Chirp coding for ultrasound imaging of polymer-shelled microbubbles

Recently, polymer-shelled microbubbles have been proposed, showing some important advantages. They offer both a mechanical stability and a distinctive feature to be easily targeted to a specific address within the body. This paper focuses on the opportunity of use polymeric microbubbles as contrast agent in medical imaging applications, where the nonlinear response is exploited and the destruction of bubbles is avoided. The major drawback of polymer-shelled microbubbles is related to the lower elasticity of their shell. Lower echoes are then expected. As a consequence, it is evident the risk that, for low/moderate pressure values, the response of these microbubbles is not sufficiently nonlinear to allow a good detection of the contrast agent presence. A non-conventional method, based on the transmission in temporal succession of three chirp pulses, is employed to detect polymeric bubbles at different acoustic pressures. The results reveal that this signal processing technique is particularly adapted for the detection of polymer-shelled microbubbles even at low acoustic pressures and low concentrations, where conventional techniques, as pulse inversion and contrast pulse sequence imaging, fail.