Shell buckling enhances subharmonic behavior of phospholipid coated ultrasound contrast agent microbubbles.

Subharmonic behavior of coated microbubbles can greatly enhance the contrast in ultrasound imaging. The threshold driving pressure above which subharmonic oscillations are initiated can be calculated from a linearized Rayleigh‐Plesset‐type equation. Earlier experimental studies on a suspension of phospholipid‐coated microbubbles showed a lower threshold than predicted from traditional elastic shell models. Here we present an experimental study of the subharmonic behavior of individual BR‐14 microbubbles (Bracco Research) with initial radii between 1.6 and 4.8 μm. The subharmonic behavior was studied as a function of the amplitude and the frequency of the driving pressure pulse. The radial response of the microbubbles was recorded with the Brandaris ultrahigh‐speed camera, while the resulting acoustic response was measured with a calibrated transducer. It is shown that the threshold pressure is minimum near a driving frequency equal to half the resonance frequency of the bubble, as expected. We found a threshold pressure as low as 10 kPa for certain bubble sizes, which can be explained by the shell buckling model proposed by [Marmottant et al., JASA (2005)]. We show that the origin of subharmonic behavior is a result of the discontinuous transition within the bubble shell from the elastic state to the tensionless buckling state.