Lorentz Force Electrical Impedance Tomography

This article describes a method called Lorentz Force Electrical Impedance Tomography. The electrical conductivity of biological tissues can be measured through their sonication in a magnetic field: the vibration of the tissues inside the field induces an electrical current by Lorentz force. This current, detected by electrodes placed around the sample, is proportional to the ultrasonic pressure, to the strength of the magnetic field and to the electrical conductivity gradient along the acoustic axis. By focusing at different places inside the sample, a map of the electrical conductivity gradient can be established. In this study experiments were conducted on a gelatin phantom and on a beef sample, successively placed in a 300 mT magnetic field and sonicated with an ultrasonic transducer focused at 21 cm emitting 500 kHz bursts. Although all interfaces are not visible, in this exploratory study a good correlation is observed between the electrical conductivity image and the ultrasonic image. This method offers an alternative to detecting pathologies invisible to standard ultrasonography

Acousto-electrical speckle pattern in Lorentz force electrical impedance tomography

Ultrasound speckle is a granular texture pattern appearing in ultrasound imaging. It can be used to distinguish tissues and identify pathologies. Lorentz force electrical impedance tomography is an ultrasound-based medical imaging technique of the tissue electrical conductivity. It is based on the application of an ultrasound wave in a medium placed in a magnetic field and on the measurement of the induced electric current due to Lorentz force. Similarly to ultrasound imaging, we hypothesized that a speckle could be observed with Lorentz force electrical impedance tomography imaging. In this study, we first assessed the theoretical similarity between the measured signals in Lorentz force electrical impedance tomography and in ultrasound imaging modalities. We then compared experimentally the signal measured in both methods using an acoustic and electrical impedance interface. Finally, a bovine muscle sample was imaged using the two methods. Similar speckle patterns were observed. This indicates the existence of an "acousto-electrical speckle" in the Lorentz force electrical impedance tomography with spatial characteristics driven by the acoustic parameters but due to electrical impedance inhomogeneities instead of acoustic ones as is the case of ultrasound imaging

Imaging of Shear Waves Induced by Lorentz Force in Soft Tissues

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this 5 study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 um. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model 10 using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample

Electromagnetic Hydrophone with Tomographic System for Absolute Velocity Field Mapping

The velocity and pressure of an ultrasonic wave can be measured by an electromagnetic hydrophone made of a thin wire and a magnet. The ultrasonic wave vibrates the wire inside a magnetic field, inducing an electrical current. Previous articles reported poor spatial resolution of comparable hydrophones along the axis of the wire. In this study, submillimetric spatial resolution has been achieved by using a tomographic method. Moreover, a physical model is presented for obtaining absolute measurements. A pressure differential of 8% has been found between piezoelectric and electromagnetic hydrophone measurements. These characteristics show this technique as an alternative to standard hydrophones

Spatially broad opening of the blood-brain barrier with an unfocused ultrasound transducer in rabbits

International audienceThe aim of this work was to study the opening of the blood-brain barrier (BBB) over a large volume using an unfocused ultrasound device in the presence of ultrasound contrast agents in rabbits. A mono-element planar 1MHz ultrasound transducer was used to perform burst sonications in 24 healthy New-Zealand white rabbits after craniectomy and during intravenous injection of Sonovue®. The transducer was operated with a pulse repetition frequency of 1Hz, and a range of pulses lengths and in situ acoustic pressures (10-35ms and 0.3-1MPa respectively). Opening of the BBB was observed in contrast-enhanced images in a 4.7T MRI, through blue dye extravasation and with confocal microscopy. Adverse effects were analyzed on histology. A significant BBB opening limited spatially to the extent of the ultrasound field was observed. BBB opening appeared during the sonication and lasted for several hours. Monitoring was possible on MRI sequences as a significant gadolinium contrast enhancement (p<0.0001). BBB opening was associated with perivascular blood red cell extravasation and transient vascular spasm. In conclusion, the BBB can be opened in large areas of the brain with low power unfocused ultrasound, with limited tissue damage, and could permit safe drug delivery in the brain. Work supported by CarThera and Région Ile-de-France

Application de l'élastographie à l'imagerie du cancer de la prostate et à sa thérapie par ultrasons focalisés

Un système d'imagerie de la prostate par élastographie ultrasonore a été développé. Il utilise un ballon rempli d'un liquide de couplage ultrasonore pour comprimer la prostate. La faisabilité de visualiser l'anatomie prostatique ainsi que des tumeurs bénignes et malignes de la prostate in vitro est démontrée. L'importance de la rapidité de l'acquisition pour obtenir des images de qualité in vivo est ensuite mise en évidence. Il est alors montré que ce système permet de détecter le cancer et de visualiser les effets de la thérapie par ultrasons focalisés de haute intensité (HIFU) in vivo. Enfin il est montré in vitro qu'il est possible de visualiser la formation d'une lésion HIFU élémentaire en utilisant seulement l'élévation de température pour former l'image.An ultrasonic imaging device for prostate elastography was developed. A balloon filled with a coupling liquid served as a compressor. In vitro, the system was capable of imaging the anatomy of the prostate as well as benign and malignant tumors. Then the major influence of the acquisition frame on the image quality in vivo was demonstrated. The system was shown to be able to detect prostate cancer and to visualize the effects of high intensity focused ultrasound (HIFU) therapy in vivo. It was finally shown in vitro that the formation of an elementary HIFU lesion could be observed by passive elastography, using only temperature elevation to create the elastogram.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

The dynamic radiation force of ultrasound (applications for material characterization and medical imaging)

L'objectif de ce travail est d'explorer les applications d'une nouvelle technique d'imagerie basée sur la force de radiation ultrasonore. Une étude théorique a été développée pour comprendre les phénomènes physiques liés à la force de radiation. Un modèle analytique a été développé qui tient en compte l'effet dynamique de la force de radiation sur des échantillons élastiques et viscoélastiques de cylindres, coques cylindriques et coques sphériques. L'idée ensuite est d'utiliser cette force de radiation pour détecter des résonances d'objets immergés dans un fluide non visqueux. Dans ce manuscrit, nous montrons que les résonances détectées expérimentalement correspondent à des modes de vibrations de compression et non pas de torsion. Pour la suite, nous avons appliqué la force de radiation pour des fins diagnostiques, particulièrement pour la détection des grains de curiethérapie qui sont utilisés dans le traitement du cancer de la prostate. L'ensemble des résultats contenus dans ce mémoire nous encourage à penser que les techniques basées sur la force de radiation dynamique, tel que la Vibro-acoustographie, pourraient devenir un outil utile en rhéologie et en médecineLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Mid-term results demonstrate salvage high-intensity focused ultrasound (HIFU) as an effective and acceptably morbid salvage treatment option for locally radiorecurrent prostate cancer.

BACKGROUND: Local occurrence of prostate cancer (PCa) after external beam radiation (EBRT) may benefit from definitive local therapy. OBJECTIVE: To evaluate the safety and efficacy of salvage high-intensity focal ultrasound (HIFU) in local PCa recurrence after EBRT and to determine prognostic factors for optimal patient selection. DESIGN, SETTING, AND PARTICIPANTS: Between 1995 and 2006, patients with a local PCa recurrence after EBRT were retrospectively included. INTERVENTION: All patients received salvage HIFU with the Ablatherm device. MEASUREMENTS: Prognostic factors (pre-EBRT risk group, androgen-deprivation [AD] use, pre-HIFU prostate-specific antigen [PSA], Gleason score and positive biopsy percentage) were studied in univariate and multivariate analyses. Progression was defined as positive biopsy and/or last PSA > nadir + 2 ng/ml and/or adjuvant therapy introduction. All complications were recorded. RESULTS AND LIMITATIONS: Some 194 HIFU sessions for 167 patients were performed. Local cancer control was achieved with negative biopsy results in 122 (73%) patients. The median PSA nadir was 0.19 ng/ml. The mean follow-up period was 18.1 mo (range: 3-121 mo). Seventy-four patients required no hormone therapy. The actuarial 5-yr overall survival rate was 84%. The actuarial 3-yr progression-free survival rate was significantly lower in three circumstances: (1) worsening of the pre-EBRT stage with 53%, 42%, and 25% for low-, intermediate-, and high-risk patients, respectively, (2) increase in the pre-HIFU PSA, and (3) use of AD during PCa management. In multivariate analyses, the risk ratio for intermediate- and high-risk patients were 1.32 and 1.96, respectively. The risk ratio was 2.8 if patients had received AD. No rectal complications were observed. Urinary incontinence accounted for 49.5% of the urinary sphincter implantations required in 11% of patients. This is a retrospective study in which the role of the PSA doubling time and the time until recurrence was not evaluated. CONCLUSIONS: Salvage HIFU is a curative treatment option for local relapse after EBRT with acceptable morbidity. Careful patient selection is imperative depending upon the aforementioned prognostic factors

Intéraction d'une onde ultrasonore de haute intensité dans les tissus biologiques en présence de bulles (application au traitement de l'insuffisance veineuse superficielle)

Le sujet de cette thèse s'inscrit dans le domaine des effets biologiques des ultrasons de haute intensité (HIFU). Une nouvelle théorie pour prédire les effets des HIFU dans les tissus est développée ainsi qu'une nouvelle application thérapeutique visant le traitement de l'insuffisance veineuse superficielle (IVS). La théorie de l'acoustique non linéaire est utilisée comme base pour décrire les effets des HIFU dans les tissus, en particulier les effets dus à la présence des bulles de cavitation. Une méthode de calcul du champ acoustique adaptée pour les sources HIFU est développée. Un modèle de prédiction de la formation de lésions biologiques par un champ HIFU est présenté où le rôle des bulles dans la formation de cette lésion est considéré. Les résultats de ce modèle sont confrontés à des résultats expérimentaux rassemblés dans la littérature. Une étude est menée sur l'utilisation des effets thermiques induits par des HIFU dans le traitement de l'IVS. Dans la plupart des cas, l'IVS a pour origine un dysfonctionnement des valvules de la veine saphène externe qui sont censées empêcher le reflux sanguin vers le système veineux superficiel. Deux approches de traitement de l'IVS avec les HIFU ont été développées : l'occlusion définitive de la veine et la correction du dysfonctionnement valvulaire. Des expériences in vitro pour chacune des deux approches sont présentées et discutées.The theme of this dissertation concerns the domain of biological effects of high intensity ultrasound (HIFU). A new theory for the prediction of effects of HIFU in biological tissues is developed, as well as a new therapeutic application which aims the treatment of the superficial venous insufficiency (SVI). The nonlinear acoustic theory is used as the basis to describe the effects of HIFU on tissues. A special attention is given to the effects dues to the presence of cavitation bubbles. A method to calculate the pressure field produced by HIFU sources is developed. A model to predict the lesion formation due to a HIFU field is presented where the contribution of bubbles is considered. The results of this model are compared to experimental results found in literature. A study is carried out on the use of thermal effects of HIFU for the treatment of the SVI. In most of cases, the main cause of the SVI is the dysfunction of valves in the great saphenous vein. The valves are supposed to stop blood reflux towards the superficial venous system. Two variants of the treatment of the SVI with HIFU were developed: the total occlusion of the vein and the correction of the dysfunctional valves. In vitro experiments for each of the two variants were carried out and are discussed.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF