Provided herein is a tag comprising an imaging sphere suspended within a matrix. In some embodiments, the matrix comprises a polymer. In some embodiments, the matrix comprises an adhesive polymer. In some embodiments, the matrix comprises a shelf-stable polymer. In some embodiments, the matrix comprises an epoxy. In some embodiments, the adhesive polymer comprises cyanoacrylate. In some embodiments, the imaging sphere comprises a cavity loaded with an imaging agent surrounded by a shell. In some embodiments, the imaging agent exhibits echogenic properties. Further provided is a method for preparing an imaging sphere comprising: depositing organic or inorganic material onto a bead surface to form a sphere with a solid core； removing the solid core by solvent extraction or calcination to form a hollow sphere； and loading the hollow sphere with an imaging agent.
