Key words
diagnostic displays - DIN 6868-157 - quality assurance - constancy tests - randomized
tests
Introduction
The findings of digital X-ray images are to a considerable extent dependent on the
quality of the monitors and the ambient conditions in the reading room. This dependence
is only minor for clearly recognizable lesions with high contrast, as they can hardly
be overlooked even under unfavorable conditions. This dependence, however, is critical
with respect to lesions in the low-contrast range. Here, the highest quality criteria
are required which are rarely met by standard PC components (monitor, display controller,
etc.). The physician can only be assured by means of acceptance and constancy tests
that the monitors meet these requirements at all times.
Nowadays flat screens are used almost exclusively (an overview of the abbreviations
used are given in [Table 1]) instead of cathode ray tube monitors. This resulted in an urgent adaptation of
the DIN standards for the acceptance and constancy testing of diagnostic monitors
to the state of the art. Shortly after publication of the new standard, DIN 6868-157
[1], there was a comprehensive overview of these changes [2]. Nevertheless, since its publication, the standard has raised numerous questions
and has led to many uncertainties among users. This has been made clear, for example,
by the many questions in the online forum “Forum Röntgenverordnung” (a forum dealing
with questions regarding the X-ray Ordinance, standards, etc.) [3]. This technical review deals critically with the new standard and describes initial
experiences with DIN 6868-157 while providing help with the application of the standard.
In doing so, the medical relevance of some test items is also scrutinized.
Table 1
Overview of abbreviations and terms.
|
Abbreviation
|
Term
|
Explanation
|
|
IDD
|
image display device
|
monitor
|
|
IDS
|
image display system
|
comprises the entire workstation with CPU, graphic card, software, and monitor(s)
|
|
E
|
Illuminance
|
describes the brightness in a room, unit: Lux (lx)
|
|
GSDF
|
Grayscale Standard Display Function
|
function which assigns luminance values which have been adapted to the non-linear
contrast sensitivity of the human eye to the digital input signal
|
|
L‘min
|
Minimum luminance
|
minimum luminance that a monitor can display (black). Includes ambient light. Unit:
cd/m²
|
|
L’max
|
Maximum luminance
|
maximum luminance that a monitor can display (white). Includes ambient light. Unit:
cd/m²
|
|
Lamb
|
Veiling glare
|
luminescence reflected onto the monitor by the ambient light. Unit: cd/m²
|
|
RC
|
Room class
|
Defines the requirements for luminance
|
Essential Changes to the previous Testing Procedures
Essential Changes to the previous Testing Procedures
A major change to the previous test situation is the consideration of the entire image
display system (IDS) as a complete workstation with a PC, software and monitors, in
contrast to an image display device (IDD). This standard has the advantage of requiring
that changes to all components that can influence image quality, such as the display
controller, are also checked and documented, and not just the monitors themselves.
[Fig. 1] shows, for example, the consequences of an incompatible display controller and the
resulting difference in brightness between the upper and lower half of the monitor.
Fig. 1 An incompatible display controller causing a difference in luminance. Luminance L1 = 192 cd/m², L2 = 184 cd/m² in test pattern TG18-UN80.
In addition to the image display system, the standard additionally includes the ambient
lighting conditions. Previously they were implicitly checked by measuring the minimum
luminance and veiling glare, whereas the new standard explicitly requires measurement
of the illuminance of the room. The lighting is regulated by the newly-introduced
room classes (RC). The room classes reflect various operating conditions of the monitors
([Table 2]). The activities carried out in the room (diagnosis, examination, etc.) determine
the room class and thus set the requirements for the maximum permitted illuminance
and the monitor used. The earlier classification of projection radiography (thorax,
skeleton, breast) into category A, and classification of fluoroscopy, computed tomography
and subtraction angiography into category B have been omitted in their previous form.
Table 2
DIN 6868-157 introduces a classification for different types of rooms, e. g. reading
room, treatment room, etc. and defines the limiting values for the illuminance accordingly.
|
Room class
|
Room
|
Activities according to DIN 6868-157
|
Illuminance
|
|
RC1
|
reading room
|
evaluation of radiographs and projections
|
≤ 50 lx
|
|
RC2
|
examination room with immediate establishment of a diagnosis
|
medical activities in the examination room in which therapy-related decisions are
made
|
≤ 100 lx
|
|
RC3
|
rooms for performing the examination
|
activities during which the examination is carried out using a dialog monitor
|
≤ 500 lx
|
|
RC4
|
observation and treatment rooms
|
activities in which a known and assessed diagnosis must be repetitively reproduced
|
≤ 1000 lx
|
|
RC5
|
dental diagnostic workstation
|
diagnosis outside the luminance requirements of a dental treatment room
|
≤ 100 lx
|
|
RC6
|
dental treatment room
|
diagnosis following the luminance requirements of a dental treatment room
|
≤ 1000 lx
|
In addition to the requirements for the acceptance test, the new standard also contains
instructions and limiting values for the constancy test, which was previously regulated
by the quality assurance guideline (QS-RL, Germ. Qualitätssicherungs-Richtlinie).
In addition, the DICOM Grayscale Standard Display Function (GSDF) that adapts the
luminance values to the sensitivity of the human eye for contrast changes is now mandatory
for medical displays. Furthermore, contact measurement using a near-field luminance
meter is now approved as an addition to the usual distance measurement in the telescope
method for the constancy test and certain parts of the acceptance test. For the first
time, limits for pixel defects were introduced. The standard is valid exclusively
for application areas within the framework of the X-ray Ordinance, that is, not applicable
to ultrasound equipment or MRI units.
Modification of the quality assurance guideline resulted in the mandatory application
of DIN 6868-157 dated December 15, 2014, thus replacing DIN V 6868-57 [4]
[5]. However, old acceptance tests according to DIN V 6868-57 remain valid during a
transitional period until 2025. This also includes an exchange of subcomponents, e. g.
the PC; only after replacing the monitors does the new standard have to be applied.
Likewise, constancy tests can continue to be performed according to QS-RL.
Acceptance and Constancy Testing
Acceptance and Constancy Testing
As before, acceptance tests are necessary during commissioning as well as after monitor
replacement. According to the new standard, an acceptance test must also be carried
out if the room class changes. Only the radiation protection responsible/commissioner
can change this if the relevant activities are modified. A constancy test is sufficient
for a change of location. Regular constancy tests need only be carried out semi-annually
instead of quarterly as before.
Test Patterns
The patterns used for testing have been adapted to DIN EN 62563-1:2014-01; test patterns
used for mammography have been supplemented by additional images of the American Association
of Physicists in Medicine (AAPM) [6]
[7]. These test patterns can be obtained in the standard 1024 × 1024 pixel format as
a bitmap via the Radiology Standards Committee (NAR, Germ. Normenausschuss Radiologie).
Based on what we know so far, other storage formats such as DICOM or other resolutions,
e. g. 1600 × 1200, which represent the minimum requirement for projection radiography
are, unfortunately, not available so the user usually needs a software application
containing the images required for acceptance and constancy testing. This can result
in expenses in the amount of several hundred euros if the testing software is not
included in the purchase of the monitors. These additional costs may be acceptable
for an acceptance tester, but this will tend to lead to misunderstandings and reduced
acceptance of the test by the operators (medical practices, etc.).
Visual Inspections
Overall Image Quality
The newly introduced test pattern TG18-OIQ (overall image quality) is used to assess
the overall image quality and geometry during the acceptance test as well as the daily
constancy test ([Fig. 2]). Various test image elements have to be assessed depending on the room class. The
greatest challenge is test element 3, which shows the lettering “QUALITY CONTROL”
in low contrast on white, gray and black fields. From left to right the individual
characters are displayed in decreasing contrast. Depending on the room and application
class, different characters of the lettering must be visible. The highest requirements
apply to mammography, where the entire lettering must be recognizable. At 12 bits,
the final letter “L” in the black field corresponds to a pixel value of 16, which
corresponds to just 0.4 % of the maximum gray scale value. Previously only the 5 %
field had to be recognizable (corresponds roughly to the “U” in “QUALITY”). It is
not clear why these requirements were so significantly raised. Thus, for example,
EUREF (European Reference Organisation for Quality Assured Breast Screening and Diagnostic
Services) does not require legibility of the entire lettering, but that the number
of observed characters remains constant [8]. The contrast of the lettering is so low that the other test image elements can
influence the evaluation of the test item. This is easy to check by covering the areas
around the black field so as not to dazzle the eye.
Fig. 2 Element 3 of the test pattern TG18-OIQ displays the letters “QUALITY CONTROL” in
a black, a gray and a white rectangle.
The value and acceptance of the currently valid procedure was examined in a blind
study. To do this, over a period of six weeks the study determined how often and with
what care the daily constancy tests were performed. Four modified versions of the
TG18-OIQ test image were developed in which either characters in the “QUALITY CONTROL”
lettering or the line pair grids in the corners and center were removed. The modified
test patterns were included into the RadiCS quality assurance software made by EIZO
and, unknown to the users, remotely controlled distributed daily to five to ten diagnostic
workstations via a server. A total of 616 tests were evaluated, of which 172 (28 %)
used modified test patterns. The completion rate during the testing period was 88 %.
This means that 12 % of all pending examinations were aborted or skipped by the examining
physician. Of a total of 148 valid tests with modified test images, only 7 (5 %) were
correctly recognized as faulty. In 141 cases (95 %), however, the test was incorrectly
rated as passed. Subsequently, the users were informed about the study, and a second
phase examined whether this could lead to a change in the performance of the constancy
test. Modified test images were distributed over a shortened two-week test period.
A total of 276 examinations were evaluated, of which 29 used modified images. The
completion rate was 85 % and could not be increased. Of a total of 25 tests with modified
test images, 7 (28 %) were correctly classified as faulty, whereas 18 (72 %) were
mistakenly considered to be passed. This shows, on the one hand, the lack of acceptance
of the constancy tests and on the other hand, the low significance of the test results.
Overall, the daily constancy test according to the new standard is considerably more
extensive than before. Whereas previously only a check of gray-scale reproduction
was prescribed, the overall picture quality is now to be checked visually by means
of several elements of the TG18-OIQ test image. The additional workload reduces the
acceptance of the test by the user, but there is no evidence of any additional benefit.
Homogeneity, Color Impression and Uniformity, Defective Pixels
In both the acceptance and constancy tests, the monitors must be tested for homogeneity
and color impression using the TG18-UN80 test pattern (uniformity, at 80 % of the
driving level). A check for dead pixels is strictly required only in the acceptance
test, but should also be checked in the constancy test.
Previously it was required that the medically used area of the display device must
not contain artifacts which influence the diagnosis. Artifacts can be caused by defective
pixels, among other things. An accurate assessment of when these pixel defects affect
the diagnosis is difficult to determine and also varies with the person carrying out
the test, therefore the new standard has defined several types of pixel defects and
sets limiting values. A distinction is made among permanently lit pixels (defect type
A), permanently dark pixels (defect type B), abnormal subpixels which do not correspond
to error types A or B (defect type C) and defect clusters (defect type D), see [Fig. 3]. The exact number of allowed pixel defects must be calculated based on the specified
limits for a resolution of 1024 × 1024 and total number of pixels of the image display.
It may be helpful for the examiner to create a table with the limiting values of the
most frequent resolutions if the software does not provide such a calculation.
Fig. 3 Display of pixels and subpixels on a gray-scale monitor and possible defects. In
a color monitor, a pixel is composed of one red, one green and one blue subpixel.
Different colors can be displayed by combining these subpixels. The arrangement of
these subpixels can differ, depending on the manufacturer.
A typical display device for mammography with a resolution of 2048 × 2560 may have
5 pixel errors of defect type A, 25 type B defects, 25 type C defects, and 5 type
D pixel defects, thus a total of 180 defective (sub)pixels. Especially when combining
several – or in the extreme case all four – defect types, these limiting values appear
to be too high. In practice, working with so many pixel defects is hardly imaginable
and would be rejected by most physicians.
Measurement Testing
As in the past, the acceptance test is carried out as a distance measurement using
the telescope method (measurement method A), but other methods can also be used in
the constancy test ([Fig. 4]). The standard permits the use of the calibrated luminance meter according to the
telescope principle of the acceptance test as well as the near range luminance meter
for contact measurement (method B), as well as meters integrated into the image display
system (method C + D). In test procedures B, C and D, the illuminance must also be
determined in order to account for the ambient light. With a suitable software, the
constancy test can be carried out automatically and, depending on the software, in
some cases even remotely controlled.
Fig. 4 Methods for measuring the luminance: a telescope method, b near range luminance meter, c front integrated luminance meter, d back integrated luminance meter. Methods b–d require an additional measurement of the illuminance.
Illuminance
An addition to DIN 6868-157 was the requirement that the illuminance must be adapted
to the environment. Requirements for the maximum illuminance are determined by the
room class.
Minimum/Maximum/Veiling Glare
The minimum and maximum luminance must be measured for the constancy tests. The ratio
of minimum and maximum luminance (maximum luminance ratio, formerly maximum contrast)
has to be determined only in the acceptance test. Absolute limits for the maximum
luminance and maximum luminance ratio are specified depending the application. The
minimum requirements for the maximum luminance ratio were raised from 100 to 250 for
projection radiography and from 40 to 100 for other application areas. The transitional
periods for legacy equipment will lead to a difference in quality between old and
new monitors in the coming years.
Luminance Response
In order to accommodate the nonlinear contrast sensitivity of the human eye, the new
standard made the grayscale standard display function obligatory for display devices
with diagnostic quality ([Fig. 5]). The human eye is more sensitive to minor relative changes in areas of higher luminance
(white) compared with low-luminance areas (black). The introduction of a standard
luminance response results in a comparable image impression on different monitors,
even among different manufacturers.
Fig. 5 The grayscale standard display function shows the changes in luminance that are necessary
for a human observer to notice a difference (“just noticeable difference”, JND). The
luminance response of the display will be adjusted accordingly.
The luminance response is determined with contact measurement at the 0 – 100 % driving
levels (test images TG18-LN8-01 to 18). A suitable software tool is necessary for
the evaluation of the measured values. If no quality assurance software is used, the
tool offered by the European Reference Organization for Quality Assisted Breast Screening
and Diagnostic Services (EUREF), for example, can be used [10]. Veiling glare Lamb has to be considered in the calculations since contact measurement is prescribed.
Exceptions were applications in dentistry (RK5 + 6), in which the GSDF is not mandatory,
since low contrasts play only a subordinate role in dentistry. In room class 3 the
GSDF has to be measured only in the acceptance test.
Display Homogeneity
In order to guarantee that the image impression is uniform across the entire monitor,
the homogeneity of the display system must be measured at fixed points at 10 % and
80 % of the maximum digital driving level during the acceptance test. The number of
measuring points depends on the screen diagonal and thus also takes into account large
monitors that replace two smaller individual monitors (e. g. one 6 MP monitor instead
of two 3 MP monitors).
According to the old standard, different test patterns could be used for checking
homogeneity, but it was prescribed that one measuring point should be placed near
each of the four corners. Following the new definition, the measuring points were
moved further towards the center; a check of the corners or edge is thus no longer
performed. Although only the medically used area of an imaging display is supposed
to be checked and not the area wich is concealed, by the menu bar of the PACS, for
example, inhomogeneity tends to appear along the edge and corners, and not in the
area to be checked according to the standard ([Fig. 6]). The larger the monitor, the further removed the measuring points are from the
edge. It would therefore be advisable to carry out the measurements at a certain distance
from the edge of the medically used area regardless of the monitor size.
Fig. 6 Left: The test pattern TG18-UN80 shows shaded edges as a typical sign of ageing.
The display passes the constancy test. Right: The part of the display that is used
for reading varies with the anatomical region (e. g. chest, breast) and the application
software (e. g. PACS).
A further modification concerns the definition of homogeneity. Previously, the deviation
of the vertices (E1 – 4) from the center (M1) was considered, whereas now the deviation
of the measuring point with the highest luminance to the point with the lowest luminance
is evaluated. At the same time, the tolerances were adapted to this changed approach.
According to the old standard tolerances of ± 15 % (application category A) or ± 20 %
(application category B) were used for the deviation of luminance of the vertices
from the center. Now, the limiting value for homogeneity within the entire display
device is 25 % (RK1 – 4) and 30 % (RK5 + 6). █Because of this revised approach monitors█
which do not meet the requirements of the prior standard may be operated according
to the new standard. According to the old standard, a monitor with 171 cd / m² in
the center (M1, z) and 135 cd / m² in one of the corners (E1, k1), exhibits a deviation of approx. 21 % thus exceeds the tolerances for application
categories A and B display devices. According to the new standard, homogeneity is
approx. 24 %, thus fulfilling requirements for all applications. Therefore, the monitor
in [Fig. 6] would pass the measurement tests of the new standard on the one hand due to the
position of the measuring points and on the other hand due to the changed limiting
values. In the case of a visual complaint by the user, this may lead to discussions
regarding warranty claims.
Multi-display Image Homogeneity
The check of the homogeneity of adjacent monitors connected to the same image display
system and which are supposed to show an identical image was also introduced. This
is a positive step, since a workstation frequently consists of several monitors, and
varying image impressions should be avoided. A similar rule already existed for mammography
in PAS 1054 [11], according to which maximum contrast and maximum luminance were compared. In the
new standard, homogeneity of multiple display systems is measured at low luminance
(10 % of the maximum driving level, test image TG18-UN10). A comparison of the whole
luminescence response would be more useful instead. It remains to be seen how these
changes will affect mammography. For other acquisition methods for which there were
no requirements regarding the homogeneity of multiple display systems the introduction
of this test item represents a tightening of the rules.
Limiting Values and Tolerances for the Constancy Test
For constancy tests, DIN 6868-157 specifies both absolute limits as well as tolerances
for deviation from the reference values. Since it is not useful to use limit values
and tolerances for each test item, and since a general listing is missing in the standard,
[Table 3] contains the limiting values and tolerances which, in the authors’ opinion, should
be used according to the new standard.
Table 3
The authors recommend the following limiting values for constancy tests according
to DIN 6868-157.
|
test
|
interval
|
method
|
test image
|
limiting value
|
tolerance
|
|
overall image quality
|
daily
|
visually
|
TG18-OIQ or TG18-QC
|
-
line pair–grid visibility
-
visibility of 5 % and 95 % fields (only dentistry)
-
visibility of characters “QUALITY CONTROL” in white and gray field (dentistry in gray
field only)
-
visibility of characters “QUALITY CONTROL” in black field:
-
entire visibility of grid
-
continuity of ramp bar
|
–
|
|
homogeneity of luminescence
|
semi-annually
|
visually
|
TG18-UN80
|
no interfering irregularities
|
–
|
|
color impression and uniformity
|
semi-annually
|
visually
|
TG18-UN80
|
|
–
|
|
minimum luminance (L’min)
|
semi-annually
|
metrologically
|
TG18-LN8 – 01
|
≥ 1.1 Lamb (1.1 times veiling luminance)
|
reference value ± 30 %
|
|
maximum luminance (L’max)
|
semi-annually
|
metrologically
|
TG18-LN8 – 18
|
-
projection radiography, mammography: ≥ 250 cd/m²
-
fluoroscopy, computed tomography: ≥ 150 cd/m²
-
mammographic stereotaxy, RK5: ≥ 200 cd/m²
-
RK6: ≥ 300 cd/m²
|
reference value ± 30 %
|
|
veiling glare (Lamb)
|
in case of abnormalities of minimum luminance: semi-annually
|
metrologically
|
–
|
–
|
reference value ± 30 %
(exception: reference value < 0.15 cd/m², then reference value + 30 %)
|
|
illuminance (E)
(alternative to veiling glare)
|
semi-annually
|
metrologically
|
–
|
-
RC1: E ≤ 50 lx
-
RC2: E ≤ 100 lx
-
RC3: E ≤ 500 lx
-
RC4: E ≤ 1000 lx
-
RC5: E ≤ 100 lx
-
RC6: E ≤ 1000 lx
|
–
|
|
homogeneity (H) of multiple display units
|
semi-annually
|
metrologically
|
TG18-UN10
|
–
|
|
|
luminescence response
|
semi-annually
|
metrologically
|
18 test images TG18-LN8-01 through TG18-LN8-18
|
–
|
-
Projection radiography, mammography: GSDF ± 10 %
-
Fluoroscopy, computed tomography: GSDF ± 15 %
|
Quality Assurance Software
Quality Assurance Software
In principle, it is possible to carry out acceptance and constancy tests without quality
assurance software. Since the DIN test images are available only in the standard 1024 × 1024
format, a software solution is usually required to generate the test images at other
resolutions. Furthermore, testing is simplified if the related test images are immediately
called up and the results documented.
Quality assurance software is provided by manufacturers of diagnostic monitors (e. g.
Barco, EIZO), measuring equipment (iba) or PACS (aycan), but also by quality assurance
service providers (diraal, mdp dental). [Table 4] shows an overview of currently available quality assurance software focused on DIN 6868-157.
Table 4
Overview of available software for quality control according to DIN 6868-157.
|
manufacturer
|
software
|
central storage of protocols?
|
miscellaneous
|
|
AYCAN
|
ayDisplayQuality Software
|
archiving in PACS
|
plug-in for aycan OsiriX PRO
|
|
barco
|
QA Web for DIN 6868 – 157
|
server
|
|
|
diraal
|
QAXRAY Pro
|
server
|
|
|
EIZO
|
RadiCS
|
server
|
|
|
goFileMaker
|
gFM-dental
|
database
|
dentistry only
|
|
iba
|
DisplayQ
|
no
|
modular; documentation and display of test images separate
|
|
mdp dental
|
KPS 2015
|
no
|
dentistry only; modular
|
|
NEC
|
GammaCompMD QA
|
server
|
cooperation with diraal
|
|
Qubyx
|
PerfectLum
|
server
|
|
The scope of functions available differs distinctly. For example, software by mdp
dental is designed only for dentistry, while other programs cover all areas of application.
Depending on the manufacturer, the software is often modularly constructed, so that
in the basic version, for example, only the constancy test is available which is sufficient
for most users, thus keeping costs down.
Some manufacturers also offer the option to save the results of the acceptance and
constancy tests centrally on a server. This can be particularly useful for larger
hospitals or multiple-site practices in order to quickly access protocols. A server
solution is generally not required for single workstations.
Prices of quality assurance software vary substantially. Some manufacturers offer
software at no cost together with the purchase of other products, such as diagnostic
monitors; with other manufacturers, on the other hand, a few hundred euros have to
be invested in addition to the cost of the workstation. Therefore, prior to purchasing
software the requirements should be carefully considered to avoid unnecessary costs.
Discussion
An adaptation of the standard to the state of the art is in principle to be welcomed.
In addition to some improvements, such as the extension of the test interval from
quarterly to semi-annual tests, several items of DIN 6868-157 must be seen critically.
Particular attention is to be paid to the daily constancy test.
Acceptance of this test among physicians is low. This is primarily due to the tests
requiring the user to only confirm the visibility in a permanently identical test
pattern, thus calling into question the usefulness of this exercise. Randomized tests
in which the user has to recognize a structure at any point on the screen and subsequently
point to it with the mouse are much closer to the actual diagnostic situation. These
tests also directly provide a convincing result in which the physician is assured
that the monitor together with the ambient lighting conditions have a high probability
of meeting quality requirements.
When evaluating the test point, a bias is generated as the user knows exactly what
should be seen, thus deviations from the standard may not be recognized for an extended
period of time. In this case as well, a randomized test, in which not only the objects
to be recognized but also their positions vary from test to test and must be recognized
by the tester, would be advantageous. In this way a subjective test would become a
semi-objective test. Possibly, these semi-objective tests could even replace the metrological
checks, which would lead to a cost reduction. There are already similar approaches,
such as the MoniQA software program [9].
In addition to the daily visual constancy test, metrological verification of homogeneity
should be seen in a critical light. On the one hand, the threshold has been raised;
on the other, the newly-defined measuring points are placed too far in the center
of the monitor. Inhomogeneity in the outer regions of the screen is thereby not detected.
It would therefore be more advantageous to carry out the measurements at a strictly
defined short distance from the edge of the medically used area.
The standard includes limits for pixel defects, providing a clear benchmark for the
manufacturer, examiner and user. Previously there was a subjective estimate of the
number of allowable pixel defects, which could lead to different opinions, especially
between manufacturers and users. While the introduction of limiting values is therefore
generally to be assessed positively, the thresholds themselves have been poorly chosen,
however. The limits are clearly too high, particularly when combining several types
of defects.
The new standard also allows built-in sensors and automated measurements for the tests.
The quality assurance software of one manufacturer supports the remote performance
of the semi-annual test without a trained inspector on-site. However it is not advisable
to allow a completely remote-controlled test in which the device checks itself. In
recent months numerous discussions have shown the dubiousness of device-internal testing
software (e. g. the emissions scandal at VW).
Quality assurance of diagnostic monitors must not only be aimed at checking compliance
with physical parameters, especially since the correlation of these parameters with
the needs of radiological activities has not always been proven. Rather, quality assurance
must demonstrate that the processing of a binary image is optimally adapted to human
visual physiology under the given ambient conditions. For this reason, the eye of
the user must absolutely be included in the test.
Summary
The transition of image displays from cathode ray tubes to systems using flat screen
monitors made an adaptation of the standard to the state of the art urgently necessary.
The new standard has led to some improvements, but many questions have been raised
by the users as a result of standard’s complexity.
Likewise, the integration of dental applications into the standard was only partially
successful. In many aspects exceptions for dentistry resulted in a considerably reduced
range of testing.
On the whole, the new standard contains numerous exceptions that make understanding
and interpretation difficult. Compared with the earlier standard, requirements were
increased for many test items, such as visibility of low contrast in the daily constancy
test. Other items, such as homogeneity requirements, were lowered.
The clash of interests during the creation of the standard has been made clear by
the numerous objections to the drafts. However, the resulting compromise has led to
further discussions and criticism since the publication of DIN 6868-157. A revision
of the standard to eliminate ambiguities is therefore to be welcomed; especially useful
would be the introduction of randomized tests.
-
The physician has to be confident that the diagnostic monitor can display all relevant
lesions,
-
therefore regular monitor testing is indispensable.
-
Since November 2014, DIN 6868-157 has governed the acceptance and constancy testing
of diagnostic monitors.
-
Numerous users are having problems implementing the new standard.
-
A revision of the standard to clarify misunderstandings appears necessary.
-
Randomized tests should be used for daily constancy testing.
