Experimental assessment of four ultrasound scattering models for characterizing concentrated tissue-mimicking phantoms

International audienceTissue-mimicking phantoms with high scatterer concentrations were examined using quantitative ultrasound techniques based on four scattering models: the Gaussian Model (GM), the Faran Model (FM), the Structure Factor Model (SFM) and the Particle Model (PM). Experiments were conducted using 10- and 17.5-MHz focused transducers on tissue-mimicking phantoms with scatterer concentrations ranging from 1 to 25%. Theoretical BSCs were first compared with the experimentally measured BSCs in the forward problem framework. The measured BSC versus scatterer concentration relationship was predicted satisfactorily by the SFM and the PM. The FM and the PM overestimated the BSC magnitude at actual concentrations greater than 2.5% and 10%, respectively. The SFM was the model that better matched the BSC magnitude at all the scatterer concentrations tested. Secondly, the four scattering models were compared in the inverse problem framework to estimate the scatterer size and concentration from the experimentally measured BSCs. The FM did not predict the concentration accurately at actual concentrations greater than 12.5%. The SFM and PM need to be associated with another quantitative parameter to differentiate between low and high concentrations. In that case, the SFM predicted the concentration satisfactorily with relative errors below 38% at actual concentrations ranging from 10 to 25%

Assessment of accuracy of the structure-factor-size-estimator method in determining red blood cell aggregate size from ultrasound spectrum backscattering coefficient

International audienceA computer simulation algorithm suitable for generating non-overlapping, isotropic and fairly identical red blood cell (RBC) clusters is presented. RBCs were stacked following the hexagonal close packing (HCP) structure to form a compact spherical aggregate. Such an aggregate was repeated and placed randomly under non-overlapping condition in three-dimensional space to mimic an aggregated blood sample. Backscattering coefficients (BSCs) were computed for samples at various cluster sizes and different hematocrits showing BSC increases with mean aggregate sizes. The accuracy of the structure factor size estimator (SFSE) method in determining mean aggregate size and packing factor were also examined. A good correlation (R2 ≥ 0.94) between the mean size of aggregates predicted by the SFSE and true size was found for each hematocrit. This study shows that for spherical aggregates there exists a region for each hematocrit where SFSE works most accurately. Typically, error of SFSE in estimating mean cluster size was < 20% for dimensions between 14-17 µm at 40% hematocrit. This study suggests that the theoretical framework of SFSE is valid under the assumption of isotropic aggregates

Ultrasound characterization of red blood cells distribution: a wave scattering simulation study

International audienceUltrasonic backscattered signals from blood contain frequency-dependent information that can be used to obtain quantitative parameters describing the aggregation state of red blood cells (RBCs). However the relation between the parameters describing the aggregation level and the backscatterer coefficient needs to be better clarified. For that purpose, numerical wave simulations were performed to generate backscattered signals that mimic the response of two-dimensional (2D) RBC distributions to an ultrasound excitation. The simulated signals were computed with a time-domain method that has the advantages of requiring no physical approximations (within the framework of linear acoustics) and of limiting the numerical artefacts induced by the discretization of object interfaces. In the simple case of disaggregated RBCs, the relationship between the backscatter amplitude and scatterer concentration was studied. Backscatter coefficients (BSC) in the frequency range 10 to 20 MHz were calculated for weak scattering infinite cylinders (radius 2.8 $\mu$m) at concentrations ranging from 6 to 36$\%$. At low concentration, the BSC increased with scatterer concentrations; at higher concentrations, the BSC reached a maximum and then decreased with increasing concentration, as it was noted by previous authors in \emph{in vitro} blood experiments. In the case of aggregated RBCs, the relationship between the backscatter frequency dependence and level of aggregation at a concentration of 24$\%$ was studied for a larger frequency band (10 - 50 MHz). All these results were compared with a weak scattering model based on the analytical computing of the structure factor

Modèle de facteur de structure pour l’estimation ultrasonore des structures cellulaires

Les techniques quantitatives ultrasonores pour l'estimation des microstructures tissulaires sont basées sur l'analyse fréquentielle des signaux rétrodiffusés par les tissus. Ces techniques ont pour objectif de différencier des tissus sains et pathologiques, de détecter des cancers ou de faire le suivi de la réponse à un traitement. Je me suis principalement intéressée à l'estimation ultrasonore des microstructures pour deux applications principales : le sang et les tumeurs. Les deux applications ont en commun une modélisation acoustique des milieux en terme de fluides hétérogènes, qu'il s'agisse d'écoulements chargés (suspensions d'agrégats de globules rouges) ou des tissus biologiques (les tissus mous sont assimilés à des fluides aux fréquences ultrasonores).La première partie de l’exposé porte sur la caractérisation ultrasonore de l'agrégation érythrocytaire pour la détection ou le suivi de l'hyperagrégation associée à des pathologies impliquant des désordres rhéologiques sanguins. Les difficultés majeures pour modéliser la rétrodiffusion par le sang est de considérer une forte concentration de particules (40%) et des agrégats de particules. Nous présentons notre contribution, en particulier la proposition d'une théorie de milieu effectif combinée au modèle de facteur de structure. Nous comparons la précision de cette théorie avec des modèles classiques pour estimer la structure des agrégats sur la base de simulations numériques (où tous les paramètres sont contrôlés) et sur des données expérimentales de sang cisaillé en écoulement.La deuxième partie de l’exposé porte sur la compréhension de la rétrodiffusion ultrasonore par les tumeurs. Les tumeurs cancéreuses résultent souvent d'une prolifération de cellules anormales formant une masse qui grossit. Les modèles classiques de diffusion ultrasonore utilisés par les techniques d'estimation des structures cellulaires supposent que les diffuseurs sont distribués de façon aléatoire (i.e. des milieux dilués), alors que les tumeurs sont des milieux à fortes concentrations cellulaires. L’enjeu scientifique est de répondre aux questions suivantes : quels sont les modèles de diffusion adaptés à un amas dense de cellules? Quelles sont les principales structures responsables de la diffusion ultrasonore au sein des tumeurs? Les problèmes que nous étudions pour l'agrégation érythrocytaire peuvent donc aider à mieux comprendre la diffusion ultrasonore par les tumeurs cancéreuses. Nous comparons différents modèles de diffusion et discutons de leurs limites à travers des expériences sur des biofantômes de cellules modélisant des tumeurs cancéreuses

Comparison of three scattering models for ultrasound blood characterization

International audienceUltrasonic backscattered signals from blood contain frequency-dependent information that can be used to obtain quantitative parameters reflecting the aggregation level of red blood cells (RBCs). The approach consists in estimating structural aggregate parameters by fitting the spectrum of the backscattered radio-frequency echoes from blood to an estimated spectrum considering a theoretical scattering model. In this study, three scattering models were examined: a new implementation of the Gaussian Model (GM), the Structure Factor Size Estimator (SFSE) and the new Effective Medium Theory combined with the Structure Factor Model (EMTSFM). The accuracy of the three scattering models in determining mean aggregate size and compactness was compared by two- and three-dimensional (2D and 3D) computer simulations where RBC structural parameters are controlled. Two clustering conditions were studied: (1) when the aggregate size varied and the aggregate compactness was fixed in both 2D and 3D cases, and (2) when the aggregate size was fixed and the aggregate compactness varied in the 2D case. For both clustering conditions, the EMTSFM was found more suitable than the GM and SFSE for characterizing RBC aggregation

Non-destructive diagnosis of the integrity of green wood using ultrasonic computed tomography

International audienceUltrasonic computed tomography (UCT) was used to assess the integrity of green wood. Woods are heterogeneous air-coupled, orthorhombic materials. Because of the difference in acoustic impedance between the material and the surrounding medium (water or coupling gel), the ultrasonic wave propagation is greatly perturbed by physical processes such as the refraction, attenuation and scattering of the waves. UCT belongs to the inverse scattering class of problems and the aim of this study was therefore to present a strategy for simply, efficiently and accurately developing a reconstruction algorithm. UCT is based on several assumptions, such as the presence of a low contrast medium (the biological medium), a large frequency range (a broadband pulse), and dense and complete sets of projections. If these conditions are ideally fulfilled, we can reconstruct images of the impedance of the medium. This technique involves an algorithm based on first-order Born approximation methods. To date we have tested the qualitative aspects of this imaging technique and part of the quantitative aspects by performing numerical simulations and on our tomographic testing ground. The latter is composed of a base supporting a rotating and translating mechanical structure that can hold several transducers, and a numerical system and computing and recording the projections. When performing non-destructive assessments on wood, we would have liked to obtain quantitative images related to acoustic parameter of wood (impedance, speed of sound, attenuation). However, since the problem is non-linear, a low-frequency method involving signal and image processing was used

05-2005 Newsletter

Minnesota State University, Mankato, Library Services Newsletter for May 2005

A 2-D anatomic breast ductal computer phantom for ultrasonic imaging

International audienceMost breast cancers (85%) originate from the epithelium and develop first in the ductolobular structures. In screening procedures, the mammary epithelium should therefore be investigated first by performing of an anatomically guided examination. For this purpose (mass screening, surgical guidance), we developed a two-dimensional anatomic phantom corresponding to an axial cross-section of the ductolobular structures, which makes it possible to better understand the interactions between the breast composition and ultrasound. The various constitutive tissues were modeled as a random inhomogeneous continuum with density and sound speed fluctuations. Ultrasonic pulse propagation through the breast computer phantom was simulated using a finite element time domain method (the phantom can be used with other propagation codes). The simulated Ductal Echographic image is compared with the Ductal Tomographic (DT) reconstruction. The preliminary results obtained show that the DT method is more satisfactory in terms of both the contrast and the resolution

Soft tissue absorption tomography with correction for scattering aberrations.

International audienceAmong the many factors involved in ultrasound attenuation phenomena, scattering effects play a major role, even in the unexpected case of soft tissues. It is proposed in this study to quantitatively evaluate the scattering affecting the measurements, before reconstructing the absorption parameter alone. The reconstruction procedure involves three steps: i/ estimating the sound speed map using a transmission tomography algorithm. This estimation procedure provides a numerical phantom of the organ probed, cleared of all dissipative components. This absorption free phantom mimics the (viscoacoustic) tissues imaged except for the density and absorption characteristics: the density a priori equals 1000 kg/m3, and the absorption is not taken into account. The impedance fluctuations in the object are therefore approximated on the basis of the sound speed contrast; ii/ synthesing the field scattered by the absorption free phantom; the attenuation observed here results solely from the scattering phenomenon. The synthesis is carried out using a finite-element time domain code simulating the ultrasonic propagation through the phantom. It provides the scattering distortion reference introduced into the log spectral absorption estimator; iii/ reducing the scattering distortions affecting the integrated absorption measured along the ray paths using a log spectral procedure. The corrected integrated absorption is then processed using a tomographic reconstruction procedure that provides an estimate of the absorption distribution. Simple numerical simulations show the improvement obtained in the absorption estimates with this approach

Coherent and incoherent ultrasound backscatter from cell aggregates

International audienceThe Effective Medium Theory (EMT) combined with the Structure Factor Model was recently developed to model the ultrasound backscatter from aggregating Red Blood Cells (RBCs) [Frances-chini, Metzger, Cloutier, IEEE UFFC, 2011]. The EMT assumes that aggregates can be treated as homogeneous effective spheres and the structure factor considers the interactions between the effective spheres. In this study, the EMT is further developed to decompose the differential backscattering cross section of a single cell aggregate into coherent and incoherent components. The coherent component corresponds to the average backscatter from the effective scatterer, and the incoherent component considers the fluctuation of the scattering wave around its average within the effective scatterer. A new theoretical expression for the incoherent component based on the structure factor is proposed and compared with another formulation based on the Gaussian direct correlation function. This theoretical improvement is assessed using computer simulations of ultrasound backscatter from aggregating cells. The consideration of the incoherent component based on the structure factor allows to approximate the simulations satisfactorily for a krag limit around 2, against a krag limit comprised between 1.07 and 1.47 with the former model considering only the coherent component