Factors affecting the appearance  
 

DENSITY

Since x-rays are used to obtain CT images, the density of a structure affects the final image of a CT in a similar manner. 

Dense structures, like bone, attenuate more of the x-ray beam and appear white.  Less dense structures, like air, attenuate less of the x-ray beam and appear black.  Soft tissue structures are of intermediate densities and appear as varying shades of gray.

The CT detectors are much more sensitive than conventional radiograph cassettes.  With the detector and beam rotation allowing for the accumulation of many data points in one slice, spatial resolution is excellent, and subtle differences in soft tissue densities are visible which are not noted on routine x-ray.