Fluence Threshold for Photothermal Bubble Generation Using Plasmonic Nanoparticles

International audienceUnder nano- to femtosecond pulsed illumination at their plasmonic resonance wavelength, metalnanoparticles efficiently absorb the incident light energy that is subsequently converted into heat. In aliquid environment, with sufficiently high pulse fluences (light energy per unit area), this heat generationmay result in the local formation of a transient nanobubble. This phenomenon has been the subjectof a decade of investigations and is at the basis of numerous applications from cancer therapy to photoacouticimaging. The aim of this article is to clarify the question of the fluence threshold requiredfor bubble formation. Using a Runge-Kutta-4 numerical algorithm modeling the heat diffusion arounda spherical gold nanoparticle, we numerically investigate the influence of the nanoparticle diameter,pulse duration (from the femto- to the nanosecond range), wavelength and Kapitza resistivity in orderto explain the observations reported in the literature

Pulsed bi-frequency method for characterization of microbubbles in the context of decompression sickness

International audienceDuring hyperbaric decompression, the absolute ambient pressure is reducing; microbubbles may be generated from pre-existing gas nuclei. An accurate monitoring of the size and of the density of the bubble population will provide a valuable means to understand the nucleation and growth processes in tissues. In this aim, an ultrasonic characterization method based on a dual frequency technique applied on a single bubble is tested. The method consists in sending two ultrasonic waves on a stationary bubble. One is a low frequency wave (30 kHz\leqflf\leq 60 kHz), which excites the bubble near its resonance frequency and the other is a high frequency (fhf=1MHz) wave that measures the changes in the acoustic cross-section induced by the low frequency activation. The resonance frequency, directly related to the radius, can be detected by looking at the spectrum. The development of an optimal sensor embedded on a diver leads to the use of a single transducer acting as an transmitter/receiver of pulsed waves. The straight forward outcome is a higher probability detection and a better radius estimation accuracy. Distinctions in the signal processing allows dedicated detection/sizing processes suitable either for bubbles circulating in the blood flow (larger bubble) or for stationary bubbles in tissues (several microns)

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

High order harmonic balance formulation of free and encapsulated microbubbles

The radial responses of free or encapsulated micro-bubbles excited by a plane wave of large wavelength are governed by NonLinear Ordinary Differential Equa- tions (NL-ODEs). The nonlinear frequency response details the harmonic con- tent of the time response and constitutes the expected outcome of a high order harmonic analysis. In this paper, a high order harmonic balance analysis of the ”RNNP” (bubble), Hoff and Marmottant (contrast agents) models are per- formed with the open-source Manla

Improved Localization and Quantification in Photoacoustic Tomography by Using Acoustic Information Content

In this letter is proposed a method for improving the localization and the quantification of the optical parameters in photoacoustic tomography of biological tissues, that are intrinsically heterogeneous in both optical and acoustic properties. It is based on the exploitation of both the photoacoustic signal, generated by the heterogeneous optical structures, and the secondary acoustic echoes due to the interaction between a primary pho-toacoustic wave generated near the tissues surface region and the heterogeneous acoustic structures. These secondary echoes can also be collected through proper measurements of the photoacoustic signals. The experimental procedure is presented as well as the method to filter the signal and the reconstruction algorithm including the account of the acoustic information

Near-Field Ultrasound Mammography

We introduce in this note a near-field formulation of the acoustic field scattered by a fluid object supposed to be weakly heterogeneous (Born approximation). This derivation is based on the Huygens-Fresnel principle that describes the scattered field as the result of the interferential scheme of all the secondary spherical waves. This derivation leads us to define a new Fourier transform that we name the Elliptical Fourier transform. The latter provides an elliptical spectrum whose harmonic components, the weighted elementary basis functions, have an elliptical spatial support. Based on these elliptical projections, we define the Elliptical Radon transform that allows us to establish a near-field extension of the Fourier Projection-Slice theorem. Thanks to these spectral and tomographic transforms, we show that it is possible to reconstruct either the impedance or the celerity maps of an acoustical model characterized in terms of impedance and celerity fluctuations.We observe that this formulation is very close to that one developed in the far field domain where the Radon transform pair is derived from an harmonic plane wave decomposition. This formulation allows us to introduce the Ductal Tomography, following the example of the Ductal Echography, that provides a systematic inspection of each mammary lobe, in order to reveal lesions at an early stage. In that aim, we develop 2D anatomic breast computer phantoms corresponding to an axial cross-section of the ductolubular structure in healthy and pathological situations. The goal is also, from the practitioner's point of view, to compare the recognized DE reference with the high potential tomographic approach.Nous introduisons par cette note une formulation originale en champ proche, du champ ultrasonore diffracté par un organe faiblement contrasté – le sein – (Approximation de Born). Cette formulation est fondée sur le principe d'Huygens-Fresnel qui construit ce champ sur la base d'un schéma interférentiel à partir des sources secondaires sphériques. Cette approche nous amène à définir une nouvelle transformée de Fourier dite «elliptique» dont le spectre (du même nom) s'établit sur une famille de fonctions de base – les composantes harmoniques – qui présentent un support spatial ellipsoïdal caractéristique des senseurs actifs multi-statiques. Ces projections elliptiques nous permettent de définir la transformée de Radon Elliptique qui fonde une extension en champ proche du théorème coupe-projection. Grâce à ces transformées spectrale et tomographique, nous montrons qu'il est possible de reconstruire les cartographies de fluctuation d'impédance en réflexion et de célérité en transmission. Nous observons que l'algorithme est très proche de celui universellement utilisé en champ lointain, basé sur une décomposition harmonique de type ondes planes. Cette formulation nous permet, à l'instar de l'échographie, de jeter les bases de la tomographie ductale offrant une inspection systématisée de chaque lobe en vue de la détection précoce du cancer du sein. Dans ce but, des fantômes numériques anatomiques 2D de sein correspondant à une coupe axiale de la structure ductolobulaire dans des situations saines et pathologiques sont développés, afin de valider les modèles précités d'analyse et d'inversion des données en champ proche. Ils permettent aussi une comparaison réaliste des procédés échographique (référence du radiologue) et tomographique, avec, pour cette dernière modalité, un avantage confirmé

Development of anthropomorphic phantoms for combined PET-ultrasound breast imaging

International audienceAs part of the development a combined PET-Ultrasound multimodal scanner for breast imaging by the Crystal Clear collaboration (The ClearPEM-Sonic project) we have developed and tested a phantom that can be used for making realistic images with both modalities. In a first step we measured the propagation velocities of the acoustic waves, the attenuation coefficient and the elasticity (Young's modulus) for several series of different samples based on gelatine and agar mixtures. We determined which preparations reproduce the acoustical and elastic properties of different body tissues of interest in breast imaging such as fat tissue, glandular tissue, fibrous tissue and carcinomat tissue. In a second step we have built phantoms where we added a small amount of FDG (WHAT IS FDG?) during the preparation of the phantom such as to give the different parts in the phantom activities similar to what is usually present in the breast during PET imaging. The phantom was than imaged on a Philips Gemini TF PET/CT and on a US scanner from ATL, HDI 5000. The images were superposed using rigid transformations to produce combined PET/US images. We also evaluated the performance of our phantoms for US elastographic imaging. Details on the procedure for producing the phantoms will be given