Validating volume flow measurements from a novel semiautomated four-dimensional Doppler ultrasound scanner

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.

Accuracy and reproducibility of a novel dynamic volume flow measurement method

In clinical practice, blood volume flow (BVF) is typically calculated assuming a perfect parabolic and axisymmetric velocity distribution. This simple approach cannot account for the complex flow configurations that are produced by vessel curvatures, pulsatility and diameter changes and, therefore, results in a poor estimation. Application of the Womersley model allows compensation for the flow distortion caused by pulsatility and, with some adjustment, the effects of slight curvatures, but several problems remain unanswered. Two- and three-dimensional approaches can acquire the actual velocity field over the whole vessel section, but are typically affected by a limited temporal resolution. The multigate technique allows acquisition of the actual velocity profile over a line intersecting the vessel lumen and, when coupled with a suitable wall-tracking method, can offer the ideal trade-off among attainable accuracy, temporal resolution and required calculation power. In this article, we describe a BVF measurement method based on the multigate spectral Doppler and a B-mode edge detector algorithm for wall-position tracking. The method has been extensively tested on the research platform ULA-OP, with more than 1700 phantom measurements at flow rates between 60 and 750 mL/min, steering angles between 10 ° and 22 ° and constant, sinusoidal or pulsed flow trends. In the averaged BVF measurement, we found an underestimation of about -5% and a coefficient of variability (CV) less than 6%. In instantaneous measurements (e.g., systolic peak) the CV was in the range 2%-8.5%. These results were confirmed by a preliminary test on the common carotid artery of 10 volunteers (CV = 2%-11%).

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 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.

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.

A generic bioheat transfer thermal model for a perfused tissue

A thermal model was needed to predict temperatures in a perfused tissue, which satisfied the following three criteria. One, the model satisfied conservation of energy. Two, the heat transfer rate from blood vessels to tissue was modeled without following a vessel path. Three, the model applied to any unheated and heated tissue. To meet these criteria, a generic bioheat transfer model (BHTM) was derived here by conserving thermal energy in a heated vascularized finite tissue and by making a few simplifying assumptions. Two linear coupled differential equations were obtained with the following two variables: tissue volume averaged temperature and blood volume averaged temperature. The generic model was compared with the widely employed empirical Pennes' BHTM. The comparison showed that the Pennes' perfusion term wC(p)(1-epsilon) should be interpreted as a local vasculature dependent heat transfer coefficient term. Suggestions are presented for further adaptations of the general BHTM for specific tissues using imaging techniques and numerical simulations.

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.