EN ISO 15708-2:2025

SLOVENSKI STANDARD
01-april-2025
Neporušitvene preiskave - Sevalne metode za računalniško tomografijo - 2. del:
Načela, oprema in vzorci (ISO 15708-2:2025)
Non-destructive testing - Radiation methods for computed tomography - Part 2:
Principles, equipment and samples (ISO 15708-2:2025)
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren für Computertomographie - Teil 2:
Grundlagen, Geräte und Proben (ISO 15708-2:2025)
Essais non destructifs - Méthodes par rayonnements pour la tomographie informatisée -
Partie 2: Principes, équipements et échantillons (ISO 15708-2:2025)
Ta slovenski standard je istoveten z: EN ISO 15708-2:2025
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 15708-2
EUROPEAN STANDARD
NORME EUROPÉENNE
January 2025
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN ISO 15708-2:2019
English Version
Non-destructive testing - Radiation methods for computed
tomography - Part 2: Principles, equipment and samples
(ISO 15708-2:2025)
Essais non destructifs - Méthodes par rayonnements Zerstörungsfreie Prüfung - Durchstrahlungsverfahren
pour la tomographie informatisée - Partie 2: Principes, für Computertomographie - Teil 2: Grundlagen, Geräte
équipements et échantillons (ISO 15708-2:2025) und Proben (ISO 15708-2:2025)
This European Standard was approved by CEN on 7 January 2025.

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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye 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
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 15708-2:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 15708-2:2025) has been prepared by Technical Committee ISO/TC 135 "Non-
destructive testing" in collaboration with 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 July 2025, and conflicting national standards shall be
withdrawn at the latest by July 2025.
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 ISO 15708-2:2019.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 15708-2:2025 has been approved by CEN as EN ISO 15708-2:2025 without any
modification.
International
Standard
ISO 15708-2
Third edition
Non-destructive testing —
2025-01
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 15708-2:2025(en) © ISO 2025

ISO 15708-2:2025(en)
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 15708-2:2025(en)
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 .6
5.5 Acquisition, reconstruction, visualization and storage system .7
6 CT system stability . 7
6.1 General .7
6.2 X-Ray Stability .7
6.3 Manipulator stability .8
7 Geometric alignment . 8
8 Sample considerations. 8
8.1 Size and shape of sample .8
8.2 Materials (including a table of X-ray voltage versus 10 % transmission) .9
Annex A (informative) CT system components . 10
Bibliography .16

iii
ISO 15708-2:2025(en)
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, 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 www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC
5, Radiographic testing., in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 138, Non-destructive testing, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 15708-2:2017), which has been technically
revised.
The main changes are as follows:
— addition of normative references;
— correction of the vacuum level for activating the turbo pump in A.1.1;
— addition of photon counting as an example under semiconductors in A.2.3;
— editorial changes.
A list of all parts in the ISO 15708 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
International Standard ISO 15708-2:2025(en)
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.
This document is applicable only to industrial imaging (i.e. non-medical applications) and provides a
consistent set of definitions of CT performance parameters, including the relationship between these
performance parameters and CT system specifications.
This document is applicable to industrial computed tomography.
This document does not apply to other techniques 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, Non-destructive testing — Radiation methods for computed tomography — Part 1: Terminology
ISO 15708-3, Non-destructive testing — Radiation methods for computed tomography — Part 3: Operation and
interpretation
ISO 15708-4, 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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
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

ISO 15708-2:2025(en)
slices) or over the complete volume. Radiographic imaging is possible because different materials have
different X-ray attenuation coefficients. In CT images, the linear X-ray attenuation coefficients are displayed
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 volumes 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
Among the radiographic techniques, CT can be an excellent examination technique whenever the primary
goal is to locate and quantify volumetric details in three dimensions. In addition, since CT 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, in which 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 separated from each other, preserving the full spatial information.
With proper calibration, dimensional inspections and material density determinations can also be carried out.
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 therefore 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. For known materials, the additional information can be
used for improved discrimination or improved characterization.
4.3 Limitations of CT
CT is an indirect test procedure and absolute CT measurements (e.g. of the size of material inhomogeneities
or of wall thicknesses) shall be based on a comparison with other absolute measurement procedures in
accordance with 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 shall 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 in 4.4.1. See ISO 15708-3
for a more detailed description.

ISO 15708-2:2025(en)
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 can correspond to a broad range of
materials.
Furthermore, there shall be sufficient X-ray transmission (≥ 10 %, see 8.2) through the sample at all
projection angles without saturating any part of the detector.
4.4 Main CT process steps
4.4.1 Acquisition
Multiple projections are systematically recorded during a CT scan: 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 acquired. The CT image quality can be improved by increasing the
number of projections in a scan.
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. These corrections are applied
to each subsequently acquired image of the CT data set. Some systems can be configured to enhance either
the X-ray settings or 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 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 note 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 certain regions of
larger objects, it introduces artefacts (due to incomplete data) which can sometimes be reduced by special
processing.
Radiographic imaging, as used for CT examination, is always affected by noise. In radiography this noise
arises from two sources:
a) intrinsic variation corresponding to photon statistics in the emission and detection of photons;
b) variations specific to instruments and processing used.
Noise in CT projections is often amplified by the reconstruction algorithm. In CT images, statistical noise
appears as random variation superimposed on the CT grey value of each voxel, limiting the 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 15708-2:2025(en)
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 techniques.
The reconstruction software can 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 specified 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 can 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 depend 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 data can be misinterpreted in this process step.
To highlight features of interest during visualisation, different digital filter operations can be performed. It
is characteristic of all these operations 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 its 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 shall 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 technique (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 can be detrimental for
specific measurement or analysis tasks, but can also 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 document.
More details are given in ISO 15708-3:2017, 5.5.

ISO 15708-2:2025(en)
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 (including any mechanical structures that influence 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. For example, in scanners designed for in vivo animal studies or for imaging
large structures, the source and detector can orbit around the sample.
In most micro-/nano- or sub-micro-tomography systems, the resolution is determined primarily by the
X-ray focal spot size. Due to geometric magnification, the detector element spacing can be much larger than
the computed voxel size, and a thicker and therefore more efficient scintillator can be used. A disadvantage
of this approach is that the sample should be located very close to the source in order to achieve high
magnification ratios. This is particularly problematic 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 can be optimised for resolution, energy, speed of acquisition or simply cost. Although a particular
system can operate in a wide range of conditions, it will operate optimally in a much smaller range and
the user should consider the main application when choosing one model over another and not simply over-
specify.
For example, a high-resolution CT system (small X-ray focal spot size) can have a considerably lower
flux output at more modest resolution settings than one designed to operate at modest resolution only.
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.
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. The resolution and scan times depend on the X-ray
exposure, i.e. the faster the scan, the worse the SNR, as well as 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 e
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