A suspension of gas-filled microbubbles can be synthesized by sonicating a lipid solution and a first gas in a reaction volume. The resulting microbubble suspension can be stored for later use, for example, for infusion into a patient for gas delivery thereto. Various techniques can improve the shelf life of the microbubbles. For example, the microbubble suspension can be freeze-dried to remove water and the first gas therefrom while leaving the microbubble shells intact. In an alternative, the microbubble suspension can be frozen. In still another alternative, the microbubble suspension can be formed with a first gas that has a low solubility, thereby creating microbubbles with increased stability. Prior to use, the microbubble suspension can be prepared by exchanging the gas in the microbubble cores, rehydrating, and/or raising the temperature of the stored microbubbles.
