Factors affecting the appearance  
 

DENSITY

CT allows soft tissue structures seen as varying shades of gray to be differentiated from each other. This is usually not possible on x-ray.

Note that the fluid filled gallbladder can be differentiated from the adjacent liver, which is higher in density and appears a lighter shade of gray.  The spleen and left kidney can be seen distinctly from each other, partially due to the excellent spatial resolution of CT, but also due to differences in density.