TECHNICAL PERFECTION OF CHEST X – RAY
The first step to read a chest x-ray is to see whether it is technically perfect. Proceed further only when it is technically perfect.For example ; the chest x – ray of a child shows double left lung in the picture given here .The subsequent x-ray taken 72 hours later is normal. It was all due to the rotational error while taking the chest x – ray. The child was being investigated for pyrexia of unknown origin and the Chest X – ray here is part of that investigation which is normal. 
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