The magnitude of radiation force on ultrasound contrast agents

TLDR

High‑speed photography with 10‑ns resolution reveals that insonified bubbles translate under radiation force while also oscillating radially. A modified Rayleigh–Plesset model predicts bubble radius dynamics, and a differential equation links bubble acceleration to acoustic radiation force and fluid drag to compute translation. Simulations and experiments show microbubbles can move several microns—e.g., a 1.5‑µm agent travels over 5 µm at >0.5 m/s during a 20‑cycle pulse—so radiation force must be considered in ultrasound imaging.

Abstract

High-speed photography of insonified bubbles with a time resolution of 10 ns allows observations of translation due to radiation force, in addition to the visualization of radial oscillations. A modified version of the Rayleigh–Plesset equation is used to estimate the radius–time behavior of insonified microbubbles, and the accuracy of this model is verified experimentally. The translation of insonified microbubbles is calculated using a differential equation relating the acceleration of the bubble to the forces due to acoustic radiation and the drag imposed by the fluid. Simulations and experiments indicate that microbubbles translate significant distances with clinically relevant parameters. A 1.5 micron radius contrast agent can translate over 5 microns during a single 20-cycle, 2.25 MHz, 380 kPa acoustic pulse, achieving velocities over 0.5 m/s. Therefore, radiation force should be considered during an ultrasonic examination because of the possibility of influencing the position and flow velocity of the contrast agents with the interrogating acoustic beam.