Editors' ChoiceUltrasound Therapy

Bubbles and Sound, What a Blast!

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Science Translational Medicine  05 Oct 2011:
Vol. 3, Issue 103, pp. 103ec163
DOI: 10.1126/scitranslmed.3003258

Bubbles are fun to play with and fascinating to observe. You can make them out of chewing gum, commercial “blowing bubbles” solution, or even just soapy dishwater. Or, as described by Zhang et al., tiny microbubbles can be created from biocompatible microdroplets by using a technique called acoustic droplet vaporization (ADV). These microbubbles have the potential to destroy cancerous tissues, demonstrating that bubbles are not only fun, but also therapeutic.

Ultrasound is traditionally used to image tissues in the body; but high-intensity focused ultrasound (HIFU) can also be applied at a focal point to agitate cells, causing them to heat to a temperature that is deadly. Such HIFU­ treatment can be used for cancer therapy and is fast, noninvasive, and does not involve radiation. Combining HIFU with microbubble contrast agents can increase the heating effects; however, control over the focal point of cell destruction remains challenging. With ADV, high-energy ultrasound can focally convert microdroplets into microbubbles. This allows only the target tissue to be heated, without damaging surrounding healthy tissue. Zhang et al. performed in vitro experiments using tissue-mimicking phantoms, as well as in vivo experiments in canine livers, to show how ADV can improve targeted HIFU thermal ablation. Lipid-coated perfluoropentane droplets were first converted into microbubbles by using ADV. Then, the resulting bubble cloud—reaching a volume of ~0.3 cm3—permitted real-time ultrasound imaging of the size, shape, and location of the lesion during HIFU treatment. Importantly, Zhang and colleagues found that the size of the lesion created by HIFU with ADV was 15 times larger in vivo than by HIFU alone. Furthermore, the lesions were created at the expected location, which is an improvement over previous techniques that used ultrasound microbubble contrast agents.

There are several advantages to this technique compared with standard HIFU without microbubbles. First, limiting the heating to a focal region (such as cancerous tissue) can speed up the heating process, leading to decreased treatment times. Second, the microbubbles block the ultrasound waves, protecting tissues behind the bubbles. Third, the newly created bubbles can last for several minutes, essentially cutting off blood flow to cancer cells. The combination of this microbubble treatment method and real-time clinical ultrasound imaging will allow doctors to treat patient tumors effectively… and what a blast that will be.

M. Zhang et al., Acoustic droplet vaporization for enhancement of thermal ablation by high intensity focused ultrasound. Acad. Radiol. 18, 1123–1132 (2011). [Abstract]

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