Ultrafast framing optical and acoustical recording of nonlinearly oscillating microbubbles

Microbubble nonlinear behavior improves detection efficacy of ultrasound contrast agents (UCAs), but not all microbubble sizes present in the dispersed UCA population will contribute to the detection signal. This paper experimentally studies the relation between size and nonlinear response of isolated lipid-coated microbubbles, held in a transparent tube. The echo from microbubbles is acoustically recorded with a sensitive custom electronic system, and simultaneously the radial excursion is optically recorded with an ultrafast framing camera. Ultrasound-induced deflation is used to sample the bubble at multiple sizes. At sizes smaller than the resonant size the bubbles show compression-only behavior accompanied by strong second harmonic scattering. Subharmonics are observed for bubble sizes larger than resonant.