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A multimodality contrast agent able to be detected with ultrasound and MRI can combine the advantages of the two diagnostic techniques. In this paper, a preliminary ultrasound assessment of two different types of innovative magnetic microbubbles is presented. In both cases, the combination of superparamagnetic iron oxide nanoparticles with polymeric microbubbles is used to produce the contrast agent. The only difference is the structure of the magnetic bubble: the nanoparticles are attached on the polymeric shell in the first case, while in the second case they are physically trapped into the membrane. After a preliminary comparison with polymer-shelled bubbles without magnetic nanoparticles, the performance of the two types of magnetic bubbles are compared through in vitro experiments. A non-conventional signal processing technique, particularly adapted to detect polymeric bubbles and based on the combination of multi-pulse excitation and chirp coding, is applied. The measurement of the contrast-to-tissue ratio reveals that the detection sensitivity of the signal processing technique depends on the structure of the magnetic microbubbles.