Measurement of volumetric flow

Color Flow Ultrasound Spatial Sampling Beam Density for Partial Volume-Corrected Three-Dimensional Volume Flow (3DVF): Theory, Simulation, and Experiment

OBJECTIVE

The purpose of this study was to quantify the accuracy of partial volume-corrected three-dimensional volume flow (3DVF) measurements as a function of spatial sampling beam density using carefully-designed parametric analyses in order to inform the target applications of 3DVF.

METHODS

Experimental investigations employed a mechanically-swept curvilinear ultrasound array to acquire 3D color flow (6.3 MHz) images in flow phantoms consisting of four lumen diameters (6.35, 4.88, 3.18 and 1.65 mm) with volume flow rates of 440, 260, 110 and 30 mL/min, respectively. Partial volume-corrected three-dimensional volume flow (3DVF) measurements, based on the Gaussian surface integration principle, were computed at five regions of interest positioned between depths of 2 and 6 cm in 1 cm increments. At each depth, the color flow beam point spread function (PSF) was also determined, using in-phase/quadrature data, such that 3DVF bias could then be related to spatial sampling beam density. Corresponding simulations were performed for a laminar parabolic flow profile that was sampled using the experimentally-measured PSFs. Volume flow was computed for all combinations of lumen diameters and the PSFs at each depth.

RESULTS

Accurate 3DVF measurements, i.e., bias less than ±20%, were achieved for spatial sampling beam densities where at least 6 elevational color flow beams could be positioned across the lumen. In these cases, greater than 8 lateral color flow beams were present. PSF measurements showed an average lateral-to-elevational beam width asymmetry of 1:2. Volume flow measurement bias increased as the color flow beam spatial sampling density within the lumen decreased.

CONCLUSION

Applications of 3DVF, particularly those in the clinical domain, should focus on areas where a spatial sampling density of 6 × 6 (lateral x elevational) beams can be realized in order to minimize measurement bias. Matrix-based ultrasound arrays that possess symmetric PSFs may be advantageous to achieve adequate beam densities in smaller vessels.

Measurement of Cerebral Metabolism Under Non-Chronic Hemodynamic Conditions

BACKGROUND

Blood flow to the brain is a critical physiological function and is useful to monitor in critical care settings. Despite that, a surrogate is most likely measured instead of actual blood flow. Such surrogates include velocity measurements in the carotid artery and systemic blood pressure, even though true blood flow can actually be obtained using MRI and other modalities. Ultrasound is regularly used to measure blood flow and is, under certain conditions, able to provide quantitative volumetric blood flow in milliliters per minute. Unfortunately, most times the resulting flow data is not valid due to unmet assumptions (such as flow profile and angle correction). Color flow, acquired in three dimensions, has been shown to yield quantitative blood flow without any assumptions (3DVF).

METHODS

Here we are testing whether color flow can perform during physiological conditions common to severe injury. Specifically, we are simulating severe traumatic brain injury (epidural hematoma) as well as hemorrhagic shock with 50% blood loss. Blood flow was measured in the carotid artery of a cohort of 7 Yorkshire mix pigs (40-60 kg) using 3DVF (4D16L, LOGIQ 9, GE HealthCare, Milwaukee, WI, USA) and compared to an invasive flow meter (TS420, Transonic Systems Inc., Ithaca, NY, USA).

RESULTS

Six distinct physiological conditions were achieved: baseline, hematoma, baseline 2, hemorrhagic shock, hemorrhagic shock plus hematoma, and post-hemorrhage resuscitation. Mean cerebral oxygen extraction ratio varied from 40.6% ± 13.0% of baseline to a peak of 68.4% ± 15.6% during hemorrhagic shock. On average 3DVF estimated blood flow with a bias of -9.6% (-14.3% root mean squared error) relative to the invasive flow meter. No significant flow estimation error was detected during phases of flow reversal, that was seen in the carotid artery during traumatic conditions. The invasive flow meter showed a median error of -11.5% to 39.7%.

CONCLUSIONS

Results suggest that absolute volumetric carotid blood flow to the brain can be obtained and potentially become a more specific biomarker related to cerebral hemodynamics than current surrogate markers.

Comment on Barbieri et al. Umbilical Vein Blood Flow in Uncomplicated Pregnancies: Systematic Review of Available Reference Charts and Comparison with a New Cohort. J. Clin. Med. 2023, 12, 3132

We would like to comment on the systemic review article published in the Journal of Clinical Medicine by Barbieri et al [...].

An ultrasound imaging and computational fluid dynamics protocol to assess hemodynamics in iliac vein compression syndrome

OBJECTIVE

Elevated shear rates are known to play a role in arterial thrombosis; however, shear rates have not been thoroughly investigated in patients with iliac vein compression syndrome (IVCS) owing to imaging limitations and assumptions on the low shear nature of venous flows. This study was undertaken to develop a standardized protocol that quantifies IVCS shear rates and can aid in the diagnosis and treatment of patients with moderate yet symptomatic compression.

METHODS

Study patients with and without IVCS had their iliac vein hemodynamics measured via duplex ultrasound (US) at two of the following three vessel locations: infrarenal inferior vena cava (IVC), right common iliac vein, and left common iliac vein, in addition to acquiring data at the right and left external iliac veins. US velocity spectra were multiplied by a weighted cross-sectional area calculated from US and computed tomography (CT) data to create flow waveforms. Flow waveforms were then scaled to enforce conservation of flow across the IVC and common iliac veins. A three-dimensional (3D), patient-specific model of the iliac vein anatomy was constructed from CT and US examination. Flow waveforms and the 3D model were used as a basis to run a computational fluid dynamics (CFD) simulation. Owing to collateral vessel flow and discrepancies between CT and US area measurements, flows in internal iliac veins and cross-sectional areas of the common iliac veins were calibrated iteratively against target common iliac flow. Simulation results on mean velocity were validated against US data at measurement locations. Simulation results were postprocessed to derive spatial and temporal values of quantities such as velocity and shear rate.

RESULTS

Using our modeling protocol, we were able to build CFD models of the iliac veins that matched common iliac flow splits within 2% and measured US velocities within 10%. Proof-of-concept analyses (1 subject, 1 control) have revealed that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein, more typical of the arterial rather than the venous circulation. These results encourage us to extend this protocol to a larger group of patients with IVCS and controls.

CONCLUSIONS

We developed a protocol that obtains hemodynamic measurements of the IVC and iliac veins from US, creates patient-specific 3D reconstructions of the venous anatomy using CT and US examinations, and computes shear rates using calibrated CFD methods. Proof-of-concept results have indicated that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein. Larger cohorts are needed to assess the relationship between venous compression and shear rates in patients with IVCS as compared with controls with noncompressed iliac veins. Further studies using this protocol may also give promising insights into whether or not to treat patients with moderate, yet symptomatic compression.

Bedside Cerebral Blood Flow Quantification in Neonates

Measurement of blood flow to the brain in neonates would be a very valuable addition to the medical diagnostic armamentarium. Such conditions such as assessment of closure of a patent ductus arteriosus (PDA) would greatly benefit from such an evaluation. However, measurement of cerebral blood flow in a clinical setting has proven very difficult and, as such, is rarely employed. Present techniques are often cumbersome, difficult to perform and potentially dangerous for very low birth weight (VLBW) infants. We have been developing an ultrasound blood volume flow technique that could be routinely used to assess blood flow to the brain in neonates. By scanning through the anterior fontanelles of 10 normal, full-term newborn infants, we were able to estimate total brain blood flows that closely match those published in the literature using much more invasive and technically demanding methods. Our method is safe, easy to do, does not require contrast agents and can be performed in the baby's incubator. The method has the potential for monitoring and assessing blood flows to the brain and could be used to routinely assess cerebral blood flow in many different clinical conditions.

Umbilical Vein Pulse Wave Spectral Analysis: A Possible Method for Placental Assessment Through Evaluation of Maternal and Fetal Flow Components

OBJECTIVES

Placental blood flow analysis is complicated by having both maternal and fetal flow components. Using the Fast Fourier Transform (FFT) of the umbilical venous pulse wave spectra (PW) envelope, we could simultaneously assess maternal/fetal blood flow in the placenta and investigate if normal and intrauterine growth restriction (IUGR)/pre-eclamptic pregnancies could be distinguished.

METHODS

This retrospective study included normal gestations (N = 11) and gestations with IUGR, pre-eclampsia, or both (N = 13). Umbilical vein PW were acquired and spectral envelopes were identified as a function of time and analyzed by FFT. Base-10 logarithms of the ratios of the maternal/fetal spectral peaks (LRSP) were compared in normal and IUGR/pre-eclamptic populations (two-tailed t-test). Body mass index (BMI), gestational age at scan time, placental position, and weight-normalized umbilical vein blood volume flow (two-tailed t-test, analysis of variance [ANOVA] analysis) were tested. P < .05 was considered significant.

RESULTS

The LRSP for normal and IUGR/pre-eclamptic pregnancies were 0.141 ± 0.180 and -0.072 ± 0.262 (mean ± standard deviation), respectively (P = .033). We detected differences between normal gestations and combinations of LRSP and weight-normalized umbilical venous blood flows. Placental effects based on LRSPs and blood flow may act synergistically in cases with both pre-eclampsia and IUGR (P = .014). No other significant associations were seen.

CONCLUSIONS

In this preliminary study, we showed that umbilical venous flow contains markers related to placental maternal/fetal blood flow, which can be used to assess IUGR and pre-eclampsia. When coupled with umbilical cord blood flow, this new marker may potentially identify the primary causes of the two conditions.

Doppler ultrasound to assess the pharmacodynamic effects of splanchnic vasoactive compounds

AIMS

In search of noninvasive biomarkers to assess the pharmacodynamic effects of portal pressure-lowering drugs, the reproducibility of flow measurements in the superior mesenteric artery was evaluated using Doppler ultrasound.

METHODS

A reproducibility study was conducted in 15 healthy male volunteers (18-50 y). Eight ultrasound measurements were performed for each subject: 2 observers each conducted 2 measurements during 2 separate visits. The following flow parameters were captured: peak systolic velocity (PSV), end diastolic velocity (EDV), pulsatility index (PI), volume flow (VF) and vessel diameter. Reproducibility was assessed by the intraclass correlation coefficient.

RESULTS

Results are presented as intraclass correlation coefficient [95% confidence interval]. The flow parameters PSV, EDV, PI and VF correlated excellently within observer during visit 1 (0.888 [0.748-0.953], 0.910 [0.793-0.962], 0.844 [0.656-0.933] and 0.916 [0.857-0.951], respectively) and visit 2 (0.925 [0.829-0.968], 0.942 [0.863-0.976], 0.883 [0.719-0.954] and 0.915 [0.855-0.951], respectively). Measurements conducted during 2 separate visits by 1 observer correlated well to excellently for PSV, EDV, PI and VF (0.756 [0.552-0.875], 0.836 [0.694-0.916], 0.807 [0.631-0.904] and 0.839 [0.783-0.882], respectively). Measurements conducted by 2 distinct observers correlated well to excellently for PSV, EDV and VF during visit 1 (0.813 [0.584-0.922], 0.884 [0.597-0.945] and 0.786 [0.575-0.899], respectively) and visit 2 (0.779 [0.498-0.912], 0.861 [0.672-0.945], 0.810 [0.553-0.926], respectively). Vessel diameter measurements were poorly reproducible.

CONCLUSION

Doppler ultrasound is a reproducible method for flow measurements in the superior mesenteric artery in a standardized group of healthy volunteers. Therefore, it is a promising technique to assess pharmacodynamic effects of splanchnic vasoactive compounds in early clinical drug development.

Comparison of Variations Between Spectral Doppler and Gaussian Surface Integration Methods for Umbilical Vein Blood Volume Flow

OBJECTIVES

We are studying a new method for estimating blood volume flow that uses 3-dimensional ultrasound to measure the total integrated flux through an ultrasound-generated Gaussian surface that intersects the umbilical cord. This method makes none of the assumptions typically required with standard 1-dimensional spectral Doppler volume flow estimates. We compared the variations in volume flow estimates between techniques in the umbilical vein.

METHODS

The study was Institutional Review Board approved, and all 12 patients gave informed consent. Because we had no reference standard for the true umbilical vein volume flow, we compared the variations of the measurements for the flow measurement techniques. At least 3 separate spectral Doppler and 3 separate Gaussian surface measurements were made along the umbilical vein. Means, standard deviations, and coefficients of variation (standard deviation/mean) for the flow estimation techniques were calculated for each patient. P < .05 was considered significant.

RESULTS

The ranges of the mean volume flow estimates were 174 to 577 mL/min for the spectral Doppler method and 100 to 341 mL/min for the Gaussian surface integration (GSI) method. The mean standard deviations (mean ± SD) were 161 ± 95 and 45 ± 48 mL/min for the spectral Doppler and GSI methods, respectively (P < .003). The mean coefficients of variation were 0.46 ± 0.17 and 0.18 ± 0.14 for the spectral Doppler and GSI methods respectively (P < 0.002).

CONCLUSIONS

The new volume flow estimation method using 3-dimensional ultrasound appears to have significantly less variation in estimates than the standard 1-dimensional spectral Doppler method.

Three-dimensional US for Quantification of Volumetric Blood Flow: Multisite Multisystem Results from within the Quantitative Imaging Biomarkers Alliance

Background Quantitative blood flow (QBF) measurements that use pulsed-wave US rely on difficult-to-meet conditions. Imaging biomarkers need to be quantitative and user and machine independent. Surrogate markers (eg, resistive index) fail to quantify actual volumetric flow. Standardization is possible, but relies on collaboration between users, manufacturers, and the U.S. Food and Drug Administration. Purpose To evaluate a Quantitative Imaging Biomarkers Alliance-supported, user- and machine-independent US method for quantitatively measuring QBF. Materials and Methods In this prospective study (March 2017 to March 2019), three different clinical US scanners were used to benchmark QBF in a calibrated flow phantom at three different laboratories each. Testing conditions involved changes in flow rate (1-12 mL/sec), imaging depth (2.5-7 cm), color flow gain (0%-100%), and flow past a stenosis. Each condition was performed under constant and pulsatile flow at 60 beats per minute, thus yielding eight distinct testing conditions. QBF was computed from three-dimensional color flow velocity, power, and scan geometry by using Gauss theorem. Statistical analysis was performed between systems and between laboratories. Systems and laboratories were anonymized when reporting results. Results For systems 1, 2, and 3, flow rate for constant and pulsatile flow was measured, respectively, with biases of 3.5% and 24.9%, 3.0% and 2.1%, and -22.1% and -10.9%. Coefficients of variation were 6.9% and 7.7%, 3.3% and 8.2%, and 9.6% and 17.3%, respectively. For changes in imaging depth, biases were 3.7% and 27.2%, -2.0% and -0.9%, and -22.8% and -5.9%, respectively. Respective coefficients of variation were 10.0% and 9.2%, 4.6% and 6.9%, and 10.1% and 11.6%. For changes in color flow gain, biases after filling the lumen with color pixels were 6.3% and 18.5%, 8.5% and 9.0%, and 16.6% and 6.2%, respectively. Respective coefficients of variation were 10.8% and 4.3%, 7.3% and 6.7%, and 6.7% and 5.3%. Poststenotic flow biases were 1.8% and 31.2%, 5.7% and -3.1%, and -18.3% and -18.2%, respectively. Conclusion Interlaboratory bias and variation of US-derived quantitative blood flow indicated its potential to become a clinical biomarker for the blood supply to end organs. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Forsberg in this issue.

Ultrasound Shear Wave Elastography and Doppler Sonography to Assess the Effect of Hydration on Human Kidneys: A Preliminary Observation

To assess the feasibility of ultrasound imaging in depicting the changes in kidney size, hemodynamics and cortex viscoelasticity after hydration, we prospectively performed 2-D ultrasound shear wave elastography (SWE) and Doppler sonography of bilateral kidneys in 30 volunteers. Kidney length, cortex shear wave velocity (SWV), shear wave dispersion (SWD), interlobar artery peak systolic velocity (PSV), end-diastolic velocity (EDV) and resistive index (RI) were measured before and 60 min after with and without drinking water (1 L). The differences in kidney length, SWV, PSV, EDV and color pixel intensity before and after hydration were significant (p < 0.001), whereas these differences were not significant without hydration (p > 0.05). SWD and RI did not significantly differ with or without hydration. Inter- and intra-observer reliability in performing SWE and Doppler sonography was good. The use of Doppler sonography and ultrasound SWE to evaluate the effect of hydration on kidney size, hemodynamics and viscoelasticity seem to be feasible.

Three-dimensional US Fractional Moving Blood Volume: Validation of Renal Perfusion Quantification

Background Three-dimensional (3D) fractional moving blood volume (FMBV) derived from 3D power Doppler US has been proposed for noninvasive approximation of perfusion. However, 3D FMBV has never been applied in animals against a ground truth. Purpose To determine the correlation between 3D FMBV and the reference standard of fluorescent microspheres (FMS) for measurement of renal perfusion in a porcine model. Materials and Methods From February 2017 to September 2017, adult pigs were administered FMS before and after measurement of renal 3D FMBV at baseline (100%) and approximately 75%, 50%, and 25% flow levels by using US machines from two different vendors. The 3D power Doppler US volumes were converted and segmented, and correlations between FMS and 3D FMBV were made with simple linear regression (r2). Similarity and reproducibility of manual segmentation were determined with the Dice similarity coefficient and 3D FMBV reproducibility (intraclass correlation coefficient [ICC]). Results Thirteen pigs were studied with 33 flow measurements. Kidney volume (mean Dice similarity coefficient ± standard deviation, 0.89 ± 0.01) and renal segmentation (coefficient of variation = 12.6%; ICC = 0.86) were consistent. The 3D FMBV calculations had high reproducibility (ICC = 0.97; 95% confidence interval: 0.96, 0.98). The 3D FMBV per-pig correlation showed excellent correlation for US machines from both vendors (mean r2 = 0.96 [range, 0.92-1.0] and 0.93 [range, 0.78-1.0], respectively). The correlation between 3D FMBV and perfusion measured with microspheres was high for both US machines (r2 = 0.80 [P < .001] and 0.70 [P < .001], respectively). Conclusion The strong correlation between three-dimensional (3D) fractional moving blood volume (FMBV) and fluorescent microspheres indicates that 3D FMBV shows excellent correlation to perfusion and good reproducibility. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Morrell et al in this issue.

Partial Volume Effect and Correction for 3-D Color Flow Acquisition of Volumetric Blood Flow

Blood volume flow (VF) estimation is becoming an integral part of quantitative medical imaging. Three-dimensional color flow can be used to measure volumetric flow, but partial volume correction (PVC) is essential due to finite beamwidths and lumen diameters. Color flow power was previously assumed to be directly proportional to the perfused fractional color flow beam area (voxel). We investigate the relationship between color flow power and fractionally perfused voxels. We simulate 3-D color flow imaging using Field II based on a 3.75-MHz mechanically swept linear array. A 16-mm-diameter tube with laminar flow was embedded into soft tissue. We investigated two study scenarios where soft tissue backscatter is 1) 40 dB higher and 2) 40 dB lower, relative to blood. Velocity and power were computed from color flow packets ( n = 16 ) using autocorrelation. Study 1 employed a convolution-based wall filter. Study 2 did not employ a wall filter. VF was computed from the resulting color flow data, as published previously. Partial volume voxels in Study 1 show lesser power than those in Study 2, likely due to wall filter effects. An "S"-shaped relationship was found between color flow power and fractionally perfused voxel area in Study 2, which could be due to an asymmetric lateral-elevational point spread function. Flow computation is biased low by 7.3% and 7.9% in Study 1 and Study 2, respectively. Uncorrected simulation estimates are biased high by 41.5% and 12.5% in Study 1 and Study 2, respectively. Our findings show that PVC improves 3-D VF estimation and that wall filter processing alters the proportionality between color flow power and fractionally perfused voxel area.

Evaluation of Umbilical Vein Blood Volume Flow in Preeclampsia by Angle-Independent 3D Sonography

OBJECTIVES

To investigate the association between umbilical vein blood volume flow and the condition of preeclampsia in an at-risk maternal patient cohort. Umbilical vein volume flow was quantified by a 3-dimensional (3D) sonographic technique that overcomes several limitations of standard sonographic flow measurement methods.

METHODS

A total of 35 patients, each with a singleton pregnancy, were recruited to provide 5 patients with preeclampsia, derived as a subset from a 26-patient at-risk group, and 9 patients with normal pregnancies. An ultrasound system equipped with a 2.0-8.0-MHz transducer was used to acquire multivolume 3D color flow and power mode data sets to compute the mean umbilical vein volume flow in patients with normal pregnancies and preeclampsia.

RESULTS

The gestational ages of the pregnancies ranged from 29.7 to 34.3 weeks in the patients with preeclampsia and from 25.9 to 34.7 weeks in the patients with normal pregnancies. Comparisons between patients with normal pregnancies and those with preeclampsia showed weight-normalized flow with a moderately high separation between groups (P = .11) and depth-corrected, weight-normalized flow with a statistically significant difference between groups (P = .035). Umbilical vein volume flow measurements were highly reproducible in the mean estimate, with an intrapatient relative SE of 12.1% ± 5.9% and an intrameasurement relative SE of 5.6% ± 1.9 %. In patients who developed pregnancy-induced hypertension or severe pregnancy-induced hypertension, umbilical vein volume flow suggested gestational hypertensive disorder before clinical diagnosis.

CONCLUSIONS

Results indicate that mean depth-corrected, weight-normalized umbilical vein volume flow is reduced in pregnancies complicated by preeclampsia and that volume flow may indicate hypertensive disorder earlier in gestation. Volume flow measurements are highly reproducible, and further study in a larger clinical population is encouraged to determine whether 3D volume flow can complement the management of preeclampsia and, in general, at-risk pregnancy.

Recent technological advancements in cardiac ultrasound imaging

About 92.1 million Americans suffer from at least one type of cardiovascular disease. Worldwide, cardiovascular diseases are the number one cause of death (about 31% of all global deaths). Recent technological advancements in cardiac ultrasound imaging are expected to aid in the clinical diagnosis of many cardiovascular diseases. This article provides an overview of such recent technological advancements, specifically focusing on tissue Doppler imaging, strain imaging, contrast echocardiography, 3D echocardiography, point-of-care echocardiography, 3D volumetric flow assessments, and elastography. With these advancements ultrasound imaging is rapidly changing the domain of cardiac imaging. The advantages offered by ultrasound imaging include real-time imaging, imaging at patient bed-side, cost-effectiveness and ionizing-radiation-free imaging. Along with these advantages, the steps taken towards standardization of ultrasound based quantitative markers, reviewed here, will play a major role in addressing the healthcare burden associated with cardiovascular diseases.

Low-Cost 3-D Flow Estimation of Blood With Clutter

Volumetric flow rate estimation is an important ultrasound medical imaging modality that is used for diagnosing cardiovascular diseases. Flow rates are obtained by integrating velocity estimates over a cross-sectional plane. Speckle tracking is a promising approach that overcomes the angle dependency of traditional Doppler methods, but suffers from poor lateral resolution. Recent work improves lateral velocity estimation accuracy by reconstructing a synthetic lateral phase (SLP) signal. However, the estimation accuracy of such approaches is compromised by the presence of clutter. Eigen-based clutter filtering has been shown to be effective in removing the clutter signal; but it is computationally expensive, precluding its use at high volume rates. In this paper, we propose low-complexity schemes for both velocity estimation and clutter filtering. We use a two-tiered motion estimation scheme to combine the low complexity sum-of-absolute-difference and SLP methods to achieve subpixel lateral accuracy. We reduce the complexity of eigen-based clutter filtering by processing in subgroups and replacing singular value decomposition with less compute-intensive power iteration and subspace iteration methods. Finally, to improve flow rate estimation accuracy, we use kernel power weighting when integrating the velocity estimates. We evaluate our method for fast- and slow-moving clutter for beam-to-flow angles of 90° and 60° using Field II simulations, demonstrating high estimation accuracy across scenarios. For instance, for a beam-to-flow angle of 90° and fast-moving clutter, our estimation method provides a bias of -8.8% and standard deviation of 3.1% relative to the actual flow rate.

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.

Ultrasound Vector Flow Imaging-Part I: Sequential Systems

This paper gives a review of the most important methods for blood velocity vector flow imaging (VFI) for conventional sequential data acquisition. This includes multibeam methods, speckle tracking, transverse oscillation, color flow mapping derived VFI, directional beamforming, and variants of these. The review covers both 2-D and 3-D velocity estimation and gives a historical perspective on the development along with a summary of various vector flow visualization algorithms. The current state of the art is explained along with an overview of clinical studies conducted and methods for presenting and using VFI. A number of examples of VFI images are presented, and the current limitations and potential solutions are discussed.

Volumetric blood flow in transjugular intrahepatic portosystemic shunt revision using 3-dimensional Doppler sonography

OBJECTIVES

Three-dimensional (3D)/4-dimensional (4D) sonographic measurement of blood volume flow in transjugular intrahepatic porto systemic shunt revision with the intention of objective assessment of shunt patency.

METHODS

A total of 17 patients were recruited (12 male and 5 female; mean age, 55 years; range, 30-69 years). An ultrasound system equipped with a 2.0-5.0-MHz probe was used to acquire multivolume 3D/4D color Doppler data sets to assess prerevision and postrevision shunt volume flow. Volume flow was computed offline based on the principle of surface integration of Doppler-measured velocity vectors in a lateral-elevational c-surface positioned at the color flow focal depth (range, 8.0-11.5 cm). Volume flow was compared to routine measurements of the prerevision and postrevision portosystemic pressure gradient. Prerevision volume flow was compared with the outcome to determine whether a flow threshold for revision could be defined.

RESULTS

Linear regression of data from revised transjugular intrahepatic portosystemic shunt cases showed an inverse correlation between the mean-normalized change in prerevision and postrevision shunt volume flow and the mean-normalized change in the prerevision and postrevision portosystemic pressure gradient (r(2) = 0.51; P = .020). Increased shunt blood flow corresponded to a decreased pressure gradient. Comparison of prerevision flows showed preliminary threshold development at 1534 mL/min, below which a shunt revision may be recommended (P = .21; area under the receiver operating characteristic curve = 0.78).

CONCLUSIONS

Shunt volume flow measurement with 3D/4D Doppler sonography provides a potential alternative to standard pulsed wave Doppler metrics as an indicator of shunt function and predictor of revision.

Hemodynamic changes in the hepatic circulation after the modulation of the splenic circulation in an in vivo human experimental model

Recent advances in liver surgery have highlighted the effects of the splenic circulation on the hepatic circulation with respect to the hepatic arterial buffer response (HABR). The aim of the present study was to investigate the actual hemodynamic effects of splenic artery embolization/ligation and splenectomy on the hepatic circulation in patients who underwent pancreaticoduodenectomy through in vivo experimental models. In vivo models of splenic artery embolization/ligation (only splenic artery clamping) and splenectomy (simultaneous clamping of both the splenic artery and the splenic vein) were created in 40 patients who underwent pancreaticoduodenectomy for various reasons. The portal venous flow velocity, the portal venous flow volume, the hepatic arterial flow velocity, and the hepatic arterial resistance index were measured with color Doppler ultrasonography. Clamping of the splenic artery induced an immediate and significant increase (16%) in the hepatic artery velocity (P < 0.001), and the portal venous flow also decreased significantly (10%, P = 0.03). Fifteen minutes after the clamping of the splenic artery, the hepatic artery velocity remained significantly increased at the level of the initial clamping, and the portal venous flow significantly decreased (16%, P < 0.001). Clamping of the splenic vein, which was performed after the clamping of the splenic artery, resulted in an immediate and significant decrease (30%) in the portal venous flow (P < 0.001), but the hepatic arterial flow was not affected. Fifteen minutes after the clamping of the splenic vein, there was no change in the portal flow, which remained significantly lower (28%) than the flow in controls, whereas the hepatic arterial flow further significantly increased (31%, P < 0.001). In conclusion, our findings indicate that both splenic artery embolization/ligation and splenectomy are effective for increasing hepatic arterial flow and decreasing portal flow, with splenectomy providing a greater advantage. The HABR underlies these hemodynamic changes.

Three-dimensional sonographic measurement of blood volume flow in the umbilical cord

OBJECTIVES

Three-dimensional (3D) umbilical cord blood volume flow measurement with the intention of providing a straightforward, consistent, and accurate method that overcomes the limitations associated with traditional pulsed wave Doppler flow measurement and provides a means by which to recognize and manage at-risk pregnancies.

METHODS

The first study involved 3D sonographic volume flow measurements in 7 healthy ewes whose pregnancies ranged from 18 to 19 weeks' gestation (7 singletons). Sonographic umbilical arterial and venous flow measurements from each fetus were compared to the corresponding average measured arterial/venous flow to assess the feasibility of measurement in a static vessel. A second complementary study involved 3D sonographic volume flow measurements in 7 healthy women whose pregnancies ranged from 17.9 to 36.3 weeks' gestation (6 singletons and 1 twin). Umbilical venous flow measurements were compared to similar flow measurements reported in the literature. Pregnancy outcomes were abstracted from the medical records of the recruited patients.

RESULTS

In the fetal sheep model, arterial/venous flow comparisons yielded errors of 10% or less for 8 of the 9 measurements. In the clinical study, venous flow measurements showed agreement with the literature over a range of gestational ages. Two of the 7 patients in the clinical study had lower flow than anticipated for gestational age; one had a subsequent diagnosis of intrauterine growth restriction, and the other had preeclampsia.

CONCLUSIONS

Accurate measurement of umbilical blood volume flow can be performed with relative ease in both the sheep model and in humans using the proposed 3D sonographic flow measurement technique. Results encourage further development of the method as a means for diagnosis and identification of at-risk pregnancies.

A 5-MHz cylindrical dual-layer transducer array for 3-D transrectal ultrasound imaging

Two-dimensional transrectal ultrasound (TRUS) is being used in guiding prostate biopsies and treatments. In many cases, the TRUS probes are moved manually or mechanically to acquire volumetric information, making the imaging slow, user dependent, and unreliable. A real-time three-dimensional (3-D) TRUS system could improve reliability and volume rates of imaging during these procedures. In this article, the authors present a 5-MHz cylindrical dual-layer transducer array capable of real-time 3-D transrectal ultrasound without any mechanically moving parts. Compared with fully sampled 2-D arrays, this design substantially reduces the channel count and fabrication complexity. This dual-layer transducer uses PZT elements for transmit and P[VDF-TrFE] copolymer elements for receive, respectively. The mechanical flexibility of both diced PZT and copolymer makes it practical for transrectal applications. Full synthetic aperture 3-D data sets were acquired by interfacing the transducer with a Verasonics Data Acquisition System. Offline 3-D beamforming was then performed to obtain volumes of two wire phantoms and a cyst phantom. Generalized coherence factor was applied to improve the contrast of images. The measured -6-dB fractional bandwidth of the transducer was 62% with a center frequency of 5.66 MHz. The measured lateral beamwidths were 1.28 mm and 0.91 mm in transverse and longitudinal directions, respectively, compared with a simulated beamwidth of 0.92 mm and 0.74 mm.

Estimation of blood velocity, volumetric flow and wall shear rate using Doppler ultrasound

Commercial ultrasound systems can make a number of measurements related to haemodynamics which are relevant to clinical practice and to clinical research. These include maximum velocity, volumetric flow and wall shear rate. Using appropriate protocols, measurements can be made averaged over the cardiac cycle, or as a function of time through the cardiac cycle. Maximum velocity underpins most of these measurements. Maximum velocity is overestimated as a result of geometric spectral broadening, by typically up to 30%, but by much larger amounts as the angle approaches 90°. Though not used in clinical practice, a simple correction technique using a string phantom can substantially reduce these errors. For volumetric flow and wall shear rate, methods such as specialist multi-gate ultrasound systems, magnetic resonance imaging and image guided modelling are available. Before resorting to these more complex methods users might consider that, with care and attention to procedure, high quality information may be obtained using commercial ultrasound systems. Manufacturers could make more use of the colour flow image for quantification of velocity, and adopt vector Doppler techniques.

Validation of endoscopic ultrasound measured flow rate in the azygos vein using a flow phantom

Increase in flow rate within the azygos vein may be used as an indicator of the degree of liver cirrhosis. The aim of this study was to evaluate the error in measurement of flow rate using a commercial endoscopic ultrasound system, using a flow phantom that mimicked azygos vein depth, diameter and flow rate. Diameter was underestimated in all cases, with an average underestimation of 0.09 cm. Maximum velocity was overestimated, by 4 ± 4% at 50°, 11 ± 3% at 60° and 23 ± 7% at 70°. The increase in error with beam-vessel angle is consistent with the error as arising from geometric spectral broadening. Flow was underestimated by amounts up to 33%, and it is noted that the overestimation caused by geometric spectral broadening is in part compensated by underestimation of diameter. It was concluded that measurement of flow rate using a commercially available endoscopic ultrasound system is dependent on the beam-vessel angle, with errors up to 33% for typical vessel depths, diameter and beam-vessel angle.

In vitro simulation of the first technique for non-invasive measurement of volumetric ophthalmic artery blood flow in humans

BACKGROUND AND OBJECTIVE

To evaluate the accuracy, reproducibility, and variability of volumetric flow measurements taken by color Doppler imaging ultrasound, using an in vitro "phantom" model to simulate the ophthalmic artery.

MATERIALS AND METHODS

An agar flow phantom with two wall-less lumens was constructed to simulate the ophthalmic artery. Velocity and volumetric flow measurements were taken for various flow rates and ultrasound probe positions. The measurements were analyzed for accuracy, reproducibility, and variability.

RESULTS

Velocity measurements were more accurate than flow measurements (8 of 24 vs 3 of 24 accurate trials). The average coefficient of variation for volumetric blood flow was 11.4% (n = 120). Volumetric flow significantly correlated with velocity (R(2) = 0.408, n = 600, P < .001). The highest correlation was achieved using the large lumen with the probe held at 75 degrees , offset to the flow (R(2) = 0.862, n = 75).

CONCLUSION

Based on an in vitro model, non-invasive color Doppler imaging recordings of volumetric flow measurements in the ophthalmic artery significantly correlated with velocity and higher correlations were found using the larger lumens, although the data showed a lack of high accuracy in measurements of flow and velocity.

In vivo comparison of three ultrasound vector velocity techniques to MR phase contrast angiography

The objective of this paper is to validate angle independent vector velocity methods for blood velocity estimation. Conventional Doppler ultrasound (US) only estimates the blood velocity along the US beam direction where the estimate is angle corrected assuming laminar flow parallel to vessel boundaries. This results in incorrect blood velocity estimates, when angle of insonation approaches 90 degrees or when blood flow is non-laminar. Three angle independent vector velocity methods are evaluated in this paper: directional beamforming (DB), synthetic aperture flow imaging (STA) and transverse oscillation (TO). The performances of the three methods were investigated by measuring the stroke volume in the right common carotid artery of 11 healthy volunteers with magnetic resonance phase contrast angiography (MRA) as reference. The correlation with confidence intervals (CI) between the three vector velocity methods and MRA were: DB vs. MRA: R=0.84 (p<0.01, 95% CI: 0.49-0.96); STA vs. MRA: R=0.71 (p<0.05, 95% CI: 0.19-0.92) and TO vs. MRA: R=0.91 (p<0.01, 95% CI: 0.69-0.98). No significant differences were observed for any of the three comparisons (DB vs. MRA: p=0.65; STA vs. MRA: p=0.24; TO vs. MRA: p=0.36). Bland-Altman plots were additionally constructed, and mean differences with limits of agreements (LoA) for the three comparisons were: DB vs. MRA=0.17 ml (95% CI: -0.61-0.95) with LoA=-2.11-2.44 ml; STA vs. MRA=-0.55 ml (95% CI: -1.54-0.43) with LoA=-3.42-2.32 ml; TO vs. MRA=0.24 ml (95% CI: -0.32-0.81) with LoA=-1.41-1.90 ml. According to the results, reliable volume flow estimates can be obtained with all three methods. The three US vector velocity techniques can yield quantitative insight into flow dynamics and visualize complex flow patterns, which potentially can give the clinician a novel tool for cardiovascular disease assessment.

Accuracy of volumetric flow rate measurements: an in vitro study using modern ultrasound scanners

OBJECTIVE

Volumetric flow measurement with Doppler ultrasound is useful in assessing blood flow as part of an evaluation of arteriovenous fistula maturity in patients undergoing hemodialysis. In this study, we assessed both accuracy and variability in volumetric flow measurements obtained using modern and commercially available ultrasound systems and an in vitro experimental setup.

METHODS

Volumetric flow measurements using duplex ultrasound were obtained by 3 users operating 5 different systems for randomized flow in the range of 100 to 1000 mL/min. Users performed 3 consecutive measurements at a given flow rate. Data were analyzed using statistical techniques to assess measurement accuracy and variability.

RESULTS

Over the span of flow rates studied, the root mean square error (RMSE) for the 5 ultrasound systems ranged from 38.8 to 79.7, 36.8 to 52.0, 73.0 to 85.3, 26.7 to 44.6, and 43.9 to 93.5 mL/min. Corresponding average RMSE values were 60.3, 42.7, 81.1, 37.2, and 64.4 mL/min, respectively. A linear regression analysis of mean interobserver measurements revealed an excellent correlation for all ultrasound systems (r(2) > 99.1%). Assessment of intraobserver measurements revealed no statistically significant differences for any ultrasound system evaluated (P > .94). Comparison of interobserver measurements indicates no statistically significant differences between any of the 5 systems (P > .14).

CONCLUSIONS

Modern ultrasound systems are reasonably accurate in blood flow measurement in an experimental setup mimicking clinically relevant blood flow ranges in a hemodialysis fistula. Users need adequate training and experience to perform multiple measurements and use appropriate techniques to minimize errors in flow measurement.

Mean volume flow estimation in pulsatile flow conditions

To verify a previously reported three-dimensional (3D) ultrasound method for the measurement of time-average volumetric blood flow, experiments were performed under pulsatile flow conditions, including in vivo investigations, and results were compared with accepted, but invasive, "gold standard" techniques. Results showed that volume averaging results in the correct time-average volume flow without the need for cardiac gating. Unlike other currently employed methods, this method is independent of Doppler angle, flow profile and vessel geometry. A GE Logiq 9 ultrasound system (GE Medical Systems, Milwaukee, WI, USA) and a four-dimensional (4D) 10L and 4D 16L probe were used to acquire 3D Doppler measurements in the femoral and carotid arteries of four canines. Two invasive blood flow meters were used (electromagnetic for one canine and ultrasonic for three canines) as the gold standard techniques. Transcutaneous color flow measurements were taken to obtain 3D volume data sets encompassing the vessel. Constant depth planes were used to integrate color flow pixels encompassing the entire vessel cross-section. Power Doppler data were used to correct for partial volume effects. An artificial stenosis was induced to vary the ambient volume flow. Unrestricted, bidirectional flow was measured as high as 400 mL min(-1). Several flow restrictions were imposed that decreased the measured volumetric flow rate to as low as 30 mL min(-1). All flow rate estimates (n=38) were plotted against results obtained via the gold standards. A general line fit resulted in y = 0.926 x - 0.87 (r(2) = 0.95), which corresponds to a 0.6% flow offset relative to the average flow rate of 142 mL min(-1), as well as a 7.4% error in the linearity of our estimate. A secondary curve fit was performed that required the slope to be 1 and the intercept to be 0, which yielded an r(2)-value of 0.93. The percent-error distribution was computed and fitted to a Gaussian function, which yielded mu=-7.04% and sigma=9.52%. Theoretical studies were conducted to estimate the expected error in our volume flow measurements as a function of number of samples (N) used for averaging pulsatile waveforms. Experiments showed the same 1/N dependence as theory. Direct comparisons of volume flow rate estimates using volumetric color Doppler and independent standards showed that our method has good accuracy under in vivo pulsatile blood flow conditions.

Haemodynamics and blood flow measured using ultrasound imaging

Visualization of, and measurements related to, haemodynamic phenomena in arteries may be made using ultrasound systems. Most ultrasound technology relies on simple measurements of blood velocity taken from a single site, such as the peak systolic velocity for assessment of the degree of lumen reduction caused by an arterial stenosis. Real-time two-dimensional (2D) flow field visualization is possible using several methods, such as colour flow, blood flow imaging, and echo particle image velocimetry; these have applications in the examination of the flow field in diseased arteries and in heart chambers. Three-dimensional (3D) and four-dimensional ultrasound systems have been described. These have been used to provide 2D velocity profile data for the estimation of volumetric flow. However, they are limited for haemodynamic evaluation in that they provide only one component of the velocity. The provision of all seven components (three space, three velocity, and one time) is possible using image-guided modelling, in which 3D ultrasound is combined with computational fluid dynamics. This method also allows estimation of turbulence data and of relevant quantities such as the wall shear stress.

Carotid stenosis assessed with a 4-dimensional semiautomated Doppler system

OBJECTIVE

The purpose of this study was to compare peak systolic velocities (PSVs) and the degree of stenosis obtained with a real-time 3-dimensional (ie, 4-dimensional) Doppler ultrasound scanner (Encore PV; VueSonix Sensors Inc, Wayne, PA) to conventional Doppler ultrasound imaging of the carotid arteries (common [CCA], internal [ICA], and external [ECA]). A secondary goal was to assess Encore volume flow measurements.

METHODS

Seventy patients referred for clinical carotid ultrasound participated in this pilot study. Peak systolic velocities of the CCA, ECA, and ICA were obtained bilaterally. The degree of stenosis in the ICA was calculated based on the ICA PSV and ICA/CCA PSV ratio. The Encore detects all 3-dimensional blood flow velocity vectors within 10-s longitudinal volumes of the ICA, ECA, and CCA. On the Encore, a reader determined the centerline of the vessels. The PSV and volume flow were then automatically calculated. The flow measurement error was obtained by comparing the CCA flow to the ICA and ECA flow. Data were compared using linear regression, intraclass correlation coefficients (ICCs), and Bland-Altman analysis.

RESULTS

Due to technical difficulties, only 59 patients (323 vessel segments) were available for analysis. There was good agreement between methods for assessing the degree of stenosis based on the ICA PSV (ICC = 0.83; P < .0001) and, to a lesser degree, on the ICA/CCA PSV ratio (ICC = 0.65; P < .0001). Peak systolic velocity measurements obtained with conventional ultrasound and the Encore correlated in all vessels (r >or= 0.32; P < .002), and Bland-Altman analysis showed reasonable variations. The Encore mean volume flow error +/- SD was -4.1% +/- 66.4% and was not biased (P = .57).

CONCLUSIONS

A new semiautomated 4-dimensional Doppler device is comparable to conventional Doppler ultrasound for assessment of carotid stenosis.