Microbubbles have a wide variety of applications in medicine, therapy and imaging. They are gas-filled bubbles (typically 3 μm in diameter and air gas of <200μL) that are usually injected intravenously. They are effective as contrast agents because they oscillate in an ultrasound beam. During this back and forth movement due to the response in the sound pressure waves, they fortunately vibrate at high frequencies used for diagnostic ultrasound (US) imaging. (1-10 MHz). This effect enhances S/N ratio and has specific spectral properties that can be taken advantage of for diagnostic improvements. One such such example is SonoVue® microbubbles commercially available at Bracco Imaging. The contract model is a bubble filled with sulfur hexafluoride.
Microbubbles are not only used for image enhancements but also as vehicles for drug delivery. They can be used to target tumours for cancer chemotherapy. Once these agents, normally coated in a thin polymer shell are delivered to the desired site, the microbubbles can be burst open by ramping up the frequency of the US and detecting local signals at tumour sites while releasing the drug.
With respect to the improvements of these agents, the major hurdles are developing technologies for site delivery and confining it there, detection development, and the process of expanding and contracting for creating various echoes. While US is one of the most common imaging modalities in medicine, one of the most exciting challenge is to bridge the gap between the use of US in molecular imaging. This is mainly due to the lack of molecular biosensors available. The most recent advancement to this was the generation of biogenic gas nanostructures as US agents[1]. Here, Shapiro et. al. have taken advantage of the fact there are certain photosynthetic micro-organisms that regulate their buoyancy by forming gas nanostructures, known as “gas vehicles” inside their own cell body. Their size can be up to nanometres in diameter and they were isolated from Anabaena flos-aquae (Ana) and Halobacteria NRC-1 (Halo). Attaching biomolecules, such as biotin to the surface of these nanostructures enables targeting capabilities.
[1] Shapiro, M. G. et al. Biogenic gas nanostructures as ultrasonic molecular reporters. Nat. Nanotechnol. 1–6 (2014).
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