A new method for analysis of motion of carotid plaques from RF ultrasound images

Fluid structure interaction analysis to stratify the behavior of different atheromatous carotid plaques

BACKGROUND

In asymptomatic carotid stenosis (ACS), different plaque types, i.e. lipidic (LP), fibrous (FP), and calcific (CP), could have different hemodynamic and structural behaviors.

METHODS

Different carotid plaques, reconstructed from medical imaging of ACS >70%, were analyzed by computing fluid structure interaction (FSI), modeling the spatial distribution of wall shear stresses (WSS), plaque displacements (D), von Mises stresses (VMS), and absorbed elastic energy (AEE) together with their maximum-in-space values at the systole (WSS<inf>syst</inf>, D<inf>syst</inf>, VMS<inf>syst</inf> and AEE<inf>syst</inf>).

RESULTS

WSS resulted significantly higher in CP, whereas D and VMS showed the highest values for LP. Regarding AEE<inf>syst</inf> stored by the plaques, LP absorbed in average 2320 J/m³, FP 408 J/m³ (470%) and CP 99 J/m³ (2240%), (P<0.01, P<0.01, and P<0.01, respectively).

CONCLUSIONS

Depending upon their nature, plaques store different deformations and inner distributions of forces, thus potentially influencing their vulnerability.

An Upgraded Siamese Neural Network for Motion Tracking in Ultrasound Image Sequences

Deep learning is heavily being borrowed to solve problems in medical imaging applications, and Siamese neural networks are the front runners of motion tracking. In this article, we propose to upgrade one such Siamese architecture-based neural network for robust and accurate landmark tracking in ultrasound images to improve the quality of image-guided radiation therapy. Although several researchers have improved the Siamese architecture-based networks with sophisticated detection modules and by incorporating transfer learning, the inherent assumptions of the constant position model and the missing motion model remain unaddressed limitations. In our proposed model, we overcome these limitations by introducing two modules into the original architecture. We employ a reference template update to resolve the constant position model and a linear Kalman filter (LKF) to address the missing motion model. Moreover, we demonstrate that the proposed architecture provides promising results without transfer learning. The proposed model was submitted to an open challenge organized by MICCAI and was evaluated exhaustively on the Liver US Tracking (CLUST) 2D dataset. Experimental results proved that the proposed model tracked the landmarks with promising accuracy. Furthermore, we also induced synthetic occlusions to perform a qualitative analysis of the proposed approach. The evaluations were performed on the training set of the CLUST 2D dataset. The proposed method outperformed the original Siamese architecture by a significant margin.

A Computational Fluid-Structure Interaction Study for Carotids With Different Atherosclerotic Plaques

Atherosclerosis is a systemic disease that leads to accumulation of deposits, known as atherosclerotic plaques, within the walls of the carotids. In particular, three types of plaque can be distinguished: soft, fibrous, and calcific. Most of the computational studies who investigated the interplay between the plaque and the blood flow on patient-specific geometries used nonstandard medical images to directly delineate and segment the plaque and its components. However, these techniques are not so widely available in the clinical practice. In this context, the aim of our work was twofold: (i) to propose a new geometric tool that allowed to reconstruct a plausible plaque in the carotids from standard images and (ii) to perform three-dimensional (3D) fluid-structure interaction (FSI) simulations where we compared some fluid-dynamic and structural quantities among 15 patients characterized by different typologies of plaque. Our results highlighted that both the morphology and the mechanical properties of different plaque components play a crucial role in determining the vulnerability of the plaque.

Faster Search Algorithm for Speckle Tracking in Ultrasound Images

Block matching techniques have been studied exhaustively for motion estimation in Ultrasound (US) images. Exhaustive Search (ES) is the most commonly used search algorithm for block matching in US images. However, ES can be computationally expensive and slow. In this paper, a faster search algorithm called the Adaptive Rood Pattern Search (ARPS) is adopted to US images along with subpixel matching to reduce the computational cost and enhance block matching. Both ES and ARPS were applied in the context of block matching based 2D speckle tracking and were compared using Number of Computations per Frame (NCF), Computational Time per Frame (CTF) and Root Mean Squared Error (RMSE) as metrics. Our simulations and experimental results proved that ARPS outperformed ES by a substantial margin. Adaptation of this technique could help improve the performance of real-time motion estimation drastically.

Motion synchronisation patterns of the carotid atheromatous plaque from B-mode ultrasound

Asynchronous movement of the carotid atheromatous plaque from B-mode ultrasound has been previously reported, and associated with higher risk of stroke, but not quantitatively estimated. Based on the hypothesis that asynchronous plaque motion is associated with vulnerable plaque, in this study, synchronisation patterns of different tissue areas were estimated using cross-correlations of displacement waveforms. In 135 plaques (77 subjects), plaque radial deformation was synchronised by approximately 50% with the arterial diameter, and the mean phase shift was 0.4 s. Within the plaque, the mean phase shifts between the displacements of the top and bottom surfaces were 0.2 s and 0.3 s, in the radial and longitudinal directions, respectively, and the synchronisation about 80% in both directions. Classification of phase-shift-based features using Random Forests yielded Area-Under-the-Curve scores of 0.81, 0.79, 0.89 and 0.90 for echogenicity, symptomaticity, stenosis degree and plaque risk, respectively. Statistical analysis showed that echolucent, high-stenosis and high-risk plaques exhibited higher phase shifts between the radial displacements of their top and bottom surfaces. These findings are useful in the study of plaque kinematics.

Frequency-constrained robust principal component analysis: a sparse representation approach to segmentation of dynamic features in optical coherence tomography imaging

Sparse representation theory is an exciting area of research with recent applications in medical imaging and detection, segmentation, and quantitative analysis of biological processes. We present a variant on the robust-principal component analysis (RPCA) algorithm, called frequency constrained RPCA (FC-RPCA), for selectively segmenting dynamic phenomena that exhibit spectra within a user-defined range of frequencies. The algorithm lacks subjective parameter tuning and demonstrates robust segmentation in datasets containing multiple motion sources and high amplitude noise. When tested on 17 ex-vivo, time lapse optical coherence tomography (OCT) B-scans of human ciliated epithelium, segmentation accuracies ranged between 91-99% and consistently out-performed traditional RPCA.

Performance comparison of rigid and affine models for motion estimation using ultrasound radio-frequency signals

Tissue motion estimation is widely used in many ultrasound techniques. Rigid-model-based and nonrigid-modelbased methods are two main groups of space-domain methods of tissue motion estimation. The affine model is one of the commonly used nonrigid models. The performances of the rigid model and affine model have not been compared on ultrasound RF signals, which have been demonstrated to obtain higher accuracy, precision, and resolution in motion estimation compared with B-mode images. In this study, three methods, i.e., the normalized cross-correlation method with rigid model (NCC), the optical flow method with rigid model (OFRM), and the optical flow method with affine model (OFAM), are compared using ultrasound RF signals, rather than the B-mode images used in previous studies. Simulations, phantom, and in vivo experiments are conducted to make the comparison. In the simulations, the root-mean-square errors (RMSEs) of axial and lateral displacements and strains are used to assess the accuracy of motion estimation, and the elastographic signal-tonoise ratio (SNRe) and contrast-to-noise ratio (CNRe) are used to evaluate the quality of axial strain images. In the phantom experiments, the registration error between the pre- and postdeformation RF signals, as well as the SNRe and CNRe of axial strain images, are utilized as the evaluation criteria. In the in vivo experiments, the registration error is used to evaluate the estimation performance. The results show that the affinemodel- based method (i.e., OFAM) obtains the lowest RMSE or registration error and the highest SNRe and CNRe among all the methods. The affine model is demonstrated to be superior to the rigid model in motion estimation based on RF signals.

A systematic literature review of the effect of carotid atherosclerosis on local vessel stiffness and elasticity

OBJECTIVE

This systematic literature review sought to determine the effects of carotid atherosclerotic plaque on local arterial stiffness.

METHODS

MedLine, EMBASE, and grey literature were searched with the following term: ("atherosclerosis" or "carotid atherosclerosis" or "carotid artery disease" or "carotid plaque") AND ("distensibility" or "elasticity" or "stiffness" or "compliance") NOT ("pulse wave velocity" or "PWV" or "carotid-ankle" or "ankle-brachial" or "augmentation index" or "cardio-ankle" or "CAVI" or "flow mediated dilation" or "FMD"). Results were restricted to English language articles reporting local arterial stiffness in human subjects with carotid atherosclerosis.

RESULTS

Of the 1466 search results, 1085 abstracts were screened and 191 full-text articles were reviewed for relevance. The results of the 50 studies that assessed some measure of carotid arterial elasticity or stiffness in patients with carotid plaque were synthesized and reviewed.

DISCUSSION

A number of different measures of carotid elasticity were found in the literature. Regardless of which metric was used, the majority of studies found increased carotid stiffness (or decreased distensibility) to be associated with carotid plaque presence, the degree of atherosclerosis, and incident stroke.

CONCLUSION

Carotid artery mechanics are influenced by the presence of atherosclerotic plaque. The clinical applicability of carotid elasticity measures may be limited by the lack of reference values and standardized techniques.

Carotid plaque hemodynamics

Internal carotid artery (ICA) plaques constitute one major source of retinal and cerebral brain embolism. Current guidelines recommend optimal treatment of cardiovascular risk factors and recanalization based on the degree of ICA stenosis. However, ICA plaque composition, motion, vascularization, and local hemodynamics have only received limited attention as potential and independent risk factors for plaque rupture. The European Carotid Surgery Trial (ECST) showed an increased risk of stroke recurrence even in moderate stenosis if the plaque surface was ulcerated in angiography. Further indicators of plaque instability and thus vulnerability were established by native or contrast-enhanced two-dimensional duplex ultrasound, CT, and MRI. Due to high soft tissue contrast, multi-contrast MRI seems to be ideally suited to identify plaque compositions that are prone to rupture, although data from large clinical trials proving the independent predictive value of plaque morphology are lacking. The role of cardiovascular risk factors for atherosclerosis of the common carotid artery is well established. Nevertheless, little is known concerning the impact of local hemodynamics on plaque development, progression, and rupture. Wall shear stress, the friction force acting on the endothelium of the vessel wall, was shown to be able to induce local atherosclerosis and vulnerable plaques in animal models. Plaque movement and deformation was limited to investigations using ultrasound in order to identify plaques at risk. Similarly, models to calculate tensile plaque stress seem to be able to identify peak mechanical stress acting on plaque surfaces that make such regions susceptible to rupture. In this review, current evidence regarding the correlation of plaque location, composition, and local hemodynamics at the carotid artery bifurcation will be presented. Moreover, the potential benefit of a future comprehensive and individual risk assessment will be discussed.

Shear wave elastography assessment of carotid plaque stiffness: in vitro reproducibility study

This study assessed inter- and intra-observer reproducibility of shear wave elastography (SWE) measurements in vessel phantoms simulating soft and hard carotid plaque under steady and pulsatile flow conditions. Supersonic SWE was used to acquire cine-loop data and quantify Young's modulus in cryogel vessel phantoms. Data were acquired by two observers, each performing three repeat measurements. Mean Young's modulus was quantified within 2-mm regions of interest averaged across five frames and, depending on vessel model and observer, ranged from 28 to 240 kPa. The mean inter-frame coefficient of variation (CV) was 0.13 (range: 0.07-0.18) for observer 1 and 0.14 (range: 0.12-0.16) for observer 2, with mean intra-class correlation coefficients (ICCs) of 0.84 and 0.83, respectively. The mean inter-operator CV was 0.13 (range: 0.08-0.20), with a mean ICC of 0.76 (range: 0.69-0.82). Our findings indicate that SWE can quantify Young's modulus of carotid plaque phantoms with good reproducibility, even in the presence of pulsatile flow.

Wall motion in the stenotic carotid artery: association with greyscale plaque characteristics, the degree of stenosis and cerebrovascular symptoms

Background

Systolic dilation of the atherosclerotic carotid artery depends on several factors including arterial compliance and the haemodynamic environment. The purpose of this study was to quantify wall motion in stenotic carotid arteries and investigate any associations with the ultrasound greyscale plaque characteristics, the degree of stenosis, and the presence of cerebrovascular symptoms.

Methods

Variations in the lumen diameters of 61 stenotic carotid arteries (stenosis range 10%-95%) from 47 patients were measured before the proximal shoulder of the atherosclerotic plaque using ultrasound image sequences over several cardiac cycles. Absolute and percentage diameter changes from diastole to systole were calculated and their relationship to the degree of stenosis, greyscale plaque characteristics, and the presence of ipsilateral hemispheric symptoms were studied.

Results

The mean absolute diameter change from diastole to systole was 0.45 mm (s.d. 0.17), and the mean percentage diameter change was 6.9% (s.d. 3.1%). Absolute and percentage diameter changes did not have a statistically significant relationship to the degree of stenosis, greyscale plaque characteristics, or the presence of ipsilateral hemispheric symptoms (p > 0.05). Parameters significantly correlated with the presence of symptoms were the degree of stenosis (p = 0.01), plaque greyscale median (p = 0.02) and the plaque surface irregularity index (p = 0.02).

Conclusions

Our study confirmed the degree of stenosis, plaque greyscale median and our surface irregularity index were significant predictors of symptoms, but found no significant correlation between diameter changes of stenosed carotid arteries and the presence of ipsilateral hemispheric symptoms.

Characterization of aortic remodeling following Kawasaki disease: toward a fully developed automatic biparametric model

PURPOSE

Mechanical properties of the arteries are essential in assessing cardiovascular diseases. New imaging modalities that allow mapping strain, shear and elasticity distributions within the arterial wall are rapidly evolving. Very recently, our group introduced an iterative optical flow-based elastography method devoted to B-mode data. In this paper, the authors propose an adaptation of the optical flow method to investigate aortic remodeling following Kawasaki disease, an early childhood vasculitis. Namely, displacement and strain of the aortic wall are used to assess aortic stiffness in this human disease model. The authors also introduce a fully developed automatic method to support postprocessing data analysis.

METHODS

The sequalae of Kawasaki disease on the ascending aorta were examined in children. The pathologic population (n = 4) was 15.00 ± 2.45 years old, while the healthy control population (n = 5) was 13.13 ± 0.18 years old. B-mode data were digitally recorded with commercially available cardiac echocardiography machines.

RESULTS

Kawasaki disease had a very significant impact on the aortic stiffness. Indeed, pathologic subjects' aortic wall strain estimate was significantly lower compared to healthy controls (2.75% ± 0.56% versus 4.24% ± 0.65%, respectively; p < 0.001). Similarly, displacement of the aortic wall was also significantly lower compared to controls (p < 0.001).

CONCLUSIONS

The potential of the optical flow-based method to quantify aortic wall remodeling in a human disease model was demonstrated. The authors now intend to extend this investigation to a larger pathologic cohort with various degrees of vasculitis severity.

Analysis and quantification of arterial wall motion from B-mode ultrasound images - comparison of block-matching and optical flow

Motion of the carotid atheromatous plaque may be responsible for plaque rupture and cerebrovascular symptoms. B-mode ultrasound allows non-invasive recording of arterial wall and plaque motion. Our aim was to analyze quantitatively patterns of arterial wall motion with different techniques. Temporal sequences of digitized B-mode ultrasound images of the carotid arteries of 10 young healthy subjects were interrogated. Arterial wall motion was analyzed using: a/ block-matching, and b/ optical flow. The motion of selected regions of the luminal surface of the arterial wall was estimated using region tracking and block-matching. The motion of areas of the arterial wall was estimated using optical flow. Waveforms showing radial and axial displacements, as well as radial and axial velocities were produced for the selected ROIs using both techniques. Both techniques produced waveforms with peaks, corresponding to cardiac cycle events, that occurred at similar time points. To study the similarity of the waveforms obtained from the two techniques, a cross-correlation coefficient was calculated. Cross-correlation coefficients were 0.72..0.22 and 0.70..0.19 for displacements and velocities, respectively, in the radial direction. In the axial direction, the coefficients were 0.32..0.39 and 0.24..0.22 for displacements and velocities, respectively. On the basis of this relative comparison of methods, we conclude that significant observations can be made for each motion analysis technique in terms of characterization of the mechanical properties of the tissue.

A texture matching method considering geometric transformations in noninvasive ultrasonic measurement of arterial elasticity

Measurement of arterial elasticity can provide an important reference for understanding arterial wall changes that may occur in the early stages of atherosclerosis. Conventional correlation-based methods for evaluating arterial wall movements consider only the translation, ignoring the rotation and deformation, which limits the accuracy of measurement of arterial displacement and its biomechanical properties. This article proposes a novel texture matching method based on ultrasonic B-mode image considering geometric transformations to accurately measure arterial displacement and acquire arterial elasticity noninvasively. The method was validated by simulated images with rotation and deformation and further by measurements in vitro arterial phantom and in vivo common carotid arteries of 20 healthy volunteers. Simulation results demonstrate that the method can improve the accuracy of measurement of arterial displacement. Experimental results show that the elastic modulus of the arterial phantom agrees well with the results obtained from mechanical tests, deviating only 4.1%. The mean elastic modulus of the common carotid arteries is 361.7 ± 93.5 kPa. The texture matching method was shown to be able to measure the displacement and elasticity of the arterial wall with complex geometric transformations and may be clinically useful for early detecting and monitoring atherosclerosis.

Stratifying risk: asymptomatic carotid disease

Stroke generates significant healthcare expenses and it is also a social and economic burden. The carotid artery atherosclerotic plaque instability is responsible for a third of all embolic strokes. The degree of stenosis has been deliberately used to justify carotid artery interventions in thousands of patients worldwide. However, the annual risk of stroke in asymptomatic carotid artery disease is low. Plaque morphology and its kinetics have gained ground to explain cerebrovascular and retinal embolic events. This review provides the readers with an insightful and critical analysis of the risk stratification of asymptomatic carotid artery disease in order to assist in selecting potential candidates for a carotid intervention.

Introduction to the biomechanics of carotid plaque pathogenesis and rupture: review of the clinical evidence

The management of patients with asymptomatic carotid disease is currently under debate and new methods are warranted for better risk stratification. The role of the biomechanical properties of the atherosclerotic arterial wall together with the effect of different stress types in plaque destabilisation has only been recently investigated. PubMed and Scopus databases were reviewed. There is preliminary clinical evidence demonstrating that the analysis of the combined effect of the various types of biomechanical stress acting on the carotid plaque may help us to identify the vulnerable plaque. At present, MRI and two-dimensional ultrasound are combined with fluid-structure interaction techniques to produce maps of the stress variation within the carotid wall, with increased cost and complexity. Stress wall analysis can be a useful tool for carotid plaque evaluation; however, further research and a multidisciplinary approach are deemed as necessary for further development in this direction.

Optical-flow-based B-mode elastography: application in the hypertensive rat carotid

Background-Ultrasound elastography is now used worldwide in tissue characterization. The primary premises of elastography are that speckle kinematics reproduces underlying tissue kinematics and that tissue motion can be inferred from speckle tracking. This implicitly assumes that speckle pattern is a material property that can be tracked with respect to time and space. It is then convenient to express the motion of such a material property in terms of total derivative, also known as optical flow (OF) equations. Aims-The present paper introduces a new iterative OF-based elastography (OFBE) method devoted to B-mode data. The first OFBE iteration computes axial and lateral displacement fields. Such displacement fields are used for data rigid registration, prior to the second OFBE iteration which computes the 2-D strain tensor. Methods-The OFBE method was validated in the common carotid artery of rat hypertension models. The effect of aging on carotid stiffness was investigated in female recombinant inbred rats (RI-17, (n=2)) in the first experiment. The outcomes of low/high-salt diets were examined in young male Dahl salt-sensitive rats (SS, n=6; SM12, n=6; SM9, n=6) in the second experiment. Results-Good concordance was observed between left and right carotid axial strain measurements with 11.4% relative error, whereas 4.6% relative error occurred between diastolic and systolic axial strain measurements. Old (80 and 85 weeks) RI-17 carotids were determined to be twice as stiff with 5.70 +/- 0.97% (strain+/-std) as young carotids (30 and 34 weeks) with 13.26 +/- 2.73%, p < 0.001. Carotid axial strain measurement also indicated that salt diets had a significant impact on SS (p=0.008) and SM12 (p < 0.001) but not on SM9 (p=0.881) rats.

Contrast-enhanced sonographic characteristics of neovascularization in carotid atherosclerotic plaques

PURPOSE

To evaluate neovascularization within carotid atherosclerotic plaques with contrast-enhanced sonography.

METHODS

We used contrast-enhanced sonography to examine 63 patients with carotid atherosclerotic plaques. The features of neovascularization within the plaques were analyzed and correlated with plaque size and echogenicity.

RESULTS

There were 81 atherosclerotic plaques, 62 of which (43 soft and 19 mixed) enhanced after injection of a contrast agent. The enhancement occurred from the carotid wall to the center of the plaque with a short-line pattern in 36 plaques, whereas 26 plaques enhanced from both the carotid wall and the carotid lumen, with a sparse spot pattern. The arrival time of contrast was shorter (p < 0.001) and time to peak was longer (p < 0.001) in the plaques than in the carotid lumen. Time to peak was shorter, whereas enhanced intensity was greater in soft plaques than in mixed plaques (p < 0.01 and p < 0.05, respectively). Among the 19 unenhanced plaques, 6 were hard, 3 were calcified, 3 were soft, and 7 were mixed. The thickness of the unenhanced plaques was <2.4 mm.

CONCLUSION

Contrast-enhanced sonography allows the noninvasive, dynamic evaluation of neovascularization within carotid plaques, and the presence of neovascularization may correlate with plaque morphology.

Noninvasive vascular elastography: toward a complementary characterization tool of atherosclerosis in carotid arteries

Only a minority of patients with carotid arterial disease have warning symptoms, because the majority of strokes are caused by previously asymptomatic lesions. Because morbidity and mortality after acute stroke are high, patients should be diagnosed and treated before symptoms develop. The hypothesis of this study is that vascular elasticity maps (or elastograms) of carotids are of predictive value for plaque characterization. The strain tensor from either cross-sectional or longitudinal ultrasound radiofrequency data were assessed by a new implementation of the Lagrangian speckle model estimator (LSME), which considers local echogenicity variations. A 26-year-old healthy male (HS1), a 40-year-old (HS2) normal female subject and two 75-year-old asymptomatic patients with severe carotid stenoses were scanned. Reproducible elastograms were obtained as a function of time over five to seven cardiac cycles. Stress-strain modulus elastograms were computed for normal subjects. Stiffening of healthy carotid walls was estimated to be 148 +/- 7 kPa and 163 +/- 30 kPa at peak-systole for HS1 and HS2, respectively. For patients with heterogeneous plaques, strain and shear elastograms revealed interesting information about plaque size, tissue composition and mechanical interaction between structures. In conclusion, the LSME provides a promising approach for strain and shear estimates to characterize vulnerable plaque.

Motion compensation for intravascular ultrasound palpography

Rupture of vulnerable plaques in coronary arteries is the major cause of acute coronary syndromes. Most vulnerable plaques consist of a thin fibrous cap covering an atheromous core. These plaques can be identified using intravascular ultrasound (IVUS) palpography, which measures radial strain by cross-correlating RF signals at different intraluminal pressures. Multiple strain images (i.e., partial palpograms) are averaged per heart cycle to produce a more robust compounded palpogram. However, catheter motion due to cardiac activity causes misalignment of the RF signals and thus of the partial palpograms, resulting in less valid strain estimates. To compensate for in-plane catheter rotation and translation, we devised four methods based on block matching. The global rotation block matching (GRBM) and contour mapping (CMAP) methods measure catheter rotation, and local block matching (LBM) and catheter rotation and translation (CRT) estimate displacements of local tissue regions. These methods were applied to nine in vivo pullback acquisitions, made with a 20 MHz phased-array transducer. We found that all these methods significantly increase the number of valid strain estimates in the partial and compounded palpograms (P < 0.008). The best method, LBM, attained an average increase of 17% and 15%, respectively. Implementation of this method should improve the information coming from IVUS palpography, leading to better vulnerable plaque detection.

Non-invasive high-frequency vascular ultrasound elastography

Non-invasive vascular elastography (NIVE) was recently introduced to characterize mechanical properties of superficial arteries. In this paper, the feasibility of NIVE and its applicability in the context of high-frequency ultrasound imaging is investigated. First, experiments were performed in vitro on vessel-mimicking phantoms. Polyvinyl alcohol cryogel was used to create two double-layer vessels with different mechanical properties. In both cases, the stiffness of the inner layer was made softer. Radial stress was applied within the lumen of the phantoms by applying incremental static pressure steps with a column of a flowing mixture of water-glycerol. The vessel phantoms were insonified at 32 MHz with an ultrasound biomicroscope to provide cross-section sequences of radio-frequency (RF) ultrasound data. The Lagrangian speckle model estimator (LSME) was used to assess the two-dimensional-strain tensors, and the composite Von Mises elastograms were computed. A new implementation of the LSME based on the optical flow equations was introduced. Deformation parameters were estimated using an inversion algorithm. For each in vitro experiment, both layers of approximately 1 mm were distinguished. Second, the use of the method for the purpose of studying small vessels (MicroNIVE) in genetically engineered rodents was introduced. Longitudinal scans of the carotid artery were performed at 40 MHz. The in vivo results give confidence in the feasibility of MicroNIVE as a potential tool to non-invasively study the impact of targeted genes on vascular remodelling in rodents.

Parameters describing motion in carotid artery plaques from ultrasound examination: A reproducibility study

We have previously developed a method for quantifying motion in carotid artery plaques from sequences of ultrasound (US) radiofrequency images. Here, we examine the intraoperator reproducibility of the results. Five independent recordings were made on each of six symptomatic and six asymptomatic patients, and processed off-line into 29 motion parameters, representing motion amplitude, stretch/compression and shear motion. For the statistical analysis, we used a linear mixed model and investigated the parameters for contributions from individual patients, contributions from recordings on each patient and contributions from heart cycles within each recording. The model was valid for seven parameters calculated over the entire heart cycle (four calculated over the systole only), which all showed good reproducibility (intraclass coefficient for variance over all patients rho(alpha) >/= 0.4). Averaging three recordings of two heart cycles each gives acceptable accuracy (normalised variance of patient means lambda < 0.3). This acquisition scheme is reasonable in a clinical situation.

Tissue Doppler imaging of carotid plaque wall motion: a pilot study

Background

Studies suggest the physical and mechanical properties of vessel walls and plaque may be of clinical value in the diagnosis and treatment of cardiovascular atherosclerotic disease. The purpose of this pilot study was to investigate the potential clinical application of ultrasound Tissue Doppler Imaging (TDI) of Arterial Wall Motion (AWM) and to quantify simple wall motion indices in normal and diseased carotid arteries.

Methods

224 normal and diseased carotid arteries (0–100% stenoses) were imaged in 126 patients (age 25–88 years, mean 68 ± 11). Longitudinal sections of the carotid bifurcation were imaged using a Philips HDI5000 scanner and L12-5 probe under optimized TDI settings. Temporal and spatial AWMs were analyzed to evaluate the vessel wall displacements and spatial gradients at peak systole averaged over 5 cardiac cycles.

Results

AWM data were successfully extracted in 91% of cases. Within the carotid bifurcation/plaque region, the maximum wall dilation at peak systole ranged from -100 to 750 microns, mean 335 ± 138 microns. Maximum wall dilation spatial gradients ranged 0–0.49, mean 0.14 ± 0.08. The AWM parameters showed a wide variation and had poor correlation with stenoses severity. Case studies illustrated a variety of pertinent qualitative and quantitative wall motion features related to the biophysics of arterial disease.

Conclusion

Our clinical experience, using a challenging but realistic imaging protocol, suggests the use of simple quantitative AWM measures may have limitations due to high variability. Despite this, pertinent features of AWM in normal and diseased arteries demonstrate the potential clinical benefit of the biomechanical information provided by TDI.