What is a CT Scanner?

What is a CT Scanner?

A computed tomography (CT) scanner is a special type of x-ray machine which rotates around a patient to obtain cross-sectional images of their body—that is, slice-by-slice views of their insides.

Most modern-day CT scanners consist of an x-ray source mounted on a circular ring, or gantry, opposite an arc of detectors. The patient lies on a table at the center of the gantry, the gantry is rapidly rotated, and as it rotates the source emits a fan-like beam of x-rays toward the detectors. Some of the x-rays are absorbed or scattered by the patient’s body while others pass straight through to the detectors. The x-ray intensities measured at each angle provide information about the density of the patient’s body along that direction, and by combining information from many different angles it is possible to generate a cross-sectional image of the patient. This process is called tomography.

CT imaging is thus quite different from traditional x-ray imaging (aka “projection radiography”) such as chest x-rays. Whereas projection radiography uses x-rays to generate a two-dimensional (2D) image of the patient directly on film or a digital detector from a single point of view, CT uses computers to reconstruct a three-dimensional (3D) image from x-rays detected from multiple points of view. This is how CT gets the name “computed tomography.”

How does a CT scanner work?

By slowly moving the patient table through a CT scanner, the computerized system can generate a series of consecutive image slices (“axial views”) which can be combined to form a 3D image of the patient. Once a 3D image has been created, it is possible to view cross-sections of the patient from any direction. Sometimes a 4D image (i.e., a “movie”) is obtained by repeatedly scanning the patient for a short period of time. This is often done to see how a patient’s tissues move during breathing.

How are CT scans used?

CT scanners are commonplace in modern health care because they are extremely useful for diagnostic purposes—that is, for noninvasively imaging patient anatomy and identifying disease. CT imaging also plays a crucial role in modern radiation therapy owing to some fundamental characteristics of the images themselves. Because a CT image is created using x-rays, it represents a physically meaningful map of how x-rays interact with the patient’s body. For this reason, CT images are commonly used as the basis for dose calculations in radiation therapy treatment planning.

As a historical note, CT scanners were once commonly referred to as CAT scanners, for “computerized axial tomography,” and this name is still frequently encountered in popular use.

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