computed tomography scan
Meaning of computed tomography scan: A plain x-ray flattens the body into one picture, while slicing it apart separates structures that would otherwise be stacked on top of each other.
Definition of computed tomography scan
A computed tomography scan is an imaging test in which an x-ray source rotates around the body while detectors on the far side record how much of the beam gets through. A computer turns those readings into a stack of cross sectional pictures, each representing a thin slice, which can be viewed one at a time or rebuilt into a view along any plane.
Why is it clearer than a plain x-ray?
A conventional x-ray sends the beam through the body once and records everything in its path on a single sheet. Ribs, lung, heart and spine all land on top of one another, and a small abnormality can be lost behind something denser. Rotating the source and measuring from every angle lets a computer work backward to how dense each small volume of tissue must have been, producing a slice in which those structures are separated. Modern machines gather the data in a continuous spiral as the table moves, the arrangement described as helical computed tomography, which is fast enough to cover the chest and abdomen in a single held breath. The slices are thin enough that the stack can be turned into a view from the side or the front, or into a three dimensional model, without scanning again.
What does contrast material add?
| Approach | What it makes visible |
|---|---|
| No contrast | Bone, bleeding, air, stones and lung tissue, all of which already stand apart |
| Injected into a vein | Blood vessels, and organs by how much blood reaches them |
| Swallowed | The outline and wall of the stomach and intestine |
| Timed after injection | Arteries alone, or the way a lesion fills and empties over successive passes |
Iodine based contrast material absorbs x-rays strongly, so wherever blood carries it the image brightens. Timing is what turns that into information, since many abnormalities are told apart not by how bright they become but by how quickly they brighten and how quickly they fade.
What are the trade offs?
Radiation dose is the main one. A single study of the chest or abdomen delivers considerably more than a plain film of the same region, which weighs differently for a one time investigation than for repeated follow up in a young person. Contrast carries its own considerations for the kidneys and can provoke allergic reactions. And detail cuts both ways: scanning finds a great many small, harmless abnormalities that would never have caused trouble, each of which then invites further imaging. Magnetic resonance imaging avoids radiation and shows soft tissue better, but takes far longer, while a positron emission tomography scan adds information about activity that no anatomical picture provides.
Also known as
Used in a sentence
A computed tomography scan of the chest was ordered to further evaluate the abnormal finding.
Good to know
A computed tomography scan involves greater radiation exposure than a conventional x-ray, a consideration weighed against the additional diagnostic detail the cross-sectional images provide.
Word origin
Where “computed tomography scan” comes from
Formed from tomo-, Greek for a cutting or slice, and -graphy, from graphein, meaning to write or record, describing the sliced, cross-sectional images the technique produces.