Sharif Digital Repository

Transmission-mode ultrasound computed tomography (USCT) has emerged as a novel, safe, and cost-effective method for quantitative imaging of biological tissues, especially in breast screening. Despite technological advancements, challenges such as reduced accuracy in heterogeneous media, incomplete acoustic field coverage, sensitivity to initial conditions, and phenomena like cycle skipping still limit image quality and reconstruction speed in these systems. In this thesis, aiming to improve the speed and performance of USCT imaging, three reconstruction algorithms have been designed, implemented, and evaluated: a travel-time-based Gauss-Newton method, frequency-domain full waveform inversion... 

The problem studied in this research is the online stochastic bipartite matching. In this problem the vertices of one side of the given graph arrive in an online manner, with respect to a probability distribution. Also the edges of the graph exist according to a given probability distribution and one should perform queries from an oracle to know about the existence of an edge. The given graph shall be weighted or unweighted. The goal here is to find a maximum matching in the graph that is as close to the omniscient optimum as possible, while the number of queries performed per vertex is limited. In the general case of the problem, there are no specific conditions, but in other versions,... 

Photoacoustic imaging is one of the nascent imaging methods that has received a lot of attention in the last two decades. There are two major imaging challenges with this method, including the finite size of the transducer element and the limited view angles of the photoacoustic tomography arrangement. To compensate for the effects of finite transducer size, we have proposed a multi-angle detection method for circular photoacoustic imaging systems based on the synthetic aperture techniques, in the proposed method, each transducer, in addition to rotating around the scan center, also has a rotation in the around its center. This rotation depends on the cut-off frequency, the diameter of the... 

The hybrid imaging technique combining ultrasound and optical tomography, based on computed tomography, has gained significant attention in the past two decades due to its non-invasive and non-ionizing advantages in medical imaging. By integrating optical and ultrasound imaging, it enables high-resolution imaging and diagnosis based on optical absorption properties of various tissues. However, challenges such as large transducer size, limited bandwidth, and unwanted artifacts in the imaging process limit the quality of reconstructed images and prevent achieving optimal resolution and contrast. This study focuses on developing processing and structural methods to overcome these challenges and... 

Accurate prediction of myocardial infarction complications is one of the major challenges in the management of cardiac patients, as the occurrence of such complications can lead to severe consequences for quality of life, length of hospitalization, treatment costs, and mortality. This study aims to develop a data-driven, machine learning-based framework to predict 12 critical post-infarction outcomes using modern algorithms. In the first step, patients were stratified into more homogeneous groups based on treatment effects through a causal clustering algorithm. Subsequently, machine learning models, including logistic regression, random forest, and gradient boosting, were trained and... 

When a crack is subjected to a compression field, it will close and its edges will get into contact with each other. Depending on the direction and magnitude of the loads and also the coefficient of friction, ‘stick’ or ‘slip’ situationsbetween the edges will occur. This type of crack is known as ‘frictional crack.’ In this project, first these cracks are studied analytically and the order of singularity is derived using asymptotic analysis and also the analytical fields are determined for both ‘isotropic’ and ‘orthotropic’ materials. Then, numerical simulations are carried out using extended finite element method which is considered as the most powerful means for analyzing the problems... 

Particulate porous mateials have always been of interest in terms of reducing shock waves effects in different protective applications. Therefore, the physics governing the flow in porous media is especially significant for which different models have been presented by the researchers. The complexities of these media have caused many existing models to be unable to properly predict the behavior of granular media under shock loadings. On the other hand, the complexity of the equations makes the numerical solution of them cumbersome and costly in a way that many researchers do not solve the whole coupled equations and reduce their number. In addition, current high-resolution TVD solutions of...