SIST EN ISO 15708-2:2019
(Main)Non-destructive testing - Radiation methods for Computed tomography - Part 2: Principles, equipment and samples (ISO 15708-2:2017)
Non-destructive testing - Radiation methods for Computed tomography - Part 2: Principles, equipment and samples (ISO 15708-2:2017)
ISO 15708-2:2017 specifies the general principles of X-ray computed tomography (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 specifications.
ISO 15708-2:2017 deals with computed axial tomography and excludes other types of tomography such as translational tomography and tomosynthesis.
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 2: Grundlagen, Geräte und Proben (ISO 15708-2:2017)
Dieses Dokument legt die allgemeinen Grundlagen der Röntgencomputertomographie (CT) sowie die ange-wendeten Geräte und grundsätzliche Überlegungen zu Proben, Materialien und Geometrie fest.
Es gilt für die industrielle Bildgebung (d. h. nicht medizinische Anwendungen) und bietet einen einheitlichen Satz von Festlegungen zu CT Leistungsparametern, einschließlich der Art des Zusammenhangs dieser Leistungsparameter mit den Spezifikationen des CT Systems.
Dieses Dokument befasst sich mit der axialen Computertomographie und schließt weitere Arten der Tomo-graphie, wie z. B. Translationstomographie und Tomosynthese, aus.
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée - Partie 2: Principes, équipements et échantillons (ISO 15708-2:2017)
Le présent document spécifie les principes généraux de la tomographie informatisée (TI) par rayonnement X, l'équipement utilisé ainsi que les considérations de base relatives à l'échantillon, aux matériaux et à la géométrie.
Il est applicable à l'imagerie industrielle (c'est-à-dire aux applications non médicales) et donne un ensemble cohérent de définitions des paramètres de performance de la TI, y compris la façon dont ces paramètres sont reliés aux spécifications du système TI.
Le présent document traite de la tomographie axiale informatisée et exclut les autres types de tomographie, tels que la tomographie par translation et la tomosynthèse.
Neporušitvene preiskave - Sevalne metode za računalniško tomografijo - 2. del: Načela, oprema in vzorci (ISO 15708-2:2017)
Standard ISO 15708-2:2017 določa splošna načela rentgenske računalniške tomografije (CT), uporabljeno opremo in temeljne zamisli glede vzorcev, materialov in geometrije.
Uporablja se za slikanje v industriji (npr. v nemedicinske namene) in podaja dosleden sklop definicij podatkov delovanja računalniške tomografije, vključno s tem, kako so podatki delovanja povezani s specifikacijami sistema računalniške tomografije.
Standard ISO 15708-2:2017 obravnava računalniško aksialno tomografijo in izključuje druge vrste tomografij, kot so translacijska tomografija in tomosinteza.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 15708-2:2019
01-julij-2019
Nadomešča:
SIST EN 16016-2:2012
Neporušitvene preiskave - Sevalne metode za računalniško tomografijo - 2. del:
Načela, oprema in vzorci (ISO 15708-2:2017)
Non-destructive testing - Radiation methods for Computed tomography - Part 2:
Principles, equipment and samples (ISO 15708-2:2017)
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 2:
Grundlagen, Geräte und Proben (ISO 15708-2:2017)
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée -
Partie 2: Principes, équipements et échantillons (ISO 15708-2:2017)
Ta slovenski standard je istoveten z: EN ISO 15708-2:2019
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
SIST EN ISO 15708-2:2019 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 15708-2:2019
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SIST EN ISO 15708-2:2019
EN ISO 15708-2
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2019
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN 16016-2:2011
English Version
Non-destructive testing - Radiation methods for Computed
tomography - Part 2: Principles, equipment and samples
(ISO 15708-2:2017)
Essais non destructifs - Méthodes par rayonnements Zerstörungsfreie Prüfung - Durchstrahlungsverfahren
pour la tomographie informatisée - Partie 2: Principes, für Computertomografie - Teil 2: Grundlagen, Geräte
équipements et échantillons (ISO 15708-2:2017) und Proben (ISO 15708-2:2017)
This European Standard was approved by CEN on 11 February 2019.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 15708-2:2019 E
worldwide for CEN national Members.
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SIST EN ISO 15708-2:2019
EN ISO 15708-2:2019 (E)
Contents Page
European foreword . 3
2
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SIST EN ISO 15708-2:2019
EN ISO 15708-2:2019 (E)
European foreword
The text of ISO 15708-2:2017 has been prepared by Technical Committee ISO/TC 135 "Non-destructive
testing” of the International Organization for Standardization (ISO) and has been taken over as
EN ISO 15708-2:2019 by Technical Committee CEN/TC 138 “Non-destructive testing” the secretariat of
which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by October 2019, and conflicting national standards shall
be withdrawn at the latest by October 2019.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 16016-2:2011.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 15708-2:2017 has been approved by CEN as EN ISO 15708-2:2019 without any
modification.
3
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SIST EN ISO 15708-2:2019
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SIST EN ISO 15708-2:2019
INTERNATIONAL ISO
STANDARD 15708-2
Second edition
2017-02
Non-destructive testing — Radiation
methods for computed tomography —
Part 2:
Principles, equipment and samples
Essais non destructifs — Méthodes par rayonnements pour la
tomographie informatisée —
Partie 2: Principes, équipements et échantillons
Reference number
ISO 15708-2:2017(E)
©
ISO 2017
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All 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. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General principles . 1
4.1 Basic principles. 1
4.2 Advantages of CT . 2
4.3 Limitations of CT . 2
4.4 Main CT process steps . 3
4.4.1 Acquisition . 3
4.4.2 Reconstruction . 4
4.4.3 Visualization and analysis . 4
4.5 Artefacts in CT images . 4
5 Equipment and apparatus . 5
5.1 General . 5
5.2 Radiation sources . 6
5.3 Detectors . 6
5.4 Manipulation . 7
5.5 Acquisition, reconstruction, visualization and storage system . 7
6 CT system stability . 7
6.1 General . 7
6.2 X-Ray Stability . 8
6.3 Manipulator stability . 8
7 Geometric alignment. 8
8 Sample considerations . 9
8.1 Size and shape of sample . 9
8.2 Materials (including table voltage/thickness of penetration) . 9
Annex A (informative) CT system components .11
Bibliography .17
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
Foreword
ISO (the International Organization 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 the 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 procedures 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
editorial 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 rights 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 explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www . i so .org/ iso/ foreword .html.
This document 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.
This 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 found on the ISO website.
iv © ISO 2017 – All rights reserved
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SIST EN ISO 15708-2:2019
INTERNATIONAL STANDARD ISO 15708-2:2017(E)
Non-destructive testing — Radiation methods for
computed tomography —
Part 2:
Principles, equipment and samples
1 Scope
This document specifies the general principles of X-ray computed tomography (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
specifications.
This document deals with computed axial tomography and excludes other types of tomography such as
translational tomography and tomosynthesis.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this 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:
Terminology
ISO 15708-3:2017, Non-destructive testing — Radiation methods for computed tomography — Part 3:
Operation and interpretation
ISO 15708-4:2017, Non-destructive testing — Radiation methods for computed tomography — Part 4:
Qualification
ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel
3 Terms and definitions
For 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 http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 General principles
4.1 Basic principles
Computed 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 volume. Radiographic imaging is possible because different materials
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SIST EN ISO 15708-2:2019
ISO 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 colour). 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 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 attenuation 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 attenuation 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 proportional to the material
density. It also depends on the energy of the X-ray beam.
4.2 Advantages of CT
This 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-ray
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 images 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 either 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 conventional 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 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 CT
CT is an indirect test procedure and measurements (e.g. of the size of material faults; of wall thicknesses
must be compared 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, the 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, statistical 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 reserved
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
CT grey values cannot be used to identify unknown materials unambiguously unless a priori 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 detector.
4.4 Main CT process steps
4.4.1 Acquisition
During 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 taken. 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-rays
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 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 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 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 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 contrast 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 can 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 voxel and limits density
resolution.
Although statistical noise is unavoidable, the signal-to-noise ratio can be improved by increasing the
number of projections and/or time of exposure for each of them, the intensity of the X-ray source or the
voxel size. However, some of these measures will decrease spatial resolution. This trade-off between
spatial resolution and statistical noise is inherent in computed tomography.
© ISO 2017 – All rights reserved 3
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
4.4.2 Reconstruction
A CT scan initially produces a number of projections of an object. The subsequent reconstruction
of the CT image from these individual projections is the main step in computed tomography, which
distinguishes this examination technique from other radiographic methods.
The reconstruction software may apply additional corrections to the CT projections during
reconstruction, e.g. reduction of noise, correction of beam hardening and/or scattered radiation.
Depending on the CT system, either individual CT slices or 3D CT images are reconstructed.
4.4.3 Visualization and analysis
This step includes all operations and data manipulations, for extracting the desired information from
the reconstructed CT image.
Visualisation can either be performed in 2D (slice views) or in 3D (volume). 2D visualisation allows the
user to examine the data slice-wise along a defined axis (generally it can be an arbitrary path).
For 3D imaging, the CT volume or selected surfaces derived from it, are used for generating the desired
image according to the optical model underlying the algorithm. The main advantage of this type of
visualisation is that the visual perception of the image corresponds well with the natural appearance of
the object for the human eye, although features may appear superimposed in the 2D-representation on
a screen.
During visualisation, additional artefacts of different origin can occur, especially in the 3D imaging
of the CT volume. Such artefacts due to sampling, filtering, classification and blending within the
visualisation software are dependent on the hardware and software used, as well as the visualisation
task at hand. Therefore such artefacts are not included in the definition of artefacts as found in 4.5.
Nevertheless, the user should be aware that misinterpretation of the data might also occur in this
process step.
To highlight features of interest during visualisation different digital filter operations can be performed.
A characteristic of all these operations is that although they enhance one or more properties of the data,
they simultaneously deteriorate other properties (for example: highlighting the edges deteriorates
recognition of inner structures of an object). Therefore digital filters should always be used cautiously
for specific tasks, being aware which benefits and which detriments they are associated with.
A computer used for 3D visualisation should be able to process the complete volume of interest in the
main memory. The corresponding monitor should have a resolution, a dynamic range and settings
sufficient for the given visualisation task. Adequate vision of the personnel is to be ensured in
accordance with ISO 9712.
4.5 Artefacts in CT images
An artefact is an artificial feature which appears on the CT image but does not correspond to a physical
feature of the sample. Artefacts result from different origins; they can be classified into artefacts
arising from the measurement itself and the equipment (artefacts due to a finite beam width, scattered
radiation, instabilities and detector peculiarities) and artefacts inherent to the method (e.g. beam
hardening). Artefacts can also be divided into acquisition artefacts (e.g. scattered radiation, ring
artefacts) and reconstruction artefacts (e.g. cone beam artefacts). Some artefacts can be eliminated
by using an appropriate measurement technique with suitable parameters, while others can only be
reduced in their extent. Artefacts may be detrimental for specific measurement or analysis tasks, but
may have no impact on certain other analyses. With this fact in mind, the type and extent of artefacts in
a data set has to be evaluated in the context of the corresponding analysis task.
Noise and the partial volume effect are not considered as artefacts in this standard.
More details are given in ISO 15708-3:2017, 5.5.
4 © ISO 2017 – All rights reserved
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SIST EN ISO 15708-2:2019
ISO 15708-2:2017(E)
5 Equipment and apparatus
5.1 General
In relation to performance, a CT system can be considered as comprising four main components: the
X-ray source, detector, sample manipulation stages (the latter including any mechanical structure that
influences image stability) and reconstruction/visualisation system.
Generally the source and detector will be fixed while the sample rotates in the beam to acquire the
necessary set of projections. In scanners for example designed for in vivo animal studies or for imaging
large structures, the source and detector may
...
SLOVENSKI STANDARD
oSIST prEN ISO 15708-2:2018
01-december-2018
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Non-destructive testing - Radiation methods for Computed tomography - Part 2:
Principles, equipment and samples (ISO 15708-2:2017)
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 2:
Grundlagen, Geräte und Proben (ISO 15708-2:2017)
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée -
Partie 2: Principes, équipements et échantillons (ISO 15708-2:2017)
Ta slovenski standard je istoveten z: prEN ISO 15708-2
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
oSIST prEN ISO 15708-2:2018 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN ISO 15708-2:2018
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oSIST prEN ISO 15708-2:2018
INTERNATIONAL ISO
STANDARD 15708-2
Second edition
2017-02
Non-destructive testing — Radiation
methods for computed tomography —
Part 2:
Principles, equipment and samples
Essais non destructifs — Méthodes par rayonnements pour la
tomographie informatisée —
Partie 2: Principes, équipements et échantillons
Reference number
ISO 15708-2:2017(E)
©
ISO 2017
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oSIST prEN ISO 15708-2:2018
ISO 15708-2:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All 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. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved
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oSIST prEN ISO 15708-2:2018
ISO 15708-2:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General principles . 1
4.1 Basic principles. 1
4.2 Advantages of CT . 2
4.3 Limitations of CT . 2
4.4 Main CT process steps . 3
4.4.1 Acquisition . 3
4.4.2 Reconstruction . 4
4.4.3 Visualization and analysis . 4
4.5 Artefacts in CT images . 4
5 Equipment and apparatus . 5
5.1 General . 5
5.2 Radiation sources . 6
5.3 Detectors . 6
5.4 Manipulation . 7
5.5 Acquisition, reconstruction, visualization and storage system . 7
6 CT system stability . 7
6.1 General . 7
6.2 X-Ray Stability . 8
6.3 Manipulator stability . 8
7 Geometric alignment. 8
8 Sample considerations . 9
8.1 Size and shape of sample . 9
8.2 Materials (including table voltage/thickness of penetration) . 9
Annex A (informative) CT system components .11
Bibliography .17
© ISO 2017 – All rights reserved iii
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oSIST prEN ISO 15708-2:2018
ISO 15708-2:2017(E)
Foreword
ISO (the International Organization 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 the right to be represented on that committee. International
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The procedures 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
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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This document 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.
This 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 found on the ISO website.
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INTERNATIONAL STANDARD ISO 15708-2:2017(E)
Non-destructive testing — Radiation methods for
computed tomography —
Part 2:
Principles, equipment and samples
1 Scope
This document specifies the general principles of X-ray computed tomography (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
specifications.
This document deals with computed axial tomography and excludes other types of tomography such as
translational tomography and tomosynthesis.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this 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:
Terminology
ISO 15708-3:2017, Non-destructive testing — Radiation methods for computed tomography — Part 3:
Operation and interpretation
ISO 15708-4:2017, Non-destructive testing — Radiation methods for computed tomography — Part 4:
Qualification
ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel
3 Terms and definitions
For 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 http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 General principles
4.1 Basic principles
Computed 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 volume. Radiographic imaging is possible because different materials
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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 colour). 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 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 attenuation 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 attenuation 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 proportional to the material
density. It also depends on the energy of the X-ray beam.
4.2 Advantages of CT
This 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-ray
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 images 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 either 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 conventional 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 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 CT
CT is an indirect test procedure and measurements (e.g. of the size of material faults; of wall thicknesses
must be compared 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, the 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, statistical 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.
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CT grey values cannot be used to identify unknown materials unambiguously unless a priori 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 detector.
4.4 Main CT process steps
4.4.1 Acquisition
During 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 taken. 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-rays
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 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 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 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 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 contrast 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 can 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 voxel and limits density
resolution.
Although statistical noise is unavoidable, the signal-to-noise ratio can be improved by increasing the
number of projections and/or time of exposure for each of them, the intensity of the X-ray source or the
voxel size. However, some of these measures will decrease spatial resolution. This trade-off between
spatial resolution and statistical noise is inherent in computed tomography.
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4.4.2 Reconstruction
A CT scan initially produces a number of projections of an object. The subsequent reconstruction
of the CT image from these individual projections is the main step in computed tomography, which
distinguishes this examination technique from other radiographic methods.
The reconstruction software may apply additional corrections to the CT projections during
reconstruction, e.g. reduction of noise, correction of beam hardening and/or scattered radiation.
Depending on the CT system, either individual CT slices or 3D CT images are reconstructed.
4.4.3 Visualization and analysis
This step includes all operations and data manipulations, for extracting the desired information from
the reconstructed CT image.
Visualisation can either be performed in 2D (slice views) or in 3D (volume). 2D visualisation allows the
user to examine the data slice-wise along a defined axis (generally it can be an arbitrary path).
For 3D imaging, the CT volume or selected surfaces derived from it, are used for generating the desired
image according to the optical model underlying the algorithm. The main advantage of this type of
visualisation is that the visual perception of the image corresponds well with the natural appearance of
the object for the human eye, although features may appear superimposed in the 2D-representation on
a screen.
During visualisation, additional artefacts of different origin can occur, especially in the 3D imaging
of the CT volume. Such artefacts due to sampling, filtering, classification and blending within the
visualisation software are dependent on the hardware and software used, as well as the visualisation
task at hand. Therefore such artefacts are not included in the definition of artefacts as found in 4.5.
Nevertheless, the user should be aware that misinterpretation of the data might also occur in this
process step.
To highlight features of interest during visualisation different digital filter operations can be performed.
A characteristic of all these operations is that although they enhance one or more properties of the data,
they simultaneously deteriorate other properties (for example: highlighting the edges deteriorates
recognition of inner structures of an object). Therefore digital filters should always be used cautiously
for specific tasks, being aware which benefits and which detriments they are associated with.
A computer used for 3D visualisation should be able to process the complete volume of interest in the
main memory. The corresponding monitor should have a resolution, a dynamic range and settings
sufficient for the given visualisation task. Adequate vision of the personnel is to be ensured in
accordance with ISO 9712.
4.5 Artefacts in CT images
An artefact is an artificial feature which appears on the CT image but does not correspond to a physical
feature of the sample. Artefacts result from different origins; they can be classified into artefacts
arising from the measurement itself and the equipment (artefacts due to a finite beam width, scattered
radiation, instabilities and detector peculiarities) and artefacts inherent to the method (e.g. beam
hardening). Artefacts can also be divided into acquisition artefacts (e.g. scattered radiation, ring
artefacts) and reconstruction artefacts (e.g. cone beam artefacts). Some artefacts can be eliminated
by using an appropriate measurement technique with suitable parameters, while others can only be
reduced in their extent. Artefacts may be detrimental for specific measurement or analysis tasks, but
may have no impact on certain other analyses. With this fact in mind, the type and extent of artefacts in
a data set has to be evaluated in the context of the corresponding analysis task.
Noise and the partial volume effect are not considered as artefacts in this standard.
More details are given in ISO 15708-3:2017, 5.5.
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5 Equipment and apparatus
5.1 General
In relation to performance, a CT system can be considered as comprising four main components: the
X-ray source, detector, sample manipulation stages (the latter including any mechanical structure that
influences image stability) and reconstruction/visualisation system.
Generally the source and detector will be fixed while the sample rotates in the beam to acquire the
necessary set of projections. In scanners for example designed for in vivo animal studies or for imaging
large structures, the source and detector may orbit around the sample, as in medical scanners.
In the majority of micro-/nano- or sub-micro-tomography systems, the resolution is determined
primarily by the X-ray focal spot size. Geometric magnification allows the detector element spacing to
be much larger than the computed voxel size and a thicker and therefore more efficient scintillator to
be used. A disadvantage of this approach is that to obtain high magnification ratios, the sample should
be located very close to the source. This is a particular problem if the sample is to be mounted in some
form of environmental chamber or, for example, an in situ loading stage. This imposes a lower limit on
the source to sample distance, thus reducing X-ray fluence (resulting in a lower signal-to-noise ratio
and/or increased acquisition time) and requiring the detector to be mounted proportionately further
away in order to achieve the same magnification factor. Alternatively, if the sample to detector distance
is low compared with the source to sample distance, the detector resolution becomes the limiting factor,
rather than the spot size. In this case, the increased source to detector distance again means reduced
X-ray fluence and high-resolution detectors tend to require thinner and hence less efficient scintillators.
CT systems may be optimised for resolution, energy, speed of acquisition or simply cost. Although a
particular system may operate over a wide range of conditions, it will operate optimally over a much
smaller range and the user should consider the prime application when selecting one model over
another and not simply over-specify.
For example, a high-resolution CT system (small X-ray focal spot size) may have a considerably lower
flux output at more modest resolution settings than one designed to operate at such resolution.
Furthermore, a high performance rotation stage for a high-resolution scanner will have a much smaller
load limit. Similarly, a system designed for high energy imaging will require a thicker phosphor screen,
giving poorer resolution compared with a thinner screen, which is adequate at lower energies.
Some CT systems can provide interchangeable X-ray target heads (transmission or reflection, see
Annex A) and/or interchangeable detectors, but these will come at a higher price.
When comparing resolution and scan times on different CT systems, it is important to consider the
signal-to-noise ratio (SNR), see ISO 15708-3:2017, 5.1.3. This is dependent on the X-ray exposure and
thus the faster the scan, the worse the SNR. It is also dependent on the sample type and geometry. A
sample with a high void volume fraction (or with a high proportion of relatively low absorbing regions),
such as a foam or cancellous bone sample, will exhibit a better SNR than a more homogeneous sample.
For a given exposure, the best SNR is obtained with the X-ray accelerating voltage set to give
approximately 10 %–20 % transmission through the sample. If the transmission is too low, the low
number of photons detected will give rise to excessive noise. Conversely, if it is too high, the contrast
(signal in SNR) will be too low. The SNR does not vary sharply with voltage however, and simulations
of X-ray attenuation in aluminium indicate that the SNR only drops by 20 % of the peak value if the
voltage is set for 35 % or 40 % transmission. For a given sample size, the required X-ray exposure to
maintain a fixed SNR is proportional to the fourth power of resolution (for a given detector). Thus, for
example, doubling the resolution will require a 16-fold increase in exposure while a 10-fold increase in
resolution will demand a 10,000 fold increase in exposure. There is therefore a critical need to use the
same or similar samples when comparing the image quality from one system with that of another.
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5.2 Radiation sources
Most industrial CT systems will use an electrically generated X-ray source, and these can be subdivided
into three main types:
— open tube (or vacuum demou
...
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