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Doppler broadening
About: Doppler broadening is a research topic. Over the lifetime, 5509 publications have been published within this topic receiving 92552 citations.
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TL;DR: The use of an Intensity‐weighted peak velocity (centroid), with calculation of the velocity which includes 95% of the spectral points, rather than the absolute peak of the spectrum, proved to be more accurate and was angle Independent in these Instruments.
Abstract: Ultrasound instruments are used to evaluate blood flow velocities in the human body. Most clinical instruments perform velocity calculations based on the Doppler principle and measure the frequency shift of a reflected ultrasound beam. Doppler-only instruments use single-frequency, single-crystal transducers. Linear- and annular-array multiple-crystal transducers are used for duplex scanning (simultaneous B-mode image and Doppler). Clinical interpretation relies primarily on determination of peak velocities or frequency shifts as identified by the Doppler spectrum. Understanding of the validity of these measurements is important for instruments in clinical use. The present study examined the accuracy with which several ultrasound instruments could estimate velocities based on the identification of the peak of the Doppler spectrum, across a range of different angles of insonation, on a Doppler string phantom. The string was running in a water tank at constant speeds of 50, 100, and 150 cm/sec and also in a sine wave pattern at 100- or 150-cm/sec amplitude. Angles of insonation were 30, 45, 60, and 70 degrees. The single-frequency, single-crystal transducers (PC Dop 842, 2-MHz pulsed-wave, 4-MHz continuous-wave) provided acceptably accurate velocity estimates at all tested velocities independent of the angle of insonation. All duplex Doppler instruments with linear-array transducers (Philips P700, 5.0-MHz; Hewlett-Packard Sonos 1000, 7.5-MHz; ATL Ultramark 9 HDI, 7.5-MHz) exhibited a consistent overestimation of the true flow velocity due to increasing intrinsic spectral broadening with increasing angle of insonation.(ABSTRACT TRUNCATED AT 250 WORDS)
33 citations
TL;DR: In this article, the effects of resonance and electron scattering on the escape of Lymanα photons during cosmological hydrogen recombination were investigated, and it was shown that due to the redistribution of photons over frequency hydrogen recombinations is sped up, leading to a frequency-dependent asymmetry between the emission and absorption profile.
Abstract: In this paper we consider the effects of resonance and electron scattering on the escape of Lymanα photons during cosmological hydrogen recombination. We pay particular attention to the influence of atomic recoil, Doppler boosting and Doppler broadening using a Fokker-Planck approximation of the redistribution function describing the scattering of photons on the Lymanα resonance of moving hydrogen atoms. We extend the computations of our recent paper on the influence of the 3d /3s-1s two-photon channels on the dynamics of hydrogen recombination, simultaneously including the full time-dependence of the problem, the thermodynamic corrections factor, leading to a frequency-dependent asymmetry between the emission and absorption profile, and the quantummechanical corrections related to the two-photon nature of the 3d/3s-1s emission and absorption process on the exact shape of the Lymanα emission profile. We show here that due to the redistribution of photons over frequency hydrogen recombination is sped up
33 citations
33 citations
TL;DR: In this paper, the calibration of a Lyman-α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 1014 atoms/cm2 and total pressure of 1.5 torr He is described.
Abstract: Atomic absorption and fluorescence spectrophotometry have been routinely used in kinetic investigations as probes of relative, rather than absolute, atom concentration. The calibration of a Lyman-α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 1014 atoms/cm2 and total pressure of 1.5 torr He is described. The photometer is characterized in terms of a two-level emission source and an absorption region in which only Doppler broadening of the transition is considered. The modifications due to pressure broadening by high pressures (500 ≤ P ≤ 1500 torr) in the absorption region are discussed in detail. Application of the technique is reported for the recombination of hydrogen atoms in the presence of six nonreactive heat bath gases. Experiments were performed in a static reaction cell at pressures of 500–1500 torr of heat bath gas, and hydrogen atoms were produced by Hg (3P1) photosensitization of H2. The technique is critically evaluated and the mechanistic implications of the hydrogen atom recombination results are examined. The measured room temperature recombination rate constants in H2, He, Ne, Ar, Kr, and N2 are 8.5 ± 1.2, 6.9 ± 1.5, 5.9 ± 1.5, 8.0 ± 0.8, 10.2 ± 0.9, and 9.6 ± 1.4, respectively, where the units are 1033 cm6/molec2 · sec.
33 citations
TL;DR: Investigation of in vivo spectral Doppler measurements of velocity waveforms found a high degree of correlation between velocities obtained by the real-time UD and the accelerated PC-MRA technique, demonstrating that to avoid overestimations of peak systolic velocity, the results for UD must be carefully corrected to compensate for errors caused by intrinsic spectral broadening.
Abstract: Ultrasound Doppler (UD) velocity measurements are commonly used to quantify blood flow velocities in vivo. The aim of our work was to investigate the accuracy of in vivo spectral Doppler measurements of velocity waveforms. Waveforms were derived from spectral Doppler signals and corrected for intrinsic spectral broadening errors by applying a previously published algorithm. The method was tested in a canine aneurysm model by determining velocities in small arteries (3–4 mm diameter) near the aneurysm where there was moderately disturbed flow. Doppler results were compared to velocity measurements in the same arteries acquired with a rapid volumetric phase contrast MR angiography technique named phase contrast vastly undersampled isotropic projection reconstruction magnetic resonance angiography (PC-VIPR MRA). After correcting for intrinsic spectral broadening, there was a high degree of correlation between velocities obtained by the real-time UD and the accelerated PC-MRA technique. The peak systolic velocity yielded a linear correlation coefficient of r = 0.83, end diastolic velocity resulted in r = 0.81, and temporally averaged mean velocity resulted in r = 0.76. The overall velocity waveforms obtained by the two techniques were also highly correlated (r = 0.89 ± 0.06). There were, however, only weak correlations for the pulsatility index (PI: 0.25) and resistive index (RI: 0.14) derived from the two techniques. Results demonstrate that to avoid overestimations of peak systolic velocities, the results for UD must be carefully corrected to compensate for errors caused by intrinsic spectral broadening.
33 citations