ISO 18939:2013

INTERNATIONAL ISO
STANDARD 18939
First edition
2013-11-01
Imaging materials — Digital hard copy
for medical imaging — Methods of
measuring permanence
Matériaux pour l’image — Photocopie numérique pour imagerie
médicale — Méthodes de mesure de la permanence
Reference number
ISO 18939:2013(E)
©
ISO 2013

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ISO 18939:2013(E)

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© ISO 2013
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Published in Switzerland
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ISO 18939:2013(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Physical test methods . 4
4.1 General . 4
4.2 Layer adhesion . 4
4.3 Binder stability test . 6
4.4 Blocking test and image interaction test . 7
5 Test methods for image stability . 9
5.1 General . 9
5.2 Thermal-ageing test (dark stability) . 9
5.3 Light chamber test .13
5.4 Image spread test .16
Annex A (informative) Light stability test conversion of units .19
Annex B (informative) Effect of residual compounds on thermally processed
radiographic images .20
Annex C (informative) Simulated thermal ageing tests .21
Annex D (informative) Greyscale evaluation based on just noticeable differences (JNDs) defined
DICOM standard display function (SDF) .22
Annex E (informative) CIE colour space parameters for evaluation of discolouration .26
Bibliography .27
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ISO 18939:2013(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 WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information.
The committee responsible for this document is ISO/TC 42, Photography.
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ISO 18939:2013(E)

Introduction
This International Standard prepared by ISO TC 42, WG 5 provides information for measuring the image
stability and other relevant properties of medical dry hard copy films with greyscale images made with
photothermographic, thermographic, and microcapsule type materials or with inkjet printing. Medical
colour images and prints on reflective material for referral purposes are not covered.
Medical dry hard copy films are employed widely for digitally recording medical images in general
radiography and mammography, because of the systems’ simplicity, flexibility, ease of use, and attendant
environmental advantages. First realizations of medical dry hardcopy systems entered the market
together with CR and DR modalities in the 1990s, starting with photothermographic, thermographic,
and microcapsule type materials. Recently, also inkjet based systems were also available. Dry hard copy
systems gained through its one-step dry processing method which obviates the need for film processing
equipment and liquid processing solutions and provides a significant saving in capital and labour costs.
Thermally processed dry hard copy films use osynthetic polymers, e.g. poly (vinylbutyral), poly
(vinyl alcohol), and poly (styrene butadiene) as binders for image forming silver clusters, instead of
gelatine being used in wet processed AgX films. This renders the binder more inert to moisture and
its deleterious effects, including oxidation. The support for thermally processed dry hard copy films is
[1][2][3][4][5][6]
normal, photographic grade PET [poly (ethylene terephthalate) safety film.
A disadvantage of thermally processed dry hard copy images is their greater potential instability caused
by the presence of unused chemicals after image formation; these are not removed by liquid processing
solutions as with conventional silver halide films. Consequently, the potential for formation of excessive
fog exists throughout the life of the thermally processed dry hard copy film. Such degradation of image
quality has occasionally been observed in the course of prolonged exposure to ambient illumination
or storage under high temperature or, most frequently, due to unintended over-exposure to light and
heat in a reader-printer (view box). Also, in case of a fire in the storage area or near a vault or safe,
the temperature sometimes increase sufficiently high to cause image degradation, even though the
temperature used for generating thermally processed dry hard copy images range well above 100 °C.
These images are considerably stable under normal user and storage conditions as well as on accelerated
[7][8][9]
ageing studies ). Hence, thermally processed dry hard copy films do not fall within the provisions
of ISO 18901 that apply to chemical fixation.
Inkjet based dry hard copy images may also be susceptible to temperature, humidity and light depending
on the details of the technical details of the inkjet printing system, its type of ink (e.g. aqueous, solvent
or wax based), the colorants (dye or pigment) and the type of ink receiver layers (porous, swellable, etc.)
of the hard copy film.
2
General radiographs are normally viewed on light boxes at a luminance level of 2 000 to 4 000 cd/m ,
[24][25]
whereas according to American College of Radiology (ACR) recommended practices, mammograms
2
and clinical quality reviews are viewed at a luminance of at least 3 000 cd/m or higher depending on
the modality. In addition, all mammograms and mammogram test images are required to be masked
completely during diagnostic inspection, so that no light directly emitted by the light box surface can
reach the observer’s eyes. The recommended level of intensity of surrounding illumination in that viewing
2
situation is below 10 cd/m . In practice, light box outputs and surrounding illumination conditions do
vary considerably and, therefore, this standard requires use of a light chamber which permits close
control of all illumination parameters, temperature, relative humidity and duration of exposure.
Everyone concerned with the preservation of records on radiographic film understands that specifying
the chemical and physical characteristics of the material does not, by itself, ensure that the records
will not deteriorate. It is also recognized that enclosure materials used to make radiographic envelopes
effects the preservation quality of records It is also essential to provide the correct storage temperature
and humidity, and protection from the hazards of fire, water, fungus, and certain atmospheric pollutants.
These aspects are considered in pertinent International Standards for storage of films, for example,
[16] [17]
ISO 18902 and ISO 18911.
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INTERNATIONAL STANDARD ISO 18939:2013(E)
Imaging materials — Digital hard copy for medical imaging
— Methods of measuring permanence
1 Scope
This International Standard establishes test methods for measuring the stability of photographic
films intended for storage of medical records. It is applicable to greyscale images on films for use
in transmission mode that are based on thermally processed materials (photothermography,
thermography, microcapsule) or created by inkjet printing. Thermally processed materials have a
base of safety polyester [poly (ethylene terephthalate)] and work predominantly with silver behenate
salts dispersed in non-gelatinous emulsions or dye-based microcapsule emulsions that are thermally
processed to produce a black-and-white image. In inkjet printing ink droplets are jetted onto a film with
an ink-receiving layer to produce a greyscale image.
This International Standard does not cover wet-processed black-and-white films or black-and-white
paper. It is not applicable to medical colour images or colour prints created by colour inkjet or dye
diffusion thermal transfer (D2T2). Neither does it cover medical greyscale images printed on reflective
materials for referral purposes or filmless systems such as picture archiving and communication
systems (PACS) in medical imaging.
This International Standard requires the arbitrary choice of “illustrative end points” for changes in
colour and perceived contrast to depict quantifiable changes due to physical ageing. Extrapolations
based on ‘illustrative end points’ do not have any proven diagnostic or clinical relevance due to the lack
of corresponding statistically significant scoring by radiologists.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 5-2, Photography and graphic technology — Density measurements — Part 2: Geometric conditions for
transmittance density
ISO 5-3, Photography and graphic technology — Density measurements — Part 3: Spectral conditions
ISO 18907, Imaging materials — Photographic films and papers — Wedge test for brittleness
ISO 18924, Imaging materials — Test method for Arrhenius-type predictions
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
adherography
imaging technology utilizing a high intensity laser beam to form a positive carbon image through
differential thermal adhesion
Note 1 to entry: This process involves fusion of a laser sensitive, carbon-containing layer with the final imaging
layer in exposed areas, followed by controlled peeling, which removes the unexposed portion. The positive image
is then made durable and permanent by the application of a transfer coat.
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ISO 18939:2013(E)

3.2
microcapsule
imaging technology in which heat-responsive microcapsules containing dye precursors are thermally
rendered to develop a dye image
Note 1 to entry: Heat-responsive microcapsules containing dye precursors are dispersed together with a
development emulsion on a polyester support. Application of computer-modulated heat that matches the density
pattern of a digital image renders the walls of the microcapsules differently permeable. The varying amounts
of developer, which penetrate the capsule walls, produce corresponding differences in dye image density. The
capsule walls revert to their impermeable state on cooling and provide protection against dye formation and dye
degradation under normal storage conditions.
3.3
phase change solid inkjet
imaging technology based on modulated deposition of micro-droplets of non-aqueous, waxy inks on a
microcellular surface of a layer coated on a polyester support
Note 1 to entry: Four shades of neutral ink are used to obtain the wide grey scale density range required for
medical images. The melting point of the ink is considerably above ambient temperature, ensuring image stability
under normal storage conditions.
3.4
photothermography
imaging technology based on thermal development of a light-induced latent image in dispersed silver salts
Note 1 to entry: The process involves a polymeric layer containing light sensitive silver halide crystals, light
insensitive silver behenate crystallites, silver soaps and a reducing agent coated on a polyester support. A latent
image formed by light exposure of the silver halide crystals catalyses an oxidation-reduction reaction between
the silver behenate and the reducing agent upon heating above 120 °C. This yields a metallic silver image by
physical development.
3.5
thermography
imaging technology based on image-wise thermal modulation and development of dispersed silver salts
Note 1 to entry: The process utilizes a polymeric layer containing a light-insensitive organic silver salt, a reducing
agent and a stabilizer, coated on a polyester support. Reduction of the organic silver salt by the reducing agent
accelerated by heat (100 °C–200 °C) yields a metallic silver image whose densities are controlled by the adjustable
temperature of print head elements. The integrity of the silver image under normal storage conditions is secured
by stabilization of the unused silver salt.
3.6
aqueous inkjet
imaging technology involving image formation with an aqueous ink by a modulated deposition of micro-
droplets on the surface of an ink absorbing layer coated on a polyester support
Note 1 to entry: Black-and-white and colour images can be produced by suitable selection of inks.
3.7
just noticeable difference levels
jnd-levels
measure of the non-linear response of the visual system to luminance stimuli defined as a table of
2
ascending photometric luminance levels (between 1 and 10 000 cd/m ), which are perceived as
[20]
equidistant with the smallest perceivable difference (“just noticeable difference”) between them
3.8
jnd-contrast
Δjnd
numerical difference between the jnd-levels of two neutral patches on a radiographic film for a given
viewing situation (intensity of light box and ambient light intensity), which is used as measure of
perceived contrast between the two patches
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ISO 18939:2013(E)

3.9
change in jnd-contrast
(Δjnd (t)/Δjnd(0)) – 1
measure of relative change in perceived contrast between two neutral patches — for example in the
course of a stability test: Δjnd(t) / Δjnd(0) – 1, i.e. [(jnd-contrast after treatment)/(jnd-contrast before
treatment)] – 1
3.10
colour changes
Δa*, Δb*
differences in the CIE colour coordinates a* and b*, e.g. in the course of incubation of dry hardcopy film
3.11
endpoints
set of numerical values defining those changes in colour (Δa*, Δb*) and jnd-contrast [(Δjnd(t)/Δjnd(0))
– 1], for given reference visual density (D ) at which time to failure is evaluated in the course of
vis
thermal-stability and light-stability tests in order to produce Arrhenius extrapolation plots following
the Arrhenius test method described in ISO 18924
3.12
diagnostic endpoint
set of endpoints, for which changes in colour (Δa*, Δb*) and jnd-contrast [(Δjnd(t)/Δjnd(0)) – 1] for given
visual density (D ), have been correlated with loss of the materials’ diagnostic function based on
vis
statistically validated psychophysical scoring by radiologists
Note 1 to entry: At the time of writing this standard document insufficient data was available to specify diagnostic
end points that could be judged as “relevant or prohibitive” from the standpoint of medical diagnostics. Diagnostic
end points need a correlation with judgments or scores by radiologists, for which a statistically relevant set
of psychometric data for a given medical application or modality is needed. Diagnostic end points depend on
a variety of factors, amongst which are — nonexclusively — type of modality, pathology under investigation,
method of image processing, printer settings and density range of the medical image.
3.13
illustrative endpoints
set of arbitrarily defined endpoints for changes in colour (Δa*, Δb*), and jnd-contrast Δjnd(t)/Δjnd(0) – 1
for a given reference visual density D , at which time to failure is evaluated in the course of thermal
vis
stability tests
Note 1 to entry: Arrhenius extrapolations based on illustrative end points do not have any proven diagnostic
or clinical relevance due to the lack of corresponding psycho-visual data. For diagnostically relevant Arrhenius
extrapolations a set of diagnostic end points would be necessary.
3.14
film base
plastic support for the emulsion and backing layers
3.15
emulsion layer
image or image-forming layer of photographic films, papers and plates
3.16
safety poly (ethylene terephthalate) base
polyester film base composed mainly of a polymer of ethylene glycol and terephthalic acid
3.17
processed dry hard copy film
dry hard copy film on which a (test) image has been written by its corresponding printer (in analogy to
the wet processing of conventional AgX based film)
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ISO 18939:2013(E)

4 Physical test methods
4.1 General
This section describes tests for layer adhesion (4.2), binder stability (4.3) as well as blocking and image
interaction (4.4).
4.2 Layer adhesion
4.2.1 General
Layer adhesion failure is tested under two conditions, namely for tape-stripping (4.2.2) and humidity
cycling (4.2.3).
4.2.2 Tape-stripping adhesion test
4.2.2.1 General
The results of the tape-stripping test may depend upon the adhesive tape used if the bonding force
between the adhesive tape and the particular film surface under test is not sufficiently high. For this
reason, a minimum bonding force is specified for this test. This bonding force shall be determined by
applying the adhesive tape to the film surface in the same manner as described in the tape-stripping
test. The tape shall be rapidly peeled back from the film surface at an angle of approximately 180°. The
peel back force required to separate the tape from the film shall be measured by a suitable device such
as a strain gauge or spring scale capable of reading the maximum force used. A bonding force of at least
0,9 N per millimetre of tape width is required.
4.2.2.2 Specimen preparation
Although the dimensions of the processed film specimen are not critical, one dimension shall be at least
150 mm to allow proper handling during the test. Four specimens shall be used for the emulsion surface
and four specimens for the backing layer, if present.
4.2.2.3 Conditioning
All specimens shall be conditioned at 23 ± 2 °C and at 50 ± 5 % relative humidity for at least 15 h. This
can be accomplished by means of an air-conditioned room or an air-conditioned cabinet. The specimens
shall be supported in such a way as to permit free circulation of the air around the film and the linear air
velocity shall be at least 150 mm/s.
4.2.2.4 Procedure
The film specimens shall not be removed from the conditioning atmosphere for testing. Apply a strip of
pressure-sensitive, plastic-base adhesive tape about 150 mm long to the surface of the processed film.
Press the tape down with thumb pressure to ensure adequate contact, leaving enough tape at one end to
grasp. No portion of the tape shall extend to the edges of the film specimens or extend to film notches.
In order to facilitate physical ageing, the adhesive-taped film specimens shall be kept for 16 h prior to
stripping. Hold the specimen firmly on a flat surface and remove the tape rapidly from the film surface.
This shall be accomplished by peeling the tape back on itself and pulling the end so that it is removed
from the film at an angle of approximately 180°.
4.2.2.5 Reporting of results
The processed film shall be examined for any evidence of removal of the emulsion layer or backing
layer, when tested.
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ISO 18939:2013(E)

4.2.3 Humidity-cycling adhesion test
4.2.3.1 General
This test evaluates the sticking, blocking and delaminating of emulsion or backing layers or transference
of paper material to the film surface.
4.2.3.2 Specimen preparation
Two specimens of processed film shall be selected from an area of high density. The preferred specimen
size is 50 mm × 50 mm, or 50 mm × film width where the size of the film permits. However, dimensions
are not critical, provided all specimens are of uniform size and proper handling is possible.
4.2.3.3 Procedure
The procedures can be followed either with two separate humidity-temperature controlled ovens or
by using two glass desiccators as described below. The physical test conditions of temperature, relative
humidity and duration of the test shall remain the same in both procedures.
NOTE Films occasionally exhibit what appear to be small pinholes in the image after processing. These can
be caused by dirt or dust particles on the emulsion surface at the time the raw film is exposed and should not be
confused with holes or cracks in the emulsion layer. The existence of such clear spots in the image prior to humidity
cycling should be noted so that their presence does not lead to a false interpretation of adhesion weakness.
4.2.3.3.1 Humidity-temperature controlled oven method
Mount the test specimens in a specimen rack and place the rack inside the oven in such a way that the
specimens are freely exposed to the required conditioning atmosphere. Place the rack in a forced-air
circulating humidity and temperature controlled oven for 8 h at 50 ± 2 °C and 80 ± 5 % relative humidity.
After 8 h, place the specimens and specimen rack for 16 h in a second humidity and temperature
controlled oven maintained at 50 ± 2 °C and 11 ± 5 % relative humidity.
The sequence of time periods of 8 h at high relative humidity and 16 h at low relative humidity shall
constitute one cycle.
NOTE This can be easily accomplished by placing the specimens in the high relative humidity chamber in the
morning and in the low humidity chamber in the evening.
Each film specimen shall be subjected to 12 humidity cycles. After this, remove the film specimens from
the specimen rack and examine the emulsion and any backing layer for any evidence of peeling, flaking,
or cracking produced as a result of the humidity-cycling treatment (see 4.2.3). During an interruption
in the cycling procedure, the film specimens shall be kept at 50 ± 2 °C and 11 ± 5 % relative humidity.
4.2.3.3.2 Glass desiccator method
Two glass desiccators with saturated aqueous salt solutions are placed in an oven that is controlled at
50 ± 2 °C: In one dessicator a saturated solution of ammonium sulfate (NH ) SO in water is provided at
4 2 4
the bottom of the jar and in another dessicator a saturated solution of lithium chloride in water.
Ensure that the saturated solutions contain an excess of undissolved crystals at 50 °C. The undissolved
crystals shall be completely covered by a layer of saturated salt solution and the surface area of the
solution should be as large as practical. The jars with salt solution shall be kept in the oven at 50 ± 2 °C
for at least 20 h prior to use to ensure attainment of equilibrium. At 50°C, the atmosphere in the jar with
ammonium sulfate (NH ) SO will reach 80 % rV, representing the high relative humidity condition,
4 2 4
whereas the atmosphere in the jar with lithium chloride will reach 11 % rH, representing the low relative
[10][11]
humidity condition
NOTE 1 The relative humidity in the desiccator method is based on the normal vapour pressure of the salt
solution, but the relative humidity tolerance cannot be specified.
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ISO 18939:2013(E)

Mount the test specimens in a specimen rack and place the rack in the first desiccator jar with the
saturated ammonium sulfate solution in such a way that the specimens are freely exposed to the
required conditioning atmosphere. After 8 h, place the specimens and specimen rack for 16 h in the
second desiccator jar with the saturated lithium chloride solution. Maintain both jars in the forced-air
circulating oven at 50 ± 2 °C.
The sequence of time periods of 8 h at high relative humidity and 16 h at low relative humidity shall
constitute one cycle.
NOTE 2 This can be easily accomplished by placing the specimens in the high relative humidity jar in the
morning and in the low humidity jar in the evening.
Each film specimen shall be subjected to 12 humidity cycles. After this, remove the film specimens from
the specimen rack and examine the emulsion and any backing layer for any evidence of peeling, flaking,
or cracking produced as a result of the humidity-cycling treatment (see 4.2.3). During an interruption in
the cycling procedure, the film specimens shall be kept at 50 ± 2 °C in the desiccator with the low relative
humidity (saturated Lithium Chloride solution).
4.2.4 Reporting of results
The film shall be examined under the magnification and lighting conditions that are normal for the
intended use of the product. The emulsion layer or backing layer of the processed film shall be examined
for layer sepa
...