Research Interest

Assessment of the safety and efficiency of microbubble exposure in diagnostic and therapeutic ultrasound

GR ter Haar, A Tokarczyk, I Rivens in collaboration with E Stride University College, London
Source of funding: EPSRC

Ultrasound contrast agents (microbubbles) are widely used in ultrasound imaging especially in echocardiography. These are 2-10 µm gas-filled bubbles which, after injection, remain in the cardiovascular system. The behaviour of contrast agents during ultrasound exposure may, however, cause beneficial (sonoporation, enhancement of gene delivery) or harmful (cell damaging) bioeffects.  This project is focused on exploring the direct interaction between ultrasound-exposed microbubbles and a blood vessel.

An experimental model using an isolated, cannulated blood vessel (~400 µm outer diameter) has been developed. This allows microscopic visualisation of vessel behaviour after exposure to ultrasound with or without contrast agents and monitoring of cavitation activity using a fibre-optic hydrophone (Precision Acoustics, UK).  Vessels have been perfused with contrast agent (SonoVue) at concentrations of zero, 104 or 107 bubbles/ml immediately before exposure to 1.7 MHz HIFU.  Vessels exposed to 4.3 MPa (peak negative pressure) or above were characterised by a reduced response to Acetylcholine (indicative of endothelial cell viability), but only the presence of contrast agents  resulted in leakage of intraluminal buffer after exposure at those pressures. A 2.5 MPa, 5 s exposure caused no damage to vessels in any experimental group and their behaviour was similar to that of control (unexposed) arterioles.  Broadband emission monitoring showed increased acoustic cavitation activity was seen when there was a reduced vessel response to Acetylcholine (indicating endothelial cell damage). Vascular permeability changes occurred only in the presence of SonoVue enhanced acoustic cavitation.  These results will be confirmed by histology using tinctorial and immunohistochemical stains.

It is intended that this model will be also used to investigate the bio-effects induced by the exposure of vessels to diagnostic US in the presence of contrast agents.

Figure 1.  Experimental system used for the exposure of rat mesenteric vessels to ultrasound in the presence of microbubble contrast agents.

Figure 2.  Typical response of a vessel to 5MPa peak negative pressure in the presence of microbubble contrast agents.  Top: graph showing the vessel’s response to Phenylephrine (indicating smooth muscle cell function) and Acetylcholine (indicating endothelial cell function) before and after ultrasound exposure.  Bottom left: fluorescence image after ultrasound exposure, showing localised uptake of FITC dye.  Bottom right: cavitation detection using a fibre optic hydrophone during ultrasound exposure.