In Computed Tomography (CT), a motorized table moves the patient through the CT imaging system. At the same time, a source of x-rays rotates within the circular opening, and a set of x-ray detectors rotates  on the far side of the patient. The x-ray source produces a narrow, fan-shaped beam, with widths ranging from 1 to 20 mm. In axial CT, which is commonly used for head scans, the table is stationary during a rotation, after which it is moved along for the next slice. In helical CT, which is commonly used for body scans, the table moves continuously as the x-ray source and detectors rotate, producing a spiral or helical scan.

CT diagram from Brenner and Hall

The illustration shows a single row of detectors, but current machines typically have multiple rows of detectors operating side by side, so that many slices (currently up to 64) can be imaged simultaneously, reducing the overall scanning time. All the data are processed by computer to produce a series of image slices representing a three-dimensional view of the target organ or body region.
Taken from Brenner DJ, Hall EJ. N Engl J Med 2007;357:2277-2284.

Radiation dose from CT can be as much as 300 times higher than the dose from a typical x-ray or “radiograph.” Controversy exists in radiology about whether this amount of radiation dose, measured in milligrays, is capable of causing cancer.