Disclosed is a suspension of gas-filled microbubbles in a physiologically acceptable liquid carrier comprising a lipid mixture of a first lipid having transition temperature of about 41 °C such as DPPC or DPPG, a second lipid having transition temperature of about 55 °C such as DSPC or DSPG, and a PEGylated DSPE such as DSPE-PEG2000, DSPE-PEG3000, or DSPE-PEG5000, and methods of preparation thereof.