ABSTRACT
The detector unit contains a large number of detectors arranged on an arc of a circle centered at the X-ray source. X-rays travel along straight lines between the source and the detectors. From the strength of the X-ray beam reaching the detector, the total X-ray attenuation along the line between the source and the detector can be estimated. If the X-ray source and detector assembly are kept stationary and the patient is moved through the gantry, an image is obtained which is similar to images obtained in traditional X-ray film radiography. When an X-ray beam passes through the body, its attenuation at any point depends on the material at that point and on the energy distribution of the beam. In computed tomography (CT), the spectrum is made up from many energy levels and it changes as the beam passes through the object. There are several other sources of error in CT besides the polychromatic nature of the X-ray beam.
