"Such microbubbles or microballoons may comprise gases such as, e.g., air, nitrogen, carbon dioxide, or hydrogen; or noble gases such as helium, argon, xenon or krypton; or sulfur-containing fluorinated gases such as sulfurhexafluoride, disulfurdecafluoride or trifluoromethylsulfurpentafluoride; or, for example, selenium hexafluoride; or halogenated silanes such as methylsilane or dimethylsilane; o" . . . . . .