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On all Lateral CXRs these are three key nomal features :
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Both domes of the diaphragm should be visible.
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The vertebral bodies should become darker (ie blacker) when proceeding from above downwards.
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Excluding the rib shadows - there should not be any abrupt change in density across the cardiac shadow.
2. On this lateral CXR, note : only one dome of the diaphragm is visible, AND the vertebral bodies appear whiter ( rather than blacker ) in the lower thoracic spine.
3. These abnormal features indicate that there is shadowing in a lower lobe. Given this patient's clinical history - this shadowing is consistent with a lower lobe pneumonia.
[ Someone who is experienced will tell you - " actually, we can go further : this lateral CXR indicates that this pneumonia is situated in the left lower lobe . . . it is not in the right lower lobe ". So, ask an experienced CXR reader why such precision is possible in this patient ].
See : The Chest X-Ray : A Survival Guide : Pages 14-31