Precision and accuracy of Doppler flow measurements. In vitro and in vivo study of the applicability of the method in human fetuses

The precision of the Doppler method for quantitative blood flow measurement in the fetal descending thoracic aorta and in the umbilical vein and for estimation of the Pulsatility Index from the velocity curve from fetal aorta was tested in vivo by examination of six pregnant women eight times. Two investigators examined each patient twice in random order upon two successive days. The diameter of the vessel was measured using planimetry on a magnified time-motion image of the diameter variations during the heart cycle, while the angle between the ultrasound Doppler beam and the vessel of interest was measured on the hard copy image. No systematic variation was found between observers, days, repeated observations or repeated readings of curves and images. The mean coefficient of variations was 5.6% for the quantitative flow per kilogram estimated fetal weight measured in the fetal descending aorta, 6.8% for the quantitative flow per kilogram estimated fetal weight measured in the umbilical vein and 9.8% for the Pulsatility Index. When the diameter of aorta was calculated as the mean of the maximal and the minimal diameter measured on the hard copy image, the mean coefficient of variation for the flow increased to 9.4%. In vitro tests of the Doppler instrument and the real-time scanner revealed a systematic overestimation of Doppler measured flow of only 4.4% compared with the true flow, and a real-time scanner underestimation of vessel diameter of only 1.1%.     

The time-distance recorder as a means of improving the accuracy of fetal blood flow measurements

The influence of pulsatile diameter changes on calculation of volume flow has been studied. In vitro studies and an animal study were carried out with a real-time imaging and pulsed Doppler velocity measurement system. For precise pulsatile diameter information a wall motion tracking device was incorporated. Whereas in vitro a high degree of accuracy was found for the measurements of volume flow, this could not be substantiated in the descending aorta of the fetal lamb, in which Doppler volume flow differed between -7.5 and 17% from magnetic volume flow. In a clinical study the relative influence of various diameter approximations on calculated fetal aortic volume flow was assessed in 16 normal third trimester pregnancies. Depending on the selected diameter approximation method it appeared that differences from 19% underestimation to 9% overestimation in calculated volume flow could be obtained when reference was made to volume flow derived from diameter and velocity information.     

Reproducibility of ultrasonic fetal volume blood flow measurements

The intraobserver reproducibility of ultrasonic volume blood flow measurements in the human fetus was evaluated in this study. A new approach, simultaneous measurement of the vessel diameter and the flow velocity with a pulsed-wave Doppler ultrasound synchronized with a real-time ultrasound phase-locked echo-tracking system, was used to estimate volume blood flow (VBF) in the fetal descending aorta. Measurements were performed in a longitudinal study on 20 normally grown fetuses. Intraobserver reproducibility of repeated estimations of mean blood flow velocities throughout gestation was very good, with high values of intraclass correlation coefficient (IntraCC 0·80–0·91) and low values of coefficient of variation (CV 4–11%). The IntraCC of repeated vessel diameter measurements throughout gestation was low (0·30–0·68), whereas the values of CV were acceptable (< 12%), with the exception of the period between 140 and 167 gestational days (CV > 12%). The lower reproducibility of vessel diameter measurement contributed directly to the relatively low reproducibility of VBF estimations overall (IntraCC 0·25–0·70; CV 17–28%), as these are calculated from a formula using both flow velocity and vessel diameter. Nevertheless, the synchronized approach gives absolute values of vessel diameter, flow velocity and VBF comparable with values reported in the human fetus previously. The new method provides, by taking the vessel wall pulsations into consideration and by measuring diameter and velocity simultaneously, a more complete information on fetal haemodynamics and fetal physiology.     

Evaluation of umbilical venous blood flow by Doppler color flow mapping and conventional ultrasonographic methods.

Ultrasonographic measurement of umbilical venous blood flow as a reflection of placental blood flow has been used for assessment of fetal well-being. Because fetal and maternal factors may prevent satisfactory measurement of umbilical venous diameter and flow velocity, an alternative method would be helpful. We found that, in 35 human fetuses between 18 and 35 weeks of gestation, that both variables for umbilical venous flow calculation can be measured from a single Doppler color frame with close measurement agreement to conventional combined cross-sectional pulsed Doppler ultrasonographic measurements. Thus, this method provides an alternative to the conventional method or may be used for a cross-check and has the potential for automated flow calculation.     

Blood flow in the fetal descending aorta; intrinsic factors affecting fetal blood flow, i.e. fetal breathing movements and cardiac arrhythmia

An ultrasonic method combining real-time ultrasonography and pulsed Doppler technique was used for the examination of blood flow in the fetal descending aorta. The mean aortic blood flow velocity in the last trimester of normal pregnancies was 29.0 cm/s; the peak maximum velocity 97.3 cm/s and the mean blood flow 238.4 ml/min/kg. The blood flow velocity did not change significantly with gestational age, the aorta diameter showed a linear growth. During labour, the aortic blood flow in undistressed fetuses was not different from the flow recorded during late pregnancy. Fetal breathing movements modulate the flow velocity signals in the descending aorta, the inferior vena cava and the umbilical vein of the fetus; therefore, when quantifying fetal blood flow, only periods without fetal breathing movements should be considered. A group of fetuses with various types of cardiac arrhythmias was examined. Postextrasystolic potentiation was found to be present already during intrauterine life. The present method enables quantitative evaluation of the hemodynamic effects of cardiac arrhythmias.     

Novel Method for Fetal and Maternal Heart Rate Measurements Using 2-D Ultrasound Color Doppler Flow Images

Fetal heart rate (FHR) and maternal heart rate (MHR) are important indicators of fetal well-being during pregnancy. A common method in clinical examination is to estimate the FHR using the Doppler shift of echoes from umbilical artery blood flow based on an ultrasound pulsed-wave (PW) Doppler technique. Similarly, a sampling gate can be located at the maternal blood flow to measure MHR using PW Doppler. Ultrasound color Doppler flow imaging (CDFI) is one of the most commonly used imaging modes for clinical fetal examinations. Color coding is employed to display the blood flow velocity and direction in color grades according to the Doppler shift. Continuous CDF images contain dynamic changes characteristics of the blood flow. The periodic characteristics can be used to obtain heart rate information. Therefore, here we propose a novel method to measure FHR and MHR simultaneously using CDF images. The proposed method calculates the histogram of color similarity of CDF images to initially extract the periodic characteristics of the CDF image sequence. The histogram of color similarity function is then processed by a bandpass filter and autocorrelation operation to reduce noise and enhance periodicity. Finally, peak detection is performed on the processed signal to obtain the period and estimate the heart rate. The proposed method can measure the FHR and MHR in parallel after selecting two regions containing the umbilical artery and maternal blood flow, respectively. Thus, the method has high computational efficiency. The proposed method was evaluated on a Doppler flow phantom and clinical CDF images and then compared with the PW Doppler method. The correlation analysis and Bland–Altman plots reveal that the proposed method agrees well with the PW Doppler. It is a sanity check method for real-time clinical FHR and MHR measurements.     

Antenatal fetal blood flow in the descending aorta and in the umbilical vein and their ratio in normal pregnancy

One hundred and sixty sequential measurements of umbilical vein blood flow (UVBF) and descending aorta blood flow (DABF) in normal fetuses were performed in utero by the duplex real-time ultrasound and pulsed Doppler technique. Throughout pregnancy from 26 to 41 weeks the blood flow velocity of the umbilical vein (UV) and the descending aorta (DA) remained relatively constant, while the diameter of UV and DA, UVBF and DABF increased with advancing gestational age. Moreover, the UVBF-to-DABF ratio was nearly constant, with a mean value of 64% from 26 weeks onwards. The DABF-to-abdominal area (DABF-to-AA) ratio was constant throughout pregnancy, with a mean value of 7.0 ml/min/cm2. We call these two constants the umbilical-aortic index and descending-aortic index. These relationships may be of help in the diagnosis of intrauterine growth retardation and other fetal disorders.     

Fetal cardiac arrhythmias and their effect on volume blood flow in descending aorta of human fetus

Using a two-dimensional linear array real-time and pulsed Doppler ultrasound system, volume blood flow measurements were made in 86 normal pregnancies, seven cases of fetal bradycardia, and seven cases of fetal tachycardia at the lower thoracic level of the fetal descending aorta. During fetal cardiac arrhythmias, volume blood flow was maintained within the normal range until the heart rates reached around 50 bpm and 230 bpm, after which the volume blood flow diminished. The changes observed suggest that the Frank-Starling mechanism is functional in the fetal myocardium and demonstrate the stability of the fetal circulation.     

Ultrasonic measurement of fetal blood velocity wave form as a secondary diagnostic test in screening for intrauterine growth retardation

Based on routine fetometry screening at 32 weeks of gestation, 80 out of 3226 singleton pregnancies were suspected of intrauterine growth retardation (IUGR) and 72 of them were subjected to repeated fetal blood flow measurements by Doppler ultrasound for evaluation of the fetal condition. The maximum blood velocity wave form recorded from the fetal descending aorta and umbilical artery was classified as normal or abnormal depending on the presence or absence of positive end-diastolic blood flow. Compared to the pregnancies with normal fetal blood flow, the group of 30 pregnancies with abnormal blood flow patterns had significantly more pregnancy complications and more operative deliveries for fetal distress. In the latter group, all newborns but one were small-for-gestational age and had low 1-min Apgar scores more frequently. Fetal blood flow measurements have a good capacity for predicting unfavorable fetal outcomes and can be recommended for clinical use. The combination of ultrasound screening and Doppler blood flow measurement has reduced the number of pregnancies requiring intensive surveillance.     

Time-domain ultrasonography during pregnancy.

Time-domain ultrasonography is an alternative to Doppler analysis of blood flow direction and velocity. The technique uses timing information between successive echo pulses to measure flow velocities directly through a color display map. Volume flow quantification also can be achieved by combining time-domain methods with M-mode ultrasonography. This report describes our preliminary experiences with fetal vascular imaging by time-domain ultrasonography, compares Doppler peak velocity correlation to time-domain sonography, and examines the in vitro accuracy of time-domain sonography combined with M-mode volume flow measurements. Excellent agreement was found between peak Doppler velocity measurements and time-domain ultrasonography for a variety of maternal and fetal vessels (N = 29). Close volume-flow correlation was observed between a flow pump and time-domain ultrasonography under constant and pulsatile conditions. Time-domain ultrasonography appears to be a useful alternative to Doppler techniques for imaging small fetal vessels and also may provide the basis for accurate volume flow measurements during pregnancy.     

Inability of color and spectral Doppler to identify fetal renal obstruction.

Color flow Doppler imaging was used to obtain fetal renal artery flow velocity waveforms in 130 normal fetuses at various gestational ages to build a database for the establishment of normal ranges for the Pulsatility Index in the fetal renal arteries throughout pregnancy. Twenty-nine cases of fetal renal tract dilation (greater than 5 mm anteroposterior diameter of the renal pelvis) of various causes were investigated. We found that measurements of the pulsatility index were not significantly altered from the normal range, so that color flow Doppler does not appear to be helpful in the differential diagnosis of fetal renal dilation.     

Changes in venous flow and intra tracheal flow in fetal breathing movements

Using pulsed Doppler ultrasound combined with a real-time B-mode image, changes in the central venous blood flow and tracheal fluid flow were recognized during fetal breathing movements. The blood flow in the umbilical vein was increased with fetal inspiration in which the abdominal wall of fetus moved inward, and decreased with expiration in which the abdominal wall moved outward. Velocity in the fetal vena cava revealed a complex blood flow, in accordance with both fetal cardiac motions and breathing movements. The tracheal flow velocity during fetal breathing was measured at a maximum of 40 cm s-1 and the flow volume was estimated at a maximum of 6 ml breath-1 in the matured fetus.     

Three-dimensional power Doppler ultrasound for the assessment of the fetal brain blood flow in normal gestation

Early identification of the abnormal fetal brain vascularization and blood flow is very important, because the deficient perfusion of the fetal brain may be related to a poor prognosis of the central nervous system (CNS) development. To assess the fetal brain vascularization and the blood flow in normal gestation, we measured the fetal brain vascularization and the brain blood flow in normal fetuses using three-dimensional (3-D) power Doppler ultrasound (US) and the quantitative 3-D power Doppler histogram analysis. This study was undertaken by a prospective and cross-sectional design. In total, 155 normal singletons with gestational age (GA) between 21 and 40 weeks were included. The 3-D power Doppler US and the quantitative 3-D histogram analyses were used to assess the fetal brain vascular indexes, i.e., vascularization index (VI), flow index (FI) and vascularization-flow index (VFI), in each case. Our results revealed that all the fetal brain VI, FI and VFI increased significantly with GA (all p<0.001). In addition, the fetal brain VI, FI and VFI were all significantly correlated with the common fetal growth indices, such as biparietal diameter, occipitofrontal diameter, head circumference, abdominal circumference, femur length and estimated fetal weight. Our study indicates that fetal brain vascularization and blood flow increase significantly with the advancement of GA as well as the fetal common growth indices during normal gestation. We believe our data may serve as a reference for further studies of the fetal brain blood flow in abnormal conditions.     

Comparison of forearm plethysmography methods with brachial artery pulsed Doppler flowmetry in man

Summary. Blood flow in the forearm was evaluated using simultaneous measurements of pulsed Doppler flowmetry and strain-gauge plethysmography in 32 normal subjects and 91 patients with sustained essential hypertension. The two determinations of blood flow were strongly correlated (r = 0·58). Measurements with strain-gauge plethysmography reflected changes in blood flow velocity but were poorly correlated with changes in arterial diameter as measured by pulsed Doppler flowmetry. The latter method permits evaluation of instantaneous variations in blood flow velocity and detection of active modifications of arterial diameter.     

A feasability study of color flow doppler vectorization for automated blood flow monitoring

An ongoing issue in vascular medicine is the measure of the blood flow. Catheterization remains the gold standard measurement method, although non-invasive techniques are an area of intense research. We hereby present a computational method for real-time measurement of the blood flow from color flow Doppler data, with a focus on simplicity and monitoring instead of diagnostics. We then analyze the performance of a proof-of-principle software implementation. We imagined a geometrical model geared towards blood flow computation from a color flow Doppler signal, and we developed a software implementation requiring only a standard diagnostic ultrasound device. Detection performance was evaluated by computing flow and its determinants (flow speed, vessel area, and ultrasound beam angle of incidence) on purposely designed synthetic and phantom-based arterial flow simulations. Flow was appropriately detected in all cases. Errors on synthetic images ranged from nonexistent to substantial depending on experimental conditions. Mean errors on measurements from our phantom flow simulation ranged from 1.2 to 40.2% for angle estimation, and from 3.2 to 25.3% for real-time flow estimation. This study is a proof of concept showing that accurate measurement can be done from automated color flow Doppler signal extraction, providing the industry the opportunity for further optimization using raw ultrasound data.     

Doppler assessment of fetal aortic isthmus blood flow in two different sonographic planes during the second half of gestation.

OBJECTIVE: To compare the reliability of Doppler blood flow measurements of the fetal aortic isthmus (AoI) according to whether the sampling plane is obtained from the traditional longitudinal aortic arch (LAA) view or the more recently described three vessels and trachea (3VT) view of the fetal upper mediastinum. METHODS: Doppler blood flow measurements of pulsatility index (PI), resistance index (RI), peak systolic (PSV), end-diastolic (EDV) and time-averaged maximum (TAMXV) velocities were performed in the AoI of 40 fetuses between 24 and 36 weeks of gestation. All measurements were sampled in two different sonographic planes of the AoI: the LAA view, at a few millimeters beyond the origin of the left subclavian artery, and the 3VT view, just before the V-shaped junction of the aortic and ductal arches. All scans were performed by the same observer. The reliability of Doppler blood flow measurements was assessed by calculating intraclass correlation coefficients (ICCs) and limits of agreement between the two different sonographic sites evaluating the AoI. RESULTS: Mean values of PI, RI, PSV, EDV and TAMXV were similar in the LAA and 3VT views. The PI and vascular velocities were reliably measured from both sonographic sites. ICCs for variability of measurements were 0.78, 0.63, 0.63, 0.60 and 0.55 for PI, RI, PSV, EDV and TAMXV, respectively. Limits of agreement revealed minimal disagreement between the two sites of evaluation of the AoI for all measurements. CONCLUSIONS: On the basis of our observations, Doppler blood flow measurements across the fetal AoI can be reliably obtained from both the 3VT and the traditional LAA sonographic views. Since the transverse upper thoracic 3VT plane is achievable in most fetal positions, Doppler study of the AoI appears to be easier than expected.     

Umbilical vein blood flow in growth-restricted fetuses.

OBJECTIVE: To determine whether umbilical blood flow is reduced in a subset of growth-restricted (IUGR) fetuses when expressed as flow per kilogram or flow per unit of specific sonographic fetal measurements. DESIGN: Prospective. SUBJECTS: Thirty-seven IUGR fetuses were examined by Doppler ultrasound within 4 h of the last non-stress test prior to delivery. This population was divided into three groups of varying clinical severity according to the characteristics of umbilical arterial pulsatility index (PI) and heart rate. METHODS: Absolute and weight-specific umbilical vein (UV) flow were calculated from measurements of UV diameter and UV mean velocity. Umbilical vein diameter, velocity and UV flow were calculated also per unit head (HC) or abdominal circumference (AC) and correlated with gestational age. RESULTS: Umbilical vein flow (UVf) per kilogram fetal weight was significantly lower in the more severe IUGR fetuses (abnormal umbilical arterial PI) than in normally grown comparable fetuses (P < 0.001). Umbilical vein flow per unit HC was significantly lower in the three groups (P < 0.001) than in the control population. The UV diameter/HC ratio was normal whereas UV velocity/HC ratio was significantly lower in IUGR fetuses than in comparable controls. CONCLUSIONS: The present study clearly establishes that umbilical venous blood flow is reduced in IUGR fetuses on a weight-specific basis. The sonographic growth parameter which best distinguishes umbilical flow differences of IUGR fetuses from normal fetuses is the head circumference.     

In Vivo Validation of Volume Flow Measurements of Pulsatile Flow Using a Clinical Ultrasound System and Matrix Array Transducer

The goal of this study was to evaluate the accuracy of a non-invasive C-plane Doppler estimation of pulsatile blood flow in the lower abdominal vessels of a porcine model. Doppler ultrasound measurements from a matrix array transducer system were compared with invasive volume flow measurements made on the same vessels with a surgically implanted ultrasonic transit-time flow probe. For volume flow rates ranging from 60 to 750 mL/min, agreement was very good, with a Pearson correlation coefficient of 0.97 (p < 0.0001) and a mean bias of ?4.2%. The combination of 2-D matrix array technology and fast processing gives this Doppler method clinical potential, as many of the user- and system-dependent parameters of previous methods, including explicit vessel angle and diameter measurements, are eliminated.     

Assessment of normal fetal liver blood flow using quantitative three-dimensional power Doppler ultrasound

Fetal liver blood flow is very important for fetal hemodynamics. To assess the development of fetal liver vascularization and blood flow in normal gestation, we measured the fetal liver vascularization and blood flow in normal fetuses using the three-dimensional (3-D) power Doppler ultrasound (US) and quantitative 3-D power Doppler histogram analysis. This study was undertaken with a prospective, cross-sectional design. In total, 196 normal singletons with gestational age between 20 and 40 weeks were included. The 3-D power Doppler US and the quantitative histogram analysis were used to assess the fetal liver vascularization index (VI), flow index (FI), vascularization-flow index (VFI) and mean greyness in each case. Our results showed that all the fetal liver VI, FI and VFI increased significantly with gestational age (GA), whereas, fetal liver mean greyness decreased with GA. Using GA as the independent variable, the linear regression equations for fetal liver VI, FI, VFI and mean greyness were VI = 0.5746 x GA - 5.8264 (r = 0.86, p < 0.0001), FI = 0.3291 x GA + 35.624 (r = 0.35, p < 0.001), VFI = 0.2905 x GA - 3.4871 (r = 0.82, p < 0.0001) and mean greyness = -0.2034 x GA + 42.315 (r = -0.20, p < 0.0001). In addition, fetal liver VI, FI, VFI and nean greyness were all significantly correlated with common fetal growth indexes, such as biparietal diameter, occipitofrontal diameter, head circumference, abdominal circumference, femur length and estimated fetal weight. Our study indicates that normal fetal liver vascularization and blood flow change significantly with the advancement of GA as well as fetal growth indexes. We believe our data may serve as a reference for further studies of fetal liver blood flow in abnormal conditions.     

Assessment of ventricular filling volumes with an automated color Doppler method: validation in a pulsatile flow model.

OBJECTIVE: Determination of ventricular filling volumes with the use of Doppler echocardiographic measurements critically depends on the presence of a circular-shaped flow area and a flat velocity profile across it because evaluation of flow volume is usually based on echocardiographic measurements of its diameter and pulsed Doppler recordings within the center of this area. The approach may be limited at the mitral and tricuspid ring levels as a result of their noncircular shape and because nonflat velocity profiles are present. The purpose of this study was to examine in a pulsatile flow model simulating ventricular inflow conditions the accuracy of an automated method based on the analysis of color Doppler flow velocities for evaluation of flow volumes. MATERIALS AND METHODS: A recently-developed automated Doppler method that takes into account the velocity distribution across a region of interest was examined in a pulsatile flow model by using flows with waveforms characteristic for ventricular inflow through tubes with elliptically-shaped cross-sectional areas. Color Doppler imaging was performed against flow direction along the major and minor axes of the tubes with major diameters ranging between 3 and 5 cm and major-to-minor diameter ratios of 1.5 and 2.0. RESULTS: A close correlation was found between flow volumes measured by the Doppler technique for registrations along the minor or major axis of the ellipses and actual values (r = 0.99, standard error of the estimate = 0.44 to 1.98 mL), with a systematic underestimation or overestimation, respectively, depending on the diameter ratio. Averaging of the data derived from 2 orthogonal measurements by using the geometric mean value yielded an excellent agreement between Doppler data and actual flow volumes. CONCLUSION: This automated color Doppler method enables reliable determination of flow volumes in a pulsatile flow model simulating ventricular inflow conditions with the use of 2 orthogonal imaging views. The data indicate that the method may improve the noninvasive evaluation of ventricular filling volumes.