"Compression-only" behavior of phospholipid-coated contrast bubbles.

Ultrasound contrast agents oscillate approximately linearly up to a certain pressure range where nonlinearity sets in. Nonlinear microbubble oscillations are exploited in ultrasound pulse-echo imaging as this improves the contrast-to-tissue ratio. Here we report the observation of a highly nonlinear response of phospholipid-coated contrast agents at pressures as low as 50 kPa, termed "compression-only" behavior, where the microbubbles only compress, yet hardly expand. Time-resolved bubble dynamics recorded through ultra high-speed imaging revealed that nearly 40% of the coated bubbles show "compression-only" behavior.