Different tissues in the body give different shades on CT scan according to their densities. These shades are described by CT number or Hounsfield unit. For example bone appear white on ct scan with HU of 1000 and air appear black having HU of -1000. There are many shades in between these two which are important in recognizing normal tissues and also pathologies.
Following CT image showing tissues with different densities. I found calcified aorta an interesting thing to pick :-p
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