Microbubble spectroscopy of ultrasound contrast agents

We present a new optical characterization of the behavior of single ultrasound contrast bubbles. The method consists of insonifying individual bubbles several times successively sweeping the applied frequency and recording movies of the bubble response up to 25 million frames per second with an ultra-high-speed camera operated in a segmented mode. The method, termed microbubble spectroscopy, enables one to reconstruct a resonance curve of the oscillating bubble in a single run. We analyze the data through a linearized model for coated bubbles. The results confirm the significant influence of the shell on the bubble dynamics: shell elasticity increases the resonance frequency by about 50%, and shell viscosity is responsible for about 70% of the total damping. The obtained value for shell elasticity is in quantitative agreement with previously reported values. The shell viscosity decreases significantly with the dilatation rate, revealing the nonlinear behavior of the phospholipid coating.