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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 (volume element).
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.
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 11C-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. 15O-oxygen and 11C-glucose have been used for brain imaging and 11C-palmitate for heart imaging. 81Rb, 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.
tomographyImaging The creation of images at planes located at specific distances from an x-ray beam. See Brain imaging, Computed tomography, Contrast-enhanced electron-beam tomography, Electron beam computed tomography, Focused appendix computed tomography, High-resolution computed tomography, Nephrotomography, Optical coherence tomography, Positron emission tomography, SPECT tomography, Spiral computed tomography.
tomography(to-mog'ra-fe) [ tomo- + -graphy]
computed axial tomographyAbbreviation: CAT
See: computed tomography
computed tomographyAbbreviation: CT
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 tomographyAbbreviation: CAT
See: computed tomography
electrical impedance tomography
electron-beam tomographyUltrafast computed tomography
full body computed tomographyAbbreviation: FBCT
CAUTION!The test exposes patients to high levels of radiation, reveals more false positive findings than true positives, and is expensive.
Heidelberg retinal tomographyAbbreviation: HRT
helical computed tomography
optical coherence tomographyAbbreviation: OCT
positron emission tomographyAbbreviation: PET
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 tomographyAbbreviation: QCT
single photon emission computed tomographyAbbreviation: SPET, SPECT
spiral computed tomographyHelical computed tomography.
ultrafast computed tomography
xenon-enhanced computed tomography
tomographyX-ray examination performed so as to produce an image of a slice of tissue. In tomography, a narrow beam and a detector, fixed relative to each other, move in a rotary manner around the patient. CT scanning is a form of (computer-assisted) tomography.
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; radiology.
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; interference.
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.