The patient may receive an injection of a "contrast material" to facilitate visualization of vascular structure. In typical examinations there are several phases each made up of 10 to 50 rotations of the x-ray tube around the patient in coordination with the table moving through the circular opening. The thickness of the fan beam may be as small as 1 millimeter or as large as 10 millimeters. The x-ray source produces a narrow, fan-shaped beam of x rays used to irradiate a section of the patient's body (Figure 4). As the patient passes through the CT imaging system, a source of x rays rotates around the inside of the circular opening.
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