Operating Manual

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16.6.2 Determination of image quality
To determine the quality of a digital image, existing codes require two different IQIs in
analogy to radioscopy. One wire- or plaque IQI for contrast, and one duplex wire IQI for the
determination of spatial resolution (unsharpness).
To establish spatial resolution a density line scan across the X-ray image at the location of
theduplex IQI should be made to determine this resolution. The resulting linear analogue
response of this scan is then interpreted to determine achieved resolution as illustrated in
figure 18-16. The criterion is that the dip between two peaks of the wire pair must be equal
to or more than 20% of the peak heights.
To avoid line interference or Moiré-effects during the line scan process of the reader, the IQI
for that purpose should be rotated for 5° with respect to edge of the CR plate or DR panel,
as required by EN 14784-1 and illustrated in figure 19-16.
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• With millions of pixels it is “normal” that over time a few pixels become less
responsive, similar to pixels of flat panel displays as used with (notebook) computers.
Usually the un-acceptable number and pattern of dead pixels is specified by the
manufacturer. Fortunately, in cases a small area of the panel is out of order, an
experienced interpreter of DR images is able to differentiate (by pattern recogni-
tion and known position on the panel) real component defects from less responsive
pixels. Also "interpolation" software is used to reduce or eliminate the effect of bad
pixels. So, when a small portion of the pixels respond typically, the corresponding
pixel values can be interpolated with data from neighbouring pixels. In document
ASTM 2597 a number of terms and definitions are given for malfunctioning pixels
or pixel clusters.
• Some flat panel DR detectors are also subject to some memory effect, in jargon
called “ghosting”. This is due to hysteresis of the scintillation layer after exposure.
The image slowly fades away, particularly in case of high energy - and dose levels.
This hysteresis causes a certain dead time of the system, from seconds to minutes
depending on the application during which the detector cannot immediately be
re-used.
16.6 Image quality and exposure energy
16.6.1 Exposure energy
To achieve the best image quality the maximum X-ray tube voltage or energy of the isotope
selected should be as low as possible. This applies for both film and digital radiography.
Figure 17-16 shows a graph taken from EN 14784-2 showing the optimum energy versus
wall thickness for different materials.
Fig. 17-16. Optimum radiation energies for best image quality
Fig. 18-16. Resolution criterion of duplex IQIs for digital X-ray images
Fig. 19-16. CR image of a weld with rotated duplex IQI
Material thickness
COPPER
STEEL
TITANIUM
ALUMINUM
mm
According to EN14784-2
kV/k
e
V
Energy
The two wires of a pair are resolved if the dip
between the peaks is greater than 20% of the
maximum intensity.
Peak
Duplex IQI
Scan line
Signal amplitude
Linear scale
Analogue signal intensity
Relevant part of CR image