Anatomy.—A bleb is a small gas-containing space within the visceral pleura or in the subpleural lung, not larger than 1 cm in diameter (25).
CT scans.—A bleb appears as a thin-walled cystic air space contiguous with the pleura. Because the arbitrary (size) distinction between a bleb and bulla is of little clinical importance, the use of this term by radiologists is discouraged. [Fleischner Society]