How images are obtained  

CT images use x- rays to obtain a thin axial image of the patient- a “slice”.  To obtain CT images, the beam of x-rays and the x-ray detector circle around the patient, continuously producing and detecting a thin beam of x-rays that pass through the patient. This is in contrast to radiographs, where a stationary x-ray beam source and detector are used.


The patient lies on a table that slowly moves through a circular tube, called the gantry. The gantry houses the generator that produces the x-rays as well as a specialized x-ray detector. The two are located 180 degrees apart from one another. The generator and detector move in unison around the patient, continuously transmitting a thin beam of x-rays through the patient. Those that are not absorbed by the patient are sensed by the detector and transmitted to a computer. The data is analyzed and an image produced which represents one "slice" of the patient.

Traditionally, with each 360 degree rotation of the x-ray beam and detector, data is obtained from that axial section of the patient.  The table then moves, and another axial section of the patient is obtained.  With newer CT scanners, the table moves continuously through the gantry, and the generator and detector continuously circle the patient.  As a result, instead of a single “slice” of the patient being obtained with each rotation, a continuous helix of data is obtained.  A specialized computer interprets the information from the detector and transforms it into images, which represent sequential “slices” of the patient.