1、BS ISO 15708-2:2017Non-destructive testing Radiation methods forcomputed tomographyPart 2: Principles, equipment and samplesBSI Standards PublicationWB11885_BSI_StandardCovs_2013_AW.indd 1 15/05/2013 15:06BS ISO 15708-2:2017 BRITISH STANDARDNational forewordThis British Standard is the UK implementa
2、tion of ISO 15708-2:2017. The UK participation in its preparation was entrusted to TechnicalCommittee WEE/46, Non-destructive testing.A list of organizations represented on this committee can beobtained on request to its secretary.This publication does not purport to include all the necessaryprovisi
3、ons of a contract. Users are responsible for its correctapplication. The British Standards Institution 2017. Published by BSI Standards Limited 2017ISBN 978 0 580 95775 8ICS 19.100Compliance with a British Standard cannot confer immunity fromlegal obligations.This British Standard was published unde
4、r the authority of theStandards Policy and Strategy Committee on 31 March 2017.Amendments/corrigenda issued since publicationDate Text affectedBS ISO 15708-2:2017 ISO 2017Non-destructive testing Radiation methods for computed tomography Part 2: Principles, equipment and samplesEssais non destructifs
5、 Mthodes par rayonnements pour la tomographie informatise Partie 2: Principes, quipements et chantillonsINTERNATIONAL STANDARDISO15708-2Second edition2017-02Reference numberISO 15708-2:2017(E)BS ISO 15708-2:2017ISO 15708-2:2017(E)ii ISO 2017 All rights reservedCOPYRIGHT PROTECTED DOCUMENT ISO 2017,
6、Published in SwitzerlandAll rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Per
7、mission can be requested from either ISO at the address below or ISOs member body in the country of the requester.ISO copyright officeCh. de Blandonnet 8 CP 401CH-1214 Vernier, Geneva, SwitzerlandTel. +41 22 749 01 11Fax +41 22 749 09 47copyrightiso.orgwww.iso.orgBS ISO 15708-2:2017ISO 15708-2:2017(
8、E)Foreword iv1 Scope . 12 Normative references 13 Terms and definitions . 14 General principles . 14.1 Basic principles. 14.2 Advantages of CT . 24.3 Limitations of CT . 24.4 Main CT process steps 34.4.1 Acquisition 34.4.2 Reconstruction 44.4.3 Visualization and analysis 44.5 Artefacts in CT images
9、45 Equipment and apparatus 55.1 General . 55.2 Radiation sources . 65.3 Detectors 65.4 Manipulation . 75.5 Acquisition, reconstruction, visualization and storage system . 76 CT system stability 76.1 General . 76.2 X-Ray Stability 86.3 Manipulator stability . 87 Geometric alignment 88 Sample consider
10、ations . 98.1 Size and shape of sample . 98.2 Materials (including table voltage/thickness of penetration) . 9Annex A (informative) CT system components .11Bibliography .17 ISO 2017 All rights reserved iiiContents PageBS ISO 15708-2:2017ISO 15708-2:2017(E)ForewordISO (the International Organization
11、for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has t
12、he right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.The proc
13、edures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editori
14、al rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent righ
15、ts identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents).Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.For an ex
16、planation on the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISOs adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL: www .iso .org/ iso/ foreword .html.This docume
17、nt was prepared by the European Committee for Standardization (CEN) (as EN 16016-2) and was adopted, under a special “fast-track procedure”, by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC 5, Radiographic testing, in parallel with its approval by the ISO member bodies.Thi
18、s second edition of ISO 15708-2 cancels and replaces ISO 15708-1:2002, of which it forms the subject of a technical revision. It takes into consideration developments in computed tomography (CT) and computational power over the preceding decade.A list of all parts in the ISO 15708 series can be foun
19、d on the ISO website.iv ISO 2017 All rights reservedBS ISO 15708-2:2017INTERNATIONAL STANDARD ISO 15708-2:2017(E)Non-destructive testing Radiation methods for computed tomography Part 2: Principles, equipment and samples1 ScopeThis document specifies the general principles of X-ray computed tomograp
20、hy (CT), the equipment used and basic considerations of sample, materials and geometry.It is applicable to industrial imaging (i.e. non-medical applications) and gives a consistent set of CT performance parameter definitions, including how those performance parameters relate to CT system specificati
21、ons. This document deals with computed axial tomography and excludes other types of tomography such as translational tomography and tomosynthesis.2 Normative referencesThe following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this
22、 document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.ISO 15708-1:2017, Non-destructive testing Radiation methods for computed tomography Part 1: TerminologyISO 15708-3:2017, Non-destr
23、uctive testing Radiation methods for computed tomography Part 3: Operation and interpretationISO 15708-4:2017, Non-destructive testing Radiation methods for computed tomography Part 4: QualificationISO 9712, Non-destructive testing Qualification and certification of NDT personnel3 Terms and definiti
24、onsFor the purposes of this document, the terms and definitions given in ISO 15708-1 apply.ISO and IEC maintain terminological databases for use in standardization at the following addresses: IEC Electropedia: available at h t t p :/ www .electropedia .org/ ISO Online browsing platform: available at
25、 h t t p :/ www .iso .org/ obp4 General principles4.1 Basic principlesComputed tomography (CT) is a radiographic inspection method which delivers three-dimensional information on an object from a number of radiographic projections either over cross-sectional planes (CT slices) or over the complete v
26、olume. Radiographic imaging is possible because different materials ISO 2017 All rights reserved 1BS ISO 15708-2:2017ISO 15708-2:2017(E)have different X-ray attenuation coefficients. In CT images, the X-ray linear attenuation coefficients are represented as different CT grey values (or in false colo
27、ur). For conventional radiography the three-dimensional object is X-rayed from one direction and an X-ray projection is produced with the corresponding information aggregated over the ray path. In contrast, multiple X-ray-projections of an object are acquired at different projection angles during a
28、CT scan. From these projection images the actual slices or volume are reconstructed. The fundamental advantage compared to radiography is the preservation of full volumetric information. The resulting CT image (2D-CT slice or 3D-CT volume), is a quantitative representation of the X-ray linear attenu
29、ation coefficient averaged over the finite volume of the corresponding volume element (voxel) at each position in the sample.The linear attenuation coefficient characterizes the local instantaneous rate at which X-rays are attenuated as they propagate through the object during the scan. The attenuat
30、ion of the X-rays as they interact with matter is the result of several different interaction mechanisms: Compton scattering and photoelectric absorption being the predominant ones for X-ray CT. The linear attenuation coefficient depends on the atomic numbers of the corresponding materials and is pr
31、oportional to the material density. It also depends on the energy of the X-ray beam.4.2 Advantages of CTThis radiographic method can be an excellent examination technique whenever the primary goal is to locate and quantify volumetric details in three dimensions. In addition, since the method is X-ra
32、y based it can be used on metallic and non-metallic samples, solid and fibrous materials and smooth and irregularly surfaced objects.In contrast to conventional radiography, where the internal features of a sample are projected onto a single image plane and thus are superposed on each other, in CT i
33、mages the individual features of the sample appear separate from each other, preserving the full spatial information.With proper calibration, dimensional inspections and material density determinations can also be made.Complete three-dimensional representations of examined objects can be obtained ei
34、ther by reconstructing and assembling successive CT slices (2D-CT) or by direct 3D CT image (3D-CT) reconstruction. Computed tomography is thus valuable in the industrial application areas of non-destructive testing, 2D and 3D metrology and reverse engineering.CT has several advantages over conventi
35、onal metrology methods: acquisition without contact; access to internal and external dimensional information; a direct input to 3D modelling especially of internal structures.In some cases, dual energy (DE) CT acquisitions can help to obtain information on the material density and the average atomic
36、 number of certain materials. In the case of known materials the additional information can be traded for improved discrimination or improved characterization.4.3 Limitations of CTCT is an indirect test procedure and measurements (e.g. of the size of material faults; of wall thicknesses must be comp
37、ared with another absolute measurement procedure, see ISO 15708-3). Another potential drawback of CT imaging is the possible occurrence of artefacts (see 4.5) in the data. Artefacts limit the ability to quantitatively extract information from an image. Therefore, as with any examination technique, t
38、he user must be able to recognize and discount common artefacts subjectively.Like any imaging system, a CT system can never reproduce an exact image of the scanned object. The accuracy of the CT image is dictated largely by the competing influences of the imaging system, namely spatial resolution, s
39、tatistical noise and artefacts. Each of these aspects is discussed briefly in 4.4.1. A more complete description will be found in ISO 15708-3.2 ISO 2017 All rights reservedBS ISO 15708-2:2017ISO 15708-2:2017(E)CT grey values cannot be used to identify unknown materials unambiguously unless a priori
40、information is available, since a given experimental value measured at a given position may correspond to a broad range of materials.Another important consideration is to have sufficient X-ray transmission through the sample at all projection angles (see 8.2) without saturating any part of the detec
41、tor.4.4 Main CT process steps4.4.1 AcquisitionDuring a CT scan, multiple projections are taken in a systematic way: the images are acquired from a number of different viewing angles. Feature recognition depends, among other factors, on the number of angles from which the individual projections are t
42、aken. The CT image quality can be improved if the number of projections of a scan is increased.As all image capture systems contain inherent artefacts, CT scans usually begin with the capture of offset and gain reference images to allow flat field correction; using black (X-rays off) and white (X-ra
43、ys on with the sample out of the field of view) images to correct for detector anomalies. The capture of reference images for distortion correction (pin cushion distortion in the case of camera-based detector systems with optical distortion), and centre of rotation correction can also take place at
44、this stage. Each subsequent captured image for the CT data set has these corrections applied to it. Some systems can be configured to either the X-ray settings or enhance the image to ensure that the background intensity level of the captured images remains constant throughout the duration of the CT
45、 scan.The quality of a CT image depends on a number of system-level performance factors, with one of the most important being spatial resolution.Spatial resolution is generally quantified in terms of the smallest separation at which two features can be distinguished as separate entities. The limits
46、of spatial resolution are determined by the design and construction of the system and by the resolution of and number of CT projections. The resolution of the CT projection is limited by the maximum magnification that can be used while still imaging all parts of the sample at all rotation angles.It
47、is important to notice that the smallest feature that can be detected in a CT image is not the same as the smallest that can be resolved spatially. A feature considerably smaller than a single voxel can affect the voxel to which it corresponds to such an extent that it appears with a visible contras
48、t so that it can be easily detected with respect to adjacent voxels. This phenomenon is due to the “partial-volume effect”.Although region-of-interest CT (local tomography) can improve spatial resolution in specified regions of larger objects, it introduces artefacts (due to incomplete data) which c
49、an sometimes be reduced with special processing.Radiographic imaging as used for CT examination is always affected by noise. In radiography this noise arises from two sources: (1) intrinsic variation corresponding to photon statistics related to the emission and detection of photons and (2) variations specific to instruments and processing used. Noise in CT projections is often amplified by the reconstruction algorithm. In the CT images statistical noise appears as a random variation superimposed on the CT grey value of each voxe
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