CT of obstructive lobar collapse.

The varied computed tomographic (CT) appearance of obstructive lobar collapse is described in 25 cases. CT is helpful in understanding the morphology and mechanisms of lobar collapse and in diagnosis., atypical cases. The final shape of the collapsed lobe is dependent on the size and location of the obstructing tumor and on the amount of retained lung fluid. Focal bulging of the fissure (S-sign) is the most helpful sign in identifying the obstructing tumor. Differential enhancement could not separate the tumor from collapsed lung in six of eight (75%) of the cases studied with intravenous contrast, a finding at variance with previous reports. The limitations of CT in evaluating mediastinal or pleural invasion in the presence of lobar collapse are discussed.