Ultrasound in reverberating and aberrating environments: applications to human transcranial, transabdominal, and super-resolution imaging

Abstract

Ultrasound imaging in the human body is degraded by effects of reverberation and aberration. The heterogeneous acoustical properties of different tissue types distort and reflect the wavefront as it travels to the target and as the echos travel back to the transducer. Transcranial imaging, has been a persistent challenge for ultrasound because the phase aberration, reverberation and attenuation from the human skull reduce the spatial resolution, to a millimeter or more, and limit contrast. Similar challenges arise in human abdominal imaging especially for patients with a large body mass index. Identifying, quantifying, and modeling these complex mechanisms of degradation are a critical component to develop rational strategies that can improve image quality. In this work, an experimental and simulation framework, calibrated to soft tissue measurements, that isolates and characterizes the individual sources of image degradation in ultrasound imaging is established. We show that using this simulation framework we can span the parameter space of image degradation in an independent or orthogonal fashion. Such flexibility offers advantages in the generation of training databases for machine learning applications as well as the development of beamforming strategies for challenging imaging scenarios. We also explored the framework's applications to lung ultrasound imaging, where the interpretation of reverberation artefacts occurring at the pleural surface is used to determine the underlying lung pathology. Using our acoustical simulation tools, B-mode images showcasing primary clinical features used in diagnostic lung imaging were successfully reproduced. These simulations establish a clear relationship of the artifacts to known underlying anatomical structures in a quantitative way. Transcranial simulations in 2D and 3D demonstrate that reverberations, whose role was previously unappreciated, are the principal source of image contrast and resolution degradation at shallow depths below 4~cm and when scattering tissue is present. Finally in the current work, the potential improvements offered by super-resolution imaging were explored by establishing the feasibility of transcranial super-resolution imaging through an intact human skull at a frequency of 2.5~MHz, with and without applying a phase correction, using with an existing clinical transducer.Doctor of Philosoph