Billy Y. S. Yiu

Vector Flow Visualization of Urinary Flow Dynamics in a Bladder Outlet Obstruction Model

Voiding dysfunction that results from bladder outlet (BO) obstruction is known to alter significantly the dynamics of urine passage through the urinary tract. To non-invasively image this phenomenon on a time-resolved basis, we pursued the first application of a recently developed flow visualization technique called vector projectile imaging (VPI) that can track the spatiotemporal dynamics of flow vector fields at a frame rate of 10,000 fps (based on plane wave excitation and least-squares Doppler vector estimation principles). For this investigation, we designed a new anthropomorphic urethral tract phantom to reconstruct urinary flow dynamics under controlled conditions (300xa0mm H2O inlet pressure and atmospheric outlet pressure). Both a normal model and a diseased model with BO obstruction were developed for experimentation. VPI cine loops were derived from these urinary flow phantoms. Results show that VPI is capable of depicting differences in the flow dynamics of normal and diseased urinary tracts. In the case with BO obstruction, VPI depicted the presence of BO flow jet and vortices in the prostatic urethra. The corresponding spatial-maximum flow velocity magnitude was estimated to be 2.43xa0m/s, and it is significantly faster than that for the normal model (1.52xa0m/s) and is in line with values derived from computational fluid dynamics simulations. Overall, this investigation demonstrates the feasibility of using vector flow visualization techniques to non-invasively examine internal flow characteristics related to voiding dysfunction in the urethral tract.

Spiral Flow Phantom for Ultrasound Flow Imaging Experimentation

As new ultrasound flow imaging methods are being developed, there is a growing need to devise appropriate flow phantoms that can holistically assess the accuracy of the derived flow estimates. In this paper, we present a novel spiral flow phantom design whose Archimedean spiral lumen naturally gives rise to multi-directional flow over all possible angles (i.e., from 0° to 360°). Developed using lost-core casting principles, the phantom geometry comprised a three-loop spiral (4-mm diameter and 5-mm pitch), and it was set to operate in steady flow mode (3 mL/s flow rate). After characterizing the flow pattern within the spiral vessel using computational fluid dynamics (CFD) simulations, the phantom was applied to evaluate the performance of color flow imaging (CFI) and high-frame-rate vector flow imaging. Significant spurious coloring artifacts were found when using CFI to visualize flow in the spiral phantom. In contrast, using vector flow imaging (least-squares multi-angle Doppler based on a three-transmit and three-receive configuration), we observed consistent depiction of flow velocity magnitude and direction within the spiral vessel lumen. The spiral flow phantom was also found to be a useful tool in facilitating demonstration of dynamic flow visualization based on vector projectile imaging. Overall, these results demonstrate the spiral flow phantom’s practical value in analyzing the efficacy of ultrasound flow estimation methods.

Arterial Phantoms with Regional Variations in Wall Stiffness and Thickness

Regional wall stiffening and thickening are two common pathological features of arteries. To account for these two features, we developed a new arterial phantom design framework to facilitate the development of vessel models that contain a lesion segment whose wall stiffness and thickness differ from those of other segments. This new framework is based on multi-part injection molding principles that sequentially casted the lesion segment and the flank segments of the vessel model using molding parts devised with computer-aided design tools. The vessel-mimicking material is created from polyvinyl alcohol cryogel, and its acoustic properties are similar to those of arteries. As a case demonstration, we fabricated a stenosed three-segment phantom composed of a central lesion segment (5.1-mm diameter, 1.95-mm wall thickness, 212.6-kPa elastic modulus) and two flank segments (6.0-mm diameter, 1.5-mm wall thickness, 133.7-kPa elastic modulus). B-mode imaging confirmed the difference in thickness between the lesion segment and flank segments of the phantom. Also, Doppler-based vessel wall displacement analysis revealed that when pulsatile flow was fed through the phantom (carotid pulse; 27u2009mL/s peak flow rate), the lesion segment distended less compared with the flank segments. Specifically, the three-beat averaged peak wall displacement in the lesion segment was measured as 0.28u2009mm, and it was significantly smaller than that of the flank segments (0.60u2009mm). It is anticipated that this new multi-segment arterial phantom can serve as a performance testbed for the evaluation of local arterial stiffness estimation algorithms.

Ultrasound Imaging and Measurement of Choroidal Blood Flow

Purpose The choroid is a vascular network providing the bulk of the oxygen and nutrient supply to the retina and may play a pivotal role in retinal disease pathogenesis. While optical coherence tomography angiography provides an en face depiction of the choroidal vasculature, it does not reveal flow dynamics. In this report, we describe the use of plane-wave ultrasound to image and characterize choroidal blood flow. Methods We scanned both eyes of 12 healthy subjects in a horizontal plane superior to the optic nerve head using an 18-MHz linear array. Plane-wave data were acquired over 10 transmission angles that were coherently compounded to produce 1000 images/sec for 3 seconds. These data were processed to produce a time series of power Doppler images and spectrograms depicting choroidal flow velocity. Analysis of variance was used to characterize peak systolic, and end diastolic velocities and resistive index, and their variability between scans, eyes, and subjects. Results Power Doppler images showed distinct arterioles within a more diffuse background. Choroidal flow was moderately pulsatile, with peak systolic velocity averaging approximately 10 mm/sec and resistive index of 0.55. There was no significant difference between left and right eyes, but significant variation among subjects. Conclusions Plane-wave ultrasound visualized individual arterioles and allowed measurement of flow over the cardiac cycle. Characterization of choroidal flow dynamics offers a novel means for assessment of the choroids role in ocular disease. Translational Relevance Characterization of choroidal flow dynamics offers a novel means for assessment of the choroids role in ocular disease.

Plane-wave vector-flow imaging of adult mouse heart

High-frequency ultrasound Doppler modes have been used extensively for murine cardiovascular (CV) studies, but traditional linear-array imaging modes are limited in terms of spatial and temporal resolution. Plane-wave imaging methods allow for high-speed vector-flow information to be obtained throughout a full image frame. Plane-wave imaging has been demonstrated in human CV studies, but its use in mouse models has received minimal attention. A Verasonics Vantage with an 18-MHz linear array was used to acquire plane-wave data at a frame rate of 30 kHz from the left ventricle of adult mice. Batches of 3 transmissions spanning ± 5 degrees were sent out. The mouse was placed supine on a heated imaging platform and then 2Du2009+u2009time data sequences. The data were beamformed using standard delay-and-sum methods and vector-flow estimates were obtained at each pixel location using a least-squares, multi-angle Doppler analysis approach. Vortex patterns in the left ventricle were visualized over several heart cycles s...

High-speed, high-frequency, vector-flow imaging of in utero mouse embryos

Plane-wave imaging has been demonstrated in humans for cardiovascular (CV) studies, but its use in mouse embryo models has received minimal attention even though the mouse is the most common experimental organism to study gene function and human disease, including CV disease (CVD). While high-frequency ultrasound Doppler modes have been used to study mouse embryo models, traditional linear-array imaging modes are limited in terms of spatial and temporal resolution which limits the amount of functional information that can be mined. The goal of this study was to obtain high-speed plane-wave data from in utero mouse embryos and then process the data to obtain vector flow information.

Multi-plane, time-synchronized color encoded speckle imaging: A new approach for aneurysm flow visualization

Inside tortuous vasculatures such as aneurysms, flow patterns are inherently 3D in nature and would vary spatiotemporally in different phases of a pulse cycle. At present, it is difficult to consistently visualize such complex flow using color Doppler imaging. 4D flow imaging may be a potential solution, but existing clinical 3D scanners have hitherto yielded mediocre visualization of 3D flow volumes, and their frame rate is inadequate in tracking fast-changing flow patterns. In this work, we aim to achieve intuitive visualization of flow volumes through the design of a novel time-resolved flow visualization technique. Our new technique is a multi-plane, time-synchronized extension of a high-frame-rate flow imaging paradigm called color encoded speckle imaging (CESI) developed by our lab earlier.

Towards real-time adaptive color flow imaging: A GPU-optimized eigen-based clutter filter core

Eigen-filters with attenuation response adapted to clutter statistics in color flow imaging (CFI) have shown improved flow detection sensitivity in the presence of tissue motion. However, its practical adoption for routine use in clinical scanners is hindered by the long processing time required to derive the eigen-components. In this work, we seek to overcome this issue by formulating a parallel-computing framework that enables fast execution of eigen-filtering so as to foster their practical adoption in CFI. Our framework readily differs from commercial eigensolvers available on numerical software packages. In particular, our algorithm is the first that is optimized for parallel execution of multiple eigen-computation tasks on small matrices (a typical scenario in CFI processing).

Time-resolved vector projectile imaging of urinary flow dynamics

Developing tools to visualize urinary flow dynamics is important, because urinary hydrodynamics is known to have a causal relationship with urethral voiding dysfunction problems. Yet, the design of such an imaging technique is challenging as fine time resolution is essential to track urine passage that can traverse at >2m/s speeds. Previously, we have overcome the time resolvability challenge in flow visualization with a new technique called vector projectile imaging (VPI), which can render flow vector fields with <1 ms time resolution. In this work, we present the first application of VPI to track urinary flow patterns during different voiding phases.

The spiral flow phantom: A new tool for vector flow estimator performance analysis

As new flow imaging methods are actively being developed, there is a growing need to devise appropriate phantoms that can holistically assess the accuracy of the derived flow estimates. Straight-tube models simply cannot serve this purpose, nor can spinning discs that simulate tissue motion instead of flow dynamics. Anthropomorphic bifurcation models are also not ideal, because their geometry does not embody all possible curvature angles and thus angle-specific estimation errors are prone to be overlooked. Here, we present a novel spiral phantom design with two unique features: 1) comprehensive multi-directional flow (i.e. from 0 to 360 deg); 2) space-time variability in flow speed. Both features are essential to foster meticulous evaluation of a flow estimators performance.