Here is a good Chest XR showing an infiltrate which shows up on the lateral projection. On the AP you can get a hint of infiltrate along the L cardiac border but it isn’t very impressive. However on the lateral you can really see the infiltrate as you get into the diaphragmatic recess. Normally on the lateral projection the vertebral bodies will become more lucent (darker) as you approach the diaphragm. One of the signs of a left lower lobe pneumonia is loss of the vertebral lucency on the lateral projection (as in this case). Other signs can be silhouetting of the diaphragm (the diaphragm will be obscured by infiltrate).
Author: Russell Jones, MD