Dynamics of ultrasound contrast agents within rat cecum vessels

A theoretical model was established to study interaction between ultrasound contrast agents (UCAs, called microbubbles) and compliant microvessels. The predicted microbubble expansion and vessel wall displacement are in good agreement with the ex vivo experimental results using the rat cecum vessels for ultrasound with a center frequency of IMHz and a PRP (peak rarefactional pressure) of 800kPa. We demonstrate that compared to the bubble oscillation within an infinite liquid or within a 200-micron tube, bubble maximum expansion within the small vessels is substantially reduced. With two-cycle 1 MHz, 0.8 MPa PRP ultrasound, the relative expansion of the shelled microbubbles with a diameter of 4 and 7.2-μm estimated by the Rayleigh-Plesset equation is 4.1 and 3.0, respectively, while within a 14-μm vessel, we observed that the bubble expansion was 2.6 and 1.3-fold, respectively. Our model predicted that the corresponding relative expansion is 2.6 and 1.8, respectively. With 0.8 MPa PRP, 1 MHZ ultrasound, a 4 pm bubble within a 25 μm vessel can displace the vessel wall up to 2 microns and induce a circumferential stress near 36kPa.