Ultrasound Contrast Agents of the first generation contained air as gas, which has a good solubility in water (blood). Under the influence of blood pressure and acoustic pressure in the ultrasound field parts of the gas are 'squeezed out' and dissolved in the blood.

Second generation Contrast Agents contain an inert lipophilic gas with very low solubility in blood, avoiding early leakage of the gas and making the microbubbles much more stable.

The shell of modern ultrasound Contrast Agents consists of a thin and flexible membrane of phospholipids arranged in a monolayer with the hydrophilic heads on the outer side (towards the surrounding liquid/blood) and the lipophilic chains on the inner side (towards the encapsulated gas).