Improved boundary segmentation of skin lesions in high-frequency 3D ultrasound.

In this article, we propose a segmentation algorithm for skin lesions in 3D high-frequency ultrasound images. The segmentation is done on melanoma and Basal-cell carcinoma tumors, the most common skin cancer types, and could be used for diagnosis and surgical excision planning in a clinical context. Compared with previously proposed algorithms, which tend to underestimate the size of the lesion, we propose two new boundary terms which provide significant improvements of the accuracy. The first is a probabilistic boundary expansion (PBE) term designed to broaden the segmented area at the boundaries, which uses the feature asymmetry criterion. The second is a curvature dependent regularization (CDR), which aims at overcoming the tendency of standard regularization to shrink segmented areas. On a clinical dataset of 12 patients annotated by a dermatologist, the proposed algorithm has a comparable Dice index but increases the sensitivity by 26%. The proposed algorithm improves the sensitivity for all lesions, and the obtained sensitivity is close to that of the intra-observer variability.

Transverse oscillations for tissue motion estimation.

This paper gives an overview of the methods developed for tissue motion estimation using transverse oscillation images (TO). TO images are specific radiofrequency ultrasound images featuring oscillations in both spatial directions. The initial studies on TO were published in the late 1990s. This paper reviews the main ideas and applications behind this motion estimation approach. First the origin and motivation of TO is briefly reviewed. Then the beamforming methods that lead to TO images are given, detailing the receive-only approach and the transmit-and-receive approach using synthetic aperture data. The different medical applications where TO has been used are discussed (blood flow, elastography and echocardiography), showing how it can improve motion estimation. Finally, the future perspectives of TO are outlined.

3D dynamical ultrasonic model of pulsating vessel walls.

The aim of this work is to introduce a novel 3-D model of pulsating vessels, through which the dynamic acoustic response of arterial regions can be predicted. Blood flow is numerically simulated by considering the fluid-dynamic displacements of the scatterers (erythrocytes), while a mechanical model calculates the wall displacement due to fluid pressure. The acoustic characteristics of each region are simulated through the FIELD software. Two numerical phantoms of a carotid artery surrounded by elastic tissue have been developed to illustrate the model. One of them includes a plaque involving a 50% stenosis. B-mode and M-mode images are produced and segmented to obtain the wall displacement profile. A cylindrical holed phantom made of cryogel mimicking material has been constructed for the model validation. In pulsatile flow conditions, fluid and wall displacements have been measured by Doppler ultrasound methods and quantitatively compared to simulated M-mode images, showing a fairly good agreement.

Axial strain imaging of intravascular data: results on polyvinyl alcohol cryogel phantoms and carotid artery.

Mapping the local elastic properties of an atherosclerotic artery is of major interest for predicting the disease evolution or an intervention outcome. These properties can be investigated by elastography, which estimates the strain distribution within a medium in response to a stress. But because diseased arteries are highly heterogeneous, a small global deformation may result in high local strains in the softest regions. For those reasons, we use in this paper the strain estimation method we recently developed to compute elastograms of original vessel-mimicking cryogel phantoms and a fresh excised human carotid artery. This adaptive method has been effectively proved to be accurate in a wider range of strains (0-7%) than commonly used gradient-based methods, and very adapted for investigating highly heterogeneous tissues. Resulting elastograms cover a wider range of strains (0-3.5%) than all previously reported intravascular elastograms, improving the discrimination between healthy and diseased regions.

Axial strain imaging using a local estimation of the scaling factor from RF ultrasound signals.

The main signal-processing techniques used in elastography compute strains as the displacement derivative. They perform well for very low deformations, but suffer rapidly from decorrelation noise. Aiming to increase the range of accurate strain measurements, we developed an adaptive method based on the estimation of strains as local scaling factors. Its adaptability makes this method appropriate for computing scaling factors resulting from larger strains or a wide spread of strain variations. First, segments corresponding to the same part of tissue are adaptively selected in the rest and stressed state echo signals. Then, local scaling factors are estimated by iteratively varying their values until reaching the zero of the phase of the complex cross-correlation function. Results from simulation and from experimental data are presented. They show how this adaptive method can track various local deformations and its accuracy for strain up to 7%.

Modeling geometric artefacts in intravascular ultrasound imaging.

A model has been developed for estimating the geometric distortions in intravascular ultrasound (IVUS) imaging caused by the position of the ultrasound catheter within the artery. Geometric distortion causes degradation on cross-sectional images of the vessel wall where, for characteristic positioning of the transducer within the vessel, a circular artery is seen on IVUS images as a noncircular vessel represented by more or less complex shapes. Artefacts, therefore, have a clinical impact on the accuracy of qualitative and quantitative intravascular analyses. The main distortions are due to the inclination and the off-centered position of the transducer within the vessel. These effects are increased by two factors: first, the point of origin of the ultrasound beam does not coincide with the rotation axis of the catheter; second, in the case of a mechanical rotating transducer, the ultrasound beam is not perpendicular to the long axis of the catheter, but has an inclination such that the transducer looks forward from the emitting point. All these parameters are taken into account in the three-dimensional (3-D) geometric model developed in this paper. The model was formulated to predict the geometric deformation for artery contour of various shapes and can model artefacts during stent implantation (Finet et al. 1998). Simulations were made for various geometric configurations and compared to in vitro and in vivo IVUS images. The model results are consistent with the experimental results. Finally, the model was used for estimating the values of the geometric parameters that cause distortions on ultrasonic images.

Artifacts in intravascular ultrasound imaging during coronary artery stent implantation.

Intravascular ultrasound imaging is able to provide direct images of the stent meshwork. However, a paradoxical question remains unanswered: Why is it not possible to correct or prevent implantation defects by ultrasound-guided implantation? We postulate that these discrepancies are due to image artifacts. We performed an in vitro experiment allowing detection, physical characterization, and computerized simulations of the various aspects of these artifacts. The width of the echo of a strut is variable, dependent on its distance from the transducer. The stent strut echo orientation is variable, and depends on the position of the transducer inside the stent. The stent contour image depends on the position of the transducer. In conclusion, knowledge of these stent intravascular ultrasound image artifacts enabled us to discriminate accurately between artifacts and real stent implantation defects, and are indispensable for accurate qualitative and quantitative analyses of stents.