Principles and Concepts of Computed Tomography

Abstract

The final CT image is actually made of a grid of tiny squares called pixels. The scale of grays assigned to each pixel represents the attenuation of x-rays by the structures in the tomographic slice. Manipulation of the gray scale allows optimal visualization of all the tissues within the slice. This control over the gray scale and the absence of structure superimposition are the advantages of CT over conventional x-ray techniques. The steps used to acquire the final CT image are collection of data from the patient (recording of transmitted x-ray intensities from many angles), computer processing of data (mathematical calculation of attenuation of each structure in the tomographic slice), image display (assigning of appropriate gray scale to CT image to evaluate all structures), and data storage (recording series of tomographic images on x-ray film and archiving image data for later review). Common image artifacts that must be identified and interpreted as such include aliasing, ring artifacts, beam hardening effect, metal, motion, partial volume averaging, and streaking from out-of-field objects.