Methods of and apparatus for preparing temperature activated gaseous precursor-filled liposomes are described. Gaseous precursor-filled liposomes prepared by these methods are particularly usefull for example, in ultrasonic imaging applications and in therapeutic drug delivery systems.Gas, gaseous precursors and perfluorocarbons are presented as novel potentiators for ultrasonic hyper-thermia. The gas, gaseous precursors and perfluorocarbons which may be administered into the vasculature, interstitially or into any body cavity are designed to accumulate in cancerous and diseased tissues. When therapeutic ultrasonic energy is applied to the diseased region heating is increased because of the greater effectiveness of sound energy absorption caused by these agents.