Acoustical behavior of single ultrasound contrast agent microbubbles

Abstract

Ultrasound contrast agents (UCA) is a relatively new tool, used in medical \nultrasound 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 \ncontain 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. \nThis way we can provide a improved image of the structure of the organs, we can more \naccurately diagnose abnormal of defected areas on them, and if the containing gas is \nreplaced with an appropriate drug, focus drug delivery can be achieved, providing potential healing of the disease. \n! Up to now, most of the studies done in order to investigate the microbubbles - \nultrasound interaction are mostly population studies, which due to their roughly approach \ncannot clearly diagnose and explain the complex nonlinear and dynamic properties of \nUCAs and their response to ultrasound field. Because of that, our knowledge on the \nUCAsʼ, is yet limited although the potential improvements at imaging and therapeutic \ntechnics that would be gained from a thorough understanding of this interactions can be very big. \n! In this thesis, a study of single microbubbles is presented, using a system \ndeveloped by the group of Ultrasound imaging in Edinburgh University. The system uses a \nspecially modified commercial ultrasound machine, phased array probes and a specially \ndesigned single microbubbles measuring system. The system has the ability to feed, \ninsonify and capture the backscattered echo from a single microbubble, with the use of \nfully calibrated and characterized ultrasound - probe system. The aim of the project is to \ncontribute in the understanding of the behavior of the UCAs when insonified with an \nultrasound wave, and the underlying physical mechanism behind that. \nThe response of two different commercially available UCAs under insonification by \nultrasound waves of varying acoustical pressure is presented. The UCAs used are Definity \nand biSphere. In total echoes from 842 biSphere and 1994 Definity microbubbles have \nben captured and the data have been analyzed with in house made matlab codes. \nAlso the response of the bubbles when insonified by consequent pulses of different \nrepetition frequency has been investigated. For this reason, echoes from other 1221 \nDefinity and 459 biSphere microbubbles have been captured and analyzed. \nThe fully understanding of the agent - ultrasound interaction mechanisms can be the basis \nfor creating improved signal processing tools for ultrasound imaging contrast enhancement \nthat would improve imaging abilities of the modality, and potentially use of UCAs as a \ntherapeutic mean.