Findings
-Aspiration may cause pneumonia in the dependent basilar segments.
-On lateral CXR, the normal spine should appear more lucent (darker) in the lower thorax.
-On lateral CXR, a positive lateral spine sign is said to be present if the lower thoracic vertebrae appear whiter.
-A positive lateral spine sign is frequently caused by basilar infiltrates.


http://eradiology.bidmc.harvard.edu/Classics/item.aspx?section=Emergency+Radiology&labelpk=33f0adab-853f-4010-a0e2-0756b3f1eac5&pk=6b5e5a96-e799-4206-b3fe-5e82667cd979