Bubble dynamics of ultrasound contrast agents

The bubble dynamics involved with microbubble contrast agents under insonification is investigated. The acoustic field of an ATL HDI‐3000 diagnostic ultrasound system in a contrast specific harmonic imaging mode is reviewed first, and its major features that are related with microbubble behavior are discussed. Issues relating to sound attenuation, mechanical index, and bubble destruction are addressed. The nonlinear oscillatory behavior of contrast microbubbles is modeled with the Gilmore equation. The acoustic pressure field of a short pulse utilized in harmonic imaging is measured with a hydrophone and used as the driving pressure of the Gilmore model. Radius‐time [R(t)] and bubble wall velocity‐time [U(t)] curves are shown. Frequency domain analysis of U(t) indicates transient resonance characteristics in both the fundamental and second harmonic components that are somewhat different from what one would expect with a continuous‐wave steady‐state response. The times for complete solution of microbubbles in water are calculated and correlated to observations seen in ultrasound images with contrast agents. Radio frequency (rf) data of scattered pulses from contrast agent microbubbles in an in‐vitro experiment were collected with a phased array. This data is used to support and explain the contrast microbubble behavior.