Assembly comprising a gas-filled microvesicle and a structural entity which is capable to associate through an electrostatic interaction to the outer surface of said microvesicle (microvesicle associated component—MAC), thereby modifying the physico-chemical properties thereof. Said MAC may optionally comprise a targeting ligand, a bioactive agent, a diagnostic agent or any combination thereof. The assembly of the invention can be formed from gasfilled microbubbles or microballoons and a MAC having a diameter of less than 100 pm, in particular a micelle and is used as an active component in diagnostically and/or therapeutically active formulations, in particular for enhancing the imaging in the field of ultrasound contrast imaging, including targeted ultrasound imaging, ultrasound-mediated drug delivery and other imaging techniques such as molecular resonance imaging (MRI) or nuclear imaging.