Comparing Carotid and Brachial Artery Stiffness: A First Step Toward Mechanical Mapping of the Arterial Tree.

Arterial stiffness is a predictor of cardiovascular mortality. It increases with age and is accelerated by hypertension and other cardiovascular risk factors. In addition to the disease state, arterial stiffness increases from the proximal to the distal arterial compartments. Concurrent assessment of various vessels from the same subjects is unavailable in the literature. The aim of this work was to quantify an intrinsic mechanical feature, namely, wall stiffness, of the common carotid artery (CCA) and brachial artery (BA). CCAs and BAs of healthy adolescents were investigated. Cine loops of CCA and BA B-mode data were digitally recorded at the same clinical examination, and arterial elastic moduli were estimated off-line with our proprietary non-invasive Imaging-based BioMarker (ImBioMark) algorithm. The 11 study subjects were 14.4 ± 1.2 years old, with normal body habitus and blood pressures 112.3 ± 10.6/63.6 ± 5.7 mm Hg. BAs had a higher elastic modulus than CCAs (arterial elastic moduli: 129.73 ± 25.67 kPa vs. 49.55 ± 14.75 kPa, p < 0.001). There was a positive correlation between the BA and CCA (slope = 0.36, intercept = 111.62 kPa, R(2) = 0.045). This article documents, for the first time, a correlation between the CCA and BA of the same subject, under the same conditions. We previously reported preliminary data for the aorta and documented the effect of aging on the CCA; we now intend to study the femoral artery as well and include age stratification to pursue our investigations. The results reported here can be seen as the first step toward mechanical mapping of the arterial tree.

Ascending Aorta Elastography After Kawasaki Disease Compared to Systemic Hypertension.

Kawasaki disease (KD) is a systemic vasculitis, classically affecting large- and medium-size arteries. The coronary arteries draw most of the clinical attention, whereas few studies have taken interest in the ascending aorta. Using a proprietary imaging-based mechanical biomarker (ImBioMark), we sought to determine aortic stiffness in KD compared to systemic hypertension (HTN) and healthy children. We evaluated parasternal long-axis views focused on the ascending aorta in 20 controls, 12 KD, and 8 HTN as a comparative clinical model of vascular stiffness. We calculated systolic and diastolic aortic wall strain with ImBioMark. Strain was tested for normality against height, systolic, and diastolic blood pressure in normal subjects. Strain from KD and HTN was normalized (Z score) accordingly. Z score comparisons were performed using nonparametric statistics. Age was similar between KD and HTN (9.1 ± 5.3 and 9.9 ± 5.3 years old; p = NS). Systolic and diastolic strain values were normally distributed against height, systolic blood pressure, and diastolic blood pressure in healthy subjects. HTN subjects had abnormal systolic and diastolic strain values (p < 0.0001). Whereas KD subjects had normal diastolic strain, systolic strain was significantly lower (p < 0.001), and systolic strain was intermediate between controls and HTN. There were no significant differences in aortic strain among KD, however, according to the presence of coronary artery aneurysms. Despite normal blood pressure, the ascending aorta in KD exhibits reduced strain during systole. This may reflect in situ rigidity of the aorta. The normal diastolic strain in KD may, in contrast, reflect normal peripheral vascular resistance.

Carotid wall elastography to assess midterm vascular dysfunction secondary to intrauterine growth restriction: feasibility and comparison with standardized intima-media thickness.

Several studies have suggested that intrauterine growth restriction (IUGR) increases the risk of cardiovascular disease and early atherosclerosis. Early detection of arteriopathy is essential to early intervention. Although arterial intima-media thickness (IMT) is considered an index of subclinical atherosclerosis in the adult, its validity in pediatric patients may be limited. We have recently introduced a novel imaging-based biomarker (ImBioMark) to assess intrinsic mechanical features of the arterial wall from B-mode ultrasound data. The aim of the work described here was to evaluate the potential of ImBioMark in investigation of cardiovascular health status at the level of the common carotid artery (CCA) in adolescents born after IUGR. We also compared ImBioMark results with automated IMT measurements, a well-established biomarker used in clinical practice and research. The potential sequelae of IUGR on the CCA were examined in a group of adolescents in comparison with healthy controls. Patients with IUGR (n = 7) were 13.85 ± 0.46 y old; the healthy controls (n = 7) were 14.58 ± 0.80 y old (p = 0.058). Cine loops of the CCA B-mode data were digitally recorded, and the arterial elastic modulus was estimated a posteriori with ImBioMark. IMT of the CCA was automatically calculated using QLAB software (Philips, Andover, MA, USA). All patients had been evaluated in utero in our fetal echocardiographic laboratory. ImBioMark detected a significant increase in CCA stiffness in patients with IUGR as compared with healthy controls: elastic modulus = 90.74 ± 11.86 versus 61.30 ± 15.94 kPa, respectively (p = 0.002). There was, however, no significant difference between patients with IUGR and controls in IMT (0.483 ± 0.067 versus 0.476 ± 0.051 mm, respectively, p = 0.831). The impact of IUGR on CCA wall dynamics was confirmed by ImBioMark. The apparent limitation of IMT measurement in this cohort may be the result of geometric arterial changes, that is, the expected thickening, below the level of detection at this age. As early detection of vascular modulation is essential to early intervention in a population at risk, we now intend to extend ImBioMark to investigate larger pathologic cohorts with various degrees of arteriopathy.

The biophysical properties of the aorta are altered following Kawasaki disease.

BACKGROUND: The long-term sequelae of Kawasaki disease (KD) are based on the coronary complications. Because KD causes generalized vasculitis, with documented aneurysms in the femoral, iliac, renal, axillary, and brachial arteries, the aim of this study was to assess the biophysical properties of the aorta (BPA) after KD. The BPA are biometric measurements representing vascular structural and dynamic changes in response to cardiac work. METHODS: Anthropometric and echocardiographic measurements of the aorta in a series of patients with KD were compared with those of healthy subjects. The BPA were calculated noninvasively by extrapolating previously validated equations that were conceived for invasive measurements. Because BPA vary with body habitus, control subjects were used to normalize BPA parameters for height to compute BPA Z-score equations. RESULTS: Between June 2007 and February 2010, BPA were recorded in 57 patients with KD >1 year after the onset of the disease, 45 without and 12 with coronary artery sequelae. The mean intervals between the acute onset of KD and enrollment were 10.0 ± 5.0 and 5.8 ± 4.5 years for patients with and without coronary artery sequelae, respectively (P = .008). Patients with KD with coronary artery sequelae had significantly altered Z scores of aortic diameter modulation, Peterson's elastic modulus, and ß stiffness index (P = .001-.016). Patients with KD without coronary artery sequelae also exhibited altered elasticity, stiffness, and pulse-wave velocity (P = .001-.026). CONCLUSIONS: Altered BPA after KD are detectible despite apparent resolution of acute vasculitis. Future directions toward determining multilevel and multilayer vascular impact, including vascular autonomous homeostasis, require thorough investigation.

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.

A compensative model for the angle-dependence of motion estimates in noninvasive vascular elastography.

PURPOSE: Atherosclerosis of peripheral cerebral arteries can lead to stroke either by stenosis formation or plaque rupture. This pathology is initiated by the alteration of arterial wall mechanical properties shown to be assessable by ultrasound elastography. Recently, noninvasive vascular elastography (NIVE) was introduced for noninvasive imaging of the mechanical properties of superficial arteries as markers of vulnerable plaques. However, NIVE motion estimates are angle-dependent, with optimal scanning angle being represented by the alignment of tissue motion with ultrasound beam orientation. The objective of this study was to introduce a model that compensates for such angle-dependence in order to reduce the bias on strain estimates, namely, when investigating longitudinal vessel segments. METHODS: The model is based on the Lagrangian speckle model estimator (LSME) because the LSME assesses the 2D-deformation matrix required to compute the scanning angle. RESULTS: Experiments on vessel-mimicking phantoms indicated that such a model enables the estimation of scanning angle with less than 3-degrees error. The method was also validated in vivo in human carotid arteries where less than 4-degrees error was observed. In both cases, the compensative model estimated the inclination angles with low variability. CONCLUSION: Angle-dependence may be an important factor to consider in avoiding potentially distort clinical diagnoses. Results, reported in this article, suggest that the LSME-based compensative model might be considered as a very interesting and promising clinical tool for NIVE applications.

Mice overexpressing both human angiotensinogen and human renin as a model of superimposed preeclampsia on chronic hypertension.

Preeclampsia is the major cause of maternal and fetal mortality/morbidity. Because hypertension is an important risk factor for preeclampsia, we investigated whether hypertensive mice that overexpress human renin and angiotensinogen develop superimposed preeclampsia. Given that the mechanisms underlying this disease are still poorly understood, animal models are of great use for elucidation. Blood pressure and proteinuria were measured by telemetry and ELISA, respectively. Heart function was evaluated by echocardiography, whereas pathological cardiac hypertrophy-related genes were assessed by real-time PCR. Soluble fms-like tyrosine kinase 1 plasma concentrations were quantitated by ELISA and placental expression by real-time PCR. Transgenic mice develop de novo proteinuria during gestation and marked blood pressure elevation, which are hallmarks of superimposed preeclampsia on chronic hypertension. Abnormal placentation present in these mothers produced a significant decrease in pup and placental weight and was associated with an increased placental expression of soluble fms-like tyrosine kinase 1. We also found heightened circulating levels of this receptor, when adjusted for placental mass, as has been observed in women who suffer from preeclampsia. Cardiac hypertrophy could be observed in the transgenic mice and was exacerbated by gestation. As a result, heart function was significantly decreased, and markers of pathological hypertrophy were increased. Our data, thus, confirm the characterization of a new model of superimposed preeclampsia on chronic hypertension. Because chronically hypertensive women are at risk of developing the pathology, our model reflects a clinical reality and is, thus, an excellent tool to elucidate the molecular mechanisms triggering this disease.

Vulnerable atherosclerotic plaque elasticity reconstruction based on a segmentation-driven optimization procedure using strain measurements: theoretical framework.

It is now recognized that prediction of the vulnerable coronary plaque rupture requires not only an accurate quantification of fibrous cap thickness and necrotic core morphology but also a precise knowledge of the mechanical properties of plaque components. Indeed, such knowledge would allow a precise evaluation of the peak cap-stress amplitude, which is known to be a good biomechanical predictor of plaque rupture. Several studies have been performed to reconstruct a Young's modulus map from strain elastograms. It seems that the main issue for improving such methods does not rely on the optimization algorithm itself, but rather on preconditioning requiring the best estimation of the plaque components' contours. The present theoretical study was therefore designed to develop: 1) a preconditioning model to extract the plaque morphology in order to initiate the optimization process, and 2) an approach combining a dynamic segmentation method with an optimization procedure to highlight the modulogram of the atherosclerotic plaque. This methodology, based on the continuum mechanics theory prescribing the strain field, was successfully applied to seven intravascular ultrasound coronary lesion morphologies. The reconstructed cap thickness, necrotic core area, calcium area, and the Young's moduli of the calcium, necrotic core, and fibrosis were obtained with mean relative errors of 12%, 4% and 1%, 43%, 32%, and 2%, respectively.

Noninvasive vascular elastography for carotid artery characterization on subjects without previous history of atherosclerosis.

BACKGROUND: Noninvasive vascular ultrasound elastography (NIVE) was recently introduced to assess mechanical properties (strain or elasticity) of peripheral vessel walls. The goal of this study was to determine strain values in subjects with normal carotid arteries and the reproducibility of these measurements. METHODS: Sixteen individuals without previous history of carotid atherosclerosis were recruited in four age categories [40-49], [50-59], [60-69], and [70-79] years old. The left and right common and internal carotids (LCC, LIC, RCC, and RIC, respectively) were independently scanned by two radiologists (RAD-A and RAD-B). The axial strain elastograms were computed with the Lagrangian speckle model estimator. RESULTS: Supported by Bland-Altman analyses, strain values between LCC and RCC were found similar with a Pearson correlation coefficient (r) of 0.83 (p < 0.0001). Equivalently, a good correlation was found between RAD-A and RAD-B for common carotids with r=0.80 (p < 0.0001). Lower strain values (p < 0.001) were measured for male common carotids (1.62 +/- 0.32%) than females (2.21 +/- 0.76%). Regarding the internal carotid strain measurements, the correlation was lower between RAD-A and RAD-B with r=0.52 (p=0.01), but drastically decreased between LIC and RIC (r=0.16, nonsignificant). Male internal carotid strain estimates (p=0.03) were lower (1.48 +/- 0.44%) than in females (1.84 +/- 0.64%). Additionally, male common and internal carotid mean elastic moduli varied from 33-106 kPa, whereas it covered a range of 25-67 kPa for females. Female carotids were more elastic (44 +/- 17 kPa) than males (58 +/- 17 kPa, p <0.001). CONCLUSION: Strain measurements in common carotids were found reproducible. However, less consistency was observed for the deeper internal carotids. The NIVE imaging method still remains to be validated with pathological cases, but it might provide a unique approach for stroke prevention and characterization of vascular stiffness.

Performance evaluation of different implementations of the Lagrangian speckle model estimator for non-invasive vascular ultrasound elastography.

Non-invasive vascular ultrasound elastography (NIVE) was recently introduced to characterize mechanical properties of carotid arteries for stroke prevention. Using the Lagrangian speckle model estimator (LSME), the four components of the 2D deformation matrix (delta), which are the axial strain (delta(yy)) and shear (delta(yx)) and the lateral strain (delta(xx)) and shear (delta(xy)), can be computed. This paper overviews four different implementations of the LSME and addresses their reliability. These implementations include two unconstrained (L&M and L&M+) and one constrained (ITER(c)) iterative algorithms, and one optical flow-based (OF-based) algorithm. The theoretical frameworks were supported by biomechanical simulations of a pathology-free vessel wall and by one single layer vessel-mimicking phantom study. Regarding simulations, the four LSME implementations provided similar biases on axial motion parameters, except the L&M that outperformed other methods with a minimum strain bias of -3%. LSME axial motion estimates showed good consistence with theory, namely the OF-based algorithm that in a specific instance estimated delta(yy) with no relative error on the standard deviation. With regards to lateral motion parameters, ITER(c) exhibited a minimum strain bias of -8.5% when ultrasound beam and motion mostly run parallel, whereas L&M performs strain and shear estimates with less than 23% bias independently of orientations. The in vitro vessel phantom data showed LSME delta(yy) and delta(yx) maps that were qualitatively equivalent to theory, and noisy delta(xx) and delta(xy) elastograms. In summary, the authors propose to promote the OF-based LSME as an optimal choice for further applications of NIVE, because of its reliability to compute both axial strain and shear motion parameters and because it outperformed the other implementations by a factor of 30 or more in terms of processing time.

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.

On the potential of the lagrangian estimator for endovascular ultrasound elastography: in vivo human coronary artery study.

Diagnosis and prognosis of coronary artery atherosclerosis evolution currently rely on plaque morphology and vessel stenosis degree. Such information can accurately be assessed with intravascular ultrasound (IVUS) imaging. A severe complication of coronary artery atherosclerosis is thrombosis, a consequence of plaque rupture or fissure, which might lead to myocardial infarction and sudden ischemic death. Plaque rupture is a complicated mechanical process, correlated with the plaque morphology, composition, mechanical properties and with the blood pressure. Extracting information on the plaque local mechanical properties may reveal relevant features about plaque vulnerability. Accordingly, endovascular elastography (EVE) was introduced to complement IVUS for investigating coronary artery diseases. In this article, in vivo elastographic data are reported for three patients (patient 1, patient 2 and patient 3) who were diagnosed with severe coronary artery stenoses. Time-sequence radio-frequency (RF) data were acquired, in the minutes preceding angioplasty, using an ultrasound scanner working with a 30 MHz mechanical rotating single-element transducer. The elastograms of the radial strain and radial shear distributions within the vessel wall were computed from pairs of successive RF images using the Lagrangian estimator (LE). A hard atherosclerotic plaque (low radial strain and shear) was identified in patient 1. High radial strain and shear values in the plaque areas for patient 2 and patient 3 suggested the presence of lipid cores (soft materials), known to be prone-to-rupture sites when located close to the lumen. To conclude, EVE allowing radial strain and shear images is an improvement over existing EVE methods that may assist IVUS in preoperative vessel lesion assessments and in endovascular therapy planning.

Estimation of polyvinyl alcohol cryogel mechanical properties with four ultrasound elastography methods and comparison with gold standard testings.

Tissue-mimicking phantoms are very useful in the field of tissue characterization and essential in elastography for the purpose of validating motion estimators. This study is dedicated to the characterization of polyvinyl alcohol cryogel (PVA-C) for these types of applications. A strict fabrication procedure was defined to optimize the reproducibility of phantoms having a similar elasticity. Following mechanical stretching tests, the phantoms were used to compare the accuracy of four different elastography methods. The four methods were based on a one-dimensional (1-D) scaling factor estimation, on two different implementations of a 2-D Lagrangian speckle model estimator (quasistatic elastography methods), and on a 1-D shear wave transient elastography technique (dynamic method). Young's modulus was investigated as a function of the number of freeze-thaw cycles of PVA-C, and of the concentration of acoustic scatterers. Other mechanical and acoustic parameters-such as the speed of sound, shear wave velocity, mass density, and Poisson's ratio-also were assessed. The Poisson's ratio was estimated with good precision at 0.499 for all samples, and the Young's moduli varied in a range of 20 kPa for one freeze-thaw cycle to 600 kPa for 10 cycles. Nevertheless, above six freeze-thaw cycles, the results were less reliable because of sample geometry artifacts. However, for the samples that underwent less than seven freeze-thaw cycles, the Young's moduli estimated with the four elastography methods showed good matching with the mechanical tensile tests with a regression coefficient varying from 0.97 to 1.07, and correlations R2 varying from 0.93 to 0.99, depending on the method.

Intravascular ultrasound image segmentation: a three-dimensional fast-marching method based on gray level distributions.

Intravascular ultrasound (IVUS) is a catheter based medical imaging technique particularly useful for studying atherosclerotic disease. It produces cross-sectional images of blood vessels that provide quantitative assessment of the vascular wall, information about the nature of atherosclerotic lesions as well as plaque shape and size. Automatic processing of large IVUS data sets represents an important challenge due to ultrasound speckle, catheter artifacts or calcification shadows. A new three-dimensional (3-D) IVUS segmentation model, that is based on the fast-marching method and uses gray level probability density functions (PDFs) of the vessel wall structures, was developed. The gray level distribution of the whole IVUS pullback was modeled with a mixture of Rayleigh PDFs. With multiple interface fast-marching segmentation, the lumen, intima plus plaque structure, and media layers of the vessel wall were computed simultaneously. The PDF-based fast-marching was applied to 9 in vivo IVUS pullbacks of superficial femoral arteries and to a simulated IVUS pullback. Accurate results were obtained on simulated data with average point to point distances between detected vessel wall borders and ground truth <0.072 mm. On in vivo IVUS, a good overall performance was obtained with average distance between segmentation results and manually traced contours <0.16 mm. Moreover, the worst point to point variation between detected and manually traced contours stayed low with Hausdorff distances <0.40 mm, indicating a good performance in regions lacking information or containing artifacts. In conclusion, segmentation results demonstrated the potential of gray level PDF and fast-marching methods in 3-D IVUS image processing.

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.

On the potential of the Lagrangian speckle model estimator to characterize atherosclerotic plaques in endovascular elastography: in vitro experiments using an excised human carotid artery.

Endovascular ultrasound (US) elastography (EVE) was introduced to supplement endovascular US echograms in the assessment of vessel lesions and for endovascular therapy planning. Indeed, changes in the vascular tissue stiffness are characteristic of vessel wall pathologies and EVE appears as a very appropriate imaging technique to outline the elastic properties of vessel walls. Recently, a model-based approach was proposed to assess tissue motion in EVE. It specifically consists of a nonlinear minimization algorithm that was adapted to speckle motion estimation. Regarding the theoretical framework, such an approach considers the speckle as a material property; this assumption then led to the derivation of the optical flow equations, which were suitably combined with the Lagrangian speckle model estimator to provide the full 2-D polar strain tensor. In this study, the proposed algorithm was validated in vitro using a fresh excised human carotid artery. The experimental setup consisted of a cardiovascular imaging system (CVIS) US scanner, working with a 30-MHz mechanical rotating single-element transducer, a digital oscilloscope and a pressuring system. A sequence of radiofrequency (RF) images was collected while incrementally adjusting the intraluminal static pressure steps. The results showed the potential of this 2-D algorithm to characterize and to distinguish an atherosclerotic plaque from the normal vascular tissue. Namely, the geometry as well as some mechanical characteristics of the detected plaque were in good agreement with histology. The results also suggested that there might exist a range of intraluminal pressures for which plaque detectability is optimal.

Automatic 3D segmentation of intravascular ultrasound images using region and contour information.

Intravascular ultrasound (IVUS) produces images of arteries that show the lumen in addition to the layered structure of the vessel wall. A new automatic 3D IVUS fast-marching segmentation model is presented. The method is based on a combination of region and contour information, namely the gray level probability density functions (PDFs) of the vessel structures and the image gradient. Accurate results were obtained on in-vivo and simulated data with average point to point distances between detected vessel wall boundaries and validation contours below 0.105 mm. Moreover, Hausdorff distances (that represent the worst point to point variations) resulted in values below 0.344 mm, which demonstrate the potential of combining region and contour information in a fast-marching scheme for 3D automatic IVUS image processing.

Adapting the Lagrangian speckle model estimator for endovascular elastography: theory and validation with simulated radio-frequency data.

Intravascular ultrasound (IVUS) is known to be the reference tool for preoperative vessel lesion assessments and for endovascular therapy planning. Nevertheless, IVUS echograms only provide subjective information about vessel wall lesions. Since changes in the vascular tissue stiffness are characteristic of vessel pathologies, catheter-based endovascular ultrasound elastography (EVE) has been proposed in the literature as a method for outlining the elastic properties of vessel walls. In this paper, the Lagrangian Speckle Model Estimator (LSME) is formulated for investigations in EVE, i.e., using a polar coordinate system. The method was implemented through an adapted version of the Levenberg-Marquardt minimization algorithm, using the optical flow equations to compute the Jacobbian matrix. The theoretical framework was validated with simulated ultrasound rf data of mechanically complex vessel wall pathologies. The results, corroborated with Ansys finite element software, demonstrated the potential of EVE to provide useful information about the heterogeneous nature of atherosclerotic plaques.

Noninvasive vascular elastography: theoretical framework.

Changes in vessel wall elasticity may be indicative of vessel pathologies. It is known, for example, that the presence of plaque stiffens the vascular wall, and that the heterogeneity of its composition may lead to plaque rupture and thrombosis. Another domain of application where ultrasound elastography may be of interest is the study of vascular wall elasticity to predict the risk of aneurysmal tissue rupture. In this paper, this technology is introduced as an approach to noninvasively characterize superficial arteries. In such a case, a linear array ultrasound transducer is applied on the skin over the region of interest, and the arterial tissue is dilated by the normal cardiac pulsation. The elastograms, the equivalent elasticity images, are computed from the assessment of the vascular tissue motion. Investigating the forward problem, it is shown that motion parameters might be difficult to interpret; that is because tissue motion occurs radially within the vessel wall while the ultrasound beam propagates axially. As a consequence of that, the elastograms are subjected to hardening and softening artefacts, which are to be counteracted. In this paper, the Von Mises (VM) coefficient is proposed as a new parameter to circumvent such mechanical artefacts and to appropriately characterize the vessel wall. Regarding the motion assessment, the Lagrangian estimator was used; that is because it provides the full two-dimensional strain tensor necessary to compute the VM coefficient. The theoretical model was validated with biomechanical simulations of the vascular wall properties. The results allow believing in the potential of the method to differentiate hard plaques and lipid pools from normal vascular tissue. Potential in vivo implementation of noninvasive vascular elastography to characterize abdominal aneurysms and superficial arteries such as the femoral and the carotid is discussed.