Mapping Myocardial Elasticity with Intracardiac Acoustic Radiation Force Impulse Methods

<p>Implemented on an intracardiac echocardiography transducer, acoustic radiation force methods may provide a useful means of characterizing the heart's elastic properties. Elasticity imaging may be of benefit for diagnosis and characterization of infarction and heart failure, as well as for guidance of ablation therapy for the treatment of arrhythmias. This thesis tests the hypothesis that with appropriately designed imaging sequences, intracardiac acoustic radiation force impulse (ARFI) imaging and shear wave elasticity imaging (SWEI) are viable tools for quantification of myocardial elasticity, both temporally and spatially. Multiple track location SWEI (MTL-SWEI) is used to show that, in healthy in vivo porcine ventricles, shear wave speeds follow the elasticity changes with contraction and relaxation of the myocardium, varying between 0.9 and 2.2 m/s in diastole and 2.6 and 5.1 m/s in systole. Infarcted tissue is less contractile following infarction, though not unilaterally stiffer. Single-track-location SWEI (STL-SWEI) is proven to provide suppression of speckle noise and enable improved resolution of structures smaller than 2 mm in diameter compared to ARFI and MTL-SWEI. Contrast to noise ratio and lateral edge resolution are shown to vary with selection of time step for ARFI and arrival time regression filter size for STL-SWEI and MTL-SWEI. </p><p>In 1.5 mm targets, STL-SWEI achieves alternately the tightest resolution (0.3 mm at CNR = 3.5 for a 0.17 mm filter) and highest CNR (8.5 with edge width = 0.7 mm for a 0.66 mm filter) of the modalities, followed by ARFI and then MTL-SWEI.</p><p>In larger, 6 mm targets, the CNR-resolution tradeoff curves for ARFI and STL-SWEI overlap for ARFI time steps up to 0.5 ms and kernels $\leq$1 mm for STL-SWEI. STL-SWEI can operate either with a 25 dB improvement over MTL-SWEI in CNR at the same resolution, or with edge widths 5$\times$ as narrow at equivalent CNR values, depending on the selection of regression filter size. Ex vivo ablations are used to demonstrate that ARFI, STL-SWEI and MTL-SWEI each resolve ablation lesions between 0.5 and 1 cm in diameter and gaps between lesions smaller than 5 mm in 3-D scans. Differences in contrast, noise, and resolution between the modalities are discussed. All three modalities are also shown to resolve ``x''-shaped ablations up to 22 mm in depth with good visual fidelity and correspondence to surface photographs, with STL-SWEI providing the highest quality images. Series of each type of image, registered using 3-D data from an electroanatomical mapping system, are used to build volumes that show ablations in in vivo canine atria. In vivo images are shown to be subject to increased noise due to tissue and transducer motion, and the challenges facing the proposed system are discussed. Ultimately, intracardiac acoustic radiation force methods are demonstrated to be promising tools for characterizing dynamic myocardial elasticity and imaging radiofrequency ablation lesions.</p>Dissertatio

Analysis of strain in the human left ventricle using real-time 3D echocardiography and optical flow

Cardiovascular disease (CVD) consistently ranks among the leading causes of death in the United States. The most common subtype of CVD, ischemic heart disease, is a frequent precursor of myocardial infarction and heart failure, most commonly affecting the left ventricle (LV). Today, echocardiography is regarded as the gold standard in screening, diagnosis, and monitoring of LV dysfunction. But while global assessment of LV function tends to be quantitative, cardiologists with specific expertise still perform many regional evaluations subjectively. However, a more objective and quantitative measure of regional function – myocardial strain – has been developed and widely studied using 2D echocardiography. With recent developments in real-time 3D echocardiography (RT3DE), it has become possible to measure strain in its native 3D orientation as well. Our laboratory’s earlier work introduced the Optical Flow (OF) method of strain analysis, which was validated on simulated echocardiograms as well as through animal studies. The principal goal of this thesis is to translate this OF-based method of strain estimation from the research setting to the patient’s bedside. We have performed a series of studies to evaluate the feasibility, accuracy, and reproducibility of OF-based myocardial strain estimation in a routine clinical setting. The first investigation focused on the optimization of RT3DE acquisition and the OF processing pipeline for use in human subjects. Subsequently, we evaluated the capacity of this technique to distinguish abnormal strain patterns in patients with CVD and varying degrees of LV dysfunction. Our analysis revealed that segmental strain measures obtained by OF may have better sensitivity and specificity than the more commonly used global LV strains. Our third validation study examined the reproducibility of these strain measures in both healthy and diseased populations. We established that OF-based strain measures demonstrate repeatability comparable to that achieved by the latest commercial software commonly used in clinical research to estimate 2D or 3D strain. These studies were driven in large part by the absence of a ground truth or accepted gold standard of 3D strain measurements in the human LV. However, cardiac magnetic resonance imaging has had considerable success in measuring some forms of strain in the human LV. We therefore began to develop an image-processing pipeline to derive strain estimates from a new pulse sequence called 3D-DENSE. We further sought to improve the OF pipeline by automating the process of tracking the LV border. To this end, we developed a level-set based technique which tracks the LV endocardium. Our evaluation of its performance on RT3DE data confirmed that this method performs within the limits of inter-observer variability. Overall, our pilot studies of OF-based strain estimation demonstrate that the technique possesses several promising features for improving cardiologists’ ability to quantify and interpret the complex three-dimensional deformations of the human LV

Utilisation de signaux hypercomplexes en estimation du mouvement et recalage multimodal

L'imagerie médicale est d'une nécessité certaine pour aider les médecins à comprendre et interpréter les comportements mécaniques et fonctionnels du corps humain. Les différentes modalités existantes fournissent des informations complémentaires qui peuvent améliorer cette compréhension. En particulier, la déformation d'organes ou de tissus peut fournir une indication sur la présence ou non d'une pathologie. Cette appréciation qualitative est facile à effectuer à l'œil nu, mais une estimation automatisée et précise de cette déformation peut être nécessaire. Le choix le plus naturel pour traiter les images est de se baser sur l'intensité des pixels. Cependant, certaines approches d'estimation du mouvement décomposent d'abord l'image en différents descripteurs, tels que la phase spatiale, qui porte l'information structurelle de l'image. L'objectif de cette thèse est d'évaluer l'apport de ce type de descripteurs dans le cadre de séquences ultrasonores (US) et de recalage multimodal entre images par résonance magnétique (IRM) et US. Pour cela, nous avons d'abord montré que pour des images US, une approche basée sur la phase issue du signal monogène constituait un bon compromis vis-à-vis de techniques de mise en correspondance de blocs ou de flux optique basé sur la phase extraite du signal analytique complexe 2D. Nous avons ensuite poursuivi cette étude en considérant les différentes informations issues du signal monogène, avec son extension au cas 3D. Cela nous a permis de proposer un estimateur de translations basé sur un autre descripteur : l'orientation principale locale. Nous avons ensuite évalué l'apport de la phase dans le cadre du recalage IRM-US basé sur l'information mutuelle. Nous avons remarqué que dans ce cas, la phase donnait de meilleurs résultats que l'intensité dans la direction latérale mais pas axiale. Finalement, nous présentons les enjeux cliniques du prolapsus génito-urinaire chez la femme. Nous avons ainsi introduit un estimateur de mise en correspondance de blocs déformables basé sur la phase, que nous avons appliqué à des séquences échographiques in vivo. Bien que cet estimateur ait tendance à minimiser le stade du prolapsus, il permet un meilleur suivi des tissus au fil de la séquence que l'estimateur de blocs déformables initial basé sur l'intensitéNowadays, medical imaging is necessary to help doctors to understand and interpret the mechanical and functional behavior of the human body. The different existing modalities provide complementary information, which can improve this comprehension. In particular, the tissue deformation provide an indication on the presence of a pathology. This qualitative appreciation is easy to perform for the human eye, but it would be useful to get an automatic and accurate estimation of this deformation. The most natural choice to process images is to use the intensities of the pixels. However, some approaches estimate the motion decomposing the image in several descriptors, such as spatial phase, which is a strucural information of the image. The aim of this thesis is to evaluate the contribution of this kind of descriptors, when they are used for motion estimation on ultrasound (US) sequences and multimodal registration, between a magnetic resonance images (MRI) and US images. For this, we first showed that for ultrasound images, an approach based on the monogenic spatial phase was a good compromise, facing block matching technics or optical flow estimation based on 2D analytic complex signal. Then, we continued this study, considering all the features extracted from the 3D monogenic signal. It allowed us to propose a translation estimator based on another descriptor : the main local orientation. Afterward, we evaluated the contribution of the phase for MR-US registration based on the mutual information. We noted that, in this case, the spatial phase gave more accurate results than the intensity-based approach in the lateral direction, but not in the axial direction. Finally, we present the clinical issues of the pelvic organ prolaps. Thus, we introduced a phase-based block deformable block matching estimator. We applied this estimator on in vivo US sequences. Although this estimator tends to minimize the degree of the pelvic floor disorders, it allows a better tissues monitoring than the intensity-based block deformable estimator all along the sequenc