What is Computed Tomography?
Conventional X ray Images
Figure 1: Chest X ray Image
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.
Computed Tomography (CT)
Figure 2: Cross-sectional Image of Abdomen
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.
How a CT system works
Figure 3: Patient in CT Imaging System
1. A motorized table moves the patient (Figure 3) through a circular opening in the CT imaging system.
2. 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.
3. 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.
4. 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.
Figure 4: CT Fan Beam
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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