Comparison of the performance of the RF cross correlation and doppler autocorrelation technique to estimate the mean velocity of simulated ultrasound signals

In pulsed Doppler systems the received RF (radio frequency) signal is multiplied by a quadrature reference signal and subsequently averaged over a short depth range to obtain a sample of the complex Doppler signal. The mean frequency of the sampled Doppler signal, obtained with the autocorrelation function, reflects the mean velocity of the scatterers moving through the sample volume. An alternative is to evaluate the two-dimensional cross correlation function of a short segment of the RF signals over subsequent lines, giving the mean velocity of the scatterers. Both methods of velocity estimation were applied to computer-generated RF signals with varying RF bandwidth, signal-to-noise ratio, and mean and width of the imposed velocity distribution. The length of the RF signal segment and the number of lines for velocity estimation (package length) affects the accuracy of the velocity estimate. It can be concluded that the cross correlation technique behaves superiorly especially for a low velocity dispersion. Furthermore, the standard deviation of the velocity estimate decreases for an increasing sample volume length and package length, while the performance of the conventional Doppler technique is rather independent of the length of the sample volume. The difference between both techniques decreases for a greater package length or for signals simulating a wide velocity distribution.     

An efficient algorithm to remove low frequency Doppler signals in digital Doppler systems.

In color flow imaging, a high flow map rate in combination with a reasonable width of the map and good velocity resolution is essential to properly appreciate the time-dependent phenomena. The velocity resolution depends on the length of the signal segment considered in combination with the settling time of the high pass filter used to eliminate transients and low frequency artifacts. The latter can be reduced by appropriate processing. This paper presents an algorithm to suppress low frequency Doppler signals effectively and efficiently, while all the data points within the segment considered contribute equally to the average Doppler frequency computed. The algorithm is applied to computer generated Doppler signals to evaluate their time and frequency behavior. It is concluded that the proposed scheme functions adequately under various signal conditions.     

Alterations in segmentary branch pulmonary artery blood flow velocimetry in fetuses of diabetic mothers.

OBJECTIVE: To evaluate pulmonary blood flow in fetuses of diabetic mothers by measuring changes in fetal segmentary pulmonary artery blood flow velocimetry throughout pregnancy. METHODS: Thirty-eight women with pregestational diabetes between weeks 18 and 38 were compared with 99 women with singleton low-risk gestations as controls. Flow velocity waveforms at the proximal middle and distal segments of the right pulmonary artery were obtained with power and color Doppler sonography in all fetuses. The pulsatility index of each segment was compared between the 2 groups. The mean value and 95% confidence interval for each segment were determined in correlation with gestational age for both groups. RESULTS: The highest mean pulsatility indices were obtained in the proximal segment of the pulmonary artery and were 2.25 in the diabetes group and 2.36 in controls. The mean pulsatility indices were significantly decreased in the middle and distal segments to 1.59 and 1.10 in the diabetes group and to 1.57 and 1.02 in controls (P < .05). There were no significant differences in pulsatility indices measured at the proximal and middle segments between the study and control groups. However, the mean pulsatility index +/- SD measured at the distal segment in the diabetic group was 9% higher than in controls (1.10 +/- 0.13 versus 1.02 +/- 0.12; P = .01). The mean pulsatility index (in the study and control groups) in each arterial segment did not change significantly throughout gestation (P > .1). CONCLUSIONS:. In human fetuses throughout gestation, the pulmonary circulation maintains stable vascular resistance in both diabetic and normal pregnancies. However, in all gestations, the pulsatility index in each segment of the pulmonary artery is unique and reflects the proximity to the heart and the impedance at each location. The significantly higher pulsatility index in the diabetes group might be related to alterations in the microcirculation of diabetic patients.     

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.     

Reliability of manual examination and frequency of symptomatic cervical motion segment dysfunction in cervicogenic headache

This study investigated the reliability of manual examination procedures and the frequency that each or multiple segments in the upper cervical spine above the C4 vertebra were the dominant source of pain in subjects with cervicogenic headache (CGH). Eighty subjects were evaluated, 60 with CGH (39 females, mean age 33 years) and arbitrarily a further 20 asymptomatic subjects (13 females, mean age 34 years) included to reduce examiner bias, but subsequently omitted from data analysis. Two experienced physiotherapists examined on the same day each subject with standard manual examination procedures, independently rating each segment in the upper cervical spine above the C4 vertebra for involvement. Examiners were blind to each other’s findings and the subject’s clinical status. Standard and adjusted Kappa coefficients were calculated for each segment in symptomatic subjects only. Chi-squared analysis for goodness of fit was used to identify the segment that was most frequently determined the predominant symptomatic segment. Manual examination above the C4 vertebra showed good reliability. The C1/2 segment was most commonly symptomatic, with a positive finding at this segment in 63% of cases. The high frequency of C1/2 involvement in CGH highlights the importance of examination and treatment procedures for this motion segment.     

Segmentary fetal branch pulmonary artery blood flow velocimetry: in utero Doppler study.

OBJECTIVE: To evaluate changes in human, fetal segmentary, pulmonary artery blood flow velocimetry throughout pregnancy. DESIGN: Ninety-nine women with a singleton, low-risk gestation between 14 and 37 weeks of pregnancy were selected to participate in a prospective, cross-sectional study. All fetuses were evaluated using power and color Doppler ultrasound. Flow velocity waveforms at three sites of the right pulmonary artery were obtained. The pulsatility index (PI) was calculated in the proximal, mid and distal segment of the pulmonary artery. Mean values and 95% confidence interval (CI) for each segment were determined in correlation with gestational age. RESULTS: A full study that included Doppler measurements of all three segments of the pulmonary artery was completed on 99 fetuses. The highest mean PI of 2.36 was obtained in the proximal segment of the right pulmonary artery (CI = 2.29-2.42), whereas in the mid and distal segments the mean PI decreased significantly to 1.57 (CI = 1.53-1.61) and 1.02 (CI = 1.0-1.0) (P < 0.001), respectively. Throughout gestation, the mean PI measurements in the proximal, middle and distal segments of the branch pulmonary artery increased slightly, but without statistical significance (r = 0.274, 0.248, 0.047), respectively; (P > 0.5). CONCLUSIONS: The data obtained suggests that pulmonary circulation maintains stable vascular resistance during gestation in the human fetus. However, the PI obtained from the separate segments of the branch pulmonary artery is unique and each differs from the other, reflecting the proximity to the heart and the peripheral impedance at each location.     

Performance of the mean frequency Doppler modulator.

The mean frequency Doppler demodulator is of interest for quantitative measurement of blood flow, particularly in small or deep-lying vessels. It is thus desirable that the various potential sources of error be considered, and that their effects on its performance be quantified where possible. This paper analyzes the effects of interfering noise, more than one vessel falling within the Doppler sample volume, Doppler filtering, frequency aliasing, and double-sidebanding of the Doppler signal. The analysis applies to either frequency offset or non-offset Doppler systems, and a variety of blood velocity distributions is considered. It is shown that, in a number of instances, the errors can be predicted and therefore corrected. Experimental results are presented confirming the theoretical analysis.     

Mean Myocardial Velocity Mapping in Quantifying Regional Myocardial Contractile Reserve in Patients with Impaired Left Ventricular Systolic Function: Doppler Myocardial Imaging Study

The aim of this study was to use Doppler myocardial imaging–derived mean myocardial velocity (MMV) at baseline and during low-dose dobutamine stress echocardiography (DSE) to quantify regional contractile reserve of the left ventricle (LV). Sixteen patients (mean age 59 ± 7 years) with coronary artery disease and regional left ventricular wall motion abnormalities were studied. During each increment of Dobutamine infusion, 6 2-dimensional transthoracic apical images were acquired in standard gray-scale and Doppler myocardial imaging modes at 30° steps over 180°. For the analysis, the LV was divided into 18 segments. For each segment, both wall motion score and MMV obtained in systole and both early and late diastole were measured at baseline and at each stage of DSE. In viable segments by wall motion score, MMV increased during DSE in systole and in early and late diastole. In contrast, in nonviable segments, MMV did not change during DSE. Mean myocardial velocity mapping is a promising new approach to quantify regional myocardial contractile reserve of the LV. (J Am Soc Echocardiogr 2000;13:96-107.)     

Adaptive estimation of the mean frequency of a doppler signal from short data windows

The color Doppler estimator (CE₁), which is calculated from the phase of the first correlation lag of the Doppler signal, is compared to the general mean frequency estimator (CE_n), which is based on a weighted summation of all the available correlation lags, for long and short Doppler data sets (typically 48 and 8 Doppler samples). A new estimator of the Doppler signal mean frequency is derived from the results of this study. It optimizes the compromise between the range of analyzable frequencies and the estimation variance for the characteristics of the Doppler signal. Demonstration is provided that the behavior of this estimator shifts from that of CE₁ to that of CE_n, according to the setting of a single parameter. An adaptive version of this estimator is implemented and applied to Doppler recordings. Applications can be contemplated for color Doppler imaging.     

A removable extraluminal Doppler probe for continuous monitoring of changes in cardiac output.

To address the problem of monitoring cardiac output in postoperative cardiac patients, a removable, extraluminal pulsed Doppler probe has been constructed which continuously reflects changes in cardiac output by monitoring blood velocity in the ascending aorta. Velocity is related to the frequency of the Doppler shift and the angle at which the ultrasound beam intersects the vessel. A 1 mm2 piezoelectric crystal mounted at a 45 degree angle on the tip of the probe is activated with a 20-MHz range-gated pulsed Doppler, and velocity is determined from Fourier analysis of quadrature data using computer software with a 50-KHz processing range. The device is designed for application to the ascending aorta at the conclusion of a cardiac surgical procedure; and, since mean diameter changes in the ascending aorta have been shown to be very small over a wide range of mean aortic pressures, cardiac output is linearly related to velocity. To test this technique, simultaneous recordings were made from the pulsed Doppler probe and an electromagnetic flow probe in five mongrel dogs over a range of cardiac outputs from 0.5 to 6 l/min for a total of 136 data points. Excellent correlation was found between the electromagnetic flow probe and pulsed Doppler probe data (average r = .98 +/- .006). Average slope between the two was 1.004 and average zero intercept was -0.025 l/min. The catheter is small, stable, has no baseline drift, and continuously and accurately reflects pulsatile changes at high and low flows.     

Measurements of fetal limb bones in early pregnancy.

The fetal humerus, radius/ulna, femur, tibia/fibula and foot were measured in 296 pregnant women at 64-108 days' menstrual age, using vaginal sonography. All limb measurements correlated significantly with gestational age and biparietal diameter independently. Ratios of limb segment lengths did not change with gestation. Reference ranges (mean and 95% data intervals) were constructed for each limb segment. These data may prove useful in prenatal diagnosis of skeletal dysplasias and in the antenatal assessment of patients at risk of chromosomal abnormalities.     

Comparison of visually evoked peak systolic and end diastolic blood flow velocity using a control system approach.

Transcranial Doppler sonography measures blood flow velocity in basal cerebral vessels with high accuracy. For quantification, time averaged mean blood flow velocities are used most because the peak systolic and end diastolic blood flow velocities mark the velocity extremes of one heart cycle. It is known, from hemodynamic measurements of the neurovascular coupling mechanism, that the end diastolic velocity is more sensitive for change in hemodynamics than the peak systolic velocity. Thus, we used a recently introduced control system approach to compare both indices for their use in functional transcranial Doppler tests focusing on hemodynamics of blood flow velocity change. We enrolled 65 healthy young volunteers without a medical history of cardiovascular risk factors, and performed a visual stimulation test. Peak systolic and end diastolic maximal blood flow velocities were used after transformation to relative data for control-system analysis. Due to Doppler artefacts, 95% of peak systolic and 86% of end diastolic data sets were analyzed. Results showed statistically significant differences for resting blood flow velocity and the control system parameter gain, attenuation and rate time, whereas the parameters' natural frequency and time delay were equal. Increase in relative blood flow velocity in the posterior cerebral artery due to visual-cortical stimulation was higher in end diastolic values than peak systolic data. Using a complex visual stimulation paradigm, the higher sensitivity of the end diastolic index is of no practical use. Being less influenced by Doppler artefacts, the peak systolic velocity index is more feasible for control-system analysis of dynamic blood flow regulation.     

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.     

Change in amplitude distributions of Doppler spectrograms recorded below the aortic valve in patients with a valvular aortic stenosis.

Amplitude distributions of Doppler spectrograms were characterized in a group of 22 patients having no aortic pressure gradient and another group of 26 patients having a stenotic aortic valve. Specifically, for each patient, the ratios of the mean amplitude in three normalized frequency bands (low, middle and high) to the mean amplitude of the Doppler spectrogram computed in selected portions of the systolic period were considered. Pulsed-wave Doppler spectrograms were recorded by positioning the sample volume in the left ventricular outflow tract, approximately 1 cm below the aortic valve. Statistically significant differences were found between the middle (p = 0.041) and high (p = 0.028) frequency bands of Doppler signals recorded from the two groups of patients. The differences observed are believed to be attributed to blood flow eddies generated below the stenotic aortic heart valve and to changes in blood flow orientation.     

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.     

Prospective assessment of the aorto-iliac segment by visual interpretation of frequency analysed doppler waveforms—A comparison with arteriography

In a group of 83 patients the prospective Doppler assessment of disease in the aorto-iliac segment, obtained from visual interpretation of the frequency analysed Doppler waveforms, is compared with subsequent arteriographic findings.The Doppler technique proved to be particularly accurate in the severely diseased vessels (sensitivity 87%, specificity 88%) but tended to underestimate the extent of the minimal/moderate disease.     

Quantitative assessment of the common femoral to popliteal arterial segment using continuous wave Doppler ultrasound

The common femoral to popliteal segments of 123 patients with radiologically confirmed arterial disease have been compared with those of 43 volunteers apparently free from disease. The comparisons have been based upon parameters derived from Doppler-shifted, continuous wave ultrasound.Simultaneous recordings of the Doppler signal were made from over the common femoral artery and the popliteal artery; these were analyzed subsequently to reveal variations of maximum frequency with time. Fifteen parameters were obtained from each segment. Of these the Pulsatility Index (P.I.) the Damping Factor(Δ), the pulse wave transit time(TT) the rise time ratio(RTR) from each segment were examined in detail. The Doppler results from the patient groups were compared also with the findings from arteriography.On the basis of these comparisons it was found that the RTR was simpler to measure than other more commonly used parameters such as Pulsatility Index and at least as sensitive for the differentiation between severe arterial disease and the mildly affected or normal state. However, none of these parameters was sufficiently sensitive to distinguish between mild to moderate disease and normal.     

Pulsed doppler techniques for measuring instantaneous maximum and mean flow velocities in carotid arteries

A 5 MHz pulsed Doppler instrument measuring instantaneous maximum and mean flow velocities is presented. The maximum velocity estimator is based on the principle of frequency variable filtering controlled by a feedback loop to follow the velocity spectrum envelope.Findings by Doppler and bilateral selective carotid arteriography in 216 patients were compared. Extracranial carotid stenoses were identified by the finding of a vessel segment with locally increased flow velocity. Peak Velocity Ratio (PVR) was calculated from maximum velocities measured in the stenosis and in more distal Internal Carotid Artery (ICA) segments. Using PVR, ICA stenoses > 20% were detected with sensitivity 96%, specificity 94%, positive accuracy 94% and negative accuracy 96%. Total ICA occlusions were identified with sensitivity 97% and specificity 99%.     

二维斑点追踪成像技术评价左室心肌节段舒张功能的研究

目的：斑点追踪成像技术(Speckle tracking imaging, STI)分析不同舒张功能状态下左室节段心肌的应变率,比较心肌节段舒张功能与整体舒张功能的关系。 方法：Doppler测量二尖瓣口血流E峰、A峰,TDI测量二尖瓣环室间隔eSp峰、侧壁eL峰。将所有病例分为三组：A组：舒张功能正常组;B组：顺应性减低;C组：假性正常化组。用STI分析左室17个节段心肌舒张早期应变率(ESR)、舒张晚期应变率(ASR),比较各组间17节段心肌舒张期应变率的变化规律。 结果：随着整体舒张功能的降低,17个节段心肌的ESR也相应降低,ASR则增大,二尖瓣环水平后间隔基底段的变化幅度大于前侧壁基底段。与A组相比,B组和C组中间段心肌与心尖段心肌ESR的变化幅度均大于基底段;B、C两组相比,心尖段心肌ESR的变化幅度最大,基底段和中间段变化不明显。 结论：STI有助于早期发现节段心肌的舒张功能受损;随着舒张功能恶化,心尖段ESR持续减低,ASR先升高后降低。     

正常成人上肢动脉彩色多普勒超声测量

目的　应用彩色多普勒超声检查和测量正常上肢动脉 ,以得出上肢动脉的超声正常值。方法　15 3名健康志愿者 3 0 6条上肢动脉 ,每条上肢分 7个节段。测量解剖学和血流动力学参数。结果　获得上肢动脉各节段各参数超声正常值 ,发现上肢动脉从近心至远心段 ,内径减少 ,管壁变薄 ,收缩期峰值流速、收缩期加速度、搏动指数值下降 (P <0 .0 5 ) ;上肢动脉各节段均可表现多种流速曲线形态 ,近心段以双向三峰所占比例大(67%锁骨下动脉段双向三峰 ) ,腋动脉以远段反向峰渐不明显 (4 5 .2 %肱动脉段单向双峰 )。结论　彩色多普勒超声能成功用于检查上肢动脉 ,发现变异和病变。