computerized tomography
Meaning of computerized tomography: The scanner captures no picture at all; it records absorption from thousands of angles, and the image is calculated afterward.
Definition of computerized tomography
Computerized tomography, usually shortened to CT, is an imaging method that rotates an x-ray source around the body and uses readings taken from many angles to compute cross-sectional images. Each image is a slice, separating structures that overlap and obscure one another on an ordinary x-ray, and the slices can be stacked into a three-dimensional view.
How does it build an image?
An x-ray tube and a bank of detectors sit opposite each other on a ring and spin around the body while the table moves steadily through, tracing a helical path. Every tissue absorbs x-rays by a characteristic amount, and each rotation records how much of the beam survived along thousands of lines through the body. A computer then solves for the absorption at every point in the slice. The values are expressed on a scale where water sits at zero, air far below it and dense bone far above, and the same dataset can be displayed in different ways to favor lung, soft tissue or bone. Iodine-based contrast given by vein makes vessels and well-supplied tissue stand out, and oral contrast outlines the bowel. This is tomography in its literal sense: imaging by section.
How does it compare with MRI?
| CT | MRI | |
|---|---|---|
| What it uses | X-rays | A strong magnetic field and radio waves |
| Speed | Seconds | Many minutes |
| Strongest at | Bone, acute bleeding, lung, trauma, calcification | Soft tissue detail, brain and spinal cord, ligaments, marrow |
| Radiation | Yes | None |
| Main constraints | Contrast reactions, kidney function | Metal implants, confined space, length of scan |
What are the trade-offs?
Radiation dose is the central one, since a scan delivers considerably more than a plain film, and dose accumulates over repeated studies. Manufacturers and departments counter this with lower-dose protocols and reconstruction methods that tolerate a weaker signal, and specialized low-dose studies exist for particular questions, as in computed tomography colonography. Incidental findings are the other cost: a detailed scan often shows something unrelated to the question asked, which then has to be resolved. Movement blurs the image, and metal implants throw bright streaks across nearby structures. Where the question concerns metabolic activity rather than anatomy, a positron emission tomography scan is combined with CT so that function and structure are recorded together.
Also known as
Used in a sentence
A CT scan of the abdomen was ordered to further evaluate the patient's persistent pain.
Good to know
A CT scan differs from magnetic resonance imaging in using x-ray radiation and in its particular strength at showing bone and acute bleeding, which is why one is chosen over the other for a given question.
Word origin
Where “computerized tomography” comes from
Formed from tomo-, from Greek tomos, meaning a cutting or slice, combined with -graphy, from Greek graphein, meaning to write or record.