Atelectasis with reverse S sign is seen in

The Golden S-sign is seen on both PA chest radiographs and on CT scans. It is named because this sign resembles a reverse S shape, and is therefore sometimes referred to as the reverse S-sign of Golden. Although typically seen with right upper lobe collapse, the S-sign can also be seen with the collapse of other lobes. It is created by a central mass obstructing the upper lobe bronchus and should raise suspicion of a primary bronchogenic carcinoma. It can also be caused by other central masses, such as metastasis, primary mediastinal tumour, or enlarged lymph nodes(Grainger&;s diagnostic radiology, 6th ed, 328)