropic. It is elongated in the streamwise direction and strongly asymmetric in the direction normal to the wall, in contrast to the isotropic, exponential model representation used in the original elliptic relaxation model. This discrepancy is the main cause of the slight amplification of the energy redistribution in the log layer as predicted by the elliptic relaxation equation. New formulations of the model are proposed in order to correct this spurious behaviour, by accounting for the rapid variations of the length scale and the asymmetrical shape of the correlation function. These formulations do not rely on the use of so-called ‘wall echo’ correction terms to damp the redistribution. The belief that the damping is due to the wall echo effect is called into question through the present DNS analysis.

Intravascular ultrasound Doppler measurement of blood flow velocity.

The authors investigated the feasibility and accuracy of intravascular Doppler sonography (IVDS) with a newly developed microprobe. The known method to determine blood flow velocity by transcutaneous Doppler sonography was transferred to an intravascular usable Doppler probe. With the improved technique, a microprobe measuring 0.3 mm in diameter can be advanced through a 5F catheter used in routine diagnostic angiography to perform intravascular Doppler sonography. In a first step, the conditions for application and measurement were studied in a flow tube model and were transferred in a second step to patients undergoing routine angiography and patients with arterial stenoses. Measurements with nondegassed liquids showed a high intensive acoustic signal, but no echo effect could be recorded in degassed liquids. IVDS of healthy vessels in 40 patients showed the same typical flow pulse curve as seen in transcutaneous measurements. The optimal position of the microprobe with respect to the contact of the vessel wall, the diastolic and systolic phase, and the distance to the tip of the catheter could be evaluated. In 95.6% of the 40 patients with healthy vessels, IVDS was successful. Fourteen patients with arterial stenotic diseases were investigated before and after percutaneous transluminal angioplasty, and the results of these investigations correlated well with the angiographical results. For percutaneously directly reachable vessels, the transcutaneous Doppler sonography is the choice for easy noninvasive and inexpensive measurement of blood flow velocity. However, for vessels that are difficult or impossible to reach percutaneously, intravascular measurement is a valid procedure.

Low-Loss Gigahertz Range Surface Acoustic Wave Filters using Harmonic Two-Phase Unidirectional Transducers

The fabrication of surface acoustic wave (SAW) filters employing the conventional interdigital transducer (IDT) involves significant loss in the transmitted signals. An inherent problem in IDT is that the minimum insertion loss is 6 dB (1) owing to bidirectionality and strong passband ripple from triple-transit echo and secondary effects. To avoid these flaws and gain a simple structure of the filter, two-phase unidirectional transducers were chosen in this study. The fabrication of devices for frequencies above 1.5 GHz is still difficult and hence expensive. In this paper, a computer simulation model is built and used to study the frequency response of two GHz harmonic filters using unidirectional transducers. The proposed design provides moderate insertion loss. A simple model to find the input impedance of the proposed transducer is discussed.

Soliton-like mode in two-dimensional excitable medium: echo effects induced by collision of excitation waves with local nonconductive medium elements.

Soliton-like mode in two-dimensional excitable medium: echo effects induced by collision of excitation waves with local nonconductive medium elements.

Reclaiming America: Nike, clean air, and the new national activism

Acknowledgments Introduction Part One: Reclaiming America outside the Electoral Process: National Anti-Sweatshop Campaigns and the Movement for a Living Wage 1. Just Don't Buy It: Challenging Nike and the Rules of the Global Economy 2. From Challenging American Sweatshops to a Movement for a Global Living Wage Part Two: Reclaiming America through the Political Process: National Environmental Activism and the Pentagon Redirection Campaign 3. The New National Environmental Activism 4. The Pentagon: Reclaiming America by Giving Peace a Chance Part Three: Resources for National Activism: Community-Based Organizations, the Media, and the Internet 5. Community-Based Nonprofit Organizations: From Demobilizers to Agents of Change 6. The Media: Mobilizing through the Echo Effect 7. The Internet: Mobilizing in Cyberspace Conclusion Notes Index

Guidelines for multipoint videoconferencing using low-cost, PC-based equipment.

We have compiled a set of guidelines for effective multipoint videoconferencing carried out with low-cost, PC-based systems. The guidelines are based on experience gained during multipoint videoconferences in epidemiology and orthopaedics/radiology. Joint data inspection and manipulation were carried out by application sharing between the PCs. The guidelines are categorized by the time of their application and also by their content. For effective multipoint videoconferences, preparation of the agenda and of digital documents is mandatory. The prevention of echo effects and half-duplex audio connections are of special importance. A protocol to optimize the order of speakers increases the efficient use of conference time. The guidelines worked well when used in a multisite research project and addressed the most important weak points of multipoint videoconferences using low-cost equipment.

Direct effects of the prostaglandins E2 and f2alpha on progesterone release by the corpus luteum of the marmoset monkey (Callithrix jacchus) studied by in vitro microdialysis.

The effects of the prostaglandins (PG) E2 and F2alpha on progesterone secretion in luteal tissue (32 corpora lutea) explanted from the mid-luteal ovary of the marmoset monkey (n=13) were investigated using an in vitro microdialysis system. Consecutive applications of 1, 10 and 100 microg/ml PGE2 resulted in a significant increase in secretion of progesterone at the maximum dose of 100 microg/ml, which was shown to be the stimulatory dose in both long-period and 20-min pulse (time to collect one fraction) applications. The response varied individually between 1.4- and 3. 4-fold above the baseline concentrations. Application of 500 microg/ml PGF2alpha led to similar hormone responses. In contrast, lower doses of PGF2alpha (0.5, 5 and 50 microg/ml) resulted in significantly increased levels of secretion of progesterone, to approximately 1.4-fold baseline values, only after the application was terminated (echo effect). Responses were less variable when a short pulse of 20 min duration was applied, instead of long applications of 1-2 h. On the basis of the passage rates measured for tritiated PGF2alpha, transfer through the dialysis membrane was assumed to be in the range of 1% for both PGs. Ultrastructurally, luteal cells lying in a sheath of five to seven cell layers around the dialysis tubing appeared intact and were interconnected by gap junctions. Vesiculation of the smooth endoplasmic reticulum was more prominent after PG treatment, indicating a stimulation of cellular synthesis/secretory activities that was in accordance with the stimulatory action of both PGs on progesterone release under these in vitro conditions.

Strong echo effect and nonlinear transient growth in shear flows

The nonlinear interaction of two disturbances excited successively in a two-dimensional Couette flow is shown to lead to a transient energy growth. This phenomenon, which is called the echo effect and exists in several other physical systems, is interesting because the energy growth appears long after the energy associated with the original disturbances has decayed. Here, the echo effect is studied analytically and numerically in a situation where the nonlinear response has the same order of magnitude as the two excitations. A system of amplitude equations describing the nonlinear interactions between three sheared modes is derived and employed to examine the physical mechanism of the echo. The qualitative validity of this system is confirmed by numerical simulations. The influence of viscous dissipation on the echo effect is also considered.

Thermal dephasing and the echo effect in a confined Bose-Einstein condensate

Thermal fluctuations of the normal component induce dephasing---reversible damping of the low-energy collective modes of a confined Bose-Einstein condensate. The dephasing rate is calculated for the isotropic oscillator trap, where the Landau damping is expected to be suppressed. This rate is characterized by a steep temperature dependence. It is weakly amplitude dependent, and is sensitive to the total number of atoms in the trap. The value of the rate belongs to the range of the damping rates observed by Jin et al. [Phys. Rev. Lett. 77, 420 (1996)]. We suggest that a reversible nature of the damping caused by the thermal dephasing in the isotropic trap can be tested by the echo effect. A reversible nature of the Landau damping is also discussed, and a possibility of observing the echo effect in an anisotropic trap is considered as well. The parameters of the echo for the isotropic trap are calculated in the weak echo limit. Results of the numerical simulations of the echo are also presented.