Operating Manual

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Lateral resolution
Lateral resolution is determined by pixel size. First
of all the pixel size of the detector and secondly the
pixel size of the display screen. The effect of pixel size
and number of pixels is illustrated in figure 26-16.
The same text is displayed in four different resolu-
tions. The text on top shows 6 pixels vertically, step
by step increasing to 50 pixels for the lowest text.
For this reason also the hardware of work stations
is specified by standards to guarantee best possible
image presentation without loss of information as
contained in the original digital data.
Hardware performance should be equal to or
better than the required exposure/detector quality.
16.8 Comparison of film, CR- and DR methods
The choice of which technique to use
depends first of all on the require-
ments with regard to the ultimate
image quality. In both the CR- and DR
methods the same IQI’s also used in
conventional radiography are applied
to check the radiographic processand
image quality. The major parameters
to compare the three methods (film,
CR and DR) are speed (dose needed
for creating the image) and image
quality (noise, resolution, contrast).
Figure 27-16 graphically illustrates
the relative image quality of different
films and digital techniques.
This overview shows that the best image quality (best IQI visibility) of CR plates is similar to
what can be achieved with medium to finer grain film (compare point A with B) but is
appoximately five times faster. At point C the quality is less than what can be achieved with
coarse grain film but the speed is more than ten times faster compared to point B. RCF films
(five tot ten times faster than D7-film) are positioned in the same range as CR plates. The
graph for DR panels is based on the results obtained with common flat panel detectors with
different numbers of pixels (25 to 400 microns). The best quality that can be achieved with
DR panels comes close to fine grain film D3 (compare point D to point E).
The graph also shows that the speed is much higher to achieve the same image quality of
D-type films. Depending on the required image quality a time saving of at least a factor 20
(D against E) and roughly 200 (F against E) can be achieved, however with poorer quality.
The range for true real-time (real instant) images shows that exposures can be made with
extremely low dose but at cost of image quality.
16.9 Impact and status of CR- and DR standards
Development of standards
The application of established NDT methods is almost
exclusively possible thanks to the existence of written stan-
dards (codes, norms, guidelines, procedures, specifica-
tions, qualification of personnel, etc.). For the introduction
and market acceptance of a new non-established NDT
method - apartfrom economic considerations - it is therefo-
re essential that standards support its use.
The standards for film radiography were written many
years ago and did not envisage digital radiography, so a
whole set of new standards is required.
In general the issue of standards lags far behind the introduction of a new method.
Development of standards starts earliest once a new NDT method is almost mature and has
shown certain market viability. The creation of a standard usually is a time consuming and
painstaking formal process. Standards are compiled by (international) working groups con-
sisting of NDT-specialists of different disciplines e.g. research and development, industry,
universities and authorities. Participation of authorities at an early stage can speed-up lega-
lisation (legislation) and release of a standard.
Sometimes industrial, economic and scientific interests do
not fully match and hamper quick release of a new stan-
dard. Even under the most favourable conditions, making a
standard is a process that takes many years; at first it requi-
res a “working document” followed by a “preliminary ver-
sion” prior to official release. Until 2005 this was the situa-
tion for CR and it still is for DR.
In addition to the many specialists involved, there is also a
large number of national and international normalisation
organisations that issue such standards. They often adopt
the content of each other’s documents, to save time, after
careful judgment of the content and give them their own
specific issue numbers and annotations.
Fig. 26-16. Effect of number of pixels on
lateral resolution
Fig. 27-16. Relative image quality and speed of the various
radiographic methods
This graph appears enlarged in the appendix.
Relative dose
Faster
Relative image quality DQE
True real-time
Radioscopy
F
D
A
C
B
D2
(fine grain)
D7 (coarse grain)
E
CR-Plates
DR-Panels
RCF-Films
D-Films
Better
Terminology
What’s in a name?
Standard
Standard practice
Recommended practice
Norm
Code
Procedure
Specification
Qualification
Guideline etc.
Major
Issuing
Organisations
ISO
EN
ASTM
ASME
JIS
IEC
DNV