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P R Hoskins
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ABSTRACT: 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.
Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 01/2010; 224(2):255-71. · 1.21 Impact Factor
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ABSTRACT: One hundred and one pregnant women with hypertensive disorders were prospectively studied with doppler ultrasound flow velocity waveforms of the umbilical artery. Results were withheld from the obstetricians so that the test did not influence obstetric management. It was found that the resistance index of the umbilical artery was higher in patients with proteinuric pregnancy induced hypertension and in patients with chronic hypertension than in patients with non-proteinuric pregnancy induced hypertension. Patients with an abnormal resistance index were more likely to be delivered prematurely, have small for dates babies, have fulminating hypertension and have an abnormal antepartum cardiotocogram. Thirteen patients had a pathological antenatal cardiotocogram and they all had an abnormal umbilical artery flow velocity wave form when the cardiotocogram was normal.
07/2009; 9(1):9-13.
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ABSTRACT: Abdominal aortic aneurysm disease progression is probably influenced by tissue stresses and blood flow conditions and so accurate estimation of these will increase understanding of the disease and may lead to improved clinical practice. In this work the blood flow and tissue stresses in axially symmetric aneurysms are calculated using a complete fluid-structure interaction as a benchmark for calculating the error introduced by simpler calculations: rigid walled for the blood flow, homogeneous pressure for the tissue stress, as well as one-way-coupled interactions. The error in the peak von Mises stress in a homogeneous pressure calculation compared with a fluid-structure interaction calculation was less than 3.5 per cent for aneurysm diameters up to 7 cm. The error in the mean wall shear stress, in a rigid-walled calculation compared with a fluid-structure interaction calculation, varied from 30 per cent to 60 per cent with increasing aneurysm diameter. These results suggest that incorporation of the fluid-structure interaction is unnecessary for purely mechanical modelling, with the aim of evaluating the current rupture probability. However, for more complex biological modelling, perhaps with the aim of predicting the progress of the disease, where accurate estimation of the wall shear stress is essential, some form of fluid-structure interaction is necessary.
Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 03/2009; 223(2):195-209. · 1.21 Impact Factor
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ABSTRACT: The paper reviews techniques for the estimation of wall stresses in arterial disease. Wall stresses are important as arterial disease progresses through a complex interplay between local biology and local mechanical stresses. The possibility then arises of using wall stresses as new diagnostic indicators in patients with arterial disease. Estimation of wall stresses using imaging systems is problematic. Developments in the last 10 years have been aimed at providing tools for estimation of wall stresses within individual patients, using a combination of three-dimensional (3D) imaging and computational modelling. For blood flow, 3D arterial lumen information is obtained from 3D imaging. Computational fluid dynamics is then used to estimate the 3D velocity field within the lumen, from which wall shear stress may be calculated. For arterial mechanics, the 3D arterial wall geometry is integrated with solid modelling to provide estimates of the strain field and stress field within the artery wall. For intraplaque stresses, this has been achieved through the use of detailed two-dimensional (2D) intraplaque geometry from MRI. Inverse techniques have been used to provide images of Young's modulus in atherosclerotic plaque using intravascular ultrasound and solid modelling. Several research centres now have processing chains to allow this technology to be used in clinical studies. In time, possibly over the next 10 years or so, robust protocols with proven clinical utility will arise which, when combined with high-performance computing, will allow image-guided modelling to be used as an adjunct to modern radiology in the same way that image-processing tools are used today.
The British journal of radiology 01/2009; 82 Spec No 1:S3-17. · 2.11 Impact Factor
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ABSTRACT: It is widely accepted that subjects with vascular disease have increased arterial stiffness and intima-media thickness (IMT) when compared with healthy controls. The aim of this study was to investigate indices of arterial stiffness and IMT in the common carotid arteries (CCAs) of subjects with and without peripheral arterial disease (PAD), in order to look for evidence of change in wall quality and quantity to explain increased stiffness that has been found in the arteries of subjects with vascular disease.
The arterial distension waveform (ADW), IMT, diameter and brachial blood pressure were measured to calculate Young's Modulus (E) and elastic modulus (Ep) in the common carotid arteries of subjects with and without PAD. 38 subjects with confirmed PAD were compared with 43 normal controls matched for age, sex, smoking and hypertension. The mean diameter (8.35mm [95% CI 7.93-8.77] vs. 6.93mm [6.65-7.20] P<0.001, increase 20%), IMT (0.99mm [0.92-1.07] vs. 0.88mm [0.82-0.93] P=0.020, increase 12.5%), Ep (315kPa [185-444] vs. 190kPa [164-216] P=0.034, increase 66%) and E (1383kPa [836-1930] vs. 744kPa [641-846] P=0.006, increase 86%) were all significantly higher in subjects with PAD.
This study suggests that increased stiffness observed in subjects with peripheral vascular disease is a result of change in both quantity and quality of the arterial wall. Changes in indices of arterial stiffness were much higher than changes in IMT and diameter. These preliminary observations may be an indication that indices of arterial stiffness are a sensitive early marker of atherosclerosis.
Atherosclerosis 12/2008; 205(2):477-80. · 3.79 Impact Factor
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D M Watts,
C J Sutcliffe,
R H Morgan,
S Meagher,
J Wardlaw,
M Connell,
M E Bastin,
I Marshall,
K V Ramnarine, P R Hoskins,
R A Black
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ABSTRACT: Doppler ultrasound is widely used in the diagnosis and monitoring of arterial disease. Current clinical measurement systems make use of continuous and pulsed ultrasound to measure blood flow velocity; however, the uncertainty associated with these measurements is great, which has serious implications for the screening of patients for treatment. Because local blood flow dynamics depend to a great extent on the geometry of the affected vessels, there is a need to develop anatomically accurate arterial flow phantoms with which to assess the accuracy of Doppler blood flow measurements made in diseased vessels. In this paper, we describe the computer-aided design and manufacturing (CAD-CAM) techniques that we used to fabricate anatomical flow phantoms based on images acquired by time-of-flight magnetic resonance imaging (TOF-MRI). Three-dimensional CAD models of the carotid bifurcation were generated from data acquired from sequential MRI slice scans, from which solid master patterns were made by means of stereolithography. Thereafter, an investment casting procedure was used to fabricate identical flow phantoms for use in parallel experiments involving both laser and Doppler ultrasound measurement techniques.
Ultrasound in Medicine & Biology 03/2007; 33(2):296-302. · 2.29 Impact Factor
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ABSTRACT: 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 . 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.
Ultrasound in Medicine & Biology 02/2007; 33(2):303-10. · 2.29 Impact Factor
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ABSTRACT: Hemodynamics factors and biomechanical forces play key roles in atherogenesis, plaque development and final rupture. In this paper, we investigated the flow field and stress field for different degrees of stenoses under physiological conditions. Disease is modelled as axisymmetric cosine shape stenoses with varying diameter reductions of 30%, 50% and 70%, respectively. A simulation model which incorporates fluid-structure interaction, a turbulence model and realistic boundary conditions has been developed. The results show that wall motion is constrained at the throat by 60% for the 30% stenosis and 85% for the 50% stenosis; while for the 70% stenosis, wall motion at the throat is negligible through the whole cycle. Peak velocity at the throat varies from 1.47 m/s in the 30% stenosis to 3.2m/s in the 70% stenosis against a value of 0.78 m/s in healthy arteries. Peak wall shear stress values greater than 100 Pa were found for > or =50% stenoses, which in vivo could lead to endothelial stripping. Maximum circumferential stress was found at the shoulders of plaques. The results from this investigation suggest that severe stenoses inhibit wall motion, resulting in higher blood velocities and higher peak wall shear stress, and localization of hoop stress. These factors may contribute to further development and rupture of plaques.
Journal of Biomechanics 01/2007; 40(16):3715-24. · 2.43 Impact Factor
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ABSTRACT: Arterial wall motion is an essential feature of a healthy cardiovascular system and it is known that wall motion is affected by age and disease. In recent years, methods have been developed for measurement of wall motion with the intention of providing diagnostically useful information. An issue with all of these techniques is the accuracy and variability of both wall motion and derived quantities such as elasticity, which requires the development of suitable test tools. In this paper, a vessel wall phantom is described for use in ultrasound studies of wall motion. The vessel was made from polyvinyl alcohol (PVA) subjected to a freeze-thaw process to form a cryogel (PVA-C). The elastic modulus, acoustic velocity and attenuation coefficient varied from 57 kPa, 1543 m s(-1) and 0.18 dB cm(-1) MHz(-1) for one freeze-thaw cycle to 330 kPa, 1583 m s(-1) and 0.42 dB cm(-1) MHz(-1) for 10 freeze-thaw cycles. Wall motion was effected by the use of pulsatile flow produced from a gear pump. The use of a downstream flow resistor removed gross distortions in the wall motion waveform, possibly by removal of reflected pressure waves. However, a low amplitude 20 Hz oscillation remained, which is unphysiologic and thought to be caused by the vibration of the distended PVA-C vessel.
Ultrasound in Medicine & Biology 10/2006; 32(9):1349-57. · 2.29 Impact Factor
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Ultrasonics Symposium, 2005 IEEE; 10/2005
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ABSTRACT: Cross sectional B-mode images were acquired from dead mice at projections through 360°. The image set was rotated about the isocentre and a set of aligned images produced. These were averaged to form a compound image. The compound images show a dramatic improvement in image quality over individual B-mode images; with much reduced speckle (2% compared with 18% in the B-mode image), and reduction in artefacts. Organs such as kidney and stomach are visible, along with skeletal anatomy. This demonstrates that the mouse skeleton does not have the same deleterious effect on the ultrasound beam as that of the human skeleton. This is probably due to the closer matching to soft tissue of acoustic properties of the cartilaginous skeleton of mice, as opposed to the high mismatch of the heavily calcified human skeleton. B-mode 360° compound imaging may he of interest as the basis for a new 3D technique providing high quality ultrasound images which are much less dependent on operator skill than is the case for real time B-mode ultrasound. A further development worth pursuing is tomographic reconstruction from projections.
Ultrasonics Symposium, 2004 IEEE; 09/2004
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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 image. For 3D segmentation the contour of the first slice is found, and this is used as the initial position for the next slice. As the initial position of the contour may be outside the artery, the pressure term is decided by comparing the feature of texture inside and outside of the contour, allowing the contour to expand or shrink. The model has been tested on 55 images from carotid arteries. The 'gold standard' boundary drawn by a radiologist and the segmented boundary showed an average difference of 0.40±0.30mm. 3D data was obtained using an anatomically correct carotid bifurcation flow phantom and gridded ready for CFD.
Biomedical Imaging: Nano to Macro, 2004. IEEE International Symposium on; 05/2004
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ABSTRACT: 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<sup>-1</sup>) the error in wall shear measurement is 62 % and at a lower scale setting (4.4 cms<sup>-1</sup>) 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.
Biomedical Imaging: Nano to Macro, 2004. IEEE International Symposium on; 05/2004
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ABSTRACT: Dual-beam vector Doppler has the potential to improve peak systolic blood velocity measurement accuracy by automatically correcting for the beam-flow Doppler angle. Using a modified linear-array system with a split receive aperture, we have assessed the angle-dependence over Doppler angles of 40 degrees -70 degrees and the reproducibility of the dual-beam blood maximum velocity estimate measured in the common carotid arteries (CCA) 1 to 2 cm prior to the bifurcation of 9 presumed-healthy volunteers. The velocity magnitude estimate was reduced by approximately 7.9% as the angle between the transmit beam and the vessel axis was increased from 40 degrees to 70 degrees. With repeat measurements made, on average, approximately 6 weeks apart, the 95% velocity magnitude limits of agreement were as follows: Intraobserver -41.3 to +45.2 cm/s; interobserver -29.6 to +46.8 cm/s. There was an 8.6 cm/s interobserver bias in velocity magnitude. We conclude that the dual-beam vector Doppler system can measure blood velocity within its scan plane with low dependence on angle and with similar reproducibility to that of single-beam systems.
Ultrasound in Medicine & Biology 03/2004; 30(2):271-6. · 2.29 Impact Factor
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ABSTRACT: A 3D Ultrasound (US) phantom scanning device has been developed to provide calibration data for image processing and computational fluid dynamics (CFD) as part of a study into the biomechanical status of diseased arteries in vivo. The cartesian scanning system moves a conventional US probe from a Philips HDI5000 US scanner over the upper surface of the phantom. The device can scan phantoms of up to 150 mm long and 75 mm wide. The transducer can be rotated about two axes and translated in one axis manually. X and Y planar movements are made automatically. Control is provided by a labVIEW based control program and a Parker L25i stepper motor driver system. scalar and rotary positional accuracies are ±50μm and ±0.1° respectively.
Engineering in Medicine and Biology Society, 2003. Proceedings of the 25th Annual International Conference of the IEEE; 10/2003
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ABSTRACT: Ultrasound (US) test phantoms incorporating tissue-mimicking materials (TMMs) play an important role in the quality control (QC) and performance testing of US equipment. Three commercially available TMMs (Zerdine from CIRS Inc.; condensed-milk-based gel from Gammex RMI; urethane-rubber-based from ATS Labs) and a noncommercial agar-based TMM, were investigated. Acoustic properties were measured over the frequency range 2.25 to 15 MHz at a range of ambient temperatures (10 to 35 degrees C). The acoustic velocity of the TMMs remained relatively constant with increasing frequency. Only the agar-based TMM had a linear increase of attenuation with frequency, with the other materials exhibiting nonlinear responses to varying degrees (f(1.08) to f(1.83)). The acoustic velocity and attenuation coefficient of all the TMMs varied with temperature, with the urethane-rubber TMM showing the greatest variation of +/- 1.2% for acoustic velocity and +/- 12% for attenuation coefficient. The data obtained in this study highlight the importance of greater knowledge of the acoustic behavior of TMMs to variations in both frequency and temperature, to ensure that accurate and precise measurements are obtained during QC and performance testing.
Ultrasound in Medicine & Biology 08/2003; 29(7):1053-60. · 2.29 Impact Factor
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ABSTRACT: In order to reduce the fluctuations in the velocity magnitude estimate, we propose a modification to the standard algorithm for reconstructing the (two component) vector velocity from the measured Doppler shifts in two directions. This uses the standard dual-beam algorithm, combined with temporal smoothing, to find only the velocity angle, then uses the single-beam algorithm to estimate the velocity magnitude. We present initial data showing the significant reduction in velocity estimate fluctuation that this hybrid method achieves compared to the standard algorithm.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 02/2003; · 1.69 Impact Factor
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ABSTRACT: Tissue Doppler Imaging (TDI) can assess tissue motion in vascular and cardiac imaging. However, a major drawback of these measurements is that the motion estimation is limited to the component along the ultrasound beam axis. Cardiac and vessel wall motion studies have shown that complex three-dimensional motions can be observed, and that there is a clinical need to fully assess the three components of the vector motion. This work describes how TDI can be extended by acquiring a real time two-component velocity vector via a dual beam vector Doppler technique. A vector Doppler velocity estimator using a small interbeam angle can suffer from both bias and large variance. This estimator is also strongly dependent on the settings of the echographic system. To reduce the large bias and variance, most vector velocity techniques use a very large ensemble length (EL) (>20), which does not allow real time implementation. We propose a new processing technique, which reduces the bias and the standard deviation of the vector velocity estimate. The new method assumes that the vector velocity angle varies slowly over the cardiac cycle. The angle can then be estimated using a large time window. The performance of this new technique has been tested experimentally using a tissue mimicking rotating phantom. It is shown that the factors influencing the results are the EL, the precision of the TDI estimates and the time window. The results indicate that the variance and bias of velocity magnitude and orientation estimates decrease with increasing EL, increasing precision of the TDI estimates and increasing time window. Using an EL of 9, 8 bits for the velocity estimate, and an observation time of one second, a 5-degree bias of the angle estimate is observed, with a variance below 7 degree averaged over all angles. A 10% bias of the velocity magnitude is observed, with a variance of 1%. In conclusion, TDI can be improved with vector Doppler providing two-dimensional tissue motion estimation, enabling more accurate biomechanical tissue property assessment.
Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE; 11/2002
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P R Hoskins
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ABSTRACT: This article will review the ability of ultrasound techniques to provide 3D information on arterial geometry, blood flow and tissue motion.3D systems. 3D datasets can be obtained by sequential acquisition of 2D slices. Ideally a transducer is required in which there is full control of beam steering within a 3D volume. This requires a 2D array consisting of several thousand elements. Prototype 2D arrays have been built which provide several 3D datasets per second. Blood velocity measurement. Current Doppler systems estimate only the component of velocity in the direction of the Doppler beam. Lack of knowledge of the direction of blood motion and also other effects associated with 'spectral broadening' limit the accuracy of velocity measurement. Improved accuracy can be obtained using vector Doppler systems using 2 or 3 beam directions; this approach is referred to as 'vector Doppler'. Tissue motion. Doppler techniques can also be used to detect tissue motion (Tissue Doppler Imaging or TDI). Motion of the artery wall can be calculated from the TDI images. It is possible to estimate simultaneously motion for adjacent diameters within the longitudinal plane, and to visualise the relative motion at different parts of the wall.
Biorheology 02/2002; 39(3-4):451-9. · 1.93 Impact Factor
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ABSTRACT: Steady fluid flow was studied in a simple bifurcation model and in a physiologically realistic model of the human carotid bifurcation. Wall shear stress (WSS) vectors were calculated from phase-contrast (PC) magnetic resonance imaging (MRI) measurements of the velocity field. Velocity measurements in the inflow regions were also used as boundary conditions for computational fluid dynamics (CFD) calculations of WSS, which were compared with those derived from MRI alone. In regions of well-behaved flow, MRI and CFD estimates of WSS were in good general agreement. In regions of disturbed flow, for example near the bifurcation, the quality of the MRI measurements was sufficient for reliable calculation of WSS vectors when a sensitive surface coil was used. The combination of MRI and CFD would seem to be a powerful technique for the investigation of flow phenomena.
Journal of Magnetic Resonance Imaging 12/2001; 14(5):563-73. · 2.70 Impact Factor
