Making of a radiographic image of a selected plane by means of reciprocal linear or curved motion of the x-ray tube and film cassette; images of all other planes are blurred (“out of focus”) by being relatively displaced on the film. SYN: conventional t., planigraphy, planography, sectional radiography, stratigraphy.
- computed t. (CT) imaging anatomic information from a cross-sectional plane of the body, each image generated by a computer synthesis of x-ray transmission data obtained in many different directions in a given plane. SYN: computerized axial t..
- computerized axial t. (CAT) SYN: computed t..
- conventional t. SYN: t..
- dynamic computed t. computed t. with rapid injection of contrast medium, usually with sequential scans at only one or a few levels; used to enhance the vascular compartment. SYN: dynamic CT.
- electron beam t. (EBT) computed t. in which the circular motion of the x-ray tube is replaced by rapid electronic positioning of the cathode ray around a circular anode, allowing full scans in tens of milliseconds.
- helical computed t. SYN: spiral computed t..
- high-resolution computed t. (HRCT) computed t. with narrow collimation to reduce volume-averaging and an edge-enhancing reconstruction algorithm to sharpen the image, sometimes with a restricted field of view to minimize the size of pixels in the region imaged; used particularly for lung imaging.
- hypocycloidal t. body section radiography using a complex film and tube motion with a pattern resembling a three-leaf clover.
- positron emission t. (PET) creation of tomographic images revealing certain biochemical properties of tissue by computer analysis of positrons emitted when radioactively tagged substances are incorporated into the tissue. Radiotracers used in PET are analogs of physiologic or pharmaceutical agents into which positron-emitting isotopes with short half-lives (2–110 min) have been incorporated. Radioisotopes are produced artificially by bombarding a stable compound with a proton beam generated by a cyclotron. The uptake and metabolism of these positron emitters mimic, at least in part, those of the radiostable natural substances to which they are analogous. Concentrated in particular organs or tissues and incorporated into metabolic processes, they can reflect biochemical function or dysfunction. The glucose analog 2-(fluorine-18)fluoro-2-deoxy-d-glucose (FDG) is widely used to locate zones of heightened energy metabolism. When a positron emitted by a radiotracer collides with an electron, the particles annihilate each other and 2 gamma rays are discharged in opposite directions (at 180°). After intravenous administration of the radiotracer, the subject is positioned within a scanner consisting of a ring of scintillation crystals that convert gamma rays into flashes of visible light. These flashes are detected and recorded electronically, and a computer program assembles the data into a three-dimensional image, color-coded to reflect concentration density.Unlike other imaging procedures, PET assesses metabolic activity and physiologic function rather than anatomic structure. Because the half-lives of the radionuclides are short and the equipment expensive, PET has not thus far been used extensively in clinical settings. But since its development in the mid-1970s, it has proved the most important tool yet devised for the experimental investigation of the living brain, whether healthy, traumatized, or diseased. Besides providing important diagnostic information in Alzheimer and other dementias, parkinsonism, and Huntington disease, PET can localize epileptic foci in preparation for surgical intervention, assess intracranial neoplasms, and help to direct therapeutic choices in acute stroke. The sensitivity and specificity of PET in determining malignancy render it valuable in oncology in avoiding biopsies for low grade tumors, in noninvasive differentiation of tumors from radiation necrosis, in early modification of ineffective chemotherapy, and in avoiding unnecessary diagnostic and therapeutic surgery. PET has been employed in cardiology to screen for coronary artery disease, to assess flow rates and flow reserve, and to distinguish viable from nonviable myocardium in bypass and transplant candidates.
- single photon emission computed t. (SPECT) tomographic imaging of metabolic and physiologic functions in tissues, the image being formed by computer synthesis of photons of a single energy emitted by radionuclides administered in suitable form to the patient.
- spiral computed t. computed t. in which the x-ray tube continuously revolves around the patient, who is simultaneously moved longitudinally; computer interpolation allows reconstruction of standard transverse scans or images in any preferred plane. SYN: helical computed t., helical CT, spiral CT.
- trispiral t. hypocycloidal t. that allows a much thinner and more uniform plane of focus; formerly used especially for inner ear t..

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to·mog·ra·phy tō-'mäg-rə-fē n, pl -phies a method of producing a three-dimensional image of the internal structures of a solid object (as the human body) by the observation and recording of the differences in the effects on the passage of waves of energy impinging on those structures called also stratigraphy see COMPUTED TOMOGRAPHY, POSITRON-EMISSION TOMOGRAPHY
to·mo·graph·ic .tō-mə-'graf-ik adj

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the technique of rotating a radiation detector around the patient so that the image obtained gives additional three-dimensional information. In plain film tomography the source of X-rays and the photographic film move round the patient to produce an image of structures at a particular depth within the body, bringing them into sharp focus, while deliberately blurring structures above and below them. In computerized tomography (CT) this technique produces an image of a slice through the body at a particular level. The visual record of this technique is called a tomogram. See also dental pantomogram, positron emission tomography, SPECT scanning.

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to·mog·ra·phy (to-mogґrə-fe) [tomo- + -graphy] the recording of internal body images at a predetermined plane by means of the tomograph; called also body section radiography.

Medical dictionary. 2011.

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