SIST EN 62494-1:2009

Medical electrical equipment - Exposure index of digital X-ray imaging systems - Part 1: Definitions and requirements for general radiography (IEC 62494-1:2008 )

This part of IEC 62494 specifies definitions and requirements for the EXPOSURE INDEX of images acquired with DIGITAL X-RAY IMAGING SYSTEMS. This standard is applicable to DIGITAL X-RAY IMAGING SYSTEMS used in general radiography for producing PROJECTION X-ray images for general applications, such as, but not exclusively: computed radiography (CR) systems based on stimulable phosphors; flat-panel detector based systems; charge-coupled device (CCD) based systems. Image intensifier based systems and systems for mammographic or dental application are not covered in this first edition.

Medizinische elektrische Geräte - Dosisindikator digitaler Röntgenbildsysteme - Teil 1: Definitionen und Anforderungen für die allgemeine Radiographie (IEC 62494-1:2008 )

Appareils électro-médicaux - Indice d'exposition des systèmes d’imagerie numérique à rayonnement X - Partie 1: Définitions et exigences pour la radiographie générale (CEI 62494-1:2008 )

La CEI 62494-1:2008 spécifie les définitions et les exigences pour l'indice d'exposition des images acquises avec des systèmes d'imagerie numérique à rayonnement X. La CEI 62494-1:2008 est applicable aux systèmes d'imagerie numérique à rayonnement X utilisés en radiographie générale pour produire des images radiologiques de projection pour les applications générales, comme par exemple, mais pas exclusivement:  - les systèmes de radiographie assistée par ordinateur (CR, computed radiography) basés sur des luminophores stimulables;  - les systèmes basés sur des détecteurs à écran plat;  - les systèmes basés sur un dispositif à transfert de charge (CCD, charge-coupled device).  Les systèmes basés sur un intensificateur d'image et les systèmes utilisés en mammographie ou pour les applications dentaires ne sont pas traités dans cette première édition. La CEI 62494-1:2008 définit l'indice d'exposition uniquement pour les images produites avec un seul événement d'irradiation. Les images produites à partir d'irradiations multiples (par exemple, images tomosynthétiques ou biénergétiques, vues multiples sur une seule plaque de radiographie assistée par ordinateur) ne sont pas traitées.

Medicinska električna oprema - Indeks izpostavljenosti pri digitalnih sistemih za rentgensko slikanje - 1. del: Definicije in zahteve za splošno radiografijo (IEC 62494-1:2008 )

General Information

• Status: Published
• Publication Date: 03-Dec-2008
• ICS: 11.040.50 - Radiographic equipment
• Technical Committee: IEMO - Electrical equipment in medical practice
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 28-Nov-2008
• Due Date: 02-Feb-2009
• Completion Date: 04-Dec-2008
• Ref Project: EN 62494-1:2008 - Medical electrical equipment - Exposure index of digital X-ray imaging systems - Part 1: Definitions and requirements for general radiography

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 62494-1:2009
01-januar-2009
0HGLFLQVNDHOHNWULþQDRSUHPD,QGHNVL]SRVWDYOMHQRVWLSULGLJLWDOQLKVLVWHPLK]D
UHQWJHQVNRVOLNDQMHGHO'HILQLFLMHLQ]DKWHYH]DVSORãQRUDGLRJUDILMR,(&

Medical electrical equipment - Exposure index of digital X-ray imaging systems - Part 1:
Definitions and requirements for general radiography (IEC 62494-1:2008 )
Medizinische elektrische Geräte - Dosisindikator digitaler Röntgenbildsysteme - Teil 1:
Definitionen und Anforderungen für die allgemeine Radiographie (IEC 62494-1:2008 )
Appareils électro-médicaux - Indice d'exposition des systèmes d’imagerie numérique à
rayonnement X - Partie 1: Définitions et exigences pour la radiographie générale (CEI
62494-1:2008 )
Ta slovenski standard je istoveten z: EN 62494-1:2008
ICS:
11.040.50 Radiografska oprema Radiographic equipment
SIST EN 62494-1:2009 en,fr
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 62494-1:2009

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SIST EN 62494-1:2009

EUROPEAN STANDARD
EN 62494-1

NORME EUROPÉENNE
November 2008
EUROPÄISCHE NORM

ICS 11.040.50


English version


Medical electrical equipment -
Exposure index of digital X-ray imaging systems -
Part 1: Definitions and requirements for general radiography
(IEC 62494-1:2008)


Appareils électromédicaux -  Medizinische elektrische Geräte -
Indice d'exposition des systèmes Dosisindikator digitaler
d’imagerie numérique à rayonnement X - Röntgenbildsysteme -
Partie 1: Définitions et exigences Teil 1: Definitionen und Anforderungen
pour la radiographie générale für die allgemeine Radiographie
(CEI 62494-1:2008) (IEC 62494-1:2008)




This European Standard was approved by CENELEC on 2008-10-01. CENELEC 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 Central Secretariat or to any CENELEC 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 CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels


© 2008 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 62494-1:2008 E

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SIST EN 62494-1:2009
EN 62494-1:2008 – 2 –
Foreword
The text of document 62B/680/CDV, future edition 1 of IEC 62494-1, prepared by SC 62B, Diagnostic
imaging equipment, of IEC TC 62, Electrical equipment in medical practice, was submitted to the
IEC-CENELEC parallel vote and was approved by CENELEC as EN 62494-1 on 2008-10-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2009-07-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2011-10-01
In this standard, the following print types are used:
– requirements, compliance with which can be tested, and definitions: in roman type;
– explanations, advice, notes, general statements, exceptions and references: in smaller type;
– TERMS DEFINED IN CLAUSE 3 OF THIS STANDARD, IN EN 60601-1 OR IN IEC/TR 60788, AS REFERENCED IN
THE INDEX OF DEFINED TERMS: SMALL CAPITALS.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 62494-1:2008 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60601-1 NOTE  Harmonized as EN 60601-1:2006 (not modified).
IEC 60601-2-43 NOTE  Harmonized as EN 60601-2-43:2000 (not modified).
IEC 62220-1 NOTE  Harmonized as EN 62220-1:2004 (not modified).
IEC 62220-1-2 NOTE  Harmonized as EN 62220-1-2:2007 (not modified).
__________

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SIST EN 62494-1:2009
– 3 – EN 62494-1:2008

Annex ZA
(normative)

Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application 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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.

Publication Year Title EN/HD Year

IEC/TR 60788 2004 Medical electrical equipment - Glossary of - -
defined terms

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SIST EN 62494-1:2009

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SIST EN 62494-1:2009
IEC 62494-1
Edition 1.0 2008-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE


Medical electrical equipment – Exposure index of digital X-ray imaging
systems –
Part 1: Definitions and requirements for general radiography

Appareils électromédicaux – Indice d'exposition des systèmes d’imagerie
numérique à rayonnement X –
Partie 1: Définitions et exigences pour la radiographie générale

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
R
CODE PRIX
ICS 11.040.50 ISBN 2-8318-9944-3
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN 62494-1:2009
– 2 – 62494-1 © IEC:2008
CONTENTS
FOREWORD.3
INTRODUCTION.5
1 Scope.7
2 Normative references .7
3 Terms and definitions .7
4 Requirements .9
4.1 Creation of ORIGINAL DATA.9
4.2 Determination of the RELEVANT IMAGE REGION and the VALUE OF INTEREST .9
4.3 Requirements for the EXPOSURE INDEX .10
4.4 Calibration of the EXPOSURE INDEX .10
4.5 Determination of the CALIBRATION FUNCTION .11
4.6 Determination of the INVERSE CALIBRATION FUNCTION .11
4.7 Requirements for the DEVIATION INDEX.11
Annex A (informative) Details on the rationale, properties and use of the EXPOSURE
INDEX .13
Annex B (informative) Details on the rationale, properties and use of the DEVIATION
INDEX .17
Annex C (normative) Beam conditions to be used for calibration.18
Bibliography.19
Terminology – Index of defined terms .20

Figure A.1 – Example of an ORIGINAL DATA radiograph with an example of the RELEVANT
IMAGE REGION outlined .14
Figure A.2 – Histogram of the ORIGINAL DATA for the radiograph shown in Figure A.1 .15
Figure A.3 – Relative IMAGE RECEPTOR AIR KERMA required to produce a fixed detector
response for the four x-ray beam qualities defined in ISO 9236-1 .16

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SIST EN 62494-1:2009
62494-1 © IEC:2008 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

MEDICAL ELECTRICAL EQUIPMENT –
EXPOSURE INDEX OF DIGITAL X-RAY IMAGING SYSTEMS –

Part 1: Definitions and requirements for general radiography


FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62494-1 has been prepared by subcommittee 62B: Diagnostic
imaging equipment, of IEC technical committee 62: Electrical equipment in medical practice.
The text of this standard is based on the following documents:
Enquiry draft Report on voting
62B/680/CDV 62B/703/RVC

Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

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SIST EN 62494-1:2009
– 4 – 62494-1 © IEC:2008
In this standard, the following print types are used:
– requirements, compliance with which can be tested, and definitions: in roman type;
– explanations, advice, notes, general statements, exceptions and references: in smaller type;
– TERMS DEFINED IN CLAUSE 3 OF THIS STANDARD, IN IEC 60601-1 OR IN IEC 60788, AS
REFERENCED IN THE INDEX OF DEFINED TERMS: SMALL CAPITALS.
A list of all parts of the IEC 62494 series, published under the general title Medical electrical
equipment – Exposure index of digital X-ray imaging systems, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

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SIST EN 62494-1:2009
62494-1 © IEC:2008 – 5 –
INTRODUCTION
The direct connection between the level of detector exposure and optical density is well
established in film-screen radiology. This is not the case in digital radiography, where almost
always a constant image characteristic is achieved using automatic image processing.
Consequently, deviations from the intended exposure, i.e., over- and underexposure, are not
noticeable by a corresponding deviation in image brightness. While considerable
underexposure results in an increased level of noise, the more alarming aspect (from a
radiation protection point of view) is that overexposure cannot be recognized easily in the
displayed image.
Therefore, various manufacturers of digital radiography systems have introduced so-called
exposure indicators for their equipment. These are numbers, determined from the original
image data of each image taken, which allow conclusions about the level of the exposure at
the image receptor. However, the exposure indicators are manufacturer or system specific,
i.e. they differ for the systems of different manufacturers in their definition and scaling. A
unified EXPOSURE INDEX for all digital radiography systems is needed to simplify its usage, e.g.
for the establishment of exposure guidelines, particularly when systems of different
manufacturers are used within the same department.
This standard defines such a concept of the EXPOSURE INDEX. What is laid down here refers to
the definition, the scale and the general requirements for the EXPOSURE INDEX. The process of
its calculation in detail (software algorithm) is excluded from this standard as to not obstruct
technical progress.
The EXPOSURE INDEX allows the OPERATOR to judge if an image was taken at a detector
exposure level suitable for the intended level of image quality. It is important to note that the
EXPOSURE INDEX, as defined in this standard, is derived from the image signal, which in turn is
usually related to the energy absorbed in the detector, i.e. the detector dose, but not directly
to the air kerma at the image receptor. The relation to IMAGE RECEPTOR AIR KERMA (air kerma
at the detector surface) is introduced only at one radiation quality through calibration.
However, this definition is appropriate as the image quality in digital radiography is
determined mainly by the signal-to-noise level, which in turn is determined by the absorbed
energy. Annex A provides more details on the rationale, properties and use of the EXPOSURE
INDEX.
The level of detector exposure needed to obtain a suitable level of image quality may vary
depending on body part, view, or the x-ray imaging system used, as may the appropriate
EXPOSURE INDEX. This standard introduces a second parameter, called DEVIATION INDEX, which
quantifies the deviation of an actual EXPOSURE INDEX from the appropriate EXPOSURE INDEX
(called TARGET EXPOSURE INDEX). While this parameter does not relate to the image receptor
dose on an absolute scale, it allows the operator an easy check whether the exposure is
considered acceptable for the specific imaging task. Annex B provides more details on the
rationale, properties and use of the DEVIATION INDEX.
The storage of the EXPOSURE INDEX (and the DEVIATION INDEX) together with the image data,
e.g., in a DICOM tag field, allows the documentation and communication of the image
receptor dose level in clinical practice.
The EXPOSURE INDEX does not obviate the use of dose parameters that describe the patient’s
REFERENCE AIR KERMA or the kerma-area
exposure to radiation, such as, for example, the
product. Because the relation between patient exposure and detector exposure is influenced
by a number of factors that are generally not known under clinical conditions, the EXPOSURE
INDEX should not be used to calculate or estimate patient dose.

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SIST EN 62494-1:2009
– 6 – 62494-1 © IEC:2008
The EXPOSURE INDEX cannot be used to control the compliance with diagnostic reference
1)
levels, which refer to patient dose [1] .
___________
1)
Figures in square brackets refer to the Bibliography.

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SIST EN 62494-1:2009
62494-1 © IEC:2008 – 7 –
MEDICAL ELECTRICAL EQUIPMENT –
EXPOSURE INDEX OF DIGITAL X-RAY IMAGING SYSTEMS –

Part 1: Definitions and requirements for general radiography



1 Scope
This part of IEC 62494 specifies definitions and requirements for the EXPOSURE INDEX of
images acquired with DIGITAL X-RAY IMAGING SYSTEMS.
This standard is applicable to DIGITAL X-RAY IMAGING SYSTEMS used in general radiography for
producing PROJECTION X-ray images for general applications, such as, but not exclusively:
• computed radiography (CR) systems based on stimulable phosphors;
• flat-panel detector based systems;
• charge-coupled device (CCD) based systems.
Image intensifier based systems and systems for mammographic or dental application are not
covered in this first edition.
This standard defines the EXPOSURE INDEX only for images generated with a single
IRRADIATION event. Images generated from multiple IRRADIATIONS (e.g., tomosynthetic or dual-
energy images, multiple views on a single CR plate) are not covered.
2 Normative references
The following referenced documents are indispensable for the application 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.
IEC/TR 60788:2004, Medical electrical equipment – Glossary of defined terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TR 60788 and the
following apply.
3.1
CALIBRATION CONDITIONS
set of conditions under which EXPOSURE INDEX calibration is done
3.2
CALIBRATION FUNCTION
function expressing the VALUE OF INTEREST as a function of the IMAGE RECEPTOR AIR KERMA that
is valid under CALIBRATION CONDITIONS
3.3
DETECTOR SURFACE
accessible area which is closest to the IMAGE RECEPTOR PLANE
NOTE After removal of all parts (including the ANTI-SCATTER GRID and components for AUTOMATIC EXPOSURE
CONTROL, if applicable) that can be safely removed from the RADIATION BEAM without damaging the digital X-ray
detector.
[IEC 62220-1-2:2007, definition 3.3]

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SIST EN 62494-1:2009
– 8 – 62494-1 © IEC:2008
3.4
DEVIATION INDEX
DI
number quantifying the deviation of the actual EXPOSURE INDEX from a TARGET EXPOSURE INDEX
3.5
DIGITAL X-RAY IMAGING DEVICE
device consisting of a digital X-ray detector including the protective layers installed for use in
practice, the amplifying and digitizing electronics, and a computer providing the ORIGINAL DATA
(DN) of the image
[IEC 62220-1:2003, definition 3.5]
NOTE This may include protecting parts, such as anti-scatter grids or AEC components
3.6
DIGITAL X-RAY IMAGING SYSTEM
X-ray equipment using a DIGITAL X-RAY IMAGING DEVICE, providing PROJECTION images in digital
format, comprising subsystems allowing to process, display, print or store the images
3.7
EXPOSURE INDEX
EI
measure of the detector response to radiation in the RELEVANT IMAGE REGION of an image
acquired with a DIGITAL X-RAY IMAGING SYSTEM
NOTE For a fixed RADIATION QUALITY, the signal generated in the detector is proportional to the IMAGE RECEPTOR
AIR KERMA (or exposure).
3.8
IMAGE RECEPTOR AIR KERMA
K
AIR KERMA at the position of the DETECTOR SURFACE, free-in-air (excluding backscatter)
3.9
INVERSE CALIBRATION FUNCTION
function expressing the IMAGE RECEPTOR AIR KERMA as a function of the VALUE OF INTEREST that
CALIBRATION CONDITIONS
is valid under
3.10
ORIGINAL DATA
DN
RAW DATA to which the corrections allowed in this standard have been applied
[IEC 62220-1:2003, definition 3.12]
NOTE The relation of the ORIGINAL DATA to the IMAGE RECEPTOR AIR KERMA may include a non-linear, e.g.,
logarithmic or square-root characteristic.
3.11
RAW DATA
pixel values read directly after the analogue-digital-conversion from the DIGITAL X-RAY IMAGING
DEVICE without any software corrections
[IEC 62220-1:2003, definition 3.14]]
3.12
RELEVANT IMAGE REGION
examination-specific sub-area or sub-areas of the image containing the diagnostically relevant
information
NOTE This is typically the region for which the exposure parameters should be optimized.

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SIST EN 62494-1:2009
62494-1 © IEC:2008 – 9 –
3.13
TARGET EXPOSURE INDEX
EI
T
expected value of the EXPOSURE INDEX when exposing the X-RAY IMAGE RECEPTOR properly
NOTE The TARGET EXPOSURE INDEX may depend on the type of detector, on the type of examination, on the
diagnostic question and on other parameters.
3.14
VALUE OF INTEREST
V
central tendency of the original data in the relevant image region
NOTE Central tendency is a statistical term depicting generally the centre of a distribution. It may refer to a
variety of measures such as the mean, the median or the mode.
4 Requirements
4.1 Creation of ORIGINAL DATA
The following image-independent corrections of the RAW DATA are allowed for the creation of
ORIGINAL DATA in advance of the processing of the data for the determination of the
CALIBRATION FUNCTION and the EXPOSURE INDEX.
All the following corrections if used shall be made as in normal clinical use:
– replacement of the RAW DATA of bad or defective pixels by appropriate data;
– a flat-field correction comprising for example:
• correction of the non-uniformity of the RADIATION FIELD;
• correction for the offset of the individual pixels;
• gain correction for the individual pixels;
• a correction for velocity variation during a scan;
– a correction for geometrical distortion.
NOTE 1 Some detectors execute linear image processing due to their physical concept. As long as this image
processing is linear and image-independent, these operations are allowed as an exception.
NOTE 2 Image correction is considered image-independent if the same correction is applied to all images
independent of the image contents.
NOTE 3 Processes that are used to enhance individual images for presentation, such as edge enhancement,
noise smoothing, and histogram equalization, are not considered correction even if they are linear and are applied
to all images independent of image content.
4.2 Determination of the RELEVANT IMAGE REGION and the VALUE OF INTEREST
The determination of the RELEVANT IMAGE REGION should be done by methods that identify the
attenuated regions of the beam that are relevant to the diagnostic purpose of the acquired
image.
The selection of the RELEVANT IMAGE REGION can be done by image segmentation, histogram
based, or other appropriate methods. The method used shall be documented.
NOTE 1 Several methods to determine the RELEVANT IMAGE REGION exist. These may be based on image
histogram evaluation, on image segmentation or a combination of both. The RELEVANT IMAGE REGION need not be a
contiguous area of the image
NOTE 2 While it is understood that the selection of the RELEVANT IMAGE REGION is an important step in the
generation of the EXPOSURE INDEX and that a single unified method may be desirable, it is not feasible at this time.
Future versions of the standard may address this issue.

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SIST EN 62494-1:2009
– 10 – 62494-1 © IEC:2008
The VALUE OF INTEREST shall be calculated using the mean, median, mode, trimmed mean,
trimean, or other recognized statistical method for the description of central tendency of the
ORIGINAL DATA in the RELEVANT IMAGE REGION. The method used shall be documented.
NOTE 3 Care should be taken in the selection of the method used to calculate the central tendency in a manner
not influenced by outlying values. Methods such as trimmed mean or trimean reduce the influence of extreme
values.
NOTE 4 Background information on the influence of the selection of the RELEVANT IMAGE REGION and the VALUE OF
INTEREST is described in Annex A.
4.3 Requirements for the EXPOSURE INDEX
The EXPOSURE INDEX EI shall be related to the VALUE OF INTEREST V according to the formula:
EI = c ⋅ g(V ) (1)
0
where g(V) is an equipment-specific INVERSE CALIBRATION FUNCTION that is defined in
–1
subclause 4.6 and c = 100 μGy is a constant.
0
NOTE 1 The INVERSE CALIBRATION FUNCTION accounts for different scalings of the ORIGINAL DATA in different
DIGITAL X-RAY IMAGING DEVICES.
The EXPOSURE INDEX shall be calculated directly after image acquisition and after any manual
adjustments of the automatic image processing (e.g., when the automatic segmentation or
histogram evaluation algorithm failed to correctly identify the RELEVANT IMAGE REGION) so that
it is available to the OPERATOR prior to image confirmation.
NOTE 2 Image confirmation is the step concluding the image acquisition process. It may happen either by a user
action or automatically. It asserts that the image has been processed properly. This is usually done by examining
the image on the display of the acquisition workstation.
If the EXPOSURE INDEX is outside the valid range of the INVERSE CALIBRATION FUNCTION (see
subclause 4.6) that effect shall be indicated.
4.4 Calibration of the EXPOSURE INDEX
The EXPOSURE INDEX EI shall be calibrated for the DIGITAL X-RAY IMAGING SYSTEM over the
specified operating range of IMAGE RECEPTOR AIR KERMA such that
EI = c ⋅ K (2)
0 CAL
where K is the IMAGE RECEPTOR AIR KERMA in μGy under the CALIBRATION CONDITIONS and
CAL
–1
c = 100 μGy is a constant.
0
CALIBRATION CONDITIONS shall be:
– homogeneous IRRADIATION of the EFFECTIVE IMAGE RECEPTION AREA;
– IMAGE RECEPTOR AIR KERMA covering the specified operating range of the DIGITAL X-RAY
IMAGING DEVICE;
– measurement of the IMAGE RECEPTOR AIR KERMA free-in-air without backscattered radiation
as specified in Annex C;
– a single fixed RADIATION QUALITY as specified in Annex C;
– VALUE OF INTEREST computed from a RELEVANT IMAGE REGION that shall be the central 10 %
of the area of the homogeneously exposed EFFECTIVE IMAGE RECEPTION AREA.
Conditions needed to verify the CALIBRATION FUNCTION, such as the time interval between
exposure and processing in the CR reader, should be supplied by the manufacturer.

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SIST EN 62494-1:2009
62494-1 © IEC:2008 – 11 –
NOTE For radiographic techniques other than the one used for calibration, the relation between the EXPOSURE
INDEX EI and the IMAGE RECEPTOR AIR KERMA K will deviate from Eq. (2) because of the energy dependence of x-ray
response of the detector, scattered radiation and possibly other effects.
4.5 Determination of the CALIBRATION FUNCTION
The CALIBRATION FUNCTION f(K) shall be determined from the relationship between the IMAGE
RECEPTOR AIR KERMA K and the VALUE OF INTEREST V for the calibration RADIATION
CAL CAL
QUALITY from a series of homogenously exposed images. The CALIBRATION FUNCTION f(K) is
defined by
V = f (K ) (3)
CAL CAL
where V is the VALUE OF INTEREST with a RELEVANT IMAG
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