tomography

tomography /to·mog·ra·phy/ (to-mog´rah-fe) the recording of internal body images at a predetermined plane by means of the tomograph.

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computed tomography  (CT), computerized axial tomography (CAT) an imaging method in which a cross-sectional image of the structures in a body plane is reconstructed by a computer program from the x-ray absorption of beams projected through the body in the image plane.

positron emission tomography  (PET) a nuclear medicine imaging method similar to computed tomography, except that the image shows the tissue concentration of a positron-emitting radioisotope.

single-photon emission computed tomography  (SPECT) a type in which gamma photon–emitting radionuclides are administered and then detected by one or more gamma cameras rotated around the patient, using the series of two-dimensional images to recreate a three-dimensional view.

ultrasonic tomography  the ultrasonographic visualization of a cross-section of a predetermined plane of the body.

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to·mog·ra·phy (t-mgr-f)

n.

Any of several techniques for making detailed x-rays of a plane section of a solid object, such as the body, while blurring out the images of other planes. Also called laminagraphy, planigraphy, planography, stratigraphy.

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tomo·graphic (tm-grfk) adj.

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tomography (tōmog´rfē),

n a radiographic technique that produces a film representing a detailed cross section of tissue structures at a predetermined depth.

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tomography

any method that produces images of single tissue planes. In conventional radiology, tomographic images (body-section radiographs) are produced by motion of the x-ray tube and film or by motion of the patient that blurs the image except in a single plane. In reconstruction tomography (CT and PET) the image is produced by a computer program. Called also laminagraphy, planigraphy, body-section technique.

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computerized axial tomography

see computed tomography (below).

computed tomography (CT)

a revolutionary radiological imaging modality that uses computer processing to generate an image (CT scan) of the tissue density in a 'slice' about 0.5 inch thick through the patient's body. Called also computerized axial tomography (CAT) and computerized transaxial tomography (CTAT).

Because CT is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. However, in some areas the injection of contrast further enhances the image. CT also has a better spatial resolution than scintillation imaging (about 1 mm for CT compared with 15 mm for a scintillation camera).

positron emission tomography (PET)

a combination of computed tomography and scintillation scanning. Natural biochemical substances or drugs tagged with a positron-emitting radioisotope are administered to the subject. After injection, the tagged substance (tracer) is localized in specific tissues like its natural analog. When the isotope decays, it emits a positron, which then annihilates with an electron of a nearby atom, producing two 511 keV gamma rays traveling in opposite directions 180° apart. When the gamma rays trigger a ring of detectors around the subject, the line between the detectors on which the decay occurred is stored in the computer. A computer program (reconstruction algorithm), like those used in computed tomography, produces an image of the distribution of the tracer in the plane of the detector ring.

Most of the isotopes used in PET scanning have a half-life of only 2 to 10 minutes. Therefore, they must be produced by an on-site cyclotron and attached chemically to the tracer and used within minutes. Because of the expense of the scanner and cyclotron, PET is used only in research centers.

ultrasonic tomography

the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography.