Method development and automated analysis of ultrasound images of phase-shift bubbles.

Abstract

Ultrasound mediated drug delivery is an important tool in the fight against cancer. A new concept called Acoustic Cluster Therapy (ACT ) is under development, and two pilot imaging studies have been performed on prostate cancer xenografts in mice. A large amount of raw ultrasound data has been recorded, but existing software can not perform the required image processing. The ACT concept is based on clusters of microbubbles and microdroplets. When exposed to diagnostic ultrasound, the microdroplets become microbubbles. This phase-shift from liquid to gas is followed by a microbubble growth to 30μm. These phase-shift bubbles get stuck in the small capillaries of the tumor vasculature. A complete program has been developed in MATLAB® to process the raw ultrasound data. The program is tailored to the unique properties of the phase-shift bubbles, and is able to reduce noise and motion artefacts, to visualize the contrast agent, and to count the number of ultrasound activated phase-shift bubbles. The program produces high quality videos, displaying both free flowing contrast agent and identified, stuck phase-shift bubbles. The program was validated against a synthesized data set, and we found that the program counted accurately up to 2 bubbles/mm2. A saturation was experienced above this threshold, and too few bubbles were counted. The program was applied to a data set of 16 tumors, divided into four groups based on different ACT cluster dose and activation ultrasound settings. A significant difference (p = 0.023) was found between the different doses, while no significant difference (p = 0.146) was found between the different activation ultrasound settings. There was neither a correlation between the tumor size and the number of stuck phase-shift bubbles. The results show very good correlation with the resultss obtained from manual counting.