Injectable Bubbles for Physiological Pressure Measurement

Microbubbles - used as contrast agents in ultrasound imaging - are important tools in biomedical research, having been used together with ultrasound to develop significant diagnostic and therapeutic techniques. It has been suggested that the dynamic behaviour of microbubbles is dependent on the surrounding fluid's ambient (hydrostatic) pressure, and the potential to non-invasively determine blood pressure has numerous medical applications. To study this dependence, a computational mathematical model was created based on Marmottant's dynamic model of a microbubble. A pulse inversion (PI) protocol was incorporated into the model to emphasize the nonlinear behaviour of the microbubble's response. The mathematical model was also used to assess the sensitivity of the microbubbles to undesirable changes in parameters other than the ambient pressure. It found that a variation in the microbubble's initial radius and surface tension would cause the most significant changes in signal energy and hence pose a risk to ambient pressure measurements. To test the practicality of detecting a change in the dynamic behaviour of microbubbles, in vitro experiments were designed and carried out using clinically available contrast agent with two different ultrasound systems. The experiments, while possessing certain limitations, confirmed that there is a change in microbubbles' dynamic behaviour when the ambient pressure is varied, in cases by as little as 7.36 mmHg (981 Pa - a 0.98% change in atmospheric pressure). The experimental results establish a proof-of-principle that future experimental work can build upon to verify the mathematical model, and hence aid in developing a non-invasive blood pressure measurement procedure.Comment: A thesis submitted in partial fulfilment of the requirements for the undergraduate degree of Master of Engineering (MEng) in the Department of Engineering Science, University of Oxfor