Tamie L Poepping

In this age of rapid medical and technological advancements, institutions are recognizing the need for interdisciplinary training. In a recent article (Hendee 2007), the need for more extensive physics education during a clinical residency in diagnostic imaging has been emphasized. Corrective measures have been recommended by the American Association of Physicists in Medicine and the Radiological Society of North America

ABSTRACT Vector Doppler ultrasound (VDUS) systems offer the potential for improved accuracy in mapping of complex flow parameters, such as recirculation, turbulence, and shear stress which are probable risk factors leading to vascular disease and stroke. Cross-beam VDUS systems were evaluated for velocity accuracy to optimize the number of receivers for the inter-beam angle, wall filter, system orientation, and complexity of flow seen in a stenosed carotid artery. Preliminary results for velocity estimation show promise for validation of numerical results.

Page 1. Investigation of Doppler Ultrasound Velocity-Based Indices for Carotid Plaque Ulcerations Using In Vitro Flow Models Emily Y. Wong,1,2 Meghan L. Thorne,1,2 Hristo N. Nikolov,1 Tamie L. Poepping,3 Richard N. Rankin,4 and David W. Holdsworth1,2,4 ...

Carotid plaque ulcerations, or irregularities in plaque surface morphology, have been identified as an independent risk factor for ischemic stroke. Our previous studies using Doppler ultrasound (DUS) have indicated significant flow disturbances distal to ulceration in the atherosclerotic carotid bifurcation, as characterized by parameters such as turbulence intensity (TI). Additional tools are needed to understand the implications of such flow

ABSTRACT Vector Doppler ultrasound (VDUS) systems offer the potential for improved accuracy in mapping of complex flow parameters, such as recirculation, turbulence and shear stress, which are probable risk factors leading to vascular disease and stroke. VDUS system configurations with 2 to 6 receivers and corresponding reconstructions of a complex velocity field were numerically evaluated.

A unique in-vitro system has been developed that incorporates both realistic phantoms and flow. The anthropomorphic carotid phantoms are fabricated in agar with stenosis severity of 30% or 70% (by NASCET standards) and one of two geometric configurations- concentric or eccentric. The phantoms are perfused with a flow waveform that simulates normal common carotid flow. Pulsed Doppler ultrasound data are

Turbulence is ubiquitous to many systems in nature, except the human vasculature. Development of turbulence in the human vasculature is an indication of abnormalities and disease. A severely stenosed vessel is one such example. In vitro modeling of common vascular diseases, such as a stenosis, is necessary to develop a better understanding of the fluid dynamics for a characteristic geometry.

An in vitro flow system has been used to assess the flow disturbances downstream of the stenosis in a family of seven carotid bifurcation phantoms modelling varying plaque build-up both axially symmetrically (concentrically) and asymmetrically (eccentrically). Radio frequency data were collected for 10 s at each of over 1000 sites within each model, and a sliding 1024-point FFT is applied

... my parents, my sister (Elham), my brother in law (Dr. M. Sarzaeem) and my brothers (Ali, Hamed and Mohammad). I also wish to thank my husband, Mahdi Tavakoli, for his endless love and support in all moments of my life. vi Page 8. Contents Certificate of Examination i ...

Clinical decision-making for the treatment of patients with diseased carotid artery is mainly based on the severity of the stenosis. However, stenosis severity alone is not a sensitive indicator, and other local factors for the assessment of stroke risk are required. Flow disturbance is of particular interest due to its proven association with increased thromboembolic activities. The objective of this study was to investigate the level of turbulence intensity (TI) with regards to certain geometrical features of the plaque - namely stenosis severity, eccentricity, and ulceration. A family of eight carotid-artery bifurcation models was examined using particle image velocimetry. Results showed a marked difference in turbulence intensity among these models; increasing degree of stenosis severity resulted in increased turbulence intensity, going from 0.12 m/s for mild stenosis to 0.37 m/s for severe stenosis (with concentric geometry). Moreover, independent of stenosis severity, eccentricity led to further elevations in turbulence intensity, increasing TI by 0.05-0.10 m/s over the counterpart concentric plaque. The presence of ulceration (in a 50% eccentric plaque) produced a larger portion of moderate turbulence intensity (~0.10 m/s) compared to the non-ulcerated model, more proximal to the bifurcation apex in the post-stenotic recirculation zone. The effect of plaque eccentricity and ulceration in enhancing the downstream turbulence has potential clinical implications for a more sensitive assessment of stroke risk beyond stenosis severity alone.

The most widely performed test for patients suspected of having carotid atherosclerosis is Doppler ultrasound (DUS). Unfortunately, limitations in sensitivity and specificity prevent DUS from being the sole diagnostic tool. Novel DUS velocity-derived parameters, such as turbulence intensity (TI), may provide enhanced hemodynamic information within the carotid artery, increasing diagnostic accuracy. In this study, we evaluate a new technique for

The carotid artery bifurcation is a common site of atherosclerosis which is a major leading cause of ischemic stroke. The impact of stenosis in the atherosclerotic carotid artery is to disturb the flow pattern and produce regions with high shear rate, turbulence, and recirculation, which are key hemodynamic factors associated with plaque rupture, clot formation, and embolism. In order to characterize the disturbed flow in the stenosed carotid artery, stereoscopic PIV measurements were performed in a transparent model with 50% stenosis under pulsatile flow conditions. Simulated ECG gating of the flowrate waveform provides external triggering required for volumetric reconstruction of the complex flow patterns. Based on the three-component velocity data in the lumen region, volumetric shear-stress patterns were derived.

Numerical analysis of the hemodynamic effect of plaque ulceration in the stenotic carotid artery bifurcation. [Proceedings of SPIE 7261, 72611V (2009)]. Emily Y. Wong, Jaques S. Milner, David A. Steinman, Tamie L. Poepping, David W. Holdsworth. Abstract. ...

Atherosclerotic disease, and the subsequent complications of thrombosis and plaque rupture, has been associated with local shear stress. In the diseased carotid artery, local variations in shear stress are induced by various geometrical features of the stenotic plaque. Greater stenosis severity, plaque eccentricity (symmetry) and plaque ulceration have been associated with increased risk of cerebrovascular events based on clinical trial studies. Using particle image velocimetry, the levels and patterns of shear stress (derived from both laminar and turbulent phases) were studied for a family of eight matched-geometry models incorporating independently varied plaque features - i.e. stenosis severity up to 70%, one of two forms of plaque eccentricity, and the presence of plaque ulceration). The level of laminar (ensemble-averaged) shear stress increased with increasing stenosis severity resulting in 2-16 Pa for free shear stress (FSS) and approximately double (4-36 Pa) for wall shear stress (WSS). Independent of stenosis severity, marked differences were found in the distribution and extent of shear stress between the concentric and eccentric plaque formations. The maximum WSS, found at the apex of the stenosis, decayed significantly steeper along the outer wall of an eccentric model compared to the concentric counterpart, with a 70% eccentric stenosis having 249% steeper decay coinciding with the large outer-wall recirculation zone. The presence of ulceration (in a 50% eccentric plaque) resulted in both elevated FSS and WSS levels that were sustained longer (∼20 ms) through the systolic phase compared to the non-ulcerated counterpart model, among other notable differences. Reynolds (turbulent) shear stress, elevated around the point of distal jet detachment, became prominent during the systolic deceleration phase and was widely distributed over the large recirculation zone in the eccentric stenoses.

ABSTRACT Stenosis severity alone is not a sensitive indicator of ischemic stroke risk; however, it remains the primary indicator for clinical decision-making. Carotid endarterectomy is strongly recommended for patients with severe stenosis, while treatment for lesser stenosis severity remains disputed. Thromboembolism is one of the major causes of ischemic stroke and has shown high correlation with hemodynamic factors, such as turbulence. Geometrical factors - such as the degree of stenosis severity, plaque eccentricity, and ulceration - can alter the local hemodynamics of the carotid artery, such as by inducing flow disturbances. The objective of this work was to investigate the impact of these geometrical features on the level of turbulence intensity using DUS and PIV. A family of carotid artery models was examined with geometries ranging from disease-free to severe stenosis, in both eccentric and concentric forms of plaque symmetry, and in the case of moderate stenosis (50%) with and without ulceration. Plaque eccentricity and ulceration were found to enhance the flow disturbances downstream of a stenosis, suggesting that clinical diagnosis should consider plaque shape and roughness in addition to stenosis severity.

The effect of lossy, MP3 (MPEG-Layer 3) compression on clinically important Doppler parameters - derived from spectral analysis of Doppler ultrasound signals - was investigated. Ten, 10-second acquisitions of gated Doppler ultrasound signal were collected in a phantom perfused with a pulsatile flow waveform. Doppler data were collected using two sample volume lengths - 1.5 mm and 10 mm. The

In this study we explore the use of colour flow imaging to quantify blood velocity and wall shear rate. A known velocity profile was obtained using a straight tube flow rig. Colour Doppler ultrasound images were obtained and velocity profiles extracted off-line. There was a small discrepancy (2.6%) in maximum velocity, which was not present after convolution with the colour flow point spread function, hence is likely to be due to velocity gradient broadening within the sample volume. There was disagreement at the edge of the vessel due to loss of velocities below the clutter filter. At a typical clinical colour flow scale (12.8 cms-1) the error in wall shear measurement is 62 % and at a lower scale setting (4.4 cms-1) the error is 11%. This improvement seems to be due to the lower level of clutter filter that is used as the velocity scale decreases.

The effect of lossy, MP3 compression on spectral parameters derived from Doppler ultrasound (US) signals was investigated. Compression was tested on signals acquired from two sources: 1. phase quadrature and 2. stereo audio directional output. A total of 11, 10-s acquisitions of Doppler US signal were collected from each source at three sites in a flow phantom. Doppler signals were digitized at 44.1 kHz and compressed using four grades of MP3 compression (in kilobits per second, kbps; compression ratios in brackets): 1400 kbps (uncompressed), 128 kbps (11:1), 64 kbps (22:1) and 32 kbps (44:1). Doppler spectra were characterized by peak velocity, mean velocity, spectral width, integrated power and ratio of spectral power between negative and positive velocities. The results suggest that MP3 compression on digital Doppler US signals is feasible at 128 kbps, with a resulting 11:1 compression ratio, without compromising clinically relevant information. Higher compression ratios led to significant differences for both signal sources when compared with the uncompressed signals.

To investigate the correlation between disease severity and Doppler spectral measurements in the carotid artery bifurcation, a unique in vitro system has been developed that mimics the human vasculature with respect to both anatomy and flow perfusion. Agar-based carotid phantoms are perfused with a blood-mimicking fluid using a computer-controlled pump and realistic pulsatile flow waveform. A three-axis translational stage allows the lumen to be interrogated with a 0.6-microL Doppler sample volume at the desired spatial intervals using a semiautomated acquisition system, to collect 10 cardiac cycles of gated quadrature data at each site. Off-line analysis, including a 1024-point FFT, produces a 4-D (i.e., time-varying 3-D) Doppler velocity data set with 1.3-cm/s velocity resolution and 12-ms temporal resolution. Using this system, in vitro flow in bifurcations with both normal and stenosed lumen geometry (from 30% to 80% stenosis by NASCET criteria) can be studied, along with the effect of factors, such as stenosis geometry (concentric vs. eccentric) and flow rate, on the observed Doppler ultrasound (US) spectra and haemodynamic patterns.

Abstract A method for segmentation of arteries in ultrasound B-mode images using a modified balloon model is presented. The external force which pulls the contour to the arterial boundary is the combination of the gradient and the second order derivative of the ...

A nonplanar wall-less anatomical flow phantom of a healthy human carotid artery is described, the construction of which is based on a lost-core technique described in the companion paper (Part I) by Watts et al. (2006). The core was made by rapid prototyping of an idealized three-dimensional computer model of the carotid artery. Flow phantoms were built using these idealized non planar carotid artery bifurcations. Physiologically realistic flow waveforms were produced with resistance index values of 0.75, 0.72 and 0.63 in the common, external and internal carotid artery branches, respectively. Distension of the common carotid using M-mode imaging was found to be at 10% of diameter. Although differences in vessel diameter between the phantom and that of the original computer model were statistically significant (p < 0.05), there was no difference (p > 0.05) in measurements made on the lost-cores and those obtained by B-mode ultrasound on the resulting flow phantoms. In conclusion, it was possible to reliably reproduce geometrically similar anatomical flow phantoms that are capable of producing realistic physiological flow patterns and distensions. (E-mail: smeagher@staffmail.ed.ac.uk)

The effect of lossy, MP3 compression on spectral parameters derived from Doppler ultrasound (US) signals was investigated. Compression was tested on signals acquired from two sources: 1. phase quadrature and 2. stereo audio directional output. A total of 11, 10-s acquisitions of Doppler US signal were collected from each source at three sites in a flow phantom. Doppler signals were digitized at 44.1 kHz and compressed using four grades of MP3 compression (in kilobits per second, kbps; compression ratios in brackets): 1400 kbps (uncompressed), 128 kbps (11:1), 64 kbps (22:1) and 32 kbps (44:1). Doppler spectra were characterized by peak velocity, mean velocity, spectral width, integrated power and ratio of spectral power between negative and positive velocities. The results suggest that MP3 compression on digital Doppler US signals is feasible at 128 kbps, with a resulting 11:1 compression ratio, without compromising clinically relevant information. Higher compression ratios led to significant differences for both signal sources when compared with the uncompressed signals.

A technique for the rapid but accurate fabrication of multiple flow phantoms with variations in vascular geometry would be desirable in the investigation of carotid atherosclerosis. This study demonstrates the feasibility and efficacy of implementing numerically controlled direct-machining of vascular geometries into Doppler ultrasound (DUS)–compatible plastic for the easy fabrication of DUS flow phantoms. Candidate plastics were tested for longitudinal speed of sound (SoS) and acoustic attenuation at the diagnostic frequency of 5 MHz. Teflon® was found to have the most appropriate SoS (1376 ± 40 m s–1 compared with 1540 m s–1 in soft tissue) and thus was selected to construct a carotid bifurcation flow model with moderate eccentric stenosis. The vessel geometry was machined directly into Teflon® using a numerically controlled milling technique. Geometric accuracy of the phantom lumen was verified using nondestructive micro-computed tomography. Although Teflon® displayed a higher attenuation coefficient than other tested materials, Doppler data acquired in the Teflon® flow model indicated that sufficient signal power was delivered throughout the depth of the vessel and provided comparable velocity profiles to that obtained in the tissue-mimicking phantom. Our results indicate that Teflon® provides the best combination of machinability and DUS compatibility, making it an appropriate choice for the fabrication of rigid DUS flow models using a direct-machining method. (E-mail: david.holdsworth@imaging.robarts.ca)

Ultrasound (US) B-mode images distal to smooth, rounded cavities, such as cysts, containing a fluid with a speed of sound mismatch to the surrounding tissue, often exhibit a “refractile” edge shadowing artefact. This usually appears as narrow, hypoechoic, shadow lines extending a significant distance distal to the lateral edges of the fluid cavity and parallel to the US beam. The true reasons for this artefact are likely to be complex and to vary from case to case, with many different explanations found in the literature. However, we present a simplified theoretical model for the phenomenon based on a pulsed, finite-beam solution of US scattering from circular fluid-filled cylinders that suggests that “edge” shadows can occur distal not only to edges but also to points where the incident beam intersects the cavity near to the critical angle. Both mechanisms support the view that edge shadows can arise from the combination of unusually high wavefront spreading and the speckle-generating nature of the surrounding tissue. In vitro data from a tissue-mimicking phantom confirms that the edge shadow structure depends on the sign of the speed of sound contrast between the cylinder fluid and the surrounding medium. (E-mail: robins@maths.usyd.edu.au)

Carotid stenosis severity is a commonly used indicator for assessing risk of stroke. However, the majority of individuals with severe carotid artery disease never suffer a stroke, and strokes can occur even with only mild or moderate stenosis. This suggests local factors (other than stenosis severity) at or near the carotid artery bifurcation may be important in determining stroke risk. In this paper we investigate the effect of stenosis geometry on flow patterns in the stenosed carotid bifurcation, using concentrically and eccentrically stenosed anthropomorphic carotid bifurcation models having identical stenosis severity. Computational simulations and experimental flow visualizations both demonstrate marked differences in flow patterns of concentric and eccentric stenosis models for moderately and severely stenosed cases, respectively. In particular, we identify post-stenotic recirculation zone size and location, and spatial extent of elevated wall shear stress as key factors differing between the two geometries. As these are also key biophysical factors promoting thrombogenesis, we propose that the stenosed carotid bifurcation geometry—or the induced flow patterns themselves—may provide more specific indicators for those plaques that are vulnerable to enhanced thromboembolic potential, and hence, increased risk of ischemic stroke. © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 8710+e, 8719Xx

Blood flow instabilities in the carotid artery bifurcation have been highly correlated to clot formation and mobilization resulting in ischemic stroke. In this work, PIV-measured flow velocities in normal and stenosed carotid artery bifurcation models were analyzed by means of proper orthogonal decomposition (POD). Through POD analysis, transition to more complex flow was visualized and quantified for increasing stenosis severity. While no evidence of transitional flow was seen in the normal model, the 50%-stenosed model started to show characteristics of transitional flow, which became highly evident in the 70% model, with greatest manifestation during the systolic phase of the cardiac cycle. By means of a model comparison, we demonstrate two quantitative measures of the flow complexity through the power-law decay slope of the energy spectrum and the global entropy. The more complex flow in the 70%-stenosed model showed a flatter slope of energy decay (-0.91 compared to -1.34 for 50% stenosis) and higher entropy values (0.26 compared to 0.17). Finally, the minimum temporal resolution required for POD analysis of carotid artery flow was found to be 100 Hz when determined through a more typical energy-mode convergence test, as compared to 400 Hz based on global entropy values.

. An investigation of the temperature response and growth of thermal lesions resulting from in vivo, interstitial laser photocoagulation at long exposures was conducted to assess extended lesion growth characteristics and test the applicability of first order unimolecular rate kinetics (Arrhenius theory) to thermal lesion growth. Irradiations were performed in vivo in rabbit muscle using a continuous 805 nm diode laser