Some aspects of the relationship between instantaneous volumetric blood flow and continuous wave Doppler ultrasound recordings--I. The effect of ultrasonic beam width on the output of maximum, mean and RMS frequency processors

Assessment of resistance vessel function in human skeletal muscle: guidelines for experimental design, Doppler ultrasound, and pharmacology

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions.

Technical recommendations for the use of carotid duplex ultrasound for the assessment of extracranial blood flow

Duplex ultrasound is an evolving technology that allows the assessment of volumetric blood flow in the carotid and vertebral arteries during a range of interventions along the spectrum of health and chronic disease. Duplex ultrasound can provide high-resolution diameter and velocity information in real-time and is noninvasive with minimal risks or contraindications. However, this ultrasound approach is a specialized technique requiring intensive training and stringent control of multiple complex settings; results are highly operator-dependent, and analysis approaches are inconsistent. Importantly, therefore, methodological differences can invalidate comparisons between different imaging modalities and studies; such methodological errors have potential to discredit study findings completely. The task of this review is to provide the first comprehensive, user-friendly technical guideline for the application of duplex ultrasound in measuring extracranial blood flow in human research. An update on recent developments in the use of edge-detection software for offline analysis is highlighted, and suggestions for future directions in this field are provided. These recommendations are presented in an attempt to standardize measurements across research groups and, hence, ultimately to improve the accuracy and reproducibility of measuring extracranial blood flow both within subjects and between groups.

Thin-beam ultrasound overestimation of blood flow: how wide is your beam?

It has been predicted that the development of thin-beam ultrasound could lead to an overestimation of mean blood velocity by up to 33% as beam width approaches 0% of vessel diameter. If both beam and vessel widths are known, in theory, this overestimation may be correctable. Therefore, we updated a method for determining the beam width of a Doppler ultrasound system, tested the utility of this technique and the information it provides to reliably correct for the error in velocity measurements, and explored how error-corrected velocity estimates impact the interpretation of in vivo data. Using a string phantom, we found the average beam width of four different probes varied across probes from 2.93 ± 0.05 to 4.41 ± 0.06 mm (mean ± SD) and with depth of insonation. Using this information, we tested the validity of a calculated correction factor to minimize the thin-beam error in mean velocity observed in a flow phantom with known diameter. Use of a correction factor reduced the overestimation from 39 ± 11 to 7 ± 9% (P < 0.05). Lastly, in vivo we explored how knowledge of beam width improves understanding of physiological flow conditions. In vivo, use of a correction factor reduced the overestimation of mean velocity from 23 ± 11 to -4 ± 9% (P < 0.05). Thus this large source of error is real, has been largely ignored by the early adaptors of Doppler ultrasound for vascular physiology studies in humans, and is correctable by the described techniques.

The removal of wall components in Doppler ultrasound signals by using the empirical mode decomposition algorithm

Doppler ultrasound systems, used for the noninvasive detection of the vascular diseases, normally employ a high-pass filter (HPF) to remove the large, low-frequency components from the vessel wall from the blood flow signal. Unfortunately, the filter also removes the low-frequency Doppler signals arising from slow-moving blood. In this paper, we propose to use a novel technique, called the empirical mode decomposition (EMD), to remove the wall components from the mixed signals. The EMD is firstly to decompose a signal into a finite and usually small number of individual components named intrinsic mode functions (IMFs). Then a strategy based on the ratios between two adjacent values of the wall-to-blood signal ratio (WBSR) has been developed to automatically identify and remove the relevant IMFs that contribute to the wall components. This method is applied to process the simulated and clinical Doppler ultrasound signals. Compared with the results based on the traditional high-pass filter, the new approach obtains improved performance for wall components removal from the mixed signals effectively and objectively, and provides us with more accurate low blood flow.

An improved spectral width Doppler method for estimating Doppler angles in flows with existence of velocity gradients

Doppler angle (i.e., beam-to-flow angle) is an important parameter for quantitative flow measurements. With known Doppler angles, volumetric flows can be obtained by the mean flow velocity times the cross-section area of the vessel. The differences or changes between prestenotic and poststenotic volumetric flows have been quantified as an indicator for assessing the clinical severity of the stenosis. Therefore, several research groups have dedicated themselves to developing user-independent methods to determine automatically the Doppler angle. Nevertheless, most of these methods were developed for narrow ultrasound beam measurements. For small vessels, where the beam width is a significant fraction of the diameter of the vessel, the effect of velocity gradients plays an important role and should not be ignored in the Doppler angle estimations. Accordingly, this paper is concerned with a method for improving the estimation of Doppler angles from spectral width Doppler (SWD) method, but correcting for velocity-gradient broadening that may arise when the beam has a nonzero width. In our method, Doppler angles were firstly calculated by SWD and then were corrected by an artificial neural network (ANN) method to neutralize the contribution of velocity gradient broadening (VGB). This SWD and ANN conjoint method has been successfully applied to estimate Doppler angles from 50 degrees to 80 degrees for constant flows in 10 mm, 4 mm and 1 mm diameter tubes, whose mean flow velocities were 15.3, 19.9 and 25.5 cm/s, respectively, and the achieved mean absolute errors of the estimated Doppler angles were 1.46 degrees , 1.01 degrees and 1.3 degrees.

Application of empirical mode decomposition to remove the wall components in Doppler ultrasound signals: a simulation study

Doppler ultrasound blood flow analysis systems normally use a high-pass filter to remove the large, low frequency components from the vessel wall from the blood flow signal. Unfortunately, the filter also removes the low frequency Doppler signals arising from slow moving blood. In this paper, we propose to use a novel technique, called the empirical mode decomposition (EMD), to remove the wall components from the mixed signals. The EMD is used to decompose a Doppler signal into finite individual components. Then the wall components are automatically identified and removed by using a strategy. This method is applied to process the simulated Doppler signals. Compared with the results based on the tradition high-pass filter, the new approach obtains improved performance for wall removal from the mixed Doppler signals, and provide us with more accurate low blood flow.

Doppler ultrasound spectral enhancement using the Gabor transform-based spectral subtraction

Most of the important clinical indices of blood flow are estimated from the spectrograms of Doppler ultrasound (US) signals. Any noise may degrade the readability of the spectrogram and the precision of the clinical indiCes, so the spectral enhancement plays an important role in Doppler US signal processing. A new Doppler US spectral enhancement method is proposed in this paper and implemented in three main steps: the Gabor transform is used to compute the Gabor coefficients of a Doppler US signal, the spectral subtraction is performed on the magnitude of the Gabor coefficients, and the Gabor expansion with the spectral subtracted Gabor coefficients is used to reconstruct the denoised Doppler US signal. The different analysis and synthesis windows are examined in the Gabor transform and expansion. The signal-to-noise ratio (SNR) improvement together with the overall enhancement of spectrograms are examined on the simulated Doppler US signals from a femoral artery. The results show the denoising method based on the orthogonal-like Gabor expansion achieves the best denoising performance. The experiments on some clinical Doppler US signals from umbilical arteries confirm the superior denoising performance of the new method.

Microembolic signal characterization using adaptive chirplet expansion

The adaptive chirplet expansion (ACE) is proposed to characterize high-intensity, transient signals from circulating microemboli. The nonnegative adaptive spectrogram based on the ACE gives a compact representation of the microembolic signal (MES) in joint-time, frequency domain. The mean instantaneous power (MIP) and mean instantaneous frequency (MIF) of MES are estimated from the adaptive spectrogram. Then, several important characteristics of MES, such as embolus-to-blood ratio (EBR), half width maximum (HWM), and embolic signal onset (ESO), are computed from the MIP, and the frequency modulation is examined in the MIF. To validate the new method, we improved the simulation model of the audio Doppler ultrasound signal. Some MESs together with a Doppler ultrasound signal from carotid blood flow are simulated in the simulation study. As a comparison, the adaptive Gabor expansion (AGE) also is implemented on these simulated signals. The experimental results of the simulation study show that the new method, based on the ACE, outperforms the AGE-based method in MES characterization. The consistent conclusion has been confirmed by the clinical study on some clinical MESs.

Directional velocity estimation using focusing along the flow direction. I: Theory and simulation

A new method for directional velocity estimation is presented. The method uses beamformation along the flow direction to generate data in which the correct velocity magnitude can be directly estimated from the shift in position of the received consecutive signals. The shift is found by cross-correlating the beamformed lines. The approach can find the velocity in any direction, including transverse to the traditionally emitted ultrasound beam. The velocity estimation is studied through extensive simulations using Field II. A 128-element, 7-MHz linear array is used. A parabolic velocity profile with a peak velocity of 0.5 m/s is simulated for different beam-to-flow angles and for different emit foci. At 45 degrees the relative standard deviation over the profile is 1.6% for a transmit focus at 40 mm. At 90 degrees the approach gave a relative standard deviation of 6.6% with a transmit focus of 80 mm, when using 8 pulse-echo lines and stationary echo canceling. Pulsatile flow in the femoral artery was also simulated using Womersley's flow model. A purely transverse flow profile could be obtained with a relative standard deviation of less than 10% over the whole cardiac cycle using 8 pulse emissions for each imaging direction, which is sufficient to show clinically relevant transverse color flow images.

Ultrasonic error compensation method to correct instrumental and systematic errors in volumetric flow measurements: a theoretical study

A computational model has been developed to observe the effects of several instrumental parameters and systematic error sources on ultrasonic volumetric flow measurements in vessels using an error compensation method proposed previously by the authors. The effects of two essential instrumental parameters, the gate length and pulse duration, on two intermediate flow results and an optimal correction factor as a function of the beam width-to-vessel diameter ratio (BWR) have been studied under steady flow conditions. The corrected flow results under typical combinations of those parameters show that by proper selection of a correction factor, the proposed method can compensate for those systematic errors. If the estimated BWR is within +/- 0.1 of the actual BWR value, the systematic error can be limited to within 6.5%. The study also shows that the compensation method can effectively reduce the errors caused by other systematic error sources such as the system sensitivity and high-pass filter function, as well as misalignment of the ultrasound beam from the center of the vessel.

Doppler power spectrum from a Gaussian sample volume

A closed-form expression for the Doppler power spectrum due solely to the range of blood velocities passing through a Gaussian sample volume placed anywhere in a vessel under conditions of axisymmetric flow, uniform backscatter and negligible intrinsic spectral broadening has been derived. The formulation presented here allows the independent specification of the sample volume position and width, in the three dimensions, and enables simple estimations of spectral shape for pulsed wave Doppler systems. Simpler expressions were derived for the cases of symmetric sample volume projections onto the vessel cross-section and/or sample volumes centred in the vessel. Closed form expressions were derived for mean frequency and spectral width in the case of a symmetric sample volume projection centred in the vessel. The effects of sample volume size and position on the Doppler spectral width and mean frequency are shown for a range of velocity profiles.

The effects of temporal bone on transcranial Doppler ultrasound beam shape

A knowledge of beam shape is desirable for many Doppler ultrasound applications, and is especially important for transcranial Doppler recordings where the beam may undergo significant distortion by the skull. Although it may not be possible to determine the precise beam shape for individual cases due to variations in the physical characteristics of the media in the beam path, information about the range of beam shapes that are likely to arise for in vivo recordings from the middle cerebral artery may in future allow limits of uncertainty to be derived, and may even allow partial correction in some cases. In order to assess the potential intersubject variation in beam sensitivity across the middle cerebral artery, the beam shapes generated by four commercial Transcranial Doppler transducers and the effects on beam shape of four different samples of temporal bone were investigated. The results demonstrate that there seems to be relatively little difference in the beam shapes of commercial transducers, but that the distortion effects of temporal bone are variable and unpredictable.

A review of the measurement of blood velocity and related quantities using Doppler ultrasound

Ultrasound systems can be used to investigate blood flow by use of the Doppler effect. The flow information may be displayed as either a real-time sonogram or a two-dimensional colour image. Estimates of maximum velocity using commercial systems are in error by typically 10-100 per cent; this is associated with the inability of the single-beam Doppler method to measure the true direction of flow, and with geometric spectral broadening. Vector Doppler systems acquire Doppler information along two beam directions and are able to measure accurately the velocity and direction of motion within the scan plane. The small beam width of modern Doppler systems means that the condition of uniform insonation, required for estimation of mean velocity from mean frequency shift, is not valid except for the very smallest vessels. Other quantities related to the velocity may also be estimated, such as the volumetric flow and wall shear stress. Flow visualization using colour flow imaging suffers from dependence of the displayed colour on the direction of blood motion. The vector Doppler technique may be extended to colour flow to give improved visualization of flow, in which there is no angle dependence within the scan plane.

Time-averaged mean velocity for volumetric blood flow measurements: an in vitro model validation study using physiological femoral artery flow waveforms

This study assesses the accuracy of the volume flow measurement of the ATL HDI 3000 duplex ultrasound scanner using a model of the femoral arterial circulation. The beam profile of the transducer was measured, and used to identify regions of the beam where there may be poor insonation characteristics. The flow measurement accuracy was not found to be influenced by the vessel depth between 1.0 cm and 8.0 cm in a 0.7 cm diameter vessel. Overall accuracy was 3%+/-9%. Vessels in excess of 0.9 cm produced larger errors. In the model system, pulse rates between 60 bpm and 120 bpm had no significant effect on the measurement accuracy (p > 0.01). The results of this study suggest that accurate measurements of femoral arterial blood flow are possible. Further work will be required to assess the accuracy of the technique in vivo.

Mean blood velocity measurement with a narrow ultrasound beam and an asymmetric velocity profile

Previous discussions of the measurement of spatial mean blood velocity using Doppler ultrasound with a narrow beam have required the velocity profile in the cross section to be symmetric about the vessel axis. A type of asymmetry is put forward which incurs no error in mean velocity measurement when the beam is directed through the point of maximum velocity. Assuming correct alignment of the beam, errors due to profile asymmetry are, therefore, related to the degree of departure of the asymmetry from this acceptable form.

The effect of refraction and assumed speeds of sound in tissue and blood on Doppler ultrasound blood velocity measurements

The combined effect of three assumptions relating to refraction, the speed of sound in tissue and the speed of sound in blood on the accuracy of Doppler ultrasound blood velocity measurements has been investigated. A theoretical relationship giving the net velocity measurement error introduced by these three assumptions has derived using a model in which tissue and blood layers are separated by straight, parallel boundaries. This net error is dependent on the assumed and actual speed of sound in tissue, the assumed speed of sound in blood and the Doppler angle, but is effectively independent of the actual speed of sound in blood. For clinical blood velocity measurements, the net error is estimated to be as much as an 8% overestimation of the actual velocity, higher than previously predicted for any of the factors individually. The relationship also predicts a net velocity measurement error in experimental flow systems and string phantoms which is dependent on the speed of sound in the liquid bath. A water bath at room temperature will give an overestimation of approximately 2%. Experimental investigations using conventional and modified string phantoms and a 5-MHz linear phased array system support these conclusions. The effect of perturbing the layers from their parallel orientation has also been considered theoretically and has provided additional support for the above conclusions. These results may help more accurate Doppler velocity measurements in both experimental and clinical settings.

A theory to correct the systematic error caused by the imperfectly matched beam width to vessel diameter ratio on volumetric flow measurements using ultrasound techniques

When a multigate procedure is used to measure volumetric flow in vessels, in addition to the flow rate result obtained from the conventional velocity profile method, a second result from an "average velocity profile method" can be obtained simultaneously. The latter method obtains the flow rate from the product of the average velocity across the profile and the cross-sectional area of the vessel. A theoretical model has been used to study the effect of the beam width to vessel diameter ratio (BWR) on these two results, as well as a third flow rate result obtained from the uniform insonation method. A theory has been established to correct the systematic error caused by the imperfectly matched BWR associated with each method. It uses a correction factor and the difference between the results from the average velocity profile method and the velocity profile method to compensate for the systematic error. The relationship between an optimal correction factor and the BWR under different flow conditions has been studied. The results using the correction theory in this model show that if the estimated BWR is within +/- 0.1 from the actual BWR value, the theoretical error in volumetric flow estimation can be limited to within 6.5% for the entire range of BWR.

Computerised transient hyperaemic response test--a method for the assessment of cerebral autoregulation

A simple bedside test has been developed to assess the state of autoregulation in subarachnoid haemorrhage patients. Transcranial Doppler was used to measure blood flow velocity in the middle cerebral artery after a brief common carotid compression. Acceleration of blood flow postcompression was interpreted as evidence of intact cerebral autoregulation. A program using the Windows environment was designed for signal analysis of the transient hyperaemic response test (THRT). The flow velocity signal from the TCD was recorded, carotid compression and release automatically detected and the test results immediately displayed and stored in a database. The program was verified in 614 tests; 552 of them were analysed off-line using previously recorded data and 62 on-line during the examination. A significant correlation was found between the results of computerised testing and the patient's neurological state.

A mean blood velocity statistic for the Doppler signal from a narrow ultrasound beam

An easily calculable statistic proportional to the instantaneous spatial mean blood velocity through a vessel cross section is derived from Doppler power spectral estimates for the case where the Doppler beam is assumed to be of negligible thickness compared to the vessel diameter. This is an alternative statistic to that derived where uniform insonation is assumed, an assumption thought to be poorer in many real cases. The main requirement is that the velocity profile is monotonic increasing from the vessel wall to the vessel center. Errors in each statistic are compared for a variety of true beam dimensions, using a variety of velocity profiles, and the new statistic is shown to incur less error for Gaussian beam response profiles with a standard deviation less than 0.4 of the vessel radius, or for rectangular response profiles with a width less than 0.65 of the vessel diameter. If an estimate can be made of the true beam dimensions and vessel diameter, a weighted sum of the two statistics can give a more accurate estimate of mean velocity. The case of a beam displaced from the center of the vessel is also considered, and errors are found to be less than 4% for a displacement of 20% of the vessel radius.

Variations in acoustical beam properties of intracoronary Doppler catheters

The limitations of coronary angiography in assessing the functional significance of coronary obstructions is well known. While the critical variable of coronary blood flow cannot be readily measured, intraluminal Doppler sonography offers useful related functional information on blood flow velocity. In order to fully evaluate Doppler signals it is essential to have exact knowledge of the transducer transmission characteristics and of the ultrasound beam topology. In an experimental set-up, the transmitter-receiver characteristics of five commonly used Doppler catheters were investigated. In comparing the beam characteristics we found inhomogeneities in the lateral beam spread. At a penetration depth of 3.0 mm the beam shape varied from a minimum of 1.25 mm up to a maximum of 3.5 mm. The mean was 2.25 mm. The different beam profiles of the investigated Doppler transducers cause an error in measuring the blood flow velocity. The blood flow velocity tends to be underestimated the more the vessel diameter and the blood flow velocity increase. Contrary to transducer design optimized for imaging, for spectral analysis of the Doppler signal it would be advantageous to have as broad a beam as possible in order to illuminate the entire vessel lumen.

Measurement of mean velocity during pulsatile flow using time-averaged maximum frequency of Doppler ultrasound waveforms

It has been suggested that mean velocity of flow could be estimated by the time-averaged maximum frequency over an integral number of cardiac cycles (Gill 1985). The present study verified this theory experimentally with a computer-controlled flow phantom. The effects of some parameters on the relationship between mean velocity and time-averaged maximum frequency were also studied. Parameters investigated included beam-vessel angle, diameter of tubing, pulsatility, flow rate and stenosis. The velocities measured by the Doppler system were compared with the actual velocities. A simple theoretical model was also developed to compare with the experimental results. The results showed that, in a long straight tube, the mean velocity can be estimated to within about 5% from the time-averaged maximum Doppler shift at various flow rates and pulsatilities. The error due to geometrical spectral broadening, especially for large beam-vessel angles, can be estimated to within 3% and therefore corrected.

The present status of ultrasonic imaging in medicine

The ultrasonic techniques that are used routinely in medical imaging have resulted from basic scientific discoveries, new methods of signal analysis and image processing, the development of transducer materials and fabrication techniques, and the application of digital electronics. The system designer is constrained by the ultrasonic properties of tissues, especially speed and attenuation: these properties determine the optimum choices of ultrasonic frequency, and spatial contrast and temporal resolutions. Ultrasonic Doppler techniques provide information about moving targets, including blood flow. Two-dimensional images of anatomical structure and blood flow can be combined in real-time displays. Other advances include sonoelasticity imaging, computed tomography, three-dimensional imaging, contrast agents and quality assurance. Contemporary ultrasonic diagnostic techniques seem to be safe, but the search for possible hazards is continuing with emphasis on thermal effects and cavitation.

Calculation of Doppler spectral power density functions

A volume integral method for the calculation of Doppler ultrasound spectral power density (spd) functions is described. Axisymmetric flow in a circular tube with a power law velocity profile is assumed. The spd function is regarded as a probability density function for scatterer velocity, and the assumptions under which this is justified are considered. It is shown that the spd function is independent of Doppler angle except in the presence of wall reflection effects. A coordinate system centered on the beam is used and this enables the integrals to be easily formulated for arbitrary beams. Irregularly shaped and nonuniform beams can be treated. For the common flow and beam patterns, which exhibit symmetry, the volume integrals can often be reduced to a single integral and evaluated directly. The method is applied and the spectra are calculated for various different cases. Results are obtained for uniform rectangular and circular insonating beams, and for nonuniform beams with Gaussian, jinc, and sinc profiles. The effects of narrow beams and wall reflection are shown. The method may be readily applied to other beam and flow patterns, and extension to more complicated situations is also discussed.

A computer controlled flow phantom for generation of physiological Doppler waveforms

A flow phantom for the generation of physiological Doppler waveforms is described. The suspension of scattering particles is driven by a gear pump powered by a stepping motor. The speed of the stepping motor is controlled by a BBC microcomputer. The waveform shape is selected from a library of waveforms from disc. Use of the microcomputer allows the waveform shape and mean flow to be easily changed. Sephadex particles suspended in a solution of glycerol were used as artificial blood. Thin walled heat shrink tubing which had been moulded around metal rods was used. Distortions in the waveforms caused by reflections from the end of the tubing were largely removed by reducing the pipe diameter to half of its value for 30 cm from the end of the pipe. There was good agreement between the control waveforms and the Doppler waveforms over a wide range of waveform pulsatility.

Selection of a spectral analysis method for the assessment of velocity distribution based on the spectral distribution of ultrasonic Doppler signals

The direct Fourier transform method, autoregressive modelling, the maximum likelihood method and the Wigner-Ville distribution were applied to the Doppler signal obtained from a fully insonated laminar model flow. The appreciation of the spectral method was based on the properties of the ratio variance/(fmean)2 (INT) of the spectrum. The basic criterion was the sensitivity of INT to the analysis parameters, especially the data window. The results of spectral analysis, as well as the properties of INT, were strongly affected by the method applied. The maximum likelihood method appeared best suited for the purpose of assessment of velocity distribution and is expected to give the best results in the case of in vivo blood flow. The performances of other discussed methods were inferior, due to their stronger incompatibility with the signal properties.

Comparison of peak and modal aortic blood flow velocities with invasive measures of left ventricular performance

The purpose of this study was to determine the accuracy of Doppler-derived modal and maximum velocity and peak and mean acceleration of ascending aortic blood for the assessment of left ventricular systolic function. Studies were performed in six anesthetized open-chest dogs. Doppler-derived modal velocity, maximum velocity, and peak and mean acceleration were compared with left ventricular dP/dt, maximum aortic blood flow, and rate of blood flow measured with an electromagnetic flow probe under varying inotropic states. Maximum Doppler velocity showed better correlation (r = 0.94, y = 0.34 + 3.95) with maximum aortic blood flow than the modal velocity (r = 0.85, y = 1.49 + 3.85x). Peak acceleration also correlated better with the rate of blood flow (r = 0.92, y = 12.3 + 4.92x) than the mean acceleration (r = 0.83, y = 12.2 + 4.27x). Modal and maximum velocity and mean and peak acceleration correlated well with left ventricular dP/dt. We conclude that peak modal and peak maximum velocity and peak and mean acceleration are accurate measurements of left ventricular function. Maximum velocity and peak acceleration are more accurate than modal velocity and mean acceleration.

The physical principles of Doppler and spectral analysis

The Doppler effect provides an ultrasonic method for the detection of echoes from moving structures, particularly flowing blood. In its most simple form, the continuous wave Doppler offers velocity information without depth resolution and is therefore used mainly for the examination of superficial structures. The pulsed Doppler, in combination with real-time imaging, provides a more flexible tool for the interrogation of selected sites in an ultrasound image for motion and flow. The recent development of Doppler flow imaging, in which limited Doppler information is displayed over an entire ultrasound image, usually in color and in real-time, promises to secure the association between Doppler and conventional ultrasound imaging techniques. Spectral analysis permits features of the Doppler signal to be identified which are associated with hemodynamic phenomena, such as flow disturbance and wave reflection. In addition, it allows the quantitative application of Doppler to the estimation of such physiological variables as velocity, flow rate, and pressure difference.

Transcranial pulsed Doppler ultrasound findings in brain stem death

Data are presented from transcranial insonation of the middle cerebral artery (MCA) performed at intervals in 23 unconscious children for whom the outcome was subsequently poor. Once an MCA signal had been observed over a 30 minute period with time averaged velocity less than 10 cm s-1 and/or a direction of flow index, DFI, defined as 1 minus the ratio of reverse to forward flow of less than 0.8, recovery to forward flow throughout diastole was never observed and no patient recovered brain stem reflexes. Recovery of forward flow in diastole, and of brain stem function, was seen in cases with time averaged MCA velocity in the range 10 to 25 cm s-1 and with reverse flow but a DFI of greater than 0.8 for short periods of time. All but one of the 13 children fulfilling clinical criteria for brain stem death had MCA signals with time averaged velocity of less than 10 cm/s and DFI of less than 0.8. This type of signal was not observed in five children who were left in a persistent vegetative state.

Failure of autoregulation of cerebral blood flow in neonates studied by pulsed Doppler ultrasound of the internal carotid artery

To reveal the influence of therapeutically induced changes of arterial blood pressure on cerebral circulation, pulsed Doppler measurements of blood velocity in the right internal carotid artery were performed in 23 neonates. A positive correlation between mean arterial blood pressure and time-averaged maximum blood velocity (change more than 0.5%/torr) could be noticed in 16 infants. These infants were supposed to have loss of autoregulation. The main characteristics in this non-autoregulating group were: gestational age less than 31 weeks, birth weight less than 1501 g and mean carotid blood velocity less than 20 cm/s. In accordance with animal experiments we assume that autoregulation does not work below a definite lower limit of brain perfusion, which is reflected by carotid blood velocity in our study. Patients below/equal 1500 g or 30 gestational weeks very often do not exceed this limit and thus do not reach the "range of autoregulation".

Influence of spectral broadening on continuous wave Doppler ultrasound spectra: a geometric approach

A model is presented that enables the detailed effects of spectral broadening to be calculated for a continuous wave (CW) Doppler system by using geometric boundary arguments. The model assumes a uniform distribution of isotropic scatterers and treats the transmitter and receiver crystals as incremental sources and receivers. Detailed results for rectangular and circular geometries are presented in order to provide a physical understanding of the manner in which spectral broadening arises. Results are given for the circular geometry, to illustrate the manner in which the received spectrum is affected by the transducer size and distance from the vessel.

Transcranial measurement of blood velocities in the basal cerebral arteries using pulsed Doppler ultrasound: velocity as an index of flow

Blood velocities have been measured transcranially, at small Doppler angles, in the middle cerebral artery of normal volunteers. Cerebral blood flow was changed by varying carbon dioxide tension. In four volunteers, the relationships between arterial pCO2 and percentage change in intensity weighted mean, median, and maximum Doppler-shifted frequencies in the internal carotid and middle cerebral arteries were linear with slopes of 2.5 and 2.8% per mm Hg change in pCO2. In 38 volunteers, the relationship between end-expiratory pCO2 and time-averaged maximum Doppler frequency was linear over the range of pCO2 20-60 mm Hg with slopes of 2.5 and 2.9 percentage change per mm Hg, for internal carotid and middle cerebral, respectively. These results are very similar to those reported using direct methods of measuring cerebral blood flow. As the transcranial Doppler method is reproducible, this indicates that changes in middle cerebral blood velocity may be used to monitor changes in flow.

Measurement of blood flow by ultrasound: accuracy and sources of error

Doppler ultrasound has now developed to the point where the rate of flow of blood in a given vessel can be measured with appropriate instrumentation. The theoretical basis of Doppler flow measurement is reviewed in this paper, with particular emphasis on the potential and actual sources of error. Three distinct approaches are identified, and the strengths and weaknesses of each discussed. The separate errors involved in estimating the vessel cross-sectional area, the angle of approach, and the Doppler shift are analyzed, together with the question of the uniformity of scattering from the blood. In vivo and in vitro tests of the accuracy obtained using a number of Doppler flow measuring instruments are then reviewed. It is concluded that the Doppler methods are capable of good absolute accuracy when suitably designed equipment is used in appropriate situations, with systematic errors of 6% of less. There are, however, considerable random errors, attributable primarily to errors in measuring the cross-sectional area and the angle of approach. Repeating the measurement of flow several times and averaging the results can reduce these random errors to an acceptable level.

Non-invasive measurement of blood flow-rate in expanded polytetrafluoroethylene grafts

Four 6 mm e-PTFE grafts were placed subcutaneously into four dogs between the carotid artery and jugular vein to model shunt flow through an access graft. Flow-rates were varied from 120 to 2000 ml/min (N = 42) by use of a cardiac stimulant (Aramine) and external constrictor. Measures of graft flow were made simultaneously by electromagnetic flowmeter (EMF) and Doppler ultrasound based on a known graft diameter, use of a flat-head probe providing a fixed beam-to-vessel angle and derivation of the mean Doppler-shift frequency from first moment computation of the frequency spectra. Correlation coefficients between the Doppler ultrasound and EMF recordings were 0.54, 0.96 and 0.92 at measurement sites just beyond the arterial/graft anastomosis, at the graft loop and just proximal to the graft/venous anastomosis, respectively. Availability of a simple and accurate non-invasive flowmeter for superficially placed e-PTFE grafts may be valuable for evaluating the systemic effects of vascular shunts, for monitoring the patency of access grafts and for assessing the function of bypass grafts for limb ischaemia.

The interpretation of continuous wave ultrasonic Doppler blood velocity signals viewed as a problem in pattern recognition

The process of inferring the state of a patient's circulation from ultrasonic Doppler waveforms may be regarded as a problem in pattern recognition. In this article, each stage in the process, as it might be applied to the analysis of Doppler signals, is examined in turn and it is shown how an automatic on-line system for arterial assessment may one day be practical using such principles.