Fuxing Zhang

IN VITRO AND PRELIMINARY IN VIVO VALIDATION OF ECHO PARTICLE IMAGE VELOCIMETRY IN CAROTID VASCULAR IMAGING

Noninvasive, easy-to-use and accurate measurements of wall shear stress (WSS) in human blood vessels have always been challenging in clinical applications. Echo particle image velocimetry (Echo PIV) has shown promise for clinical measurements of local hemodynamics and wall shear rate. Thus far, however, the method has only been validated under simple flow conditions. In this study, we validated Echo PIV under in vitro and in vivo conditions. For in vitro validation, we used an anatomically correct, compliant carotid bifurcation flow phantom with pulsatile flow conditions, using optical particle image velocimetry (optical PIV) as the reference standard. For in vivo validation, we compared Echo PIV-derived 2-D velocity fields obtained at the carotid bifurcation in five normal subjects against phase-contrast magnetic resonance imaging (PC-MRI)-derived velocity measurements obtained at the same locations. For both studies, time-dependent, 2-D, two-component velocity vectors; peak/centerline velocity, flow rate and wall shear rate (WSR) waveforms at the common carotid artery (CCA), carotid bifurcation and distal internal carotid artery (ICA) were examined. Linear regression, correlation analysis and Bland-Altman analysis were used to quantify the agreement of different waveforms measured by the two techniques. In vitro results showed that Echo PIV produced good images of time-dependent velocity vector maps over the cardiac cycle with excellent temporal (up to 0.7 ms) and spatial (∼0.5 mm) resolutions and quality, comparable with optical PIV results. Further, good agreement was found between Echo PIV and optical PIV results for velocity and WSR measurements. In vivo results also showed good agreement between Echo PIV velocities and phase contrast MRI velocities. We conclude that Echo PIV provides accurate velocity vector and WSR measurements in the carotid bifurcation and has significant potential as a clinical tool for cardiovascular hemodynamics evaluation.

Development of a custom-designed echo particle image velocimetry system for multi-component hemodynamic measurements: system characterization and initial experimental results

We have recently developed an ultrasound-based velocimetry technique, termed echo particle image velocimetry (Echo PIV), to measure multi-component velocity vectors and local shear rates in arteries and opaque fluid flows by identifying and tracking flow tracers (ultrasound contrast microbubbles) within these flow fields. The original system was implemented on images obtained from a commercial echocardiography scanner. Although promising, this system was limited in spatial resolution and measurable velocity range. In this work, we propose standard rules for characterizing Echo PIV performance and report on a custom-designed Echo PIV system with increased spatial resolution and measurable velocity range. Then we employed this system for initial measurements on tube flows, rotating flows and in vitro carotid artery and abdominal aortic aneurysm (AAA) models to acquire the local velocity and shear rate distributions in these flow fields. The experimental results verified the accuracy of this technique and indicated the promise of the custom Echo PIV system in capturing complex flow fields non-invasively.

An improved approach for obtaining thermodynamic descriptions of intermetallic phases: application to the Cr–Ta system

Some shortcomings of phenomenological phase diagram calculations are addressed in the present note: (i) the formalism currently used for describing ordered intermetallic phases and (ii) the criteria used for assessing the quality of a thermodynamic description. It is shown how the formalism used for ordered intermetallic phases can be improved by replacing the use of sublattice L parameters with a configurational independent term, which has its physical origin in the atomic size mismatch of the participating elements. It is also suggested that an examination of solid state phase diagrams provides an additional criterion for the acceptance of good thermodynamic descriptions over the criteria used at present. The Cr–Ta system is used as an example to show how this improved approach can be used to enhance the quality of the phenomenological thermodynamic modelling of ordered intermetallic phases.

Evaluation of segmentation algorithms for vessel wall detection in echo particle image velocimetry

Recent in-vitro and in-vivo validation studies confirmed the accuracy of echo particle image velocimetry (echo PIV), a simple non-invasive means of measuring multi-component blood velocity vectors. Echo PIV should also be useful for direct measurement of wall shear stress (WSS) in clinical studies. However, calculation of WSS requires accurate delineation of vessel walls in ultrasound images, which may be problematic when conventional segmentation techniques are used. In this paper, we proposed two methods for segmenting contrast enhanced B-mode images. The first is based on the intensity profile of ultrasound images, termed intensity-based edge detection (IBED) and the second based on the movement of microbubbles, termed movement-based quadratic difference (MBQD). The parameters related with the two methods were optimized over large sets of microbubble images acquired from human carotid vessels using an echo PIV system (Illumasonix LLC, Boulder, CO). A validation study on the two algorithms was carried out against manual delineations on both common carotid artery (CCA) and carotid bifurcation images, with 20 frames for each group. The inter-observer variability of three manual delineations, in pixels (about 80 µm/pixel), was 0.9±0.4, 1.3±0.6, 1.3±0.6 on CCA images, and 2.5±1.0, 3.9±1.1, 2.3±1.1 on bifurcation images. The absolute difference (mean±SD) between each computer-generated contour and the ground truths, taken as the average of three manual delineations, were 1.3±0.8, 3.8±0.8, 5.3±0.5 on CCA images, and 2.3±0.9, 4.6±1.3, 6.3±0.6 on bifurcation images, for the MBQD, IBED and active contour methods, respectively. The MBQD method shows comparable performance with manual delineations on particle images even with poor intima-media layer quality.

Cluster/site approximation calculation of the ordering phase diagram for Cd–Mg alloys

Abstract We have used the modified cluster/site approximation in a phenomenological calculation of the phase diagram and single phase thermodynamic properties for cadmium–magnesium alloys in the order/disorder region. Values for the three cluster energies differ little from those obtained by LMTO-ASA calculations on unrelaxed alloys of ideal c/a ratio. Only two other parameters contained in contributions to the Gibbs energy are required. One comes from a configuration independent term and the other is an entropy factor coming from the cluster/site approximation.

Noninvasive wall shear stress measurements in human carotid artery using echo particle image velocimetry: Initial clinical studies

Wall shear stress (WSS) has been shown to be important to endothelial cell function and gene expression. Previous studies have shown that fluid dynamics might be closely related to the initialization of atherosclerotic plaques which preferentially originate in areas of disturbed flow in human vessels, where WSS is low or oscillatory. We recently developed a novel ultrasound-based technique, termed echo particle image velocimetry (echo PIV), by which the multi-component hemodynamic information in human cardiovascular system could be assessed. In this paper, we show that echo PIV was successfully employed to measure hemodynamic information in the right common carotid artery (rCCA) of ten healthy volunteers and that the values show good agreement with phase-contrast magnetic resonance imaging (PC-MRI) measurements with mean absolute differences (mean±SD) of 10.0%±9.8%, 10.1%±8.8% and 17.0%±15.3% for velocity, flow rate and WSS, respectively. In particular, the mean WSS (dynes/cm^2) in rCCA of ten volunteers was found to be 9.2±2.0 by echo PIV, and 8.0±1.4 by PC-MRI, both in agreement with published data. We further showed that calculating WSS by either peak/mean velocity or flow rate together with arterial diameter was invalid for in-vivo measurements due to invalidity of assuming parabolic velocity profile in carotid artery. We found that the peak velocity across radial direction in rCCA was about 1.6 times of the mean velocity, not 2 times as it should be in parabolic distribution. In conclusion, echo PIV demonstrated several advantages over traditional techniques in terms of both temporal and spatial resolution when measuring WSS in human vessels.

Systematic validation of the echo particle image velocimetry technique using a patient specific carotid bifurcation model

Full field opaque flow measurement technique has significant applications in evaluating details of cardiovascular hemodyamics non-invasively. Echo Particle Image Velocimetry (Echo PIV) is a simple-to-use method that has shown promising results in our previous in vitro studies on measurements of blood flow characteristics. So far, however, no systematic validation of Echo PIV in a realistic vascular anatomy against reference-standard methods has been done. In this paper, a patient-specific carotid bifurcation model was created using biplane angiography data from an adult human male. The model reproduced vascular geometry accurately, while allowing for both Optical PIV and Echo PIV to be performed on the same flow field. A pulsatile pump and a compliance chamber were used to produce physiologically realistic flow conditions with a peak velocity around 70 cm/s, heart rate of 75 beats/min, mean Reynolds number of 1484, and a Womersley number of 16.1. The Echo PIV measurements were validated by Optical PIV technique, a gold standard for velocity field measurement, in terms of velocity, flow rate and wall shear rate profiles at a few different positions in carotid artery. Results show that the Echo PIV measurements agreed well with Optical PIV results, with a mean error 7.7% +/- 3.5% for velocity profiles, and a mean error 9.2% +/- 5.7% for wall shear rate profiles.

In Vivo Validation of Echo Partical Image Velocimetry (Echo PIV) in Human Carotid Arteries Using Phase-Contrast MRI

Accurate non-invasive measurements of dynamic wall shear stresses (WSS) in the cardiovascular system should allow clinicians to evaluate the progression of atherosclerosis, estimate vulnerability of plaques, and assess hemodynamic changes in the proximity of implants such as vascular grafts and stents that may contribute to restenosis. Although computational methods have been used to obtain blood flow characteristics from patients, these methods are difficult to apply in routine fashion, are prone to errors due to incorrect application of boundary conditions and are time and resource intensive. Ultrasound Doppler methods [1] allow simple measurement of blood flow velocities but are not acceptable for shear measurements because they do not provide multiple-component velocity vectors, are prone to angulation error and have relatively poor spatial resolution. Phase contrast magnetic resonance imaging (PC-MRI) velocimetry [2] does provide good spatial resolution and multiple-component velocity vectors in vivo and is considered the gold-standard. However, PC-MRI is expensive, time consuming and possesses relatively poor temporal resolution.Copyright

12B-4 Real-Time Multi-Component Hemodynamic Measurement in Vascular Aneurysms Using Echo Particle Image Velocimetry: Comparison of In Vitro and Computational Results

Abdominal aortic aneurysms (AAAs) are localized balloon-shaped expansions commonly found in the infrarenal segment of the abdominal aorta, between the renal arteries and the iliac bifurcation. From a biomechanical standpoint, AAA rupture risk is related to mechanical and hemodynamic factors such as localized flow fields and velocity patterns, and flow- induced stresses within the fluid and in the aneurysm structure. Thus, localized hemodynamics proximal, within and distal to AAA formations play an important role in modulating the disease process, and non-invasive and easy-to-implement methods to characterize and quantify these complex hemodynamics would be tremendously useful. Based on the synthesis of two existing technologies, particle image velocimetry (PIV) and brightness-mode (B-mode) contrast ultrasound echo imaging, we have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (Echo PIV), to perform accurate non-invasive measurement of velocity profiles, multiple velocity vectors and local shear rate in arteries and hearts by identifying and tracking flow tracers (ultrasound contrast microbubbles) within the flow fields. Here, we examine the utility of Echo PIV for quantifying the complex hemodynamics found within aneurysms. Three-dimensional computational fluid dynamics (CFD) simulations were used for comparison. The Echo PIV system was used to measure velocity vectors within in vitro fusiform AAA models. Ultrasound contrast microbubbles (Optisonreg, Amersham, UK) were seeded into the flows. To verify Echo PIV results, 3D computational fluid dynamics (CFD) was used on an identical aneurysm model. The computational solid model was imported into ICEM-CFD (ANSYS Inc., PA) and meshed using 35,000 hexahedral elements. The same flow conditions were used for both in vitro and CFD studies. Comparison results show that Echo PIV measures the flow field accurately and provides clear delineation of the complex flow patterns within the AAA model. To verify the performance of the Echo PIV system on a point-by-point basis, we compared velocity profiles at the bulge center from both CFD and Echo PIV results. Difference in velocity magnitudes was < 5 %, verifying the accuracy of Echo PIV in this model. Echo PIV was also able to resolve the multiple velocity scales seen within the vascular aneurysm (< 2 cm/sec to > 20 cm/sec). These results show that our custom-designed Echo PIV system is capable of accurately providing detailed multi-component, angle- independent velocity vectors within vascular aneurysms.

Addition of Particle Tracking Techniques to Improve Two-Dimensional Echo PIV for Opaque Flow Measurement

The ability to measure multi-component velocity vectors in opaque flows, especially in blood flows, would have a number of important benefits. Over the last few years, we have developed a new technique, echo particle imaging velocimetry (echo PIV), based on the synthesis of two technologies: PIV and digital brightness-mode (B-mode) ultrasound contrast imaging. Recently-reported results using a custom-developed Echo PIV system [1] showed the utility of this technique in accurately measuring two-dimensional velocity vectors in a variety of opaque flows.Copyright