Ultrasound attenuation by encapsulated microbubbles: time and pressure effects.

Ultrasound (US) contrast agents (UCA) consist of artificial encapsulated microbubbles filled with low-diffusivity gas. This study evaluated, both experimentally and theoretically, the behavior of a cloud of encapsulated microbubbles while the surrounding pressure was modified within the physiological range. The theoretical analysis included calculation of US attenuation caused by a bubble cloud. The radius and gas content of each bubble were determined from a solution of a diffusion problem. Shell permeability and rigidity were taken into account. Both experiments and theory demonstrated that, for fixed ambient pressures, higher pressures result in increased rate of attenuation decay. Pulsatile ambient pressure induces pulsations of attenuation of the same frequency. In general, theoretical predictions are in good agreement with experimental data.