Guillaume Zahnd

Measurement of Two-Dimensional Movement Parameters of the Carotid Artery Wall for Early Detection of Arteriosclerosis: A Preliminary Clinical Study

The aim of this study was to clinically investigate the association between the risk factors of early-stage atherosclerosis and the two-dimensional (2-D) movement of the artery wall. To meet this objective, a speckle tracking approach for the estimation of the 2-D trajectory of the vessel wall was proposed and applied to B-mode ultrasound (US) sequences of the left common carotid artery (CCA). A deformable skeleton model was also introduced in the block matching scheme. Finally, the 2-D movements of both proximal and distal walls were investigated in three different local regions, with 1.5 × 0.3 mm(2) kernel blocks. A clinical study was conducted in which two different populations (26 young healthy volunteers and 26 older diabetic patients) were studied. The results show that the mean amplitude value of the diameter change ΔD, of the longitudinal displacement of the proximal wall ΔX(p) and of the longitudinal displacement of the distal wall ΔX(d) were 0.65 ± 0.17 vs. 0.41 ± 0.12 mm (p < 0.001), 0.48 ± 0.21 vs. 0.26 ± 0.18 mm (p < 0.001) and 0.48 ± 0.20 vs. 0.35 ± 0.23 mm (p = 0.006) for the young healthy volunteers and the older diabetic patients, respectively. The results of the three dynamic parameters ΔD, ΔX(p) and ΔX(d) were systematically and significantly lower for the diabetic subjects, respectively 37%, 46% and 27%. The method introduced in this feasibility study might constitute a pertinent approach to assess the presence of early-stage arteriosclerosis by the noninvasive estimation of the 2-D motion of the intima-media complex in the CCA.

Longitudinal displacement of the carotid wall and cardiovascular risk factors: associations with aging, adiposity, blood pressure and periodontal disease independent of cross-sectional distensibility and intima-media thickness

The recently discovered longitudinal displacement of the common carotid arterial wall (i.e., the motion along the same plane as the blood flow), may be associated with incident cardiovascular events and represents a novel and relevant clinical information. At present, there have only been a few studies that have been conducted to investigate this longitudinal movement. We propose here a method to assess noninvasively the wall bi-dimensional (two-dimensional [2-D], cross-sectional and longitudinal) motion and present an original approach that combines a robust speckle tracking scheme to guidance by minimal path contours segmentation. Our method is well suited to large clinical population studies as it does not necessitate strong imaging prerequisites. The aim of this study is to describe the association between the longitudinal displacement of the carotid arterial wall and cardiovascular risk factors, among which periodontal disease. Some 126 Indigenous Australians with periodontal disease, an emerging risk factor, and 27 healthy age- and sex-matched non-indigenous control subjects had high-resolution ultrasound scans of the common carotid artery. Carotid intima-media thickness and arterial wall 2-D motion were then assessed using our method in ultrasound B-mode sequences. Carotid longitudinal displacement was markedly lower in the periodontal disease group than the control group (geometric mean (IQR): 0.15 mm (0.13) vs. 0.42 mm (0.30), respectively; p < 0.0001), independent of cardiovascular risk factors, cross-sectional distensibility and carotid intima-media thickness (p < 0.0001). A multivariable model indicated that the strongest correlates of carotid longitudinal displacement in adults with periodontal disease were age (β-coefficient = -.235, p = .03), waist (β-coefficient = -.357, p = 0.001), and pulse pressure (β-coefficient = .175, p = 0.07), independent of other cardiovascular risk factors, cross-sectional distensibility and pulse wave velocity. Carotid longitudinal displacement, estimated with our approach, is impaired in the periodontal disease group, independent of established cardiovascular risk factors and other noninvasive measures of arterial stiffness, and may represent an important marker of cardiovascular risk.

Evaluation of a Kalman-based block matching method to assess the bi-dimensional motion of the carotid artery wall in B-mode ultrasound sequences

We aim at investigating arterial diseases at early stage, by assessing the longitudinal (i.e. in the same direction as the blood flow) motion of the intima-media complex. This recently evidenced phenomenon has been shown to provide relevant and complementary information about vascular health. Our method assesses the longitudinal and radial motion from clinical in vivo B-mode ultrasound sequences. To estimate the trajectory of a selected point during the cardiac cycle, we introduce a block matching method that involves a temporal update of the reference block using a pixel-wise Kalman filter. The filter uses the initial gray-level of the pixel as control signal to avoid divergence due to cumulating errors. The block and search-window sizes are adapted to the tissue of interest. The method was evaluated on image sequences of the common carotid artery, acquired in 57 healthy volunteers and in 25 patients at high cardiovascular risk. Reference trajectories were generated for each sequence by averaging the tracings performed by three observers. Six different computerized techniques were also compared to our method. With a pixel size of 30 μm, the average absolute motion estimation errors were 84 ± 107 μm and 20 ± 19 μm for the longitudinal and radial directions, respectively. This accuracy was of the same order of magnitude as the inter- and intra-observers variability, and smaller than for the other methods. The estimated longitudinal motion amplitude was significantly reduced in at-risk patients compared with healthy volunteers (408 ± 281 μm vs. 643 ± 274 μm, p<0.0001). Our method can constitute a reliable and time-saving technique to investigate the arterial stiffness in clinical studies, in the objective to detect early-stage atherosclerosis.

Progressive attenuation of the longitudinal kinetics in the common carotid artery: preliminary in vivo assessment.

Longitudinal kinetics (LOKI) of the arterial wall consists of the shearing motion of the intima-media complex over the adventitia layer in the direction parallel to the blood flow during the cardiac cycle. The aim of this study was to investigate the local variability of LOKI amplitude along the length of the vessel. By use of a previously validated motion-estimation framework, 35 in vivo longitudinal B-mode ultrasound cine loops of healthy common carotid arteries were analyzed. Results indicated that LOKI amplitude is progressively attenuated along the length of the artery, as it is larger in regions located on the proximal side of the image (i.e., toward the heart) and smaller in regions located on the distal side of the image (i.e., toward the head), with an average attenuation coefficient of -2.5 ± 2.0%/mm. Reported for the first time in this study, this phenomenon is likely to be of great importance in improving understanding of atherosclerosis mechanisms, and has the potential to be a novel index of arterial stiffness.

Minimal-path contours combined with speckle tracking to estimate 2D displacements of the carotid artery wall in B-mode imaging

We present here an enhancement of our Contour and Speckle Tracking (CST) method, dedicated to assess the bi-dimensional (radial and longitudinal) movement of the common carotid artery walls in ultrasound B-mode sequences, in an attempt to detect early-stage atherosclerosis. CST is based on a speckle tracking approach guided by contour segmentation. We introduce a convolution operator dedicated to find the interfaces of the intima-media complex using a minimal-path approach, and we propose a block matching framework with a simple update scheme. Our method has been evaluated on 12 sequences of healthy carotid arteries, versus manually traced reference contours and 2D trajectories. A classical block matching speckle tracking (BMST) algorithm has also been applied for comparison. The accuracy of the segmentation was 30 ± 17 μm and 29 ± 21 μm for the lumen-intima and the media-adventitia interfaces, respectively. The mean longitudinal and radial dispersion error was 36 ± 24 μm and 14 ± 8 μm vs 103 ± 102 μm and 60 ± 33 μm, for CST and BMST, respectively.

Real time US-tagging combined with phase-based optical flow applied to 2D motion estimation of the carotid artery wall

Early detection of cardiovascular diseases (CVD) is possible by assessing the dynamic properties of the common carotid artery, i.e. the 2D (radial and longitudinal) motion of the wall layers. B-mode imaging is widely used, however this modality does not provide a high definition in the longitudinal direction (i.e. parallel to the probe) due to the aperture problem and the homogeneity of the tissues. To increase the definition in the longitudinal direction, we present here a specific beamforming method dedicated to estimate the 2D motion of the carotid artery wall. The generated US-tagged images present radiofrequency oscillations in both directions. The 2D motion is assessed with a specific phase-based optical flow method. The method was implemented in real time on a research echograph and applied in vivo on a healthy carotid over 3 cardiac cycles. The results show that our method is more accurate compared to classical block matching on B-mode images. Index Terms- Carotid Artery, US-Tagging, Phased-Based Optical Flow Matching, Motion Estimation, Block Matching, Real Time, Beamforming, RF2D.

Intravascular Imaging and Computer Assisted Stenting, and Large-Scale Annotation of Biomedical Data and Expert Label Synthesis

In this work we propose a technique to automatically estimate circular cross-sections of the vessels in CT scans. First, a circular contour is extracted for each slice of the CT by using the Hough transform. Afterward, the locations of the circles are optimized by means of a parametric snake model, and those circles which best fit the contours of the vessels are selected by applying a robust quality criterion. Finally, this collection of circles is used to estimate the local probability density functions of the image intensity inside and outside the vessels. We present a large variety of experiments on CT scans which show the reliability of the proposed method.

Semi-automated Quantification of Fibrous Cap Thickness in Intracoronary Optical Coherence Tomography

Acute coronary syndrome represents a leading cause of death. Events are triggered by rupture of atheromatic plaques, as a result of disruption of the overlying fibrous cap. Pathological studies have shown that cap thickness is a critical component of plaque stability. Therefore, assessment of fibrous cap thickness could be a valuable tool for estimating the risk of future events. To aid preoperative planning and peri-operative decision making, intracoronary optical coherence tomography imaging can provide very detailed information about arterial wall structure. However, manual interpretation of the images is laborious, subject to variability, and therefore not always sufficiently reliable for immediate decision of treatment. We present a novel semi-automatic computerized interventional imaging tool to quantify coronary fibrous cap thickness in optical coherence tomography. The most challenging issue when estimating cap thickness is caused by the diffuse nature of the anatomical abluminal interface to be detected. Our method can successfully extract the fibrous cap contours using a robust dynamic programming framework based on a geometrical a priori. Validated on a dataset of 90 images from 11 patients, our method provided a good agreement for minimum cap thickness with the reference tracings performed by a medical expert (35.7 ±33.3 μm, R=.68) and was similar to inter-observer reproducibility (35.2 ±33.1 μm, R=.66), while being significantly faster and fully reproducible. This tool demonstrated promising performances and could potentially be used for online identification of high risk-plaques.

Real-time specific beamforming applied to motion trajectory estimation in ultrasound imaging

In this paper, a motion trajectory estimation method with specific ultrasound RF images is proposed. These specific RF images with axial and lateral oscillations are obtained using an unconventional beamforming technique. The first aim of this paper is to present an experimental setup of real-time implementation of this beamforming technique providing phase images sensitive to subpixel motions. The second aim of the paper is to introduce a method of trajectory estimation. The proposed multi-frame motion estimation method uses a spatio-temporal prediction technique and an analytic estimator of 2-D spatial shifts dedicated to phase images.

Contour segmentation of the intima, media, and adventitia layers in intracoronary OCT images application to fully automatic detection of healthy wall regions

PurposeQuantitative and automatic analysis of intracoronary optical coherence tomography images is useful and time-saving to assess cardiovascular risk in the clinical arena.MethodsFirst, the interfaces of the intima, media, and adventitia layers are segmented, by means of an original front propagation scheme, running in a 4D multi-parametric space, to simultaneously extract three non-crossing contours in the initial cross-sectional image. Second, information resulting from the tentative contours is exploited by a machine learning approach to identify healthy and diseased regions of the arterial wall. The framework is fully automatic.ResultsThe method was applied to 40 patients from two different medical centers. The framework was trained on 140 images and validated on 260 other images. For the contour segmentation method, the average segmentation errors were