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.
(CT) (computerized axial tomography
(CAT)) a radiologic imaging modality that uses computer processing to generate an image (CAT scan
) of the tissue density in a “slice” as thin as 1 to 10 mm in thickness through the patient's body. These images are spaced at intervals of 0.5 to 1 cm. Cross-sectional anatomy can be reconstructed in several planes without exposing the patient to additional radiation.
Since its introduction in 1972, the use of this modality has grown rapidly. Because it is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. It also has a better spatial resolution than scintillation imaging (about 1 mm for CAT compared to 15 mm for a scintillation camera).
A CAT scan is divided into a square matrix of pixels
(picture elements). The newer CAT scanners use a high resolution matrix with 256 × 256 or 512 × 512 pixels. The region of the tissue slice corresponding to a pixel has a cross-sectional area of 1 × 1 mm to 2 × 2 mm; because of the thickness of the slice, it has a finite height and is therefore referred to as a voxel
The actual measurements made by the scanner are the x-ray attenuations along thousands of rays traversing the slice at all angles. The attenuation value for a ray is the sum of the values for all of the voxels it passes through. A computer program called a reconstruction algorithm
can solve the problem of assigning attenuation values for all the pixels that add up to the measured values along each ray.
The attenuation values are converted to CAT numbers by subtracting the attenuation value of water and multiplying by an arbitrary coefficient to produce values ranging from −1000 for air to +1000 for compact bone with water as 0. CT numbers are sometimes expressed in Hounsfield units
, named after Godfrey Hounsfield, the inventor of the CT scanner; Hounsfield and Allan Cormack were co-winners of the Nobel Prize in physiology or medicine in 1979 for the development of computerized axial tomography.
Computed tomography. Relative position of the x-ray tube, patient, and detectors in a fourth generation CT unit.
electron beam computed tomography
(EBCT) ultrafast computed tomography
done with a scanner in which the patient is surrounded by a large circular anode that emits x-rays as the electron beam is guided around it.
extended narrow tomography tomography involving an increase in amplitude and increase in exposure angle resulting in greater thinness of the cut for examination.
linear tomography tomography in which the tube and film move in the same direction.
narrow angle tomography a type of tomography that results in thicker sections for examination.
pluridirectional tomography tomography in which there is a great deal of movement in a variety of directions.
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 by injection; the tagged substance (tracer) then becomes localized in specific tissues like its natural analogue. 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 degrees 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. However, PET is important because it provides information that cannot be obtained by other means. By labeling the blood with 11
C-carbon monoxide, which binds to hemoglobin, images can be obtained showing the regional perfusion of an organ in multiple planes. By using labeled metabolites, images can be obtained showing metabolic activity of an organ. 15
O-oxygen and 11
C-glucose have been used for brain imaging and 11
C-palmitate for heart imaging. 81
Rb, which is distributed like potassium, is also used for heart imaging. By using labeled neurotransmitters, hormones, and drugs the distribution of receptors for these substances in the brain and other organs can be mapped.
single-photon emission computed tomography (SPECT) a type of tomography in which gamma photon–emitting radionuclides are administered to patients and then detected by one or more gamma cameras rotated around the patient. From the series of two-dimensional images produced, a three-dimensional image can be created by computer reconstruction. The technique improves resolution of, and decreases interference by, overlapping organs. It is used particularly for assessment of cardiac disease, stroke, and liver disease; for staging of cancer; and to diagnose physical abnormalities through evaluation of function.
the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography
tomography /to·mog·ra·phy/ (to-mog´rah-fe) the recording of internal body images at a predetermined plane by means of the tomograph.
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.
tomography (to-mog'ra-fe) [ tomo- + -graphy]
A radiographic technique that selects a level in the body and blurs out structures above and below that plane, leaving a clear image of the selected anatomy. This is accomplished by moving the x-ray tube in the opposite direction from the imaging device around a stationary fulcrum defining the plane of interest. Tube movements can be linear, curvilinear, circular, elliptical, figure eight, hypocycloidal, or trispiral. With the exception of renal tomography most tomographic procedures have been replaced by computed tomography (CT). Synonym: body section radiography
; body section roentgenography
computed axial tomography Abbreviation: CAT
See: computed tomography
computed tomography Abbreviation: CT
A computerized x-ray scanning system that produces a sectional anatomic image. It is achieved by digital processing of x-ray attenuation coefficients from a 360° wedge scan of ionizing radiation. There is considerable use of data from the attenuation coefficients in diagnosis. Computed tomography is colloquially called a cat scan.
CAUTION!CT scans expose patients to radiation on the order of 10 mSv per scan. Educational materials about the potential risks and benefits of scanning should be provided to patients to ensure that scans are performed safely and carefully.
computerized axial tomography Abbreviation: CAT
See: computed tomography
electrical impedance tomography
Cross-sectional body imaging that reconstructs pictures of internal organs based on measurements of their electrical activity as detected by electrodes placed on the surface of the body.
electron-beam tomographyUltrafast computed tomography
full body computed tomography Abbreviation: FBCT
An examination from head to toe of the body with computed tomographic imaging, promoted as a screening test for cancer and other illnesses.
CAUTION!The test exposes patients to high levels of radiation, reveals more false positive findings than true positives, and is expensive.
Heidelberg retinal tomography Abbreviation: HRT
A confocal laser scanning system that produces three-dimensional images of the posterior segment of the eye. It is used to diagnose and treat glaucoma.
helical computed tomography
Computed tomographic (CT) images that are obtained as the CT table moves continuously during a single, held breath. Detailed evaluation of dynamic internal features is feasible with this technique. Synonym: spiral computed tomography
optical coherence tomography Abbreviation: OCT
A radiographical method used to obtain high-resolution cross-sectional images of tissues and their defects, e.g., of the structures of the eye.
POSITRON EMISSION TOMOGRAPHY: PET SCAN revealing lung cancer
PET SCAN OF BRAIN
positron emission tomography Abbreviation: PET
Reconstruction of brain sections by using positron-emitting radionuclides. By using several different radionuclides, researchers can measure regional cerebral blood flow, blood volume, oxygen uptake, and glucose transport and metabolism, and can locate neurotransmitter receptors. PET has been used with fludeoxyglucose F 18 to identify and localize regional lymph node metastases and to help assess response to therapy.
The images produced by PET are in colors that indicate the degree of metabolism or blood flow. The highest rates appear red, those lower appear yellow, then green, and the lowest rates appear blue. The images in various disease states may then be compared to those of normal subjects. Three- and four-dimensional reconstructions are often achieved through the use of computed tomography (CT) with the same machine. See: illustration
quantitative computed tomography Abbreviation: QCT
A method for determining the bone mineral density of a three-dimensional bony specimen, e.g., in the vertebral bodies or the forearms. It is used in the diagnosis of osteopenia and osteoporosis.
single photon emission computed tomography Abbreviation: SPET, SPECT
A medical imaging method for reconstructing sectional images of radiotracer distributions. See: nuclear medicine scanning test; positron emission tomography
spiral computed tomographyHelical computed tomography.
ultrafast computed tomography
Computed tomographic scanning that produces images by rotating the x-ray beam at targets placed around a patient, instead of moving a patient on a gantry through the scanner. The technique minimizes patient movement artifacts and decreases scanning times to about 50 to 100 msec. It is capable of providing good resolution of vascular structures, such as the aorta and the coronary arteries. Synonym: electron-beam tomography
xenon-enhanced computed tomography
Computed tomographic scanning that uses the inert gas xenon to improve the visual distinction between healthy and abnormal tissues, esp. to visualize blood flow to different regions of the brain in stroke.
A radiographic technique for making a detailed X-ray image of a selected plane section of the body while blurring out the images of other planes. The data can be manipulated to represent three-dimensional images of structures.computed tomography (CT)
A radiographic method of viewing a three-dimensional image of a layer of body structures, which is constructed by a computer from a series of plane cross-sectional X-ray images made along an axis. The images indicate the X-ray absorption (called attenuation) of tissues (e.g. bones attenuate most, lungs attenuate least and blood vessels are in between). The X-rays are received by numerous gas or solid-state detectors and computers are used to store, process and manipulate the information received from these detectors. The method yields far better differentiation of tissues than conventional radiography thus providing more precise diagnostic information. Usage includes the detection of orbital fractures, orbital cellulitis, intraorbital calcification, cerebral haemorrhage and orbital tumours. Syn
. computerized axial tomography (CAT); CAT scan; CT scan. See glaucoma detection
; magnetic resonance imaging
.confocal scanning laser tomography See confocal scanning laser ophthalmoscope
.optical coherence tomography (OCT)
A non-invasive, optical diagnostic imaging technique, which enables in vivo cross-sectional tomographic visualization of internal microstructure in biological systems. OCT is analogous to ultrasound imaging except that it uses light rather than sound, thus achieving approximately 1-100✕ higher image resolution. This is accomplished by using polychromatic (broad bandwidth) or tunable light sources in combination with interferometric techniques to detect depth resolved reflectivity profiles, due to subtle refractive index changes. Several adjacent one-dimensional optical A-scans are combined into two- or three-dimensional tomograms for quantitative analysis of the optic nerve head topography, peripapillary fibre layer thickness, macular retinal thickness, as well as corneal visualization. Quantitative results are compared with an age-matched normative database. OCT can be used for early diagnosis of retinal diseases (e.g. cystoid macular oedema, central serous retinopathy, retinal detachments, macular hole), better understanding of retinal pathogenesis, monitoring of nerve fibre layer thickness and optic nerve head changes in glaucomatous eyes, as well as corneal thickness changes following refractive surgery. Most recent developments enable several 10 thousand measurements per second, allowing three-dimensional retinal images nearly free of motion artifact. In combination with improved resolution this technique has the potential to perform non-invasive optical biopsy of the human retina, i.e. visualization of intraretinal morphology in retinal pathologies approaching the level achieved with histopathology. See glaucoma detection
.positron emission tomography (PET)
A neuroimaging technique in which a positron-emitting isotope incorporated into a metabolically active molecule (e.g. fluorodeoxyglucose) is injected intravenously and used as radioactive tracers to generate images of regional cerebral blood flow and glucose consumption contained in the tracers and thus, indirectly brain function. The emitted positron collides with an electron, giving rise to two photons, which strike detectors placed around the head. Tomographic images can be used to construct a three-dimensional image of the relative concentration of the tracer within the brain. PET has been used to study normal and abnormal brain function and to assess tumours, stroke, cortical lesions and also mapping of the visual cortex. See fMRI magnetic resonance imaging
; functional neuroimaging
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.
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.
the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography