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Principles of CT Operation: What is X-ray absorption?

What is X-ray absorption?

Physically, the contrast originates from different X-ray absorptions of different object areas which can be caused by varying thickness as well as by varying material:

In the below image object A is of the same thickness as object B but yields a higher absorption due to higher density or higher atomic number. Object C consists of the same material as object B but absorbs less radiation than the thicker object B.