Operating Manual

161
Exposure parameters
The quality of a digital image is affected by a number of factors. The final image cannot be
better than the quality of the X-ray information arriving at the detector. Just as with con-
ventional film radiography, this inherent loss of information is determined by a variety of
parameters. These are; the X-ray spectrum (kV, filters, and screens), the part to be inspec-
ted (thickness, material) and exposure conditions (focal distance, backscatter, exposure
time). The overall effect is visible as loss of contrast and sharpnes of the final image on the
screen of the workstation. Some optimisation is possible for digital systems, the majority of
measures quite similar to those appropriate for good conventional film techniques.
MTF (Modulation Transfer Function)
No imaging system is perfect. All imaging systems record their inputs imperfectly. One obvi-
ous shortcoming - of paramount importance for radiography - is a reduction in sharpness by
imperfect contrast transmission throughout the total imaging chain. The scientific method
to quantify performance (fidelity) to transfer contrast information is characterised by the
“Modulation Transfer Function”, MTF for short. MTF describes the relation between
contrast and spatial frequency.
In practice MTF characterises the unsharp-
ness (blurriness) that a digital system adds
to an image, thus indicating the level of
distortion of contrast/sharpness in the
resulting image as illustrated in figure 21-16.
The graph shows the distortion of contrast
of a square wave (black-white) input and
output for an ideal MTF of 1 (100%) and a
low MTF. Each step in an imaging chain has
an individual MTF.
The MTF of a complete system is the product of the MTF's of the individual steps. In the end
of an imaging process the effect is visible in the amount of loss of image quality. MTF for a
total system typically ranges from 0 to 1 (0 to 100 %). Sharp features and small flaw indi-
cations will be more easily visible in images produced with a system that has a high MTF.
Figure 22-16 graphically shows the
image distortion of for example an
ideal pin-shaped detail that through
successive distortions by the steps in
the system, is presented as a blurred
spot on the screen of the work station.
Every step in the process widens the
detail that was ideal in the beginning
with a simultaneous reduction of
contrast and sharpness.
160
16.6.3 Indicators of image quality - MTF and DQE
Factors influencing image quality
In the process of making a radiograph three factors influence the ultimate image quality:
1. exposure conditions
2. detector performance/efficiency
3. performance of the processing equipment to form an image
To enable quantification of the quality of digital radiographs and the hardware used to
create them, two notions are in use: MTF and DQE.
Image quality definitions
Image quality is the total result of resolution/sharpness, contrast resolution and noise.
Conventional X-ray films exhibit an extremely high intrinsic resolution due to the fine gra-
nularity of the radiation sensitive crystals (a few microns in size). The resolution of the
resulting image is far better than the human eye can resolve. Hence for film contrast IQI’s
provide an adequate measure of indicating resolution and image quality that meets the qua-
lification needs of industry, thus there is no need for any additional resolving criteria for tra-
ditional film. However, digital radiography has a much coarser intrinsic resolution (typically
50 microns or more) so a different situation exists compared to film radiography.
To select or purchase the proper digital system, information that quantifies the resolving
power of a digital system is needed. Although generic methods to measure optical resolu-
tion exist, they have not yet been fully specified for digital radiographic systems.
To fulfil the need for quantifica-
tion of resolution, prior to antici-
pated release of future stan-
dards, suppliers of digital radio-
graphy systems already proacti-
vely use methods and definit-
ions
that are common in other
sciences
.
Resolution is defined as the smallest separation (distance) between two objects that the
human eye can distinguish. Because the human eye is not easily quantifiable, an objective
method to indicate resolution is needed. Resolution is dependent on contrast (grey levels)
and separation (distance).
Resolution is expressed as the number of line pairs (black and white) that can be distin-
guished in one mm, see figure 20-16.
Fig. 20-16. Resolution: line pairs per mm
Fig. 21-16. MTF and resulting contrast distortion
Fig. 22-16. MTF causing step-wise image distortion
1 mm
1 Lp/mm
1 mm
4 Lp/mm
Ideal MTF input = output
Low MTF with distorted output
Initial information
Process steps
Image contrast
Final image
on monitor
screen