Acoustical behavior of single ultrasound contrast agent microbubbles

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Δερμιτζάκης, Αριστείδης
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Ultrasound contrast agents (UCA) is a relatively new tool, used in medical ultrasound imaging to enhance the diagnostic abilities of ultrasound and potentially offer a therapeutic value in medical ultrasounds. UCAs are bi or single-layered bubbles, that contain a highly backscattering gas. By injecting them into the blood flow, we locally increase the echogenicity and therefor enhance the contrast of the blood-tissue interface. This way we can provide a improved image of the structure of the organs, we can more accurately diagnose abnormal of defected areas on them, and if the containing gas is replaced with an appropriate drug, focus drug delivery can be achieved, providing potential healing of the disease. ! Up to now, most of the studies done in order to investigate the microbubbles - ultrasound interaction are mostly population studies, which due to their roughly approach cannot clearly diagnose and explain the complex nonlinear and dynamic properties of UCAs and their response to ultrasound field. Because of that, our knowledge on the UCAsʼ, is yet limited although the potential improvements at imaging and therapeutic technics that would be gained from a thorough understanding of this interactions can be very big. ! In this thesis, a study of single microbubbles is presented, using a system developed by the group of Ultrasound imaging in Edinburgh University. The system uses a specially modified commercial ultrasound machine, phased array probes and a specially designed single microbubbles measuring system. The system has the ability to feed, insonify and capture the backscattered echo from a single microbubble, with the use of fully calibrated and characterized ultrasound - probe system. The aim of the project is to contribute in the understanding of the behavior of the UCAs when insonified with an ultrasound wave, and the underlying physical mechanism behind that. The response of two different commercially available UCAs under insonification by ultrasound waves of varying acoustical pressure is presented. The UCAs used are Definity and biSphere. In total echoes from 842 biSphere and 1994 Definity microbubbles have ben captured and the data have been analyzed with in house made matlab codes. Also the response of the bubbles when insonified by consequent pulses of different repetition frequency has been investigated. For this reason, echoes from other 1221 Definity and 459 biSphere microbubbles have been captured and analyzed. The fully understanding of the agent - ultrasound interaction mechanisms can be the basis for creating improved signal processing tools for ultrasound imaging contrast enhancement that would improve imaging abilities of the modality, and potentially use of UCAs as a therapeutic mean.
Microbubbles, Ultrasound, Acoustic response