The invention relates to a process for the preparation of beads comprising a biocompatible hydrophobic polymer, a perfluorocarbon and optionally a metal compound, comprising the steps of:a) adding the perfluorocarbon and optionally the metal compound to a solution of the biocompatible hydrophobic polymer in a polar solvent to provide a first liquid mixture,b) adding the first liquid mixture to an aqueous solution of a biocompatible surfactant under sonication to obtain a second liquid mixture,c) maintaining the sonication of the second liquid mixture while cooling,d) evaporating the polar solvent from the second liquid mixture to obtain a suspension of beads comprising the biocompatible hydrophobic polymer, the perfluorocarbon and optionally the metal compound,e) separating the beads from the suspension and preparing a water suspension of the beads andf) freeze-drying the water suspension to obtain the beads,wherein the addition of the first liquid mixture to the biocompatible surfactant in step b) is performed within a period of at most 10 seconds,wherein the sonication in step b) and the sonication in step c) are performed by a probe at an amplitude of at least 240 µm for 1-10 minutes andwherein the weight ratio of the biocompatible surfactant to the biocompatible hydrophobic polymer is at least 3:1.
