Doppler Ultrasound Velocity Measurements Research Articles

Analysis of finite element and finite volume methods for fluid-structure interaction simulation of blood flow in a real stenosed artery

This paper presents a qualitative and quantitative comparison between the finite element and the finite volume methods for the fluid-structure interaction simulation of blood flow through a real stenosed artery. The artery geometry corresponds to a severely stenosed (around 75% lumen reduction) portion of the brachiocephalic trunk, located immediately upstream of the bifurcation of this vessel into the right subclavian and right common carotid arteries. The patient-specific geometry was segmented from medical images of a computerized tomography scanner from an individual with the subclavian steal syndrome. Doppler ultrasound velocity measurements were used to determine and impose patient-specific boundary conditions. The numerical simulations were performed in commercial software, Ansys and COMSOL, with a comparative second order discretization for the pressure, velocity and displacement variables. The results of this research disclosed a reasonable overall agreement between the predicted hemodynamics for both approaches. The finite volume method software (Ansys) proved to be more efficient in computational time and memory requirements.

A hybrid echocardiography-CFD framework for ventricular flow simulations.

Image-based CFD is a powerful tool to study cardiovascular flows while 2D echocardiography (echo) is the most widely used noninvasive imaging modality for the diagnosis of heart disease. Here, echo is combined with CFD, that is, an echo-CFD framework, to study ventricular flows. To achieve this, the previous 3D reconstruction from multiple 2D echo at standard cross sections is extended by: (a) reconstructing aortic and mitral valves from 2D echo and closing the left-ventricle (LV) geometry by approximating a superior wall; (b) incorporating the physiological assumption of the fixed apex as a reference (fixed) point in the 3D reconstruction; and (c) incorporating several smoothing algorithms to remove the nonphysical oscillations (ringing) near the basal section. The method is applied to echo from a baseline LV and one after inducing acute myocardial ischemia (AMI). The 3D reconstruction is validated by comparing it against a reference reconstruction from many echo sections while flow simulations are validated against the Doppler ultrasound velocity measurements. The sensitivity study shows that the choice of the smoothing algorithm does not change the flow pattern inside the LV. However, the presence of the mitral valve can significantly change the flow pattern during the diastole phase. In addition, the abnormal shape of a LV with AMI can drastically change the flow during diastole. Furthermore, the hemodynamic energy loss, as an indicator of the LV pumping performance, for different test cases is calculated, which shows a larger energy loss for a LV with AMI compared to the baseline one.

Delineating the Role of Cerebral Blood Flow and Hypocapnia on Neuromuscular Function

Neuromuscular fatigue, defined as the inability of a muscle to maintain a given level of force regardless of whether or not a task can be sustained, has been attributed, in part, to central mechanisms. Although reductions in cerebral blood flow (CBF) may be implicated in the development of central fatigue, the contribution from hypocapnic-reductions (i.e., PETCO2) in CBF versus reductions in CBF per se has yet to be isolated. PURPOSE: To examine the hypothesis that hypocapnia, independent of concomitant reductions in CBF, cause impairments in neuromuscular function. METHODS: Neuromuscular function, as indicated by motor evoked potentials (MEP), M-waves and cortical voluntary activation (cVA) of the flexor carpal radialis muscle during isometric wrist flexion, was assessed in 8 males (31±12 y) during 3 separate conditions: 1) cyclooxygenase inhibition using Indomethacin (Indo) to selectively reduce CBF; 2) controlled hyperventilation-induced hypocapnia (Hypocapnia); and 3) isocapnic hyperventilation (Isocapnia). CBF was estimated using transcranial Doppler ultrasound velocity measurements of the middle cerebral artery. Change from baseline and comparison of the 3 conditions was assessed using two-way repeated measures ANOVA. Relationships between variables were assessed using Pearson‘s correlations. RESULTS: The experimental conditions successfully isolated CBF and PETCO2. MEP amplitude (% M-wave amplitude) increased in hypocapnia (14.6±9.5%, p < 0.01) in comparison to Indo (-2.0±2.8%) and Isocapnia (0.6±1.8%); however, no significant changes were observed in M wave (p = 0.666) or cVA (p = 0.117). Correlation analysis revealed that MEP amplitude was inversely associated with PETCO2 (r = -0.54, p < 0.01) and cVA was weakly associated with CBF (r = 0.30, p < 0.05). CONCLUSIONS: Increased motor cortex excitability appears to be exclusive to hypocapnia but not reductions in CBF per se. However, a weak relationship between reductions in CBF and cVA exists, such that further research is required to confirm this finding and explore the potential implications. Supported by NSERC Discovery 227912-2012.

Doppler string phantom for assessment of clinical doppler ultrasound velocity measurement

Purpose: The Doppler string phantom provides accurate velocity of the string motion; it can be used to calibrate Doppler ultrasound (US) velocity measurements and to evaluate variations due to intrinsic spectral broadening. We developed a semi‐automated method to estimate the mode velocity (Vmode) and peak velocity (Vmax) based on duplex US images from a string phantom, and use them to assess clinical Doppler US velocity measurement. Methods : Steady motion of a rubber O‐ring (20 – 110 cm/s) in a CIRS Doppler String phantom (Model 043) was studied using GE LOGIQ E9 system with a 9L probe. 5 s of Doppler spectral data was averaged to generate a mean spectral profile. It was fitted by a Gaussian function and Vmode was defined as the velocity of the Gaussian peak, while Vmax is defined as the velocity at which the spectral profile falls to within 1 SD of the background. Vmode and Vmax were evaluated against the prescribed motor velocity. Repeatability and variation to scanning parameters were analyzed and reported in % range, i.e. (max – min) / mean. Results: Vmode and Vmax had good repeatability over six days (6.0% for Vmode, 2.9% for Vmax). Gain, compression, scale, sample volume (SV) depth and length, frequency and beam steering all had minimal impact on Vmode and Vmax (variations ≤ 4.4%). Doppler angle θ had minimal effect on Vmode (2.2%) but a strong effect on Vmax (26% increase as θ increased from 10° to 60°). Vmode was linearly correlated with but overestimated the motor velocity (Pearson’s r = 1.05, R 2 = 1). Conclusion : This study developed a simple yet robust Vmode and Vmax estimation method. Combined with a string phantom, these velocity estimators are shown to be a useful tool to evaluate clinical Doppler US system performance. For the tested system, only Doppler angle has an appreciable impact on Vmax estimation. -------------------------------------------- Cite this article as : Zhang Y, Lynch T, Hangiandreou NJ. Doppler string phantom for assessment of clinical doppler ultrasound velocity measurement. Int J Cancer Ther Oncol 2014; 2(2):020246. DOI:10.14319/ijcto.0202.46

Application of Womersley Model to Reconstruct Pulsatile Flow From Doppler Ultrasound Measurements

A Womersley model-based assessment of pulsatile rigid-tube flow is presented. Multigate Doppler ultrasound was used to measure axial velocities at many radial locations along a single interrogation beam going through the center of a stiff tube. However, a large impediment to Doppler ultrasound diagnostics and resolution close to the wall is considerable noise due to the presence of the wall-fluid interface as well as many other effects, such as spectral broadening, coherent scattering, time resolution, and Doppler angle uncertainty. Thus, our confidence in measured signals is questionable, especially in the wall vicinity where the important oscillatory shear stresses occur. In order to alleviate known biases and shortcomings of the pulsed Doppler ultrasound measurements we have applied Womersley's laminar axisymmetric rigid-tube approximation to reconstruct velocity profiles over the entire flow domain and specifically close to wall, enabling unambiguous determination of the shear stresses. We employ harmonic analysis of the measured velocity profiles at all or selected trusted tube radial locations over one or more periods. Each of estimated Fourier coefficients has a unique counterpart in the respective pressure gradient component. From ensemble-averaged cross-sectional pressure gradient components we compute velocity profiles, volume flow rate, wall shear stress, and other flow parameters. Estimation of the pressure gradients from spatially resolved pulsed Doppler ultrasound velocity measurements is an added benefit of our reconstruction method. Multigate pulsed Doppler ultrasound scanners offer powerful capabilities to noninvasively and nonintrusively measure velocity profiles for hemodynamic and other fluid flow applications. This flow reconstruction method can also be tailored for use with other flow diagnostic modalities, such as magnetic resonance imaging (MRI) and a wide class of optical methods.

Effect of Beam-Flow Angle on Velocity Measurements in Modern Doppler Ultrasound Systems

The purpose of this article is to examine the effect of different beam-flow angles on the accuracy of Doppler ultrasound velocity measurements in modern ultrasound systems. A flow phantom was used to create a steady flow of water in a 4.3-mm-diameter tube. Using three different modern university-grade ultrasound systems, flow was measured at 30°, 40°, 50°, 60°, 70°, 80°, and 88° beam-flow angles twice by two radiologists in consensus using a convex and linear probe. Measured flow ratio, defined as measured velocity divided by estimated actual velocity, was calculated. Intraprobe, interprobe, and intermachine mean variation of measured flow ratio were calculated. Measured flow ratio increased as beam-flow angles increased. Measured flow ratios for the angles 30°, 40°, 50°, 60°, 70°, 80°, and 88° were 0.90, 0.97, 1.10, 1.22, 1.62, 2.34, and 10.29, respectively. Intraprobe, interprobe, and intermachine variation did not show marked differences. For angles grouped as 30-40°, 50-60°, 70°, and 80-88°, intraprobe variation was 12%, 15%, 15%, and 26%; interprobe variation was 20%, 16%, 13%, and 26%; and intermachine variation was 16%, 16%, 17%, and 54%, respectively. As beam-flow angle increased, an increase in spectral broadening was also noted. There is no simple cutoff beam-flow value, such as the well-quoted less than 60°, at which velocity measurements can be considered accurate. For follow-up imaging, beam-flow angle differences should be considered, and the same beam-flow angles should be used when possible. Follow-up imaging by different sonography machines is feasible.

Increased internal carotid artery peak systolic velocity is associated with presence of significant atherosclerotic plaque instability independent of degree of ICA stenosis

The aim of our work was to study the relationship between Doppler ultrasound velocity measurements and the presence of histologic features of plaque instability in carotid atherosclerosis, in particular, intraplaque hemorrhage (IPH). Consecutive patients undergoing carotid endarterectomy in a one-year period were included. All patients were examined by duplex ultrasonography and carotid angiography. Endarterectomy specimens were examined histologically for features of plaque instability. The quantity of IPH was measured by digital image analysis. The associations between Peak Systolic Velocity (PSV), end-diastolic velocity (EDV), degree of ICA stenosis, shape and length of the lesion and the features of plaque instability and quantity of IPH were assessed. Seventy-four patients (20 asymptomatic, 54 symptomatic) were included. PSV was independently associated with the presence of significant IPH [p < 0.001, OR = 1.04 (95% CI = 1.01–1.06)], as was the degree of angiographic ICA stenosis [p < 0.05, OR = 0.98 (95% CI = 0.92–1.6)]. Neither EDV nor the shape of the lesion was associated with IPH (p = 0.26 and p = 0.38, respectively). A close correlation was observed between PSV and the quantity of IPH (r2 = 0.68, p < 0.0001). A significant association was observed between PSV and the presence of plaque ulceration (p < 0.05); however, this was not found to be independent of the quantity of IPH and the degree of ICA stenosis [p = 0.17, OR = 1.28 (95% CI = 0.6–2.44)]. PSV at the site of ICA stenosis appears to be associated with the quantity of intraplaque hemorrhage, independent of the angiographic degree of ICA stenosis. We propose that the role of Doppler velocity measurements extends beyond measurement of the degree of ICA stenosis. Increased ICA peak systolic velocity by itself may be an indicator of atherosclerotic plaque instability.

Increased internal carotid artery peak systolic velocity is associated with presence of significant atherosclerotic plaque instability independent of degree of ICA stenosis

The aim of our work was to study the relationship between Doppler ultrasound velocity measurements and the presence of histologic features of plaque instability in carotid atherosclerosis, in particular, intraplaque hemorrhage (IPH). Consecutive patients undergoing carotid endarterectomy in a one-year period were included. All patients were examined by duplex ultrasonography and carotid angiography. Endarterectomy specimens were examined histologically for features of plaque instability. The quantity of IPH was measured by digital image analysis. The associations between Peak Systolic Velocity (PSV), end-diastolic velocity (EDV), degree of ICA stenosis, shape and length of the lesion and the features of plaque instability and quantity of IPH were assessed. Seventy-four patients (20 asymptomatic, 54 symptomatic) were included. PSV was independently associated with the presence of significant IPH [p < 0.001, OR = 1.04 (95% CI = 1.01–1.06)], as was the degree of angiographic ICA stenosis [p < 0.05, OR = 0.98 (95% CI = 0.92–1.6)]. Neither EDV nor the shape of the lesion was associated with IPH (p = 0.26 and p = 0.38, respectively). A close correlation was observed between PSV and the quantity of IPH (r 2 = 0.68, p < 0.0001). A significant association was observed between PSV and the presence of plaque ulceration (p < 0.05); however, this was not found to be independent of the quantity of IPH and the degree of ICA stenosis [p = 0.17, OR = 1.28 (95% CI = 0.6–2.44)]. PSV at the site of ICA stenosis appears to be associated with the quantity of intraplaque hemorrhage, independent of the angiographic degree of ICA stenosis. We propose that the role of Doppler velocity measurements extends beyond measurement of the degree of ICA stenosis. Increased ICA peak systolic velocity by itself may be an indicator of atherosclerotic plaque instability.

Common carotid artery: variability of Doppler US velocity measurements.

To determine if Doppler ultrasound (US) measurements of systolic velocity in the common carotid artery vary markedly with distance from the bifurcation. Bilateral, insonation angle-corrected velocity was measured with US in the common carotid artery. Measurements were obtained 1, 2, 3, 4, and 5 cm from the carotid artery bifurcation in 20 volunteers (aged 25-43 years) with no history of neurologic or cardiovascular disease. Velocity in the common carotid artery increased with distance (toward the aorta) from the bifurcation (mean increase, 9 cm/sec for each centimeter of distance from the bifurcation, up to the 4-cm distance; P < .001). Velocity increased more rapidly in subjects who were younger (P < .001) and in those with a lower pulse rate (P = .001). The distance from the bifurcation at which common carotid artery velocity is measured should be standardized. This may increase the accuracy of the systolic velocity ratio for assessment of internal carotid artery stenosis and decrease some of the reported variability between different centers that perform US of the carotid artery.

Blood flow in the portal vein: velocity quantitation with phase-contrast MR angiography.

Quantitative phase-contrast magnetic resonance (MR) angiography of the portal vein was prospectively evaluated in 79 fasting patients and 23 healthy volunteers. Images were obtained during a 12-second breath-hold acquisition in the coronal (n = 102) and axial (n = 11) planes. Pathologic correlation was available in 55 of 79 patients and included findings of cholangiocarcinoma, cirrhosis, hepatocellular carcinoma, hepatitis, and metastatic disease. Forty-one patients had correlative Doppler ultrasound (US) findings. MR and US findings correlated as to flow direction in all cases. In nine patients, Doppler US velocity measurements were available and closely correlated with MR findings. A comparison of axial and coronal portal venous phase-contrast measurements in 11 patients revealed no substantial difference with regard to the plane used. Quantitative phase-contrast MR angiography is a simple and rapid technique for the assessment of portal venous patency, flow direction, and flow velocity and, combined with high-resolution conventional MR imaging, may obviate the current use of both computed tomographic and US examinations.

Accuracy of colour Doppler ultrasound velocity measurements in small vessels

Colour Doppler ultrasound offers the possibility of imaging small vessels not visible by B-mode alone. The colour Doppler image of velocities allows the course of small vessels to be imaged in the X-Y plane of the scan provided the Doppler frequency shift is of sufficient magnitude. This permits alignments of the Doppler cursor, allowing angle correction to provide true velocity measurements from the Doppler shift obtained. Before attempting to make velocity measurements, however, it is essential to be aware of the possible error in the Z plane caused by the thickness of the Doppler sample volume. To quantify this source of error, hydrophone and flow-rig measurements were performed on an Acuson 128 colour Doppler scanner with both 5 MHz linear-array and 3.5 MHz phased-array transducers. Measurements of the transmitted pulses using a point hydrophone showed that both probes employ approximately 3.5 MHz Doppler pulses (in both colour and pulsed Doppler modes). The two transducers have the same axial resolution. In colour Doppler mode the axial length of the sample volume increases automatically with depth by up to 0.5 mm. Measurements of colour and pulsed Doppler signal strength were obtained in a controlled flow rig. Both transducers produced accurate colour flow images of the phantom at their optimum depths; flow velocity errors due to Z-plane thickness are < 5%. There was, however, substantial error outside these optimum conditions (up to 20%).