Preparation of dispersions of particles for use as contrast agents in ultrasound imaging

A method of making a suspension of particles in a first fluid of a size suitable for responding to ultrasound, by forcing a second fluid (2) through an array of pores (30) into the first fluid (1), the pores being of substantially uniform diameter, and the pressure of the second fluid and a flow rate of the first fluid across the pores being arranged so that shear forces at the pores cause the second fluid to be suspended as substantially monodisperse particles in the first fluid. This can enable a more monodisperse suspension. The array of pores can be an etched silicon array. The suspension can be used as a contrast agent, or can be an emulsion to make droplets of a precursor material, which can be formed into capsules with a core and a shell. A liquid core can be converted to a gas to provide hollow shells.