Operating Manual

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Remark: MTF and DQE are used to characterise detectors and systems. Some users may
find these scientific notions rather abstract and hard to understand. While very useful in
selecting a detector for a particular application, in practice they do not replace the duplex
IQI as final indicator of image quality for CR- and DR applications.
Noise, image averaging and DQE
Noise is a dominant factor in the DQE value. System noise can be reduced by signal avera-
ging resulting in improved image quality as illustrated by the images of figure 24-16.
Noise in turn depends on dose, thus the time needed for an exposure to create an
image that might include signal averaging to achieve the required image quality.
Reduction of noise by averaging the signals from a number of exposures increases the image
quality but reduces the DQE value due to the longer exposure time.
16.7 Resolution – number of bits
Resolution is a key word connected with digital radiography. Apart from all digital proces-
sing inside the system it is ultimately the image resolution that determines its quality.
Two resolutions are of importance:
1. depth resolution – the number of grey levels in which a signal is presented
2. lateral resolution – the pixel size
Bit depth
For depth resolution, to present the densities in a map like image, usually 12 bit (2
12
) is
applied. This corresponds to 4096 different grey levels, which corresponds (when for con-
venience divided by 1000), to 4 in film technology. The effect of the number of bits is
illustrated in figure 25-16. Image A shows a 1 bit (2
1
) 2 level image and hardly contains any
information. Image B shows a 2 bits (2
2
) image equal to density 4 grey levels with still lots
of missing details. Image C shows a 12 bit image, providing more than sufficient informa-
tion and showing all details even far beyond what the human eye can distinguish.
DQE (Defective Quantum Efficiency)
Just as it is more difficult to discern fine
detail when an object is dimly lit, it is also
more difficult to observe defect indications
in a noisy X-ray image with limited detected
dose. The effects of contrast and noise on
defect recognition is illustrated in figure 23-16.
Thus, in addition to the two factors (exposu-
re parameters and MTF) already mentioned
there is an additional loss of image quality if
some of the X-rays are not absorbed during
the primary detection process determined by
the ability of the detector to accurately trans-
fer the information present in the incident X-rays. MTF does not take into account the inhe-
rent noise resulting from the X-ray dose available for an image or noise added by the detec-
tion system. The measure that combines MTF with detection efficiency and noise conside-
rations is known as “Detective Quantum Efficiency”, or DQE for short.
In mathematical terms DQE can best be thought of as the square of the signal-to-noise ratio
(SNR
out
) of the X-ray contrast measured by the detector, divided by the square of the
contrast-SNR
in
incident on the detector, for each spatial frequency:
DQE (f) = (SNR
out
)2 / (SNR
in
)2 otherwise: DQE ≈ Image quality / Dose
So, DQE indicates a detector system's ability to accurately represent all of the contrast infor-
mation present in the incident X-ray field as a function of spatial frequency. A perfect detec-
tor will give a DQE of one (100%) over all frequencies, while a poor detector has a DQE that
approaches zero.
For example, assume there are two detectors with different DQE's. If the same incident dose
is applied, the detector with the higher DQE will give a larger SNR
out
and better image qua-
lity. Alternatively, the same image quality can be achieved with the other detector as well, but
requiring a higher dose, which translates to increased exposure time or higher tube current.
In general, DQE consolidates many individual parameters (resolution, efficiency/exposure
time, noise etc.) into a single parameter describing the overall plate- or panel performance.
Therefore specifying DQE for a detector will also help determine both the final image quali-
ty and inspection times required for a given application. Like MTF, DQE can range from 0.0
to 1.0; numbers in practice vary from 0.05 to 0.9.
In summary: MTF quantifies the maximum possible resolution of the total system, but DQE
quantifies the actual performance of the detector including its resolution, noise and dose
efficiency (exposure efficiency). The DQE function characterises the final image quality ver-
sus the inspection time required for a given application.
Increasing contrast
Decreasing noise
WORST
BEST
Fig. 23-16. Defect recognition versus contrast and noise
Fig. 24-16. Effect of image-averaging on noise and image quality
Fig. 25-16. Depth resolution by number of bits
DR image - one shot DR image - 16 shots
A
B
1 bit 2 bits 12 bits