Contrast agents are widely used in imaging, but until recently they had little place in ultrasonography. This has changed with the introduction of

microbubbles, small (typically 3 μm in diameter) gas filled bubbles that are usually injected intravenously.

How do microbubbles work?

Microbubbles work by resonating in an ultrasound beam, rapidly contracting and expanding in response to the pressure changes of the sound wave. They

vibrate particularly strongly at the high frequencies used for diagnostic ultrasound imaging. This makes them several thousand times more reflective than

normal body tissues. In this way they enhance both grey scale images and flow mediated Doppler signals. The resonance that microbubbles produce has

several special properties that can be exploited to improve diagnosis. Multiple harmonic signals or overtones are produced, similar to a musical instrument.

Ultrasound scanners can be tuned to “listen” to these harmonics, producing strong preferential imaging of the microbubbles in an image. The selective

excitation produced can also destroy microbubbles relatively easily, an effect that can be useful both in imaging and in emerging therapeutic applications.

Regardless of the shell or gas core composition, microbubble size is fairly uniform. They lie within in a range of 1-4 micrometers in diameter. That makes

them smaller than red blood cells, which allows them to flow easily through the circulation as well as the microcirculation. Depending on the type of

microbubbles used, contrast enhanced ultrasound is of 2 types:

• Targeted
• Untargeted