Rjg Ruud van Sloun

Compressed sensing for ultrasound computed tomography.

Ultrasound computed tomography (UCT) allows the reconstruction of quantitative tissue characteristics, such as speed of sound, mass density, and attenuation. Lowering its acquisition time would be beneficial; however, this is fundamentally limited by the physical time of flight and the number of transmission events. In this letter, we propose a compressed sensing solution for UCT. The adopted measurement scheme is based on compressed acquisitions, with concurrent randomised transmissions in a circular array configuration. Reconstruction of the image is then obtained by combining the born iterative method and total variation minimization, thereby exploiting variation sparsity in the image domain. Evaluation using simulated UCT scattering measurements shows that the proposed transmission scheme performs better than uniform undersampling, and is able to reduce acquisition time by almost one order of magnitude, while maintaining high spatial resolution.

Cumulative phase delay imaging for contrast-enhanced ultrasound tomography.

Standard dynamic-contrast enhanced ultrasound (DCE-US) imaging detects and estimates ultrasound-contrast-agent (UCA) concentration based on the amplitude of the nonlinear (harmonic) components generated during ultrasound (US) propagation through UCAs. However, harmonic components generation is not specific to UCAs, as it also occurs for US propagating through tissue. Moreover, nonlinear artifacts affect standard DCE-US imaging, causing contrast to tissue ratio reduction, and resulting in possible misclassification of tissue and misinterpretation of UCA concentration. Furthermore, no contrast-specific modality exists for DCE-US tomography; in particular speed-of-sound changes due to UCAs are well within those caused by different tissue types. Recently, a new marker for UCAs has been introduced. A cumulative phase delay (CPD) between the second harmonic and fundamental component is in fact observable for US propagating through UCAs, and is absent in tissue. In this paper, tomographic US images based on CPD are for the first time presented and compared to speed-of-sound US tomography. Results show the applicability of this marker for contrast specific US imaging, with cumulative phase delay imaging (CPDI) showing superior capabilities in detecting and localizing UCA, as compared to speed-of-sound US tomography. Cavities (filled with UCA) which were down to 1u2009mm in diameter were clearly detectable. Moreover, CPDI is free of the above mentioned nonlinear artifacts. These results open important possibilities to DCE-US tomography, with potential applications to breast imaging for cancer localization.

Ultrasound coefficient of nonlinearity imaging

Imaging the acoustical coefficient of nonlinearity, β, is of interest in several healthcare interventional applications. It is an important feature that can be used for discriminating tissues. In this paper, we propose a nonlinearity characterization method with the goal of locally estimating the coefficient of nonlinearity. The proposed method is based on a 1-D solution of the nonlinear lossy Westerfelt equation, thereby deriving a local relation between β and the pressure wave field. Based on several assumptions, a β imaging method is then presented that is based on the ratio between the harmonic and fundamental fields, thereby reducing the effect of spatial amplitude variations of the speckle pattern. By testing the method on simulated ultrasound pressure fields and an in vitro B-mode ultrasound acquisition, we show that the designed algorithm is able to estimate the coefficient of nonlinearity, and that the tissue types of interest are well discriminable. The proposed imaging method provides a new approach to β estimation, not requiring a special measurement setup or transducer, that seems particularly promising for in vivo imaging.

Viscoelasticity Mapping by Identification of Local Shear Wave Dynamics

Estimation of soft tissue elasticity is of interest in several clinical applications. For instance, tumors and fibrotic lesions are notoriously stiff compared with benign tissue. A fully quantitative measure of lesion stiffness can be obtained by shear wave (SW) elastography. This method uses an acoustic radiation force to produce laterally propagating SWs that can be tracked to obtain the velocity, which in turn is related to Young’s modulus. However, not only elasticity, but also viscosity plays an important role in the propagation process of SWs. In fact, viscosity itself is a parameter of diagnostic value for the detection and characterization of malignant lesions. In this paper, we describe a new method that enables imaging viscosity from SW elastography by local model-based system identification. By testing the method on simulated data sets and performing in vitro experiments, we show that the ability of the proposed technique to generate parametric maps of the viscoelastic material properties from SW measurements, opening up new possibilities for noninvasive tissue characterization.

Towards Dynamic Contrast Specific Ultrasound Tomography

We report on the first study demonstrating the ability of a recently-developed, contrast-enhanced, ultrasound imaging method, referred to as cumulative phase delay imaging (CPDI), to image and quantify ultrasound contrast agent (UCA) kinetics. Unlike standard ultrasound tomography, which exploits changes in speed of sound and attenuation, CPDI is based on a marker specific to UCAs, thus enabling dynamic contrast-specific ultrasound tomography (DCS-UST). For breast imaging, DCS-UST will lead to a more practical, faster, and less operator-dependent imaging procedure compared to standard echo-contrast, while preserving accurate imaging of contrast kinetics. Moreover, a linear relation between CPD values and ultrasound second-harmonic intensity was measured (coefficient of determinationu2009=u20090.87). DCS-UST can find clinical applications as a diagnostic method for breast cancer localization, adding important features to multi-parametric ultrasound tomography of the breast.

Sparsity-driven super-localization in clinical contrast-enhanced ultrasound

Super-resolution (SR) ultrasound enables detailed assessment of the fine vascular network by pinpointing individual microbubbles (MBs), using ultrasound contrast agents (UCAs). The information in SR images is determined by the density of localized MBs and their localization accuracy. To obtain high densities, one can evaluate extremely sparse subsets of MBs across thousands of frames by using a very low MB dose and imaging for a very long time, which is impractical for clinical routine. While ultrafast imaging somewhat alleviates this problem, long acquisition times are still required to enhance the full vascular bed. As a result, localization accuracy remains hampered by patient motion. Recently, Sparsity-based Ultrasonic Super resolution Hemodynamic Imaging (SUSHI) achieved comparable spatial resolution with a sub-second temporal resolution. However, in the current implementation of SUSHI this temporal resolution was achieved using very high frame-rate, e.g. plane-wave imaging, which is not currently widely available in clinical scanners. The aim of this work is twofold. First, to attain a high MB localization accuracy on dense contrast-enhanced ultrasound (CEUS) data using a clinical dose of UCA and a widespread clinical scanner. Second, to retain a high resolution by motion compensation.

Effects of perfusion and vascular architecture on contrast dispersion: Validation in ex-vivo porcine liver under machine perfusion

Dynamic contrast enhanced ultrasound (DCE-US) enables imaging of cancer angiogenesis by quantification of perfusion and dispersion. Although increased perfusion may be found in areas of active angiogenesis due to increased demands for blood supply, decreased perfusion may be caused by the decreased efficiency and functionality, typical of cancer angiogenic microvasculature. Contrast dispersion, mainly determined by the flow profile in large vessels and by the multipath trajectories in the microvasculature, may thus represent a suitable alternative to characterize cancer angiogenesis. Based on a model of the contrast transport kinetics as a convective-dispersion process, several DCE-US methods have been proposed estimating dispersion for characterization of cancer angiogenic vasculature. Although dispersion imaging has shown promising in a clinical context, its physical link with variations in flow and vascular architecture has never been shown. The objective of this work is thus to investigate the influence of flow and underlying vascular architecture on the estimation of dispersion in an ex-vivo machine-perfused pig liver.

Contrast enhanced ultrasound tomography by means of the cumulative phase delay between second harmonic and fundamental component

Standard dynamic contrast-enhanced ultrasound (DCE-US) imaging detects and estimates ultrasound-contrastagent (UCA) concentration based on the amplitude of the harmonic components generated during ultrasound (US) propagation through UCAs. However, harmonic generation is not specific to UCAs, as it also occurs for US propagating through tissue. Moreover, nonlinear artifacts affect standard DCE-US imaging, causing contrast to tissue ratio (CTR) reduction, and resulting in possible tissue misclassification and misinterpretation of UCA concentrations. Especially for US tomography, no contrast-specific modality exists. Recently, a new marker for UCAs was discovered. A cumulative phase delay (CPD) between the second harmonic (2H) and fundamental (F0) component is in fact observable for US propagating through UCAs, and is absent in tissue. In this paper, CPD based tomographic US images are presented and compared to standard US tomography. By testing its applicability on gelatin phantoms with cylindrical cavities filled with a SonoVue® UCA dilution, we showed that CPD tomography outperforms standard US tomography by speed of sound and dispersion analysis, yielding a superior CTR: 32.2 dB vs. 0.06 and 4.6 dB, respectively.

Imaging the ultrasonic coefficient of nonlinearity: The impact of speed of sound variations

Imaging the acoustical coefficient of nonlinearity, β, is of interest in a number of healthcare interventional applications. For cardiac ablation therapy, detecting the protective fibrofatty layer between the atrial wall and the esophagus would provide risk assessment with respect to esophageal injury. The coefficient of nonlinearity β is an important feature that can be used for discriminating between tissues. Starting from a 1D inhomogeneous generalized lossy nonlinear Westervelt equation, we derived an analytical solution for β, which is then further adapted to work in echo mode. By considering the ratio between the harmonic and fundamental fields, the influence of spatial variations in backscatter coefficient and spatial absorption variations are mitigated. By testing the method on simulations that range from a 1D plane wave excitation with inhomogeneous nonlinearity and absorption, to a 3D linear array scan that also considers inhomogeneities in speed of sound, we show that the designed algorithm is able to estimate the coefficient of nonlinearity, and that the tissue types of interest are well discriminable. The proposed imaging method provides a new approach to β estimation, not requiring a special measurement setup or transducer, that seems particularly promising for in vivo imaging.

Cumulative phase delay imaging - A new contrast enhanced ultrasound modality

Recently, a new acoustic marker for ultrasound contrast agents (UCAs) has been introduced. A cumulative phase delay (CPD) between the second harmonic and fundamental pressure wave field components is in fact observable for ultrasound propagating through UCAs. This phenomenon is absent in the case of tissue nonlinearity and is dependent on insonating pressure and frequency, UCA concentration, and propagation path length through UCAs. In this paper, ultrasound images based on this marker are presented. The ULA-OP research platform, in combination with a LA332 linear array probe (Esaote, Firenze Italy), were used to image a gelatin phantom containing a PVC plate (used as a reflector) and a cylindrical cavity measuring 7u2005mm in diameter (placed in between the observation point and the PVC plate). The cavity contained a 240u2005µL/L SonoVueO® UCA concentration. Two insonating frequencies (3u2005MHz and 2.5u2005MHz) were used to scan the gelatine phantom. A mechanical index MI = 0.07, measured in water at the cavity locatio...