ADVERTORIAL
Introduction to Direct
Digital Radiography (DR)
Kat Evans
Veterinary practices are frequently described as having
digital radiography, however, this can be a broad and
often inaccurate description. To prevent confusion, it
is recommended to use the terminology Direct Digital
Radiography (DR) or Computed Radiography (CR)
when describing ‘digital radiography’. In this article,
we will discuss the theory behind DR, and answer
some frequently asked questions.
What is DR?
Put simply, DR systems convert x-rays directly into
a digital signal after they have passed through the
patient. This information is then relayed to a computer
to enable you to see the radiograph on a monitor. This
differs from CR which requires a cassette to be read
or processed to enable visualisation of the image.
All digital systems will also apply algorithms, which are
also known as LUTs (Look Up Tables). These take the
raw image, which is normally very flat, and enhance it
to give us a usable image. Algorithms, and therefore
image appearance, can vary enormously depending
upon the quality of the equipment from different
manufacturers. Different DR systems will use different
receptor technologies to capture the raw image, but
most are based around a layer of Thin Film Transistors,
known as flat panel detectors. These are further sub-
divided into direct conversion and indirect conversion
detectors.
Figure 1. Direct conversion DR detector
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Direct conversion DR detector
In direct conversion detectors (Figure 1) x-ray photons
hit the receptor and interact with the atoms of the
photoconductor, converting the energy from the
x-ray photons into an electrical charge. The electrical
charge is then used by the thin film transistor array to
produce a digital image. The information that is used
to create the image is the location and strength with
which the photons hit the TFT layer. This interaction
works most effectively if the incoming x-ray photons
have a high energy level, i.e. are produced using a high kV.
Physical manufacturing limitations restrict the
resolution achievable by direct conversion. An array
of tiny detectors is used because it is physically
impossible to manufacture one single large detector
and inevitably this results in small ‘gaps’ between
the individual detectors. The photoconductor is
designed to maximise resolution by channelling
incoming ions towards the detectors. There are a
range of photoconductors used (the most common
is amorphous selenium) which vary in their sensitivity
to radiation, resolution and their ability to cope with
environmental stresses.
Indirect conversion DR receptor
Indirect DR receptors (Figure 2) have more layers than
direct DR receptors. There is a layer of scintillation
Figure 2. Indirect conversion DR receptor