SIST EN 14784-1:2005
(Main)Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 1: Classification of systems
Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 1: Classification of systems
This European standard specifies fundamental parameters of computed radiography systems with the aim of enabling satisfactory and repeatable results to be obtained economically. The techniques are based both on fundamental theory and test measurements. This European standard specifies the performance of computed radiography (CR) systems and the measurement of the corresponding parameters for the system scanner and phosphor imaging plate (IP). It describes the classification of these systems in combination with specified metal screens for industrial radiography. It is intended to ensure that the quality of images - as far as this is influenced by the scanner-IP system - is in conformity with the requirements of Part 2 of this standard. The standard relates to the requirements of film radiography defined in EN 584-1 and ISO 11699-1.
This European standard defines system tests at different levels. More complicated tests are described, which allow the determination of exact system parameters. They can be used to classify the systems of different suppliers and make them comparable for users. These tests are specified as manufacturer tests. Some of them require special tools, which are usually not available in user laboratories. Therefore, simpler user tests are also described, which are designed for a fast test of the quality of CR systems and long term stability.
There are several factors affecting the quality of a CR image including geometrical unsharpness, signal/noise ratio, scatter and contrast sensitivity. There are several additional factors (e.g. scanning parameters), which affect the accurate reading of images on exposed IPs using an optical scanner.
Zerstörungsfreie Prüfung - Industrielle Computer-Radiographie mit Phosphor-Speicherfolien - Teil 1: Klassifizierung der Systeme
In diesem Dokument werden für Systeme der Computer-Radiographie die grundlegenden Parameter mit dem
Ziel festgelegt, auf wirtschaftliche Art und Weise zufrieden stellende und wiederholbare Ergebnisse zu
erreichen. Die Techniken basieren sowohl auf den grundlegenden Theorien als auch auf Testmessungen. Es
werden die Leistungsfähigkeit von Systemen der Computer-Radiographie (CR-Systeme) sowie die Messung
der entsprechenden Parameter des zugehörigen Abtastsystems und der Phosphor-Speicherfolie (IP)
spezifiziert. Für diese Systeme wird eine Klassifizierung in Kombination mit festgelegten Metallfolien für die
industrielle Durchstrahlungsprüfung beschrieben. Durch dieses Dokument soll sichergestellt werden, dass die
Bildgüte, so weit sie von der Kombination aus Speicherfolie und Abtastsystem beeinflusst wird, den
Anforderungen von Teil 2 dieses Dokuments entspricht. Das Dokument bezieht sich auf die Anforderungen an
Filmsysteme für Durchstrahlungsprüfungen, die in EN 584-1 und ISO 11699-1 definiert werden.
In diesem Dokument werden Systemtests unterschiedlichen Aufwands festgelegt. Es werden komplizierte
Tests beschrieben, mit denen die Bestimmung exakter Systemparameter möglich ist. Sie können zur
Klassifizierung der Systeme von unterschiedlichen Lieferanten angewendet werden, um die Vergleichbarkeit
der Systeme für die Anwender zu erleichtern. Diese Tests werden als Herstellertests bezeichnet. Für einige
dieser Herstellertests sind spezielle Geräte erforderlich, mit denen das Prüflaboratorium des Anwenders im
Allgemeinen nicht ausgestattet ist. Aus diesem Grund werden auch einfachere Tests beschrieben, die für den
Anwender als Schnelltest zur Ermittlung der Qualität von CR-Systemen und ihrer Langzeitstabilität
vorgesehen sind.
Essais non destructifs - Radiographie industrielle numérisée avec des plaques-images au phosphore - Partie 1 : Classification des systemes
La présente Norme européenne spécifie les parametres fondamentaux de radiographie numérique avec écrans photostimulables permettant d’obtenir des résultats satisfaisants et reproductibles de façon économique. Les techniques sont basées a la fois sur la théorie fondamentale et sur des mesurages d’essai. Le présent document spécifie les performances de systemes de radiographie numérique et le mesurage des parametres correspondants relatifs au lecteur numériseur du systeme et a l'écran photostimulable a mémoire. Il décrit la classification de ces systemes combinés a des écrans renforçateurs métalliques spécifiés pour la radiographie industrielle. Il est destiné a assurer la conformité de la qualité des images aux exigences de la partie 2 du présent document, dans la mesure ou cette qualité dépend du systeme lecteur numériseur écrans photostimulables utilisé. Le présent document est lié aux exigences de la radiographie par film définie dans l’EN 584-1 et l’ISO 11699-1.
La présente Norme européenne définit des essais de systemes a différents niveaux. Des essais plus complexes sont décrits, lesquels permettent de déterminer les parametres exacts des systemes. Ces essais peuvent etre utilisés pour classer les systemes de différents fournisseurs et les rendre comparables pour les utilisateurs. Ces essais sont spécifiés comme étant des essais fabricant. Quelques-uns nécessitent des outils spéciaux, qui ne sont généralement pas disponibles dans les laboratoires utilisateurs. Par conséquent, des essais utilisateurs plus simples sont également décrits, lesquels sont conçus pour tester rapidement la qualité des systemes de radiographie numérique et leur stabilité a long terme.
Plusieurs facteurs affectent la qualité de l’image radiographique numérique, entre autres le flou géométrique, le rapport signal/bruit, le diffusé et la sensibilité au contraste.
Neporušitveno preskušanje – Industrijska računalniška radiografija s hranjenjem na fosfornih slikovnih ploščah – 1. del: Klasifikacija sistemov
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 14784-1:2005
01-november-2005
1HSRUXãLWYHQRSUHVNXãDQMH±,QGXVWULMVNDUDþXQDOQLãNDUDGLRJUDILMDVKUDQMHQMHP
QDIRVIRUQLKVOLNRYQLKSORãþDK±GHO .ODVLILNDFLMDVLVWHPRY
Non-destructive testing - Industrial computed radiography with storage phosphor imaging
plates - Part 1: Classification of systems
Zerstörungsfreie Prüfung - Industrielle Computer-Radiographie mit Phosphor-
Speicherfolien - Teil 1: Klassifizierung der Systeme
Essais non destructifs - Radiographie industrielle numérisée avec des plaques-images
au phosphore - Partie 1 : Classification des systemes
Ta slovenski standard je istoveten z: EN 14784-1:2005
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
SIST EN 14784-1:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST EN 14784-1:2005
---------------------- Page: 2 ----------------------
SIST EN 14784-1:2005
EUROPEAN STANDARD
EN 14784-1
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2005
ICS 19.100
English Version
Non-destructive testing - Industrial computed radiography with
storage phosphor imaging plates - Part 1: Classification of
systems
Essais non destructifs - Radiographie industrielle Zerstörungsfreie Prüfung - Industrielle Computer-
numérisée avec des plaques-images au phosphore - Partie Radiographie mit Phosphor-Speicherfolien - Teil 1:
1 : Classification des systèmes Klassifizierung der Systeme
This European Standard was approved by CEN on 1 July 2005.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14784-1:2005: E
worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
Contents
Page
Foreword.3
1 Scope .4
2 Normative references .4
3 Terms and definitions .4
4 Personnel qualification .6
5 CR quality indicators.6
6 Procedure for quantitative measurement of image quality parameters .8
7 CR System Classification and Interpretation of Results .15
Annex A (informative) Example for I measurement .18
IPx
Annex B (informative) Example of CR test phantom .22
Annex C (informative) Guidance for application of various tests and test methods .25
Bibliography .27
2
---------------------- Page: 4 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
Foreword
This European Standard (EN 14784-1:2005) has been prepared 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 February 2006, and conflicting national standards shall be withdrawn
at the latest by February 2006.
EN 14784 comprises a series of European Standards for industrial computed radiography with storage
phosphor imaging plates which is made up of the following:
EN 14784-1 Non-destructive testing – Industrial computed radiography with storage phosphor imaging plates
– Part 1: Classification of systems
EN 14784-2 Non-destructive testing – Industrial computed radiography with storage phosphor imaging plates
– Part 2: General principles for testing of metallic materials using X-rays and gamma rays
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland
and United Kingdom.
3
---------------------- Page: 5 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
1 Scope
This European Standard specifies fundamental parameters of computed radiography systems with the aim of
enabling satisfactory and repeatable results to be obtained economically. The techniques are based both on
fundamental theory and test measurements. This document specifies the performance of computed
radiography (CR) systems and the measurement of the corresponding parameters for the system scanner and
storage phosphor imaging plate (IP). It describes the classification of these systems in combination with
specified metal screens for industrial radiography. It is intended to ensure that the quality of images - as far as
this is influenced by the scanner-IP system - is in conformity with the requirements of Part 2 of this document.
The document relates to the requirements of film radiography defined in EN 584-1 and ISO 11699-1.
This European Standard defines system tests at different levels. More complicated tests are described, which
allow the determination of exact system parameters. They can be used to classify the systems of different
suppliers and make them comparable for users. These tests are specified as manufacturer tests. Some of
them require special tools, which are usually not available in user laboratories. Therefore, simpler user tests
are also described, which are designed for a fast test of the quality of CR systems and long term stability.
There are several factors affecting the quality of a CR image including geometrical un-sharpness, signal/noise
ratio, scatter and contrast sensitivity. There are several additional factors (e.g. scanning parameters), which
affect the accurate reading of images on exposed IPs using an optical scanner.
The quality factors can be determined most accurately by the manufacturer tests as described in this
document. Individual test targets, which are recommended for practical user tests, are described for quality
assurance. These tests can be carried out either separately or by the use of the CR Phantom (Annex B). This
CR Phantom incorporates many of the basic quality assessment methods and those associated with the
correct functioning of a CR system, including the scanner, for reading exposed plates and in correctly erasing
IPs for future use of each plate.
The CR System classes in this document do not refer to any particular manufacturers Imaging Plates. A CR
system class results from the use of a particular imaging plate together with the exposure conditions –
particularly total exposure – the scanner type and the scanning parameters.
2 Normative references
The following referenced documents are indispensable for the application of this European standard. For
dated references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 462-5, Non-destructive testing ― Image quality of radiographs ― Part 5: Image quality indicators (duplex
wire type), determination of image unsharpness value.
EN 584-1, Non destructive testing ― Industrial radiographic film ― Part 1: Classification of film systems for
industrial radiography.
3 Terms and definitions
For the purposes of this European Standard, the following terms and definitions apply:
3.1
computed radiography system (CR system)
complete system of a storage phosphor imaging plate (IP) and corresponding read-out unit (scanner or
reader) and system software, which converts the information of the IP into a digital image
4
---------------------- Page: 6 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
3.2
computed radiography system class
particular group of storage phosphor imaging plate systems, which is characterised by a SNR (Signal-to-Noise
Ratio) range shown in Table 1 and by a certain basic spatial resolution value (e.g. derived from duplex wire
IQI) in a specified exposure range.
3.3
CEN speed S
CEN
defines the speed of CR systems and is calculated from the reciprocal dose value, measured in Grays, which
is necessary to obtain a specified minimum SNR of a CR system
3.4
signal-to-noise ratio (SNR)
quotient of mean value of the linearised signal intensity and standard deviation of the noise at this signal
intensity. The SNR depends on the radiation dose and the CR system properties.
3.5
modulation transfer function (MTF)
normalised Magnitude of the Fourier-transform (FT) of the differentiated edge spread function (ESF) of the
linearised PSL (photo stimulated luminescence) intensity, measured perpendicular to a sharp edge. MTF
describes the contrast transmission as a function of the object size. MTF characterises the un-sharpness of
the CR system in dependence on the scanning system and IP-type.
3.6
CR phantom
device containing an arrangement of test targets to evaluate the quality of a CR system - as well as monitoring
the quality of the chosen system
3.7
laser beam jitter
lack of smooth movement of the plate laser-scanning device, causing lines in the image consisting of a series
of steps
3.8
scanner slippage
slipping of an IP in a scanner transport system resulting in fluctuation of intensity of horizontal image lines
3.9
aliasing
pre-sampled high spatial frequency signals beyond the Nyquist frequency (given by the pixel distance)
reflected back into the image at lower spatial frequencies
3.10
gain/amplification
opto-electrical gain setting of the scanning system
3.11
linearised signal intensity
numerical signal value of a picture element (pixel) of the digital image, which is proportional to the radiation
dose. The linearised signal intensity is zero, if the radiation dose is zero.
3.12
basic spatial resolution
read-out value of un-sharpness measured with duplex wire IQI according to EN 462-5 divided by 2 as effective
pixel size of CR system
5
---------------------- Page: 7 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
4 Personnel qualification
It is assumed that industrial computed radiography is performed by qualified and capable personnel. In order
to prove this qualification, it is recommended to certify the personnel according to EN 473 or ISO 9712.
5 CR quality indicators
5.1 Description of CR quality indicators for user and manufacturer tests
5.1.1 General
The following is a description of CR quality indicators, which will be identified by reference to this document.
5.1.2 Contrast sensitivity quality indicator
The description of the selected contrast sensitivity targets corresponds to ASTM E1647-98a (see for details
Annex B.4).
5.1.3 Duplex wire quality indicator
The description of the duplex wire quality indicator corresponds to EN 462-5. The IQI shall be positioned at a
5°angle to the direction of the scanned lines (fast scan direction) or the perpendicular direction (slow scan
direction).
5.1.4 Converging line pair quality indicator
The target consist of 5 converging strips of lead (0,03 mm thickness) which can be used for spatial resolution
test by reading the limit of recognisable line pairs. It shall cover a range from 1,5 to 20 line pairs per mm
(lp/mm). Two quality indicators shall be used, one in direction of the scanned lines and the other one in the
perpendicular direction.
5.1.5 Linearity quality indicators
Rulers of high absorbing materials are located on the perimeter of the scanned range. Two quality indicators
shall be used, one in direction of the scanned lines and the other in the perpendicular direction. The scaling
shall be at least in mm.
5.1.6 T-target
This CR quality indicator consists of a thin plate of brass or copper (≤ 0,5 mm thick) with sharp edges. This
plate is manufactured in a T-shape with 5 mm wide segments. The T should have a size of at least
50 mm × 70 mm. It shall be aligned perpendicular and parallel respectively to the direction of the scanned
lines (see Figure B.1).
5.1.7 Scanner slipping quality indicator
It consists of a homogenous strip of aluminium of 0,5 mm thickness. It has a shape of a rectangle (see
Figure B.1) and shall be aligned perpendicular and parallel respectively to the direction of the scanned lines.
5.1.8 Shading quality indicator
Different shading quality indicators may be used.
One type is based on the homogeneous exposure of an imaging plate (IP) with a thin Al-plate (0,5 mm to
1,0 mm) above the IP. The exposure shall be made with low energy radiation (50 keV to 100 keV).
6
---------------------- Page: 8 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
Another type is the shading quality indicator of the CR-test phantom (see Annex B).
5.1.9 Central beam alignment quality indicator (BAM-snail)
The alignment quality indicator consists of a roll (1,5 mm to 2,0 mm thick) of thin lead foil separated by a
spacer of 0,1 mm to 0,2 mm of low absorbing material; (see Annex B.3). Honeycomb material may also be
used.
5.2 Application procedures for CR quality indicators
5.2.1 General
The CR quality indicators are designed for fast evaluation of the quality of a CR system as well as for a
periodical quality control. Annex C gives a guidance for application of various tests and test methods.
5.2.2 Exposure of CR quality indicators (user test)
The CR quality indicators should be positioned in a special arrangement as described in Annex B in the CR
phantom. The CR quality indicators can be applied separately or all together in the CR phantom. The selected
set of CR quality indicators or the CR phantom is placed on the cassette, which contains an Imaging Plate.
The radiation source is set at a distance of 1 metre and the beam is aligned with the centre of the plate. Above
a radiation energy of 100 keV a lead screen of 0,1 mm shall be applied between CR quality indicators or CR
phantom and the IP to reduce scattered radiation. Test exposures are made and the radiation and CR system
functions are optimised and the final image to be evaluated is agreed.
The exposure time and the parameter setting of the CR scanning unit determine the image quality as well as
the type of imaging plate. These values and the type of IP have to be documented and agreed as well as the
radiation energy (keV, gamma-source type), dose (e.g. in mAs) and quality (pre-filters, tube type and tube
window).
NOTE High exposure time and low gain setting yield high contrast resolution and SNR. Furthermore, the contrast
sensitivity is higher for large pixel size setting (high un-sharpness) than for small pixel size setting (low un-sharpness).
5.2.3 Initial assessment of CR quality indicators (user test)
For initial quality assessment, examine the radiographic image(s) of the CR phantom or the separated quality
indicators on the monitor (or hard copy) for the features described in 5.1.2 to 5.1.9 and 6.3.2, 6.3.3, 6.4.1 to
6.4.7. The results can provide the basis of agreement between the contracting parties.
5.2.4 Periodical control (user test)
The CR quality indicators 5.1.2 to 5.1.8 (alignment by 5.1.9) or the CR phantom shall be radiographed and the
results examined at any interval agreed between the contracting parties. For periodical control, ensure that the
agreed quality values of the tests 6.3.2, 6.3.3, and 6.4.1 to 6.4.7 are achieved.
5.3 Imaging plate fading
The Intensity of the stored image in the imaging plate will decrease over time. This effect is known as image
fading. The measurement of fading characteristics shall be done by performing the following steps:
a) expose a plate homogeneously using typical exposure conditions. For documentation the following
parameters shall be recorded: kV, SDD, pre-filter and plate material and thickness. The exposed image
shall have an intensity between 70% and 90% of the maximum possible intensity of the CR-reader at
lowest gain and under linearised condition;
b) read-out the imaging plate 5 minutes after exposure;
c) set the linearised read-out intensity of this measurement as reference (100 %);
7
---------------------- Page: 9 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
d) always expose the imaging plate with the same X-ray parameters (kV, mA*s, distance);
e) change the time between exposure and read-out. The time interval between exposure and readout will be
doubled for every measurement; steps are 15 min, 30 min, 1h, 2h, 4h, etc. up to 128 h or depending on
the application;
f) plot the linearised read-out intensity (grey value) versus time between exposure and read-out of the
imaging plate.
The fading effect has to be considered to ensure correct exposure conditions.
To enable reproducible test results it is important to consider fading effects, which influence the required
exposure time. The time between exposure and read-out for all tests shall correspond to the typical
application of the CR system.
6 Procedure for quantitative measurement of image quality parameters
6.1 Measurement of the normalized Signal-to-Noise Ratio
6.1.1 Step Exposure Method (manufacturer test)
6.1.1.1 General
CR System evaluation depends on the combined properties of the phosphor imaging plate (IP) type, the
scanner used and the selected scan parameters. Therefore, all measurements shall be performed with the
same IP type, scanner and scan parameters and documented. The applied test equipment (Figure 1) and
algorithm corresponds to EN 584-1 and ISO 11699-1.
Key
1 X-ray tube
2 Cu-Filter
3 Collimator
4 Diaphragm
5 IP in a cassette
Figure 1 — Scheme of experimental arrangement for the step exposure method
For measurement of the SNR, the following steps are taken (see also EN 584-1).
8
---------------------- Page: 10 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
6.1.1.2 The IP, with a front and back screen from lead of 0,1 mm thickness in the typical exposure cassette,
shall be positioned in front of an X-ray tube with tungsten anode. Make the exposures with an 8 mm copper
filter at the X-ray tube and the kilo voltage set such that the half value layer in copper is 3,5 mm. The kilo
voltage setting will be approximately 220 kV.
6.1.1.3 Determine the required exact kilo voltage setting by making an exposure (or an exposure rate)
measurement with the detector placed at a distance of at least 750 mm from the tube target and an 8-mm
copper filter at the tube. Then make a second measurement with a total of 11,5 mm of copper at the tube.
These filters should be made of 99,9 % pure copper.
6.1.1.4 Calculate the ratio of the first and second readings. If this ratio is not 2, adjust the kilo voltage up
or down and repeat the measurements until a ratio of 2 (within 5 %) is obtained. Record the setting of the kilo
voltage for use with the further IP tests.
6.1.1.5 The sensitive layer of the IP shall face the X-ray source. For gamma radiography with Ir-192, the
measurements shall be carried out with 0,3 mm lead screens in front and behind the IP. Also 8 mm Cu shall
be used for pre-filtering (see Figure 1).
6.1.1.6 The scanner shall read with a dynamic of ≥ 12 Bit and operate at its highest spatial resolution - or
a spatial resolution for which the classification shall be carried out. Background and anti-shading correction
may be used before the analysis of data, if it relates to the standard measurement procedure for all
measurements. In this case the procedure shall be carried out and documented for all gain and latitude
ranges and all read-out pixel sizes if any of these parameters change the SNR-analysis.
6.1.1.7 IPs are exposed in a similar way to film radiography and under the conditions described: intensity
and a noise (σ ) or SNR over dose curve shall be measured. It is especially important that the exposure of
PSL
the IP for the SNR measurements be spatially uniform. Any non-uniformities in X-ray transmission of the
cassette front, or defects in the Pb foil or in the phosphor itself could influence the SNR measurement. No
major scratches or dust shall be visible in the measurement area. Therefore exercise considerable care in
selection and placement of the aperture, and selection and maintenance of the cassette, the lead screens and
the phosphor screen. To achieve a uniform region of interest on to the IP, the following standard protocol is
recommended. Other approaches may be used as long as a uniform exposure is created. At least 12 areas
2
(test areas) of ≥ 400 mm are evenly exposed on the same IP over the full working range of dose. Due to the
different construction principles of scanners, the measurement shall be performed for all possible pixel sizes.
The digital read-out intensity values (grey values) shall be calibrated in such a way, that they are linear in
relation to the radiation dose that corresponds to the photo stimulated luminescence (PSL) intensity of the
exposed IPs. These calibrated grey values shall be used for the calculation of the SNR. In order to get a
reliable result at least six measurements shall be made on different samples, and the results are to be
averaged for each of the 12 or more dose levels measured.
6.1.1.8 The signal intensity I and standard deviation σ shall be computed from a region without
meas PSL
shading or artifacts. Sample SNR values shall be taken in different regions of the image area under test to
ensure that SNR values are within 10% stable. The size of the ROI used to measure the mean intensity and
the noise shall be at least 20 by 55 pixels and it should be an area ROI. An example technique for assuring
reliable signal to noise measurements is described below. This can be achieved using a commonly available
image-processing tool. The signal and noise shall be calculated from a data set of 1100 values or more per
exposed area. The data set is subdivided into 55 groups or more with 20 values per group. For each group
with index i, the value I is calculated as mean of the unfiltered group values and the value σ is
meas_i PSLi
calculated from the same group values. An increased number of groups yields a better (lower) uncertainty of
the result. Due to the filtering effect of this grouping procedure, the σ -values are corrected by the following
PSLi
equation:
σ = 1,0179⋅σ (1)
PSLi_corr PSLi
NOTE The values σ are multiplied with 1,0179 to correct for the following median unbiased estimation. Assume k
PSLi
is the number of consecutive observations within a group and C is the critical value of the chi-square distribution for α =
0,5 with k-1 degrees of freedom. In case of 20 observations the values σ shall be multiplied with 1,0179 for statistical
PSLii
9
---------------------- Page: 11 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
correction). The factor 1,0179 corresponds to the correction sqrt ((k-1)/c) for grouping with a group size of 20 elements
(k = 20) for application of a median procedure (c = 18,33765)
6.1.1.9 The final value I is obtained by the median of all I values. The final σ value is
meas meas_i PSL
obtained by the median of all σ values. σ shall be calculated as reference value to a resolution of
PSLi_corr PSL
100 µm, measured with a circular aperture, or 88,6 µm measured with a squared aperture. The final value
σ is calculated by
PSL100
σ = σ ⋅()SR / 88,6 (2)
PSL100 PSL max
where
SR is the maximum value of basic spatial resolution (in µm) measured in both directions
max
perpendicular and parallel to the scanning directions of the laser.
NOTE EN 584-1 requires the use of a micro-photo densitometer with circular aperture of 100 µm diameter for the
measurement of granularity σ . Because the pixels in digital images are organised in squares, the corresponding pixel
D
2
size is calculated by sqrt ((100 µm) π / 4) = 88,6 µm.
6.1.1.10 The normalised SNR is calculated by
SNR = I /σ (3)
meas PSL100
6.1.2 Step Wedge Method (manufacturer test and enhanced user test)
6.1.2.1 General
The measurement of the SNR can be performed with less accuracy using a step wedge. This method may
also be of interest for users to determine the contrast sensitivity quantitatively:
6.1.2.2 For that purpose a step wedge of Cu, with at least 12 equally increasing steps, may be used as in
the arrangement shown in Figure 2. The maximum thickness of the step wedge shall absorb 90 % of the
radiation of the central beam, which requires a thickness of 11,7 mm. To cover a range of two or more orders
of magnitude at least two suitable and different exposures, with adequate exposure time or tube current (mA),
shall be made. The distance between step wedge and IP shall be ≥ 500 mm to reduce the influence of
scattered radiation. A magnification of 2x is recommended. A beam collimator shall be used. X-ray voltage
and filtering shall be selected according to 6.1.1.2.
NOTE X-ray penetration through Cu-steps of different thickness is distorted by beam hardening and suitable
adjustment of exposure is required.
6.1.2.3 The projected area of each step shall be about 20 mm × 20 mm (≥ 400 mm²). No values shall be
taken from areas near the edges. At least two times the geometric un-sharpness shall be left between the
edges of the projected area and the area for data acquisition.
6.1.2.4 All details for the measurement and calculation of the SNR shall correspond to 6.1.1.3 to 6.1.1.10.
The graphical analysis shall be based on the plot of SNR = f (log (Exposure) - µ · w · log (e)), where µ
Cu Cu Cu
is the absorption coefficient, w is the wall thickness of the corresponding step of the step wedge and the
Cu
value "Exposure" is calculated from exposure time (seconds), multiplied by tube current (mA); see also Annex
A.
NOTE For accurate plots it is necessary to consider the wall thickness dependence of µ (beam hardening). The
Cu
influence of scattered radiation should be reduced by exact collimation. Different exposures with different exposure time or
mA-settings are recommended for the required plot. The exposure value (mAs) of the different exposures should deviate
between 5 to 8 times to allow an overlap of the measured data. A waiting time of 30 minutes is recommended between
exposure and scan of the IPs to avoid influences by fading effects.
10
---------------------- Page: 12 ----------------------
SIST EN 14784-1:2005
EN 14784-1:2005 (E)
Key
1 X-ray tube
2 Cu-Filter
3 Collimator
4 Cu-step wedge
5 IP in a cassette
Figure 2 — Scheme for the measurement of the SNR by the step wedge method
6.1.3 Contrast sensitivity measurement (manufacturer and user test)
ASTM E 1647-98a contrast sensitivity gauges are useful for visual and computer aided determination of
contrast sensitivity for a selected wall thickness. Four levels of contrast sensitivity can be measured: 1 %, 2 %,
3 % and 4 %, independent of the imaging spatial resolution limitations. For interpretation see
ASTM E 1647-98a. If image processing is available, a profile (width: 1 pixel) shall be taken through the target.
The average noise of the profile shall be less than or equal to the difference in the intensity between the full
and reduced wall thickness at the read-out percentage. The exposure conditions (kV, mAs, filters, distance,
exposure time, date) and CR system settings and -type shall be documented.
6.2 Measurement of minimum read-out intensity of computed radiographs (manufacturer
procedure)
Each CR-image shall have better or equal normalised SNR than defined by the minimum SNR -values of
IPx
Table 1. Because these SNR-values cannot be measured easily, the minimum SNR -values shall be
IPx
achieved by the application of minimum read-out intensities I .
Ipx
NOTE A classical quality assurance procedure in film radiography is
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.