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

A theoretical model has been developed to predict the effect of ultrasonic beam width on the output of maximum frequency, mean frequency and rms frequency processors when interrogating blood vessels containing either parabolic or plug flow. It is shown that the only way to measure blood flow velocities, and obtain waveforms that are proportional to instantaneous volumetric flow (irrespective of changes in velocity profile) is to use an ultrasonic beam which is uniform over the whole area of the vessel, and to process the Doppler shift signal with a mean frequency detector.     

Some aspects of the relationship between instantaneous volumetric blood flow and continuous wave doppler ultrasound recordings—III. The calculation of doppler power spectra from mean velocity waveforms, and the results of processing these spectra with maximum, mean, and rms frequency processors

A method of predicting velocity profiles and hence Doppler relative power spectra (RPS) from mean volumetric flow waveforms using an extension of Womersley's theory is described. The effect on the RPS of using an ultrasound beam which is smaller than the blood vessel is calculated, and comparisons are made between RPS found in this way and experimental RPS measured in a dog model. Finally the effect of making ultrasonic Doppler measurements on complex velocity profiles with different combinations of processing technique and ultrasonic beam size are considered.     

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.     

Implementation of spectral width Doppler in pulsatile flow measurements

In this paper, we present an automatic beam-vector (Doppler) angle and flow velocity measurement method and implement it in pulsatile flow measurements using a clinical Doppler ultrasound system. In current clinical Doppler ultrasound flow velocity measurements, the axis of the blood vessel needs to be set manually on the B-scan image to enable the estimation of the beam-vector angle and the beam-vector angle corrected flow velocity (the actual flow velocity). In this study, an annular array transducer was used to generate a conical-shaped and symmetrically focused ultrasound beam to measure the flow velocity vectors parallel and perpendicular to the ultrasound beam axis. The beam-vector angle and flow velocity is calculated from the mode frequency (f(d)) and the maximum Doppler frequency (f(max)) of the Doppler spectrum. We develop a spectrum normalization algorithm to enable the Doppler spectrum averaging using the spectra obtained within a single cardiac cycle. The Doppler spectrum averaging process reduces the noise level in the Doppler spectrum and also enables the calculation of the beam-vector angle and flow velocity for pulsatile flows to be measured. We have verified the measurement method in vivo over a wide range of angles, from 52 degrees to 80 degrees, and the standard deviations of the measured beam-vector angles and flow velocities in the carotid artery are lower than 2.2 degrees and 12 cm/s (about 13.3%), respectively.     

血流切变率的多普勒估计方法

目的：研究血液动力学的一个重要参数—血流剪切率。方法：本文提出了一种利用超声多普勒技术估计血流剪切率的方法，这种方法先估计多普勒信号的平均频率曲线和最大频率曲线，然后计算血流的速度剖面，最后得到时变的血流剪切率。结果：文中还给出这方面研究的实验及结果。结论：本文提出的方法是无损估计血流剪切率的有效方法。     

In vitro validation of volumetric blood flow measurement using Doppler flow wire

Determination of any volumetric blood flow requires assessment of mean blood flow velocity and vessel cross-sectional area. For evaluation of coronary blood flow and flow reserve, however, assessment of average peak velocity alone is widely used, but changes in velocity profile and vessel area are not taken into account. We studied the feasibility of a new method for calculation of volumetric blood flow by Doppler power using a Doppler flow wire. An in vitro model with serially connected silicone tubes of known lumen diameters (1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mm) and pulsatile blood flow ranging from 10 to 200 mL/min was used. A Doppler flow wire was connected to a commercially available Doppler system (FloMap(R), Cardiometrics) for online calculation of the zeroth (M(0)) and the first (M(1)) Doppler moment, as well as mean flow velocity (V(m)). Two different groups of sample volumes (at different gate depths) were used: 1. two proximal sample volumes lying completely within the vessel were required to evaluate the effect of scattering and attenuation on Doppler power, and 2. distal sample volumes intersecting completely the vessel lumen to assess the vessel cross-sectional area. Area (using M(0)) and V(m) (using M(1)/M(0)) obtained from the distal gates were corrected for scattering and attenuation by the data obtained from the proximal gates, allowing calculation of absolute volumetric flow. These results were compared to the respective time collected flow. Correlation between time collected and Doppler-derived flow measurements was 0.98 (p < 0.0001), with a regression line close to the line of equality indicating an excellent agreement of the two measurements in each individual tube. The mean paired flow difference between the two techniques was 1.5 +/- 9.0 mL/min (ns). Direct volumetric blood flow measurement from received Doppler power using a Doppler flow wire system is feasible. This technique may potentially be of great clinical value because it allows an accurate assessment of coronary flow and flow reserve with a commercially available flow wire system.     

The influence of variations in blood flow on pulsed doppler ultrasonic heating of the cerebral cortex of the neonatal pig

Pulsed Doppler ultrasound examination of the fetal cerebral circulation may cause potentially harmful temperature elevations in brain tissue immediately beneath the insonated segment of the skull. This study measured the effect of variations in cerebral blood flow on ultrasonic heating of the cerebral cortex of anaesthetised, neonatal pigs. Wide and narrow ultrasound beams were used. Pulsed ultrasound exposures were delivered in 90 s bursts at 5.8 micros pulse length, pulse repetition frequency 8 kHz and centre frequency 3.5 MHz. Studies were performed with the target at the focus of a fixed, stationary beam of 0.3 cm -6 dB beam width (narrow beam) and I(spta) 1.4 W/cm(2) (n = 11), or with the target in the near field of a fixed, stationary beam of 1.6 cm -6 dB beam width (wide beam) and I(spta) 3.6 W/cm(2)(n = 5). The 90 s ultrasound exposures were performed under three different conditions of ambient cerebral blood flow: baseline (during normocarbic, normoxic conditions), increased (during hypercarbic, hypoxic conditions) and absent (postmortem). Cerebral blood flow was measured using the radiolabelled microsphere technique. In the narrow beam studies, cerebral blood flow during baseline was 34 +/- 4 ml/min/100 g, rising to 109 +/- 32 ml/min/100 g during the increased phase (p < 0.001); in the wide beam studies baseline flows were 29 +/- 9 ml/min/100 g, whereas flows in the increased phase were 128 +/- 32 ml/min/100 g (p < 0.001). There was no difference in the heating curves for normal, increased and absent cerebral blood flow for exposure to the narrow beam, when mean temperature increases of 1.5 degrees C at 90 s were recorded in each case (p > 0.21, power > 0.8). However, the heating curves for the wide beam were significantly different for the three rates of blood flow with mean temperature increases of 1.9 degrees C (normal flow), 1.7 degrees C (increased flow) and 2.4 degrees C (no flow) recorded at 90 s (p < 0.05).     

Orientation-independent rapid pulsatile flow measurement using dual-angle Doppler OCT

Doppler OCT (DOCT) can provide blood flow velocity information which is valuable for investigation of microvascular structure and function. However, DOCT is only sensitive to motion parallel with the imaging beam, so that knowledge of flow direction is needed for absolute velocity determination. Here, absolute volumetric flow is calculated by integrating velocity components perpendicular to the B-scan plane. These components are acquired using two illumination beams with a predetermined angular separation, produced by a delay encoded technique. This technology enables rapid pulsatile flow measurement from single B-scans without the need for 3-D volumetric data or knowledge of blood vessel orientation.OCIS codes: (110.4500) Optical coherence tomography, (170.3880) Medical and biological imaging     

Development of a Flexible Implantable Sensor for Postoperative Monitoring of Blood Flow

We have developed a blood flow measurement system using Doppler ultrasound flow sensors fabricated of thin and flexible piezoelectric‐polymer films. These flow sensors can be wrapped around a blood vessel and accurately measure flow. The innovation that makes this flow sensor possible is the diffraction‐grating transducer. A conventional transducer produces a sound beam perpendicular to its face; therefore, when placed on the wall of a blood vessel, the Doppler shift in the backscattered ultrasound from blood theoretically would be 0. The diffraction‐grating transducer produces a beam at a known angle to its face; therefore, backscattered ultrasound from the vessel will contain a Doppler signal. Flow sensors were fabricated by spin coating a poly(vinylidene fluoride–trifluoroethylene) copolymer film onto a flexible substrate with patterned gold electrodes. Custom‐designed battery‐operated continuous wave Doppler electronics along with a laptop computer completed the system. A prototype flow sensor was evaluated experimentally by measuring blood flow in a flow phantom and the infrarenal aorta of an adult New Zealand White rabbit. The flow phantom experiment demonstrated that the error in average velocity and volume blood flow was less than 6% for 30 measurements taken over a 2.5‐hour period. The peak blood velocity through the rabbit infrarenal aorta measured by the flow sensor was 118 cm/s, within 1.7% of the measurement obtained using a duplex ultrasound system. The flow sensor and electronics operated continuously during the course of the 5‐hour experiment after the incision on the animal was closed.     

Assessment of the effect of vessel curvature on Doppler measurements in steady flow

Blood vessel curvature is responsible for the appearance of nonaxial velocity components and for minor changes in the pattern of the axial flow. All the velocity components are expected to contribute to the Doppler signal produced by the ultrasound (US) backscattered by the insonated blood cells, the axial velocity, contributing to the actual volumetric blood flow, and the transverse velocity, causing the recirculating vortices. A detailed, separate analysis of the velocity components is, therefore, mandatory to quantify how vessel curvature can affect results and clinical diagnosis. Both experimental in vitro measures and numerical simulations were performed on a curved tube and the Doppler power spectra so obtained were compared. The satisfactorily agreement of the above spectra shows that the nonaxial velocity components are easily detectable with clinical equipment and that their amplitude, as expected, is not negligible and can bias Doppler measurements and resulting clinical diagnosis.     

经颅彩色多普勒超声在诊断颅内动脉狭窄中的应用

目的探讨经颅彩色多普勒超声（TCCD）在颅内动脉狭窄诊断中的价值。方法对30例疑有颅内动脉狭窄的患者应用TCCD观察颅内各动脉的形态、走行，应用频谱多普勒测量各条血管的血流参数，包括收缩期峰值流速、舒张末流速、平均流速、阻力指数，并与数字减影血管造影（DSA）结果进行对比分析。结果16例TCCD诊断为颅内动脉狭窄，14例经DSA证实。TCCD诊断颅内动脉狭窄的特征是狭窄处彩色血流束变细，典型者呈“束腰征”；频谱多普勒显示狭窄处血流速度异常增高，频谱形态呈湍流，同时音频信号响亮。狭窄严重者彩色血流束连续性欠佳或者中断，血流速度不增快或者明显减低。血管闭塞者血流不显示。结论TCCD对于颅内动脉狭窄具有一定的诊断价值，可作为早期筛检性诊断方法。     

Doppler color flow imaging

By simultaneous processing of frequency, phase, and amplitude information in the backscattered ultrasound signal, new instruments now permit the real-time display of high-resolution grey scale images of tissue combined with the simultaneous display of flow data from vessels within the scan plane. Doppler Color Flow Imaging, or DCFI, using such processing, permits blood flow direction and relative velocity to be detected and displayed in a color encoded display from throughout the ultrasound image. We have tested a new Doppler color flow imaging system over a period of two years to evaluate the carotid arteries, peripheral arteries and veins, and dialysis fistulas. In the abdomen and pelvis we have imaged blood flow to the liver, spleen, kidneys, uterus and renal transplants. Our experience in over 500 patients leads us to conclude that DCFI has significant advantages over conventional duplex Doppler sonography for blood flow evaluation. For examination of carotid and peripheral vessels, we have found DCFI to permit more rapid assessment in both normal and abnormal states. Areas of vessel narrowing or turbulent flow may be identified rapidly and accurately, and vessel orientation may be determined precisely, allowing accurate calculation of blood flow velocity from Doppler frequency shifts. The system we have used has adequate penetration and sensitivity to allow imaging of hepatic and renal blood flow and is extremely promising as a method of imaging organ perfusion and in the detection of abnormalities of perfusion that accompany disease, such as transplant rejection. Tumor vascularity may also be identified with DCFI, opening the possibility of additional clinical applications.     

超声多普勒功率及其在检测颅内血流分布上的应用

本文结合背向散射超声多普勒信号模型对超声多普勒功率的概念及其特性进行了阐述，并提出了一种用超声多普勒功率来检测颅内血管血流分布的方法。实验结果证明，该方法能在噪声背景下灵敏准确地反应沿声束指向一维空间各个位置是否存在血流以及血流强度的大小。这一方法已被用于经颅多普勒（ＴＣＤ）脑血流分析仪中，解决了现有经颅多普勒检查时只能凭经验盲目搜寻颅内血管的问题，并提供了颅内血管分布的相对位置信息以帮助临床医生更方便准确地判定所检测的是哪一根血管。     

A viscoelastic model of arterial wall motion in pulsatile flow: implications for Doppler ultrasound clutter assessment

The existing computational model studies of pulsatile blood flow in arteries have assumed either rigid wall characteristics or elastic arterial wall behavior with wall movement limited to the radial direction. Recent in vivo studies have identified significant viscoelastic wall properties and longitudinal wall displacements over the cardiac cycle. Determining the nature of these movements is important for predicting the effects of ultrasound clutter in Doppler ultrasound measurements. It is also important for developing an improved understanding of the physiology of vessel wall motion. We present an analytically-based computational model based on the Womersley equations for pulsatile blood flow within elastic and viscoelastic arteries. By comparison with published in vivo data of the human common carotid artery as well as uncertainty and sensitivity analyses, it is found that the predicted waveforms are in reasonable quantitative agreement. Either a pressure, pressure gradient or volumetric flow rate waveform over a single cardiac cycle is used as an input. Outputs include the pressure, pressure gradient, radial and longitudinal fluid velocities and arterial wall displacements, volumetric flow rate and average longitudinal velocity. It is concluded that longitudinal wall displacements comparable to the radial displacements can be present and should be considered when studying the effects of tissue movement on Doppler ultrasound clutter.     

An integrated system for the non-invasive assessment of vessel wall and hemodynamic properties of large arteries by means of ultrasound.

OBJECTIVES: To integrate methods for non-invasive assessment of vessel wall properties (diastolic diameter, distension waveform and intima-media thickness) and hemodynamic properties (blood flow velocity and shear rate distribution) of large arteries by means of dedicated ultrasound signal processing. METHODS: we have developed an arterial laboratory (ART-lab) system. ART-lab consists of software running on a standard personal computer, equipped with a data acquisition card for the acquisition of radio frequency (RF) ultrasound signals obtained with a conventional echo scanner. It operates either (1) off-line or (2) in real-time. Real-time operation is restricted to the assessment of vessel wall properties because of limitations in computational power. RESULTS: This paper provides an overview of ART-lab ultrasound radio frequency data acquisition and dedicated RF-signal processing methods. The capabilities of the system are illustrated with some typical applications. CONCLUSIONS: ART-lab in real-time mode is a useful tool for monitoring arterial vessel wall dynamics, while off-line it can be employed to investigate the elastic vessel wall properties in combination with hemodynamics, such as blood flow velocity and shear rate distribution.     

The relationship between time averaged intensity weighted mean velocity, and time averaged maximum velocity in neonatal cerebral arteries

It is usual practice to calculate the mean velocity of blood flow in neonatal cerebral vessels from the intensity weighted mean (IWM) Doppler shift signal. Theoretically however the maximum frequency envelope could be used for similar purposes, and indeed may have certain advantages. The purpose of this study was to confirm the suitability of the maximum frequency method. Good quality Doppler recordings from the anterior cerebral and middle cerebral arteries of both term and very low birth weight babies were analyzed by both methods and compared. A small number of recordings were then deliberately degraded, either by the addition of noise or by the use of inappropriate filters, and reanalyzed. The results of these comparisons suggest that the maximum frequency follower should be the follower of choice for neonatal cerebral blood flow studies.     

Pulsed Doppler with B-mode imaging for quantitative blood flow measurement.

A technique is described, using the UI Octoson and a frequency-offset pulsed Doppler system, to obtain fully quantitative blood flow measurements in deep-lying vessels. By uniformly insonating the vessel and using a mean frequency Doppler demodulator, average velocity is obtained regardless of the velocity profile. B-scan imaging provides the necessary anatomic information to calculate volume flow from this average velocity. Results of in vitro flow measurement tests indicate accuracies of ± 14% rms error, ± 32% maximum error. The causes of error appear to be well understood, and in a number of cases they can be corrected. Preliminary clinical measurements of fetal umbilical vein flow and adult right branch portal vein flow are also presented.     

Interaction between secondary velocities,flow pulsation and vessel morphology in the common carotid artery

The common carotid artery (CCA), one of the vessels more frequently investigated by ultrasound (US), is often modeled as a straight tube in quasi-laminar flow regimens. Experimental investigations based on a prototype multigate system show that blood velocity profiles are parabolic during diastole and early systole, and flat during the systolic peak. However, during late systole/beginning of diastole, they have an "M" shape, where the velocity near the walls is higher than in the vessel center. Moreover, the profile shape changes when the sound beam direction is moved over a given cross-section; thus, suggesting a nonaxisymmetrical velocity distribution, which contradicts the straight tube assumption. The purpose of this paper was twofold. First, the actual velocity distribution in "normal" CCAs was reconstructed. The analysis of several velocity profiles confirms that the velocity distribution is markedly asymmetrical, especially during the deceleration phase following the systolic peak. Second, a tentative explanation for such behavior is given by correlating it with the growth of secondary flows caused by the slight vessel curvature and viscous effects. This explanation is supported by the comparison between in vitro results and numerical solution of the Navier-Stokes equations in laminar pulsed-flow regimens.     

Investigation of Ultrasound-Measured Flow Velocity,Flow Rate and Wall Shear Rate in Radial and Ulnar Arteries Using Simulation

Parameters of blood flow measured by ultrasound in radial and ulnar arteries, such as flow velocity, flow rate and wall shear rate, are widely used in clinical practice and clinical research. Investigation of these measurements is useful for evaluating accuracy and providing knowledge of error sources. A method for simulating the spectral Doppler ultrasound measurement process was developed with computational fluid dynamics providing flow-field data. Specific scanning factors were adjusted to investigate their influence on estimation of the maximum velocity waveform, and flow rate and wall shear rate were derived using the Womersley equation. The overestimation in maximum velocity increases greatly (peak systolic from about 10% to 30%, time-averaged from about 30% to 50%) when the beam–vessel angle is changed from 30° to 70°. The Womersley equation was able to estimate flow rate in both arteries with less than 3% error, but performed better in the radial artery (2.3% overestimation) than the ulnar artery (15.4% underestimation) in estimating wall shear rate. It is concluded that measurements of flow parameters in the radial and ulnar arteries with clinical ultrasound scanners are prone to clinically significant errors.     

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.