SIST EN ISO 15708-3:2019
(Main)Non-destructive testing - Radiation methods for computed tomography - Part 3: Operation and interpretation (ISO 15708-3:2017)
Non-destructive testing - Radiation methods for computed tomography - Part 3: Operation and interpretation (ISO 15708-3:2017)
ISO 15708-3:2017 presents an outline of the operation of a computed tomography (CT) system and the interpretation of results with the aim of providing the operator with technical information to enable the selection of suitable parameters.
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 15708-3:2017 deals with computed axial tomography and excludes other types of tomography such as translational tomography and tomosynthesis.
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 3: Durchführung und Auswertung (ISO 15708-3:2017)
Dieses Dokument bietet einen Überblick über den Betrieb eines Computertomographie (CT )Systems und die Auswertung der Ergebnisse, um dem Anwender technische Informationen zur Auswahl geeigneter Para¬meter zur Verfügung zu stellen.
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 3: Fonctionnement et interprétation (ISO 15708-3:2017)
Le présent document fournit une présentation générale du fonctionnement d'un système de tomographie informatisée (TI) et de l'interprétation des résultats afin de donner à l'utilisateur des informations techniques pour sélectionner les paramètres adaptés.
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 - 3. del: Delovanje in razlaga (ISO 15708-3:2017)
Standard ISO 15708-3:2017 opisuje delovanje sistema računalniške tomografije (CT) in razlago rezultatov z namenom zagotavljanja tehničnih podatkov uporabniku za omogočanje izbire ustreznih parametrov.
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 15708-3:2017 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 15708-3:2019
01-julij-2019
Nadomešča:
SIST EN 16016-3:2012
Neporušitvene preiskave - Sevalne metode za računalniško tomografijo - 3. del:
Delovanje in razlaga (ISO 15708-3:2017)
Non-destructive testing - Radiation methods for computed tomography - Part 3:
Operation and interpretation (ISO 15708-3:2017)
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 3:
Durchführung und Auswertung (ISO 15708-3:2017)
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée -
Partie 3: Fonctionnement et interprétation (ISO 15708-3:2017)
Ta slovenski standard je istoveten z: EN ISO 15708-3:2019
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
SIST EN ISO 15708-3:2019 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 15708-3:2019
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SIST EN ISO 15708-3:2019
EN ISO 15708-3
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2019
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN 16016-3:2011
English Version
Non-destructive testing - Radiation methods for computed
tomography - Part 3: Operation and interpretation (ISO
15708-3:2017)
Essais non destructifs - Méthodes par rayonnements Zerstörungsfreie Prüfung - Durchstrahlungsverfahren
pour la tomographie informatisée - Partie 3: für Computertomografie - Teil 3: Durchführung und
Fonctionnement et interprétation (ISO 15708-3:2017) Auswertung (ISO 15708-3:2017)
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 15708-3:2019 E
worldwide for CEN national Members.
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SIST EN ISO 15708-3:2019
EN ISO 15708-3:2019 (E)
Contents Page
European foreword . 3
2
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SIST EN ISO 15708-3:2019
EN ISO 15708-3:2019 (E)
European foreword
The text of ISO 15708-3:2017 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 15708-3:2019 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 16016-3:2011.
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 15708-3:2017 has been approved by CEN as EN ISO 15708-3:2019 without any
modification.
3
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SIST EN ISO 15708-3:2019
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SIST EN ISO 15708-3:2019
INTERNATIONAL ISO
STANDARD 15708-3
First edition
2017-02
Non-destructive testing — Radiation
methods for computed tomography —
Part 3:
Operation and interpretation
Essais non destructifs — Méthodes par rayonnements pour la
tomographie informatisée —
Partie 3: Fonctionnement et interprétation
Reference number
ISO 15708-3:2017(E)
©
ISO 2017
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SIST EN ISO 15708-3:2019
ISO 15708-3:2017(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 15708-3:2019
ISO 15708-3:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Operational procedure . 1
4.1 General . 1
4.2 CT system set-up . 2
4.2.1 General. 2
4.2.2 Geometry . 2
4.2.3 X-ray source . 3
4.2.4 Detector . 3
4.3 Reconstruction parameters . 3
4.4 Visualization . 3
4.5 Analysis and interpretation of CT images . 4
4.5.1 General. 4
4.5.2 Feature testing/defect testing . 4
4.5.3 Dimensional testing . 4
5 Requirements for acceptable results . 7
5.1 Image quality parameters . 7
5.1.1 Contrast . 7
5.1.2 Noise . 8
5.1.3 Signal to noise ratio . 9
5.1.4 Contrast to noise ratio . 9
5.1.5 Spatial resolution .10
5.2 Suitability of testing .12
5.3 CT examination interpretation and acceptance criteria .12
5.4 Records and reports .12
5.5 Artefacts .13
5.5.1 General.13
5.5.2 Beam hardening artefacts .13
5.5.3 Edge artefacts .14
5.5.4 Scattered radiation . .15
5.5.5 Instabilities .15
5.5.6 Ring artefacts .15
5.5.7 Centre of rotation error artefacts .16
5.5.8 Motion artefacts .17
5.5.9 Artefacts due to an insufficient number of projections .18
5.5.10 Cone beam artefacts .18
Annex A (informative) Spatial resolution measurement using line pair gauges .19
Bibliography .23
© ISO 2017 – All rights reserved iii
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SIST EN ISO 15708-3:2019
ISO 15708-3:2017(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 16016-3)
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 first edition of ISO 15708-3 cancels and replaces ISO 15708-2:2002, 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 15708-3:2019
INTERNATIONAL STANDARD ISO 15708-3:2017(E)
Non-destructive testing — Radiation methods for
computed tomography —
Part 3:
Operation and interpretation
1 Scope
This document presents an outline of the operation of a computed tomography (CT) system and the
interpretation of results with the aim of providing the operator with technical information to enable
the selection of suitable parameters.
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 15708-2:2017, Non-destructive testing — Radiation methods for computed tomography — Part 2:
Principle, equipment and samples
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 Operational procedure
4.1 General
For target-oriented computer tomography (CT) inspection procedures, the test and measurement tasks
are defined in advance with regard to the size and type of features/defects to be verified; for example,
through the specification of appropriate acceptance levels and geometry deviations. In the following,
the process steps of a CT application are described and information on its implementation provided.
© ISO 2017 – All rights reserved 1
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SIST EN ISO 15708-3:2019
ISO 15708-3:2017(E)
4.2 CT system set-up
4.2.1 General
The CT system set-up is oriented towards the requirements for the given task. The required spatial
resolution (taking into account the tube focal spot size), contrast resolution, voxel size and the CT image
quality can be derived from these requirements. The quality of the CT image is determined by different
parameters, which under certain circumstances counteract each other.
In the following, system parameters are described and information is provided on setting up a CT
system for inspection. Due to the interactions of the different system parameters, it may be necessary
to run through the set-up steps several times in order to acquire optimal data.
The optimal energy is that which gives the best signal-to-noise ratio and not necessarily that which
gives the clearest radiograph (the dependency of the detector efficiency on the energy is to be taken into
account). However, in order to differentiate between materials of different chemical composition it may
be necessary to adjust the accelerating voltage to maximise the difference in their linear attenuation
coefficients.
4.2.2 Geometry
The source-detector and source-object distances and thus also the beam angle used should be
specified. In order to achieve high resolutions, the projection can be magnified onto the detector.
The magnification is equal to the ratio of the source-detector distance to the source-object distance.
Increasing source-detector distance leads to a reduced intensity at the detector and thus to a reduced
signal to noise ratio. Accordingly, this also applies when using detectors with improved detector
resolution, which can result in a reduction of the signal-to-noise ratio due to the reduced intensity per
pixel. In general, for this reason, minimisation of the source-object distance is to be preferred.
In order to obtain high beam intensity at the detector, the source-detector distance should be selected
so that it is as small as possible taking into account the required resolution so that the beam cone still
fully illuminates the detector. In the case of 3D-CT, the (in general vertical) total cone beam angle
measured parallel to the rotation axis should typically be less than 15°, but this is specimen dependant,
in order to minimise reconstruction-determined (Feldkamp) distortions of the 3D model. In addition,
these restrictions do not apply for the perpendicular (in general horizontal) beam angle. For a higher
geometric magnification, the object must be positioned as near as possible to the source, taking into
consideration the limit on sharpness imposed by focal spot size. The rotation of the object must take
place at at least 180 °plus beam angle of the X-ray beam, whereby an improved data quality is the result
of an increasing number of angular increments. For this reason, the object is typically turned through
360 °. Ideally, the number of angular increments should be at least π/2 × matrix size (uneven number of
projections per 360°) where the matrix size is the number of voxels across the sample diameter or the
largest dimension. For more information, refer to 5.5.
The number of projections should be > π × matrix size for best reconstruction quality (even or uneven
number of projections per 360°).
In order to obtain as complete information as possible on the specimen, the requirement in general for a
CT is that the object (or the interesting section of the object) is completely mapped in each projection on
the detector. For large components that exceed the beam cone, a so-called measurement range extension
is used. This measurement range extension is accomplished by laterally displacing either the object
or the detector, recording the projection data in sequential measurements, and finally concatenating
(joining) them. Under certain circumstances, it is also possible to only scan a part of the object (region-
of-interest CT), which may lead to a restricted data quality in the form of so-called truncations.
A possible deviation of the recording geometry (offset between the projected axis of rotation and the
centre line of the image) must be corrected for in order to obtain a reconstruction which is as precise
as possible. This can be achieved by careful realignment of the system or be corrected using software.
2 © ISO 2017 – All rights reserved
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SIST EN ISO 15708-3:2019
ISO 15708-3:2017(E)
4.2.3 X-ray source
At the X-ray source, the maximum beam energy and tube current are to be set such that sufficient
penetration of the test object and tube power with a sufficiently small focal spot are ensured. The
required voltage shall be determined by the maximum path length in the material to be X-rayed
in accordance with ISO 15708-2:2017, 8.2. For the best measurement results, an attenuation ratio of
approx. 1:10 should be used. That is the grey level through the sample should be about 10 % of the
white level (both measured with respect to the dark level). The optimal range can be achieved through
the use of pre-filters. It should be noted that every pre-filter reduces the intensity. Pre-filters have the
additional advantage of reducing beam hardening, though further improvements can be made with
software correction.
4.2.4 Detector
The following detector settings need to be set appropriately for the sample being scanned:
— exposure time (frame rate);
— number of integrations per projection;
— digitisation gain and offset;
— binning.
If necessary, corrections for offset, gain and bad pixels (which may depend on X-ray settings) should be
applied.
The individual CT projection is determined by the detector properties: its geometric resolution,
its sensitivity, dynamics and noise. The gain and exposure time can be adjusted together with the
radiation intensity of the source so that the maximum digitised intensity does not exceed 90 % of the
saturation level.
To reduce scattered radiation, a thin filter, grid or lamellae can be used directly in front of the detector
(intermediate-filtering).
The ideal acquisition time is dependent on the required quality of the CT image and it is often limited by
the time available for inspection.
4.3 Reconstruction parameters
The volumetric region to be reconstructed, the size of the CT image (in terms of voxels) as well as its
dynamic range (which should take into account the detector dynamic range) shall be specified. In order
to achieve sufficient CT image quality, settings for the reconstruction algorithm or corrections should
be optimised.
The volumetric region is defined by the number of voxels along the X, Y and Z axes.
4.4 Visualization
Using volume visualisation, the CT image can be presented as a 3D object. Individual grey values can
be assigned any colour and opacity values to highlight or hide materials with different X-ray densities.
Zooming, scrolling, setting contrast, brightness, colour and lighting facilitate an optimal presentation
of the CT image. In addition, it is possible to place user-defined sectional planes through the object
in order to examine the internal structure, or to interactively visualise the CT image, for example by
rotating and moving it as a 3D object. Image processing can be applied to CT images to improve feature
recognition.
It may not be possible to load the whole CT image at full resolution into memory at once.
© ISO 2017 – All rights reserved 3
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SIST EN ISO 15708-3:2019
ISO 15708-3:2017(E)
4.5 Analysis and interpretation of CT images
4.5.1 General
Typical features for inspection are pores, cavities, cracks, inclusions, impurities or inhomogeneous
material distributions.
Typical measurement tasks are obtaining dimensional properties (such as length or wall thickness) or
calculating object morphology.
4.5.2 Feature testing/defect testing
Features in the sample generally give rise to changes in CT grey level within the CT image. Analysis
of CT images is performed by qualified personnel using software. A suitable contrast range or an
automatic or manual calibration is used. The position, CT grey value and dimensions of features can
be determined. Several tools are available for this, including manual ones or automatic tools such as
strobe lines or gauges that engage at grey value thresholds or edges. For examining the structure and
location of assembled components, a qualitative comparison of CT images without determination of the
dimensions can suffice.
For an automatic determination using visualisation software tools (for example fault analyses), a
calibration via the specification of a grey value range is, in general, required for the sample material
to be measured. The specification of the grey levels can be done manually using histograms or in an
interactive manner.
The detectability of features depends on the size of the feature relative to the geometric resolution and
the contrast resolution compared with the contrast difference of the feature from the base material, as
well as the quality of the image (signal to noise ratio, etc.) and any possible interference effects between
adjacent voxels (partial volume effect). For the detectability of singular pores, cavities or cracks, their
minimum extent should typically be 2 to 3 times the demagnified pixel size (at the position of the
sample).
4.5.3 Dimensional testing
4.5.3.1 General
Depending on the task, there are various methods currently in use for determining geometric features.
Point-to-point distances can be manually determined in the CT slices or more complex features can be
extracted with the help of analysis software.
The measurement of the geometric properties of an object using CT is an indirect procedure, in which
the dimensional measurement takes place in or is derived from CT images. For this reason, in order to
facilitate precise measurements, an accurate knowledge of two important variables is necessary:
— the precise image scale or voxel size and
— the boundary surface of two materials, for example the component surface (material-to-air
transition), which can be determined via a CT grey value threshold in the CT image.
4.5.3.2 Determination of precise image scale
The precise image scale or voxel size must be determined through the measurement of a suitable
calibration standard (together with the measurement object and directly before/after the object
inspection) or using a reference geometry at the object. For this, the voxel size or magnification, M,
specified by the CT system is compared with the actual available and precisely determined (using the
reference body/geometry) voxel size or magnification M*. Thus, for example, the exact voxel size can be
determined with high precision via measurements without the disturbing influence of other variables
(for example, the precise position of the co
...
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01-december-2018
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Non-destructive testing - Radiation methods for computed tomography - Part 3:
Operation and interpretation (ISO 15708-3:2017)
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomografie - Teil 3:
Durchführung und Auswertung (ISO 15708-3:2017)
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée -
Partie 3: Fonctionnement et interprétation (ISO 15708-3:2017)
Ta slovenski standard je istoveten z: prEN ISO 15708-3
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
oSIST prEN ISO 15708-3:2018 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
oSIST prEN ISO 15708-3:2018
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oSIST prEN ISO 15708-3:2018
INTERNATIONAL ISO
STANDARD 15708-3
First edition
2017-02
Non-destructive testing — Radiation
methods for computed tomography —
Part 3:
Operation and interpretation
Essais non destructifs — Méthodes par rayonnements pour la
tomographie informatisée —
Partie 3: Fonctionnement et interprétation
Reference number
ISO 15708-3:2017(E)
©
ISO 2017
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oSIST prEN ISO 15708-3:2018
ISO 15708-3:2017(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 15708-3:2018
ISO 15708-3:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Operational procedure . 1
4.1 General . 1
4.2 CT system set-up . 2
4.2.1 General. 2
4.2.2 Geometry . 2
4.2.3 X-ray source . 3
4.2.4 Detector . 3
4.3 Reconstruction parameters . 3
4.4 Visualization . 3
4.5 Analysis and interpretation of CT images . 4
4.5.1 General. 4
4.5.2 Feature testing/defect testing . 4
4.5.3 Dimensional testing . 4
5 Requirements for acceptable results . 7
5.1 Image quality parameters . 7
5.1.1 Contrast . 7
5.1.2 Noise . 8
5.1.3 Signal to noise ratio . 9
5.1.4 Contrast to noise ratio . 9
5.1.5 Spatial resolution .10
5.2 Suitability of testing .12
5.3 CT examination interpretation and acceptance criteria .12
5.4 Records and reports .12
5.5 Artefacts .13
5.5.1 General.13
5.5.2 Beam hardening artefacts .13
5.5.3 Edge artefacts .14
5.5.4 Scattered radiation . .15
5.5.5 Instabilities .15
5.5.6 Ring artefacts .15
5.5.7 Centre of rotation error artefacts .16
5.5.8 Motion artefacts .17
5.5.9 Artefacts due to an insufficient number of projections .18
5.5.10 Cone beam artefacts .18
Annex A (informative) Spatial resolution measurement using line pair gauges .19
Bibliography .23
© ISO 2017 – All rights reserved iii
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oSIST prEN ISO 15708-3:2018
ISO 15708-3:2017(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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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This document was prepared by the European Committee for Standardization (CEN) (as EN 16016-3)
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 first edition of ISO 15708-3 cancels and replaces ISO 15708-2:2002, 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 15708-3:2017(E)
Non-destructive testing — Radiation methods for
computed tomography —
Part 3:
Operation and interpretation
1 Scope
This document presents an outline of the operation of a computed tomography (CT) system and the
interpretation of results with the aim of providing the operator with technical information to enable
the selection of suitable parameters.
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 15708-2:2017, Non-destructive testing — Radiation methods for computed tomography — Part 2:
Principle, equipment and samples
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 Operational procedure
4.1 General
For target-oriented computer tomography (CT) inspection procedures, the test and measurement tasks
are defined in advance with regard to the size and type of features/defects to be verified; for example,
through the specification of appropriate acceptance levels and geometry deviations. In the following,
the process steps of a CT application are described and information on its implementation provided.
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4.2 CT system set-up
4.2.1 General
The CT system set-up is oriented towards the requirements for the given task. The required spatial
resolution (taking into account the tube focal spot size), contrast resolution, voxel size and the CT image
quality can be derived from these requirements. The quality of the CT image is determined by different
parameters, which under certain circumstances counteract each other.
In the following, system parameters are described and information is provided on setting up a CT
system for inspection. Due to the interactions of the different system parameters, it may be necessary
to run through the set-up steps several times in order to acquire optimal data.
The optimal energy is that which gives the best signal-to-noise ratio and not necessarily that which
gives the clearest radiograph (the dependency of the detector efficiency on the energy is to be taken into
account). However, in order to differentiate between materials of different chemical composition it may
be necessary to adjust the accelerating voltage to maximise the difference in their linear attenuation
coefficients.
4.2.2 Geometry
The source-detector and source-object distances and thus also the beam angle used should be
specified. In order to achieve high resolutions, the projection can be magnified onto the detector.
The magnification is equal to the ratio of the source-detector distance to the source-object distance.
Increasing source-detector distance leads to a reduced intensity at the detector and thus to a reduced
signal to noise ratio. Accordingly, this also applies when using detectors with improved detector
resolution, which can result in a reduction of the signal-to-noise ratio due to the reduced intensity per
pixel. In general, for this reason, minimisation of the source-object distance is to be preferred.
In order to obtain high beam intensity at the detector, the source-detector distance should be selected
so that it is as small as possible taking into account the required resolution so that the beam cone still
fully illuminates the detector. In the case of 3D-CT, the (in general vertical) total cone beam angle
measured parallel to the rotation axis should typically be less than 15°, but this is specimen dependant,
in order to minimise reconstruction-determined (Feldkamp) distortions of the 3D model. In addition,
these restrictions do not apply for the perpendicular (in general horizontal) beam angle. For a higher
geometric magnification, the object must be positioned as near as possible to the source, taking into
consideration the limit on sharpness imposed by focal spot size. The rotation of the object must take
place at at least 180 °plus beam angle of the X-ray beam, whereby an improved data quality is the result
of an increasing number of angular increments. For this reason, the object is typically turned through
360 °. Ideally, the number of angular increments should be at least π/2 × matrix size (uneven number of
projections per 360°) where the matrix size is the number of voxels across the sample diameter or the
largest dimension. For more information, refer to 5.5.
The number of projections should be > π × matrix size for best reconstruction quality (even or uneven
number of projections per 360°).
In order to obtain as complete information as possible on the specimen, the requirement in general for a
CT is that the object (or the interesting section of the object) is completely mapped in each projection on
the detector. For large components that exceed the beam cone, a so-called measurement range extension
is used. This measurement range extension is accomplished by laterally displacing either the object
or the detector, recording the projection data in sequential measurements, and finally concatenating
(joining) them. Under certain circumstances, it is also possible to only scan a part of the object (region-
of-interest CT), which may lead to a restricted data quality in the form of so-called truncations.
A possible deviation of the recording geometry (offset between the projected axis of rotation and the
centre line of the image) must be corrected for in order to obtain a reconstruction which is as precise
as possible. This can be achieved by careful realignment of the system or be corrected using software.
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4.2.3 X-ray source
At the X-ray source, the maximum beam energy and tube current are to be set such that sufficient
penetration of the test object and tube power with a sufficiently small focal spot are ensured. The
required voltage shall be determined by the maximum path length in the material to be X-rayed
in accordance with ISO 15708-2:2017, 8.2. For the best measurement results, an attenuation ratio of
approx. 1:10 should be used. That is the grey level through the sample should be about 10 % of the
white level (both measured with respect to the dark level). The optimal range can be achieved through
the use of pre-filters. It should be noted that every pre-filter reduces the intensity. Pre-filters have the
additional advantage of reducing beam hardening, though further improvements can be made with
software correction.
4.2.4 Detector
The following detector settings need to be set appropriately for the sample being scanned:
— exposure time (frame rate);
— number of integrations per projection;
— digitisation gain and offset;
— binning.
If necessary, corrections for offset, gain and bad pixels (which may depend on X-ray settings) should be
applied.
The individual CT projection is determined by the detector properties: its geometric resolution,
its sensitivity, dynamics and noise. The gain and exposure time can be adjusted together with the
radiation intensity of the source so that the maximum digitised intensity does not exceed 90 % of the
saturation level.
To reduce scattered radiation, a thin filter, grid or lamellae can be used directly in front of the detector
(intermediate-filtering).
The ideal acquisition time is dependent on the required quality of the CT image and it is often limited by
the time available for inspection.
4.3 Reconstruction parameters
The volumetric region to be reconstructed, the size of the CT image (in terms of voxels) as well as its
dynamic range (which should take into account the detector dynamic range) shall be specified. In order
to achieve sufficient CT image quality, settings for the reconstruction algorithm or corrections should
be optimised.
The volumetric region is defined by the number of voxels along the X, Y and Z axes.
4.4 Visualization
Using volume visualisation, the CT image can be presented as a 3D object. Individual grey values can
be assigned any colour and opacity values to highlight or hide materials with different X-ray densities.
Zooming, scrolling, setting contrast, brightness, colour and lighting facilitate an optimal presentation
of the CT image. In addition, it is possible to place user-defined sectional planes through the object
in order to examine the internal structure, or to interactively visualise the CT image, for example by
rotating and moving it as a 3D object. Image processing can be applied to CT images to improve feature
recognition.
It may not be possible to load the whole CT image at full resolution into memory at once.
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4.5 Analysis and interpretation of CT images
4.5.1 General
Typical features for inspection are pores, cavities, cracks, inclusions, impurities or inhomogeneous
material distributions.
Typical measurement tasks are obtaining dimensional properties (such as length or wall thickness) or
calculating object morphology.
4.5.2 Feature testing/defect testing
Features in the sample generally give rise to changes in CT grey level within the CT image. Analysis
of CT images is performed by qualified personnel using software. A suitable contrast range or an
automatic or manual calibration is used. The position, CT grey value and dimensions of features can
be determined. Several tools are available for this, including manual ones or automatic tools such as
strobe lines or gauges that engage at grey value thresholds or edges. For examining the structure and
location of assembled components, a qualitative comparison of CT images without determination of the
dimensions can suffice.
For an automatic determination using visualisation software tools (for example fault analyses), a
calibration via the specification of a grey value range is, in general, required for the sample material
to be measured. The specification of the grey levels can be done manually using histograms or in an
interactive manner.
The detectability of features depends on the size of the feature relative to the geometric resolution and
the contrast resolution compared with the contrast difference of the feature from the base material, as
well as the quality of the image (signal to noise ratio, etc.) and any possible interference effects between
adjacent voxels (partial volume effect). For the detectability of singular pores, cavities or cracks, their
minimum extent should typically be 2 to 3 times the demagnified pixel size (at the position of the
sample).
4.5.3 Dimensional testing
4.5.3.1 General
Depending on the task, there are various methods currently in use for determining geometric features.
Point-to-point distances can be manually determined in the CT slices or more complex features can be
extracted with the help of analysis software.
The measurement of the geometric properties of an object using CT is an indirect procedure, in which
the dimensional measurement takes place in or is derived from CT images. For this reason, in order to
facilitate precise measurements, an accurate knowledge of two important variables is necessary:
— the precise image scale or voxel size and
— the boundary surface of two materials, for example the component surface (material-to-air
transition), which can be determined via a CT grey value threshold in the CT image.
4.5.3.2 Determination of precise image scale
The precise image scale or voxel size must be determined through the measurement of a suitable
calibration standard (together with the measurement object and directly before/after the object
inspection) or using a reference geometry at the object. For this, the voxel size or magnification, M,
specified by the CT system is compared with the actual available and precisely determined (using the
reference body/geometry) voxel size or magnification M*. Thus, for example, the exact voxel size can be
determined with high precision via measurements without the disturbing influence of other variables
(for example, the precise position of the component surface (grey value threshold) in the CT image)
for the centre distances of a test piece (e.g. dumbbell, see Figure 1). In this procedure, it must be taken
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into account that the CT grey values of the test item can, under certain circumstances, be influenced by
the accompanying reference bodies (for example, through changes to the contrast ratios, interferences
and artefacts). Using the actual voxel sizes determined in this way, the visualisation software can be
correspondingly scaled/corrected as regards the voxel size specified by the system.
Figure 1 — Reference objects (dumbbell)
4.5.3.3 Threshold value determination
In order to be able to carry out dimensional measurements, the component surface or material contact
surface must be determined in the CT image. The component surface is generally derived from the
transition from solid object to surrounding air. The boundary surface is defined via a threshold value
and is thus dependent on the materials and the X-ray settings. This threshold may be specified globally
for the entire CT image as an average grey value of, for example, the material and air. This is sometimes
known as the “iso50 threshold”. A global threshold value or calibration using the iso50 method is
suitable for many measurement tasks on objects made from homogeneous materials.
A global threshold is not suitable for objects made from several materials. In these cases, different
thresholds should be used according to the materials either side of the boundary. Even in the case of
objects made from homogeneous materials, beam hardening, scattering and other artefacts can result
in local dimming or lightening in the CT image which would distort the measurement results. The grey
value threshold, for example, for surfaces in the inside of the component thus frequently differs from
that for surfaces on the outside of the component. The threshold can, if necessary, be determined locally
from grey levels either side of the boundary. A determination of the overall component surface via
locally determined threshold values, while more time consuming, is more tolerant towards contrast
variations and artefact influences.
4.5.3.4 Adjustment of geometrically primitive bodies
In addition to simple point-to-point operations (see 4.5.3), methods from coordinate measurement
technology, such as reference geometry adjustment may be employed. In this connection, so-called
geometric primitive bodies or reference elements (for example planes, cylinders, spheres or similar)
are fitted, using software, to object contours of interest in the correspondingly calibrated data. At the
reference elements, geometric features (for example, diameter, distances, angles, etc.) are determined
directly or by combining reference elements. By fitting to the typically several thousand measurement
points of the corresponding data, there is thus, due to the statistic averaging and reduction of the user
influence, an often much higher precision than via the manual distance measurement of two points.
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4.5.3.5 Generation of geometric data
So-called triangular models can be extracted from the voxels and calibrated grey value threshold.
These models represent the calibrated threshold value-isosurface, i.e. the material surface in the form
of linked triangles. The triangular model contains – as part of the extraction process precision (see
below) – the geometry information on the object surface. It consists of only two types of information:
the so-called vertices and the information as to which vertices belong to a triangle. The vertices are
3D points, which lie on the threshold value-isosurface. The quantity of all vertices is also designated a
point cloud. It is initially the linking information, i.e. the information as to which three vertices in each
case form a surface triangle, which d
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