A CPU-GPU Hybrid Approach for Accelerating Cross-correlation Based Strain Elastography

Elastography is a non-invasive imaging modality that uses ultrasound to estimate the elasticity of soft tissues. The resulting images are called 'elastograms'. Elastography techniques are promising as cost-effective tools in the early detection of pathological changes in soft tissues. The quality of elastographic images depends on the accuracy of the local displacement estimates. Cross-correlation based displacement estimators are precise and sensitive. However cross-correlation based techniques are computationally intense and may limit the use of elastography as a real-time diagnostic tool. This study investigates the use of parallel general purpose graphics processing unit (GPGPU) engines for speeding up generation of elastograms at real-time frame rates while preserving elastographic image quality. To achieve this goal, a cross-correlation based time-delay estimation algorithm was developed in C programming language and was profiled to locate performance blocks. The hotspots were addressed by employing software pipelining, read-ahead and eliminating redundant computations. The algorithm was then analyzed for parallelization on GPGPU and the stages that would map well to the GPGPU hardware were identified. By employing optimization principles for efficient memory access and efficient execution, a net improvement of 67x with respect to the original optimized C version of the estimator was achieved. For typical diagnostic depths of 3-4cm and elastographic processing parameters, this implementation can yield elastographic frame rates in the order of 50fps. It was also observed that all of the stages in elastography cannot be offloaded to the GPGPU for computation because some stages have sub-optimal memory access patterns. Additionally, data transfer from graphics card memory to system memory can be efficiently overlapped with concurrent CPU execution. Therefore a hybrid model of computation where computational load is optimally distributed between CPU and GPGPU was identified as an optimal approach to adequately tackle the speed-quality problem in real-time imaging. The results of this research suggest that use of GPGPU as a co-processor to CPU may allow generation of elastograms at real time frame rates without significant compromise in image quality, a scenario that could be very favorable in real-time clinical elastography

Novel 3D Ultrasound Elastography Techniques for In Vivo Breast Tumor Imaging and Nonlinear Characterization

Breast cancer comprises about 29% of all types of cancer in women worldwide. This type of cancer caused what is equivalent to 14% of all female deaths due to cancer. Nowadays, tissue biopsy is routinely performed, although about 80% of the performed biopsies yield a benign result. Biopsy is considered the most costly part of breast cancer examination and invasive in nature. To reduce unnecessary biopsy procedures and achieve early diagnosis, ultrasound elastography was proposed.;In this research, tissue displacement fields were estimated using ultrasound waves, and used to infer the elastic properties of tissues. Ultrasound radiofrequency data acquired at consecutive increments of tissue compression were used to compute local tissue strains using a cross correlation method. In vitro and in vivo experiments were conducted on different tissue types to demonstrate the ability to construct 2D and 3D elastography that helps distinguish stiff from soft tissues. Based on the constructed strain volumes, a novel nonlinear classification method for human breast tumors is introduced. Multi-compression elastography imaging is elucidated in this study to differentiate malignant from benign tumors, based on their nonlinear mechanical behavior under compression. A pilot study on ten patients was performed in vivo, and classification results were compared with biopsy diagnosis - the gold standard. Various nonlinear parameters based on different models, were evaluated and compared with two commonly used parameters; relative stiffness and relative tumor size. Moreover, different types of strain components were constructed in 3D for strain imaging, including normal axial, first principal, maximum shear and Von Mises strains. Interactive segmentation algorithms were also evaluated and applied on the constructed volumes, to delineate the stiff tissue by showing its isolated 3D shape.;Elastography 3D imaging results were in good agreement with the biopsy outcomes, where the new classification method showed a degree of discrepancy between benign and malignant tumors better than the commonly used parameters. The results show that the nonlinear parameters were found to be statistically significant with p-value \u3c0.05. Moreover, one parameter; power-law exponent, was highly statistically significant having p-value \u3c 0.001. Additionally, volumetric strain images reconstructed using the maximum shear strains provided an enhanced tumor\u27s boundary from the surrounding soft tissues. This edge enhancement improved the overall segmentation performance, and diminished the boundary leakage effect. 3D segmentation provided an additional reliable means to determine the tumor\u27s size by estimating its volume.;In summary, the proposed elastographic techniques can help predetermine the tumor\u27s type, shape and size that are considered key features helping the physician to decide the sort and extent of the treatment. The methods can also be extended to diagnose other types of tumors, such as prostate and cervical tumors. This research is aimed toward the development of a novel \u27virtual biopsy\u27 method that may reduce the number of unnecessary painful biopsies, and diminish the increasingly risk of cancer

Modulography: elasticity imaging of atherosclerotic plaques

Modulography is an experimental elasticity imaging method. It has potential to become an all-in-one in vivo tool (a) for detecting vulnerable atherosclerotic coronary plaques, (b) for assessing information related to their rupture-proneness and (c) for imaging their elastic material composition. Modulography determines a cross-sectional image of the elasticity distribution (=Young's modulus) from deformation (=strain) that is processed from intravascular ultrasound (IVUS) measurements. By looking at this image, cardiologists and other researchers can directly identify and characterize soft and stiff plaque-components of thin-cap fibroatheromas and of heterogeneous plaques. As a diagnostic and pharm

Characterization of carotid artery plaques using noninvasive vascular ultrasound elastography

L'athérosclérose est une maladie vasculaire complexe qui affecte la paroi des artères (par l'épaississement) et les lumières (par la formation de plaques). La rupture d'une plaque de l'artère carotide peut également provoquer un accident vasculaire cérébral ischémique et des complications. Bien que plusieurs modalités d'imagerie médicale soient actuellement utilisées pour évaluer la stabilité d'une plaque, elles présentent des limitations telles que l'irradiation, les propriétés invasives, une faible disponibilité clinique et un coût élevé. L'échographie est une méthode d'imagerie sûre qui permet une analyse en temps réel pour l'évaluation des tissus biologiques. Il est intéressant et prometteur d’appliquer une échographie vasculaire pour le dépistage et le diagnostic précoces des plaques d’artère carotide. Cependant, les ultrasons vasculaires actuels identifient uniquement la morphologie d'une plaque en termes de luminosité d'écho ou l’impact de cette plaque sur les caractéristiques de l’écoulement sanguin, ce qui peut ne pas être suffisant pour diagnostiquer l’importance de la plaque. La technique d’élastographie vasculaire non-intrusive (« noninvasive vascular elastography (NIVE) ») a montré le potentiel de détermination de la stabilité d'une plaque. NIVE peut déterminer le champ de déformation de la paroi vasculaire en mouvement d’une artère carotide provoqué par la pulsation cardiaque naturelle. En raison des différences de module de Young entre les différents tissus des vaisseaux, différents composants d’une plaque devraient présenter différentes déformations, caractérisant ainsi la stabilité de la plaque. Actuellement, les performances et l’efficacité numérique sous-optimales limitent l’acceptation clinique de NIVE en tant que méthode rapide et efficace pour le diagnostic précoce des plaques vulnérables. Par conséquent, il est nécessaire de développer NIVE en tant qu’outil d’imagerie non invasif, rapide et économique afin de mieux caractériser la vulnérabilité liée à la plaque. La procédure à suivre pour effectuer l’analyse NIVE consiste en des étapes de formation et de post-traitement d’images. Cette thèse vise à améliorer systématiquement la précision de ces deux aspects de NIVE afin de faciliter la prédiction de la vulnérabilité de la plaque carotidienne. Le premier effort de cette thèse a été dédié à la formation d'images (Chapitre 5). L'imagerie par oscillations transversales a été introduite dans NIVE. Les performances de l’imagerie par oscillations transversales couplées à deux estimateurs de contrainte fondés sur un modèle de déformation fine, soit l’ « affine phase-based estimator (APBE) » et le « Lagrangian speckle model estimator (LSME) », ont été évaluées. Pour toutes les études de simulation et in vitro de ce travail, le LSME sans imagerie par oscillation transversale a surperformé par rapport à l'APBE avec imagerie par oscillations transversales. Néanmoins, des estimations de contrainte principales comparables ou meilleures pourraient être obtenues avec le LSME en utilisant une imagerie par oscillations transversales dans le cas de structures tissulaires complexes et hétérogènes. Lors de l'acquisition de signaux ultrasonores pour la formation d'images, des mouvements hors du plan perpendiculaire au plan de balayage bidimensionnel (2-D) existent. Le deuxième objectif de cette thèse était d'évaluer l'influence des mouvements hors plan sur les performances du NIVE 2-D (Chapitre 6). À cette fin, nous avons conçu un dispositif expérimental in vitro permettant de simuler des mouvements hors plan de 1 mm, 2 mm et 3 mm. Les résultats in vitro ont montré plus d'artefacts d'estimation de contrainte pour le LSME avec des amplitudes croissantes de mouvements hors du plan principal de l’image. Malgré tout, nous avons néanmoins obtenu des estimations de déformations robustes avec un mouvement hors plan de 2.0 mm (coefficients de corrélation supérieurs à 0.85). Pour un jeu de données cliniques de 18 participants présentant une sténose de l'artère carotide, nous avons proposé d'utiliser deux jeux de données d'analyses sur la même plaque carotidienne, soit des images transversales et longitudinales, afin de déduire les mouvements hors plan (qui se sont avérés de 0.25 mm à 1.04 mm). Les résultats cliniques ont montré que les estimations de déformations restaient reproductibles pour toutes les amplitudes de mouvement, puisque les coefficients de corrélation inter-images étaient supérieurs à 0.70 et que les corrélations croisées normalisées entre les images radiofréquences étaient supérieures à 0.93, ce qui a permis de démontrer une plus grande confiance lors de l'analyse de jeu de données cliniques de plaques carotides à l'aide du LSME. Enfin, en ce qui concerne le post-traitement des images, les algorithmes NIVE doivent estimer les déformations des parois des vaisseaux à partir d’images reconstituées dans le but d’identifier les tissus mous et durs. Ainsi, le dernier objectif de cette thèse était de développer un algorithme d'estimation de contrainte avec une résolution de la taille d’un pixel ainsi qu'une efficacité de calcul élevée pour l'amélioration de la précision de NIVE (Chapitre 7). Nous avons proposé un estimateur de déformation de modèle fragmenté (SMSE) avec lequel le champ de déformation dense est paramétré avec des descriptions de transformées en cosinus discret, générant ainsi des composantes de déformations affines (déformations axiales et latérales et en cisaillement) sans opération mathématique de dérivées. En comparant avec le LSME, le SMSE a réduit les erreurs d'estimation lors des tests de simulations, ainsi que pour les mesures in vitro et in vivo. De plus, la faible mise en oeuvre de la méthode SMSE réduit de 4 à 25 fois le temps de traitement par rapport à la méthode LSME pour les simulations, les études in vitro et in vivo, ce qui pourrait permettre une implémentation possible de NIVE en temps réel.Atherosclerosis is a complex vascular disease that affects artery walls (by thickening) and lumens (by plaque formation). The rupture of a carotid artery plaque may also induce ischemic stroke and complications. Despite the use of several medical imaging modalities to evaluate the stability of a plaque, they present limitations such as irradiation, invasive property, low clinical availability and high cost. Ultrasound is a safe imaging method with a real time capability for assessment of biological tissues. It is clinically used for early screening and diagnosis of carotid artery plaques. However, current vascular ultrasound technologies only identify the morphology of a plaque in terms of echo brightness or the impact of the vessel narrowing on flow properties, which may not be sufficient for optimum diagnosis. Noninvasive vascular elastography (NIVE) has been shown of interest for determining the stability of a plaque. Specifically, NIVE can determine the strain field of the moving vessel wall of a carotid artery caused by the natural cardiac pulsation. Due to Young’s modulus differences among different vessel tissues, different components of a plaque can be detected as they present different strains thereby potentially helping in characterizing the plaque stability. Currently, sub-optimum performance and computational efficiency limit the clinical acceptance of NIVE as a fast and efficient method for the early diagnosis of vulnerable plaques. Therefore, there is a need to further develop NIVE as a non-invasive, fast and low computational cost imaging tool to better characterize the plaque vulnerability. The procedure to perform NIVE analysis consists in image formation and image post-processing steps. This thesis aimed to systematically improve the accuracy of these two aspects of NIVE to facilitate predicting carotid plaque vulnerability. The first effort of this thesis has been targeted on improving the image formation (Chapter 5). Transverse oscillation beamforming was introduced into NIVE. The performance of transverse oscillation imaging coupled with two model-based strain estimators, the affine phase-based estimator (APBE) and the Lagrangian speckle model estimator (LSME), were evaluated. For all simulations and in vitro studies, the LSME without transverse oscillation imaging outperformed the APBE with transverse oscillation imaging. Nonetheless, comparable or better principal strain estimates could be obtained with the LSME using transverse oscillation imaging in the case of complex and heterogeneous tissue structures. During the acquisition of ultrasound signals for image formation, out-of-plane motions which are perpendicular to the two-dimensional (2-D) scan plane are existing. The second objective of this thesis was to evaluate the influence of out-of-plane motions on the performance of 2-D NIVE (Chapter 6). For this purpose, we designed an in vitro experimental setup to simulate out-of-plane motions of 1 mm, 2 mm and 3 mm. The in vitro results showed more strain estimation artifacts for the LSME with increasing magnitudes of out-of-plane motions. Even so, robust strain estimations were nevertheless obtained with 2.0 mm out-of-plane motion (correlation coefficients higher than 0.85). For a clinical dataset of 18 participants with carotid artery stenosis, we proposed to use two datasets of scans on the same carotid plaque, one cross-sectional and the other in a longitudinal view, to deduce the out-of-plane motions (estimated to be ranging from 0.25 mm to 1.04 mm). Clinical results showed that strain estimations remained reproducible for all motion magnitudes since inter-frame correlation coefficients were higher than 0.70, and normalized cross-correlations between radiofrequency images were above 0.93, which indicated that confident motion estimations can be obtained when analyzing clinical dataset of carotid plaques using the LSME. Finally, regarding the image post-processing component of NIVE algorithms to estimate strains of vessel walls from reconstructed images with the objective of identifying soft and hard tissues, we developed a strain estimation method with a pixel-wise resolution as well as a high computation efficiency for improving NIVE (Chapter 7). We proposed a sparse model strain estimator (SMSE) for which the dense strain field is parameterized with Discrete Cosine Transform descriptions, thereby deriving affine strain components (axial and lateral strains and shears) without mathematical derivative operations. Compared with the LSME, the SMSE reduced estimation errors in simulations, in vitro and in vivo tests. Moreover, the sparse implementation of the SMSE reduced the processing time by a factor of 4 to 25 compared with the LSME based on simulations, in vitro and in vivo results, which is suggesting a possible implementation of NIVE in real time

2D AND 3D breast elastography with a combined ultrasound/digital tomosynthesis mammography system

Ultrasound elastography is a novel imaging method which acts as a surrogate to manual palpation to evaluate elastic properties of tissue. In this dissertation, elastography was conducted through a mammographic paddle in conjunction with a combined ultrasound/digital tomosynthesis mammography system to improve breast lesion characterization. Imaging through a mammographic paddle may adversely affect ultrasound image quality by reducing spatial resolution, increasing attenuation, and decreasing contrast. Thus, appropriate paddle choice is essential to create high quality through-paddle ultrasound images and strain images. Sonographic image quality through mammographic paddles of varying materials and thicknesses was compared with direct-contact (no paddle) image quality. TPX plastic paddles ≤ 2.5 mm thick performed best; when employed in vivo, 83% of cases produced image quality as good or better than their direct-contact analogues. Through-paddle 2D elastography was conducted through the best paddle using a 1D ultrasound transducer at 7.5 MHz and performance was compared with freehand elastography. For small strain step sizes (< 0.5%), through-paddle elastography produced correlation coefficients and strain SNR comparable to freehand elastography. Ultimately, the aim of elastography is to acquire high quality strain estimates in 3D to fully characterize tissue. Thus, data acquisition techniques were extended to a small 3D volume. Compared with its 2D analogue, 3D elastography created higher correlation coefficients for strain step sizes ≥ 1% and at least 35% improvement in strain SNR for all step sizes. These early successes indicate that through-paddle elastography can create high quality elastograms which might aid in breast lesion characterization. Next, through-paddle elastography was performed in 20 human subjects with varying breast masses. This dissertation introduced the elasticity characteristic “differential correlation coefficient,” which exploits the severe decorrelation observed in cysts under compression to differentiate cystic and solid masses. When applied in a clinical setting, this characteristic demonstrated potential to reduce the malignancy rating of a complicated cyst, changing management options from biopsy to imaging follow-up. Additional elastographic appearances of breast masses were evaluated, including lesion size, stiffness, margin regularity, and boundary flow. These studies suggest that elastography has potential to improve characterization of breast masses beyond x-ray tomography and sonography alone.NIH RO1-CA91713 and R21- CA109440http://deepblue.lib.umich.edu/bitstream/2027.42/107472/1/Booi_dissertation_FINAL.pdf-1Description of Booi_dissertation_FINAL.pdf : R.C. Booi, Ph.D. Thesi

Avaliação ecográfica da morfologia muscular perante situações de sobrecarga agudas e crónicas

Introduction: Ultrasound (US) has an important role in musculoskeletal (MSK) evaluation, allowing the study of muscle morphology and function. Muscle thickness (MT) and muscle echo-intensity (EI) are two important parameters that may quantify muscle structural adaptations to a variety of stimuli. US elastography can also offer semi-quantitative and/or quantitative assessment of tissue stiffness providing relevant information about adaptations of muscle mechanical properties. Purpose: The general aim of the studies presented in this thesis is to explore the potential of quantitative US imaging for assessing the adaptations and responses of the muscle tissue to increased contractile activity using B-mode US and US elastography. The studies were centred on the quadriceps femoris muscle and addressed the study of the effect of strength training and of acute muscle contractile activity on MT, EI and muscle stiffness. Materials and methods: Three different studies were conducted and reported along this thesis. A total of 64 young adults of both genders participated in the studies. The first study (N = 20) evaluated the intra- and inter-session (one week apart) reproducibility of MT and EI parameters and the role of plane of view (transverse vs. longitudinal) and ROI dimension on measurements’ accuracy using the intraclass correlation coefficient [ICC(3,1)], the standard error of measurement (SEM), and the smallest detectable change (SDC). Bland-Altman analysis was used to study the level of agreement between plane views and ROI sizes. The second study (N = 28) investigated the effect of a 15-week strength program on MT and EI in several regions of the heads of the quadriceps femoris. This study included a control group and two training groups performing concentric or eccentric strength training. During this study, changes in vastus lateralis’ (VL) stiffness in response to strength training were evaluated using quasi-static elastography (QSE). In the final study (N = 16), acute changes in VL’s stiffness associated with passive stretching, performance of short but intense contractile activity, and muscle isometric contractions were investigated by means of supersonic shear wave imaging (SSI). Results: Moderate to very high reliability was found for MT (intra-session, ICCs: 0.82- 0.99; inter-session, ICCs: 0.70-0.98) and EI (intra-session, ICCs: 0.74-0.97; inter-session, ICCs: 0.48-0.94). In general, reliability for MT and EI measures was higher in the transverse plane and when using a larger ROI, respectively. Measurements of EI taken with a small versus a large ROI are associated with a small bias and larger limits of Morphological ultrasound evaluation in acute and chronic muscle overloading agreement (LoA). In study 2, 15 weeks of strength training increased MT in the majority but not in all of the scanned regions. Strength training failed in changing EI in most of the quadriceps femoris, excepting in the VI and some regions of the VL. Strength training significantly increased VL’s stiffness. No differences were observed in our quantitative US parameters between concentric and eccentric training. The final study demonstrated an acute increase of around 10% in VL’s shear modulus as a result of performing maximal isometric, concentric, and eccentric contractions. The shear modulus of the VL also increased when the knee moved from 10º to 50º and then to 90º flexion. Finally, a linear relationship between the shear modulus and the level of isometric muscle contraction was observed. Conclusions: Ultrasound measures of MT and EI show moderate to very high reliability. The reliability and agreement of MT and EI measurements are improved in transverse scans and with larger ROIs. QSE could demonstrate an increase in muscle stiffness as a result of strength training. SSI proved to be a good method to investigate muscle mechanical properties changes associated with muscle function. These results emphasise the value of an objective and quantifiable muscle US evaluation for studying muscle adaptation to exercise training and muscle function, in general.Introdução: A ultrassonografia tem um papel importante na avaliação músculoesquelética, permitindo o estudo da morfologia e função muscular. A espessura muscular e a eco- intensidade muscular são dois parâmetros importantes que podem quantificar as adaptações estruturais musculares, quando o musculo é submetido a determinados estímulos. A elastografia por ultrassonografia pode, também, oferecer uma avaliação semi-quantitativa e/ou quantitativa da rigidez do tecido, fornecendo informações relevantes sobre as adaptações das propriedades mecânicas musculares. Objetivo: O objetivo geral, dos estudos apresentados nesta tese, é explorar o potencial da imagem quantitativa ultrassonográfica, de forma a avaliar as adaptações e as respostas do tecido muscular ao aumento da atividade contrátil, usando a elastografia e a ultrassonografia em modo-B. Os estudos foram centrados no músculo do quadricípite femoral e abordaram o estudo do efeito do treino de força e da atividade contrátil muscular na espessura muscular, eco-intensidade e rigidez muscular. Materiais e métodos: Três diferentes estudos foram realizados e descritos ao longo desta tese. Um total de 64 jovens adultos de ambos os géneros participaram dos estudos. No primeiro estudo (N = 20), foi analisada a reprodutibilidade da espessura muscular e da eco-intensidade dos quatro músculos que compõem o quadricípite femoral. Para isso foram adquiridas três imagens em modo B, nos planos longitudinal e transversal, em dois momentos distintos. A eco-intensidade foi medida usando dois tamanhos diferentes de região de interesse, um representado por uma forma retangular, medindo 70 mm2 e um outro representando o máximo do músculo apresentado na imagem ultrassonográfica, evitando as fáscias superficial e profundas do mesmo. A precisão das medidas foi, então, analisada usando o Coeficiente de correlação intra-classe [ICC (3,1)], o erro padrão de medição (SEM) e a menor alteração detectável (SDC). A análise de Bland-Altman foi utilizada para estudar o nível de concordância entre os planos de imagem ultrassonográficos e os diferentes tamanhos da região de interesse. No segundo estudo (N = 28), analisou-se o efeito de um programa de treino de força, com duração de 15 semanas, sobre espessura muscular e ecointensidade em três diferentes regiões de cada um dos quatro músculos que representam o quadricípite femoral: reto femoral, vasto intermédio, vasto medial e vasto lateral. Este estudo incluiu um grupo de controlo e dois grupos de treino, em que um realizou um protocolo de treino concêntrico e o outro de treino excêntrico. Durante este estudo, as alterações na rigidez do vasto lateral, em resposta ao treino de força foram avaliadas usando a elastografia quasi-statica, semi-quantitativa. No último estudo (N = 16), foram analisadas as alterações agudas na rigidez de vasto lateral associadas ao alongamento passivo, ao desempenho de atividade contrátil de curta duração, mas intensa e às contrações isométricas musculares usando a elastografia de onda supersónica por cisalhamento. Resultados: Foi encontrada uma alta ou muito alta reprodutibilidade para espessura muscular (intra-sessão, ICCs: 0,82-0,99; inter-sessão, ICCs: 0,70-0,98) e eco-intensidade (intra-sessão, ICCs: 0,74-0,97; inter-sessão, ICCs: 0,48-0,94). Em geral, a reprodutibilidade para os valores da espessura muscular foi maior no plano transversal e no que diz respeito aos valores da eco-intensidade verificou-se uma melhor reprodutibilidade quando foi utilizada uma região de interesse de maiores dimensões. Um pequeno viés e menores valores de concordância caracterizam as medidas de cointensidade obtidas com uma região de interesse maior ou menor. No estudo 2, os participantes submetidos a 15 semanas de treino de força revelaram o aumento da sua espessura na maioria das regiões musculares avaliadas, mas não em todas. Não foram encontradas alterações significavas dos valores da eco-intensidade com a realização do treino de força na maioria dos músculos do quadricípite femoral, excepto para o vasto intermédio e para algumas regiões do vasto lateral.Por outro lado, o treino de força aumentou significativamente a rigidez do vasto lateral. Não foram observadas diferenças significativas nos parâmetros quantitativos ultrassonográficos entre o treino concêntrico e excêntrico. O último estudo demonstrou um aumento agudo de cerca de10% nos valores da rigidez do vasto lateral como resultado da realização de contrações máximas isométricas, concêntricas e excêntricas. Os valores da rigidez do vasto lateral também aumentaram durante a flexão do joelho de 10º para 50º e posteriormente para 90º. Finalmente, observou-se uma relação linear entre os valores de rigidez do vasto lateral e o nível de contração muscular isométrica do quadricípite femoral. Conclusões: As medidas ultrassonográficas da espessura muscular e eco-intensidade mostram uma reprodutibilidade moderada a muito alta. A reprodutibilidade e a concordância das medidas de espessura muscular e eco-intensidade são maiores no plano transversal e quando é utilizada uma região de interesse de maior dimensão. A elastografia semi-quantitativa mostrou existir um aumento significativo na rigidez muscular como resultado do treino de força. A elastografia por onda de cisalhamento supersónica é um bom método para investigar as alterações das propriedades mecânicas musculares associadas à função muscular. Estes resultados enfatizam a importância de uma avaliação objetiva e quantificável dos músculos por ultrassonografia, para estudar a adaptação muscular ao treino e função muscular, no geral

Évaluation de la biomécanique cardiovasculaire par élastographie ultrasonore non-invasive

L’élastographie est une technique d’imagerie qui vise à cartographier in vivo les propriétés mécaniques des tissus biologiques dans le but de fournir des informations diagnostiques additionnelles. Depuis son introduction en imagerie ultrasonore dans les années 1990, l’élastographie a trouvé de nombreuses applications. Cette modalité a notamment été utilisée pour l’étude du sein, du foie, de la prostate et des artères par imagerie ultrasonore, par résonance magnétique ou en tomographie par cohérence optique. Dans le contexte des maladies cardiovasculaires, cette modalité a un fort potentiel diagnostique puisque l’athérosclérose modifie la structure des tissus biologiques et leurs propriétés mécaniques bien avant l’apparition de tout symptôme. Quelle que soit la modalité d’imagerie utilisée, l’élastographie repose sur : l’excitation mécanique du tissu (statique ou dynamique), la mesure de déplacements et de déformations induites, et l’inversion qui permet de recouvrir les propriétés mécaniques des tissus sous-jacents. Cette thèse présente un ensemble de travaux d’élastographie dédiés à l’évaluation des tissus de l’appareil cardiovasculaire. Elle est scindée en deux parties. La première partie intitulée « Élastographie vasculaire » s’intéresse aux pathologies affectant les artères périphériques. La seconde, intitulée « Élastographie cardiaque », s’adresse aux pathologies du muscle cardiaque. Dans le contexte vasculaire, l’athérosclérose modifie la physiologie de la paroi artérielle et, de ce fait, ses propriétés biomécaniques. La première partie de cette thèse a pour objectif principal le développement d’un outil de segmentation et de caractérisation mécanique des composantes tissulaires (coeur lipidique, tissus fibreux et inclusions calciques) de la paroi artérielle, en imagerie ultrasonore non invasive, afin de prédire la vulnérabilité des plaques. Dans une première étude (Chapitre 5), nous présentons un nouvel estimateur de déformations, associé à de l’imagerie ultrarapide par ondes planes. Cette nouvelle méthode d’imagerie permet d’augmenter les performances de l’élastographie non invasive. Dans la continuité de cette étude, on propose une nouvelle méthode d’inversion mécanique dédiée à l’identification et à la quantification des propriétés mécaniques des tissus de la paroi (Chapitre 6). Ces deux méthodes sont validées in silico et in vitro sur des fantômes d’artères en polymère. Dans le contexte cardiaque, les ischémies et les infarctus causés par l’athérosclérose altèrent la contractilité du myocarde et, de ce fait, sa capacité à pomper le sang dans le corps (fonction myocardique). En échocardiographie conventionnelle, on évalue généralement la fonction myocardique en analysant la dynamique des mouvements ventriculaires (vitesses et déformations du myocarde). L’abscence de contraintes physiologiques agissant sur le myocarde (contrairement à la pression sanguine qui contraint la paroi vasculaire) ne permet pas de résoudre le problème inverse et de retrouver les propriétés mécaniques du tissu. Le terme d’élastographie fait donc ici référence à l’évaluation de la dynamique des mouvements et des déformations et non à l’évaluation des propriétés mécanique du tissu. La seconde partie de cette thèse a pour principal objectif le développement de nouveaux outils d’imagerie ultrarapide permettant une meilleure évaluation de la dynamique du myocarde. Dans une première étude (Chapitre 7), nous proposons une nouvelle approche d’échocardiographie ultrarapide et de haute résolution, par ondes divergentes, couplée à de l'imagerie Doppler tissulaire. Cette combinaison, validée in vitro et in vivo, permet d’optimiser le contraste des images mode B ainsi que l’estimation des vitesses Doppler tissulaires. Dans la continuité de cette première étude, nous proposons une nouvelle méthode d’imagerie des vecteurs de vitesses tissulaires (Chapitre 8). Cette approche, validée in vitro et in vivo, associe les informations de vitesses Doppler tissulaires et le mode B ultrarapide de l’étude précédente pour estimer l’ensemble du champ des vitesses 2D à l’intérieur du myocarde.Elastography is an imaging technique that aims to map the in vivo mechanical properties of biological tissues in order to provide additional diagnostic information. Since its introduction in ultrasound imaging in the 1990s, elastography has found many applications. This method has been used for the study of the breast, liver, prostate and arteries by ultrasound imaging, magnetic resonance imaging (MRI) or optical coherence tomography (OCT). In the context of cardiovascular diseases (CVD), this modality has a high diagnostic potential as atherosclerosis, a common pathology causing cardiovascular diseases, changes the structure of biological tissues and their mechanical properties well before any symptoms appear. Whatever the imaging modality, elastography is based on: the mechanical excitation of the tissue (static or dynamic), the measurement of induced displacements and strains, and the inverse problem allowing the quantification of the mechanical properties of underlying tissues. This thesis presents a series of works in elastography for the evaluation of cardiovascular tissues. It is divided into two parts. The first part, entitled « Vascular elastography » focuses on diseases affecting peripheral arteries. The second, entitled « Cardiac elastography » targets heart muscle pathologies. In the vascular context, atherosclerosis changes the physiology of the arterial wall and thereby its biomechanical properties. The main objective of the first part of this thesis is to develop a tool that enables the segmentation and the mechanical characterization of tissues (necrotic core, fibrous tissues and calcium inclusions) in the vascular wall of the peripheral arteries, to predict the vulnerability of plaques. In a first study (Chapter 5), we propose a new strain estimator, associated with ultrafast plane wave imaging. This new imaging technique can increase the performance of the noninvasive elastography. Building on this first study, we propose a new inverse problem method dedicated to the identification and quantification of the mechanical properties of the vascular wall tissues (Chapter 6). These two methods are validated in silico and in vitro on polymer phantom mimicking arteries. In the cardiac context, myocardial infarctions and ischemia caused by atherosclerosis alter myocardial contractility. In conventional echocardiography, the myocardial function is generally evaluated by analyzing the dynamics of ventricular motions (myocardial velocities and deformations). The abscence of physiological stress acting on the myocardium (as opposed to the blood pressure which acts the vascular wall) do not allow the solving the inverse problem and to find the mechanical properties of the fabric. Elastography thus here refers to the assessment of motion dynamics and deformations and not to the evaluation of mechanical properties of the tissue. The main objective of the second part of this thesis is to develop new ultrafast imaging tools for a better evaluation of the myocardial dynamics. In a first study (Chapter 7), we propose a new approach for ultrafast and high-resolution echocardiography using diverging waves and tissue Doppler. This combination, validated in vitro and in vivo, optimize the contrast in B-mode images and the estimation of myocardial velocities with tissue Doppler. Building on this study, we propose a new velocity vector imaging method (Chapter 8). This approach combines tissue Doppler and ultrafast B-mode of the previous study to estimate 2D velocity fields within the myocardium. This original method was validated in vitro and in vivo on six healthy volunteers

Characterization of Carotid Plaques with Ultrasound Non-Invasive Vascular Elastography (NIVE) : Feasibility and Correlation with High-Resolution Magnetic Resonance Imaging

L’accident vasculaire cérébral (AVC) est une cause principale de décès et de morbidité dans le monde; une bonne partie des AVC est causée par la plaque d’athérosclérose carotidienne. La prévention de l’AVC chez les patients ayant une plaque carotidienne demeure controversée, vu les risques et bénéfices ambigus associés au traitement chirurgical ou médical. Plusieurs méthodes d’imagerie ont été développées afin d’étudier la plaque vulnérable (dont le risque est élevé), mais aucune n’est suffisamment validée ou accessible pour permettre une utilisation comme outil de dépistage. L’élastographie non-invasive vasculaire (NIVE) est une technique nouvelle qui cartographie les déformations (élasticité) de la plaque afin de détecter les plaque vulnérables; cette technique n’est pas encore validée cliniquement. Le but de ce projet est d’évaluer la capacité de NIVE de caractériser la composition de la plaque et sa vulnérabilité in vivo chez des patients ayant des plaques sévères carotidiennes, en utilisant comme étalon de référence, l’imagerie par résonance magnétique (IRM) à haute-résolution. Afin de poursuivre cette étude, une connaissance accrue de l’AVC, l’athérosclérose, la plaque vulnérable, ainsi que des techniques actuelles d’imagerie de la plaque carotidienne, est requise. Trente-et-un sujets ont été examinés par NIVE par ultrasonographie et IRM à haute-résolution. Sur 31 plaques, 9 étaient symptomatiques, 17 contenaient des lipides, et 7 étaient vulnérables selon l’IRM. Les déformations étaient significativement plus petites chez les plaques contenant des lipides, avec une sensibilité élevée et une spécificité modérée. Une association quadratique entre la déformation et la quantité de lipide a été trouvée. Les déformations ne pouvaient pas distinguer les plaques vulnérables ou symptomatiques. En conclusion, NIVE par ultrasonographie est faisable chez des patients ayant des sténoses carotidiennes significatives et peut détecter la présence d’un coeur lipidique. Des études supplémentaires de progression de la plaque avec NIVE sont requises afin d’identifier les plaques vulnérables.Stroke is a leading cause of death and morbidity worldwide, and a significant proportion of strokes are caused by carotid atherosclerotic plaque rupture. Prevention of stroke in patients with carotid plaque poses a significant challenge to physicians, as risks and benefits of surgical or medical treatments remain equivocal. Many imaging techniques have been developed to identify and study vulnerable (high-risk) atherosclerotic plaques, but none is sufficiently validated or accessible for population screening. Non-invasive vascular elastography (NIVE) is a novel ultrasonic technique that maps carotid plaque strain (elasticity) characteristics to detect its vulnerability; it has not been clinically validated yet. The goal of this project is to evaluate the ability of ultrasound NIVE strain analysis to characterize carotid plaque composition and vulnerability in vivo in patients with significant plaque burden, as determined by the reference standard, high resolution MRI. To undertake this study, a thorough understanding of stroke, atherosclerosis, vulnerable plaque, and current non-invasive carotid plaque imaging techniques is required. Thirty-one subjects underwent NIVE and high-resolution MRI of internal carotid arteries. Of 31 plaques, 9 were symptomatic, 17 contained lipid and 7 were vulnerable on MRI. Strains were significantly lower in plaques containing a lipid core compared to those without lipid, with high sensitivity and moderate specificity. A quadratic fit was found between strain and lipid content. Strains did not discriminate symptomatic patients or vulnerable plaques. In conclusion, ultrasound NIVE is feasible in patients with significant carotid stenosis and can detect the presence of a lipid core. Further studies of plaque progression with NIVE are required to identify vulnerable plaques