Whats bibasilar atelectasis? why would you need ct to find it?

Lung not expanded: Means that atelectatic lung is not fully expanded. Atelectasis can be minor where there are linear areas not fully expanded or more severe where full lobe or segment of lobe of lung collapsed. Don't necessarily need ct. Often diagnosed on chest x ray.

Lung base atelectasi: Bibasilar atelectasis typically refers to atelectasis at the lung bases. This is typically seen on chest xray or ct. Ct is not necessary for diagnosis. This mild atelectasis is typically from incomplete inspiration at the time of the exam and would resolve. It can be from prior infection or surgery, with chronic scarring. More severe form of atelectasis may indicate more significant disease.