A CT scanner uses an X-ray source with a narrow slit that generates a fan-beam, wide enough to encompass the
extent of the body, but only a few millimeters in thickness [Fig. TF6-3(a)]. Instead of recording the attenuated X-ray
beam on film, it is captured by an array of some 700 detectors. The X-ray source and the detector array are mounted
on a circular frame that rotates in steps of a fraction of a degree over a full 360◦ circle around the patient, each time
recording an X-ray attenuation profile from a different angular perspective. Typically, 1,000 such profiles are recorded
per each thin traverse slice of anatomy. In today’s technology, this process is completed in less than 1 second. To
image an entire part of the body, such as the chest or head, the process is repeated over multiple slices (layers), which
typically takes about 10 seconds to complete.