Mean Doppler Frequency Research Articles

Subsample volume processing of doppler ultrasound signals

Processing of Doppler signals produced by pulsed Doppler systems is based on the assumption that the phase of the received high frequency ultrasound signals changes linearly with depth. However, the random spatial distribution of scatterers is not in accordance with this basic assumption. Consequently, averaging of the demodulated signal over an observation window, covering a few periods of the received signal, does not improve the estimate for the instantaneous quadrature components of the Doppler signal originating from a given depth. Hence, the accuracy of the Doppler velocity estimate is independent of the length of the observation window employed. However, splitting the observation window in subsample volumes, each with a length of one period at the emission frequency, and combining the Doppler signals of the subsample volumes at the last stage of signal processing, i.e., mean Doppler frequency estimation using the autocorrelation technique, results in a considerable reduction of the variance of the velocity estimate. Using a computer simulation of the signal processing involved, it is demonstrated that with subsample volume processing the variance of the velocity estimate attains the same variance as is expected for the RF cross correlation technique.

Experimental evaluation of intrinsic and nonstationary ultrasonic Doppler spectral broadening in steady and pulsatile flow loop models

Intrinsic and nonstationary Doppler spectral broadening, and the skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. The relative Doppler bandwidth invariance theorem was experimentally verified with an in vitro steady laminar blood flow model. It is shown that the relative bandwidth is both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, the authors demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, is characterized.

A linear prediction approach to Doppler mean frequency retrieval in the presence of ground clutter

Linear-prediction-based methods are applied to mean Doppler frequency estimation of weather radar signals in the presence of ground clutter when only a small number of samples is available for processing. The ability of these methods to resolve weather signals from ground clutter in unfavorable conditions, i.e., when the clutter is stronger and its spectrum is narrower than that of the signal, is shown. Given a priori information about the ground clutter, Prony's method can be applied and simplified to derive a convenient formula for mean Doppler frequency estimation. This estimator may be considered as a generalization of the pulse-pair (PP) estimator and it can give satisfactory results in resolving weather signals from ground clutter for signal-to-noise ratios (SNRs) above 20 dB. The eigendecomposition-based minimum-norm (MN) method is applied to lower the SNR threshold to 10 dB. The performance of these estimators is compared with the signal-only lower bound and with that of a processor consisting of various ground clutter filters and the PP estimator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Synthetic aperture radar interferometry applied to ship‐generated internal waves in the 1989 Loch Linnhe experiment

Interferometer synthetic aperture radar images collected during the 1989 Loch Linnhe experiment showed mean Doppler variations across the phase of ship‐generated internal waves that corresponded to “velocity” variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave features of the order of 5 to 10 cm/s and virtually ruled out the existence of surface currents as large as the interferometer‐inferred values. In this paper we show how the pixel‐to‐pixel phase difference measured by the Jet Propulsion Laboratory interferometer is related to the mean Doppler frequency of the backscattered field. Model calculations are used to show how this frequency can sometimes change by a large amount, even when rather small surface currents are present. In particular, for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations based on our wave‐current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to large velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity estimates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thus for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave propagation direction, the mean Doppler shifts (and the phase differences) are dominated mostly by advection, and interferometer current estimates are more accurate. C band computations predict that if the antenna spacing is small enough so that the fields from the two antennas remain correlated, then the C band interferometer current estimates will be better than those at L band.

Angle-of-arrival oscillations in the mesosphere as seen by medium frequency (MF) radar

The Saskatoon MF radar operates continuously in the spaced antenna mode with 5 min resolution, primarily for horizontal wind analysis at mesospheric heights. Mean Doppler frequency and antenna-pair phase difference are available as byproducts. Clear oscillations are sometimes seen in the latter. Spectral analysis shows that these also occur in the Doppler and extend over a range of heights. The two cases shown are almost certainly perturbations in specular reflecting layers moving with horizontal velocity close to that of the wind. Downward phase propagation is apparent, and when the oscillations are present, the wind appears to be decelerating. The tentative explanation of a breaking gravity wave decelerating the mean flow is put forward, although the relations between frequency, and horizontal and vertical wavelengths, contradict linear gravity wave theory.

Effects of Indomethacin on Cerebral Hemodynamics at Rest and during Endotracheal Suctioning in Preterm Neonates

The effect of therapeutic doses of indomethacin versus placebo (saline) on cerebral hemodynamics was studied in nine mechanically ventilated preterm infants. Recordings were made at rest and during endotracheal suctioning. Indomethacin at a dose of 0.2 mg/kg body weight was infused over 5 min. Compared to placebo, there was a significant decrease in Doppler mean frequency in the anterior cerebral artery within minutes of starting the indomethacin infusion during the resting period after the first (p less than 0.01), second (p less than 0.001) and third (p less than 0.025) doses. This effect has persisted during endotracheal suctioning. Thus, indomethacin alters cerebral hemodynamics by decreasing Doppler mean frequency. In view of these findings, and particularly because of the alteration in the resting cerebral hemodynamics, the recommendation of prophylactic treatment with indomethacin remains questionable.

Measurements of sea spikes in microwave backscatter at moderate incidence

Previous authors have associated large events, or “sea spikes”, in the radar cross section of the sea surface with steep or breaking waves. Here we present a method based on an intensity threshold and the mean Doppler frequency to identify and quantify sea spikes likely to be associated with large‐scale wave breaking. This method is applied to microwave measurements of Ku band made with a dual‐polarized Doppler scatterometer at a moderate incidence angle (45°) from a fixed platform in the North Sea. For VV and HH polarization the mean normalized radar cross section σ0, the frequency of sea spike occurrence N, and the sea spike contribution to the mean normalized radar cross section, σSS0, are correlated with direct measurements of the friction velocity, u*, in the range 13 ≤ u* ≤ 49 cm s−1. The friction velocity exponents for both N and σSS0 indicate an approximately cubic dependence, while the residual radar cross section σres0 = σ0 ‐ σSS is approximately proportional to friction velocity for u* greater than approximately 20 cm s−1. At high friction velocities, (u* ≈ 40–50 cm s−1) these data show that the contribution of large sea spikes to the mean radar cross section for measurements between 25° and 45° relative to the wind is in the range of 5–10% for VV polarization and 10–20% for HH polarization.

Measurement of arterial blood flow by Doppler ultrasound

A review is given of quantitative techniques and clinical applications of arterial Doppler ultrasound. The currently available Doppler equipment of stand-alone continuous wave and pulsed wave units, duplex systems and colour flow systems is briefly described. Doppler ultrasound can be divided into procedures concerned with waveform analysis, volume flow measurement and more recently colour flow imaging. Arterial Doppler waveform analysis is considered for a number of areas including carotid, lower limb, renal and renal transplant, obstetrics, adult cerebral, neonatal cerebral, and tumour studies. Using a duplex scanner volume flow in arteries can be measured from estimates of vessel cross sectional area, mean Doppler frequency and beam-vessel angle. The errors associated with each of these measurements is discussed, and reports of experimentally determined in vivo accuracy of volume flow measurements made using this technique are considered. Other volume flow measurement techniques including the promising attenuation compensation method are also explored.

Effects of transducer beam geometry and flow velocity profile on the Doppler power spectrum: A theoretical study

A theoretical model is used to show how the Doppler spectrum for various axisymmetric velocity profiles is affected by beam misalignment and incomplete insonation. Results are presented for both circular and square beam geometries. Moreover, a closed-form expression is derived for the power spectral density received by an on-axis transducer with a Gaussian beam profile. It is shown that the error incurred in measuring the mean Doppler frequency with such a profile will generally be bounded by the results for the circular and square beam geometries. The effects of an ideal high-pass filter on the mean Doppler frequency and the backscattered Doppler power are examined. It is shown that such a filter can introduce large differences in the measured systolic to diastolic power ratios. Finally, theoretical expressions and results are presented for the spectral broadening index (SBI), normalized spectral variance (NSV), coefficient of kurtosis (CK), the coefficient of skewness (CS) as functions of the axisymmetric velocity profile shape assuming complete uniform insonation.

Algorithms for fast computation of the intensity weighted mean Doppler frequency.

Algorithms for fast computation of the intensity weighted mean Doppler frequency.

Monitoring middle cerebral artery blood velocity during carotid endarterectomy

Transcranial pulsed Doppler ultrasound and spectral analysis were used to monitor blood velocities in the middle cerebral artery of nineteen patients (mean age 61 +/- 9 years) during carotid endarterectomy. A Javid shunt was used in all patients. The intensity weighted mean Doppler frequency for each spectral sweep (at 5 ms intervals) was time-averaged over the cardiac cycle to obtain a mean value for blood velocity in the middle cerebral artery. The range of such values found in the 19 patients was: 12-38 cm s-1 after anaesthesia (baseline); 12-69 cm s-1 during diathermy; 0-30 cm s-1 during carotid clamping; 16-32 cm s-1 during shunting and 18-60 cm s-1 in the recovery room. The average change in middle cerebral artery blood velocity from baseline values showed significant increases during diathermy (P less than 0.005), shunting (P less than 0.05) and in the recovery room (P less than 0.005). Clamping of the internal carotid artery showed a significant decrease in middle cerebral artery blood velocities of all patients (P less than 0.005), three of whom showed no flow in the middle cerebral artery during clamping. Abnormally high amplitude Doppler signals at the commencement of shunting were detected in 17 of the 19 patients. Such Doppler signals are consistent with turbulent blood flow or the introduction of micro-air bubbles by the shunt. Backbleeding in the internal carotid artery before insertion of the shunt was associated with diminished flow in the ipsilateral middle cerebral artery of ten patients, oscillatory forward/reverse flow in three patients and cessation of flow in the remaining six patients.

A comparison of digital and analog methods of Doppler spectral analysis for quantifying flow.

Ultrasonic methods can be used for calculating flow when the mean Doppler frequency is representative of spatial average velocity. We have examined the capabilities of two commercially available methods of Doppler spectral analysis for providing measurements of spatial average velocity and flow. In a steady state flow model, Doppler audio spectra were recorded using a 5-MHz duplex scanner. Fast Fourier transform (FFT) spectral analysis was used to determine mean (M), mode (MO), and maximum (MAX) frequencies. An analog method (offset zero crossing detector = ZC) was used to determine root mean square (RMS) frequencies. The results of comparing Doppler flow estimates (QM, QMO, QMAX and QRMS) with direct flow measurements (n = 10; range = 128-1098 ml/min) were (1) QM = 0.67Q + 23 ml/min (SEE = 36 ml/min); (2) QMO = 0.96Q + 152 ml/min (SEE = 32 ml/min); (3) QMAX = 1.19Q + 171 ml/min (SEE = 23 ml/min); and (4) QRMS = 0.93Q + 76ml/min (SEE = 92 ml/min). Estimates of flow using M and RMS frequencies were adversely affected by experimental conditions likely to result in turbulence. We conclude that application of commercially available FFT determined M frequencies could result in significant errors in calculations of spatial average velocity and flow. Alternatively, FFT determined MO frequencies and ZC determined RMS frequencies resulted in accurate estimates of flow in this model. This study demonstrates the importance of evaluating the capabilities of commercially available methods of Doppler spectral analysis when using ultrasound for determining velocity and flow.

Mini laser-Doppler (blood) flow monitor with diode laser source and detection integrated in the probe

A mini laser-Doppler instrument for monitoring capillary flow or flow in tubes, in which a small solid state laser and two photodiodes are integrated in the probe, is described. The instrument is meant for use with skin or arterial blood flow. The mean Doppler frequency derived from the normalized first moment of the frequency distribution, measured with this instrument, appears to be linearly dependent on the flow velocity in the 0–2-mm sec−1 region.

An Adaptive MTI for Weather Clutter Suppression

A digital realization of an adaptive clutter-locking loop is presented. The purpose of the loop is to estimate the mean Doppler frequency of the clutter. The clutter spectrum is then shifted toward the zero Doppler by this estimate. A fixed moving target indicator (MTI) canceler following the loop suppresses the shifted clutter. Experimental simulations illustrate the feasibility of the loop. Results indicate that the proposed canceler works significantly better than a fixed canceler, while not as well as the 10-pulse moving target detector (MTD) processor. However, the complexity of the MTD is significantly more than the relatively simple adaptive processor presented here.

Continuous measurement of intestinal mucosal blood flow by laser-Doppler velocimetry

To measure blood flow in the intestinal mucosa we built a laser-Doppler flowmeter that consists of a helium-neon laser, an electronic circuit, and a pair of fiber-optic light guides that conduct laser light to the tissue and carry the backscattered light to a photodetector. Because light scattered by moving red blood cells experiences a shift in its frequency, we measured blood flow by detecting the mean Doppler frequency. In isolated loops of canine small bowel, we raised perfusion pressure and found the increases in laser mucosal blood flow were significantly correlated with total blood flow measured by an electromagnetic probe. During infusions of isoproterenol (a selective vasodilator of the mucosa), laser mucosal blood flow increased before total flow increased. Similarly, adenosine (a selective dilator of the muscularis) increased total flow, whereas local mucosal blood flow fell or was unchanged. In addition, reactive hyperemia was sometimes observed in the mucosa but not in the muscularis. These observations indicate that the laser-Doppler technique measures blood flow in the surface tissue and does not reflect blood flow throughout the other tissues of the bowel wall. Instrumental problems identified in this study were 1) the difficulty of calibrating the laser mucosal blood flowmeter in absolute units, 2) the uncertainty of the volume of tissue in which local mucosal blood flow is measured, and 3) the problem of maintaining contact between the optical probe and the tissue. Nevertheless, the method holds great promise because it can detect small ischemic areas, because it could be used in combination with endoscopy, and because it yields a continuous measurement of blood flow in either the muscularis or mucosa.

Backscatter radar measurements of storm-time electric field changes in the equatorial electrojet

The mean Doppler frequency ( f ̄ D ) of the VHP backscatter radar signals from type II plasma irregularities in the equatorial electrojet is a measure of the driving electric field in the electrojet. The measured Doppler frequency values are used to delineate the complex time variations of the electric field and current in the equatorial electrojet during the two geomagnetic storms of 25 April and 18 September in 1979. Using a recently evolved empirical method, the ionospheric and the magnetospheric current contributions to the storm-time surface magnetic field perturbations at Thumba have been estimated; and the characteristic time variations during the initial, main and recovery phases of the magnetic storms are presented. The most significant conclusions of the present study are: (a) the daytime SSC-related increases in ΔH at electrojet latitudes have a large contribution from an impulsive change of electrojet electric field during the SSC; (b) during the initial phase, both the magnetospheric current and electrojet current exhibit rapid fluctuations which are in antiphase; (c) the D st , values seem to underestimate, by nearly a factor of two, the asymmetric or partial ring current contribution around the time when the main phase decrease in AH reaches its peak level; and (d) the ionosphere and magnetosphere seem to respond as a single electrodynamic system to the electrical disturbances of all scales which originate in the interaction of solar wind with the outer magnetosphere.

A continuous wave doppler velocimeter for monitoring blood flow in the popliteal artery, compared with venous occlusion plethysmography of the calf

A transcutaneous Doppler velocimeter has been used for monitoring changes in blood flow in the popliteal artery during and after exercise of the calf muscles on a calf-ergometer. The instrument and the positioning of the probe are described. The validity of the Doppler measurements has been assessed by comparing results after exercise and after 5 min arterial occlusion with venous occlusion plethysmography. For 10 healthy volunteers calibration lines were found which are curved. This can be ascribed partly to alinearity of the Doppler instrument and partly to changes of the diameter of the popliteal artery. Displacement of the probe with respect to the artery, which influences the measured velocity signal, can be detected and to a certain extent corrected by taking into account the intensity of the Doppler signal. The largest deviation of a data point from the corresponding calibration line varies for the ten subjects between 90 and 170 Hz in the high flow range (mean Doppler frequency around 1000 Hz) and between 20 and 60 Hz in the low flow range (mean Doppler frequency below 200 Hz).

Design and Evaluation of an Adaptive MTI Filter

The design and evaluation of an adaptive moving target indicator (MTI) filter, the adaptive canceler for extended clutter (ACEC) is dealt with, taking into consideration adaptivity to clutter mean Doppler frequency. This consideration is one of the most important operational requirements in adaptive MTI's and permits a relatively simple hardware implementation as compared to more general optimization and adaptivity criteria (briefly described). The ACEC's algorithm compensates in real time for the clutter mean Doppler frequency. Performances have been obtained by digital computer simulation in various operational conditions.

MTI Optimization in a Multiple-Clutter Environment

A means of optimizing a moving target indicator (MTI) filter for rejecting several types of clutter, which are generated by different mechanisms such as by rain or the ground, is formulated. lt is found that the optimal performance of such a filter depends on the spectral density functions, average radar cross sections, and the relative mean Doppler frequencies of each type of clutter. lt is shown that the optimal improvement factor of such a filter is bounded by the weighted average (weighted in accordance with the radar cross sections of the clutter types) of the improvement factor for the individual clutter type. lt is also shown that the improvement factor of such a filter is a function of the relative mean Doppler frequency f0 between the clutter types. As f0 increases, the performance of the MTI system degrades. The worst improvement factor occurs when f0 is equal to half of the radar pulse-repetition frequency (PRF).

New approach to m.t.i. clutter locking

Proposals are made for new techniques, for clutter locking in m.t.i. radar systems, that adapt automatically to rapid variations in the clutter bias velocity with range. The proposed schemes are based upon what is believed to be the novel principle of correcting for the clutter bias velocity after the radar returns have been read-out from the vector canceller stores and demand no additional storage. The basic arrangements have been analysed in detail and procedures have been established with which to design systems to meet specified performances. An analysis based on generalised symmetric and skew clutter spectra has shown that the performances of these systems are very little affected by the shape of the spectrum. Using these procedures, a design has been derived for a clutter-locking arrangement for a pulse-staggering radar in which the mean Doppler frequency of the clutter can rise to 30% of the p.r.f. Addition of the proposed clutter-locking arrangement is expected to give an overall improvement factor which will be better than 23 dB at all Doppler shifts, and when averaged over all shifts will be 25 dB. In order to confirm the design analysis and to establish the degree of smoothing required in range, a program was written to simulate this particular design and to inject noiselike clutter having either a Gaussian or a highly skew spectrum. The results of this simulation corresponded satisfactorily with the predicted performance and yielded the result that the range smoothing required corresponded to only 4 range cells. The design awaits experimental verification.