Ultrasound contrast agents: optical and acoustical characterization

This thesis describes the characterization of the dynamics and the acoustic responses of single BR14 (Bracco Research S.A., Geneva, Switzerland) ultra- sound contrast agent microbubbles under the in°uence of ultrasound. In Ch. 2 of this thesis we investigate the small amplitude behavior of isolated microbubbles acoustically. To ensure that the measured acoustic response orig- inates from one bubble only, it requires the isolation of a single microbubble within an ultrasound beam. Furthermore to operate in the linear regime, the measurements should be performed at low acoustic driving pressure amplitudes, which puts stringent requirements on the sensitivity of the receiving transducer. In this chapter we present a study where both these challenges are met. In Ch. 3 and Ch. 4 the Brandaris ultrahigh-speed camera is used to investi- gate the radial dynamics of di®erently sized phospholipid coated microbubbles optically. Here, we present an experimental study where the driving pressure amplitude and frequency is varied to investigate the dependence of the non- linear subharmonic and \compression-only" behavior of phospholipid coated microbubbles on the oscillation amplitude of the bubble wall. A comparison between the experimental results and full numerical simulations presented in this chapter show the initial surface tension of the bubble determines both its subharmonic and its \compression-only" behavior. In the ¯nal chapter, Ch. 5, the optical characterization methods of Ch. 3 and Ch. 4 are combined with the acoustical characterization method presented in Ch. 2. The radial dynamics and the acoustic response of individual isolated pul- sating microbubbles are simultaneously recorded using the Brandaris ultrahigh- speed camera and a sensitive calibrated receive transducer. In this chapter we show how the bubble sound emission can be predicted from the recorded radial dynamics. Vice versa the radial dynamics of a microbubble can be determined from the remotely recorded acoustic response.