Parallel integral projection transform for straight electrode localization in 3-D ultrasound images.

In surgical practice, small metallic instruments are frequently used to perform various tasks inside the human body. We address the problem of their accurate localization in the tissue. Recent experiments using medical ultrasound have shown that this modality is suitable for real-time visualization of anatomical structures as well as the position of surgical instruments. We propose an image-processing algorithm that permits automatic estimation of the position of a line-segment-shaped object. This method was applied to the localization of a thin metallic electrode in biological tissue. We show that the electrode axis can be found through maximizing the parallel integral projection transform that is a form of the Radon transform. To accelerate this step, hierarchical mesh-grid algorithm is implemented. Once the axis position is known, localization of the electrode tip is performed. The method was tested on simulated images, on ultrasound images of a tissue mimicking phantom containing a metallic electrode, and on real ultrasound images from breast biopsy. The results indicate that the algorithm is robust with respect to variations in electrode position and speckle noise. Localization accuracy is of the order of hundreds of micrometers and is comparable to the ultrasound system axial resolution.

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.

Experimental three dimensional strain estimation from ultrasonic sectorial data.

Most of the studies devoted to elastography are focused on the estimation of the axial component of the strain. However when subjected to any load, whatever the direction, soft biological media deform in the three spatial dimensions. The aim of our work is to build a three dimensional strain mapping from data acquired with a 3D clinical sectorial probe. The estimation of radial strain is based on the estimation of local scaling factors. A method of cross-correlation of interpolated signals between adjacent radiofrequency lines was used to estimate the angular displacement and strain. For the sectorial strain estimation, the same displacement estimation technique has been implemented. The method has been tested on experimental data acquired on calibrated phantoms and compared to simulation.

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.

On the effect of lung filtering and cardiac pressure on the standard properties of ultrasound contrast agent.

The goal standard of contrast echocardiography is the absolute measure of myocardial perfusion using a contrast agent. Actually, several contrast agents are developed. All these agents show left ventricular opacification after intravenous injection. However, none of these agents shows an acceptable enhancement of the myocardium yet using conventional imaging techniques. The explanation of this phenomenon should be easy by measuring the acoustic characteristics of the contrast agent and then making a comparison of these characteristics with those of the myocardium. In this study we present definitions of standard acoustic parameters of ultrasound contrast agent, the backscatter coefficient Bs and the scattering-to-attenuation ratio STAR. Afterwards, considering an intravenous injection of the contrast agent, and taking into account the effects of lung filtering and cardiac pressure, the standard properties of contrast agents are determined in different sites: right ventricle (before lung passage), left ventricle (after lung passage and taking into account the pressure effect) and in the coronary system. Calculations showed that the acoustic properties are considerably influenced by these two effects: lung filtering and cardiac pressure. Comparison of these properties with the tissue properties (myocardium) is then performed. This determines the contribution of the contrast agent to the enhancement of the tissue visualization. The simulations are performed on Albunex microspheres. The results reveal that the difference between scattering of the myocardium and scattering of intravenously injected Albunex is too slight to be visible on an echographic image.

Standard properties of ultrasound contrast agents.

A standardization procedure for in vitro acoustic characterization of ultrasound contrast agents is presented. One new acoustic parameter for particular importance is retained: This is STAR, scattering-to-attenuation ratio, for quantification of the effectiveness of the contrast agent. The STAR expresses the ability of the contrast agent to enhance the visualization of the tissue containing the contrast agent and, at the same time, represents the degree of its absorption. So, it is desirable to produce a contrast agent with high STAR, having good scattering properties to improve the image visualization, and low attenuation to image the underlying biological structures and to avoid shadowing. In this study, we present methods for calculations and measurements of the STAR and comparison between different contrast agents.

Ultrasound contrast agent in intravascular echography: an in vitro study.

The intravascular ultrasound image of the intraluminal contour depends on the difference between acoustic impedances of the media which create the endoluminal interface. There are several limitations to the visualization and detection of this interface. These limitations are due to artifacts encountered during image formation and to anatomical complexity. The purpose of this study is to obtain intraluminal contour enhancement using ultrasound contrast agent (UCA). Therefore, our objective was to address the feasibility of this technique by documenting the following: (i) the acoustic properties of UCA at 30 MHz; (ii) in vitro experimentation with tube or postnecrotic artery; and (iii) suitable digital processing. The images obtained with UCA (enhanced image quality) and subtracted from those without UCA provided, after simple digital processing, accurate visualization of the arterial lumen. The image obtained exhibits an even, high-contrast intraluminal edge. Such characteristics facilitate contour extraction by the automated contour detection procedures.

Use of an analytic signal to model interaction between an acoustic wave and a moving target with a time-dependent velocity.

The formalism of the analytic signal and its complex envelope is used to model the interaction between an acoustic wave and a moving target in applications involving the Doppler effect. Specifically, it is shown that modeling by means of analytic signal and related concepts is suitable for the interaction between a probing acoustic wave and the scatters included in the investigated medium through which the wave propagates. When such a scatterer is a moving target, the interaction can be viewed as an angular modulation: phase modulation or frequency modulation, depending on which parameter is used as the carried message. The act of demodulating by means of a two-channel lock-in amplifier follows this theoretical modeling closely because this device simply extracts the complex envelope from the modulated signal. In the case of a target with time-dependent velocity, this modeling is very useful because the experimental results are actually the instantaneous phase and the instantaneous frequency of the complex envelope associated with the modulated signal. In this sense the physical phenomenon of interaction between an acoustical wave and a moving target (Doppler effect) can be depicted easily and clearly. Experimental results obtained with a continuous probing wave interacting with a single bubble moving in a fluid at rest are given to illustrate the proposed approach.