Carotid stenosis assessed with a 4-dimensional semiautomated Doppler system

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.

Volumetric assessment of the carotid bifurcation: an alternative concept to stenosis grading

BACKGROUND

To design a volumetric method for the assessment of carotid atheromatosis (CA) based on computed tomography (CT) angiography and three-dimensional (3D) reconstructions; to analyze the accuracy and optimal threshold values to differentiate between equivalent degrees of severity by duplex scanning and CT angiography (two-dimensional maximum intensity projection [2D MIP]; internal carotid artery stenosis [ICS] <50%; ICS >50%); and to assess the method's suitability to detect progression of CA.

METHODS

DESIGN

suitability and accuracy of a new diagnostic method.

POPULATION

90 carotid bifurcations (45 patients) were assessed with duplex scanning and CT angiography, and reevaluated after 12 ± 2 months follow-up. Determinations: Assessment of internal carotid artery (ICA) stenosis degree with duplex scanning and 2D MIP CT angiography projections. Volumetric assessment of carotid bifurcation by CT angiography (contrast volume [mm(3)] and density [Hounsfield units, H.U.] between 2-cm below and 1-cm above the anatomic bifurcation of the carotid artery [BifV], and its ratio with 1-cm segment of the common carotid artery [CCV]).

STATISTICAL ANALYSIS

descriptive statistics; intraobserver and interobserver agreement (Bland-Altman plot and intraclass correlation coefficient [ICC], accuracy of 3D volumetry and duplex scanning as referred to MIP 2D CT angiography as gold standard: sensitivity (Sens), specificity (Sp), kappa index, and receiver operating characteristic curves (ROCs).

RESULTS

Estimation of MIP 2D images (CT angiography) confirmed the findings of duplex scanning in 23 of 30 ICS <50% and 48 of 53 ICS >50% (Sens, 0.91; Sp, 0.77% kappa = 0.68). Three-dimensional volumetric assessment of carotid bifurcation showed an intraobserver and interobserver agreement with an ICC of 0.96 (95% confidence interval [CI], 0.904-0.985) and 0.94 (95% CI, 0.822-0.977), respectively. The BifV-to-CCV ratio was 5.2 ± 1.8 in the ICS <50% group versus 3.8 ± 1.3 in the ICS >50% group (P = 0.001). The optimal cutoff point of the BifV-to-CCV relationship was identified from the ROC curve in 4.1 (Sens, 0.75; Sp, 0.75; kappa, 0.46). At 12 months, a decrease of the average BifV with regard to the baseline value (475.45 [155.6] mm(3) × H.U. vs. 501.3 [171.9] mm(3) × H.U.; P = 0.04) was observed. CA progression was detected in 32 bifurcations (14 ICS <50%; 18 ICS> 50%), with a reduced bifurcation volume of 137.8 (71.4) mm(3) × H.U.; P < 0.001.

CONCLUSIONS

Volumetric assessment of carotid bifurcation is a new concept based on assessing plaque burden rather than its hemodynamic effect or maximum stenosis; thus, justifying its moderate accuracy with regard to ICS conventional ICA grading based on biplanar images. This method can be especially useful in plaque progression studies given its accuracy to detect minor changes in the arterial lumen.

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.

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.