What is Computed Tomography?
Conventional X ray Images
All x-ray imaging is based on the absorption of x rays as they pass through
the different parts of a patient's body. Depending on the amount absorbed
in a particular tissue such as muscle or lung, a different amount of x
rays will pass through and exit the body. The amount of x rays absorbed
contributes to the radiation dose to the patient.
During conventional x-ray imaging, the exiting x rays interact with a
detection device (x-ray film or other image receptor) and provide a 2-dimensional
projection image of the tissues within the patient's body - an x-ray produced
"photograph" called a "radiograph." The chest x ray
(Figure 1) is the most common medical imaging examination. During this
examination, an image of the heart, lungs, and other anatomy is recorded
on the film.
Figure 1: Chest X ray Image
Computed Tomography (CT)
Although also based on the variable absorption of x rays by different
tissues, computed tomography (CT) imaging, also known as "CAT scanning"
(Computerized Axial Tomography), provides a different form of imaging
known as cross-sectional imaging. The origin of the word "tomography"
is from the Greek word "tomos" meaning "slice" or
"section" and "graphe" meaning "drawing."
A CT imaging system produces cross-sectional images or "slices"
of anatomy, like the slices in a loaf of bread. The cross-sectional images
(Figure 2) are used for a variety of diagnostic
and therapeutic purposes.
Figure 2: Cross-sectional Image of Abdomen
How a CT system works
- A motorized table moves the patient (Figure 3) through a circular
opening in the CT imaging system.
Figure 3: Patient in CT Imaging System
- As the patient passes through the CT imaging system, a source of x
rays rotates around the inside of the circular opening. A single rotation
takes about 1 second. 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). The thickness of the fan beam may be as small as 1 millimeter or
as large as 10 millimeters. 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 patient may receive an injection of a "contrast material"
to facilitate visualization of vascular structure.
Figure 4: CT Fan Beam
- Detectors on the exit side of the patient record the x rays exiting
the section of the patient's body being irradiated as an x-ray "snapshot"
at one position (angle) of the source of x rays. Many different "snapshots"
(angles) are collected during one complete rotation.
- The data are sent to a computer to reconstruct all of the individual
"snapshots" into a cross-sectional image (slice) of the internal
organs and tissues for each complete rotation of the source of x rays.
Advances in Technology and Clinical Practice
Today most CT systems are capable of "spiral" (also called
"helical") scanning as well as scanning in the formerly more
conventional "axial" mode. In addition, many CT systems are
capable of imaging multiple slices simultaneously. Such advances allow
relatively larger volumes of anatomy to be imaged in relatively less time.
Another advancement in the technology is electron beam CT, also known
as EBCT. Although the principle of creating cross-sectional images is
the same as for conventional CT, whether single- or multi-slice, the EBCT
scanner does not require any moving parts to generate the individual "snapshots."
As a result, the EBCT scanner allows a quicker image acquisition than
conventional CT scanners.
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