CT常用词汇表.docVIP

GLOSSARY This glossary contains descriptions of commonly-used technical terms in CT as an aid to understanding the Guidelines. The bold-faced typed words in the explanatory text indicates that they occur elsewhere in the glossary. artefact (structured noise): The appearance in the CT image of details not present in the scanned object. The main components of structured noise are due to a form of partial volume effect and to beam hardening. Both effects usually result in streaking artefacts, which are observed in regions of high contrast when there is a sharp discontinuity in object density, such as at air-tissue, air-bone and metal-tissue boundaries. Streaking will also arise from mechanical misalignment within the scanner and, in clinical practice, from patient motion and the use of high-density contrast media. attenuation: Reduction of the radiation intensity, upon passage through matter, resulting from all types of interaction. back projection: Mathematical procedure for the reconstruction of the CT image, based on the smearing of the individual rays within a view (projection) back along the direction in which they were measured. Spatial filtration (convolution) of the raw data is necessary before back projection in order to reduce artefacts. beam hardening: The process of filtration of a polychromatic beam by the preferential absorption of lower energy photons in tissue, with a subsequent increase in effective energy. The associated artefacts are of particular significance in quantitative computed tomography (QCT). calibration of a CT-scanner: Correction procedures used to take account of variations in beam intensity or detector efficiency in order to achieve homogeneity within the field of view and accuracy of CT number. Calibration procedures include scanning air or an appropriate test phantom. collimation: Geometrical limitation of the extent of the radiation beam in the z-direction. computed tomography dose index (CTDI): Integral along a line parallel to the