【国外标准】 Standard Guide for Computed Tomography (CT)

5.1 Computed tomography (CT) is a radiographic reconstruction method that provides a sensitive technique whenever the primary goal is to locate and size planar and volumetric detail in three dimensions.5.2 CT provides quantitative volume images as a function of density and element number (attenuation coefficient) by means of computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional images of specific areas of a scanned object, allowing the user to see inside the object without cutting. CT is considered much easier to interpret than conventional radiographic data due to the elimination of overlapping structures. The new user can learn quickly to read CT data because the images correspond more closely to the way the human mind visualizes three-dimensional structures than conventional projection radiography. Further, because CT slices and volumes are digital, they may be enhanced, analyzed, compressed, archived, input as data into performance calculations, compared with digital data from other NDE modalities, or transmitted to other locations for remote viewing.1.1 CT is a radiographic examination technique that generates digital images in three dimensions of an object, including the interior structure. Because of the relatively good penetrability of X-rays, CT permits the nondestructive physical and, to a limited extent, chemical characterization of the internal structure of materials. Also, since the method is X-ray based, it applies equally well to metallic and non-metallic specimens, solid and fibrous materials, and smooth and irregularly surfaced objects.1.2 This guide is intended to satisfy two general needs for users of industrial CT equipment: (1) the need for a tutorial guide addressing the general principles of X-ray CT as they apply to industrial imaging; and (2) the need for a consistent set of CT performance parameter definitions, including how these performance parameters relate to CT system specifications.1.3 This guide does not specify CT examination techniques, such as the best selection of scan parameters, the preferred implementation of scan procedures, or the establishment of accept/reject criteria for a new object.1.4 Units—No units are mentioned in this document. However, for CT, values are typically stated in SI units and are regarded as standard.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.