Frequently Asked Questions - Computed Tomography (CT)

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What is Computed Tomography (CT)?

"Computed tomography (formerly known as computerized axial tomography or CAT scan) measures the weakening, or attenuation, of x-ray beams as they pass through sections of an object from hundreds of different angles. From the evidence of these measurements, a computer is able to reconstruct two and three dimensional images of the objects interior." (Eisenberg, 1992)

1992 Eisenberg, E. L. Radiology. An Illustrated History, A. S. Patterson (ed.), Mosby Year Book, pp. 467-468.

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"The relationship between the CT number of a structure and its physical density is defined by the equation Q = CT + 1000 (Q in mg/cm3, CT in HU (Hounsfield Units)). This equation holds true as long as the effective atomic number of the substance (i.e. its atomic composition) in the displayed voxel is similar to that of water. This applies with reasonable accuracy to fluids, soft tissues, and less to fat but not to iodinated contrast media of calcified structures. For this reason, the CT number of a soft-tissue structure is often used interchangeably with "density"." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.

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"Hounsfield units are a unit of measure that represents the different density levels of tissues and or other substances. The density of pure water was arbitrarily set at 0 HU and that of air at -1000 HU." (Hofer, 2000)

2000 Hoffer, M. CT Teaching Manual, Georg Thieme Verlag, pp.12-13.

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"A CT image is composed of a square image matrix that ranges in size from 256 X 256 to 1024 X 1024 picture elements or pixels. Since a CT section has a finite thickness, each pixel actually represents a small volume element, or voxel. The size of this voxel depends on the matrix size, the selected field of view (FOV), and the section thickness." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.

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"Abbreviation for Digital Imaging and Communications in Medicine, a joint standard of the American College of Radiology and National Equipment Manufacturers Association; specifies entities (or objects) and functions (or services) to allow communication between various image sources and other computer devices, such as archives or workstations." (PDR, Pugh et al. (eds.), 2000)

2000 Physicians Desktop Reference (PDR) Medical Dictionary, N.B. Pugh, B. Werner, T. W. Filardo, P. W. Binns, L. G. Francis, R. Lukens, B. Montgomery, B. L. Ferretti, K. J. Cradle, D. Workman (eds.), Lippincott Williams & Wilkens, Second Edition, pp. 1-2098.

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A micron is a unit of measurement equal to one millionth of one meter.

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"In position, not extending beyond the focus or level of origin." (PDR, Pugh et al. (eds.), 2000)

2000 Physicians Desktop Reference (PDR) Medical Dictionary, N.B. Pugh, B. Werner, T. W. Filardo, P. W. Binns, L. G. Francis, R. Lukens, B. Montgomery, B. L. Ferretti, K. J. Cradle, D. Workman (eds.), Lippincott Williams & Wilkens, Second Edition, pp. 1-2098.

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"A shaded surface display (SSD) is a surface-rendered image that provides a realistically looking three-dimensional view of the surface of a structure of interest within the acquired volume set." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.

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"Maximum intensity projection (MIP) and minimum intensity projection (MinIP) are volume-rendering techniques in which suitable editing methods are used to define the volume of interest (VOI). All of the CT image data set may be used, or the volume may be confined to a region of interest (ROI). In the most difficult case, only selected organ systems are included or excluded from VOI. The actual images are generated by projecting the volume of interest into a viewing plane and displaying the maximum CT numbers (for MIP) or the minimum CT numbers (for MinIP) that are encountered along the direction of the projection, called the viewing angle. Both techniques ensure that optimum contrast is produced between small, high-contrast structures and surrounding tissues." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.

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"Multiplanar reformations (MPR's) are two-dimensional reformatted images that are reconstructed secondarily in arbitrary planes from the stack of axial image data." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.

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"VRT assigns a range of opacity values to CT numbers and thus yields a better definition of object contours or semitransparent display of structures." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.