The Correlation Between Wall Shear Stress and Plaque Composition in Advanced Human Carotid Atherosclerosis.

The role of wall shear stress (WSS) in atherosclerotic plaque development is evident, but the relation between WSS and plaque composition in advanced atherosclerosis, potentially resulting in plaque destabilization, is a topic of discussion. Using our previously developed image registration pipeline, we investigated the relation between two WSS metrics, time-averaged WSS (TAWSS) and the oscillatory shear index (OSI), and the local histologically determined plaque composition in a set of advanced human carotid plaques. Our dataset of 11 carotid endarterectomy samples yielded 87 histological cross-sections, which yielded 511 radial bins for analysis. Both TAWSS and OSI values were subdivided into patient-specific low, mid, and high tertiles. This cross-sectional study shows that necrotic core (NC) size and macrophage area are significantly larger in areas exposed to high TAWSS or low OSI. Local TAWSS and OSI tertile values were generally inversely related, as described in the literature, but other combinations were also found. Investigating the relation between plaque vulnerability features and different combinations of TAWSS and OSI tertile values revealed a significantly larger cap thickness in areas exposed to both low TAWSS and low OSI. In conclusion, our study confirmed previous findings, correlating high TAWSS to larger macrophage areas and necrotic core sizes. In addition, our study demonstrated new relations, correlating low OSI to larger macrophage areas, and a combination of low TAWSS and low OSI to larger cap thickness.

Plaque Composition as a Predictor of Plaque Ulceration in Carotid Artery Atherosclerosis: The Plaque At RISK Study.

BACKGROUND AND PURPOSE: Plaque ulceration is a marker of previous plaque rupture. We studied the association between atherosclerotic plaque composition at baseline and plaque ulceration at baseline and follow-up. MATERIALS AND METHODS: We included symptomatic patients with a carotid stenosis of <70% who underwent MDCTA and MR imaging at baseline (n = 180). MDCTA was repeated at 2 years (n = 73). We assessed the presence of ulceration using MDCTA. Baseline MR imaging was used to assess the vessel wall volume and the presence and volume of plaque components (intraplaque hemorrhage, lipid-rich necrotic core, and calcifications) and the fibrous cap status. Associations at baseline were evaluated with binary logistic regression and reported with an OR and its 95% CI. Simple statistical testing was performed in the follow-up analysis. RESULTS: At baseline, the prevalence of plaque ulceration was 27% (49/180). Increased wall volume (OR = 12.1; 95% CI, 3.5-42.0), higher relative lipid-rich necrotic core (OR = 1.7; 95% CI, 1.3-2.2), higher relative intraplaque hemorrhage volume (OR = 1.7; 95% CI, 1.3-2.2), and a thin-or-ruptured fibrous cap (OR = 3.4; 95% CI, 1.7-6.7) were associated with the presence of ulcerations at baseline. In 8% (6/73) of the patients, a new ulcer developed. Plaques with a new ulceration at follow-up had at baseline a larger wall volume (1.04 cm3 [IQR, 0.97-1.16 cm3] versus 0.86 cm3 [IQR, 0.73-1.00 cm3]; P = .029), a larger relative lipid-rich necrotic core volume (23% [IQR, 13-31%] versus 2% [IQR, 0-14%]; P = .002), and a larger relative intraplaque hemorrhage volume (14% [IQR, 8-24%] versus 0% [IQR, 0-5%]; P < .001). CONCLUSIONS: Large atherosclerotic plaques and plaques with intraplaque hemorrhage and lipid-rich necrotic cores were associated with plaque ulcerations at baseline and follow-up.

Simultaneous Morphological and Flow Imaging Enabled by Megahertz Intravascular Doppler Optical Coherence Tomography.

We demonstrate three-dimensional intravascular flow imaging compatible with routine clinical image acquisition workflow by means of megahertz (MHz) intravascular Doppler Optical Coherence Tomography (OCT). The OCT system relies on a 1.1 mm diameter motorized imaging catheter and a 1.5 MHz Fourier Domain Mode Locked (FDML) laser. Using a post processing method to compensate the drift of the FDML laser output, we can resolve the Doppler phase shift between two adjoining OCT A-line datasets. By interpretation of the velocity field as measured around the zero phase shift, the flow direction at specific angles can be qualitatively estimated. Imaging experiments were carried out in phantoms, micro channels, and swine coronary artery in vitro at a speed of 600 frames/s. The MHz wavelength sweep rate of the OCT system allows us to directly investigate flow velocity of up to 37.5 cm/s while computationally expensive phase-unwrapping has to be applied to measure such high speed using conventional OCT system. The MHz sweep rate also enables a volumetric Doppler imaging even with a fast pullback at 40 mm/s. We present the first simultaneously recorded 3D morphological images and Doppler flow profiles. Flow pattern estimation and three-dimensional structural reconstruction of entire coronary artery are achieved using a single OCT pullback dataset.

An MRI-based method to register patient-specific wall shear stress data to histology.

Wall shear stress (WSS), the frictional force exerted on endothelial cells by blood flow, is hypothesised to influence atherosclerotic plaque growth and composition. We developed a methodology for image registration of MR and histology images of advanced human carotid plaques and corresponding WSS data, obtained by MRI and computational fluid dynamics. The image registration method requires four types of input images, in vivo MRI, ex vivo MRI, photographs of transversally sectioned plaque tissue and histology images. These images are transformed to a shared 3D image domain by applying a combination of rigid and non-rigid registration algorithms. Transformation matrices obtained from registration of these images are used to transform subject-specific WSS data to the shared 3D image domain as well. WSS values originating from the 3D WSS map are visualised in 2D on the corresponding lumen locations in the histological sections and divided into eight radial segments. In each radial segment, the correlation between WSS values and plaque composition based on histological parameters can be assessed. The registration method was successfully applied to two carotid endarterectomy specimens. The resulting matched contours from the imaging modalities had Hausdorff distances between 0.57 and 0.70 mm, which is in the order of magnitude of the in vivo MRI resolution. We simulated the effect of a mismatch in the rigid registration of imaging modalities on WSS results by relocating the WSS data with respect to the stack of histology images. A 0.6 mm relocation altered the mean WSS values projected on radial bins on average by 0.59 Pa, compared to the output of original registration. This mismatch of one image slice did not change the correlation between WSS and plaque thickness. In conclusion, we created a method to investigate correlations between WSS and plaque composition.

The effect of the heart rate lowering drug Ivabradine on hemodynamics in atherosclerotic mice.

The heart rate lowering drug Ivabradine was shown to improve cardiac outcome in patients with previous heart failure. However, in patients without heart failure, no beneficial effect of Ivabradine was observed. Animal studies suggested a preventive effect of Ivabradine on atherosclerosis which was due to an increase in wall shear stress (WSS), the blood flow-induced frictional force exerted on the endothelium, triggering anti-inflammatory responses. However, data on the effect of Ivabradine on WSS is sparse. We aim to study the effect of Ivabradine on (i) the 3D WSS distribution over a growing plaque and (ii) plaque composition. We induced atherosclerosis in ApoE-/- mice by placing a tapered cast around the right common carotid artery (RCCA). Five weeks after cast placement, Ivabradine was administered via drinking water (15 mg/kg/day) for 2 weeks, after which the RCCA was excised for histology analyses. Before and after Ivabradine treatment, animals were imaged with Doppler Ultrasound to measure blood velocity. Vessel geometry was obtained using contrast-enhanced micro-CT. Time-averaged WSS during systole, diastole and peak WSS was subsequently computed. Ivabradine significantly decreased heart rate (459 ± 28 bpm vs. 567 ± 32 bpm, p < 0.001). Normalized peak flow significantly increased in the Ivabradine group (124.2% ± 40.5% vs. 87.3% ± 25.4%, p < 0.05), reflected by an increased normalized WSS level during systole (110.7% ± 18.4% vs. 75.4% ± 24.6%, p < 0.05). However, plaque size or composition including plaque area, relative necrotic core area and macrophage content were not altered in mice treated with Ivabradine compared to controls. We conclude that increased WSS in response to Ivabradine treatment did not affect plaque progression in a murine model.

Temporal and spatial changes in wall shear stress during atherosclerotic plaque progression in mice.

Wall shear stress (WSS) is involved in atherosclerotic plaque initiation, yet its role in plaque progression remains unclear. We aimed to study (i) the temporal and spatial changes in WSS over a growing plaque and (ii) the correlation between WSS and plaque composition, using animal-specific data in an atherosclerotic mouse model. Tapered casts were placed around the right common carotid arteries (RCCA) of ApoE-/- mice. At 5, 7 and 9 weeks after cast placement, RCCA geometry was reconstructed using contrast-enhanced micro-CT. Lumen narrowing was observed in all mice, indicating the progression of a lumen intruding plaque. Next, we determined the flow rate in the RCCA of each mouse using Doppler Ultrasound and computed WSS at all time points. Over time, as the plaque developed and further intruded into the lumen, absolute WSS significantly decreased. Finally at week 9, plaque composition was histologically characterized. The proximal part of the plaque was small and eccentric, exposed to relatively lower WSS. Close to the cast a larger and concentric plaque was present, exposed to relatively higher WSS. Lower WSS was significantly correlated to the accumulation of macrophages in the eccentric plaque. When pooling data of all animals, correlation between WSS and plaque composition was weak and no longer statistically significant. In conclusion, our data showed that in our mouse model absolute WSS strikingly decreased during disease progression, which was significantly correlated to plaque area and macrophage content. Besides, our study demonstrates the necessity to analyse individual animals and plaques when studying correlations between WSS and plaque composition.

Multiline 3D beamforming using micro-beamformed datasets for pediatric transesophageal echocardiography.

Until now, no matrix transducer has been realized for 3D transesophageal echocardiography (TEE) in pediatric patients. In 3D TEE with a matrix transducer, the biggest challenges are to connect a large number of elements to a standard ultrasound system, and to achieve a high volume rate (>200 Hz). To address these issues, we have recently developed a prototype miniaturized matrix transducer for pediatric patients with micro-beamforming and a small central transmitter. In this paper we propose two multiline parallel 3D beamforming techniques (µBF25 and µBF169) using the micro-beamformed datasets from 25 and 169 transmit events to achieve volume rates of 300 Hz and 44 Hz, respectively. Both the realizations use angle-weighted combination of the neighboring overlapping sub-volumes to avoid artifacts due to sharp intensity changes introduced by parallel beamforming. In simulation, the image quality in terms of the width of the point spread function (PSF), lateral shift invariance and mean clutter level for volumes produced by µBF25 and µBF169 are similar to the idealized beamforming using a conventional single-line acquisition with a fully-sampled matrix transducer (FS4k, 4225 transmit events). For completeness, we also investigated a 9 transmit-scheme (3 × 3) that allows even higher frame rates but found worse B-mode image quality with our probe. The simulations were experimentally verified by acquiring the µBF datasets from the prototype using a Verasonics V1 research ultrasound system. For both µBF169 and µBF25, the experimental PSFs were similar to the simulated PSFs, but in the experimental PSFs, the clutter level was ~10 dB higher. Results indicate that the proposed multiline 3D beamforming techniques with the prototype matrix transducer are promising candidates for real-time pediatric 3D TEE.

Real-time volumetric lipid imaging in vivo by intravascular photoacoustics at 20 frames per second.

Lipid deposition can be assessed with combined intravascular photoacoustic/ultrasound (IVPA/US) imaging. To date, the clinical translation of IVPA/US imaging has been stalled by a low imaging speed and catheter complexity. In this paper, we demonstrate imaging of lipid targets in swine coronary arteries in vivo, at a clinically useful frame rate of 20 s-1. We confirmed image contrast for atherosclerotic plaque in human samples ex vivo. The system is on a mobile platform and provides real-time data visualization during acquisition. We achieved an IVPA signal-to-noise ratio of 20 dB. These data show that clinical translation of IVPA is possible in principle.

High shear stress relates to intraplaque haemorrhage in asymptomatic carotid plaques.

BACKGROUND AND AIMS: Carotid artery plaques with vulnerable plaque components are related to a higher risk of cerebrovascular accidents. It is unknown which factors drive vulnerable plaque development. Shear stress, the frictional force of blood at the vessel wall, is known to influence plaque formation. We evaluated the association between shear stress and plaque components (intraplaque haemorrhage (IPH), lipid rich necrotic core (LRNC) and/or calcifications) in relatively small carotid artery plaques in asymptomatic persons. METHODS: Participants (n = 74) from the population-based Rotterdam Study, all with carotid atherosclerosis assessed on ultrasound, underwent carotid MRI. Multiple MRI sequences were used to evaluate the presence of IPH, LRNC and/or calcifications in plaques in the carotid arteries. Images were automatically segmented for lumen and outer wall to obtain a 3D reconstruction of the carotid bifurcation. These reconstructions were used to calculate minimum, mean and maximum shear stresses by applying computational fluid dynamics with subject-specific inflow conditions. Associations between shear stress measures and plaque composition were studied using generalized estimating equations analysis, adjusting for age, sex and carotid wall thickness. RESULTS: The study group consisted of 93 atherosclerotic carotid arteries of 74 participants. In plaques with higher maximum shear stresses, IPH was more often present (OR per unit increase in maximum shear stress (log transformed) = 12.14; p = 0.001). Higher maximum shear stress was also significantly associated with the presence of calcifications (OR = 4.28; p = 0.015). CONCLUSIONS: Higher maximum shear stress is associated with intraplaque haemorrhage and calcifications.

Intraplaque Hemorrhage and the Plaque Surface in Carotid Atherosclerosis: The Plaque At RISK Study (PARISK).

BACKGROUND AND PURPOSE: An important characteristic of vulnerable plaque, intraplaque hemorrhage, may predict plaque rupture. Plaque rupture can be visible on noninvasive imaging as a disruption of the plaque surface. We investigated the association between intraplaque hemorrhage and disruption of the plaque surface. MATERIALS AND METHODS: We selected the first 100 patients of the Plaque At RISK study, an ongoing prospective noninvasive plaque imaging study in patients with mild-to-moderate atherosclerotic lesions in the carotid artery. In carotid artery plaques, disruption of the plaque surface (defined as ulcerated plaques and/or fissured fibrous cap) and intraplaque hemorrhage were assessed by using MDCTA and 3T MR imaging, respectively. We used a χ(2) test and multivariable logistic regression to assess the association between intraplaque hemorrhage and disrupted plaque surface. RESULTS: One hundred forty-nine carotid arteries in 78 patients could be used for the current analyses. Intraplaque hemorrhage and plaque ulcerations were more prevalent in symptomatic compared with contralateral vessels (hemorrhage, 38% versus 11%; P < .001; and ulcerations, 27% versus 7%; P = .001). Fissured fibrous cap was more prevalent in symptomatic compared with contralateral vessels (13% versus 4%; P = .06). After adjustment for age, sex, diabetes mellitus, and degree of stenosis, intraplaque hemorrhage was associated with disrupted plaque surface (OR, 3.13; 95% CI, 1.25-7.84) in all vessels. CONCLUSIONS: Intraplaque hemorrhage is associated with disruption of the plaque surface in patients with a carotid artery stenosis of <70%. Serial studies are needed to investigate whether intraplaque hemorrhage indeed increases the risk of plaque rupture and subsequent ischemic stroke during follow-up.

Carotid plaque elasticity estimation using ultrasound elastography, MRI, and inverse FEA - A numerical feasibility study.

The material properties of atherosclerotic plaques govern the biomechanical environment, which is associated with rupture-risk. We investigated the feasibility of noninvasively estimating carotid plaque component material properties through simulating ultrasound (US) elastography and in vivo magnetic resonance imaging (MRI), and solving the inverse problem with finite element analysis. 2D plaque models were derived from endarterectomy specimens of nine patients. Nonlinear neo-Hookean models (tissue elasticity C1) were assigned to fibrous intima, wall (i.e., media/adventitia), and lipid-rich necrotic core. Finite element analysis was used to simulate clinical cross-sectional US strain imaging. Computer-simulated, single-slice in vivo MR images were segmented by two MR readers. We investigated multiple scenarios for plaque model elasticity, and consistently found clear separations between estimated tissue elasticity values. The intima C1 (160 kPa scenario) was estimated as 125.8 ± 19.4 kPa (reader 1) and 128.9 ± 24.8 kPa (reader 2). The lipid-rich necrotic core C1 (5 kPa) was estimated as 5.6 ± 2.0 kPa (reader 1) and 8.5 ± 4.5 kPa (reader 2). A scenario with a stiffer wall yielded similar results, while realistic US strain noise and rotating the models had little influence, thus demonstrating robustness of the procedure. The promising findings of this computer-simulation study stimulate applying the proposed methodology in a clinical setting.

Live Observation of Atherosclerotic Plaque Disruption in Apolipoprotein E-Deficient Mouse.

AIM: The actual occurrence of spontaneous plaque rupture in mice has been a matter of debate. We report on an in vivo observation of the actual event of possible plaque disruption in a living ApoE(-/-) mouse. METHODS AND RESULTS: During live contrast-enhanced ultrasonography of a 50-week-old ApoE(-/-) male mouse, symptoms suggesting plaque disruption in the brachiocephalic artery were observed. Histological analysis confirmed the presence of advanced atherosclerotic lesions with dissections and intraplaque hemorrhage in the affected brachiocephalic trunk, pointing towards plaque rupture as the cause of the observed event. However, we did not detect a luminal thrombus or cap rupture, which is a key criterion for plaque rupture in human atherosclerosis. CONCLUSION: This study reports the real-time occurrence of a possible plaque rupture in a living ApoE(-/-) mouse.

Intravital microscopy of localized stem cell delivery using microbubbles and acoustic radiation force.

The use of stem cells for the repair of damaged cardiac tissue after a myocardial infarction holds great promise. However, a common finding in experimental studies is the low number of cells delivered at the area at risk. To improve the delivery, we are currently investigating a novel delivery platform in which stem cells are conjugated with targeted microbubbles, creating echogenic complexes dubbed StemBells. These StemBells vibrate in response to incoming ultrasound waves making them susceptible to acoustic radiation force. The acoustic force can then be employed to propel circulating StemBells from the centerline of the vessel to the wall, facilitating localized stem cell delivery. In this study, we investigate the feasibility of manipulating StemBells acoustically in vivo after injection using a chicken embryo model. Bare stem cells or unsaturated stem cells (<5 bubbles/cell) do not respond to ultrasound application (1 MHz, peak negative acoustical pressure P_ = 200 kPa, 10% duty cycle). However, stem cells which are fully saturated with targeted microbubbles (>30 bubbles/cell) can be propelled toward and arrested at the vessel wall. The mean translational velocities measured are 61 and 177 µm/s for P- = 200 and 450 kPa, respectively. This technique therefore offers potential for enhanced and well-controlled stem cell delivery for improved cardiac repair after a myocardial infarction.

Quantitative contrast-enhanced ultrasound of intraplaque neovascularization in patients with carotid atherosclerosis.

PURPOSE: Intraplaque neovascularization (IPN) is an increasingly studied marker of the vulnerable atherosclerotic plaque, and contrast-enhanced ultrasound (CEUS) is an in vivo imaging technique for the assessment of IPN. The purpose of this study was to test novel quantification methods for the detection of carotid IPN using CEUS. MATERIALS AND METHODS: 25 patients with established carotid atherosclerosis underwent bilateral carotid CEUS using a Philips iU-22 ultrasound system with an L9 - 3 transducer. Visual scoring of IPN was performed using a 3-point score. Quantification of IPN was performed using novel custom developed software. In short, regions of interest were drawn over the atherosclerotic plaques. After motion compensation, several IPN features were calculated. Statistical analysis was performed using Spearman's rho. Reproducibility of the quantification features was calculated using intra-class correlation coefficients and mean differences between calculations. RESULTS: 45 carotid arteries were available for the quantification of IPN. The quantification of IPN was feasible in all 45 carotid plaques. The IPN area, IPN area ratio and neovessel count had a good correlation with the visual IPN score (respectively ρ = 0.719, ρ = 0.538, ρ = 0.474 all p < 0.01). The intra-observer and inter-observer agreement was good to excellent (p < 0.01). The intra-observer and inter-observer variability was low. CONCLUSION: The quantification of carotid IPN on CEUS is feasible and provides multiple features on carotid IPN. Accurate quantitative assessment of IPN may be important to recognize and to monitor changes during therapy in vulnerable atherosclerotic plaques.

Geometry-based pressure drop prediction in mildly diseased human coronary arteries.

Pressure drop (△p) estimations in human coronary arteries have several important applications, including determination of appropriate boundary conditions for CFD and estimation of fractional flow reserve (FFR). In this study a △p prediction was made based on geometrical features derived from patient-specific imaging data. Twenty-two mildly diseased human coronary arteries were imaged with computed tomography and intravascular ultrasound. Each artery was modelled in three consecutive steps: from straight to tapered, to stenosed, to curved model. CFD was performed to compute the additional △p in each model under steady flow for a wide range of Reynolds numbers. The correlations between the added geometrical complexity and additional △p were used to compute a predicted △p. This predicted △p based on geometry was compared to CFD results. The mean △p calculated with CFD was 855±666Pa. Tapering and curvature added significantly to the total △p, accounting for 31.4±19.0% and 18.0±10.9% respectively at Re=250. Using tapering angle, maximum area stenosis and angularity of the centerline, we were able to generate a good estimate for the predicted △p with a low mean but high standard deviation: average error of 41.1±287.8Pa at Re=250. Furthermore, the predicted △p was used to accurately estimate FFR (r=0.93). The effect of the geometric features was determined and the pressure drop in mildly diseased human coronary arteries was predicted quickly based solely on geometry. This pressure drop estimation could serve as a boundary condition in CFD to model the impact of distal epicardial vessels.

Plaque At RISK (PARISK): prospective multicenter study to improve diagnosis of high-risk carotid plaques.

BACKGROUND: Patients with symptomatic carotid artery stenosis are at high risk for recurrent stroke. To date, the decision to perform carotid endarterectomy in patients with a recent cerebrovascular event is mainly based on degree of stenosis of the ipsilateral carotid artery. However, additional atherosclerotic plaque characteristics might be better predictors of stroke, allowing for more precise selection of patients for carotid endarterectomy. AIMS AND HYPOTHESIS: We investigate the hypothesis that the assessment of carotid plaque characteristics with magnetic resonance imaging, multidetector-row computed tomography angiography, ultrasonography, and transcranial Doppler, either alone or in combination, may improve identification of a subgroup of patients with < 70% carotid artery stenosis with an increased risk of recurrent stroke. METHODS: The Plaque At RISK (PARISK) study is a prospective multicenter cohort study of patients with recent (<3 months) neurological symptoms due to ischemia in the territory of the carotid artery and < 70% ipsilateral carotid artery stenosis who are not scheduled for carotid endarterectomy or stenting. At baseline, 300 patients will undergo magnetic resonance imaging, multidetector-row computed tomography angiography, and ultrasonography examination of the carotid arteries. In addition, magnetic resonance imaging of the brain, ambulatory transcranial Doppler recording of the middle cerebral artery and blood withdrawal will be performed. After two-years, imaging will be repeated in 150 patients. All patients undergo a follow-up brain magnetic resonance imaging, and there will be regular clinical follow-up until the end of the study. STUDY OUTCOMES: The combined primary end-point contains ipsilateral recurrent ischemic stroke or transient ischemic attack or new ipsilateral ischemic brain lesions on follow-up brain magnetic resonance imaging.

Single pulse frequency compounding protocol for superharmonic imaging.

Second harmonic imaging is currently accepted as the standard in commercial echographic systems. A new imaging technique, coined as superharmonic imaging (SHI), combines the third till the fifth harmonics, arising during nonlinear sound propagation. It could further enhance the resolution and quality of echographic images. To meet the bandwidth requirement for SHI a dedicated phased array has been developed: a low frequency subarray, intended for transmission, interleaved with a high frequency subarray, used in reception. As the bandwidth of the elements is limited, the spectral gaps in between the harmonics cause multiple (ghost) reflection artifacts. A dual-pulse frequency compounding method aims at suppressing those artifacts at a price of a reduced frame rate. In this study we explore a possibility of performing frequency compounding within a single transmission. The traditional frequency compounding method suppresses the ripples by consecutively emitting two short Gaussian bursts with a slightly different center frequency. In the newly proposed method, the transmit aperture is divided into two parts: the first half is used to send a pulse at the lower center frequency, while the other half simultaneously transmits at a slightly higher center frequency. The suitability of the protocol for medical imaging applications in terms of the steering capabilities was performed in a simulation study with INCS and the hydrophone measurements. Moreover, an experimental study was carried out to find the optimal parameters for the clinical imaging protocol. The latter was subsequently used to obtain the images of a tissue mimicking phantom containing strongly reflecting wires. Additionally, the images of a human heart in the parasternal projection were acquired. The scanning aperture with the developed protocol amounts to approximately 90°, which is sufficient to capture the cardiac structures in the standard anatomical projections. The theoretically estimated and experimentally measured grating lobe levels are equal to -28.3 dB and -35.9 dB, respectively. A considerable improvement in the axial resolution of the SHI component (0.73 mm) at -6 dB in comparison with the third harmonic (2.23 mm) was observed. A similar comparison in terms of the lateral resolution slightly favored the superharmonic component by 0.2 mm. Additionally, the images of the tissue mimicking phantom exhibited the absence of the multiple reflection artifacts. The in-vivo acquisition allows one to clearly observe the dynamic of the mitral valve leaflets. The new method is equally effective in eliminating the ripple artifacts associated with SHI as the dual-pulse technique, while the full frame rate is maintained.

The influence of axial image resolution on atherosclerotic plaque stress computations.

Biomechanical models are used extensively to study risk factors, such as peak stresses, for vulnerable atherosclerotic plaque rupture. Typically, 3D patient-specific arterial models are reconstructed by interpolating between cross sectional contour data which have a certain axial sampling, or image, resolution. The influence of the axial sampling resolution on computed stresses, as well as the comparison of 3D with 2D simulations, is quantified in this study. A set of histological data of four atherosclerotic human coronary arteries was used which were reconstructed in 3D with a high sampling (HS) and low sampling (LS) axial resolution, and 4 slices were treated separately for 2D simulations. Stresses were calculated using finite element analysis (FEA). High stresses were found in thin cap regions and regions of thin vessel walls, low stresses were found inside the necrotic cores and media and adventitia layers. Axial sampling resolution was found to have a minor effect on general stress distributions, peak plaque/cap stress locations and the relationship between peak cap stress and minimum cap thickness. Axial sampling resolution did have a profound influence on the error in computed magnitude of peak plaque/cap stresses (±15.5% for HS vs. LS geometries and ±24.0% for HS vs. 2D geometries for cap stresses). The findings of this study show that axial under sampling does not influence the qualitative stress distribution significantly but that high axially sampled 3D models are needed when accurate computation of peak stress magnitudes is required.

Chirp resonance spectroscopy of single lipid-coated microbubbles using an "acoustical camera".

An acoustical method was developed to study the resonance of single lipid-coated microbubbles. The response of 127 SonoVue microbubbles to a swept sine excitation between 0.5 and 5.5 MHz with a peak acoustic pressure amplitude of 70 kPa was measured by means of a 25 MHz probing wave. The relative amplitude modulation in the signal scattered in response to the probing wave is approximately equal to the radial strain induced by the swept sine excitation. An average damping coefficient of 0.33 and an average resonance frequency of 2.5 MHz were measured. Microbubbles experienced an average peak radial strain of 20%.

Counter-propagating wave interaction for contrast-enhanced ultrasound imaging.

Most techniques for contrast-enhanced ultrasound imaging require linear propagation to detect nonlinear scattering of contrast agent microbubbles. Waveform distortion due to nonlinear propagation impairs their ability to distinguish microbubbles from tissue. As a result, tissue can be misclassified as microbubbles, and contrast agent concentration can be overestimated; therefore, these artifacts can significantly impair the quality of medical diagnoses. Contrary to biological tissue, lipid-coated gas microbubbles used as a contrast agent allow the interaction of two acoustic waves propagating in opposite directions (counter-propagation). Based on that principle, we describe a strategy to detect microbubbles that is free from nonlinear propagation artifacts. In vitro images were acquired with an ultrasound scanner in a phantom of tissue-mimicking material with a cavity containing a contrast agent. Unlike the default mode of the scanner using amplitude modulation to detect microbubbles, the pulse sequence exploiting counter-propagating wave interaction creates no pseudoenhancement behind the cavity in the contrast image.